It has become widely accepted that particles with long-range hopping do not undergo Anderson localization. However, several recent studies demonstrated localization of particles with long-range hopping. In particular, it was recently shown that the effect of long-range hopping in one-dimensional (1D) lattices can be mitigated by cooperative shielding, which makes the system behave effectively as one with short-range hopping. Here, we show that cooperative shielding, demonstrated previously for 1D lattices, extends to 3D lattices with isotropic long-range $r^{-\alpha }$ hopping, but not to 3D cubic lattices with anisotropic long-range hopping. The specific anisotropy we consider corresponds to the interaction between dipoles aligned along one of the principal axes of the lattice. We demonstrate the presence of localization in 3D lattices with uniform ($\alpha =0$) isotropic long-range hopping and the absence of localization with uniform anisotropic long-range hopping by using the scaling behavior of eigenstate participation ratios. We use the scaling behavior of participation ratios and energy-level statistics to show that the existence of delocalized, extended nonergodic, or localized states in the presence of disorder depends on both the exponents $\alpha$ and the anisotropy of the long-range hopping amplitudes.

The isothermal magnetic entropy changes Δ$S$ (i.e., the magnetocaloric potential) are studied in ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/{\mathrm{Ni}}_{67}\mathrm{C}{\mathrm{u}}_{33}/{\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}/{\mathrm{Mn}}_{80}{\mathrm{Ir}}_{20}$ stacks at temperatures near the Curie point of the ${\mathrm{Ni}}_{67}\mathrm{C}{\mathrm{u}}_{33}$ spacer by applying magnetic fields of a few milli-Tesla. Such fields were sufficient for toggling magnetic moments in the soft ferromagnetic layer (${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$). It is found that this switching provides a significant enhancement of Δ$S$ in the heterostructure system with respect to that achieved in a single ${\mathrm{Ni}}_{67}\mathrm{C}{\mathrm{u}}_{33}$ film under such weak magnetic fields. Our finding is believed to have the potential to be utilized in magnetocaloric devices (e.g., thin-film coolers) that would be based on ferromagnetic/paramagnetic/ferromagnetic heterostructures and would operate with moderate magnetic fields.

The chemical and magnetic structures of the series of compounds ${\mathrm{Ca}}_{2-x}{\mathrm{La}}_x{\mathrm{RuO}}_4$ $[x=0$, 0.05(1), 0.07(1), 0.12(1)] have been investigated using neutron diffraction and resonant elastic x-ray scattering. Upon La doping, the low-temperature S-$Pbca$ space group of the parent compound is retained in all insulating samples $[x\le 0.07(1\left)\right]$, but with significant changes to the atomic positions within the unit cell. These changes can be characterized in terms of the local ${\mathrm{RuO}}_6$ octahedral coordination: with increasing doping, the structure, crudely speaking, evolves from an orthorhombic unit cell with compressed octahedra to a quasitetragonal unit cell with elongated ones. The magnetic structure on the other hand, is found to be robust, with the basic $\mathbf{k}=(0,0,0)$, $\mathbf{b}$-axis antiferromagnetic order of the parent compound preserved below the critical La doping concentration of $x\approx 0.11$. The only effects of La doping on the magnetic structure are to suppress the A-centred mode, favoring the B mode instead, and to reduce the Néel temperature somewhat. Our results are discussed with reference to previous experimental reports on the effects of cation substitution on the $d^4$ Mott insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$, as well as with regard to theoretical studies on the evolution of its electronic and magnetic structure. In particular, our results rule out the presence of a proposed ferromagnetic phase, and suggest that the structural effects associated with La substitution play an important role in the physics of the system.

The metamagnetic transition between the antiferromagnetic and paramagnetic state in UIrGe has been studied at various temperatures by magnetization, heat capacity, and magnetocaloric-effect measurements on a single crystal in static and pulsed magnetic fields applied along the orthorhombic $b$ axis. A first-order transition is observed at temperatures below 13 K and a second-order one at higher temperatures up to the Néel temperature $\left(T_{\mathrm{N}}=16.5\mathrm{K}\right)$. The first-order transition is accompanied by a dramatic increase of the Sommerfeld coefficient. Magnetization measurements extended to the paramagnetic range revealed an anomalous S shape (inflection point at a magnetic field $H_{\mathrm{m}})$ in magnetization isotherms at temperatures above 13 K and a temperature dependence of susceptibility with a maximum at $T_{\max }$ well above $T_{\mathrm{N}}$. The lines representing the temperature-induced evolution of $H_{\mathrm{m}}$ and field-induced evolution of $T_{\max }$, respectively, are bound for the point in the magnetic phase diagram at which the order of metamagnetic transition changes. A tentative scenario explaining these anomalies by antiferromagnetic correlations or short-range order in the paramagnetic state is discussed.

While it was speculated that $5d^4$ systems would possess nonmagnetic $J=0$ ground state due to strong spin-orbit coupling (SOC), all such systems have invariably shown the presence of magnetic moments so far. A puzzling case is that of ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$, which in spite of having a perfectly cubic structure with largely separated ${\mathrm{Ir}}^{5+}\left(d^4\right)$ ions, has consistently hosted weak magnetic moments. We show from muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements that a change in the magnetic environment of the implanted muons in ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ occurs as the temperature is lowered below 10 K indicating magnetic response has a temperature dependence unlikely for a $J=0$ system. Interestingly, the estimated value of SOC obtained by fitting the resonant inelastic x-ray scattering (RIXS) spectrum of ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ with an effective atomic many-body Hamiltonian is found to be as high as 0.39 eV, comparable to the reported value of SOC in other magnetic $d^4$ iridates with noncubic distortion. We argue that in addition to strong SOC, the presence of intersite hopping triggers delocalization of holes, resulting in the spontaneous generation of magnetic moments. Our theoretical calculations further indicate that these moments favor the formation of spin-orbital singlets in the case of ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$, which is manifested in $\mu \mathrm{SR}$ experiments measured down to 60 mK.

We introduce the general formalism to describe spin torques induced by the supercurrents injected from the adjacent superconducting electrodes into the spin-textured ferromagnets. By considering the adiabatic limit for the equal-spin superconducting correlations in the ferromagnet, we show that the supercurrent can generate both the fieldlike spin-transfer torque and the spin-orbital torque. These dissipationless spin torques are expressed through the current-induced corrections to the effective field derived from the system energy. The general formalism is applied to show that the supercurrent can either shift or move the magnetic domain walls depending on their structure and the type of spin-orbital interaction in the system. These results can be used for the prediction and interpretation of the experiments studying magnetic texture dynamics in superconductor/ferromagnet/superconductor Josephson junctions and other hybrid structures.

An atomistic effective Hamiltonian technique is employed to investigate the polarization switching mechanism of the (metastable but long-living) ferroelectric state of ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$ relaxor ferroelectric, resulting from the application of a dc electric field along the $\left[\overline{1}\overline{1}\overline{1}\right]$ direction—that is, opposite to the initial polarization. It is predicted that such switching is of inhomogeneous type. It involves the creation of intermediate short-range ordered, relaxor-like phases in between long-range ordered states inside which an infinite cluster exists and has dipoles near either the initial polarization direction (for shorter times) or the field's direction (for longer times). In contrast, dipoles belonging to finite clusters or being isolated can deviate away from [111] and $\left[\overline{1}\overline{1}\overline{1}\right]$, and, in fact, rotate in average from [111] to $\left[\overline{1}\overline{1}\overline{1}\right]$ when time increases. Such rotations govern the reversal of the polarization from [111] to $\left[\overline{1}\overline{1}\overline{1}\right]$ occurring within the intermediate relaxor-like states' region, while always resulting in the overall cancellation of any Cartesian component of the polarization that is perpendicular to [111]. These rotations occurring at the atomic scale also naturally imply that some fundamental assumptions of the original nucleation-limited-switching (NLS) model are not valid, despite the fact that we numerically further find that the whole temporal behavior of the macroscopic polarization can be well fitted by the general formula associated with NLS. In other words, we have stumbled into a novel switching or at least a switching that should be denoted as a generalized NLS model. Finally, three different electric-field regimes are predicted, with each of them having its own dependency of the switching time on the magnitude of the applied electric field and only one of them obeying the Merz's law. The existence of these three regimes is explained in terms of specific microscopic features.

The self-consistent harmonic approximation (SCHA) allows the computation of free energy of anharmonic crystals considering both quantum and thermal fluctuations. Recently, a stochastic implementation of the SCHA has been developed, tailored for applications that use total energy and forces computed from first principles. In this paper, we extend the applicability of the stochastic SCHA to complex crystals, i.e., systems in which symmetries do not fix the inner coordinates and require the optimization of both the lattice vectors and the atomic positions. To this goal, we provide an expression for the evaluation of the pressure and stress tensor within the stochastic SCHA formalism. Moreover, we develop a more robust free-energy minimization algorithm, which allows us to perform the SCHA variational minimization very efficiently in systems having a broad spectrum of phonon frequencies and many degrees of freedom. We test and illustrate the approach with an application to the phase XI of water ice using density-functional theory. We find that the SCHA reproduces extremely well the experimental thermal expansion of ice in the whole temperature range between 0 and $270\mathrm{K}$, in contrast with the results obtained within the quasiharmonic approximation, that underestimates the effect by about 25%.

We present results of measurements of resistivity of ${\mathrm{CeAuSb}}_2$ under the combination of $c$-axis magnetic field and in-plane uniaxial stress. In unstressed ${\mathrm{CeAuSb}}_2$ there are two magnetic phases. The low-field A phase is a single-component spin-density wave (SDW), with $\mathbf{q}=(\eta ,\pm \eta ,1/2)$, and the high-field B phase consists of microscopically coexisting $(\eta ,\eta ,1/2)$ and $(\eta ,-\eta ,1/2)$ spin-density waves. Pressure along a $\langle 100\rangle$ lattice direction is a transverse field to both of these phases and so initially has little effect, however it eventually induces new low- and high-field phases in which the principal axes of the SDW components appear to have rotated to the $\langle 100\rangle$ directions. Under this strong $\langle 100\rangle$ compression, the field evolution of the resistivity is much smoother than at zero strain: In zero strain, there is a strong first-order transition, while under strong $\langle 100\rangle$ it becomes much broader. We hypothesize that this is a consequence of the uniaxial stress lifting the degeneracy between the (100) and (010) directions.

An external electric field can modify the strength of the spin-orbit interaction between spins of ions in magnetic crystals. This influence leads to a spin-wave frequency shift that is linear in both the applied electric field and the wave vector of the spin wave. Here we study theoretically the external electric field as a means of control of the spin-wave power flow in thin ferromagnets. The spin-wave group velocity and focusing patterns are obtained from the slowness (isofrequency) curves by evaluating their curvature at each point of the reciprocal space. We show that the combination of the magnetodipole interaction and the electric field can result in nonreciprocal unidirectional caustic beams of dipole-exchange spin waves. We demonstrate that the degree of asymmetry of the spin-wave power flow can be tuned with the external electric field. Our findings open a novel avenue for spin-wave manipulation and development of electrically tunable magnonic devices.

The upper critical field for the field along the b axis of the orthorhombic ferromagnetic superconductor UCoGe has a particular $S$ shape, akin to the reentrant superconducting phase of URhGe. In order to explore the evolution of the superconducting phase under this transverse magnetic field, we report the thermal conductivity and resistivity measurements, revealing a possible field-induced vortex liquid phase, and supporting a field-induced change of the superconducting order parameter.

We analyze a two-dimensional Kondo lattice model with special emphasis on non-Hermitian properties of the single-particle spectrum, following a recent proposal by Kozii and Fu. Our analysis based on the dynamical mean-field theory elucidates that the single-particle spectral weight shows the exceptional points (EPs). Correspondingly, the spectral weight exhibits the band touching, resulting in a structure similar to the Fermi arc. Furthermore, we find an intriguing phenomenon arising from the periodicity of the lattice. The EPs generated by two distinct Dirac points merge and change into a hybrid point, which vanishes as the exchange interaction is increased. Accordingly, the paramagnetic phase in the low-temperature region shows a significant difference from noninteracting fermions: the imaginary part of the self-energy yields the Fermi loop without any defective points.

We elucidate the reduction of the winding number caused by the on-site disorder in a next-nearest-neighbor XY model. When disorder becomes strong enough, Majorana edge modes become critically extended, beyond which they collapse into Anderson localized (AL) states in the bulk, resulting in a topological Anderson insulating state. We identify a resilience threshold $W_t$ for every pair of Majorana fermions (MFs). In response to increasing disorder, every pair of MFs collapse into AL states in the bulk beyond their resilience threshold. For very strong disorder, all Majorana fermions collapse, and a topologically trivial state is obtained. We show that the threshold values are related to the localization length of Majorana fermions, which can be efficiently calculated by an appropriate modification of the transfer-matrix method. At the topological transition point, the localization length of the zero modes diverges, and the system becomes scale invariant. The number of peaks in the localization length as a function of disorder strength determines the number of zero modes in the clean state before disorder is introduced. This finding elevates the transfer-matrix method to the level of a tool for the determination of the topological index of both clean and disordered systems.

Using a semi-ab-initio theoretical method we examine the temperature-dependent linewidth of the Raman modes in bulk and monolayer transition-metal dichalcogenides (TMDs) ${\mathrm{MoS}}_2, {\mathrm{WS}}_2$, and ${\mathrm{MoTe}}_2$. It is found that different Raman modes show different linewidths and different temperature dependencies with respect to each other for a given sample and across different TMDs. We explain these characteristics as arising from a combination of phonon density of states, Raman mode frequency, and the relative contributions of temperature-independent mass-defect scattering and temperature-dependent intrinsic anharmonic interactions. Reported measurements for samples prepared under experimental conditions have been explained by adding frequency-dependent inhomogeneity-related background contribution to our theoretical results for the pure and homogeneous samples.

We design a driven superconducting box with four spins $S=1/2$ (qubits) such that coupled devices can give insight on the occurrence of quantum spin liquids and many-body Majorana states. Within one box or island, we introduce a generalized nuclear magnetic resonance algorithm to realize our models and study numerically the spin observables in time as well as the emergent gauge fields. We discuss the stability of the box towards various detuning effects and we include dissipation effects through a Lindblad master equation. Coupling boxes allows us to realize quantum spin-liquid phases of Kitaev ${\mathcal{Z}}_2$ spin models in various geometries with applications in the toric code. Quantum phase transitions and Majorana physics might be detected by measuring local susceptibilities. We show how to produce a Néel state of fluxes by coupling boxes and we address the role of local impurity fluxes leading to random Ising models. We also present an implementation of the Sachdev-Ye-Kitaev Majorana model in coupled ladder systems.

This work reports the measurement of electron $g$-factor anisotropy $\left(\left|\mathrm{\Delta }g\right|=|g_{001}-g_{1\overline{1}0}|\right)$ for phosphorous donor qubits in strained silicon (sSi = Si/${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x)$ environments. Multimillion-atom tight-binding simulations are performed to understand the measured decrease in $\left|\mathrm{\Delta }g\right|$ as a function of $x$, which is attributed to a reduction in the interface-related anisotropy. For $x<7\%$, the variation in $\left|\mathrm{\Delta }g\right|$ is linear and can be described by ${\eta }_{x}x$, where ${\eta }_x\approx 1.62\times {10}^{-3}$. At $x=20\%$, the measured $\left|\mathrm{\Delta }g\right|$ is $1.2\pm 0.04\times {10}^{-3}$, which is in good agreement with the computed value of $1\times {10}^{-3}$. When strain and electric fields are applied simultaneously, the strain effect is predicted to play a dominant role on $\left|\mathrm{\Delta }g\right|$. Our results provide useful insights on the spin properties of sSi:P for spin qubits, and more generally for devices in spintronics and valleytronics areas of research.

Multiband transport in superconductors is interesting both from an academic as well as an application point of view. It has been postulated that interband scattering can significantly affect the carrier dynamics in these materials. In this article we present a detailed study of the electrical transport properties of the high-mobility two-dimensional electron gas residing at the interface of ${\mathrm{LaAlO}}_3\text{/}{\mathrm{SrTiO}}_3$, a prototypical multiband superconductor. We show, through careful measurements of the gate dependence of the magnetoresistance and resistance fluctuations at ultralow temperatures, that transport in the superconducting regime of this system has contributions from two bands which host carriers of very different characters. We identify a gate-voltage tunable Lifshitz transition in the system and show that the resistance fluctuations have strikingly different features on either side of it. At low carrier densities, resistance noise is dominated by number-density fluctuations arising from trapping-detrapping of charge carriers from defects in the underlying ${\mathrm{SrTiO}}_3$ substrate, characteristic of a single-band semiconductor. Above the Lifshitz transition, the noise presumably originates from interband scattering. Our paper highlights the importance of interband scattering processes in determining the transport properties of low-dimensional systems and projects resistance fluctuation spectroscopy as a viable technique for probing the charge carrier dynamics across a Lifshitz transition.

A simple, unconventional, nonempirical, constraint-based orbital-free generalized gradient approximation (GGA) noninteracting kinetic energy density functional is presented along with illustrative applications. The innovation is adaptation of constraint-based construction to the essential properties of pseudodensities from the pseudopotentials that are essential in plane-wave-basis ab initio molecular dynamics. This contrasts with constraining to the qualitatively different Kato-cusp-condition densities. The single parameter in the proposed functional is calibrated by satisfying Pauli potential positivity constraints for pseudoatom densities. In static lattice tests on simple metals and semiconductors, the LKT (for the authors' initials) functional outperforms the previous best constraint-based GGA functional, VT84F [Phys. Rev. B 88, 161108(R) (2013)], is generally superior to a recently proposed meta-GGA, is reasonably competitive with parametrized two-point functionals, and is substantially faster.

The unusual Raman spectrum of ${\mathrm{MgB}}_2$ and its formidable temperature dependence are successfully reproduced by means of a parameter-free ab initio nonadiabatic theory that accounts for the electron-hole pair scattering mechanisms with the system phonons. This example turns out to be a prototypical case where a strong nonadiabatic renormalization of the phonon frequency is partially washed out by the aforementioned scattering events, bringing along a characteristic temperature dependence. Both electron-hole pair lifetime and energy renormalization effects due to dynamical electron-phonon coupling turn out to play a crucial role. This theory could aid in comprehending other Raman spectra characterized with unconventionally strong electron-phonon interactions.