Here we report the observation of bulk superconductivity in single crystals of the two-dimensional kagome metal ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. Magnetic susceptibility, resistivity, and heat capacity measurements reveal superconductivity below $T_c=0.93\mathrm{K}$, and density functional theory (DFT) calculations further characterize the normal state as a ${\mathbb{Z}}_2$ topological metal. Our results demonstrate that the recent observation of superconductivity within the related kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is likely a common feature across the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$: K, Rb, Cs) family of compounds and establishes them as a rich arena for studying the interplay between bulk superconductivity, topological surface states, and likely electronic density wave order in an exfoliable kagome lattice.

Scandium-alloying of aluminum nitride (AlScN) enhances the piezoelectric properties of the material and increases the performance of piezoelectric microelectromechanical systems (MEMS). However, this enhancement is caused by the destabilization of the wurtzite phase and so far the stability of AlScN thin films has not been sufficiently studied. Stability is especially important for piezoelectric devices because changes to the film microstructure or residual stress can lead to drastic changes in the device behavior. The stability of AlScN is investigated by annealing sputtered films and characterizing the resulting changes. It is found that the wurtzite phase of ${\mathrm{Al}}_{0.7}{\mathrm{Sc}}_{0.3}\mathrm{N}$ is stable at least up to $1000{}^{\circ }\mathrm{C}$ and annealing increases the crystal quality, reaching a maximum at $800{}^{\circ }\mathrm{C}$. When annealed for more than 100 h at $1000{}^{\circ }\mathrm{C}$, argon used in sputtering segregates into the grain boundaries and causes compressive strains and formation of rock-salt phase. Additionally, annealing at $1000{}^{\circ }\mathrm{C}$ for 5 h reduces the average tensile stress by approximately 1 GPa.

First-principles calculations reveal that sodium boride $\left({\mathrm{NaB}}_3\right)$ undergoes a phase transition from a tetragonal $P4/mbm$ phase to an orthorhombic $Pbam$ phase at $\sim 16$ GPa, accompanied by a counterintuitive lattice expansion along the crystallographic $a$ axis. This unusual compression behavior is identified as negative linear compressibility (NLC), which is dominantly attributed to the symmetry breaking of the boron framework. Meanwhile, the $P4/mbm$ and $Pbam$ phases form superionic conductors after undergoing a peculiar swap state at high temperature. Specifically, under “warm” conditions the Na cation pairs exhibit a rare local exchange (rotation) behavior, which may be originated from the asymmetric energy barriers of different diffusion paths. The study of the ${\mathrm{NaB}}_3$ compound sheds light on a material with a combination of NLC and ion transportation at extreme conditions.

We calculate the strength of the effective on-site Coulomb interaction (Hubbard $U$) in two-dimensional transition-metal (TM) dihalides $M{X}_2$ and trihalides $M{X}_3$ ($M=\mathrm{Ti}$, V, Cr, Mn, Fe, Co, Ni; $X=\mathrm{Cl}$, Br, I) from first principles using the constrained random-phase approximation. The correlated subspaces are formed from $t_{2g}$ or $e_g$ bands at the Fermi energy. Elimination of the efficient screening taking place in these narrow bands gives rise to sizable interaction parameters $U$ between the localized $t_{2g}$ ($e_g$) electrons. Due to this large Coulomb interaction, we find $U/W>1$ (with the bandwidth $W$) in most TM halides, making them strongly correlated materials. Among the metallic TM halides in the paramagnetic state, the correlation strength $U/W$ reaches a maximum in $\mathrm{Ni}{X}_2$ and $\mathrm{Cr}{X}_3$ with values much larger than the corresponding values in elementary TMs and other TM compounds. Based on the Stoner model and the calculated $U$ and $J$ values, we discuss the tendency of the electron spins to order ferromagnetically.

We report the anomalous Nernst effect (ANE) in large-grain ceramics, nanogranular ceramics, and nanogranular thin films of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_3$, measured over the temperature range 5–300 K. The study is complemented with thermopower, resistivity, and magnetic measurements. The temperature-dependent ANE below the Curie temperature $T_{\mathrm{C}}$ (240–250 K) is analyzed with the help of longitudinal resistivity and Seebeck using a previously proposed formula derived by a combination of Onsager reciprocity, the Mott formula, and the relation between transverse and longitudinal resistivity, ${\rho }_{xy}\propto {\rho }_{xx}^n$. We observe a characteristic exponent n ∼ 0.4 in agreement with the universal scaling for the bad-metal-type conduction regime. The nanogranular samples are characterized by higher resistivity, lower saturated magnetization, and a higher coercive field compared to large-grain ceramics. On the other hand, the magnitude of the ANE is independent of grain size. This observation likely insinuates that the characteristic length scale characterizing the ANE in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_3$ is below the grain size of nanogranular samples ∼40 nm. Therefore, the ANE associated with the bad-metal regime is independent of barriers associated with grain surface, which are responsible for activated resistivity and lowered magnetization due to a magnetically “dead” layer. The observation that the advantage of the higher coercive field of nanogranular samples is not deteriorated by a lower ANE is important for possible applications in zero magnetic field.

${\left({\mathrm{Co}}_x{\mathrm{Mn}}_{1-x}\right)}_3{\mathrm{O}}_4$ is a promising candidate material for solar thermochemical energy storage. A high-temperature model for this system would provide a valuable tool for evaluating its potential. However, predicting phase diagrams of complex systems with ab initio calculations is challenging due to the varied sources affecting the free energy, and with the prohibitive amount of configurations needed in the configurational entropy calculation. In this work we compare three different machine-learning (ML) approaches for sampling the configuration space of ${\left({\mathrm{Co}}_x{\mathrm{Mn}}_{1-x}\right)}_3{\mathrm{O}}_4$, including a simpler ML approach, which would be suitable for application in high-throughput studies. We use experimental data for a feature of the phase diagram to assess the accuracy of model predictions. We find that with some methods, data pretreatment is needed to obtain accurate predictions due to inherently composition-imbalanced training data for a mixed phase. We highlight that the important entropy contributions depend on the physical regimes of the system under investigation and that energy predictions with ML models are more challenging at compositions where there are energetically competing ground state crystal structures. Similar methods to those outlined here can be used to screen other candidate materials for thermochemical energy storage.

When two solids start rubbing together, frictional sliding initiates in the wake of slip fronts propagating along their surfaces in contact. This macroscopic rupture dynamics can be successfully mapped on the elastodynamics of a moving shear crack. However, this analogy breaks down during the nucleation process, which develops at the scale of surface asperities where microcontacts form. Recent atomistic simulations revealed how a characteristic junction size selects if the failure of microcontact junctions either arises by brittle fracture or by ductile yielding. This work aims at bridging these two complementary descriptions of the onset of frictional slip existing at different scales. We first present how the microcontact failure observed in atomistic simulations can be conveniently “coarse grained” using an equivalent cohesive law. Taking advantage of a scalable parallel implementation of the cohesive element method, we study how the different failure mechanisms of the microcontact asperities interplay with the nucleation and propagation of macroscopic slip fronts along the interface. Notably, large simulations reveal how the failure mechanism prevailing in the rupture of the microcontacts (brittle versus ductile) significantly impacts the nucleation of frictional sliding and, thereby, the interface frictional strength. This work paves the way for a unified description of frictional interfaces connecting the recent advances independently made at the micro- and macroscopic scales.

There is a long-standing question about the molecular configuration of interfacial water molecules in the proximity of solid surfaces, particularly carbon atoms, which plays a crucial role in electrochemistry and biology. In this study, the dielectric, structural, and dynamical properties of confined water placed between two parallel graphene walls at different interdistances from the angstrom scale to a few tens of nanometer have been investigated using molecular dynamics. For the dielectric properties of water, we show that the dielectric constant of the perpendicular component of water drastically decreases under sub-2-nm spatial confinement. The dielectric constant data obtained through linear response and fluctuation-dissipation theory are consistent with recent reported experimental results [L. Fumagalli et al., Science 360, 1339 (2018)]. By determining the charge density as well as fluctuations in the number of atoms, we provide a molecular rationale for the behavior of the perpendicular dielectric response function. We also interpret the behavior of the dielectric response in terms of the presence of dangling O-H bonds of water. By examining the residence time and lateral diffusion constant of water under confinement, we reveal that the water molecules tend to keep their hydrogen bond networks at the interface of water-graphene. We also found consistency between lateral diffusion and the $z$-component of variance in the center of mass of the system as a function of confinement.

The magnetic proximity effect is one of the powerful approaches to realize quantum anomalous Hall effect in topological insulators (TIs) targeting at high temperatures. Various TI/ferromagnetic-insulator (FMI) heterostructures have extensively been investigated by using a van der Waals epitaxial growth technique of TI films grown on FMI substrates. However, FMI materials which can be used as a substrate are strictly limited due to the lattice mismatching between TI and FMI, not succeeding to boost the quantization temperature to be higher. Here, we show that a large anomalous Hall effect, comparable to the best value so far reported for the heterostructure interfaces fabricated by epitaxial growth techniques, is realized for ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$ (BSTS)/${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT) heterostructure by transferring an epitaxially grown BSTS thin film floating on a ultrapure water directly on a CGT substrate (wet transfer method). The fundamental discussions about the nature of magnetic proximity effect were given in the aspect of the quality of the CGT substrate and the atomic orientation of the TI/CGT interface. The large magnetic proximity effect shown in our present studies can be applicable for a variety of FMI substrates and therefore can pave a route for promoting magnetic TIs both in basic science and applications.

Cadmium arsenide $\left({\mathrm{Cd}}_3{\mathrm{As}}_2\right)$ hosts massless Dirac electrons in its ambient-condition tetragonal phase. We report x-ray diffraction and electrical resistivity measurements of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ upon cycling pressure beyond the critical pressure of the tetragonal phase and back to ambient conditions. We find that, at room temperature, the transition between the low- and high-pressure phases results in large microstrain and reduced crystallite size, both on rising and falling pressure. This leads to nonreversible electronic properties, including self-doping associated with defects and a reduction of the electron mobility by an order of magnitude due to increased scattering. This paper indicates that the structural transformation is sluggish and shows a sizable hysteresis of over 1 GPa. Therefore, we conclude that the transition is first-order reconstructive, with chemical bonds being broken and rearranged in the high-pressure phase. Using the diffraction measurements, we demonstrate that annealing at $\sim 200\hspace{0.17em}{}^{\circ }\mathrm{C}$ greatly improves the crystallinity of the high-pressure phase. We show that its Bragg peaks can be indexed as a primitive orthorhombic lattice with $a_{\mathrm{HP}}\approx 8.68 \AA ,$ $b_{\mathrm{HP}}\approx 17.15 \AA ,$ and $c_{\mathrm{HP}}\approx 18.58 \AA$. The diffraction study indicates that, during the structural transformation, a new phase with another primitive orthorhombic structure may also be stabilized by deviatoric stress, providing an additional venue for tuning the unconventional electronic states in ${\mathrm{Cd}}_3{\mathrm{As}}_2$.

Thermoelectric conversion, which is the generation of electricity from waste heat, can play an important role in renewable energy use. Lowering the dimensionality of semiconductor thermoelectric materials is a promising approach for improving thermoelectric performance, and ultimately one-dimensional (1D) semiconductor materials have the potential to exhibit maximized performance because of the presence of a 1D electronic structure, such as the van Hove singularity (vHs) in the density of states. However, experimentally verifying the effect of the 1D nature on the thermoelectric performance in semiconductor nanomaterials has been difficult because we cannot observe any traces of the 1D electronic structure in terms of conventional thermoelectric parameters, such as the Seebeck coefficient or power factor. Here, we show that a thermoelectric parameter, the thermoelectrical conductivity ($L_{12}$), is strongly correlated with the electronic structure and exhibits a unique 1D trace with single-walled carbon nanotubes (SWCNTs). We experimentally clarify that the $L_{12}$ of high-purity semiconducting SWCNTs has a peak structure with a chemical potential in the vicinity of the vHs. For comparison, the $L_{12}$ of monolayer molybdenum disulfides and graphene, which are chosen as 2D models, shows a different behavior, simply exhibiting constant values. Furthermore, we find that theoretical calculations support these $L_{12}$ behaviors, which are consistent with the expected behaviors of 1D and 2D electronic structures. Our results demonstrate that $L_{12}$ is a very good parameter for evaluating the traces of dimensionalities, thereby advancing the elucidation of the fundamental thermoelectric properties necessary for the development of low-dimensional materials.

Pyrite ${\mathrm{FeS}}_2$ 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 p-n 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 p-n junction in ${\mathrm{FeS}}_2$ crystals by fabricating metallic ${\mathrm{CoS}}_2$ 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 enable a host of previously inaccessible observations and measurements, including determination of donor activation energies (which are as low as 5 meV for Co), observation of an unexpected second activated transport regime, realization of electron mobility up to $2100\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{–}1}{\mathrm{s}}^{\text{–}1}$, elucidation of very different mobilities in Co- and S-vacancy-doped cases, and observation of an abrupt temperature-dependent crossover to bulk Efros-Shklovskii variable-range hopping, accompanied by an unusual form of nonlinear Hall effect. Aspects of the results are interpreted with the aid of first-principles electronic structure calculations on both Co- and S-vacancy-doped ${\mathrm{FeS}}_2$. This work thus demonstrates unequivocal mitigation of the internal p-n junction in pyrite single crystals, with important implications for both future fundamental studies and photovoltaic devices.

Structural heterogeneity of amorphous solids presents difficult challenges that stymie the prediction of plastic events, which are intimately connected to their mechanical behavior. Based on a perturbation analysis of the potential energy landscape, we derive the atomic nonaffinity as an indicator with intrinsic orientation, which quantifies the contribution of an individual atom to the total nonaffine modulus of the system. We find that the atomic nonaffinity can efficiently characterize the locations of the shear transformation zones, with a predicative capacity comparable to the best indicators. More importantly, the atomic nonaffinity, combining the sign of the third-order derivative of energy with respect to coordinates, reveals an intrinsic softest shear orientation. By analyzing the angle between this orientation and the shear loading direction, it is possible to predict the protocol-dependent response of one shear transformation zone. Employing this method, the distribution of orientations of shear transformation zones in model two-dimensional amorphous solids can be measured. The resulting plastic events can possibly be understood from a simple model of independent plastic events occurring at variously oriented shear transformation zones. These results shed light on the characterization and prediction of the mechanical response of amorphous solids.

Mesoporous silica, known as SBA-15, possesses a hexagonal pore structure, with 8-nm-diameter pores and high surface area $\left(\sim 700{\mathrm{m}}^2/\mathrm{g}\right)$. The mesoporosity of SBA-15 is excellent to be filled with luminescent chromophores aiming at optical and photonic applications. SBA-15 powder infiltrated with Rhodamine B (RB) was prepared with different RB loads. Using the second harmonic of a pulsed Nd:yttrium aluminum garnet (YAG) laser (8 ns, 10 Hz) as the excitation source, the light emission spectra of the samples were investigated. Typical incoherent feedback random laser behavior was observed. Tunable random laser emissions from 579 to 585 nm were obtained depending on the excitation laser spot diameter and the RB load. The results indicate that mesoporous SBA-15 infiltrated with RB is a promising material for the development of solid-state random lasers.

$\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ has recently been the subject of intensive study, due to the prediction of axion insulator, Weyl semimetal, and quantum anomalous Hall insulator phases, depending on the structure and magnetic ordering. Experimental results have confirmed some aspects of this picture, but several experiments have seen zero-gap surfaces states at low temperature, in conflict with expectations. In this work, we develop a first-principles-based tight-binding model that allows for arbitrary control of the local spin direction and spin-orbit coupling, enabling us to accurately treat large unit-cells. Using this model, we examine the behavior of the topological surface state as a function of temperature, finding a gap closure only above the Néel temperature. In addition, we examine the effect of magnetic domains on the electronic structure, and we find that the domain wall zero-gap states extend over many unit-cells. These domain wall states can appear similar to the high-temperature topological surface state when many domain sizes are averaged, potentially reconciling theoretical results with experiments.

Accurate methods and an efficient workflow for computing and documenting dislocation core energies are developed and applied to $\frac{1}{2}\langle 111\rangle$ and $\langle 100\rangle$ dislocations in five body-centered cubic (bcc) metals W, Ta, V, Mo, and $\alpha$-Fe represented by 13 model interatomic potentials. For each dislocation type, dislocation core energies are extracted for a large number of dislocation characters thoroughly sampling the entire 2-space of crystallographic line orientations of the bcc lattice. Of particular interest, core energies of the $\frac{1}{2}\langle 111\rangle \left\{110\right\}$ dislocations are found to be distinctly asymmetric with respect to the sign of the character angle, whereas core energies of $\langle 100\rangle \left\{110\right\}$ junction dislocations exhibit marked cusps for line orientations vicinal to the closed-packed $\langle 111\rangle$ directions. Our findings furnish substantial insights for developing accurate models of dislocation core energies employed in mesoscale dislocation dynamics simulations of crystal plasticity.

The temperature-dependent ferroelectric properties of super-tetragonal ${\mathrm{BiFeO}}_3$ are investigated using surface-sensitive low-energy electron microscopy (LEEM). We use epitaxial oxide ${\mathrm{BiFeO}}_3/{\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_3$ bilayers grown by pulsed laser deposition on ${\mathrm{YAlO}}_3$ substrates. Ferroelectric, micrometer-scale domains are written by piezoresponse force microscopy and subsequently observed by LEEM from room temperature up to about 950 K. Kelvin probe force microscopy and LEEM spectroscopy reveal that the surface potential is efficiently (>50%) screened by adsorbates that are only released after annealing above 873 $\pm$ 50 K in ultrahigh vacuum. The surface structure and chemistry of the ferroelectric thin films are analyzed using scanning transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy, discarding the occurrence of a putative “skin layer” effect. While its magnetic and structural transitions were reported in the literature, the true, ferroelectric Curie temperature of super-tetragonal ${\mathrm{BiFeO}}_3$ has not been determined so far. Here, we measure a Curie temperature of 930 $\pm$ 30 K for the super-tetragonal ${\mathrm{BiFeO}}_3$ surface and corroborate it with volume-sensitive, temperature-dependent x-ray diffraction measurements. These results suggest that LEEM can be used as a powerful tool to probe surface charge and ferroelectric transitions in ultrathin films.

We report a Cu $K$- and $L_3$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$, with a focus on post-growth annealing effects. For the parent compound ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ $(x=0)$, a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of $R{E}_2{\mathrm{CuO}}_4$ $(\mathrm{RE}=\text{rare earth})$ can become metallic and superconducting at sufficiently high electron concentrations without Ce doping. For $x=0.16$, a Ce concentration for which it is known that oxygen reduction destroys long-range antiferromagnetic order and induces superconductivity, we find that the high-energy spin excitations of non-superconducting as-grown and superconducting annealed crystals are nearly identical. This finding is in stark contrast to the significant changes in the low-energy spin excitations previously observed via neutron scattering.

The relationship between the nucleation process and thermal hysteresis width in reversible thermoelastic martensitic transformations remains unclear, particularly as the volume of transforming material decreases. Understanding the number density and nature of defects which serve as nucleation sites in this class of materials requires a quantitative analysis of nucleation site potency distributions in different classes of materials with different intrinsic barriers to nucleation. Here, we investigate the size dependence of hysteresis in microscale ${\mathrm{Ni}}_{43}{\mathrm{Co}}_7{\mathrm{Mn}}_{39}{\mathrm{Sn}}_{11}$ alloy particles (radius $4.4\text{–}19.0\mu \mathrm{m}$) during reversible martensitic transformations by collecting temperature-dependent magnetization of 126 individual alloy particles. Size-dependent hysteresis is quantified by a power law model and attributed to friction-induced energy dissipation. In samples with ideal nucleation-limited transformations, martensitic transformation temperatures on cooling decreased with decreasing particle volume due to the low probability of including relatively sparse high-energy nucleation sites. Nucleation site potency distributions are quantified as a function of thermodynamic driving force and compared against potency distributions for thermoelastic martensitic transformations in other classes of materials and for burst martensitic transformations. Across different classes of materials, as the energy barrier associated with the martensitic transformation increases, number densities of defects with sufficient potency to nucleate the transformation decrease dramatically. This finding suggests that very different kinds of defects may be responsible for nucleation of martensitic phase transformations in different material systems.

Electronic states of a correlated material can be effectively modified by structural variations delivered from a single-crystal substrate. In this paper, we show that the CrN films grown on MgO (001) substrates have a (001) orientation, whereas the CrN films on $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ (0001) substrates are oriented along the (111) direction parallel to the surface normal. Transport properties of CrN films are remarkably different depending on crystallographic orientations. The critical thickness for the metal-insulator transition in CrN 111 films is significantly larger than that of CrN 001 films. In contrast to CrN 001 films without apparent defects, scanning transmission electron microscopy results reveal that CrN 111 films exhibit strain-induced structural defects, e.g., the periodic horizontal twinning domains, resulting in an increased electron scattering facilitating an insulating state. Understanding the key parameters that determine the electronic properties of ultrathin conductive layers is highly desirable for future technological applications.