Semiconducting CoSbS is well known for its thermoelectric performance at high temperatures but it is also of interest because of its colossal low-temperature thermopower. Here, we address the temperature dependence of its optical response over a broad spectral range, from which we reveal several ingredients determining the thermoelectric properties at cryotemperatures. We discover coherent phonon modes and with the additional support of scanning transmission electron microscopy investigations we provide evidence for in-gap impurity states driving the formation of correlated electrons in the valence band. Their implications, with respect to the high thermoelectric power at low temperatures, are discussed within the framework of the phonon-drag transport of low-mobility heavy quasiparticles.

Spin-pumping and spin-orbit torque efficiencies in ferromagnetic/heavy metal bilayers, which usually consist of the polycrystalline grains, are related to the spin current transmission from ferromagnets to heavy metals and vice versa, respectively. In this work, epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}\left(001\right)/\mathrm{Pt}\left(111\right)$ films were fabricated to investigate the spin-pumping efficiency and spin-orbit torque efficiency by ferromagnetic resonance and harmonic Hall resistance measurement techniques, respectively. Ferromagnetic resonance results show that the incremental magnetic damping constants in epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}\left(001\right)/\mathrm{Pt}\left(111\right)$, compared with those of the epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}\left(001\right)/\mathrm{Al}{\mathrm{O}}_x$ reference sample, depend on whether the external magnetic field is applied along the in-plane easy or hard axes of ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}$. The Pt thickness-dependent anisotropic damping constant was ascribed to the anisotropic spin current absorption in epitaxial Pt(111) layer. When electric currents were applied along the easy and hard axes of ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}$ in epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}\left(001\right)/\mathrm{Pt}\left(111\right)$ bilayers, a large difference between spin-orbit torques generated from Pt(111) was observed by a harmonic Hall resistance measurement method. Both of the results suggest that spin current transmission efficiency is related to the anisotropic spin Hall effect of epitaxial Pt(111) layer due to the different spin-orbit interaction energies along the different crystallographic directions of Pt.

We report the low-temperature properties of $\mathrm{Sr}{\mathrm{Nd}}_2{\mathrm{O}}_4$, a geometrically frustrated magnet. Magnetization and heat capacity measurements performed on polycrystalline samples indicate the appearance of a magnetically ordered state at $T_{\mathrm{N}}=2.28\left(4\right)\mathrm{K}$. Powder neutron diffraction measurements reveal that an antiferromagnetic state with the propagation vector $\mathbf{q}=\left(0\frac{1}{2}\frac{1}{2}\right)$ is stabilized below this temperature. The magnetic order is incomplete, as only one of the two ${\mathrm{Nd}}^{3+}$ sites carries a significant magnetic moment while the other site remains largely disordered. The presence of a disordered magnetic component below $T_{\mathrm{N}}$ is confirmed with polarized neutron diffraction measurements. In an applied magnetic field, the bulk property measurements indicate a phase transition at about 30 kOe. We construct a tentative $H\text{−}T$ phase diagram of $\mathrm{Sr}{\mathrm{Nd}}_2{\mathrm{O}}_4$ from these measurements.

We report on a sequence of field-induced transitions in the zigzag ladder compound ${\mathrm{BaDy}}_2{\mathrm{O}}_4$ studied with powder neutron diffraction and magnetization measurements. In agreement with the previously published results, the low-temperature zero-field structure is characterized by two half-integer propagation vectors ${\mathbf{k}}_1=\left[\frac{1}{2}0\frac{1}{2}\right]$ and ${\mathbf{k}}_2=\left[\frac{1}{2}\frac{1}{2}\frac{1}{2}\right]$. However, on application of an external magnetic field, the Bragg peaks corresponding to the zero-field structure lose their intensity rather rapidly and disappear completely in a field of 2.5 kOe. In the intermediate fields, 2.5 to 22.5 kOe, new peaks are observed characterized by the propagation vector ${\mathbf{k}}^{\prime }=\left[00\frac{1}{3}\right]$ corresponding to an up-up-down (uud) structure as well $\mathbf{k}=0$ ferromagnetic peaks. This regime of fields corresponds to a pronounced plateau in the magnetization curve. Remarkably, the uud structure survives heating to at least 1.4 K, three times higher temperature than the $T_{\mathrm{N}}$ of 0.48 K for the zero-field structure. Above 22.5 kOe, the ${\mathbf{k}}^{\prime }$ peaks disappear while the $\mathbf{k}=0$ peaks gain significant intensity indicating an increase in the polarization of the system. The analysis of the intensities of the field-induced reflections allows for a clear identification of the magnetic structures in both the intermediate- and high-field regimes.

We present a comprehensive study of a “temporal photonic crystal” or “time crystal” with a square profile of both its permittivity $\varepsilon \left(t\right)$ and permeability $\mu \left(t\right)$. The continuity of the displacement field $D\left(t\right)e^{ikx}$ and magnetic field $B\left(t\right)e^{ikx}$(where $k$ is the wave number) across the (simultaneous) discontinuities of $\varepsilon \left(t\right)$ and $\mu \left(t\right)$ facilitates the Krönig-Penney methodology of Solid State physics, leading to an analytic photonic band structure (PBS) that relates the wave frequency $\omega$ to $k$. It is periodic in $\omega$, with the period given by the modulation frequency $\mathrm{\Omega }$ and exhibits $k$ bands separated by $k$ gaps. The PBS depends qualitatively on three parameters: the strengths of the electric and magnetic modulations $m_{\varepsilon }$ and $m_{\mu }$ and $\tau =t_1/(t_1+t_2)$ associated with the intervals $t_1$ and $t_2$ that comprise a modulation period $T(=2\pi /\mathrm{\Omega })$. For equal electric and magnetic modulations the PBS is composed of straight lines and there are no $k$ gaps. This can be explained by the fact that for $m_{\varepsilon }=m_{\mu }$ the wave impedance is continuous at the abrupt interfaces of $\varepsilon \left(t\right)$ and $\mu \left(t\right)$; hence no reflections occur. Conversely, the existence of gaps for $m_{\varepsilon }\ne m_{\mu }$ can then be associated with diffraction occurring for the frequencies $\omega =(n/2)\mathrm{\Omega }$ in the presence of discontinuities of the wave impedance. In comparison to harmonic modulation, where only the first gap is appreciable, for square modulation large gaps, that increase with the difference $|m_{\varepsilon }-m_{\mu }|$, exist even between distant bands. In the particular case $m_{\mu }=-m_{\varepsilon }$ [with $\varepsilon \left(t\right)$ and $\mu \left(t\right)$ oscillating out of phase], all the gaps have equal widths. The field $D\left(t\right)$ displays the Bloch-Floquet behavior, namely, oscillations of frequency $\mathrm{\Omega }$ being modulated by an envelope of frequency $\omega (<\mathrm{\Omega })$. For $\omega =(n/2)\mathrm{\Omega }$, the fields are standing waves. We also studied the optical response to a monochromatic wave incident at the modulated slab, namely, the spectral behavior of the frequency combs that are transmitted and reflected by the slab, as well as the field profile inside the slab. Especially interesting is the case of equal modulations $m_{\varepsilon }=m_{\mu }$ where only the fundamental harmonic $n=0$ is transmitted if the modulations are out of phase, while no light at all is reflected if they are in phase.

Several early transition metal dipnictides (TMDPs) have been found to host topological semimetal states and exhibit large magnetoresistance (MR). In this paper, we use angle-resolved photoemission spectroscopy (ARPES) and magnetotransport to study the electronic properties of a TMDP ${\mathrm{ZrP}}_2$. We find that ${\mathrm{ZrP}}_2$ exhibits an extremely large and unsaturated MR of up to 40 000% at 2 K, which originates from an almost perfect electron-hole (e-h) compensation. Our band structure calculations further show that ${\mathrm{ZrP}}_2$ hosts a topological nodal loop in proximity to the Fermi level. Based on the ARPES measurements, we confirm the results of our calculations and determine the surface band structure. This paper establishes ${\mathrm{ZrP}}_2$ as a platform to investigate near-perfect e-h compensation and its interplay with topological band structures.

Quantum system abruptly driven from its stationary phase can reveal nontrivial dynamics upon approaching a new final state. We investigate here such dynamics for a correlated quantum dot sandwiched between the metallic and superconducting leads, considering two types of quenches feasible experimentally. The first one is related to a sudden change of the coupling between the dot and the superconducting lead, while the other one is associated with an abrupt shift of the quantum dot energy level. Using the time-dependent numerical renormalization group method, we examine and quantify the interplay between the proximity induced electron pairing with correlations caused by the on-dot Coulomb repulsion. We determine and discuss the time-dependent charge occupancy, on-dot pair correlation, transient currents, and analyze the evolution of the subgap quasiparticles, which could be empirically observed in the tunneling conductance. To get some insight into the dynamics of a biased junction, we make use of a mean-field approximation. We reveal the signatures of the time-dependent 0-$\pi$ transition and demonstrate that the evolution of local observables exhibits damped quantum oscillations with frequencies given by the energies of Andreev bound states.

We investigate the Hubbard Hamiltonian on ladders where the number of sites per rung alternates between two and three. These geometries are bipartite with nonequal or equal number of sites on the two sublattices. Thus they share a key feature of the Hubbard model in a class of lattices which Lieb has shown analytically to exhibit long-range ferrimagnetic order while being amenable to powerful numeric approaches developed for quasi-one-dimensional geometries. The density matrix renormalization group (DMRG) method is used to obtain the groundstate properties, e.g., excitation gaps, charge and spin densities as well as their correlation functions at half filling. We show the existence of long-range ferrimagnetic order in the one-dimensional ladder geometries. Our work provides detailed quantitative results which complement the general theorem of Lieb for generalized bipartite lattices. It also addresses the issue of how the alternation between quasi-long-range order and spin liquid behavior for uniform ladders with odd and even numbers of legs might be affected by a regular alternation pattern.

The noncentrosymmetric $R\mathrm{Al}\mathit{Pn}$ ($R=\mathrm{rare}\mathrm{earth}, Pn=\mathrm{Si}$, Ge) family, predicted to host nonmagnetic and magnetic Weyl states, provides an excellent platform for investigating the relation between magnetism and Weyl physics. By using high-field magnetotransport measurements and first principles calculations, we have unveiled herein both type-I and type-II Weyl states in the nonmagnetic LaAlSi. By a careful comparison between experimental results and theoretical calculations, nontrivial Berry phases associated with Shubnikov–de Haas oscillations are ascribed to the electron Fermi pockets related to both types of Weyl points located ∼0.1 eV above and exactly on the Fermi level, respectively. Under high magnetic field, signatures of Zeeman splitting are also observed. These results indicate that, in addition to the importance for exploring intriguing physics of multiple Weyl fermions, LaAlSi as a comparison with magnetic Weyl semimetals in the $R\mathrm{Al}\mathit{Pn}$ family would also yield valuable insights into the correlation between magnetism and Weyl physics.

Exotic physics often emerges around quantum criticality in metallic systems. Here we explore the nature of topological phase transitions between 3D double-Weyl semimetals and insulators (through annihilating double-Weyl nodes with opposite chiralities) in the presence of Coulomb interactions. From renormalization-group (RG) analysis, we find a non-Fermi-liquid quantum critical point (QCP) between the double-Weyl semimetals and insulators when artificially neglecting short-range interactions. However, it is shown that this non-Fermi-liquid QCP is actually unstable against nematic ordering when short-range interactions are correctly included in the RG analysis. In other words, the putative QCP between the semimetals and insulators is preempted by the emergence of nematic phases when Coulomb interactions are present and thus double-Weyl fermions cannot directly annihilate with each other. We further discuss the possible experimental relevance of the nematicity-preempted QCP to double-Weyl candidate materials ${\text{HgCr}}_2{\text{Se}}_4$ and ${\text{SrSi}}_2$.

We study the effects of electronic correlations on fragile topology using dynamical mean-field theory. Fragile topological insulators (FTIs) offer obstruction to the formation of exponentially localized Wannier functions, but they can be trivialized by adding certain trivial degrees of freedom. For the same reason, FTIs do not host symmetry-protected flow of edge states between bulk bands in cylindrical boundary conditions but are expected to have a spectral flow between the fragile bands and other bands under certain twisted boundary conditions. We here analyze commonly observed effects of strong correlations, such as the Mott insulator transition and magnetism, on a known model hosting fragile topology. We show that in the nonmagnetic case, fragile topology, along with the twisted boundary states, is stable with interactions below a critical interaction strength. Above this interaction strength, a transition to the Mott insulating phase occurs, and the twisted boundary states disappear. Furthermore, by applying a homogeneous magnetic field, the fragile topology is destroyed. However, we show that a magnetic field can induce a topological phase transition which converts a fragile topological insulator to a Chern insulator. Finally, we study ferromagnetic solutions of the fragile topological model.

Since the discovery of the pseudogap and Fermi arc states in underdoped cuprates, the understanding of such non-Fermi-liquid states and the associated violation of Luttinger's theorem have been the central theme in correlated electron systems. However, still lacking is a well-accepted theoretical framework to unambiguously explain these metallic states that are clearly beyond Landau's Fermi liquid and Luttinger's theorem of a Fermi surface and electron filling. Here, we design a lattice model of orthogonal metals with fermion and Ising matter fields coupled to topological order and, by solving the model via unbiased quantum Monte Carlo simulation at generic electron fillings, find that the system gives birth to phenomena of the Fermi arc and pseudogap in the single-particle spectrum that go beyond the Luttinger sum rule with broken Fermi surface but no symmetry breaking. The pseudogap and Fermi arcs coexist with a background of a deconfined $Z_2$ gauge field, and we further find that the confinement transition of the gauge field triggers a superconductivity instability and that the hopping of the gauge-neutral fermions brings the “large” Fermi surface back from the Fermi arc state. Our unbiased numerical results provide a concrete model realization and theoretical framework for the coupling between gauge field and fermions and, in the process, generate the rich phenomena of the pseudogap, the Fermi arc, and superconductivity in generic correlated electron systems.

Demonstrating the ability of existing quantum platforms to perform certain computational tasks intractable to classical computers represents a cornerstone in quantum computing. Despite the growing number of such proposed “quantum supreme” tasks, it remains an important challenge to identify their direct applications. In this work, we describe how the approach proposed by Tangpanitanon et al. [arXiv:2002.11946] for demonstrating quantum supremacy in generic driven analog many-body systems, such as those found in cold atom and ion setups, can be extended to explore dynamical quantum phase transitions. We show how key quantum supremacy signatures, such as the divergence between the output distribution and the Porter Thomas distribution, can be used as effective order parameters. We apply this approach to a periodically driven disordered one-dimensional Ising model and show that we can accurately capture the transition between the driven thermalized and many-body localized phases. This approach also captures the transition towards the Floquet prethermalized regime for high-frequency driving. Revisiting quantum phases of matter under the light of the recent discussions about quantum supremacy draws a link between complexity theory and analog many-body systems.

We investigate the $5d$ transition metal oxide ${\mathrm{BaOsO}}_3$ within a combination of density functional theory and dynamical mean-field theory, using a matrix-product-state impurity solver. ${\mathrm{BaOsO}}_3$ has four electrons in the $t_{2g}$ shell akin to ruthenates but stronger spin-orbit coupling (SOC) and is thus expected to reveal an interplay of Hund's metal behavior with SOC. We explore the paramagnetic phase diagram as a function of SOC and Hubbard interaction strengths, identifying metallic, band (van Vleck) insulating, and Mott insulating regions. At the physical values of the two couplings, we find that ${\mathrm{BaOsO}}_3$ is still situated inside the metallic region and has a moderate quasiparticle renormalization $m^{*}/m\approx 2$, consistent with specific heat measurements. SOC leads to a splitting of a van Hove singularity close to the Fermi energy and a subsequent reduction of electronic correlations (found in the vanishing SOC case), but the SOC strength is insufficient to push the material into an insulating van Vleck regime. In spite of the strong effect of SOC, ${\mathrm{BaOsO}}_3$ can be best pictured as a moderately correlated Hund's metal.

We have observed the effect of pseudomagnetic field originating from the polaritonic analog of spin-orbit coupling [transverse electric and transverse magnetic (TE-TM) splitting] on a polariton condensate in a ring-shaped microcavity. The effect gives rise to a stable four-leaf pattern around the ring as seen from the linear polarization measurements of the condensate photoluminescence. This pattern is found to originate from the interplay of the cavity potential, energy relaxation, and TE-TM splitting in the ring. Our observations are compared to the dissipative one-dimensional spinor Gross-Pitaevskii equation with the TE-TM splitting energy, which shows good qualitative agreement.

We show that electrically poled ferroelectric matrix considerably enhances the localized magnetostrictive deformations in a ferroelectric-ferrimagnetic composite. Magnetostrain measurements performed on Dy-free and Dy-modified $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3\text{−}\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ (BF-PT) ferroelectric ceramics revealed no measurable macroscopic strain in poled and unpoled Dy-free nonferromagnetic specimens. Dy-modified BF-PT, on the other hand, exhibit ferrimagnetic dysprosium-iron garnet (DyIG) as precipitates and exhibit a macroscopic strain of −4 ppm in the poled state. Despite the small (6%) volume fraction of DyIG, the macroscopic strain in Dy-modified BF-PT is almost 50% of the strain of pure DyIG. Our results suggest that the amplification of the localized magnetostrictive deformation in the ferrimagnetic islands by the neighboring ferroelectric regions is caused by the magnetostrictive stress-induced motion of ferroelectric-ferroelastic domains of the poled ferroelectric matrix.

The band topology of a superconductor is known to have profound impact on the existence of Majorana zero modes in vortices. As iron-based superconductors with band inversion and $s_{\pm }$-wave pairing can give rise to time-reversal invariant second-order topological superconductivity, manifested by the presence of helical Majorana hinge states in three dimensions, we are motivated to investigate the interplay between the second-order topology and the vortex lines in both weak- and strong-Zeeman-field regimes. In the weak-Zeeman-field regime, we find that vortex lines far away from the hinges are topologically nontrivial in the weakly doped regime, regardless of whether the second-order topology is present or not. However, when the superconductor falls into the second-order topological phase and a topological vortex line is moved close to the helical Majorana hinge states, we find that their hybridization will trivialize the vortex line and transfer robust Majorana zero modes to the hinges. Furthermore, when the Zeeman field is large enough, we find that the helical Majorana hinge states are changed into chiral Majorana hinge modes and thus a chiral second-order topological superconducting phase is realized. In this regime, the vortex lines are always topologically trivial, no matter how far away they are from the chiral Majorana hinge modes. By incorporating a realistic assumption of inhomogeneous superconductivity, our findings can explain the recent experimental observation of the peculiar coexistence and evolution of topologically nontrivial and trivial vortex lines in iron-based superconductors.

We propose a class of nodal line semimetals that host an eightfold-degenerate double Dirac nodal line (DDNL) with negligible spin-orbit coupling. We find only 5 of the 230 space groups host the DDNL. The DDNL can be considered as a combination of two Dirac nodal lines, and has a trivial Berry phase. This leads to two possible but completely different surface states, namely, a torus surface state covering the whole surface Brillouin zone and no surface state at all. Based on first-principles calculations, we predict that the hydrogen storage material LiBH is an ideal DDNL semimetal, where the line resides at Fermi level, is relatively flat in energy, and exhibits a large linear energy range. Interestingly, both the two novel surface states of DDNL can be realized in LiBH. Further, we predict that with a magnetic field parallel to DDNL, the Landau levels of DDNL are doubly degenerate due to a Kramers-like degeneracy and have a doubly degenerate zero mode.

Usually, cooling a metal sample down to cryogenic temperatures leads to immobilization of the surface atoms. In this study, we demonstrate a movement of Ni adatoms at $4.6\mathrm{K}$ on Ni films grown on a Re(0001) single crystal, while the surface is rigid at room temperature. The mobility is observed from 2 to 20 atomic-layer-thick films. Measurements at intermediate temperatures reveal an increasing mobility with decreasing temperature. The observed velocity of advancing steps is consistent with a model considering a sudden release of material, eventually triggered by the tip, followed by free diffusion. According to the model, an increasing length scale for the mobilization event with decreasing temperature, tentatively explained by epitaxial strain, is responsible for the inverse mobilization behavior.