We study the features of the superconductivity nucleation and vortex configurations in superconductors with modulated disorder. Using the Ginzburg-Landau–type theory with spatially varying diffusion coefficient, we uncover and explain the switching between the vortex-defect attraction to the repulsion upon the increase in the external magnetic field. It is shown that for rather weak applied magnetic fields, a superconducting nucleus localized near the region with the suppressed diffusion coefficient possesses a nonzero vorticity whereas the increase in the magnetic field can lead to a transition into the state with zero winding number. We demonstrate the manifestations of this switching phenomenon in superconductors with a large number of defects by performing numerical simulations of the vortex structures in superconductors with periodic spatial profiles of the diffusion coefficient. The obtained results clarify the physics of the vortex arrangement in several classes of the superconducting materials including one-dimensional superlattices and nanopatterned superconductors with regular arrays of the defects characterized by the increased concentration of nonmagnetic impurities. Based on analytical estimates within the framework of the Ginzburg-Landau theory, we show that the predicted switching in the pinning mechanism can manifest itself in the change of the slope of the critical current as a function of the external magnetic field.

Sodium, a textbook example of a nearly-free-electron metal, exhibits unforeseen pressure-induced behavior including the stabilization of numerous polymorphs—some possessing extremely complex unit cells—as well as a metal-to-insulator transition to the iconic $hP4$ phase. However, until recently, most of the experimental and theoretical studies on solid sodium have been restricted to the low-temperature regime. Herein, using ab initio evolutionary structure searches coupled with quasiharmonic calculations, we discover seven new phases of sodium that are more stable than the known $hP4$ phase at pressure-temperature conditions that were recently attained in ramp-compression experiments. From these, our calculations suggest that $\mathit{P}{6}_3/\mathit{m}$ Na is the ground state between $\approx 250$ GPa at 710 K and $\approx 350$ GPa at 1900 K. Electronic structure calculations show that this phase is a topological semimetal with a Dirac nodal surface that is protected by a nonsymmorphic symmetry $S_{2z}$ and an electride owing to its non-nuclear charge localized within one-dimensional honeycomb channels and zero-dimensional cages. Our results highlight the complexity of dense sodium's free-energy landscape and intricate electronic structure that emerges at finite temperatures and conditions where ionic cores overlap.

Magnon eigenmodes in easy-plane antiferromagnetic insulators are linearly polarized and are not expected to carry any net spin angular momentum. Motivated by recent nonlocal spin transport experiments in the easy-plane phase of hematite, we perform a series of micromagnetic simulations in a nonlocal geometry at finite temperatures. We show that by tuning an external magnetic field, we can control the magnon eigenmodes and the polarization of the spin transport signal in these systems. We argue that a coherent beating oscillation between two orthogonal linearly polarized magnon eigenmodes is the mechanism responsible for finite spin transport in easy-plane antiferromagnetic insulators. The sign of the detected spin signal is also naturally explained by the proposed coherent beating mechanism. Our finding opens a path for on-demand control of the spin signal in a large class of easy-plane antiferromagnetic insulators.

We performed inelastic neutron scattering (INS) experiments to measure spin dynamics in a polycrystalline sample of the spin tube candidate ${\mathrm{CsCrF}}_4$. The compound exhibits a successive phase transition from a paramagnetic phase through an intermediate-temperature (IT) phase of a ${120}^{\circ }$ structure to a low-temperature (LT) phase of another ${120}^{\circ }$ structure. An elaborate comparison of observed and calculated neutron spectra in the LT phase reveals that the spin Hamiltonian is identified as antiferromagnetic spin tubes including perturbative terms of intertube interaction, Dzyaloshinskii-Moriya interaction, and single-ion anisotropy. A phase diagram for the ground state is classically calculated. A set of parameters in the spin Hamiltonian obtained from the INS spectra measured in the LT phase is quite close to the boundary of the phase of the ${120}^{\circ }$ structure of the IT phase. The INS spectra measured in the IT phase are, surprisingly, the same as those in the LT phase on the level of powder-averaged spectra, even though the magnetic structures in the IT and LT phases are different. Identical dynamical structures compatible with two different static structures are observed. No difference in the observed spectra indicates no change in the spin Hamiltonian with the temperature, suggesting that the origin of the successive phase transition is a spin-entropy-driven mechanism in the spin system located near the phase boundary.

The observation of time dependent magnetic order in the spin-chain compound ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, a promising candidate for showing out-of-equilibrium magnetization behavior, has received tremendous attention for over a decade. The present study uses resonant magnetic scattering close to the cobalt K-absorption edge to probe directly the time, temperature, and magnetic field dependence of the magnetic order in a ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ single crystal. Energy dependences of the magnetic signal and their simulation suggest that the resonant x-ray scattering (RXS) signal at the pre-edge contains contributions only from quadrupole (E2E2) and dipolar-quadrupolar (E1E2) transition processes. Interestingly, the flat intensity variation as a function of azimuthal angle over a wide angular range cannot be explained by considering the ordering of magnetic dipole moments alone. The observed magnetic order below the Néel temperature $T_N$ in the present RXS study differs significantly from the reported neutron diffraction results, indicating possible contributions from high-order moments in addition to the magnetic dipole moment.

The magnetic behavior of two-dimensional (2D) periodic ensembles of dipole-coupled nanoparticles (NPs) is investigated theoretically by considering all possible structural organizations in 2D Bravais lattices, namely, square, triangular, rectangular, rhombic, and oblique lattices. The different interaction-energy landscapes (ELs) are characterized by determining the local minima that define the stable and metastable magnetic configurations as well as the transition states connecting them. The topology of the resulting ergodic ensemble of stationary states is analyzed from both local and energy perspectives by calculating the corresponding kinetic networks and disconnectivity graphs. For all lattices, the magnetic orders of the ground-state and low-lying metastable configurations are identified, including the elementary relaxation processes between them. A remarkable profound dependence of the collective magnetic behavior on the structural arrangement of the NPs is revealed. Square and triangular nanostructures are extremely good structure seekers, showing starlike kinetic networks and disconnectivity graphs with palm-tree form. Their continuously degenerate ground states are throughout-reaching hubs connected to nearly all excited magnetic configurations over a single first-order saddle point. Rhombic ensembles have double-funnel ELs in which the time-inversion-related ground states play the role of hubs with exceedingly high connectivity densities. These systems need to undergo very few elementary transitions with small downward energy barriers to relax from any configuration towards one of the ground states. However, the energy barriers between the two funnels are quite large and, furthermore, they increase as the number of particles in the unit cell is increased. Therefore, ergodicity breaking is expected in the thermodynamic limit. Finally, the rectangular and oblique lattices show contrasting ground-state orders, the former consists of an antiferromagnetic alternation of head-to-tail chains of spins while the latter is ferromagnetic. Nevertheless, the ELs of these nanostructures are qualitatively very similar. In both cases, all excited magnetic configurations consist of independent flipping of the chains of spins that are formed along the direction of the shorter Bravais vector. The whole energy spectrum of metastable magnetic configurations can thus be mapped to a one-dimensional Ising model. As a result, the kinetic network of stationary points is latticelike and the disconnectivity graphs have a willow-tree form. In conclusion, the collective magnetic behaviors of nanostructures having different point-group symmetries are contrasted and related by varying the parameters of the lattices that interpolate between them.

Helical edge states in quantum spin Hall (QSH) materials are central building blocks of topological matter design and engineering. Despite their principal topological protection against elastic backscattering, the level of operational stability depends on manifold parameters such as the band gap of the given semiconductor system in the “inverted” regime, temperature, disorder, and crystal orientation. We theoretically investigate electronic and transport properties of QSH edge states in large gap 1-$T^{\prime } {\mathrm{WTe}}_2$ monolayers. We explore the impact of edge termination, disorder, temperature, and interactions on experimentally addressable edge state observables, such as local density of states and conductance. We show that conductance quantization can remain surprisingly robust even for heavily disordered samples because of an anomalously small edge state decay length and additional protection related to the large direct gap allowed by glide symmetry. From the simulation of temperature-dependent resistance, we find that moderate disorder enhances the stability of conductance by localizing bulk states. We evaluate the edge state velocity and Luttinger liquid parameter as functions of the chemical potential, finding prospects for physics beyond linear helical Luttinger liquids in samples with ultraclean and well-defined edges.

The chiral anomaly effect has been regarded as a hallmark of Weyl semimetals. While most research focuses on the change of electrical conductivity due to the chiral charge-pumping effect, these transport measurements are accompanied by other effects such as weak antilocalization which are difficult to exclude. Also, the magnetic field intensity at which the energy dispersion of the Landau level starts to display the chiral anomaly effect is hard to identify. Here, we report the evolutionary process of the energy dispersion of the zeroth Landau level under increasing magnetic fields, and we elucidate the relationship between the chiral Landau level and the Fermi level via magneto-optical measurements in Weyl semimetal TaP. Two series of Landau-level transitions have been extracted and fitted well to a massless Weyl fermion model and a classical semiconductor model, respectively. The field-induced chiral Landau-level transition is obtained from a comparison between the spectra under Voigt geometry and Faraday geometry in an increasing magnetic field. The chiral nature of the zeroth Landau level is found to enhance with the increase of magnetic field and clearly manifests in the magneto-optical spectrum above 13 T. The state transition of the zeroth Landau level can be explained by the strength of the field-induced chiral anomaly effect and the position of the Fermi level. These magneto-optical results unveil the chiral Landau-level transitions in TaP, which serves as an ideal material platform to analyze the chiral anomaly effect.

Ultrashort-pulse laser excitation and associated thermal damage of single-layer metallic films have been extensively studied both theoretically as well as experimentally. However, study of ultrafast heat transfer mechanism in bimetallic films, a topic of immense current interest for designing next-generation optical components with better thermal resilience, is limited to a low fluence regime. Moreover, available results for damage threshold (DT) of bimetallic films have several ambiguities. The current work is an attempt towards development of comprehensive theory for the DT of Au-based bimetallic films, analyzing its dependence on associated physical parameters and bridging the gap in existing literature. This has been achieved by developing fully implicit two-temperature model-based Python code. Excellent agreement of the predicted temperature profiles with recently reported [E. L. Gurevich, Y. Levy, S. V. Gurevich, and N. M. Bulgakova, Phys. Rev. B 95, 054305 (2017)] femtosecond laser irradiated Au film validates the code. Subsequently, the code is utilized to investigate the influence of different metallic substrates, $M$ ($M$ = Ni, Cr, Cu) on ultrafast melting damage of $\mathrm{Au}\text{/}M$ films excited by single-pulse femtosecond laser. Simulations carried out for increasing Au layer thickness revealed a nonmonotonic variation of DT for all substrates. This study also brought out an important observation that for every substrate metal, there exists an optimum Au thickness for maximizing the incipient and the complete melting threshold. Maximum achievable enhancement for $\mathrm{Au}\text{/}M$ films over pure Au are found to be 30%, 22%, and 14% with Cu, Ni, and Cr substrates, respectively. The observed distinction in maximum DT among three different $\mathrm{Au}\text{/}M$ targets is explained by developing appropriate thermophysical models. Finally, an empirical function is proposed here for expressing nonmonotonic variation of two-layer DTs. Very good agreement between the substrate-specific peak positions predicted by the proposed function and effective electron diffusion length arising from thermophysical analysis makes the modeling robust and applicable for a wide range of laser wavelengths as well as arbitrary combinations of bimetallic films.

Tensor networks provide a useful tool to describe low-dimensional complex many-body systems. Finding efficient algorithms to use these methods for finite-temperature simulations in two dimensions is a continuing challenge. Here, we use the class of recently introduced isometric tensor network states, which can also be directly realized with unitary gates on a quantum computer. We utilize a purification ansatz to efficiently represent thermal states of the transverse field Ising model. By performing an imaginary-time evolution starting from infinite temperature, we find that this approach offers a different way with low computational complexity to represent states at finite temperatures.

Recent ab initio studies of electron transport in GaAs have reported that electron-phonon (e-ph) interactions beyond the lowest order play a fundamental role in charge transport and noise phenomena. Inclusion of the next-leading-order process in which an electron scatters with two phonons was found to yield good agreement for the high-field drift velocity, but the characteristic nonmonotonic trend of the power spectral density of current fluctuations (PSD) with electric field was not predicted. The high computational cost of the ab initio approach necessitated various approximations to the two-phonon scattering term, which were suggested as possible origins of the discrepancy. Here we report a semianalytical transport model of two-phonon electron scattering via the Fröhlich mechanism, allowing a number of the approximations in the ab initio treatment to be lifted while retaining the accuracy to within a few percent. We compare the calculated and experimental transport and noise properties as well as scattering rates measured by photoluminescence experiments. We find quantitative agreement within 15% for the drift velocity and $25\%$ for the $\mathrm{\Gamma }$ valley scattering rates, and agreement with the $\mathrm{\Gamma }\text{−}\mathrm{L}$ intervalley scattering rates within a factor of two. Considering these results and prior studies of current noise in GaAs, we conclude that the most probable origin of the nonmonotonic PSD trend versus electric field is the formation of space-charge domains rather than intervalley scattering as has been assumed.

The Chern insulator, as the most prominent topological state of matter, is characterized by the Chern number $\mathcal{C}$, which in principle can take any integer values. So far, most realizations of the Chern insulator are limited to $\left|\mathcal{C}\right|=1$, and achieving higher Chern numbers remains challenging. Here, we propose a general scheme to realize the Chern state with arbitrarily large Chern number in an electric circuit. Our proposal is based on the possibility of precise control on long-range couplings, which is a unique advantage of circuit systems. Besides simulations, we present an experimental demonstration of a $\mathcal{C}=3$ state fabricated on a printed circuit board and directly image its topological chiral edge modes via AC sweep analysis. Our work opens a horizon for exploring exotic topological states of matter in classical systems.

A recent experiment has unveiled an anomalously strong electron-electron attraction in the one-dimensional copper-oxide chain ${\mathrm{Ba}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_{3+\delta }$. While the effect of the near-neighbor electron attraction $V$ in the one-dimensional extended Hubbard chain has been examined recently, its effect in the Hubbard model beyond the one-dimensional chain remains unclear. Here, we report a density matrix renormalization group study of the extended Hubbard model on long four-leg cylinders on the square lattice. We find that the near-neighbor electron attraction $V$ can notably enhance the long-distance superconducting correlations while simultaneously suppressing the charge-density-wave (CDW) correlations. Specifically, for a modestly strong electron attraction, the superconducting correlations become dominant over the CDW correlations with a Luttinger exponent $K_{sc}\sim 1$ and strong divergent superconducting susceptibility. Our results provide a promising way to realize long-range superconductivity in the doped Hubbard model. The relevance of our numerical results to cuprate materials is also discussed.

In this paper we calculate the exchange interaction constant between the manganese ion inner electronic $d$-shell and GaAs valence band bound hole using their microscopic multiparticle wave functions. We reveal its parametric dependence on crystal lattice deformations and find that it could be about and even more than dozens of a percent when the strain tensor reaches values of ${10}^{-3}-{10}^{-2}$. This fact is in accordance with the previous hypothesis of deformation dependence of Mn acceptors in a GaAs fine energy structure obtained from Raman spectroscopy; we show that this dependence has the same magnitude. Also, we resolve here the problem of a substantial high-temperature mismatch between well-developed theory and experimental data for the static magnetic susceptibility of Mn ions in GaAs. We show by numerical estimates and calculations that quite a strong parametric dependence of the exchange coupling value on GaAs lattice expansion determines the high-temperature (above 50 K) magnetic susceptibility reduction as well.

Heusler alloys have attracted interest in various fields of functional materials since their properties can quite easily be tuned by composition. Here, we have investigated the relatively new class of all-$3d$ Heusler alloys in view of their potential as permanent magnets. To identify suitable candidates, we performed a high-throughput study using an electronic structure database to search for $X_{2}YZ$-type Heusler systems with tetragonal symmetry and high magnetization. For the alloys which passed our selection filters, we have used a combination of density functional theory calculations and spin dynamics modeling to investigate their magnetic properties including the magnetocrystalline anisotropy energy and exchange interactions. The candidates which fulfilled all the search criteria served as input for the investigation of the temperature dependence of the magnetization and determination of the Curie temperature. Based on our results, we suggest that ${\mathrm{Fe}}_2\mathrm{NiZn}$, ${\mathrm{Fe}}_2\mathrm{NiTi}$, and ${\mathrm{Ni}}_2\mathrm{CoFe}$ are potential candidates for permanent magnets with large out-of-plane magnetic anisotropy (1.23, 0.97, and 0.82 MJ/${\mathrm{m}}^3$, respectively) and high Curie temperatures lying more than 200 K above the room temperature. We further show that the magnitude and direction of anisotropy are very sensitive to the strain by calculating the values of anisotropy energy for several tetragonal phases. Thus application of strain can be used to tune the anisotropy in these compounds.

Flow of the normal fluid component of He II about a cylinder and flat plate in a channel with a square cross-section was observed by tracking clouds of ${\text{He}}_2^{*}$ excimers. Flow was produced using a lithographically patterned heater. The direction and speed of flow projected onto a plane parallel to the axis of the channel and perpendicular to the heater exhibited significant change with time even after the heater was turned off. Flow was recorded in movies. Excimers moved at least as fast as could be recorded by the camera. Velocity vector field maps of flow suggest formation of structures downstream of the cylinder object that are consistent with parts of eddies. In this paper, we establish a foundation to observe normal component flow of a quantum fluid over centimeters.

Binary metal sulfides are a large material family with distinct physical behavior. Here, we report the superconductivity observed in PbS by means of pressure engineering. PbS undergoes a series of electronic phase transitions upon compression, and superconductivity occurs above 23.9 GPa in the high-pressure CsCl-type cubic phase. Further compression suppresses the superconductivity while the $T_c$ is reduced from $\sim 12.0$ K at 23.9 GPa to 6.8 K at 44.3 GPa. Marginally higher $T_c\approx 12.3$ K is observed at 19.1 GPa during pressure release. The magnetic field effect verifies the superconductivity with possible multiband features. It is noted that PbS is one of the few binary metal sulfide superconductors with a $T_c$ above 10 K. This work demonstrates that PbS has a high $T_c$ under a moderate pressure due to the chemical “precompression” effect, which is very close to the $T_c$ in pure sulfur at $\sim 100$ GPa. PbS also shows unexpected multiband behavior in the cubic structure, benefiting the study of pressure-induced superconductivity and the underlying mechanism in metal sulfide compounds.

Unlike ferromagnetism, antiferromagnetism cannot readily be included in the quasiclassical Keldysh theory because of the rapid spatial variation in the directions of of the magnetic moments. The quasiclassical framework is useful because it separates the quantum effects occurring at length scales comparable to the Fermi wavelength from other length scales, and has successfully been used to study a wide range of phenomena involving both superconductivity and ferromagnetism. Starting from a tight-binding Hamiltonian, we develop general quasiclassical equations of motion and boundary conditions, which can be used to describe two-sublattice metallic antiferromagnets in the dirty limit. The boundary conditions are applicable also for spin-active boundaries that can be either compensated or uncompensated. Additionally, we show how nonuniform or dynamic magnetic textures influence the equations and we derive a general expression for observables within this framework.

Germanium telluride (GeTe) and its variants are promising compounds as high figure of merit thermoelectric materials due to their low lattice thermal conductivity. The strong anharmonicity and the intrinsic Ge vacancies are shown to be the origin of the low thermal conductivity. While the anharmonic effect on the lattice thermal conductivity has been systematically studied using the perturbation theory, the vacancy disorder has been treated perturbatively mostly as an extreme case of phonon-isotope scattering. This simplification ignores realistic features such as the nonbonding character and the detailed local environments near the vacancies. In this study, we calculate the lattice thermal conductivity of the cubic GeTe by considering the anharmonicity and the vacancy disorder on the same footing using the machine-learning potential molecular dynamics. We obtain the spectral function via the nonperturbative approaches, the velocity autocorrelation function, and the phonon unfolding scheme to investigate the effect of the vacancies on the lattice thermal conductivity. We find that the reduction in the lattice thermal conductivity by the vacancies originates from the momentum-dependent scattering of the acoustic phonon modes and the momentum-independent scattering of the optical phonon modes as determined by the strength of the anharmonicity.

We investigate the experimental signatures of Raman spectroscopy of bimagnon and trimagnon excitations in the distorted triangular lattice antiferromagnets $\alpha \text{−}L{\mathrm{Cr}}_2{\mathrm{O}}_4$ $(L=\mathrm{Sr}, \mathrm{Ca})$. Motivated by Raman scattering experiments, we utilize spin wave theory to analyze the nearly ${120}^{\circ }$ spin-3/2 spiral-ordered antiferromagnetic ground state to compute the single-magnon density of states, single-magnon dispersion, and bimagnon and trimagnon Raman spectra (polarized and unpolarized). We perform calculations of the Heisenberg antiferromagnetic Hamiltonian that incorporates magnetic interactions (exchange, anisotropy, and interlayer coupling) and lattice distortion within a four-sublattice unit cell. We investigate the Hamiltonian for both model parameter sets and experimentally proposed magnetic interactions for $\alpha \text{−}L{\mathrm{Cr}}_2{\mathrm{O}}_4$ $(L=\mathrm{Sr}, \mathrm{Ca})$. It is found that Raman scattering is capable of capturing the effect of the rotonlike $M$ and $M^{\prime }$ points on the bimagnon Raman spectrum. Our calculation confirms the connection between single-magnon rotonlike excitation energy and the bimagnon Raman excitation spectrum observed experimentally. The roton energy minimum in momentum space is half of the energy of a bimagnon excitation signal. The experimental magnetic Raman scattering result displays two peaks which have Raman shifts of 15 and 40 meV. Theoretical modeling and analysis of the experimental spectrum of $\alpha \text{−}{\mathrm{SrCr}}_2{\mathrm{O}}_4$ within our distorted Heisenberg Hamiltonian lattice suggest that the low-energy peak at 15 meV is associated with the bimagnon excitation, whereas the high-energy peak around 40 meV is primarily a trimagnon excitation. Based on our fitting procedure we propose a set of magnetic interaction parameters for $\alpha \text{−}{\mathrm{SrCr}}_2{\mathrm{O}}_4$. These parameters reproduce not only the experimental Raman spectrum but also the inelastic neutron scattering response (including capturing high-energy magnon branches). We also compute the unpolarized bimagnon and trimagnon Raman spectra for $\alpha \text{−}{\mathrm{CaCr}}_2{\mathrm{O}}_4$. In contrast to its Sr cousin the Ca-based material has an enhanced bimagnon response, with the high-energy peak still dominated by the trimagnon excitation. Furthermore, the polarization sensitivity of the Raman spectrum can be utilized to distinguish the bimagnon and trimagnon excitation channels.