We conclusively show that a pseudogap state can arise at $T>T_c$, for reasonable pairing interaction strength, from order parameter fluctuations in a two-dimensional minimal model of $d$-wave superconductivity. The occurrence of the pseudogap requires neither strong correlation nor the presence of competing order. We study a model with attractive nearest-neighbor interaction and establish our result using a combination of a cluster-based Monte Carlo method for the order parameter field and a twisted-boundary scheme to compute the momentum-resolved spectral function. Apart from a dip in the density of states that characterizes the pseudogap, the momentum and frequency resolution on our effective lattice size $\sim 160\times 160$ allows two major conclusions: (i) at $T<T_c$, despite the presence of thermal phase fluctuations, the superconductor has only nodal Fermi points while all non-nodal points on the normal state Fermi surface show a two-peak spectral function with a dip at $\omega =0$, and (ii) for $T>T_c$, the Fermi points develop into arcs, characterized by a single quasiparticle peak, and the arcs connect up to recover the normal state Fermi surface at a temperature $T^{*}>T_c$. We show the variation of $T_c$ and $T^{*}$ with coupling strength and provide detailed spectral results at a coupling where $T^{*}\sim 1.5T_c$.

We study how nonhelical spin textures affect the proximity-induced superconductivity of topological insulator (TI)-superconductor (SC) interface states. In particular we calculate the conductance of lateral heterojunctions which comprise a TI surface that is only partially covered by a superconducting material. Interface potentials at the TI-SC interface may lead to a Fermi velocity and spin texture mismatch between the two regions of the lateral heterojunction. By enforcing the conservation of current across the interface, we derive the boundary conditions and calculate the conductance in both the normal and superconducting state. The total Andreev conductance is calculated for both $s$-wave and spin-triplet parent SCs, and for several examples of nonhelical spin textures which lead to different Fermi surface mismatches between the two planar regions of the heterojunction. We find that for spin-triplet SCs, nonzero conductance signatures only appear for certain combinations of nonhelical spin textures and parent superconducting material.

We report the crystal growth, structural characterization, and physical properties of an off-stoichiometric van der Waals telluride ${\mathrm{Ta}}_{1.2}{\mathrm{Os}}_{0.8}{\mathrm{Te}}_4$, which is isostructural with the noncentrosymmetric ${\mathrm{WTe}}_2$, a prototypical type-II Weyl semimetal. Transport measurements have been carried out in magnetic fields up to 9 T. A low-temperature resistivity upturn is observed along all three crystallographic directions at zero field. The along-chain ${\rho }_a$ and interchain ${\rho }_b$ are found to increase logarithmically at low temperatures, while in the low-$T$ metallic regime, both longitudinal and transverse transports support a three-dimensional Fermi liquid ground state. As temperature is further increased, successive crossovers to electronic states of reduced dimensionality can be seen in both interchain resistivities. The transverse magnetoresistance is weak but anisotropic, the analysis of which indicates the Kohler's rule is severely violated over a large temperature range. The positive linear field dependence of Hall resistivity at selected temperatures indicates the dominance of single-band hole-type charge carriers, consistent with the results of the angle-resolved photoemission spectroscopy. Our results not only establish this transition-metal telluride family as a good system for studying various dimensional crossover phenomena, but also offer the potential for exploring topologically nontrivial phases with reduced dimensionality, given its structural similarity to ${\mathrm{WTe}}_2$.

We study the temperature dependence of the absorption of microwaves by random-anisotropy magnets. It is governed by strong metastability due to the broad distribution of energy barriers separating different spin configurations. At a low microwave power, when the heating is negligible, the spin dynamics is close to linear. It corresponds to the precession of ferromagnetically ordered regions that are in resonance with the microwave field. Previously, we showed [Phys. Rev. B 103, 214414 (2021)] that in this regime a dielectric substance packed with random magnets would be a strong microwave absorber in a broad frequency range. Here we demonstrate that on increasing the power, heating and over-barrier spin transitions come into play, resulting in the nonlinear behavior. At elevated temperatures the absorption of microwave power decreases dramatically, making the dielectric substance with random magnets transparent for the microwaves.

Ferromagnet (FM)/heavy metal (HM) bilayers are core structures for current-induced magnetization switching and chiral magnetic structure generation. Static and dynamic properties of the FM moment depend substantially on interfacial perpendicular magnetic anisotropy (iPMA) and interfacial Dzyaloshinskii-Moriya interaction (iDMI). Therefore, it is of crucial importance to control iPMA and iDMI, and to understand their underlying physics. Here we experimentally show that both iPMA and iDMI exhibit similar Slater-Pauling-like dependence on the FM variation as the saturation magnetization ($M_S$) does. We measure $M_S$, iPMA, and iDMI of the FM/HM bilayers with HM fixed to Pt and FM varied from Mn (electron number $Z=25$) to Ni $\left(Z=28\right)$, including their alloys for fractional $Z$. Our result indicates that the density of states structure important for the Slater-Pauling dependence is crucial also for iPMA and iDMI. This provides a useful method to engineer chiral magnetic textures.

Twisted bilayers of high-$T_c$ cuprate superconductors have been argued to form topological phases with spontaneously broken time reversal symmetry $\mathcal{T}$ for certain twist angles. A key outstanding challenge is to identify unambiguous signatures of these topological phases in experiments. With this goal in mind we theoretically investigate a suite of Josephson phenomena between twisted cuprate layers. At intermediate twist angles we find an unusual nonmonotonic temperature dependence of the critical current, which we attribute to the unconventional sign structure of the $d$-wave order parameter. The onset of the $\mathcal{T}$-broken phase near the ${45}^{\circ }$ twist is marked by a crossover from the conventional $2\pi$-periodic Josephson relation $J\left(\phi \right)\simeq J_{c}sin\phi$ to a $\pi$-periodic function as the single-pair tunneling becomes dominated by a second-order cotunneling process. Despite this fundamental change, the critical current remains a smooth function of the twist angle $\theta$ and temperature $T$ implying that a measurement of $J_c$ alone will not be a litmus test for the $\mathcal{T}$-broken phase. We show that clear signatures of the $\mathcal{T}$-broken phase appear when $J_c$ is measured in the presence of an applied magnetic field or radiofrequency drive: The resulting Fraunhofer oscillations acquire additional nodes and fractional Shapiro steps become visible. We discuss these results in light of recent experiments on twisted bilayers of the high-$T_c$ cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$.

The entanglement spectrum is a useful tool to study topological phases of matter, and contains valuable information about the ground state of the system. Here, we study its properties for free non-Hermitian systems for both point-gapped and line-gapped phases. While the entanglement spectrum only retains part of the topological information in the former case, it is very similar to Hermitian systems in the latter. In particular, it not only mimics the topological edge modes, but also contains all the information about the polarization, even in systems that are not topological. Furthermore, we show that the Wilson loop is equivalent to the many-body polarization and that it reproduces the phase diagram for the system with open boundaries, despite being computed for a periodic system.

In quantum statistical mechanics, finite-temperature phase transitions are typically governed by classical field theories. In this context, the role of quantum correlations is unclear: recent contributions have shown how entanglement is typically very short-ranged, and thus uninformative about long-ranged critical correlations. In this work, we show the existence of finite-temperature phase transitions where a broader form of quantum correlation than entanglement, the entropic quantum discord, can display genuine signatures of critical behavior. We consider integrable bosonic field theories in both two- and three-dimensional lattices, and show how the two-mode Gaussian discord decays algebraically with the distance even in cases where the entanglement negativity vanishes beyond nearest-neighbor separations. Systematically approaching the zero-temperature limit allows us to connect discord to entanglement, drawing a generic picture of quantum correlations and critical behavior that naturally describes the transition between entangled and discordant quantum matter.

Two-dimensional Janus structures with vertical intrinsic electric fields exhibit many interesting physical properties that are not possible with symmetric materials. We systematically investigate the structural, electronic, and transport properties of quintuple-layer atomic Janus ${\mathrm{Ga}}_2\mathrm{S}{X}_2$ ($X=$ O, S, Se, Te) monolayers using the first-principles calculations. The stability of the Janus structures is evaluated via the analysis of their phonon dispersion curves, cohesive and formation energies, and elastic constants. The existence of a vertical internal electric field due to the lack of mirror symmetry has resulted in a vacuum level difference between the two sides of the investigated Janus structures. At the ground state, while the Janus ${\mathrm{Ga}}_2\mathrm{S}{\mathrm{O}}_2$ is metallic, the other three configurations (${\mathrm{Ga}}_2{\mathrm{S}}_3, {\mathrm{Ga}}_2\mathrm{S}{\mathrm{Se}}_2$, and ${\mathrm{Ga}}_2\mathrm{S}{\mathrm{Te}}_2$) are semiconductors with indirect band gaps. The electronic properties of ${\mathrm{Ga}}_2\mathrm{S}{X}_2$ monolayers can be altered by strain engineering. In particular, the indirect–direct band gap and semiconductor-metal phase transitions were observed when the biaxial strain was introduced. The carrier mobilities of the semiconducting Janus monolayers are directional anisotropic due to their anisotropy of the deformation potential constant. It is found that all three semiconducting Janus monolayers have high electron mobility, up to 930.34 ${\mathrm{cm}}^2$/V s, which is suitable for applications in next-generation electronic devices.

We study the energy spectrum and transport property of a one-dimensional Kitaev quantum ring in a threading magnetic field. It is demonstrated that the magnetic field can effectively induce topological phase transitions for the ring in the topologically nontrivial phase at the zero magnetic field. However, for the ring in the topologically trivial phase at the zero field, there is no topological phase transition, and the energy spectrum of the system is always gapped. The magnetic field can control the appearance and disappearance of Majorana zero-energy states in the Kitaev quantum ring, when one half of the ring is in the topologically nontrivial phase and the other half is in the topologically trivial phase. Furthermore, we calculate the transport properties of the ring connected by two semi-infinite leads. It is found that the resonant peaks of transmission coefficient $T_{\text{QT}}$ correspond to the critical points of topological phase transition. In addition, we extend our findings to a more realistic quantum ring adopting a semiconductor nanowire with high spin-orbit coupling, superconducting $s$-wave pairing, and Zeeman splitting, and prove that our findings are universal.

Low temperature structural and superfluid properties of ${}^4\mathrm{He}$ confined in cylindrical nanopores are theoretically investigated by means of first-principle quantum Monte Carlo (QMC) simulations. We vary the density of ${}^4\mathrm{He}$ inside the pore, as well as the pore diameter and the potential describing the interaction of each ${}^4\mathrm{He}$ atom with the pore surface. Accordingly, the ${}^4\mathrm{He}$ fluid inside the pore forms either a single channel along the axis, or a series of concentric cylindrical shells, with varying degrees of shell overlap. In the limit of pore length greatly exceeding its radius, the ${}^4\mathrm{He}$ fluid always displays markedly one-dimensional behavior, with no “dimensional crossover” above some specific pore radius and/or as multiple concentric shells form, in contrast to what was recently claimed by other authors [Phys. Rev. B 101, 104505 (2020)]. Indeed, the predicted robustness of one-dimensional physics suggests that this system may offer a broadly viable pathway to the experimental observation of exotic behavior of, e.g., junctions of interacting Tomonaga-Luttinger liquids, in an appropriately designed network of nanopores.

Monolayer two-dimensional crystalline systems with broken inversion symmetry have shown unique optical response properties. We calculate the nonreciprocal photocurrent in the nonlinear response of two-dimensional models involving the Berry connection in the presence of impurity scattering processes. Our results are based on quantum kinetic theory, where we calculate the nonequilibrium distribution function of the valence and conduction bands. We find that the significant current peak emerges at the interband absorption threshold in the electron-doped system and that the peak is remarkably sensitive to the electron density as well as the effective band masses. The current originates from trigonal warping and increases when the warping hopping parameter increases as well the band masses. A nonlinear Hall response is equally obtained for certain model Hamiltonian parameters. Our findings provide an alternative platform for nonlinear light-matter interaction engineering.

A crossover between different power-law relaxation behaviors of many-body periodically driven integrable systems has come to light in recent years. We demonstrate, using integrable quantum systems, that similar kinds of dynamical transitions may also occur in the relaxation of such systems following a sudden quench. Particularly, we observe two distinct power-law relaxation behaviors following a sudden quench in the integrable XY model, depending upon whether the quenched Hamiltonian lies in the commensurate or the incommensurate phase. The relaxation behavior for quenches at and near the boundary line, called the disorder line (DL), separating these phases is also characterized. The relaxation at the DL shows a scaling exponent different from that for quenches to either of the commensurate or incommensurate phases. The transitions occur through a crossover from the commensurate or incommensurate scaling behavior to the DL scaling behavior. The crossover time diverges as a power law as the parameters of the final quenched Hamiltonian approach the DL. The transitions are also observed to be robust under weak integrability-breaking perturbations but disappear following strongly chaotic quenches.

Quenching has recently emerged as a transformative tool for dynamically evolving quantum systems into nonequilibrium phases with desirable properties. In the context of strongly correlated systems, such a tool can significantly alter their microscopic behavior and manifest a variety of interesting phenomena at the macroscopic level. We show that the staggered magnetic field can induce a nonequilibrium steady conducting state from the deep Mott insulating phase. The underlying mechanism can be abstracted from two-particle subspace, that is, the resonant staggered field combined with on-site interaction forced the particles to pair up and move like a free single particle. Correspondingly, the bound pair can have large energy bandwidth and exhibits the following dynamical behaviors. (i) When an electric field is applied, fast bound pair Bloch oscillation occurs, while a single electron is frozen. (ii) When a quenching resonant field is applied to an initial antiferromagnetic Mott insulating state, the final state becomes a doublon conducting state manifested by the nonzero $\eta$ correlator and large charge fluctuation. Our findings indicate that the cooperation of electron-electron correlation and modulated external field can provide an alternative pathway to access a new exotic quantum state that is absent in the ground-state phase diagram.

Magnon-phonon scattering (MPS) has attracted widespread attention in quantum heat/spin transport across the ferromagnetic/nonmagnetic (F/N) interfaces, with the rapid progress of experiments on spin caloritronics in recent years. However, the lack of theoretical methods, accounting for the MPS rigorously, has seriously hindered investigations on the quantum heat transport in magnetic nanostructures with broken translational symmetry, such as F/N interfaces. In this paper, we propose a theoretical formalism of the nonequilibrium Green function to incorporate the MPS into the quantum heat transport for three-dimensional ferromagnetic nanostructures, rigorously, through a diagrammatic perturbation analysis. A computational scheme is developed for the first-principles simulation of quantum heat transport in practical magnetic nanostructures, and a generalized formalism of heat flow is presented for the analysis of the elastic and inelastic process of heat transport. A thermal rectification driven by MPS is observed in the numerical simulation of heat transport across the F/N interface based on the ${\text{CrI}}_3$ monolayer, which is consistent with recent studies. In this paper, we open the gate to first-principles investigations of quantum heat transport in magnetic nanostructures and pave the way for the theoretical design of magnetic thermal nanodevices.

Resonant inelastic x-ray scattering (RIXS) spectra of hematite ($\alpha \text{−}\mathrm{Fe}{}_2{\mathrm{O}}_3$) were measured at the Fe $L_3$ edge for heteroepitaxial thin films which were undoped and doped with 1% Ti, Sn, or Zn, in the energy-loss range in excess of 1 eV to study electronic transitions. The spectra were measured for several momentum transfers q, conducted at both low temperature ($T=14$ K) and room temperature. While we cannot rule out dispersive features possibly owing to propagating excitations, the coarse envelopes of the general spectra did not appreciably change shape with q, implying that the bulk of the observed $L$-edge RIXS intensity originates from (mostly) nondispersive ligand field excitations. Summing the RIXS spectra over q and comparing the results at $T=14$ K to those at $T=300$ K revealed pronounced temperature effects, including an intensity change and energy shift of the $\approx 1.4$ eV peak, a broadband intensity increase of the 3–4 eV range, and higher energy features. The q-summed spectra and their temperature dependencies are virtually identical for nearly all of the samples with different dopants, save for the temperature dependence of the Ti-doped sample's spectrum, which we attribute to being affected by a large number of free charge carriers. Comparing with magnetization measurements for different temperatures and dopings likewise did not show a clear correlation between the RIXS spectra and the magnetic ordering states. To clarify the excited states, we performed spin multiplet calculations which were in excellent agreement with the RIXS spectra over a wide energy range and provide detailed electronic descriptions of the excited states. The implications of these findings to the photoconversion efficiency of hematite photoanodes is discussed.

We identify a mechanism to convert skyrmions and antiskyrmions into their antiferromagnetic (AFM) counterparts via interface engineering. The key idea is to combine properties of an antiferromagnet and a spin-orbit (SO) coupled metal. Utilizing hybrid Monte Carlo (HMC) simulations for a generic microscopic electronic Hamiltonian for the interfacial layers, we explicitly show the emergence of AFM skyrmions and AFM antiskyrmions. We further show that an effective spin Hamiltonian provides a simpler understanding of the results. We discuss the role of electronic itinerancy in determining the nature of magnetic textures, and demonstrate that the mechanism also allows for a tuning of antiskyrmion size without changing the SO coupling.