Monoclinic hafnia ($m\text{−}{\mathrm{HfO}}_2$) with its high dielectric permittivity ($\kappa$) and larger band gap of $\sim 5.5\text{−}5.9\mathrm{eV}$ deposited as a few nanometers thick $m\text{−}{\mathrm{HfO}}_2$ thin film on a Si substrate was introduced as an effective gate oxide in complementary metal-oxide-semiconductor devices by Intel in 2007. However, the existence and complex anisotropic excitation characters of this centrosymmetric monoclinic crystal structure—which involve both single-particle and collective electron excitations such as plasmons, and include electron-hole (excitonic) and electron-electron (self-energy) interactions—remain elusive. Therefore, the electronic nature of the material needs to be explored in depth for applications in semiconductor technology. In this study, spatially and momentum-resolved electron energy-loss spectroscopy (EELS) in conjunction with first-principles calculations of the electronic band structure and dielectric function have been employed to investigate electronic excitations in $m\text{−}{\mathrm{HfO}}_2$. The phase purity and crystallinity of $m\text{−}{\mathrm{HfO}}_2$ were confirmed by x-ray diffraction and EELS. Low-loss EELS performed using $aloof$ electron beam setups and energy-filtered transmission electron microscopy spectrum imaging (EFTEM-SI) revealed spectral features at 13.5 and 16 eV energy loss assigned to surface plasmons and volume plasmons (VPs), respectively. Surface exciton polaritons (SEPs) with surface resonances associated with excitonic onsets above the band gap were also observed at $\sim 7.5$ and $\sim 28\mathrm{eV}$ energy loss. The surface excitation character of these features was confirmed by EFTEM-SI and relativistic calculations of energy versus momentum ($E\text{−}k$) maps. Using collection-angle (β) and momentum ($q$)-resolved EELS, it was found that the SEP intensity at $\sim 7.5\mathrm{eV}$ energy loss is a function of β and $q$, and no anisotropic shape for the VP is observed along the [100], [010], and [001] directions. Furthermore, the peak at $\sim 48\mathrm{eV}$ energy loss was assigned to a semicore $\mathrm{Hf}5p$ plasmon involving multiplet resonant processes. All the VPs, the SEPs at 28 eV energy loss, and the $\mathrm{Hf}5p$ plasmons at 48 eV energy loss display parabolic dispersion behavior with an energy shift of $1–3\mathrm{eV}$.

Stacking of two-dimensional van der Waals (vdW) materials has emerged as a vital approach for investigating the complex interplay between coexisting materials, leading to novel electronic, photonic, and spintronic phenomena such as strong electronic correlation, enhanced Zeeman splitting of excitons, and the magnetic proximity effect. While most of the previous focus was on the electronic and photonic properties of vdW hetero-/homostructures, the study of magnetism in hetero-/homostructures remains lacking, though phenomena such as layer-dependent magnetism already highlighted the critical role of interlayer coupling in magnetism of layered systems. Here, we report an interesting two-step magnetic hysteresis loop in the homostructure of the layered vdW magnet ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT). The two coercivities in the homostructure differ from that of each constituent FGT layer, indicating that the two constituents interact mutually but do not merge into a thicker uniform material. The Curie temperature ($T_{\mathrm{C}}$) of the homostructure is in between that of the thinner and thicker constituents, further implying that the two constituent layers interact yet without forming a naturally occurring thicker layer. Our results highlight the unexplored opportunities that the rich interplay within vdW magnet homostructures can be employed to create emerging magnetic properties and develop novel multistate devices.

Screened range-separated hybrid (SRSH) functionals within generalized Kohn-Sham density functional theory (GKS-DFT) have been shown to restore a general $1/\left(r\varepsilon \right)$ asymptotic decay of the electrostatic interaction in dielectric environments. Major achievements of SRSH include an improved description of optical properties of solids and correct prediction of polarization-induced fundamental gap renormalization in molecular crystals. The density functional tight-binding method (DFTB) is an approximate DFT that bridges the gap between first-principles methods and empirical electronic structure schemes. While purely long-range corrected RSH are already accessible within DFTB for molecular systems, this work generalizes the theoretical foundation to also include screened range-separated hybrids, with conventional pure hybrid functionals as a special case. The presented formulation and implementation is also valid for periodic boundary conditions (PBC) beyond the $\mathrm{\Gamma }$ point. To treat periodic Fock exchange and its integrable singularity in reciprocal space, we resort to techniques successfully employed by DFT, in particular a truncated Coulomb operator and the minimum image convention. Starting from the first-principles Hartree-Fock operator, we derive suitable expressions for the DFTB method, using standard integral approximations and their efficient implementation in the $\mathrm{dftb}+$ software package. Convergence behavior is investigated and demonstrated for the polyacene series as well as two- and three-dimensional materials. Benzene and pentacene molecular and crystalline systems show the correct polarization-induced gap renormalization by SRSH-DFTB at heavily reduced computational cost compared to first-principles methods.

We introduce a simplified method to model the interatomic interactions of high entropy alloys based on a lookup table of cluster energies. These interactions are employed in replica exchange Monte Carlo simulations with histogram analysis to obtain thermodynamic properties across a broad temperature range. Kikuchi's cluster variation method entropy formalism is applied to directly calculate entropy from statistics on short- and long-range chemical order, and we discuss the convergence of the entropy as clusters of differing size are included. A high temperature series expansion aids in our understanding of the convergence. Computer codes implementing these methods, and supporting data, are freely available on the internet.

A general method is presented for modeling high-entropy alloys as ensembles of randomly sampled, ordered configurations on a given lattice. Statistical mechanics is applied post hoc to derive the ensemble properties as a function of composition and temperature, including the free energy of mixing and local structure. Random sampling is employed to address the high computational costs needed to model alloys with a large number of components. Doing so also provides rigorous convergence criteria, including the quantification of noise due to random sampling, and an estimation of the number of additional samples required to lower this noise to the desired levels. Binary to five-component alloys of the group-IV chalcogenides are used as case examples, for which the predicted miscibility shows excellent agreement with experiment. This method is well-suited for calculating the configurational thermodynamics, local structure, and ensemble properties of complex alloys, and it is attractive for materials with temperature-dependent, short-range order.

Electrical switching of Néel order in an antiferromagnetic insulator is desirable as a basis for memory applications. Unlike electrically driven switching of ferromagnetic order via spin-orbit torques, electrical switching of antiferromagnetic order remains poorly understood. Here we investigate the low-field magnetic properties of 30-nm-thick, $c$-axis-oriented $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ Hall devices using a diamond nitrogen-vacancy center scanning microscope. Using the canted moment of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ as a magnetic handle on its Néel vector, we apply a saturating in-plane magnetic field to create a known initial state before letting the state relax in low field for magnetic imaging. We repeat this procedure for different in-plane orientations of the initialization field. We find that the magnetic field images are characterized by stronger magnetic textures for fields along $\left[\overline{1}\overline{1}20\right]$ and $\left[11\overline{2}0\right]$, suggesting that despite the expected 3-fold magnetocrystalline anisotropy, our $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ thin films have an overall in-plane uniaxial anisotropy. We also study current-induced switching of the magnetic order in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. We find that the fraction of the device that switches depends on the current pulse duration, amplitude, and direction relative to the initialization field.

Geometric frustration of magnetic ions can lead to a quantum spin liquid ground state where long-range magnetic order is avoided despite strong exchange interactions. The physical realization of quantum spin liquids comprises a major unresolved area of contemporary materials science. One prominent magnetically frustrated structure is the kagome lattice. The naturally occurring minerals herbertsmithite $\left[\mathrm{Zn}{\mathrm{Cu}}_3{\left(\mathrm{O}\mathrm{H}\right)}_6{\mathrm{Cl}}_2\right]$ and Zn-substituted barlowite $\left[\mathrm{Zn}{\mathrm{Cu}}_3{\left(\mathrm{O}\mathrm{H}\right)}_6\mathrm{Br}\mathrm{F}\right]$ both feature perfect kagome layers of spin-$1/2$ copper ions and display experimental signatures consistent with a quantum spin liquid state at low temperatures. To investigate other possible candidates within this material family, we perform a systematic first-principles combinatorial exploration of structurally related compounds [$A{\mathrm{Cu}}_3{\left(\mathrm{O}\mathrm{H}\right)}_6{B}_2$ and $A{\mathrm{Cu}}_3{\left(\mathrm{O}\mathrm{H}\right)}_{6}BC$] by substituting nonmagnetic divalent cations ($A$) and halide anions ($B, C$). After optimizing such structures using density functional theory, we compare various structural and thermodynamic parameters to determine which compounds are most likely to favor a quantum spin liquid state. Convex hull calculations using binary compounds are performed to determine feasibility of synthesis. We also estimate the likelihood of interlayer substitutional disorder and spontaneous distortions of the kagome layers. After considering all of these factors as a whole, we select several promising candidate materials that we believe deserve further attention.

The change in the electronic structure of graphite during electrochemical lithiation was investigated using solid-state NMR. The ${}^{13}\mathrm{C}$ NMR peak positions for the lithiated graphite compounds stages 3L - 1 were assigned. The dense stage ${\mathrm{LiC}}_{12}$ and ${\mathrm{LiC}}_6$ compounds were shown via ${}^{13}\mathrm{C}$ and ${}^7\mathrm{Li}$ to be metallic in nature, with Korringa relaxation being the dominant relaxation mechanism for both nuclei. The ${}^7\mathrm{Li}$ NMR shifts in graphite can be described by a direct Fermi-contact Knight shift, while the ${}^{13}\mathrm{C}$ NMR shifts can be described by the spin-dipolar Knight shift and isotropic Knight shift. The isotropic ${}^{13}\mathrm{C}$ NMR Knight shift of ${\mathrm{LiC}}_6$ was smaller than expected for a Knight shift caused purely by the core polarization of $1s$ orbitals. This showed that there must be a contribution of $2s$ orbitals at the Fermi level. The density of states at the Fermi level $\rho \left(E_F\right)$ for ${\mathrm{LiC}}_6$ was estimated and the values, which are dependent on the Stoner enhancement factor $\alpha$, agree reasonably well with those found in literature.

Two-dimensional (2D) materials exhibit a wide range of remarkable phenomena, many of which owe their existence to the relativistic spin-orbit coupling (SOC) effects. To understand and predict properties of materials containing heavy elements, such as the transition-metal dichalcogenides (TMDs), relativistic effects must be taken into account in first-principles calculations. We present an all-electron method based on the four-component Dirac Hamiltonian and Gaussian-type orbitals (GTOs) that overcomes complications associated with linear dependencies and ill-conditioned matrices that arise when diffuse functions are included in the basis. Until now, there has been no systematic study of the convergence of GTO basis sets for periodic solids either at the nonrelativistic or the relativistic level. Here we provide such a study of relativistic band structures of the 2D TMDs in the hexagonal (2H), tetragonal (1T), and distorted tetragonal (1T') structures, along with a discussion of their SOC-driven properties (Rashba splitting and ${\mathbb{Z}}_2$ topological invariants). We demonstrate the viability of our approach even when large basis sets with multiple basis functions involving various valence orbitals (denoted triple- and quadruple-$\zeta$) are used in the relativistic regime. Our method does not require the use of pseudopotentials and provides access to all electronic states within the same framework. Our study paves the way for direct studies of material properties, such as the parameters in spin Hamiltonians, that depend heavily on the electron density near atomic nuclei where relativistic and SOC effects are the strongest.

Mixed halide chemistry has recently been utilized to tune the intrinsic magnetic properties of transition-metal halides—one of the largest families of magnetic van der Waals materials. Prior studies have shown that the strength of exchange interactions, hence the critical temperature, can be tuned smoothly with halide composition for a given ground state. Here we show that the ground state itself can be altered by a small change of halide composition in ${\mathrm{FeCl}}_{3-x}{\mathrm{Br}}_x$. Specifically, we find a threefold jump in the Néel temperature and a sign change in the Weiss temperature at $x=0.08$ corresponding to only $3\%$ bromine doping. Using neutron scattering, we reveal a change of the ground state from spiral order in ${\mathrm{FeCl}}_3$ to $A$-type antiferromagnetic order in ${\mathrm{FeBr}}_3$. From first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. These results demonstrate how varying the halide composition can tune the competing interactions and change the ground state of a spiral spin liquid system.

Vibrational dynamics in halide double perovskites govern several key aspects including carrier recombination and transport properties. Here, we present comprehensive vibrational studies investigated through micro-Raman spectroscopy to understand how octahedral cation substitution in a wide range of metal halide double perovskites ${\mathrm{Cs}}_2{B}^{\prime }{B}^{″}{\mathrm{Cl}}_6$ ($B^{\prime }={\mathrm{Ag}}_{1-x}{\mathrm{Na}}_x$; $B^{″}={\mathrm{Bi}}_{1-x}{\mathrm{In}}_x$) influence lattice vibrations. A significant enhancement in $F_{2g}$ mode intensity—a key factor in determining the cation ordering—is observed with ${\mathrm{Na}}^{+}$ substitution. In contrast to a generally observed trend, despite similar ionic sizes of ${\mathrm{Na}}^{+}$ and ${\mathrm{Bi}}^{3+}$, an increase in cationic ordering is observed as ${\mathrm{Na}}^{+}$ substitutes at ${\mathrm{Ag}}^{+}$ site in ${\mathrm{Cs}}_2{\mathrm{Ag}}_{1-x}{\mathrm{Na}}_x\mathrm{Bi}{\mathrm{Cl}}_6$ and ${\mathrm{Cs}}_2{\mathrm{Ag}}_{1-x}{\mathrm{Na}}_x\mathrm{In}{\mathrm{Cl}}_6$. The $F_{2g}$ mode intensity depends on $B^{\prime }$-site cationic ordering (${\mathrm{Ag}}^{+}$ or ${\mathrm{Na}}^{+}$), while its vibrational energy is governed by the $B^{″}$-site cations (${\mathrm{Bi}}^{3+}$ or ${\mathrm{In}}^{3+}$). The symmetric stretching vibrations depicted by $A_{1g}$ mode are mainly influenced by $\left[B^{3+}\text{−}{X}_6\right]$ octahedra. The reduction in the linewidth of symmetric-stretching LO phonon mode ($A_{1g}$) and the disappearance/diminishing of asymmetric-stretching vibrations ($E_{\mathrm{g}}$) further substantiates the improved cationic ordering. The changes in the vibrational mode intensities with $B^{\prime }$-site substitution (${\mathrm{Ag}}^{+}, {\mathrm{Na}}^{+}$) and the appearance of distinct octahedral modes with $B^{″}$-site substitution (${\mathrm{Bi}}^{3+}, {\mathrm{In}}^{3+}$) allow us to disseminate different octahedral contributions to the vibrational dynamics in the lattice. Further, the vibrational analyses on double perovskites with different choices of $B^{\prime }$ and $B^{″}$ cations and $X$ anion reveal the origin of asymmetric stretching ($E_{\mathrm{g}}$). This mode mainly prevails when sublattice distortions in the lattice exist. Thus, asymmetric-stretching mode can be a measure of sublattice distortion in the double perovskite, and a highly ordered system would exhibit very minimal or no asymmetric vibrations.

Sheared multicomponent bulk metallic glasses are characterized by both chemical and structural disorder that define their properties. We investigate the behavior of the local, microstructural elastic modulus across the plastic yielding transition in six Ni-based multicomponent glasses, that are characterized by compositional features commonly associated with solid solution formability. We find that elastic modulus fluctuations display consistent percolation characteristics pointing towards universal behavior across chemical compositions and overall yielding sharpness characteristics. Elastic heterogeneity grows upon shearing via the percolation of elastically soft clusters within an otherwise rigid amorphous matrix, confirming prior investigations in granular media and colloidal glasses. We find clear signatures of percolation transition with spanning clusters that are universally characterized by scale-free characteristics and critical scaling exponents. The spatial correlation length and mean cluster size tend to diverge prior to yielding, with associated critical exponents that exhibit fairly weak dependence on compositional variations as well as macroscopic stress-strain curve details.

The newly discovered kagome superconductors $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) exhibit superconductivity, charge order, and band topology simultaneously. To explore the intricate interplay between the superconducting and charge-density-wave (CDW) orders, we investigate the doping evolution of underlying electronic structures in doped topological kagome superconductors $\mathrm{Cs}{\left({\mathrm{V}}_{1-x}{\mathrm{Ti}}_x\right)}_3{\mathrm{Sb}}_5$ where the Ti dopant introduces hole-like charge carriers. Despite the absence of the CDW phase transition in doped compounds even for the lowest doping level of $x=0.047$, the superconductivity survives in all doped samples with enhanced critical temperatures. The high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements reveal that the Ti dopant in the kagome plane lowers the chemical potential, pushing the van Hove singularity (VHS) at $M$ point above the Fermi level. First-principle simulations corroborate the doping evolution of the band structure observed in ARPES, and affirm that the CDW instability does not occur once the VHS moves above the Fermi level, explaining the absence of the CDW ordering in our doped samples $\mathrm{Cs}{\left({\mathrm{V}}_{1-x}{\mathrm{Ti}}_x\right)}_3{\mathrm{Sb}}_5$. Our results also demonstrate a competition between the CDW and superconducting orders in the kagome-metal superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, although the superconductivity is likely inconsequential of the CDW order.

Low-buckled silicene, germanene, and stanene are group-IV graphene allotropes. They form a honeycomb lattice out of two interpenetrating ($A$ and $B$) triangular sublattices that are vertically separated by a small distance ${\mathrm{\Delta }}_z$. The atomic numbers $Z$ of silicon, germanium, and tin are larger than that of carbon ($Z_{\text{C}}=6$), making them the first experimentally viable two-dimensional topological insulators. Those materials have a twice-energy-degenerate atomistic structure characterized by the buckling direction of the $B$ sublattice with respect to the $A$ sublattice [whereby the $B$ atom either protrudes above (${\mathrm{\Delta }}_z>0$) or below (${\mathrm{\Delta }}_z<0$) the $A$ atoms], and the consequences of that energy degeneracy on their elastic and electronic properties have not been reported thus far. Here, we uncover ferroelastic, bistable behavior on silicene, which turns into an average planar structure at about 600 K. Furthermore, the creation of electron and hole puddles obfuscates the zero-temperature spin-orbit coupling (SOC)-induced band gaps at temperatures as low as 200 K, which may discard silicene as a viable two-dimensional topological insulator for room-temperature applications. Germanene, on the other hand, never undergoes a low-buckled-to-planar two-dimensional transformation, becoming amorphous at around 675 K instead, and preserving its SOC-induced band gap despite band broadening. Stanene undergoes a transition onto a crystalline three-dimensional structure at about 300 K, preserving its SOC-induced electronic band gap up to that temperature. Unlike what is observed in silicene and germanene, stanene readily develops a higher-coordinated structure with a high degree of structural order. The structural phenomena are shown to have far-reaching consequences for the electronic and vibrational properties of those two-dimensional topological insulators.

Research efforts on the description of the low-temperature magnetic order and electronic properties of ${\mathrm{U}}_3{\mathrm{O}}_8$ have been inconclusive so far. Reinterpreting neutron scattering results, we use group representation theory to show that the ground state presents collinear out-of-plane magnetic moments, with antiferromagnetic coupling both in-layer and between layers. Charge order relieves the initial geometric frustration, generating a slightly distorted honeycomb sublattice with Néel-type order. The precise knowledge of the characteristics of this magnetic ground state is then used to explain the fine features of the band gap. In this system, spin-orbit coupling (SOC) is of critical importance, as it strongly affects the electronic structure, narrowing the gap by $\sim 38\%$, compared to calculations neglecting SOC. The predicted electronic structure actually explains the salient features of recent optical absorption measurements, further demonstrating the excellent agreement between the calculated ground state properties and experiment.

Yttrium iron garnet (${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$; YIG) has a unique combination of low magnetic damping, high spin-wave conductivity, and insulating properties that make it a highly attractive material for a variety of applications in the fields of magnetics and spintronics. While the room-temperature magnetization dynamics of YIG have been extensively studied, there are limited reports correlating the low-temperature magnetization dynamics to the material structure or growth method. Here we investigate liquid phase epitaxy grown YIG films and their magnetization dynamics at temperatures down to 10 K. We show there is a negligible increase in the ferromagnetic resonance linewidth down to 10 K, which is unique when compared with YIG films grown by other deposition methods. From the broadband ferromagnetic resonance measurements, polarized neutron reflectivity, and scanning transmission electron microscopy, we conclude that these liquid phase epitaxy grown films have negligible rare-earth impurities present, specifically the suppression of Gd diffusion from the ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG) substrate into the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ film, and therefore negligible magnetic losses attributed to the slow-relaxation mechanism. Overall, liquid phase epitaxy YIG films have a YIG/GGG interface that is five times sharper and have ten times lower ferromagnetic resonance linewidths below 50 K than comparable YIG films by other deposition methods. Thus, liquid phase epitaxy grown YIG films are ideal for low-temperature experiments/applications that require low magnetic losses, such as quantum transduction and manipulation via magnon coupling.

The magnetic properties of the quasicrystal approximant ${\mathrm{Au}}_{65}{\mathrm{Ga}}_{21}{\mathrm{Tb}}_{14}$ were investigated using magnetization and neutron diffraction experiments. The temperature dependences of the magnetic susceptibility and magnetization curve indicate dominant ferromagnetic interactions, whereas a whirling antiferromagnetic order was observed in neutron diffraction experiments. In the antiferromagnetic phase, the magnetic moments are aligned almost perpendicular to a pseudofivefold symmetry axis, which corresponds to the easy-axis direction of a Tb atom. Magnetic properties similar to those of ${\mathrm{Au}}_{72}{\mathrm{Al}}_{14}{\mathrm{Tb}}_{14}$ in spite of the substantial difference in the Au concentration suggest the robustness of the easy-axis anisotropy against the chemical environment.

We discovered superconductivity at $T_{\mathrm{c}}$ = 12 K in a layered compound ${\mathrm{La}}_2{\mathrm{IOs}}_2$ with an osmium honeycomb network. Despite heavy constituent elements unfavorable for a phonon mediated mechanism, $T_{\mathrm{c}}$ is the highest among lanthanoid iodides made of lighter elements such as ${\mathrm{La}}_2{\mathrm{IRu}}_2$ with $T_{\mathrm{c}}$ = 4.8 K. Electronic anomalies are observed below 60 K similar to those observed in ${\mathrm{La}}_2{\mathrm{IRu}}_2$ below 140 K. ${\mathrm{La}}_2{\mathrm{IOs}}_2$ is a layered $5d$ electron system providing a platform to investigate the interplay between the electronic anomaly, superconductivity, and strong magnetic field.