We studied the role of reduced dimensionality and disorder in the superconducting properties of wafer-scale aluminum (Al) nanofilms. This new generation of ultrathin films were grown using molecular beam epitaxy and depict normal-state sheet resistance at least 20 times lower than the quantum resistance $h/\left(4e^2\right)$. Defying general expectations, the superconducting transition temperature of our films increases with decreasing Al film thickness, reaching 2.4 K for a 3.5-nm-thick Al film grown on GaAs: twice that of bulk Al (1.2 K). Surface phonon softening is shown to impact superconductivity in pure ultrathin films, offering a route for materials engineering in two dimensions.

In recent years, superconductivity in two-dimensional (2D) layered metal borides has aroused much interest. Here, based on first-principles calculations, we theoretically report four 2D hydrogenated metal diborides: $M_2{\mathrm{B}}_2\mathrm{H}(M=\mathrm{Al}$, Mg, Mo, W), and investigate their geometrical structures, electronic structures, phonon dispersions, thermal stability, dynamic stability, electron-phonon coupling (EPC), superconducting properties, and so on. Results reveal that the introduction of hydrogen atoms expands the frequency range of the phonon spectrum of monolayer $M_2{\mathrm{B}}_2$, and significantly increases the EPC. We systematically analyze the specific origins of superconductivity in these hydrogenated low-dimensional systems. The obtained EPC constants $\lambda$ of $M_2{\mathrm{B}}_2\mathrm{H}(M=\mathrm{Al}$, Mg, Mo, W) are 2.93, 0.86, 1.09, and 1.40, and the corresponding superconducting transition temperatures $\left(T_c\right)$ are 52.6, 23.2, 21.5, and 18.6 K, respectively. By further applying electron/hole doping or biaxial tensile strain, the $T_c$ can be further increased, with the highest $T_c$ of 60.2 K in ${\mathrm{Al}}_2{\mathrm{B}}_2\mathrm{H}$ under 0.4% biaxial tensile strain. The predicted $M_2{\mathrm{B}}_2\mathrm{H}$ provides a new platform for 2D superconductivity and may have potential applications in 2D nanodevices.

By redirecting the incident wave energy back toward the source or sensor, retroreflectors mitigate signal losses, render a higher signal-to-noise transmission with improved sensing performance, and are highly desirable in acoustic applications such as remote sensing and detection, medical ultrasound imaging, and underwater communications. In this paper, we propose a different and efficient approach to realize a wide-angle high-efficiency retroreflector that is based on the design frame of acoustic metagrating. By integrating a many-objective optimization algorithm with a deep learning neural network, we construct a comprehensive and efficient optimization framework to intelligently design the metagrating so that its seven different diffraction orders work synergistically to realize the intended high-efficiency retroreflection functionality over a broad and continuous range of incident angles. Compared with the existing approaches based on corner cubes, Luneburg lenses, or dual-layer metasurfaces, the single-layer configuration of the retroreflector has a relatively simple unit-cell structure and can be easily extended to higher working frequencies. Both the proposed design paradigm itself and the high-performance retroreflector configuration may find applications in various scenarios including ultrasonic detection and imaging, music performance monitoring, and underwater communications.

The radio-frequency permittivity-temperature profiles of acceptor-donor codoped ${\mathrm{TiO}}_2$-based rutiles commonly produce higher than expected values ($>{10}^3$ at 300 K) often with conflicting interpretations. A combination of dielectric spectroscopy (DS) and impedance spectroscopy (IS) with different electrode materials is used to reinvestigate the electrical properties of ${\mathrm{Fe}}^{3+}\text{−}{\mathrm{Nb}}^{5+}$-doped rutile in the form of ${\mathrm{FeTiNbO}}_6$ ceramics that show permittivity-temperature characteristics that are consistent with relaxor ferroelectrics (RFE). IS results reveal semiconducting grains with an activation energy of $\sim 0.16\mathrm{eV}$, and relative permittivity of similar magnitude and temperature dependence to undoped ${\mathrm{TiO}}_2(<250)$. Reducing the work function of the electrode material by replacing Au with InGa has a dramatic effect on the IS and DS data. We propose the apparent RFE behavior observed by DS and previously attributed to the formation of nanoclustering of the cations is an extrinsic effect primarily associated with the development of Schottky barriers between the semiconducting ceramics and Au contacts.

Owing to the location of the Dirac point, which is around the center of its wide bulk band gap, ${\mathrm{TlBiSe}}_2$ would be one of the most promising topological insulators for spintronics devices material. However, like many other topological insulators, defects, such as vacancy formed during the crystal growth, dope electrons into ${\mathrm{TlBiSe}}_2$ and make its bulk metallic. Here, we show the achievement of bulk insulating both $n$-type and $p$-type ${\mathrm{TlBiSe}}_2$ by photo-induced doping, a method carried out by a combination of photo-irradiation and ${\mathrm{H}}_2\mathrm{O}$ adsorption. We also show that the main trigger of this photo-induced doping is the excitation of the outermost $d$ core level of the chalcogen atoms of the topmost layer as in the case of ${\mathrm{Bi}}_2{X}_3$, where $X=\mathrm{Se}$ or Te.

Adjusting the electrical and optical properties of two-dimensional materials by pressure is an important method to realize their applications in the fields of electronics and optoelectronic devices. Here, we report the highly tunable transport properties of the layered semiconductor $\mathrm{Hf}{\mathrm{S}}_2$ under high pressure, including metallization and superconducting transition. $\mathrm{Hf}{\mathrm{S}}_2$ is one of the IVB group transition-metal dichalcogenides, exhibiting a large band gap in ambient condition. The bandgap decreases from $\sim 2.0$ to $\sim 1.1\mathrm{eV}$ within $15\mathrm{GPa}$ by pressure tuning, and $\mathrm{Hf}{\mathrm{S}}_2$ becomes metallic at $\sim 60\mathrm{GPa}$. Moreover, a superconducting transition is observed under $\sim 126\mathrm{GPa}$, and the superconducting critical temperature ($T_{\mathrm{c}}$) increases with further compression, reaching $\sim 4.4\mathrm{K}$ at $\sim 153\mathrm{GPa}$. Hall-effect measurements show that pressure changes the carrier-concentration type from electron dominated to hole dominated at $\sim 145\mathrm{GPa}$. Two structural transitions are found at moderate pressure, and one of them agrees with the changes observed in absorption and Raman spectroscopy measurements. Manipulation of the transport and optical properties of $\mathrm{Hf}{\mathrm{S}}_2$ by pressure provides important information for its practical applications.

A $2\times 2\times 2$ charge density wave (CDW) was recently observed deep inside the antiferromagnetic phase of a kagome metal FeGe, which significantly enhances its spin-polarization. A key question is whether the CDW in FeGe is driven by its electronic correlation and magnetism. Here, we address this problem using density functional theory and its combination with $U$ as well as dynamical mean-field theory. Our calculations show that large dimerization ($\sim 1.3$ Å) of Ge1 sites along the $c$ axis will enhance electronic correlation of the Fe-$3d$ orbitals and, as a result, it enhances the spin-polarization and saves more magnetic exchange energies. We find that the balance between magnetic energy saving and structural energy cost via partially dimerizing Ge1 sites in an enlarged superstructure could induce a new local minimum in total energies. The response to the large partial Ge1-dimerization will induce additional small modulations ($<0.05$ Å) of other sites in the kagome and honeycomb layers, which further reduces the total energy and leads to a stable $2\times 2\times 2$ CDW ground state in FeGe. Our results are in good agreement with the existing experiments and reveal a different unconventional CDW mechanism driven by primarily saving magnetic energies via the interplay of structure, electronic correlation, and magnetism.

The nodal-line semimetals NaAlSi and NaAlGe have significantly different ground states despite having similar electronic structures: NaAlSi exhibits superconductivity $<7\mathrm{K}$, while NaAlGe exhibits semiconductive electrical conductivity at low temperatures, indicating the formation of a pseudogap at $\sim 100\mathrm{K}$. The origin of the pseudogap in NaAlGe is unknown but may be associated with excitonic instability. We investigated hole-doping effects on the ground state in the solid solution $\mathrm{Na}\left({\mathrm{Al}}_{1–x}{\mathrm{Zn}}_x\right)\mathrm{Ge}$ and discovered that the pseudogap is suppressed continuously with increasing Zn content, followed by the appearance of a superconducting dome with the highest transition temperature of 2.8 K. This superconductivity most likely results from excitonic fluctuations.

Compact light-emitting quantum dot monolayers find wide-ranging applications in photovoltaics to display technology. A key aspect of display technology is the light extraction capability and the quantum efficiency of these layers. The usual Purcell enhancement requiring large-scale proximal metal nanostructures would completely disrupt the packing of such ordered layers and hence cannot be used. In this report, we demonstrate a nonmonotonic temperature dependence of emission enhancement in compact tiny, fully absorbing metal nanoparticle embedded quantum dot layers, which is captured by a non-Markovian treatment of emitter-metal nanoparticle interactions. We also demonstrate how this nonmonotonic behavior with temperature is dominated by the electron-phonon interactions in the metal nanoparticles. Significantly, the observed photoluminescence enhancement maximum of $\sim 10$ occurs at a temperature of $\sim 150\mathrm{K}$ which is much higher compared to earlier reports. The results suggest the possibility of using our platform in various display, photonic, and sensing devices with higher sensitivity and energy efficiency.

We present a computational investigation into the effects of chemical doping with 15 different elements on phase stability and superconductivity in the $\mathrm{La}{\mathrm{H}}_{10}$ structure. Most doping elements were found to induce softening of phonon modes, enhancing electron-phonon coupling and improving critical superconducting temperature while weakening dynamical stability. Unlike these dopants, Ce was found to extend the range of dynamical stability for $\mathrm{La}{\mathrm{H}}_{10}$ by eliminating the Van Hove singularity near the Fermi level. The doped compound, ${\mathrm{La}}_{0.75}{\mathrm{Ce}}_{0.25}{\mathrm{H}}_{10}$, maintains high-temperature superconductivity. We also demonstrate that different Ce doping configurations in the $\mathrm{La}{\mathrm{H}}_{10}$ structure have a minimal effect on energetic stability and electron-phonon coupling strength. Our findings suggest that Ce is a promising dopant to stabilize $\mathrm{La}{\mathrm{H}}_{10}$ at lower pressures while preserving its high-temperature superconductivity.

Hydrogen diffusion in metals and alloys plays an important role in the discovery of new materials for fuel cell and energy storage technology. While analytic models use hand-selected features that have clear physical ties to hydrogen diffusion, they often lack accuracy when making quantitative predictions. Machine learning models are capable of making accurate predictions, but their inner workings are obscured, rendering it unclear which physical features are truly important. To develop interpretable machine learning models to predict the activation energies of hydrogen diffusion in metals and random binary alloys, we create a database for physical and chemical properties of the species and use it to fit six machine learning models. Our models achieve root-mean-squared errors between 98–119 meV on the testing data and accurately predict that elemental Ru has a large activation energy, while elemental Cr and Fe have small activation energies. By analyzing the feature importances of these fitted models, we identify relevant physical properties for predicting hydrogen diffusivity. While metrics for measuring the individual feature importances for machine learning models exist, correlations between the features lead to disagreement between models and limit the conclusions that can be drawn. Instead grouped feature importance, formed by combining the features via their correlations, agree across the six models and reveal that the two groups containing the packing factor and electronic specific heat are particularly significant for predicting hydrogen diffusion in metals and random binary alloys. This framework allows us to interpret machine learning models and enables rapid screening of new materials with the desired rates of hydrogen diffusion.

We investigate the suitability of nearly half-metallic ferrimagnetic quaternary Heusler alloys, $\text{CoCrMn}Z$ ($Z$ = Al, Ga, Si, Ge), to assess their feasibility as electrode materials for MgO-based magnetic tunnel junctions (MTJs). The low magnetic moments of these alloys originating from the antiferromagnetic coupling between Mn and Cr spins ensure a negligible stray field in spintronics devices as well as a lower switching current required to flip their spin direction. We confirmed the mechanical stability of these materials with the evaluated values of elastic constants and by the absence of any imaginary frequency in their phonon dispersion curves. The influence of swapping disorders on the electronic structures and their relative stability is also discussed. High spin polarization of the conduction electrons is observed in the case of $\text{CoCrMn}Z$/MgO heterojunctions, independent of terminations at the interface. Based on our ballistic transport calculations, large coherent tunneling of the majority-spin $s$-like ${\mathrm{\Delta }}_1$ states can be expected through the MgO barrier. The calculated tunneling magnetoresistance (TMR) ratios are on the order of 1000%. Very high Curie temperatures specifically for CoCrMnAl and CoCrMnGa, which are comparable to bcc Co, could also yield a weaker temperature dependence of TMR ratios for CoCrMnAl/MgO/CoCrMnAl (001) and CoCrMnGa/MgO/CoCrMnGa (001) MTJs.

To explore martensitic phase transition materials with outstanding performance, phase stability, magnetism, and ductility of all-$d$-metal Heusler compounds ${\mathrm{Co}}_2\mathrm{NiT}$ ($T=t\mathrm{ransition}$ metal) were symmetrically investigated by density functional theory calculations. Different from $p\text{−}d$ covalent hybridization-dominated conventional Heusler compounds, all-$d$-metal Heusler compounds, i.e., ${\mathrm{Co}}_2\mathrm{NiT}$ with transition metal atoms Ti, V, Cr, Nb, Mo, Ta, and W, prefer $L{2}_1$-type structure rather than ${\mathrm{Hg}}_2\mathrm{CuTi}$-type structure. Moreover, inherent atomic radii become the decisive factor when determining the lattice sizes. Owing to the Kohn anomaly, high-temperature martensitic phase transitions can occur in ${\mathrm{Co}}_2\mathrm{NiT}$ compounds, which was also confirmed by the peak-and-valley-characterized density of states near the Fermi level and deep softening of transverse acoustic phonon modes as well as negative tetragonal shear elastic constant. Importantly, compared with conventional Heusler compounds, the replacement of nondirectional $d\text{−}d$ metallic bonding for $p\text{−}d$ covalent hybridization boosts more uniform distribution of electrons and enhances metallicity, which consequently improves the ductility of ${\mathrm{Co}}_2\mathrm{NiT}$ compounds. Comparatively, herein, it was found that, although magnetism is not the necessary condition for the occurrence of martensitic phase transition, the loss of magnetism can destabilize the cubic structure. Therefore, tuning magnetism can serve as another effective way to adjust martensitic phase transition in all-$d$-metal Heusler compounds. The small volume change ratio during martensitic phase transition in ${\mathrm{Co}}_2\mathrm{NiTi}$ and ${\mathrm{Co}}_2\mathrm{NiV}$ compounds endows them with more extensive application prospects. Hopefully, this paper can help better understand the mechanism of martensitic phase transition and its correlation to magnetism and origin of improved ductility in all-$d$-metal Heusler compounds ${\mathrm{Co}}_2\mathrm{NiT}$, which may accelerate the exploration of excellent high-temperature martensitic phase transition materials.

Doping at particular sites is a common method for increasing the thermoelectric efficiency of materials, by tuning carrier concentration and electronic structure. A secondary effect of doping, as well as defects, is to induce a volume change, usually referred to as chemical pressure, that may affect the thermoelectric efficiency. Theoretical investigations usually ignore the role of volume change in thermoelectric improvement, mostly for computational limitations. In this work, we address the role of chemical pressure on the thermoelectric properties of TaFeSb, $M\text{OsSb}$ ($M=\text{Ta,}\text{Nb}$), and $N\text{RuAs}$ ($N=\text{Ta,}\text{Nb,}\text{V}$) by using ab initio electronic structure calculations. We calculate the effect of both negative and positive pressure on the electronic structure, the Seebeck coefficient, electrical and thermal conductivity, as well as the power factor and thermoelectric performance. We argue that volume change, occurring because of defects or doping, should be regarded as an essential parameter to determine the thermoelectric efficiency accurately, as exemplified by TaFeSb. Among the investigated compounds, TaRuAs stands out for the peculiar behavior of electronic and thermoelectric properties with respect to volume change. NbOsSb also stands out, as the sole compounds whose thermoelectric efficiency is maximal in the ground state and cannot be increased via a moderate volume change. Overall, we predict that TaRuAs can be an excellent candidate for thermoelectric applications, due to its large thermoelectric efficiency at zero pressure and the possibility of increasing it by a small volume change. Direct calculations of $\mathrm{Ta}\mathrm{Ru}{\mathrm{As}}_{0.875}{\mathrm{Bi}}_{0.125}$ demonstrate the improved thermoelectric properties while also providing an estimate of the accuracy of our chemical-pressure-based modeling of the doping process.

We examine the structural relaxation of glassy materials at finite temperatures, considering the effect of activated rearrangements and long-range elastic interactions. Our three-dimensional mesoscopic relaxation model shows how the displacements induced by localized relaxation events can result in faster-than-exponential relaxation. Thermal activation allows for local rearrangements, which generate elastic responses and possibly cascades of new relaxation events. To study the interplay between these elastically dominated and thermally dominated dynamics, we introduce tracer particles that follow the displacement field induced by the local relaxation events, and we also incorporate Brownian motion. Our results reveal that the dynamic exponents and shape parameter of the dynamical structure factor depend on this competition and display a crossover from faster-than-exponential to exponential relaxation as temperature increases, consistent with recent observations in metallic glasses. Additionally, we find the distribution of waiting times between activations to be broadly distributed at low temperatures, providing a measure of dynamical heterogeneities characteristic of glassy dynamics.

Experimental data on compressive strength ${\sigma }_{\max }$ versus strain rate ${\dot{\varepsilon }}_{\mathrm{eng}}$ for metallic glasses undergoing uniaxial compression show varying strain rate sensitivity. For some metallic glasses, ${\sigma }_{\max }$ decreases with increasing ${\dot{\varepsilon }}_{\mathrm{eng}}$, while for others, ${\sigma }_{\max }$ increases with increasing ${\dot{\varepsilon }}_{\mathrm{eng}}$, and for certain alloys ${\sigma }_{\max }$ versus ${\dot{\varepsilon }}_{\mathrm{eng}}$ is nonmonotonic. To understand their strain rate sensitivity, we conduct molecular dynamics simulations of metallic glasses undergoing uniaxial compression at finite strain rates and coupled to heat baths with a range of temperatures $T_0$ and damping parameters $b$. In the $T_0\to 0$ and $b\to 0$ limits, we find that the compressive strength ${\sigma }_{\max }$ versus temperature $T$ obeys a “chevron-shaped” scaling relation. In the low-strain-rate regime, ${\sigma }_{\max }$ decreases linearly with increasing $T$, whereas ${\sigma }_{\max }$ grows as a power law with decreasing $T$ in the high-strain-rate regime. For $T_0>0, {\sigma }_{\max }\left(T\right)$ deviates from the scaling curve at low strain rates, but ${\sigma }_{\max }\left(T\right)$ rejoins the scaling curve as the strain rate increases. Enhanced dissipation reduces compression-induced heating, which causes ${\sigma }_{\max }\left(T\right)$ to deviate from the $b\to 0$ scaling behavior for intermediate strain rates, but ${\sigma }_{\max }\left(T\right)$ converges to the high-strain-rate power-law scaling behavior at sufficiently high strain rates. Determining ${\sigma }_{\max }\left(T\right)$ as a function of $b$ and $T_0$ provides a general framework for explaining the strain rate sensitivity of metallic glasses under compression.

We study the energetics of quasiparticle excitations in ${\mathrm{CsPbBr}}_3$ perovskite nanocrystals using path integral molecular dynamics simulations. Employing detailed molecular models, we elucidate the interplay of anharmonic lattice degrees of freedom, dielectric confinement, and electronic correlation on exciton and biexciton binding energies of a range of nanocrystal sizes. We find generally good agreement with some experimental observations of binding energies and additionally explain the observed size-dependent Stokes shift. The explicit model calculations are compared with simplified approximations to rationalize the lattice contributions to binding. We find that polaron formation significantly reduces exciton binding energies, whereas these effects are negligible for biexciton interactions. While experimentally the binding energy of biexcitons is uncertain, based on our study, we conclude that biexcitons are bound in ${\mathrm{CsPbBr}}_3$.

We investigate ferromagnetic and insulating thin films of the $B$-site ordered double perovskite ${\mathrm{Nd}}_2{\mathrm{NiMnO}}_6$ (NNMO) grown by radio frequency off-axis magnetron sputtering. The films grow epitaxially strained on a selection of substrates and display a strain-independent and bulklike $T_{\text{C}}$ of $200\mathrm{K}$ at a thickness of 30 unit cells. We explore the thickness dependence of the NNMO/${\mathrm{SrTiO}}_3$(001) system and find ferromagnetism down to ultralow thicknesses of only 3 unit cells $\left(\sim 1.2\mathrm{nm}\right)$. Below 10 unit cells, the magnetic properties deteriorate due to an interfacial charge transfer caused by the polar discontinuity at the NNMO/${\mathrm{SrTiO}}_3$ interface. A detailed x-ray magnetic circular dichroism study allows us to separate the magnetic components into a robust ferromagnetic Ni/Mn sublattice and a paramagnetic Nd sublattice.

We study the spin injection from the ferromagnetic metallic oxide ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (LSMO) into the normal metal Ag. LSMO is a highly resistive metallic oxide, but also fully spin polarized and therefore holds promise of generating highly polarized spin currents. Because of the high resistivity of the ferromagnet, and the lower resistivity of the normal metal, no conductivity mismatch issues are expected, as we demonstrate with model calculations. Our measurements unambiguously show spin-coherent injection and detection. LSMO appears to be a useful material for spin injection, but we also find the presence of an intrinsic tunnel barrier and a lower-than-expected degree of spin polarization, indicating a spin-active interface.