Interactions of charge carriers with lattice vibrations, or phonons, play a critical role in unconventional electronic transport of metals and semimetals. Recent observations of phonon-mediated collective electron flow in bulk semimetals, termed electron hydrodynamics, present new opportunities in the search for strong electron-electron interactions in high carrier density materials. Here we present the general transport signatures of such a second-order scattering mechanism, along with analytical limits at the Eliashberg level of theory. We study electronic transport, using ab initio calculations, in finite-size channels of semimetallic ZrSiS and ${\mathrm{TaAs}}_2$ with and without topological band crossings, respectively. The order of magnitude separation between momentum-relaxing and momentum-conserving scattering length scales across a wide temperature range make both of them promising candidates for further experimental observation of electron hydrodynamics. More generally, our calculations suggest that the hydrodynamic transport regime does not, to first order, rely on the topological nature of the bands. Finally, we discuss general design principles guiding future search for hydrodynamic candidates, based on the analytical formulation and our ab initio predictions. We find that systems with strong electron-phonon interactions, reduced electronic phase space, and suppressed phonon-phonon scattering at temperatures of interest are likely to feature hydrodynamic electron transport. We predict that layered and/or anisotropic semimetals composed of half-filled $d$ shells and light group V/VI elements with lower crystal symmetry are promising candidates to observe hydrodynamic phenomena in the future.

The discovery of fascinating ways to control and manipulate antiferromagnetic materials have garnered considerable attention as an attractive platform to explore novel spintronic phenomena and functionalities. Layered antiferromagnets (AFMs) exhibiting interesting magnetic structures can serve as an attractive starting point to establish novel functionalities down to the two-dimensional limit. In this work, we explore the magnetoresistive properties of the spin-ladder AFM ${\mathrm{TaFe}}_{1.25}{\mathrm{Te}}_3$. Magnetization studies reveal an anisotropic magnetic behavior resulting in the stabilization of a spin-flop configuration for $H\perp$ (10-1) plane (i.e., out-of-plane direction). Angle-dependent longitudinal and transverse magnetoresistances show an unusual anharmonic behavior. A significant anisotropic enhancement of magnetoresistance when $H\perp$ (10-1) plane compared to $H\left|\right|$ (10-1) directions has been observed. The present results deepen our understanding of the magnetoresistive properties of low-dimensional layered AFMs, and point towards the possibility of utilizing these novel material systems for antiferromagnetic spintronics.

The binary ruthenate, ${\mathrm{RuO}}_2$, has been the subject of intense interest due to its itinerant antiferromagnetism and strain-induced superconductivity. The strain mechanism and its effect on the microscopic electronic states leading to the normal and superconducting state, however, remain undisclosed. Here, we investigate highly strained epitaxial (110) ${\mathrm{RuO}}_2$ films using polarization-dependent oxygen $K$-edge x-ray absorption spectroscopy (XAS). Through the detection of pre-edge peaks arising from O:$2p$ - Ru:$4d$ hybridization, we uncover the effects of epitaxial strain on the orbital/electronic structure near the Fermi level. Our data show robust strain-induced shifts of orbital levels and a reduction of hybridization strength. Furthermore, we reveal a pronounced in-plane anisotropy of the electronic structure along the $\left[110\right]/\left[1\overline{1}0\right]$ directions naturally stemming from the symmetry-breaking epitaxial strain of the substrate. The $B_{2g}$ symmetry component of the epitaxially enforced strain breaks a sublattice degeneracy, resulting in an increase of the density of states at the Fermi level ($E_F$), possibly paving the way to superconductivity. These results underscore the importance of the effective reduction from tetragonal to orthorhombic lattice symmetry in (110) ${\mathrm{RuO}}_2$ films and its relevance towards the superconducting and magnetic properties.

Developing data-driven machine-learning interatomic potential for materials containing many elements becomes increasingly challenging due to the vast configuration space that must be sampled by the training data. We study the learning rates and achievable accuracy of machine-learning interatomic potentials for many-element alloys with different combinations of descriptors for the local atomic environments. We show that for a five-element alloy system, potentials using simple low-dimensional descriptors can reach meV/atom-accuracy with modestly sized training datasets, significantly outperforming the high-dimensional SOAP descriptor in data efficiency, accuracy, and speed. In particular, we develop a computationally fast machine-learned and tabulated Gaussian approximation potential (tabGAP) for Mo–Nb–Ta–V–W alloys with a combination of two-body, three-body, and a new simple scalar many-body density descriptor based on the embedded atom method.

Alloying has been a tradition in materials science that has enabled groundbreaking discoveries in semiconductor technologies, optics, and photovoltaics, among others. While alloying in traditional systems is relatively well established, the effects of alloying in the emerging van der Waals (vdW) two-dimensional (2D) magnets are still in their infancy. Using ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2$ as a testbed system, our results show that chemical vapor transport of stoichiometric mixtures of $\mathrm{Te}{\mathrm{Cl}}_4$, Co, and Ni enables the synthesis of highly crystalline vdW magnetic alloys with excellent control over the Ni concentration ($x$) without any tellurium impurities or phase separation. The method is advantageous compared to binary $\mathrm{Co}{\mathrm{Cl}}_2$ and $\mathrm{Ni}{\mathrm{Cl}}_2$ precursor mixtures which only produce small-sized crystals with a large compositional variation. Magnetic measurements show that the degree of magnetic anisotropy, Weiss temperature, and Néel temperature ($T_{\mathrm{N}}$) strongly correlate to the Ni concentration, offering a tune-knob to engineer the magnetic behavior of transition metal dihalides. First-principles calculations offer further insights into how the increasing Ni content influences the interlayer and intralayer magnetic couplings and the resulting magnetic response. Overall, our findings provide an important avenue toward metal cation alloying in dihalide 2D vdW magnets and offer means to tune their magnetic behavior on demand.

Indium antimonide has attracted extensive interest due to its optical and electronic applications, and functions as a constituent block in exploring Majorana physics. Here, through molecular-beam epitaxy, we achieve van der Waals epitaxy of high-quality InSb islands on graphene-covered SiC(0001) substrate. As characterized with spectroscopic imaging scanning tunneling microscopy (STM), the islands have tens of nanometers in thickness and a 2 × 2 surface reconstruction, exhibiting a narrow band gap of 0.315 ± 0.005 eV. Double concentric rings, whose diameters vary with the bias voltage of the STM tip, are observed in the spatially resolved conductance maps, which are explained as a charging process of solitary defects due to tip-induced band bending. Furthermore, high-temperature sample annealing induces a 3 × 3 surface reconstruction, which possesses a larger gap of 0.40 ± 0.01 eV. Our work demonstrates the feasibility of van der Waals epitaxy of the InSb traditional semiconductor and enriches the understanding of charging process in single defects.

Predicting crystal structure has been one of the core issues in materials research. X-ray diffraction data can generally be used to fit and refine the positions of atoms within the unit cell. However, for a system with part of its atoms diffusing or hopping below the melting temperature, the atoms inside the crystal begin to vibrate, diffuse, or hop when its temperature gradually rises. Even using the partial-occupancy concept, it becomes difficult or impossible to fit the structure due to the trial-and-error characteristic of refinement. More importantly, the limited partial-occupancy positions are insufficient and inappropriate to describe material systems containing diffusing or hopping atoms. To address this shortcoming, we proposed to perform molecular dynamics simulations and then add up all the positions of diffusing atoms in each time step to form a three-dimensional atomic density distribution into the primitive or conventional unit cell. The maximum atomic density points correspond to the partial-occupancy positions in x-ray diffraction data. The connected atomic density distribution identifies and defines the diffusing paths for mobile atoms within the crystal.

High-temperature ferromagnetism and its control in ultrathin materials are of critical interest in disruptive quantum technologies. However, intrinsic ferromagnetic ordering in recently discovered two-dimensional materials is limited to cryogenic temperatures due to fundamental spin fluctuation in reduced dimensions. While chromium trihalides order below 45 K, we report a dramatic manipulation of long-range ferromagnetic order above room temperature in ${\mathrm{CrBr}}_3$ monolayer predicted within the first-principles long-range Heisenberg XXZ model. Gate-induced charge carrier doping externally modulates various magnetic interactions in the Mott insulating monolayer, and a nontrivial magnetic phase diagram emerges. Under varied hole density, the monolayer remains ferromagnetic with Curie temperature increasing monotonically, while electron doping triggers a complex evolution of magnetic phases. The microscopic details are investigated, and the high-temperature ferromagnetism is attributed to the substantial increase in effective ferromagnetic exchange and magnetic anisotropies under an experimentally attainable hole density. Such electrically controlled ferromagnetism in ${\mathrm{CrBr}}_3$ stands for experimental verification and presents enormous new possibilities for spintronic and magnetoelectric quantum devices based on atomically thin crystals.

Two-dimensional transition-metal dichalcogenide (${\mathrm{MX}}_2$) vacancy formation energetics are extensively investigated. Within an ab initio approach, we study the ${\mathrm{MX}}_2$ systems, with M = Mo, W, Ni, Pd, and Pt, and X = S, Se, and Te. Here we classify that chalcogen vacancies are always energetically favorable over the transition-metal ones. However, for late transition metals Pd $4d$, and Pt $5d$ the metal vacancies are experimentally achievable, bringing up localized magnetic moments within the semiconducting matrix. By quantifying the localization of the chalcogen vacancy states we show that it rules the intra- and intervacancy interactions that establish both the number of vacancy states neatly lying within the semiconducting gap, as well as its electronic dispersion and spin-orbit coupling splitting. Combining different vacancies and phase variability $1T$ and $1H$ of the explored systems allows us to construct a guiding picture of the vacancy state localization.

LiB, a predicted layered compound analogous to the ${\mathrm{MgB}}_2$ superconductor, has been recently synthesized via cold compression and quenched to ambient pressure, yet its superconducting properties have not been measured. According to prior isotropic superconductivity calculations, the critical temperature ($T_{\mathrm{c}}$) was expected to be only 10–15 K. Using the anisotropic Migdal-Eliashberg formalism, we show that the $T_{\mathrm{c}}$ may actually exceed 32 K. Our analysis of the contribution from different electronic states helps explain the detrimental effect of pressure and doping on the compound's superconducting properties. In the search for related superconductors, we screened Li-Mg-B binary and ternary layered materials and found metastable phases with $T_{\mathrm{c}}$ close to or even 10–20% above the record 39 K value in ${\mathrm{MgB}}_2$. Our reexamination of the Li-B binary phase stability reveals a possible route to synthesize the LiB superconductor at lower pressures readily achievable in multianvil cells.

Finding a low-cost, stable solar absorber with suitable optical properties is one of the key aspects in promoting solar cell research. Perovskite-structured ${\mathrm{CaMnO}}_3$, composed of earth-abundant elements, is an environmentally friendly multifunctional material especially featuring thermoelectricity and G-type antiferromagnetism. In this paper, we explore the potential of $\mathrm{CaMn}{\mathrm{O}}_{3\text{−}\delta }$(CMO) as an absorber layer in all oxide solar cells. Solution-processed CMO shows strong light absorption capacity in the wavelength range of 400–750 nm with the estimated optical bandgap of 1.77 eV. First-principles calculations show $d\text{−}d$ transitions within the upper Mott-Hubbard bands, and lower Mott-Hubbard bands constitute the electronic bandgap, whereas the $p\text{−}d$ transitions between the O $2p$ and Mn $3d$ bands contribute to the optical absorption in the visible region. Furthermore, a gradual increase in the current value was observed under illumination in the photoconductivity measurement. We have designed a CMO absorber-layer-based all oxide solar cell using a conventional mesostructured ${\mathrm{TiO}}_2$ photo-anode and NiO hole transport layer (HTL). The suitable band alignment of the CMO absorber with both ${\mathrm{TiO}}_2$ and NiO enables us to achieve an average open-circuit voltage $\left(V_{\mathrm{OC}}\right)$ of 0.92 V with the maximum value of 1.03 V, which is among the highest reported values so far for oxide-based solar cells. Solar cell parameters were also validated using simulations. Our results add a direction in the search for low-cost, stable solar absorber materials.

We investigate the electronic structure of two ${\mathrm{Ir}}^{5+}$ honeycomb iridates, ${\mathrm{Sr}}_3{\mathrm{CaIr}}_2{\mathrm{O}}_9$ and ${\mathrm{NaIrO}}_3$, by means of resonant x-ray techniques. We confirm that ${\mathrm{Sr}}_3{\mathrm{CaIr}}_2{\mathrm{O}}_9$ realizes a large spin-orbit driven nonmagnetic $J_{\mathrm{eff}}=0$ singlet ground state despite sizable tetragonal distortions of Ir coordinating octahedra. However, the resonant inelastic x-ray spectra of ${\mathrm{NaIrO}}_3$ are drastically different from expectations for a Mott insulator with octahedrally coordinated ${\mathrm{Ir}}^{5+}$. We find that the data for ${\mathrm{NaIrO}}_3$ can be best interpreted as originating from a narrow gap nonmagnetic $S=0$ band insulating ground state. Our results highlight the complex role of the ligand environment in the electronic structure of honeycomb iridates and the essential role of x-ray spectroscopy to characterize electronic ground states of insulating materials.

Space is the operating environment of a multitude of systems that our society is heavily reliant on today, however, maintaining operability there necessitates special consideration of the electronic systems' tolerance of space radiation. Electronic systems are critically dependent on the electronic properties of their semiconductor components, which are modified by space radiation with an adverse impact on the space system performance. What innate property allows some semiconductors to sustain little damage while others accumulate defects rapidly with dose is poorly understood, which limits the extent to which radiation tolerance can be implemented as a design criterion. To gain insight into what properties are drivers of semiconductor radiation tolerance, the first step is to generate a dataset of the relative radiation tolerance of a broad sampling of semiconductors. To accomplish this, Rutherford backscatter channeling experiments are used to compare the displaced lattice atom buildup in InAs, InP, GaP, GaN, ZnO, MgO, and Si as a function of stepwise alpha particle dose. With this experimental information on radiation-induced incorporation of interstitial defects in hand, hybrid density functional theory electron densities (and their derived quantities) are calculated and their gradient and Laplacian are evaluated to obtain key fundamental information about the interactions in each material. It is shown that simple, undifferentiated values (which are typically used to describe bond strength) are insufficient to predict radiation tolerance. Instead, the curvature of the electron density at bond critical points provides a measure of radiation tolerance consistent with the experimental results obtained. This curvature and associated forces surrounding bond critical points have the potential to disfavor the localization of displaced lattice atoms at these points, favoring their diffusion toward perfect lattice positions. With this criterion to predict radiation tolerance, simple density functional theory simulations can be conducted on potential new materials to gain insight into how they may operate in demanding high radiation environments.

In this report, we investigated a new rare-earth-based one-dimensional Ising spin chain magnet $\mathrm{Dy}{\mathrm{Ni}}_5{\mathrm{Ge}}_3$ by means of magnetization, specific heat, and powder neutron diffraction measurements. Due to the crystalline electrical field splitting, the magnetic Dy ions share an Ising-like ground doublet state. Owning to the local point symmetry, these Ising moments form into two canted magnetic sublattices, which were further confirmed by the angle-dependent magnetization measurement. In zero fields, two successive antiferromagnetic phase transitions were found at temperatures $T_{\mathrm{N}1}=6\mathrm{K}$ and $T_{\mathrm{N}2}=5\mathrm{K}$, respectively. Only part of the moments are statically ordered in this intermediate state between $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$. Powder neutron diffraction experiments at different temperatures were performed as well. An incommensurate magnetic propagation vector of ${\mathbf{k}}_{\mathbf{m}}=(0.5,0.4,0.5)$ was identified. The refined spin configurations through the irreducible representation analysis confirmed that these Ising spins are canted in the crystal $ab$ plane.

Two-dimensional (2D) van der Waals heterostructures (vdWHs) are attractive candidates for realizing tunnel field-effect transistors (TFETs) for low-power applications. In this work, using first-principles calculations based on density functional theory (DFT), we explore heterostructures composed of 2D $\mathrm{Mo}{\mathrm{Te}}_2$ and $\mathrm{Sn}{\mathrm{Se}}_2$. Our calculations reveal that upon forming the heterostructures, the valence band top of $\mathrm{Mo}{\mathrm{Te}}_2$ and the conduction band bottom of $\mathrm{Sn}{\mathrm{Se}}_2$ are almost aligned, forming the nearly broken-gap or type-III band alignment which is highly promising for TFETs. Interestingly, we find that the band alignment can be tuned by applying external electric fields. For positive electric fields, $\mathrm{Mo}{\mathrm{Te}}_2$ $(\mathrm{Sn}{\mathrm{Se}}_2)$ band-edge positions are shifted upward (downward) with respect to the Fermi level, and more electrons are expected to tunnel from $\mathrm{Mo}{\mathrm{Te}}_2$ to $\mathrm{Sn}{\mathrm{Se}}_2$. Overall, our simulations provide fundamental insights into the electronic properties of $\mathrm{Mo}{\mathrm{Te}}_2/\mathrm{Sn}{\mathrm{Se}}_2$ stacks, and pave the way for the design and fabrication of future $\mathrm{Mo}{\mathrm{Te}}_2/\mathrm{Sn}{\mathrm{Se}}_2$-based TFETs.

The magnetic structures of the double perovskites ${\mathrm{Pr}}_2{\mathrm{NiIrO}}_6$ and ${\mathrm{Nd}}_2{\mathrm{NiIrO}}_6$ have been reinvestigated using high-intensity neutron power diffraction and compared with previously published results by Kayser et al. [Acta Mater. 207, 116684 (2021)] and Ferreira et al. [Phys. Rev. Mat. 5, 064408 (2021)]. The strongly enhanced statistics of neutron diffraction data used in this paper help us to disentangle the complicated coexisting magnetic orderings of the three sublattices in ${\mathrm{Pr}}_2{\mathrm{NiIrO}}_6$ and ${\mathrm{Nd}}_2{\mathrm{NiIrO}}_6$. A mainly ferrimagnetic ordering with additional antiferromagnetic components with propagation vector $k=0$ has been confirmed in both ${\mathrm{Pr}}_2{\mathrm{NiIrO}}_6$ and ${\mathrm{Nd}}_2{\mathrm{NiIrO}}_6$ compounds below the magnetic orderings $T_C=110$ and 120 K, respectively. ${\mathrm{Nd}}_2{\mathrm{NiIrO}}_6$ further undergoes a second magnetic transition from a ferrimagnet with $k=0$ to an antiferromagnetic state with $k=$ $(\frac{1}{2},$ $\frac{1}{2}, 0)$ below $T_N=15\mathrm{K}$. A partial persistence of the $k=0$ type order leads, however, to a coexistence of both magnetic phases below $T_N$ not detected previously. The coexistence points to the energies of the magnetic ground states being very similar. Ordered magnetic moments of ${\mathrm{Ir}}^{4+}$ amounting at 1.5 K to $\sim 0.6{\mu }_B$ in ${\mathrm{Pr}}_2{\mathrm{NiIrO}}_6$ and $0.4{\mu }_B$ in ${\mathrm{Nd}}_2{\mathrm{NiIrO}}_6$ are clearly detected and are compared with the expected value for the $J_{\mathrm{eff}}=$ $\frac{1}{2}$ model.

Unconventional chiral quasiparticles with twofold, threefold, and fourfold degeneracies and Chern number ($C$) = $\pm 2$ have recently sparked interest because of their topological chirality in momentum space. Herein, based on first-principles calculations and symmetry analysis, we propose 14 synthesized topological phonon systems with space group $P2{}_{1}3$ (No. 198) that host two types of almost-ideal double-Weyl points, which are the fourfold degenerate charge-2 Dirac point and the threefold degenerate spin-1 Weyl point. Based on the projected local density of states on the [100] and [101] surfaces of these selected realistic materials, we discovered two visible winding around the projected double-Weyl points, demonstrating the appearance of phononic double-helical surface arcs. Furthermore, we constructed a $\mathit{k}·\mathit{p}$ model to determine how the two types of double-Weyl point phonons coexist in the system with the $P2{}_{1}3$ space group. We believe that this work provides directions for investigating double-Weyl point phonons and an ideal platform for investigating phononic double-helical surface states in realistic materials.

Triangular lattice delafossite compounds built from magnetic lanthanide ions are a topic of recent interest due to their frustrated magnetism and realization of quantum disordered magnetic ground states. Here we report the evolution of the structure and electronic ground states of $\mathrm{RbCe}{X}_2$ compounds, built from a triangular lattice of ${\mathrm{Ce}}^{3+}$ ions, upon varying their anion character $(X_2={\mathrm{O}}_2$, ${\mathrm{S}}_2, \mathrm{SeS}, {\mathrm{Se}}_2, \mathrm{TeSe}, {\mathrm{Te}}_2)$. This includes the discovery of a new member of this series, ${\mathrm{RbCeO}}_2$, that potentially realizes a quantum disordered ground state analogous to ${\mathrm{NaYbO}}_2$. Magnetization and susceptibility measurements reveal that all compounds manifest mean-field antiferromagnetic interactions and, with the exception of the oxide, possess signatures of magnetic correlations onset below 1 K. The crystalline electric field level scheme is explored via neutron scattering and ab initio calculations in order to model the intramultiplet splitting of the $J=5/2$ multiplet. In addition to the two excited doublets expected within the $J=5/2$ manifold, we observe one extra local mode present across the sample series. This added mode shifts downward in energy with increasing anion mass and decreasing crystal field strength, suggesting a long-lived anomalous mode endemic to anion motion about the ${\mathrm{Ce}}^{3+}$ sites.