Metal hydrides are important across diverse applications such as hydrogen storage, batteries, gas sensors, nuclear reactions and high-temperature superconductivity. Previous computational studies of metal hydrides under extreme pressures, e.g., O(10^2) GPa, usually treat them as stoichiometric compounds without considering interstitial lattice disorder. As pressures become more moderate in the O(10^0) GPa and below range, hydrogen disorder at interstitial lattice sites becomes prominent, e.g. in the N-doped Lu hydride that was recently claimed superconducting near 1 GPa. Further adding compositional complexity from alloying and/or multi-element interstitial occupation makes elucidating pressure- and temperature-dependent observables intractable by first-principles calculations alone. We therefore propose a lattice graph neural network surrogate modeling approach to predict configuration- and pressure-dependent equation-of-state properties. Their efficiency permits Monte Carlo simulations to calculate Gibbs energies and pressure-dependent phase diagrams, thereby revealing insights into the synthesis conditions required for achieving desired phase equilibria. We demonstrate this concept for the compositionally complex cubic Lu(H, N,Va)3 system where three constituents (hydrogen, nitrogen and vacancy) have disordered multi-element interstitial occupancies and insights into pressure-dependent phase equilibria are critically needed, e.g., N-doping levels can significantly lower dehydrogenation temperatures and provide a new strategy to optimize hydrogen-storage alloys. This work can improve the thermodynamic understanding of the Lu-H-N system and help rational synthesis of N-doped Lu hydrides, but more generally demonstrates an efficient approach to model pressure-dependent thermodynamics of multi-component solid solutions.

Granular metals, consisting of nanoscale conducting and insulating regions, have been studied for more than 50 years for fundamental and applied research. Granular metals exhibit non-linear conductivity vs frequency behavior, consistent with the universal power law response, and have recently been suggested for high-pass filter applications. Here, we report that cobalt–silicon nitride (Co–SiNx) granular metals with optimized sputter conditions and post-growth annealing exhibit an exceptional 109 increase in conductivity at 1 MHz compared to the DC conductivity. The improved frequency response is correlated with structural and chemical improvements examined via scanning transmission electron microscopy and x-ray photoemission spectroscopy. While we focus on improvements for high-pass filter applications, the structural, chemical, and electronic control demonstrated here will benefit a variety of granular metal and nanoparticle applications.

An increasing magnetic field perpendicular to an undoped semiconductor surface at low temperature is known to strengthen the binding of localized electrons to stationary ions, as the wavefunction's tails evolve from exponential to Gaussian. It is also known that application of a high bias voltage to a depleted semiconductor can liberate bound charge and induce a large drop in electrical resistance. We connect these established results to experimental electrical transport measurements on off-state germanium Schottky-barrier metal–oxide–semiconductor field-effect transistor (MOSFETs) with an aluminum oxide insulating dielectric and platinum germanide contacts. We make measurements at the three distinct orientations of the magnetic field with respect to the substrate and the current. At 6 K, we observe sharp attenuation of current by more than 2 orders of magnitude, within 60 mT, at a crossover magnetic field perpendicular to the substrate. A 1 T magnetic field attenuates the current by more than 4 orders of magnitude. The strength of the attenuation and the value of the crossover field are controlled by both the gate–source and drain–source voltages. The attenuation is much weaker when the magnetic field is parallel to the current. Finally, we orient the magnetic field parallel to the substrate, but perpendicular to the current, allowing us to distinguish charge hopping at the oxide interface from charge hopping in the bulk. This large off-state magnetoresistance can be exploited for cryogenic magnetic- and photo-detection, and for high-bias, low-leakage MOSFETs.

Metal hydrides with a high hydrogen content have long been considered for materials-based hydrogen storage and but are now attracting attention as potential high-temperature superconductors.1 Recently, we showed that cryo-milling lanthanum improves hydrogen diffusion, yielding a higher hydrogen-to-metal ratio of lanthanum hydride up to LaH4.2 Another strategy for improving the thermodynamics and hydrogen desorption kinetics of metal hydrides is nanoconfinement within porous hosts.3,4 However, this has not previously been achieved for lanthanum hydride (LaHx) due to the difficulty of isolating nanoparticles of La or La-hydrides. In this work we demonstrate that LaHx can be infiltrated into two porous carbons: nitrogen-doped CMK-3 (NCMK-3) and undoped CMK-3, both with average pore size 4-5 nm. Transmission electron microscopy (TEM) images reveal that LaHx species are distributed uniformly throughout these hosts in NCMK-3 and CMK-3 with rod and spherical morphologies, respectively. X-ray absorption spectroscopy (XAS) and X-ray photoelectron microscopy (XPS) were used to probe the coordination environment of the LaHx species and composition lanthanum and nitrogen. Sieverts measurements indicate that LaHx@NCMK-3 desorbs up to 0.7 wt % hydrogen, which is higher than the non-nitrogen functionalized CMK-3 (0.5 wt%H). Density Functional Theory (DFT) and ab initio molecular dynamics (AIMD) calculations predict that host-guest interaction energies are favorable for porous carbon with pyridinic, pyrolic, or pyridonic nitrogen defects on a graphene surface, consistent with experimental data. Moreover, high-pressure experiments were conducted to understand the tunability of hydrogen content in presence of ammonia borane as a hydrogen source. These revealed that the as-prepared materials underwent an increase in H:La ratio from 1.5 to 3.0 with pressure. Our results demonstrate that nitrogen-doped nanoporous carbons can confine lanthanum hydrides, favor higher H:La ratios, and could serve as a platform for developing superconducting materials at relatively low pressures (compared with diamond anvil cells) and temperatures. -W. Guan, R. J. Helmley, V. Viswanathan Combining pressure and electrochemistry to synthesize superhydrides PNAS 2021, 118, e2110470118. Duwal, V. Stavila, C. Spataru, M. Shivanna, P. Allen, T. Elmslie, T. C. Seagle, J. Jeffries, N. Velisavljevic, J. Smith, P. Chow, Y. Xiao, Y. Meng, M. Somayazulu, P. A. Sharma Enhancement of hydrogen absorption and hypervalent metal hydride formation in lanthanum using cryogenic ball milling Phys. Rev. Mater., 2024, submitted. Stavila, S. Li, C. Dun, M. A. T. Marple, H. E. Mason, J. L. Snider, et al. Angew. Chem. Int. Ed. 2021, 60, 25815-25824. Schneemann, L. F. Wan, A. S. Lipton, Y.-S. Liu, J. L. Snider, A. A. Baker, et al. ACS Nano 2020, 14, 10294-10304.

Skyrmions and antiskyrmions are nanoscale swirling textures of magnetic moments formed by chiral interactions between atomic spins in magnetic non-centrosymmetric materials and multilayer films with broken inversion symmetry. These quasiparticles are of interest for use as information carriers in next-generation, low-energy spintronic applications. To develop skyrmion-based memory and logic, we must understand skyrmion-defect interactions with two main goals -- determining how skyrmions navigate intrinsic material defects and determining how to engineer disorder for optimal device operation. Here, we introduce a tunable means of creating a skyrmion-antiskyrmion system by engineering the disorder landscape in FeGe using ion irradiation. Specifically, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au$^{4+}$ ions at varying fluences, inducing amorphous regions within the crystalline matrix. Using low-temperature electrical transport and magnetization measurements, we observe a strong topological Hall effect with a double-peak feature that serves as a signature of skyrmions and antiskyrmions. These results are a step towards the development of information storage devices that use skyrmions and anitskyrmions as storage bits and our system may serve as a testbed for theoretically predicted phenomena in skyrmion-antiskyrmion crystals.

Granular metals (GMs), consisting of metal nanoparticles separated by an insulating matrix, frequently serve as a platform for fundamental electron transport studies. However, few technologically mature devices incorporating GMs have been realized, in large part because intrinsic defects (e.g., electron trapping sites and metal/insulator interfacial defects) frequently impede electron transport, particularly in GMs that do not contain noble metals. Here, we demonstrate that such defects can be minimized in molybdenum–silicon nitride (Mo–SiNx) GMs via optimization of the sputter deposition atmosphere. For Mo–SiNx GMs deposited in a mixed Ar/N2 environment, x-ray photoemission spectroscopy shows a 40%–60% reduction of interfacial Mo-silicide defects compared to Mo–SiNx GMs sputtered in a pure Ar environment. Electron transport measurements confirm the reduced defect density; the dc conductivity improved (decreased) by 104–105 and the activation energy for variable-range hopping increased 10×. Since GMs are disordered materials, the GM nanostructure should, theoretically, support a universal power law (UPL) response; in practice, that response is generally overwhelmed by resistive (defective) transport. Here, the defect-minimized Mo–SiNx GMs display a superlinear UPL response, which we quantify as the ratio of the conductivity at 1 MHz to that at dc, Δσω. Remarkably, these GMs display a Δσω up to 107, a three-orders-of-magnitude improved response than previously reported for GMs. By enabling high-performance electric transport with a non-noble metal GM, this work represents an important step toward both new fundamental UPL research and scalable, mature GM device applications.

Near room temperature superconductivity of metal superhydrides has been shown both theoretically and experimentally at high pressures (>100 GPa). Taking advantage of room temperature superconductivity for engineering applications requires decreasing the pressure of formation while retaining the superconducting hydride phase. We implanted lanthanum thin films with various doses of hydrogen ions at ambient pressure in order to form a lanthanum hydride phase. We found evidence for granular superconductivity below 5 K consistent with the phase coexistence of lanthanum hydride and lanthanum. As the H+ dose increased, TC decreased from 4.6 K to 3.2 K with broader superconducting transitions. Transmission electron microscopy showed increased substrate damage with increased ion dose and confirmed the granular structure of the films. Although a superhydride phase requires a higher H+ dose than what was attained in this work, we have demonstrated that ion implantation at ambient pressure is a feasible technique for superconducting lanthanum hydride formation.

Topological insulator-magnetic insulator (TI-MI) heterostructures hold significant promise in the field of spintronics, offering the potential for manipulating magnetization through topological surface state-enabled spin-orbit torque. However, many TI-MI interfaces are plagued by issues such as contamination within the magnetic insulator layer and the presence of a low-density transitional region of the topological insulator. These interfacial challenges often obscure the intrinsic behavior of the TI-MI system. In this study, we addressed these challenges by depositing sputtered Bi2Te3 on liquid phase epitaxy grown Y3Fe5O12/Gd3Ga5O12. The liquid phase epitaxy grown Y3Fe5O12 has been previously shown to have exceptional interface quality, without an extended transient layer derived from interdiffusion processes of the substrate or impurity ions, thereby eliminating rare-earth impurity-related losses in the MI at low temperatures. At the TI-MI interface, highresolution depth-sensitive polarized neutron reflectometry confirmed the absence of a low-density transitional growth region of the TI. By overcoming these undesirable interfacial effects, we isolate and probe the intrinsic low-temperature magnetization dynamics and transport properties of the TI-MI interface. Our findings revealed strong spin pumping at low temperatures, accompanied by an additional in-plane anisotropy. The enhanced spin pumping at low temperatures is correlated with the observed suppression of bulk conduction and the weak antilocalization in the TI film, highlighting the interplay between the transport and spin pumping behavior in the TI-MI system.

Silicon nitride has long been employed in the microfabrication of thermal sensors due to its favorable material properties and the ease with which it facilitates surface micromachining. While a variety of studies have utilized thin silicon nitride membranes for high sensitivity thermal measurements, limited reports exist on the physical characteristics of membranes and platforms in a thickness limit much less than 100 nm. Herein, we report on the development of low-stress, suspended silicon nitride platform devices that enable thermal characterization of membranes ranging from 120 nm to less than 10 nm in thickness, providing thermal conductivities as low as 1.1 W m(-1 )K(-1 ) near room temperature. Applications of these platforms may enable appreciable enhancement in the performance of devices reliant upon environmental thermal isolation including bolometers, calorimeters, and gas sensors, among others.

Gold–germanium (AuxGe1−x) solid solutions have been demonstrated as highly sensitive thin film thermometers for cryogenic applications. However, little is known regarding the performance of the films for thicknesses less than 100 nm. In response, we report on the resistivity and temperature coefficient of resistance (TCR) for sputtered films with thicknesses ranging from 10 to 100 nm and annealed at temperatures from 22 to 200 °C. The analysis is focused upon composition x=0.17, which demonstrates a strong temperature sensitivity over a broad range. The thinnest films are found to provide an enhancement in TCR, which approaches 20% K−1 at 10 K. Furthermore, reduced anneal temperatures are required to crystallize the Ge matrix and achieve a maximum TCR for films of reduced thickness. These features favor the application of ultra-thin films as high-sensitivity, on-device thermometers in micro- and nanolectromechanical systems.

Vanadium oxide films are widely employed as thermal detectors in uncooled infrared detection systems due to their high temperature coefficient of resistance near room temperature. One strategy toward maximizing detectivity and reducing the thermal time constant in these systems is to minimize the system platform dimensions. This approach necessitates thinner film thicknesses (≪100 nm), for which there is little information regarding thermal sensing performance. Herein, we report on the sensitivity of reactively sputtered vanadium oxide thin film resistive thermometers nominally ranging from 100 to 25 nm and assess the influence of thermal annealing. We demonstrate that films in this minimum limit of thickness maintain a high temperature coefficient while additionally providing an enhancement in characteristics of the noise equivalent power.

The low temperature anomalous Hall effect in amorphous sputtered SmxCo1−x films is investigated. The microstructure and amorphous nature of the films were characterized by local area diffraction through transmission electron microscopy. The Sm/Co ratio was changed to elucidate the origin of the anomalous Hall effect, which is a proxy for spin polarized transport. The anomalous Hall effect obeys a scaling relation that points to an intrinsic Berry-curvature based mechanism, consistent with recent reports on other amorphous magnetic materials. The anomalous Hall angle increases with increasing Sm content and reaches large values of ∼1%. This observation is related to recent measurements of the local structure in amorphous Sm–Co alloys and suggests that rare earth elements can be utilized to improve electrical control over spin polarization in amorphous magnetic films.