We introduce coherent inelastic neutron scattering with isotope substitution as a technique for measuring site-specific information on the vibrational dynamics of amorphous solids. The technique is used to extract experimentally the partial vibrational density of states $G_{\alpha }\left(E\right)$ for both Ge and Se in the prototypical network-forming glass ${\mathrm{GeSe}}_2$, where $\alpha$ specifies the chemical species and $E$ denotes the energy. The efficacy of the approximations used in interpreting the experimental data is validated by using a first-principles model, for which the set of true partial vibrational density of states is directly accessible. Our approach offers an opportunity for exploring accurately the vibrational dynamics of disordered materials.

We use two different fully vectorial microscopic models featuring nonresonant and resonant scattering, respectively, to demonstrate the Anderson localization transition for elastic waves in three-dimensional (3D) disordered solids. Critical parameters of the transition determined by finite-time and finite-size scaling analyses suggest that the transition belongs to the 3D orthogonal universality class. Similarities and differences between the elastic-wave and light scattering in strongly disordered media are discussed.

We establish a phase diagram of a model in which scalar waves are scattered by resonant point scatterers pinned at random positions in the free three-dimensional space. A transition to Anderson localization takes place in a narrow frequency band near the resonance frequency provided that the number density of scatterers $\rho$ exceeds a critical value ${\rho }_c\simeq 0.08k_0^3$, where $k_0$ is the wave number in the free space. The localization condition $\rho >{\rho }_c$ can be rewritten as $k_0{\ell }_0<1$, where ${\ell }_0$ is the on-resonance mean free path in the independent-scattering approximation. At mobility edges, the decay of the average amplitude of a monochromatic plane wave is not purely exponential and the growth of its phase is nonlinear with the propagation distance. This makes it impossible to define the mean free path $\ell$ and the effective wave number $k$ in a usual way. If these last are defined as an effective decay length of the intensity and an effective growth rate of the phase of the average wave field, the Ioffe-Regel parameter ${\left(k\ell \right)}_c$ at the mobility edges can be calculated and takes values from 0.3 to 1.2 depending on $\rho$. Thus, the Ioffe-Regel criterion of localization $k\ell <{\left(k\ell \right)}_c=\mathrm{const}.\sim 1$ is valid only qualitatively and cannot be used as a quantitative condition of Anderson localization in three dimensions.

In this paper, we detect the coupled oscillation of magnetic vortices in ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ gourd-shaped thin films electrically to evaluate the nonadiabatic spin-transfer torque (STT) coefficient $\beta$, which is important for understanding a variety of STT-related phenomena. We excite a translational mode in coupled magnetic vortices by applying an alternating electrical current that causes both STT and Oersted field torque. Since the coupled oscillation modes due to the STT and Oersted field torque have totally different eigenfrequencies, the coupled oscillation spectrum can be used to evaluate $\beta$, which is generally difficult to distinguish from the Oersted field contributions. Based on the ratio between the amplitudes of the spectrum's Lorentzian and anti-Lorentzian components, the value of $\beta$ is found to be $0.085\pm 0.016$, which is 4.3 times larger than the Gilbert damping constant $\alpha$.

We have investigated the electronic structure of monolayer Cr deposited on Ag(001), using angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations. It is favorable for the Cr layer to be buried one layer deep, resulting in a 1 Ag/1 Cr/Ag(001) sandwich structure with the moments on the Cr atoms aligned antiferromagnetically in a $c(2\times 2)$ arrangement. Both experiment and theory agree well on the measured/calculated band dispersions and their intensities. Two relatively flat bands arise from antiferromagnetic Cr $3d$ states; in addition, quantum well states with strong curvatures are prominent near both the $\overline{\mathrm{X}}$ and $\overline{\mathrm{M}}$ points of the surface Brillouin zone. Our results suggest that the buried Cr monolayer in Ag(001) could serve as a prototypical system to study two-dimensional antiferromagnetism in the presence of weak electronic correlation effects.

Electro-optical response of a current-carrying monolayer graphene is studied theoretically. Our calculation takes into account full (diagonal and nondiagonal) conductivity tensor obtained from a particle-conserving out-of-equilibrium distribution function of doped graphene. Our analytical and numerical results indicate that the presence of a moderate dc current throughout a doped graphene channel induces large Kerr rotations within a frequency range which can be tuned up to the midinfrared frequency range.

We consider normal-superconductor (NS) and superconductor-normal-superconductor (SNS) junctions based on one-dimensional nanowires with Rashba spin-orbit coupling and proximity-induced $s$-wave spin-singlet superconductivity and analytically demonstrate how both even- and odd-frequency and spin-singlet and -triplet superconducting pair correlations are always present. In particular, by using a fully quantum mechanical scattering approach, we show that Andreev reflection induces mixing of spatial parities at interfaces, thus being the unique process which generates odd-frequency pairing; on the other hand, both Andreev and normal reflections contribute to even-frequency pairing. We further find that locally neither odd-frequency nor spin-triplet correlations are induced, but only even-frequency spin-singlet pairing. In the superconducting regions of NS junctions, the interface-induced amplitudes decay into the bulk, with the odd-frequency components being generally much larger than the even-frequency components at low frequencies. The odd-frequency pairing also develops short- and long-period oscillations due to the chemical potential and spin-orbit coupling, respectively, leading to a visible beating feature in their magnitudes. Moreover, we find that in short SNS junctions at $\pi$-phase difference and strong spin-orbit coupling, the odd-frequency spin-singlet and -triplet correlations strongly dominate with an alternating spatial pattern for a large range of subgap frequencies.

We study thermodynamic manifestations of the chiral anomaly in disordered Weyl semimetals. We focus, in particular, on the effect which we call “adiabatic dechiralization,” the phenomenon in which a change in temperature and/or an absorption or release of heat results from applying parallel electric and magnetic fields that change the imbalance of quasiparticles with different chiralities (at different Weyl nodes). This effect is similar to that of adiabatic demagnetization, which is commonly used as a method of low-temperature refrigeration. We describe this phenomenon quantitatively and discuss experimental conditions favorable for its observation. A related phenomenon, which we analyze and which is readily observable in experiments, is the dependency of the heat capacity of a Weyl semimetal on parallel electric and magnetic fields.

Direct-gap materials hold promises for excitonic insulators. In contrast to indirect-gap materials, here the difficulty to distinguish from a Peierls charge density wave is circumvented. However, direct-gap materials still suffer from a divergence of polarizability when the band gap approaches zero, leading to a diminishing exciton binding energy. We propose that one can decouple the exciton binding energy from the band gap in materials where band-edge states have the same parity. First-principles calculations of two-dimensional GaAs and experimentally mechanically exfoliated single-layer ${\mathrm{TiS}}_3$ lend solid support to this principle.

Large nonsaturating magnetoresistance has been observed in various materials and electron-hole compensation has been regarded as one of the main mechanisms. Here we present a detailed study of the angle-dependent Shubnikov–de Haas effect on large magnetoresistance material pyrite-type ${\mathrm{PtBi}}_2$, which allows us to experimentally reconstruct its Fermi-surface structure and extract the physical properties of each pocket. We find its Fermi surface contains four types of pockets in the Brillouin zone: three ellipsoidlike hole pockets $\alpha$ with $C_4$ symmetry located on the edges ($M$ points), one intricate electron pocket $\beta$ merged from four ellipsoids along [111] located on the corners ($R$ points), and two smooth and cambered octahedrons $\gamma$ (electron) and $\delta$ (hole) on the center ($\mathrm{\Gamma }$ point). The deduced carrier densities of electrons and holes from the volume of pockets prove carrier compensation. This compensation at low temperatures is also supported by fitting the field dependence of Hall and magnetoresistance at different temperatures. We conclude that the compensation is the main mechanism for the large nonsaturating magnetoresistance in pyrite-type ${\mathrm{PtBi}}_2$. We found the hole pockets α may contribute major mobility because of their light masses and anisotropy to relatively avoid large-angle scattering at low temperature. This may be a common feature of semimetals with large magnetoresistance. The found subquadratic magnetoresistance in high field is probably due to field-dependent mobilities, another feature of semimetals under high magnetic fields.

We present density functional embedding for diffusion Monte Carlo calculations (DMC). Using two test systems, an H chain and a Be slab, we demonstrate the feasibility and show that the approach can give high-quality results. DMC embedded in potentials from density functional embedding theory thus presents an alternative to conventional embedded quantum chemical methods, such as embedded complete active space calculations. This is especially true for cases in which the cluster size becomes too large for accurate wave-function theory based methods or for cases in which basis-set superposition errors become dominant. Furthermore, we show that embedded DMC can easily be used for systems in which the cluster of interest is a one-dimensional or two-dimensional periodic object and cannot be described as isolated impurity. This extends the applicability of the embedding approach to new fields.

We present a theoretical study of spectral, magnetic, and structural properties of the manganese selenide under pressure within the DFT+DMFT method combining density functional theory with dynamical mean-field theory. Our results reveal that a high-spin to low-spin transition occurs upon compression at volumes below 33 ${\AA }^3$. The spin-state transition is accompanied by a metal-insulator transition and a structural phase transition with the large volume collapse of $\sim 20\%$ from the cubic B1 structure to the high-pressure MnP-type B31 structure at about 27 ${\AA }^3$. We find that the spin-state transition is the main driving force for the structural transition.

In this work, we study the interlayer interactions between sheets of blue phosphorus with quantum Monte Carlo (QMC) methods. We find that as previously observed in black phosphorus, interlayer binding of blue phosphorus cannot be described by van der Waals (vdW) interactions alone within the density functional theory framework. Specifically, while some vdW density functionals produced reasonable binding curves, none of them could provide a correct, even qualitatively, description of charge redistribution due to interlayer binding. We also show that small systematic errors in common practice QMC calculations, such as the choice of optimized geometry and finite-size corrections, are non-negligible given the energy and length scales of this problem. We mitigate some of the major sources of error and report QMC-optimized lattice constant, stacking, and interlayer binding energy for blue phosphorus. It is strongly suggested that these considerations are important and quite general in the modeling of two-dimensional phosphorus allotropes.

Defect formation energies and transition levels are critical in determining doping behavior and recombination in semiconductor applications. Hybrid functionals are often used to overcome the band gap and delocalization errors of standard density-functional theory, and it is tempting to presume that the defect properties are correctly predicted once the hybrid functional mixing parameter reproduces the experimental band gap. However, pronounced spin-orbit coupling (SOC) effects can have an additional important role, which is clearly shown in this work by analyzing SOC effects originating from the $\mathrm{Te}-p$ orbitals in CdTe. In this work, we therefore use a hybrid functional that reproduces the experimental band gap when SOC is included, requiring a larger mixing parameter $\alpha =0.33$ compared to the conventional choice of $\alpha =0.25$. This hybrid functional was then used to predict defect properties, e.g., formation energy, transition level, and defect equilibrium. For defect states that do not directly involve the $\mathrm{Te}-p$ orbitals, such as the Cd interstitial (${\mathrm{Cd}}_{\mathrm{i}}$), we find that the effect of SOC on the defect levels can be captured by simply considering the SOC-induced band-edge shift. This is not the case for the A center ($\mathrm{C}{\mathrm{l}}_{\mathrm{Te}}-V_{\mathrm{Cd}}$ defect pair), where the localized acceptor state formed by $\mathrm{Te}-p$ orbitals is more directly affected by the SOC. For this defect, a mixing parameter as large as $\alpha =0.40$ is required to reproduce the experimental acceptor level. Regarding the implications for photovoltaics, we suggest that the ${\mathrm{Cd}}_{\mathrm{i}}$, which is the dominant compensating donor, could play an important role as a recombination center. While $\mathrm{C}{\mathrm{d}}_{\mathrm{i}}$ is usually thought of as a benign shallow donor, our predicted defect levels in the fully band-gap-corrected calculations are deep enough to raise a concern, and we propose a recombination mechanism for electron capture by $\mathrm{C}{\mathrm{d}}_{\mathrm{i}}$.

This paper studies the reflection and transmission properties of longitudinal waves incident on one-dimensional, finite-length time-space modulated beam. The Young's modulus of the so-called time-space modulated beam is modulated in both time and space in the form of a traveling wave. The objectives of this paper are (I) to provide more insights into the nonreciprocal transmission through modulated media, (II) to study the influence of the modulation wave on the nonreciprocal transmission, and (III) to quantify the one-way energy isolation effect. Our results show that the strong nonreciprocal transmission observed within the stop bands of the two fundamental Bloch modes of the modulated beam is caused by the fact that, within these frequency bands, incident waves from opposite directions will excite different Bloch modes in the modulated beam. At frequencies where strong reflection is observed, the incident waves excite not only the zero-order evanescent mode but also some higher-order propagative modes in the modulated beam, leading to unavoidable wave transmission when isolation is what we desired. This undesired transmission becomes more significant as the velocity of modulation wave increases when the waves are incident on the modulated beam in the same direction as the modulation wave. The nonreciprocal property of modulated media may be explored to realize one-way energy isolation. Energy balance resulting from the reflection and transmission is studied to quantify this application.

The double-perovskite compounds ${\mathrm{Sr}}_2{\mathrm{CuTeO}}_6$ and ${\mathrm{Sr}}_2{\mathrm{CuWO}}_6$ are magnetically described as quasi-two-dimensional spin-$\frac{1}{2}$ square-lattice $J_1\text{−}{J}_2$ Heisenberg antiferromagnets with predominant $J_1$ and $J_2$ exchange interactions, respectively. We report the low-temperature magnetic properties of ${\mathrm{Sr}}_2{\mathrm{CuTe}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_6$ with randomness in the magnitudes of $J_1$ and $J_2$. It was found that the low-temperature specific heat for $0.1\le x\le 0.5$ has a large component proportional to the temperature $T$ above 1.2 K, although the low-temperature specific heat for the two parent systems is approximately proportional to $T^3$. With decreasing temperature below 1.2 K, the $T$-linear component decreases rapidly toward zero, which is insensitive to the magnetic field up to 9 T. This is suggestive of the singlet excitation decoupled from the magnetic field. The NMR spectrum for $x=0.2$ exhibits no long-range order down to 1.8 K. These results indicate that the ground state of ${\mathrm{Sr}}_2{\mathrm{CuTe}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_6$ is a valence-bond-glass state with singlet gaps.

We report structural, magnetic, magnetodielectric, and specific heat properties of hexagonal polymorph of ${\mathrm{Ce}}_2{\mathrm{O}}_3$. The $A$-type hexagonal ${\mathrm{Ce}}_2{\mathrm{O}}_3$ crystallizes in the $P\overline{3}2/m1$ space group and shows antiferromagnetic ordering at $T_{\mathrm{N}}\approx 6.2$ K as detected by magnetic susceptibility measurements. The magnetic ordering is accompanied by a $\lambda$-shape specific heat anomaly with a peak maximum at $T\approx 6.1$ K. An isotropic dielectric constant demonstrates a large magnetocapacitance effect at $T=T_{\mathrm{N}}$, which saturates at $\sim 80$ kOe. In the magnetically ordered phase the relaxation rate of the dielectric polarization increases with the magnetic field and shows divergence as the temperature approaches $T_{\mathrm{N}}$. The magnetic calculations indicate that ${\mathrm{Ce}}_2{\mathrm{O}}_3$ undergoes “easy plane” AFM ordering, which is also supported by the neutron diffraction data.

The superfluid to insulator quantum phase transition of a three-dimensional particle-hole symmetric system of disordered bosons is studied. To this end, a site-diluted quantum rotor Hamiltonian is mapped onto a classical (3+1)-dimensional $XY$ model with columnar disorder and analyzed by means of large-scale Monte Carlo simulations. The superfluid-Mott insulator transition of the clean, undiluted system is in the four-dimensional $XY$ universality class and shows mean-field critical behavior with logarithmic corrections. The clean correlation length exponent $\nu =1/2$ violates the Harris criterion, indicating that disorder must be a relevant perturbation. For nonzero dilutions below the lattice percolation threshold of $p_c=0.688392$, our simulations yield conventional power-law critical behavior with dilution-independent critical exponents $z=1.67\left(6\right)$, $\nu =0.90\left(5\right)$, $\beta /\nu =1.09\left(3\right)$, and $\gamma /\nu =2.50\left(3\right)$. The critical behavior of the transition across the lattice percolation threshold is controlled by the classical percolation exponents. Our results are discussed in the context of a classification of disordered quantum phase transitions, as well as experiments in superfluids, superconductors, and magnetic systems.

The effective attractive interaction between electrons, mediated by electron-phonon coupling, is a well-established mechanism of conventional superconductivity. In metals exhibiting a Fermi surface, the critical temperature of superconductivity is exponentially smaller than the characteristic phonon energy. Therefore, such superconductors are found only at temperatures below a few kelvin. Systems with flat energy bands have been suggested to cure the problem and provide a route to room-temperature superconductivity, but previous studies are limited to only BCS models with an effective attractive interaction. Here we generalize Eliashberg's theory of strong-coupling superconductivity to systems with flat bands and relate the mean-field critical temperature to the microscopic parameters describing electron-phonon and electron-electron interaction. We also analyze the strong-coupling corrections to the BCS results and construct the phase diagram exhibiting superconductivity and magnetic phases on an equal footing. Our results are especially relevant for novel quantum materials where electronic dispersion and interaction strength are controllable.