Chris Leighton

D21. 00005: Dilute Magnetic Moments and their Influence on Electronic Transport in Single-Crystal PdCo 1-x Cr x O 2*

W21. 00004: Spin Signal Optimization in Metallic Non-Local Spin Valves via Tuned Interface Resistance*

Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for reasons that remain poorly understood. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 μm. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we report a different approach to PdCoO2 crystal growth, using chemical vapor transport methods to achieve order-of-magnitude gains in size, the highest structural qualities yet reported, and record residual resistivity ratios ( > 440). Nevertheless, detailed mass spectrometry measurements on these materials reveal that they are not ultrapure in a general sense, typically harboring 100s-of-parts …

Artificial spin ice, consisting of nanostructured magnetic elements, provides an excellent playground for exploring many-body phenomena. In our study, we focus on the influence of a complex geometrical transformation and the introduction of a topological defect within a small patch of the classical square ice lattice. By using magnetic imaging techniques, we unveil the existence of preferential magnetic flux channels. These channels persist due to the system's inherent topological nature that leads to a modification of the energy landscape. This topological transformation offers an exciting avenue for designing and fine-tuning controllable emergent flux channels, creating new opportunities for further research.* Work at Yale University was funded by the US Department of Energy (DOE), Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Grant No. DE-SC0020162. Work at the …

Pr-containing perovskite cobaltites exhibit unusual valence transitions, coupled to coincident structural, spin-state, and metal-insulator transitions. Heteroepitaxial strain was recently used to control these phenomena in the model (Pr 1− y Y y) 1− x Ca x Co O 3− δ system, stabilizing a nonmagnetic insulating phase under compression (with a room-temperature valence/spin-state/metal-insulator transition) and a ferromagnetic (FM) metallic phase under tension, thus exposing a potential spin-state quantum-critical point. The latter has been proposed in cobaltites and can be probed in this system as a function of a disorder-free variable (strain). We study this here via thickness-dependent strain relaxation in compressive SrLaAl O 4 (001)/(Pr 0.85 Y 0.15) 0.70 Ca 0.30 Co O 3− δ epitaxial thin films to quasicontinuously probe structural, electronic, and magnetic behaviors across the nonmagnetic-insulator/FM-metal …

K21. 00003: Decoupling the spin-state and insulator-metal transitions via heteroepitaxial strain in LaCoO 3-δ*

K21. 00004: Oral: Anomalous Transport and Interfacial Charge Transfer in Pr-based Cobaltite Heterostructures*

Perovskite cobaltites have emerged as archetypes for electrochemical control of materials properties in electrolyte-gate devices. Voltage-driven redox cycling can be performed between fully oxygenated perovskite and oxygen-vacancy-ordered brownmillerite phases, enabling exceptional modulation of the crystal structure, electronic transport, thermal transport, magnetism, and optical properties. The vast majority of studies, however, have focused heavily on the perovskite and brownmillerite end points. In contrast, here we focus on hysteresis and reversibility across the entire perovskite ↔ brownmillerite topotactic transformation, combining gate-voltage hysteresis loops, minor hysteresis loops, quantitative operando synchrotron X-ray diffraction, and temperature-dependent (magneto)transport, on ion-gel-gated ultrathin (10-unit-cell) epitaxial La0.5Sr0.5CoO3−δ films. Gate-voltage hysteresis loops combined with …

D21. 00007: Thermal and Magnetic Properties of the Metallic Delafossite Triangular-Lattice Antiferromagnet PdCrO 2*

Epitaxial cobaltites have emerged as exemplary materials for electrochemical gating, in large part due to their topotactic perovskite (P)↔ brownmillerite (BM) transformations. SrCoO3− δ, for example, can be cycled between metallic ferromagnetic P SrCoO3 and insulating BM SrCoO2. 5, realizing exceptional modulation of electronic, thermal, and optical properties. It is often presumed that such cycling also generates ferromagnetic–antiferromagnetic (F-AF) modulation due to the G-type AF order in bulk SrCoO2. 5. Little is understood about magnetism in thin-film BM SrCoO2. 5, however, meaning that the true magnetic property modulation is unclear. We address this here through a neutron diffraction study of BM La1− xSrxCoO2. 5 films at x= 0.5 and 1.0. Lightly compressively strained SrCoO2. 5 films are shown to retain G-type AF order, albeit with suppressed Néel temperature (∼ 340 K). Of high interest for AF …

The pyrite-structure transition-metal disulfide Ni S 2 is in principle a model cubic antiferromagnetic Mott insulator that can be doped through insulator-metal transitions with both electrons and holes (in Ni 1–x Cu x S 2 and Ni 1–x Co x S 2), eventually inducing superconductivity and ferromagnetism, respectively. Magnetism and transport have proven challenging to understand in Ni S 2, however. The antiferromagnetic spin structure below∼ 39 K is complex due to frustration, while unexplained weak ferromagnetism emerges below∼ 30 K. Surface conduction is also now understood to dominate in Ni S 2 at low temperatures, raising questions about the interpretation of decades of prior data. Here, we present a complete study of the surface magnetotransport phenomena that emerge at low temperatures in high-quality single-crystal Ni S 2, which turn out to be strikingly rich. On cooling, isotropic magnetoresistance due …

Materials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion-gel-gated epitaxial films of the perovskite cobaltite La1− xSrxCoO3− ???? (LSCO) with 0.00≤ x≤ 0.70 offer a new route to significant, voltage-tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig-consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As-grown perovskite films are high-index insulators for x< 0.18 but lossy metals for x> 0.18, due to a percolation insulator-metal transition. Positive-voltage gating of LSCO transistors with x> 0.18 reveals a metal-insulator transition from the metallic perovskite phase to a high-index (n> 2.5), low-loss insulating phase, accompanied by a perovskite to oxygen-vacancy-ordered brownmillerite transformation at high x. At x< 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower-index, lower-loss insulating perovskite state with ????n> 0.6. In simulations of plasmonic metasurfaces, these metal-insulator and insulator-insulator transitions support significant, varied mid-infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room-temperature phase-change materials for applications including dynamic thermal imaging, camouflage, and optical memories.

Solid-state control of the thermal conductivity of materials is of exceptional interest for novel devices such as thermal diodes and switches. Here, we demonstrate the ability to continuously tune the thermal conductivity of nanoscale films of La0.5Sr0.5CoO3-δ (LSCO) by a factor of over 5, via a room-temperature electrolyte-gate-induced non-volatile topotactic phase transformation from perovskite (with δ ≈ 0.1) to an oxygen-vacancy-ordered brownmillerite phase (with δ = 0.5), accompanied by a metal-insulator transition. Combining time-domain thermoreflectance and electronic transport measurements, model analyses based on molecular dynamics and Boltzmann transport equation, and structural characterization by X-ray diffraction, we uncover and deconvolve the effects of these transitions on heat carriers, including electrons and lattice vibrations. The wide-range continuous tunability of LSCO thermal conductivity …

We have experimentally investigated and imaged how the topology of a frustrated artificial spin ice nanomagnet array can affect and constrain its kinetics. Studying vertex-frustrated Santa Fe artificial spin ice, in which magnetic excitations are correlated in thermally active one-dimensional emergent strings, we imaged the thermally-driven motion of these emergent string excitations in real time. At high temperatures, the string kinetics include topologically complex changes, which are highly constrained at lower temperatures. This system provides an unusually accessible venue in which to study the effects of topology on ergodicity through direct measurement of the thermal kinetics.

The coexistence of different properties in the same material often results in exciting physical effects. At low temperatures, the pyrite transition-metal disulphide NiS2 hosts both antiferromagnetic and weak ferromagnetic orders, along with surface metallicity dominating its electronic transport. The interplay between such a complex magnetic structure and surface-dominated conduction in NiS2, however, is still not understood. A possible reason for this limited understanding is that NiS2 has been available primarily in bulk single-crystal form, which makes it difficult to perform studies combining magnetometry and transport measurements with high spatial resolution. Here, NiS2 nanoflakes are produced via mechanical cleaving and exfoliation of NiS2 single crystals and their properties are studied on a local (micron-size) scale. Strongly field-asymmetric magnetotransport features are found at low temperatures, which …

We study the collective behavior of interacting arrays of nanomagnetic tripods. These objects have six discrete moment states, in contrast to the usual two states of an Ising-like moment. Our experimental data demonstrate that triangular lattice arrays form a “tripod ice” that exhibits charge ordering among the effective vertex magnetic charges, in direct analogy to artificial kagome spin ice. The results indicate that the interacting tripods have effective moments that act as emergent local variables, with strong connections to the well-studied Potts and clock models. In addition, the tripod moments display a tendency toward a nearest neighbor alignment in our thermalized samples that separates this system from kagome spin ice. Our results open a path toward the study of the collective behavior of nonbinary moments that is unavailable in other physical systems.

Ergodic kinetics, which are critical to equilibrium thermodynamics, can be constrained by a system’s topology. We studied a model nanomagnetic array in which such constraints visibly affect the behavior of the magnetic moments. In this system, magnetic excitations connect into thermally active one-dimensional strings whose motion can be imaged in real time. At high temperatures, our data showed the merging, breaking, and reconnecting of strings, resulting in the system transitioning between topologically distinct configurations. Below a crossover temperature, the string motion is dominated by simple changes in length and shape. In this low-temperature regime, the system is energetically stable because of its inability to explore all possible topological configurations. This kinetic crossover suggests a generalizable conception of topologically broken ergodicity and limited equilibration.

Artificial spin ice (ASI) systems composed of arrays of frustrated interacting nanomagnets offer a valuable platform for designing and exploring collective magnetic phenomena. However, most ASI geometries explored to date employ stadium-shaped magnetic islands with Ising-like character, ie, the island moments are binary. We report the study of an artificial spin system based on a new type of artificial moment: a three-fold symmetric Y-shape island. The resulting non-binary nanomagnetic building block effectively behaves as a multipolar magnetic element, giving rise to behavior akin to a Potts-like 6-state clock model. These new magnetic objects enable the exploration of a variety of novel magnetic phases, opening a path to new collective phenomena and kinetics beyond the traditional ASI binary moment systems.

Materials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion-gel-gated epitaxial films of the perovskite cobaltite La 1− x Sr x CoO 3− δ (LSCO) with 0.00≤ x≤ 0.70 offer a new route to significant, voltage-tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig-consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As-grown perovskite films are high-index insulators for x< 0.18 but lossy metals for x> 0.18, due to a percolation insulator-metal transition. Positive-voltage gating of LSCO transistors with x> 0.18 reveals a metal-insulator transition from the metallic perovskite phase to a high-index (n> 2.5), low-loss insulating phase, accompanied by a perovskite to oxygen-vacancy-ordered brownmillerite transformation at high x. At x< 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower-index, lower-loss insulating perovskite state with Δn> 0.6. In simulations of plasmonic metasurfaces, these metal-insulator and insulator-insulator transitions support significant, varied mid-infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room-temperature phase-change materials for applications including dynamic thermal imaging, camouflage, and optical memories.

For applications such as spin accumulation sensors for next-generation hard disk drive read heads, and for fundamental research, it is desirable to increase the spin signal in metallic non-local spin valves, which are central devices in spintronics. To this end, here, we report on the integration of high-spin-polarization Co–Fe binary alloy ferromagnetic injectors and detectors in Al-based non-local spin valves. Room-temperature deposition on amorphous substrates from an alloy target is shown to generate smooth, polycrystalline (110-textured), solid-solution body-centered-cubic Co 75 Fe 25 films, which we characterize by energy dispersive x-ray spectroscopy, x-ray diffraction, x-ray reflectivity, atomic force microscopy, and electronic transport. Simple integration into transparent-interface Al non-local spin valves is then shown to realize up to a factor of∼ 5 enhancement of the spin signal relative to Co, with full …

We demonstrate reversible voltage control between metal/insulator and insulator/insulator lateral patterns of epitaxial perovskite oxides and oxyfluorides via electrolytic gating, leading to electric field modulation of anisotropic transport and optical responses. Lateral stripes of SrFeO 2.5/SrFeO 2 F were patterned on conducting SrTiO 3: Nb substrates through lithographically-defined topochemical fluorination reactions. Using a back-contact geometry, biasing across an ionic gel results in a conversion of the SrFeO 2.5 brownmillerite regions to SrFeO 3 perovskite, while SrFeO 2 F remains unchanged after biasing. The application of an oxidizing bias (V G=-3 V) converts the all-insulating SrFeO 2.5/SrFeO 2 F pattern into SrFeO 3/SrFeO 2 F metal/insulator lateral structures with highly anisotropic in-plane transport and strong optical contrast between stripes. The application of a positive bias (V G= 1 V) results in reduction …

Materials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion‐gel‐gated epitaxial films of the perovskite cobaltite La1−xSrxCoO3−δ (LSCO) with 0.00 ≤ x ≤ 0.70 offer a new route to significant, voltage‐tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig‐consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As‐grown perovskite films are high‐index insulators for x < 0.18 but lossy metals for x > 0.18, due to a percolation insulator‐metal transition. Positive‐voltage gating of LSCO transistors with x > 0.18 reveals a metal‐insulator transition from the metallic perovskite phase to a high‐index (n > 2.5), low‐loss insulating phase, accompanied by a …

These files contain data along with associated output from instrumentation supporting all results reported in Zhang, Yingying; Postiglione, William M.; Xie, Rui; Zhang, Chi; Zhou, Hao; Chaturvedi, Vipul; Heltemes, Kei; Zhou, Hua; Feng, Tianli; Leighton, Chris; Wang, Xiaojia Wide-range continuous tuning of the thermal conductivity of La0.5Sr0.5CoO3-delta films via room-temperature ion-gel gating. Solid-state control of the thermal conductivity of materials is of exceptional interest for novel devices such as thermal diodes and switches. Here, we demonstrate the ability to continuously tune the thermal conductivity of nanoscale films of La0.5Sr0.5CoO3-delta (LSCO) by a factor of over 5, via a room-temperature electrolyte-gate-induced non-volatile topotactic phase transformation from perovskite (with  ≈ 0.1) to an oxygen-vacancy-ordered brownmillerite phase (with  = 0.5), accompanied by a metal-insulator transition. Combining time-domain thermoreflectance and electronic transport measurements, model analyses based on molecular dynamics and Boltzmann transport, and structural characterization by X-ray diffraction, we uncover and deconvolve the effects of these transitions on heat carriers, including electrons and lattice vibrations. The wide-range continuous tunability of LSCO thermal conductivity enabled by low-voltage (below 4 V) room-temperature electrolyte gating opens the door to non-volatile dynamic control of thermal transport in perovskite-based functional materials, for thermal regulation and management in device applications. Authors to whom correspondence should be addressed are Chris Leighton (leighton@umn.edu) and …

Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for reasons that remain poorly understood. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 um. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we first report a new approach to PdCoO2 crystal growth, using chemical vapor transport methods to achieve order-of-magnitude gains in size, the highest structural qualities yet reported, and record residual resistivity ratios (>440). Nevertheless, the first detailed mass spectrometry measurements on these materials reveal that they are not ultrapure, typically harboring 100s-of-parts-per-million impurity levels. Through quantitative crystal-chemical analyses, we resolve this apparent dichotomy, showing that the vast majority of impurities are forced to reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly pure (~1 ppm impurity concentrations). These purities are shown to be in quantitative agreement with measured residual resistivities. We thus conclude that a previously unconsidered “sublattice purification” mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites. This dataset contains all digital data in the published paper of the same name.

Reversible voltage control of electronic, magnetic, and optical properties over extraordinary ranges has recently been demonstrated in electrolyte-gated perovskite cobaltite films. This is achieved by reversibly cycling between perovskite (P) and oxygen-vacancy-ordered brownmillerite (BM) phases via electrochemical control. Most studies focus on the end states alone, however, ie P and BM, and little has been reported on the detailed gate voltage (V g) dependence. Here, using ion gel gating of 10 unit-cell-thick La 0.5 Sr 0.5 CoO 3-δ films, we reveal that probing hysteresis loops around the P-BM transformation provides a wealth of new information. We combine source-drain current and gate current hysteresis loops with operando synchrotron X-ray diffraction, and transport and magnetotransport measurements. These reveal asymmetric hysteresis, non-monotonic transformation rates, and limits on reversibility …

Pr-based cobaltites exhibit extraordinary phenomena where abrupt valence shifts trigger coupled structural/spin-state/metal-insulator transitions. Recent work achieved strain control of these phenomena in thin films, with epitaxial compression even stabilizing room-temperature transitions. Here, we study the thickness dependence of these effects in the model system compressively strained YAl O 3 (101)/(Pr 0.85 Y 0.15) 0.7 Ca 0.3 Co O 3− δ (001). Transport data reveal highly unusual behavior where thicker films exhibit two transitions: one at the fully strained temperature (∼ 245 K) and one near the bulk (∼ 135 K). High-resolution x-ray diffraction confirms that this is due to anomalous strain relaxation where, immediately above the critical thickness (∼ 30 nm), a film region with a near-bulk lattice parameter coexists with a fully strained region. Scanning transmission electron microscopy then reveals striking images …

Direct detection of spontaneous spin fluctuations, or “magnetization noise,” is emerging as a powerful means of revealing and studying magnetic excitations in both natural and artificial frustrated magnets. Depending on the lattice and nature of the frustration, these excitations can often be described as fractionalized quasiparticles possessing an effective magnetic charge. Here, by combining ultrasensitive optical detection of thermodynamic magnetization noise with Monte Carlo simulations, we reveal emergent regimes of magnetic excitations in artificial “tetris ice.” A marked increase of the intrinsic noise at certain applied magnetic fields heralds the spontaneous proliferation of fractionalized excitations, which can diffuse independently, without cost in energy, along specific quasi-1D spin chains in the tetris ice lattice.

Pyrite FeS2 has extraordinary potential as a low-cost, nontoxic, sustainable photovoltaic but has underperformed dramatically in prior solar cells. The latter devices focus on heterojunction designs, which are now understood to suffer from problems associated with FeS2 surfaces. Simpler homojunction cells thus become appealing but have not been fabricated due to the historical inability to understand and control doping in pyrite. While recent advances have put S-vacancy and Co-based n-doping of FeS2 on a firm footing, unequivocal evidence for bulk p-doping remains elusive. Here, we demonstrate the first unambiguous and controlled p-type transport in FeS2 single crystals doped with phosphorus (P) during chemical vapor transport growth. P doping is found to be possible up to at least ∼100 ppm, inducing ∼1018 holes/cm3 at 300 K, while leaving the crystal structure and quality unchanged. As the P doping is …

Direct detection of equilibrium spin fluctuations, or''magnetization noise'', is emerging as a powerful means of revealing and studying topologically-protected magnetic excitations in both natural and artificial frustrated magnets. Depending on the lattice geometry and nature of the frustration, these excitations can often be described as quasiparticles that behave as mobile magnetic charges. Here, using ultrasensitive optical detection of spontaneous magnetization noise in a thermally active system, supported by additional insight from Monte Carlo simulations, we reveal new regimes of mobile magnetic excitations in the vertex-frustrated artificial spin ice lattices, including tetris and Shakti geometries. A substantial increase of the detected noise for specific directions and ranges of applied in-plane magnetic field heralds the proliferation of these excitations, which can diffuse freely, without any cost in energy, along …

Metallic delafossite materials (eg, PdCo O 2, PtCo O 2), have attracted much recent attention due to record-high oxide conductivities, the origins of which remain unclear. Relatively little attention has been paid to their related thermal properties, however. We address this here via wide temperature range experimental studies of the crystal structure, thermal expansion, and specific heat of single-crystal PdCo O 2, combined with density-functional theory (DFT) calculations of the electronic and phononic densities of states, and thus thermal properties. PdCo O 2 is shown to retain the R 3 m space group from 12 to 1000 K, exhibiting a-and c-axis thermal expansion in good quantitative agreement with DFT-based lattice dynamics calculations. The Co-O bond lengths additionally elucidate the stability of the low-spin state of the nominally Co 3+ ions, which is a notable difference between the edge-shared Co-O octahedra in …

Electrolyte-gate transistors are a powerful platform for control of material properties, spanning semiconducting behavior, insulator-metal transitions, superconductivity, magnetism, optical properties, etc. When applied to magnetic materials, for example, electrolyte-gate devices are promising for magnetoionics, wherein voltage-driven ionic motion enables low-power control of magnetic order and properties. The mechanisms of electrolyte gating with ionic liquids and gels vary from predominantly electrostatic to entirely electrochemical, however, sometimes even in single material families, for reasons that remain unclear. In this Perspective, we compare literature ionic liquid and ion gel gating data on two rather different material classes—perovskite oxides and pyrite-structure sulfides—seeking to understand which material factors dictate the electrostatic vs electrochemical gate response. From these comparisons, we …

Pyrite FeS 2 is a low-cost, sustainable, non-toxic 1-eV-gap semiconductor with unrealized potential in applications such as photovoltaics. From the fundamental perspective, issues such as surface conduction and the deep-donor nature of sulfur vacancies have strongly hindered the study of low T transport and phenomena such as the insulator-metal transition (IMT). In this work, a novel CoS 2-based contact scheme is used to access bulk transport, enabling wide-T-range transport studies of thoroughly characterized Co-doped FeS 2 single crystals. The IMT is found to occur near 3× 10 17 cm-3 Hall density, with Efros-Shklovskii variable-range hopping below this, and electron-electron-interaction-corrected conductivity above this. In the vicinity of the IMT a host of intriguing phenomena emerge, including non-linear Hall effect with a non-monotonic T dependence, and non-saturating non-parabolic positive low-T …

Doped perovskite cobaltites (eg, La 1− x Sr x CoO 3) have been extensively studied for their spin-state physics, electronic inhomogeneity, and insulator-metal transitions. Ferromagnetically interacting spin-state polarons emerge at low x in the phase diagram of these compounds, eventually yielding long-range ferromagnetism. The onset of long-range ferromagnetism (x≈ 0.18) is substantially delayed relative to polaron percolation (x≈ 0.05), however, generating a troubling inconsistency. Here, Monte Carlo simulations of a disordered classical spin model are used to establish that previously ignored magnetic frustration is responsible for this effect, enabling faithful reproduction of the magnetic phase diagram.

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-delta, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. This dataset contains all digital data published in the Nature Communications paper of the same name.

Applied magnetic fields are an important tuning parameter for artificial spin ice (ASI) systems, as they can drive phase transitions between different magnetic ground states or tune through regimes with high populations of emergent magnetic excitations (eg, monopole-like quasiparticles). Here, using simulations supported by experiments, we investigate the thermodynamic properties and magnetic phases of square and quadrupolar ASI as a function of applied in-plane magnetic fields. Monte Carlo simulations are used to generate field-dependent maps of the magnetization, the magnetic specific heat, the thermodynamic magnetization fluctuations, and the magnetic order parameters, all under equilibrium conditions. These maps reveal the diversity of magnetic orderings and the phase transitions that occur in different regions of the phase diagrams of these ASIs, and are experimentally supported by magnetooptical …

We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator NiS 2. Through temperature-and gate-voltage-dependent electronic transport measurements, a gating-induced three-dimensional metallic state is observed at positive gate bias on single-crystal surfaces. Based on transport, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an electrochemical gating mechanism involving a substantial decrease in the S: Ni ratio over hundreds of nanometers, which is both nonvolatile and irreversible. Such findings are in striking contrast to the reversible, volatile, two-dimensional electrostatic gate effect previously seen in pyrite FeS 2. We attribute this stark difference in electrochemical vs electrostatic gating response in NiS 2 and FeS 2 to the much larger S diffusion coefficient in NiS 2. The gating irreversibility, on the …

The reversible topotactic transformation between perovskite (P) SrCoO3-δ and oxygen-vacancy-ordered brownmillerite (BM) SrCoO2. 5 has attracted much attention recently due to wide modulation of electronic, magnetic, and optical properties, particularly when voltage-triggered in electrolyte gating. SrCoO 3 is relatively unstable, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P to BM transformations across almost the entire La1-x Sr x CoO3-δ phase diagram (0< x< 0.7). Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish the P to BM transformation at nearly all x, including x< 0.5, where both P and BM are highly air-stable. The transformation threshold voltage decreases significantly with x, but also with epitaxial strain (both tensile and compressive), providing desirable tunability for devices …

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications …

Antimony selenide (Sb2Se3) is a promising absorber material for thin-film photovoltaics. However, certain areas of fundamental understanding of this material remain incomplete and this presents a barrier to further efficiency gains. In particular, recent studies have highlighted the role of majority carrier type and extrinsic doping in drastically changing the performance of high efficiency devices (Hobson et al 2020 Chem. Mater. 32 2621–30). Herein, Sn-doped Sb2Se3 bulk crystals are shown to exhibit p-type conductivity using Hall effect and hot-probe measurements. The measured conductivities are higher than those achieved through native defects alone, but with a carrier density (up to 7.4 × 1014 cm−3) several orders of magnitude smaller than the quantity of Sn included in the source material. Additionally, a combination of ultraviolet, x-ray and hard x-ray photoemission spectroscopies are employed to obtain a …

Long-range ordering is typically associated with a decrease in entropy. Yet, it can also be driven by increasing entropy in certain special cases. Here we demonstrate that artificial spin-ice arrays of single-domain nanomagnets can be designed to produce such entropy-driven order. We focus on the tetris artificial spin-ice structure, a highly frustrated array geometry with a zero-point Pauling entropy, which is formed by selectively creating regular vacancies on the canonical square ice lattice. We probe thermally active tetris artificial spin ice both experimentally and through simulations, measuring the magnetic moments of the individual nanomagnets. We find two-dimensional magnetic ordering in one subset of these moments, which we demonstrate to be induced by disorder (that is, increased entropy) in another subset of the moments. In contrast with other entropy-driven systems, the discrete degrees of freedom in …

Motivated by the existence of superconductivity in pyrite-structure CuS, we explore the possibility of ionic-liquid-gating-induced superconductivity in the proximal antiferromagnetic Mott insulator NiS. A clear gating-induced transition from a two-dimensional insulating state to a three-dimensional metallic state is observed at positive gate bias on single crystal surfaces. No evidence for superconductivity is observed down to the lowest measured temperature of 0.45 K, however. Based on transport, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an electrochemical gating mechanism involving a substantial decrease in the S:Ni ratio (over hundreds of nm), which is both non-volatile and irreversible. This is in striking contrast to the reversible, volatile, surface-limited, electrostatic gate effect in pyrite FeS. We attribute this stark difference in electrochemical vs. electrostatic gating response in NiS and FeS to the much larger S diffusion coefficient in NiS, analogous to the different behaviors observed among electrolyte-gated oxides with differing O-vacancy diffusivities. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S; this is in contrast to the better understood oxide case, where electrolysis of atmospheric HO provides an O reservoir. This study of NiS thus provides new insight into electrolyte gating mechanisms in functional materials, in a previously unexplored limit.

The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach using irreversible, plastic deformation of single crystals. We show that compressive plastic deformation induces low-dimensional superconductivity well above the superconducting transition temperature (Tc) of undeformed SrTiO3, with evidence of possible superconducting correlations at temperatures two orders of magnitude above the bulk Tc. The enhanced superconductivity is correlated with the appearance of self-organized dislocation structures, as revealed by diffuse neutron and X-ray scattering. We also observe deformation-induced signatures of quantum-critical ferroelectric fluctuations and inhomogeneous ferroelectric order using Raman scattering. Our results suggest that strain surrounding the self-organized dislocation structures …

OBJECTIVES: Wilson’s disease (WD) is a rare autosomal recessive disease caused by ATPB7 gene mutations that alter copper metabolism. Despite high prevalence, few studies have addressed the epidemiology and clinical characteristics of WD in Turkey. We analyzed data of 868 WD patients (Ferenci score>= 3) diagnosed between 1993-2021 in 43 hepatology centers in Turkey.MATERIALS & METHODS: A total of 868 patients (57.9% Male and 42.1% Female) were analyzed. 40 patients were adults. The mean diagnostic age was 118.78±65.734 months. The prevalence of consanguineous marriage was 59%(74.8% 1 st degree, 19.4% 2 nd degree and 5.8%>= 3 rd degree). 45.8% of the patients had family history for WD (80.7% 1 st degree, 8.5% 2 nd degree and 10.8%>= 3 rd degree). 22.9% of the patients were diagnosed with family screening. Clinical manifestations, biochemical, genetic and histological findings are summarized in Tables 1, 2.RESULTS: Zinc monotherapy was initiated in 11 patients (1.2%). 113 patients (13%) were treated with D-penicillamine (DP) alone. 530 patients (61.1%) received combination of DP and zinc treatments. 23 patients (2.6%) only received Trientine treatment while 73 patients were treated with Trientine in combination with zinc (8.4%). DP side effects were reported in 8.5% of the patients. Main side effects were urticarial rash (2.3%), proteinuria (1.4%) and deterioration of neurological symptoms (0.7%). 87 patients (% 10) underwent liver transplantation (LT)(63.1% living donor, 36.9% cadaveric donor). 31/70 (% 44) of patients presented with acute liver failure (ALF) underwent LT. Mortality rate after LT …

CdO has drawn much recent interest as a high-room-temperature-mobility oxide semiconductor with exciting potential for mid-infrared photonics and plasmonics. Wide-range modulation of carrier density in CdO is of interest both for fundamental reasons (to explore transport mechanisms in single samples) and for applications (in tunable photonic devices). Here, we thus apply ion-gel-based electrolyte gating to ultrathin epitaxial CdO (001) films, using transport, x-ray diffraction, and atomic force microscopy to deduce a reversible electrostatic gate response from− 4 to+ 2 V, followed by rapid film degradation at higher gate voltage. Further advancing the mechanistic understanding of electrolyte gating, these observations are explained in terms of low oxygen vacancy diffusivity and high acid etchability in CdO. Most importantly, the 6-V-wide reversible electrostatic gating window is shown to enable ten-fold modulation …

Hot probe data demonstrating p-type conductivity in Sb2Se3; Hall effect data showing conductivity type, carrier density and carrier mobility in Sb2Se3; Capacitance-voltage data showing carrier density in Sb2Se3; Secondary ion mass spectroscopy data showing Sn incorporation into Sb2Se3.

Pr-based cobaltites such as Pr1-x Ca x CoO3-δ and (Pr1-y Y y) 1-x Ca x CoO3-δ exhibit remarkable first-order coupled spin-state/metal-insulator/structural transitions driven by a unique Pr valence transition. Such phenomena offer highly appealing device functionality, but have thus far been confined to cryogenic temperatures in bulk materials. Here, we combine epitaxy on multiple substrates with transport, magnetometry, polarized neutron reflectometry, and temperature-dependent synchrotron X-ray diffraction and electron energy loss spectroscopy to demonstrate complete control over the electronic ground state of (Pr1-y Y y) 1-x Ca x CoO3-δ films. Compressive strain realizes enhancement of the spin-state/metal-insulator transition to at least 245 K, with synchrotron diffraction and electron energy loss spectroscopy confirming a Pr/Co valence shift. Conversely, tensile strain stabilizes metallic ferromagnetism with …

We study the temperature and magnetic field dependence of the total magnetic moment of large-area permalloy artificial square spin ice arrays. The temperature dependence and hysteresis behavior are consistent with the coherent magnetization reversal expected in the Stoner-Wohlfarth model, with clear deviations due to interisland interactions at small lattice spacing. Through micromagnetic simulations, we explore this behavior and demonstrate that the deviations result from increasingly complex magnetization reversal at small lattice spacing, induced by interisland interactions, and depending critically on details of the island shapes. These results establish new means to tune the physical properties of artificial spin ice structures and other interacting nanomagnet systems, such as patterned magnetic media.

Dynamic control of patterned properties in perovskite oxide films can enable new architectures for electronic, magnetic, and optical devices. In this study, it is shown that SrFeO3‐δ/SrFeO2F laterally‐heterostructured films enable voltage‐controlled tunable and reversible metal‐insulator patterned properties using room‐temperature ion gel gating. Specifically, SrFeO3‐δ film regions can be toggled between insulating HxSrFeO2.5 and metallic SrFeO3 by electrochemical redox, while SrFeO2F regions remain robustly insulating and are unaffected by ion gel gating. Various gating architectures are also compared and establish the advantages of employing a conductive substrate as the contacting electrode, as opposed to at the film surface, thereby achieving complete and reversible reduction and oxidation among SrFeO3‐δ, HxSrFeO2.5, and SrFeO3. This approach to voltage‐modulated patterned electronic, optical …

The Elliott-Yafet theory of spin relaxation in nonmagnetic metals predicts proportionality between spin and momentum relaxation times for scattering centers such as phonons. Here, we test this theory in Al nanowires over a very large thickness range (8.5–300 nm), finding that the Elliott-Yafet proportionality “constant” for phonon scattering in fact exhibits a large, unanticipated finite-size effect. Supported by analytical and numerical modeling, we explain this via strong phonon-induced spin relaxation at surfaces and interfaces, driven in particular by enhanced spin-orbit coupling.

Pr-based cobaltites such as Pr1-x Ca x CoO3-δ and (Pr1-y Y y) 1-x Ca x CoO3-δ exhibit remarkable first-order coupled spin-state/metal-insulator/structural transitions driven by a unique Pr valence transition. While such phenomena are restricted to cryogenic temperatures in bulk, recent work of ours stabilized a valence transition to T vt= 245 K in compressively strained (Pr1-y Y y) 1-x Ca x CoO3-δ films. Here, we explore the effects of strain relaxation in such films. Careful analysis of temperature-dependent resistivity reveals splitting of the valence transition into two with increasing film thickness, one at the fully strained T vt and one at the bulk T vt. In-plane and out-of-plane lattice parameters from specular X-ray diffraction and asymmetric reciprocal space maps support this picture, evidencing partial strain relaxation with increasing thickness. Remarkably, in the ultrathin limit below~ 10 unit cells, T vt remains constant …

Advances in the synthesis and characterization of colloidal magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behavior, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic system, for example, achieved quantitative understanding of the superparamagnetic blocking temperature–particle diameter relationship. This hinged, however, on the critical assumption of a ferromagnetic NP volume lower than the chemical volume due to a non-ferromagnetic dead shell indirectly deduced from magnetometry. Here, we determine both the chemical and magnetic average internal structures of Ni NP ensembles via unpolarized, half-polarized, and fully polarized small-angle neutron scattering (SANS) measurements and analyses coupled with X-ray diffraction and magnetometry. The postulated nanometric magnetic dead shell is …

Pyrite FeS 2 is an outstanding candidate for a low-cost, nontoxic, sustainable photovoltaic material, but efficient pyrite-based solar cells are yet to materialize. Recent studies of single crystals have shed much light on this by uncovering a p-type surface inversion layer on n-type (S-vacancy doped) crystals, and the resulting internal pn junction. This leaky internal junction likely plays a key role in limiting efficiency in pyrite-based photovoltaic devices, also obscuring the true bulk semiconducting transport properties of pyrite crystals. Here, we demonstrate complete mitigation of the internal pn junction in FeS 2 crystals by fabricating metallic CoS 2 contacts via a process that simultaneously diffuses Co (a shallow donor) into the crystal, the resulting heavy n doping yielding direct Ohmic contact to the interior. Low-temperature bulk transport studies of controllably Co-and S-vacancy doped semiconducting crystals then …

The nonlocal spin valve (NLSV) enables unambiguous study of spin transport, owing to its ability to isolate pure spin currents. A key principle of NLSV operation is that the “spin signal” is invariant under application of in-plane magnetic fields (above the ferromagnetic contact saturation field). Yet, for certain ferromagnet/normal metal pairings in NLSVs, an unexpected field enhancement of the spin signal occurs, presenting a challenge that has, thus far, been difficult to resolve with existing models. By correlating the extracted spin transport parameters with material, temperature, and field dependencies, in this work we identify field quenching of magnetic impurity scattering as the origin of this effect, confirmed by excellent agreement between our results and field-dependent Kondo theory. In addition to addressing this long-standing mystery, our findings highlight a potential systematic underestimation of spin transport …

Pyrite structure transition-metal disulfides exhibit diverse ground states vs d-band filling, spanning diamagnetic semiconducting, ferromagnetic metallic, antiferromagnetic Mott insulating, and superconducting in Fe S 2, Co S 2, Ni S 2, and Cu S 2. Ni S 2 is particularly interesting and poorly understood as its Mott insulating behavior is accompanied by complex antiferromagnetic ordering below∼ 38 K and perplexing weak ferromagnetism below∼ 30 K. Temperature-, pressure-, and composition-dependent insulator-metal transitions also occur, particularly in bandwidth-controlled Ni S 2− x Se x, hole-doped Ni 1− x Co x S 2, etc. Here, we use high-quality chemical-vapor-transport-grown Ni S 2 single crystals characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetometry, and extensive transport and magnetotransport measurements, to generate new insight into this system. In particular …

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being shown capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that such devices either electrically tune a known ferromagnet, or electrically induce ferromagnetism from another magnetic state, eg, antiferromagnetic. Here, we provide proof-of-principle that ferromagnetism can be voltage-induced even from a diamagnetic, ie, zero-spin state, suggesting that useful magnetic phases could be electrically-induced in``non-magnetic''materials. We use ionic-liquid-gated diamagnetic FeS 2 as a model system, showing that as little as 1 V induces a reversible insulator-metal transition, driven by electrostatic …

We explore the magnetically ordered ground state of the isovalently substituted Mott insulator Y 1− x La x Ti O 3 for x≤ 0.3 via single-crystal growth, magnetometry, neutron diffraction, x-ray magnetic circular dichroism, muon spin rotation, and small-angle neutron scattering (SANS). We find that the decrease in the magnetic transition temperature on approaching the ferromagnetic-antiferromagnetic phase boundary at the La concentration x c≈ 0.3 is accompanied by a strong suppression of both bulk and local ordered magnetic moments, along with a volumewise separation into magnetically ordered and paramagnetic regions. The thermal phase transition does not show conventional second-order behavior since neither a clear signature of dynamic critical behavior nor a power-law divergence of the magnetic correlation length is found for the studied substitution range; this finding becomes increasingly obvious with …

Electrolyte-gate-induced perovskite to brownmillerite transformations in La1-xSrxCoO3-d (LSCO) has been shown to be a facile technique to toggle between disparate electronic and magnetic phases in a single perovskite oxide thin film. Here we study the doping (Sr concentration), and strain (epitaxially imparted from the substrate) dependence of this topotactic transformation in LSCO thin films across almost the entire phase diagram. This repository page serves as a place to store the Figure plots and raw data from the cited publication.

Rare-earth titanates RTiO 3 (R= La... Lu or Y) are prototypical Mott-Hubbard insulators and are of great interest for studying the correlations among electronic, magnetic, and orbital orders. Among them, DyTiO 3 (DTO) has the highest Curie temperature (T c) of 60 K, as well as typically high coercive field and remanent magnetization. Nevertheless, due to its highly distorted structure, the growth of high-quality DTO films remains challenging. In this study, we demonstrate the first hybrid molecular beam epitaxy growth of DTO films on LaAlO 3 (001) and DyScO 3 (110) substrates. Films grown at optimized stoichiometric conditions show clear ferromagnetism with bulk-like T c= 60 K. Spectroscopic analysis confirms these DTO films as nominally stoichiometric. By systematically tuning the Dy flux, we find that small deviations in cation stoichiometry lead to simultaneous disappearance of film peaks in X-ray diffraction and …

The random substitutional solid solution between the antiferromagnetic (AFM) full-Heusler alloy Ru 2 MnSn and the ferromagnetic (FM) full-Heusler alloy Ru 2 FeSn provides a rare opportunity to study FM-AFM phase competition in a near-lattice-matched, cubic system, with full solubility. At intermediate x in Ru 2 Mn 1− x Fe x Sn this system displays suppressed magnetic ordering temperatures, spatially coexisting FM and AFM order, and strong coercivity enhancement, despite rigorous chemical homogeneity. Here, we construct the most detailed temperature-and x-dependent understanding of the magnetic phase competition and coexistence in this system to date, combining wide-temperature-range neutron diffraction and small-angle neutron scattering with magnetometry and specific heat measurements on thoroughly characterized polycrystals. A complete magnetic phase diagram is generated, showing FM-AFM …

One-dimensional strings of local excitations are a fascinating feature of the physical behavior of strongly correlated topological quantum matter. Here we study strings of local excitations in a classical system of interacting nanomagnets, the Santa Fe Ice geometry of artificial spin ice. We measured the moment configuration of the nanomagnets, both after annealing near the ferromagnetic Curie point and in a thermally dynamic state. While the Santa Fe Ice lattice structure is complex, we demonstrate that its disordered magnetic state is naturally described within a framework of emergent strings. We show experimentally that the string length follows a simple Boltzmann distribution with an energy scale that is associated with the system’s magnetic interactions and is consistent with theoretical predictions. The results demonstrate that string descriptions and associated topological characteristics are not unique to quantum …

The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach: irreversible, plastic deformation of single crystals. We show that compressive plastic deformation of SrTiO 3 (STO) induces low-dimensional superconductivity significantly above the superconducting transition temperature (T c) of undeformed samples. We furthermore present evidence for unusual normal-state transport behaviour that suggests superconducting correlations at temperatures two orders of magnitude above the bulk T c. The superconductivity enhancement is correlated with the appearance of self-organized dislocation structures, revealed by diffuse neutron and X-ray scattering. These results suggest that T c in STO is strongly influenced by the local strain around dislocations, consistent with a theory of superconductivity enhanced by …

The wide band gap semiconducting perovskite BaSn O 3 is of high current interest due to outstanding room temperature mobility at high electron density, fueled by potential applications in oxide, transparent, and power electronics. Due in part to a lack of lattice-matched substrates, BaSn O 3 thin films suffer from high defect densities, however, limiting electron mobility. Additionally, the vast majority of BaSn O 3 thin film research has focused on pulsed laser deposition or molecular beam epitaxy. Here, we present an exhaustive optimization of the mobility of Ba 0.98 La 0.02 Sn O 3 films grown by a scalable, high-throughput method: high-pressure-oxygen sputter deposition. Considering target synthesis conditions, substrate selection, buffer layer structure, deposition temperature, deposition rate, thickness, and postdeposition annealing conditions, and by combining high-resolution x-ray diffraction, reciprocal space …

We have measured magnetic-field-induced avalanches in a square artificial spin ice array of interacting nanomagnets. Starting from the ground state ordered configuration, we imaged the individual nanomagnet moments after each successive application of an incrementally increasing field. The statistics of the evolution of the moment configuration show good agreement with the canonical one-dimensional random field Ising model. We extract information about the microscopic structure of the arrays from our macroscopic measurements of their collective behavior, demonstrating a process that could be applied to other systems exhibiting avalanches.

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in some praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials. Here, we show that in thin films of the perovskite (Pr1-yYy) 1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valencedriven spin-state/structural/metal-insulator transitions to at least 245 K, potentially within reach of room temperature. The technological implications of this result are accompanied by fundamental

Artificial spin ices (ASIs) are interacting arrays of lithographically defined nanomagnets in which novel, frustrated magnetic phases can be intentionally designed. A key emergent description of fundamental excitations in ASIs is that of magnetic monopoles—mobile quasiparticles that carry an effective magnetic charge. Here, we demonstrate that the archetypal square ASI lattice can host, in specific regions of its magnetic phase diagram, plasmalike regimes containing a high density of mobile magnetic monopoles. These regimes result from the magnetic field-tunable tension on the Dirac strings connecting mobile monopoles. By passively “listening” to spontaneous monopole noise under conditions of strict thermal equilibrium, we reveal their intrinsic dynamics and show that monopole kinetics are most diffusive (that is, minimally correlated) in the plasma regime. These results open the door to on-demand monopole …

Ultrafast electron microscopy (UEM) with in situ femtosecond (fs) photoexcitation capabilities has been used to study local photoinduced structural dynamics in a variety of nanoscale and nanostructured materials [1-3]. For quantitative photoexcitation conditions so one can determine, for example, the precise energy density absorbed by the specimen, one must have a precise and accurate way to determine the pump-laser spot characteristics at the specimen position. This proves challenging owing to the restricted access and confined space of the region between the objective-lens pole pieces, thus typically necessitating the use of ex situ extrapolation methods. For example, a simple method commonly employed is to measure the laser-beam parameters ex situ on the laser table and then extrapolate the properties via calculation along the remaining pathlength and intervening optics to the specimen position in the …

The appearance of one-dimensional strings of local excitations represents an interesting feature of the physical behavior of strongly correlated topological quantum matter. Here we demonstrate that strings of local excitations can also describe the physics of a classical thermal system of interacting nanomagnets, the Santa Fe Ice geometry of artificial spin ice. We measure the moment configuration of the nanomagnets, both after annealing near the ferromagnetic Curie point and in a thermally dynamic state. While the Santa Fe Ice lattice structure is complex, we demonstrate that its disordered magnetic state is naturally described within a framework of emergent strings. We show experimentally that the string length follows a simple Boltzmann distribution with an energy scale that is associated with the system’s magnetic interactions and that is consistent with theoretical predictions. The results demonstrate that string …

Tensile-strained LaCo O 3− δ thin films are ferromagnetic, in sharp contrast to the zero-spin bulk, although no clear consensus has emerged as to the origin of this phenomenon. While magnetism has been heavily studied, relatively little attention has been paid to electronic transport, due to the insulating nature of the strain-stabilized ferromagnetic state. Here, structure, magnetism, and transport are studied in epitaxial LaCo O 3− δ films (10–22-nm thick) on various substrates (from 1.4% compressive to 2.5% tensile strain), using synchrotron x-ray diffraction, scanning probe and transmission electron microscopy, magnetometry, polarized neutron reflectometry, resistivity, and Hall effect. High quality, smooth films are obtained, exhibiting superstructures associated with both oxygen vacancy ordering and periodic in-plane ferroelastic domains. Consistent with prior work, ferromagnetism with an approximately 80–85 K …

Systems of interacting nanomagnets known as artificial spin ices are models for exotic behavior due to their accessibility to geometries and measurement modalities that are not available in natural materials 1, 2, 3, 4, 5. Despite being fundamentally composed of binary moments, these systems often display collective phenomena associated with emergent higher-order frustration. We have studied the vertex-frustrated Santa Fe ice 6, examining its moment structure both after annealing near the ferromagnetic Curie point, and in a thermally dynamic state. We experimentally demonstrate the existence of a disordered string ground state, in which the magnetic structure can be understood through the topology of emergent strings of local excitations. We also show that the system can support a long-range-ordered ground state for certain ratios of local interactions. Both states are accessible via moment reversals only through topological surgery, 7 ie, the breaking of pairs of crossed strings and their reattachment in topologically inequivalent configurations. While we observe instances of topological surgery in our experimental data, such events are energetically suppressed, and we find that an apparent kinetic bottleneck associated with topological surgery precludes the system from achieving either ground state through local moment flips. Santa Fe ice thus represents an unusual instance of competition between topological complexity and ordering, suggesting analogous structures in the quantum realm.

We present measurements of in-plane thermal and electrical conductivity in thermally evaporated gold thin-film samples ranging in thickness from≈ 20 to> 300 nm, performed using a micromachined silicon-nitride membrane thermal isolation platform. In both≈ 300-nm-thick films grown in a single Au deposition and a sample built up to> 300 nm by many sequential depositions of thinner layers, we observe strong “violations” of the Wiedemann-Franz law that relates electrical and thermal conductivities. While electrical conductivity behaves essentially as expected, thermal conductivity first rises with growing total film thickness, and then surprisingly drops as the film becomes thicker. The sharp reduction of thermal conductivity decreases the Lorenz number L for≈ 300− nm-thick samples to less than half the Sommerfeld value over the entire 78–300-K temperature range studied. Such violation near room temperature …

Ionic liquid (IL) gating has proven remarkably effective in voltage control of superconductivity, insulator-metal transitions, and magnetism. This is in large part due to its high electric fields, and thus large accumulated surface charge densities (> 10 14 cm-2). Recent studies, however, emphasize the importance of distinguishing electrostatic from electrochemical gating mechanisms in such devices. Here, we present a detailed study of the transport dynamics of IL-gated FeS 2 single crystals, where a positive gate voltage is observed to induce a remarkable insulator-metal and diamagnetic-ferromagnetic transition. This transition is found to be highly reversible in transport, which, given the delicate nature of surface conduction in FeS 2 [1], strongly evidences an electrostatic gating mechanism. Hysteretic gate voltage sweeps suggest the electrostatic electron accumulation and depletion to be spatially non-uniform, with a …

Pyrite FeS 2 is a potentially ideal absorber for thin film solar cells as it is composed of earth-abundant, inexpensive elements, has a suitable band gap (0.95 eV), and is strongly absorptive. Lack of doping control, however, has precluded p-n homojunctions. Heterojunction solar cells have disappointing efficiencies (≤ 3%), likely limited by a leaky surface inversion layer. The ability to controllably n-and p-dope FeS 2 would make possible a homojunction solar cell that could, uniquely, avoid this surface inversion entirely. Recently, we established S vacancies as n-dopants in FeS 2 and achieved transport control in bulk crystals. Here, we demonstrate p-doping by introducing phosphorus (P) during growth. Increasing P concentrations above~ 40 ppm triggers a majority carrier inversion from n-to p-type. Thermal activation energy, room temperature hole density, and mobility in p-type crystals are~ 170 meV,~ 10 18 cm-3 …

Data includes temperature-dependent electronic transport (sheet resistance, electron density, and mobility) of ion-gel-gated BaSnO3 thin films of various thicknesses and growth methods. The mobility vs electron density experimental data and the fitting results (fits) are also provided.

Pyrite FeS2 has long been considered a potentially ideal photovoltaic material, but solar cells utilizing pyrite exhibit low open-circuit voltages (VOC) and have failed to achieve conversion efficiencies >3%. The recent discovery of a conductive p-type surface layer on n-type pyrite single crystals raises the intriguing possibility that the low VOC results from a leaky internal p–n junction between the surface and interior. Here, we reveal this internal junction, for the first time, through horizontal electronic transport measurements on sulfur vacancy (VS)- and Co-doped n-type pyrite single crystals. We observe a steep increase in resistance upon cooling heavily VS-doped crystals below ∼200 K, as the dominant charge transport crosses over from interior to surface conduction. The frequently employed two-resistor equivalent circuit model for lightly-doped pyrite crystals cannot reproduce this steep rise, but it can be accounted …

D42. 00002: Tuning Electrochemical Response in Ion-Gel-Gated La 1-x Sr x CoO 3-δ Films via Sr-doping and strain*AbstractPresenter:VIPUL CHATURVEDI(Chemical Engineering and Material Science, University of Minnesota)Authors:VIPUL CHATURVEDI(Chemical Engineering and Material Science, University of Minnesota)William M Postiglione(Chemical Engineering and Material Science, University of Minnesota)Biqiong Yu(School of Physics and Astronomy, University of Minnesota)Wojciech Tabis(Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Krakow, Poland)

Iron pyrite (FeS 2) is a low-cost, earth-abundant, non-toxic semiconductor with attractive electronic and optical properties, both for fundamentals, and applications (eg. photovoltaics). Sulfur vacancies (V S)[1] and Co [2] are the only well-established n-dopants in FeS 2. V S are deep donors, however, preventing facile study of phenomena such as the insulator-metal transition (IMT), while surface conduction complicates transport at low Co doping. In this work the problem of surface conduction is circumvented via a contacting scheme that provides access to the bulk, enabling wide-T-range transport studies of Co-doped FeS 2 single crystals. An IMT is found at~ 4Χ10 17 cm-3 Hall density, with Efros-Shklovskii variable-range hopping and/or activated transport below this, and electron-electron interaction-corrected metallic conductivity above it. A number of unexpected features also occur, however, including non …

Electrolyte-based transistors utilizing ionic liquids/gels have been highly successful in the study of charge-density-controlled phenomena, particularly in oxides. Experimental probes beyond transport have played a significant role, despite challenges in their application in electric double-layer transistors. Here, we demonstrate the application of synchrotron soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) as operando probes of the charge state and magnetism in ion-gel-gated ferromagnetic perovskite films. Electrochemical response via oxygen vacancies at positive gate bias in LaAlO3 (001)/La0. 5Sr0. 5CoO3Ād is used as a test case. XAS/XMCD measurements of 4–25 unit-cell-thick films first probe the evolution of hole doping (from the O K-edge pre-peak) and ferromagnetism (at the Co L-edges), to establish a baseline. Operando soft XAS/XMCD of electrolyte-gated films is …

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that these devices either electrically tune a known ferromagnet or electrically induce ferromagnetism from another magnetic state, e.g., antiferromagnetic. Here, we demonstrate that ferromagnetism can be voltage-induced even from a diamagnetic (zero-spin) state suggesting that useful magnetic phases could be electrically induced in “nonmagnetic” materials. We use ionic liquid–gated diamagnetic FeS2 as a model system, showing that as little as 1 V induces a reversible insulator-metal transition by electrostatic surface inversion. Anomalous …

The Renewable Electricity for Minnesota’s Future (REMnF) program objectives are to advance strategic renewable electric energy research and development toward commercialized products that can stimulate Minnesota’s economy, improve renewable energy technology, and potentially reduce rates for Xcel customers. At the University of Minnesota, under the guidance of the Institute on the Environment (IonE), four projects were selected for their potential to achieve these objectives. IonE then convened an advisory board and worked with the project teams to identify useful partners and potential strategies for commercialization.

P48. 00007: Influence of plastic deformation on the transport and magnetic properties of strontium titanate*AbstractPresenter:Sajna Hameed(School of Physics and Astronomy, University of Minnesota Twin Cities)Authors:Sajna Hameed(School of Physics and Astronomy, University of Minnesota Twin Cities)Damjan Pelc(School of Physics and Astronomy, University of Minnesota Twin Cities)Zachary Anderson(School of Physics and Astronomy, University of Minnesota Twin Cities)Chris Leighton(Chemical Engineering and Materials Science, University of Minnesota)Martin Greven(School of Physics and Astronomy, University of Minnesota Twin Cities)Strontium titanate (SrTiO 3, STO) is one of the most important transition metal oxides: it is a model perovskite with a structural phase transition, and also one of the most commonly used single crystal substrates for epitaxial growth of oxide thin films. Superconductivity in …

A sulfur vacancy-related defect has been recently experimentally identified as the source of unintentional n-type doping in pyrite FeS2, a potential earth-abundant, nontoxic, ultralow-cost absorber for thin film solar cells. Historically, however, theory has not supported this possibility, as simple S mono-vacancies have high formation energies, as well as donor state energies inconsistent with experiment. Here, we use density functional theory to perform a detailed and systematic study of S vacancies in pyrite, considering not only mono-vacancies but also multiple forms of vacancy clusters. We first confirm that the S mono-vacancy indeed produces a donor state too far from the conduction band minimum to explain recent experiments. Four configurations of S di-vacancies are then investigated, leading to the finding that S–S dimer vacancies induce an elevated donor state near the middle of the gap. Importantly …

Thanks to advances in chemical synthesis that enable control over the size, structure, properties, and functionalization, magnetic nanoparticles (NPs) present unique opportunities in areas as diverse as data storage, cancer treatment, and biomedical imaging. While superparamagnetism dominates the properties of magnetic NPs, a quantitative understanding of superparamagnetic blocking in NP assemblies remains elusive. We address this challenge here via comprehensive magnetic characterization and analysis of soft ferromagnetic NP ensembles based on Ni. NPs were synthesized by the injection of a Ni–oleylamine (OAm) complex into 200 °C trioctylphosphine (TOP), with size control achieved via the TOP:OAm ratio, reaction time, and differential centrifugation. X-ray diffraction, electron microscopy, and various spectroscopies reveal polycrystalline/twinned face-centered-cubic Ni NPs with mean diameters …

The carrier-type of the emerging photovoltaic Sb2Se3 was evaluated for both thin films and bulk crystals via a range of complementary techniques. X-ray photoelectron spectroscopy (XPS), hot probe, Hall effect, and surface photovoltage spectroscopy showed films and crystals synthesized from the Sb2Se3 granulate material to be n-type with chlorine identified as an unintentional n-type dopant via secondary ion mass spectrometry analysis. The validity of chlorine as a dopant was confirmed by the synthesis of intrinsic crystals from metallic precursors and subsequent deliberate n-type doping by the addition of MgCl2. Chlorine was also shown to be a substitutional n-type shallow dopant by density functional theory calculations. TiO2/Sb2Se3 n–n isotype heterojunction solar cells with 7.3% efficiency are subsequently demonstrated, with band alignment analyzed via XPS.

Recent advances in complex oxide heterostructures have realized extraordinary control over oxygen vacancies (V O), including strain-tuned V O order, and electric-field-controlled transformations between perovskite and V O-ordered structures. Perovskite cobaltites such as La 1− x S r x Co O 3− δ provide a prime example, recent work demonstrating that strain engineering of V O ordering induces large (∼ 10 7 erg/c m 3) perpendicular magnetic anisotropy. Here we show that V O-ordered epitaxial La 0.5 Sr 0.5 Co O 3− δ films exhibit not only strong magnetic anisotropy, but also a giant form of anisotropic magnetoresistance (AMR). This has magnetic field, temperature, and angular dependencies in quantitative accord with conventional AMR, but with AMR ratios up to an extraordinary 40.3%, 20 times enhanced over bulk cobaltites, and∼ 10–100 times larger than typical transition metals. This giant AMR has no …

The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach: irreversible, plastic deformation of single crystals. We show for the superconductor SrTiO that compressive plastic deformation induces low-dimensional superconductivity significantly above the superconducting transition temperature () of undeformed samples, with evidence of superconducting correlations at temperatures two orders of magnitude above the bulk . The superconductivity enhancement is correlated with the appearance of self-organized dislocation structures, as revealed by diffuse neutron and X-ray scattering. We also observe signatures of deformation-induced quantum-critical ferroelectric fluctuations and inhomogeneous ferroelectric order via Raman scattering. These results suggest that the strain surrounding the self-organized dislocation structures induces local ferroelectricity and quantum-critical dynamics that strongly influence , consistent with a theory of superconductivity enhanced by soft polar fluctuations. More broadly, our results demonstrate the promise of plastic deformation and dislocation engineering as tools to manipulate electronic properties of quantum materials.

The wide-gap semiconducting perovskite BaSnO 3 has attracted attention since the discovery of outstanding mobility at high electron densities, spurred on by potential applications in oxide, transparent, and power electronics. Despite progress, much remains to be understood in terms of mobility-limiting scattering in BaSnO 3 thin films and thus mobility optimization. Here, we apply solid-state ion-gel-based electrolyte gating to electrostatically control electron density over a wide range (10 18 cm− 3 to> 10 20 cm− 3) in BaSnO 3 films. Temperature-and gate-voltage-dependent transport data then probe scattering mechanisms and mobility vs electron density alone, independently of sample-to-sample defect density variations. This is done on molecular-beam-epitaxy-and sputter-deposited films as a function of thickness, initial chemical doping, and initial mobility. Remarkably universal behavior occurs, the mobility first …

Recent years have seen increasingly impressive demonstrations of all-electrical control of magnetism, including electrolyte-gating-induced ferromagnetism in non-ferromagnetic materials. These demonstrations, however, involve induction of ferromagnetism from some other finite-spin magnetic state, eg., antiferromagnetic, paramagnetic, etc. In this work we use ionic liquid gating, which can induce electron densities> 10 14 cm-2, to achieve voltage-induced ferromagnetism in diamagnetic (ie., zero-spin) FeS 2 single crystals. Temperature-dependent transport measurements establish a remarkably reversible positive-bias-induced insulator-metal transition, accompanied by inversion of the FeS 2 surface conduction channel to n-type. Anomalous Hall effect measurements then reveal an accompanying onset of voltage-induced soft 2D ferromagnetism, with Curie temperature up to~ 20 K. These results are supported by …