Conventional $n$-dimensional topological superconductors (TSCs) have protected gapless $(n-1)$-dimensional boundary states. In contrast to this, second-order TSCs are characterized by topologically protected gapless $(n-2)$-dimensional states with the usual gapped $(n-1)$ boundaries. Here, we study a second-order TSC with a two-dimensional (2D) magnetic topological insulator proximity coupled to a high-temperature superconductor, where Majorana bound states (MBSs) are localized at the corners of a square sample with gapped edge modes. Due to the mirror symmetry of the hybrid system considered here, there are two MBSs at each corner for both cases: $d$-wave and $s_{\pm }$-wave superconducting pairing. We present the corresponding topological phase diagrams related to the role of the magnetic exchange interaction and the pairing amplitude. A detailed analysis, based on edge theory, reveals the origin of the existence of MBSs at the corners of the 2D sample, which results from the sign change of the Dirac mass emerging at the intersection of any two adjacent edges due to pairing symmetry. Possible experimental realizations are discussed. Our proposal offers a promising platform for realizing MBSs and performing possible non-Abelian braiding in 2D systems.

We consider the steady-state behavior of observables in the Su-Schrieffer-Heeger (SSH) model and in the one-dimensional transverse field quantum Ising model after a sudden quantum quench of the parameter controlling the gap. In the thermodynamic limit, and for times $t\to \infty$, we find nonanalyticities even in simple local observables as a function of the quench parameter, that is, a nonequilibrium quantum phase transition. We trace the appearance of this nonequilibrium quantum phase transition to an effective metal-insulator transition which occurs on the level of the generalized Gibbs ensemble (describing the steady state of the equilibrated system). Studying whether these transitions are robust, we find, in the paradigmatic case of the SSH model, that they persist for both quantum quench protocols of finite duration in time as well as thermal initial states, while they are washed out in the presence of fermion-fermion interactions and for finite system size.

Polarized-neutron diffraction (PND) experiments have revealed that the pseudogap state of the cuprates exhibits unusual intra-unit-cell (IUC) magnetism. At a qualitative level, the data indicate a moment direction that is neither perpendicular nor parallel to the $\mathrm{Cu}{\mathrm{O}}_2$ layers, yet an accurate measurement at a high-symmetry momentum point in a structurally simple compound has been lacking. Such a measurement would be crucial, as it would help to narrow down the scenarios for the microscopic origin of the IUC magnetism. Here we report PND results with unprecedented accuracy for the IUC magnetic order in the simple-tetragonal single-$\mathrm{Cu}{\mathrm{O}}_2$-layer compound $\mathrm{HgB}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$. At the pseudogap temperature, we find evidence for magnetic critical scattering. Deep in the ordered state, we determine the moment direction to be 70° ± 10° away from the normal to the $\mathrm{Cu}{\mathrm{O}}_2$ layers, which rules out both purely planar loop currents and high-symmetry Dirac multipoles, the two most prominent theoretical proposals for the microscopic origin of the IUC magnetism. However, the data are consistent with Dirac multipoles of lower symmetry or, alternatively, with a particular configuration of loop currents that flow on the faces of the $\mathrm{Cu}{\mathrm{O}}_6$ octahedra.

We report an inelastic neutron scattering study of magnetic excitations in a quantum paramagnet driven into a magnetically ordered state by chemical substitution, namely $\mathrm{Ni}{\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_2·4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2$ with $x=0.21\left(2\right)$. The measured spectrum is well accounted for by the generalized spin wave theory (GSWT) approach [M. Matsumoto and M. Koga, J. Phys. Soc. Jpn. 76, 073709 (2007)]. This analysis allows us to determine the effective Hamiltonian parameters for a direct comparison with those in the previously studied parent compound and “underdoped” system. The issue of magnon lifetimes due to structural disorder is also addressed.

Layered ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ is a promising two-dimensional (2D) material for van der Waals bonded ferromagnetic devices since its ferromagnetism can be maintained upon exfoliating the bulk down to a few layers. However, the magnetic interaction properties and phase-transition type of this system remain controversial. In this work, we systematically study the magnetic properties and critical behavior of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ single crystal. The angle dependence of magnetization indicates that ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ has a weak magnetic anisotropy along the $c$ axis but no magnetic anisotropy in the $ab$ plane. Based on the field-dependent magnetic entropy change $\left[\mathrm{\Delta }{S}_M(T,H)\right]$, the critical exponents of $\beta =0.177\left(9\right)$, $\gamma =1.746\left(8\right)$, and $\delta =10.869\left(5\right)$ are obtained around $T_C=66.4\left(3\right)$ K. With these critical exponents, the $M(T,H)$ and $\mathrm{\Delta }{S}_M(T,H)$ curves follow the universality scaling laws, implying their high reliability and self consistency. The determined critical exponents are close to the 2D-Ising model with spatial dimensionality $d=2$ and spin dimensionality $n=1$. These results suggest that for 2D-Ising-like ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$, the weak magnetic anisotropy is crucial for the stabilization of the long-range ferromagnetic ordering, which is also consistent with the prediction of renormalized spin wave theory.

First-principles calculations of high-temperature spin dynamics in solids in the context of nuclear magnetic resonance (NMR) are a long-standing problem, whose conclusive solution can significantly advance the applications of NMR as a diagnostic tool for material properties. In this work, we propose a hybrid quantum-classical method for computing NMR free induction decay(FID) for spin-$1/2$ lattices. The method is based on the simulations of a finite cluster of spins $1/2$ coupled to an environment of interacting classical spins via a correlation-preserving scheme. Such simulations are shown to lead to accurate FID predictions for one-, two-, and three-dimensional lattices with a broad variety of interactions. The accuracy of these predictions can be efficiently estimated by varying the size of quantum clusters used in the simulations.

${\mathrm{Sr}}_2{\mathrm{FeOsO}}_6$ is an insulating double perovskite compound which undergoes antiferromagnetic transitions at 140 ($T_{\text{N1}}$) and 67 K ($T_{\text{N2}}$). To study the underlying electronic and magnetic interactions giving rise to this behavior we have performed inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) experiments on polycrystalline samples of ${\mathrm{Sr}}_2{\mathrm{FeOsO}}_6$. The INS data reveal that the spectrum of spin excitations remains ungapped below $T_{\text{N1}}$, however below $T_{\text{N2}}$ a gap of 6.8 meV develops. The RIXS data reveals splitting of the $T_{2g}$ multiplet consistent with that seen in other $5d^3$ osmium based double perovskites. Together these results suggest that spin-orbit coupling is important for ground state selection in $3d\text{−}5d^3$ double perovskite materials.

We study the spectrum of an impurity coupled to a Fermi sea (e.g., minority atom in an ultracold gas, exciton in a solid) by attraction strong enough to form a molecule/trion. We introduce a diagrammatic scheme which allows treating a finite mass impurity while reproducing the Fermi-edge singularity in the immobile limit. For large binding energies the spectrum is characterized by a semicoherent repulsive polaron and an incoherent molecule-hole continuum, which is the lowest-energy feature in the single-particle spectrum. The previously predicted attractive polaron seems not to exist for strong binding.

The magnetoresistance of ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ and ${\mathrm{Co}}_{83}{\mathrm{Gd}}_{17}$ ferromagnetic thin films is measured in Corbino disk geometry, and compared to the magnetoresistance of the same films measured in the Hall-bar geometry. The symmetry of the magnetoresistance profiles is drastically modified by changing the geometry of the sample, i.e., by changing the boundary conditions. These properties are explained in a simple model, showing that the Corbino magnetoresistance is defined by the potentiostatic boundary conditions while the Hall-bar magnetoresistance is defined by galvanostatic boundary conditions.

Magnetotransport is investigated in thin films of the half-metallic ferrimagnet ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ in pulsed magnetic fields of up to 58 T. A nonvanishing Hall signal is observed over a broad temperature range, spanning the compensation temperature (155 K), where the net magnetic moment is strictly zero, the anomalous Hall conductivity is $6673{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$, and the coercivity exceeds $9\mathrm{T}$. Molecular field modeling is used to determine the intra- and intersublattice exchange constants, and from the spin-flop transition we infer the anisotropy of the electrically active sublattice to be $216\mathrm{kJ}{\mathrm{m}}^{-3}$ and predict the magnetic resonance frequencies. Exchange and anisotropy are comparable and hard-axis applied magnetic fields result in a tilting of the magnetic moments from their collinear ground state. Our analysis is applicable to collinear ferrimagnetic half-metal systems.

Superconductivity occurring at low densities of mobile electrons is still a mystery since the standard theories do not apply in this regime. We address this problem by using a microscopic model for ferroelectric (FE) modes, which mediate an effective attraction between electrons. When the dispersion of modes, around zero momentum, is steep, forward scattering is the main pairing process and the self-consistent equation for the gap function can be solved analytically. The solutions exhibit unique features: Different momentum components of the gap function are decoupled, and at the critical regime of the FE modes, different frequency components are also decoupled. This leads to effects that can be observed experimentally: The gap function can be nonmonotonic in temperature and the critical temperature can be independent of the chemical potential. The model is applicable to lightly doped polar semiconductors, in particular, strontium titanate.

We report two distinct superconducting states with different crystal structures and a crossover from a type-II to a type-I superconductor (SC) in $(\mathrm{Ba},\mathrm{Sr})\mathrm{B}{\mathrm{i}}_3$. The superconducting parameters are revealed to classify two SCs: $\mathrm{BaB}{\mathrm{i}}_3$ is in the weak-coupling limit on the basis of $\mathrm{\Delta }C/{\gamma }_{\mathrm{n}}{T}_{\mathrm{c}}\sim 0.67$ and $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}\sim 3.28$ while $\mathrm{SrB}{\mathrm{i}}_3$ is a strong-coupling SC with $\mathrm{\Delta }C/{\gamma }_{\mathrm{n}}{T}_{\mathrm{c}}\sim 2.41$ and $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}\sim 6.09$. A large Kadowaki-Woods ratio $(R_{\mathrm{KW}}\sim 3.53a_0)$ suggests an enhanced electron-electron scattering in $\mathrm{BaB}{\mathrm{i}}_3$. With increasing the pressure, the $T_{\mathrm{c}}$ of $\mathrm{BaB}{\mathrm{i}}_3$ decreases linearly at first, and then shows an abrupt increase up to 6.2 K at 0.88 GPa. This behavior can be attributed to a pressured-induced structural transition and the resulting variations of spin-orbit coupling and Fermi structures. $T_{\mathrm{c}}$ of $\mathrm{SrB}{\mathrm{i}}_3$ is suppressed monotonously by pressure. The Ginzburg-Landau parameter ${\kappa }_{\mathrm{GL}}$ of $\mathrm{BaB}{\mathrm{i}}_3$ decreases from 10.35 at ambient pressure (AP) to 0.86 at 1.75 GPa, and then tends to saturation. ${\kappa }_{\mathrm{GL}}$ of $\mathrm{SrB}{\mathrm{i}}_3$ decreases from 0.76 at AP to $1/\phantom{1\sqrt{2}}\sqrt{2}$ at 1.20 GPa, which manifests pressured-induced crossover from a type-II to a type-I SC. Possible physical mechanisms are proposed.

We investigate the interplay between the structural reconstruction and the magnetic properties of Fe double layers on Ir (111) substrate using first-principles calculations based on density functional theory and mapping of the total energies on an atomistic spin model. We show that if a second Fe monolayer is deposited on Fe/Ir (111), the stacking may change from hexagonal-close-packed to bcc (110)-like accompanied by a reduction of symmetry from trigonal to centered rectangular. Although the bcc-like surface has a lower coordination, we find that this is the structural ground state. This reconstruction has a major impact on the magnetic structure. We investigate in detail the changes in the magnetic exchange interaction, the magnetocrystalline anisotropy, and the Dzyaloshinskii-Moriya interaction depending on the stacking sequence of the Fe double layer. Based on our findings, we suggest a technique to engineer Dzyaloshinskii-Moriya interactions in multilayer systems employing symmetry considerations. The resulting anisotropic Dzyaloshinskii-Moriya interactions may stabilize higher-order skyrmions or antiskyrmions.

We present a theoretical approach for spin-polarized hot-electron transport, as it occurs after excitation by ultrafast optical pulses in heterostructures formed by ferromagnetic and normal metals. We formulate a spin-dependent particle-in-cell model that solves the Boltzmann equation for excited electrons. It includes lifetimes and transmission coefficients as parameters, which can be taken from ab initio calculations or experiment, and can be easily extended to multilayer systems. This approach is capable of describing electron transport in the ballistic, superdiffusive, and diffusive regime including secondary-carrier generation. We apply the model to optically excited carriers in Fe/Au bilayers and Fe/Au/Fe spin-valve structures and analyze theoretically the hot-electron transport dynamics probed in recent experiments on spin valves. We find contributions to the demagnetization dynamics induced in Fe/Au/Fe trilayers regardless of the parallel or antiparallel magnetic alignment of the Fe layers.

We study a quantum impurity possessing both translational and internal rotational degrees of freedom interacting with a bosonic bath. Such a system corresponds to a “rotating polaron,” which can be used to model, e.g., a rotating molecule immersed in an ultracold Bose gas or superfluid helium. We derive the Hamiltonian of the rotating polaron and study its spectrum in the weak- and strong-coupling regimes using a combination of variational, diagrammatic, and mean-field approaches. We reveal how the coupling between linear and angular momenta affects stable quasiparticle states, and demonstrate that internal rotation leads to an enhanced self-localization in the translational degrees of freedom.

We report an intense near-infrared pump, $c$-axis terahertz probe measurement on an electron-doped cuprate superconductor ${\mathrm{Pr}}_{0.88}{\mathrm{LaCe}}_{0.12}{\mathrm{CuO}}_4$ with $T_c=22$ K. The major effect of the intense pump is to induce a splitting of Josephson plasma edge below $T_c$, similar to the observation on hole-doped cuprate ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_4$ in a superconducting state. The photoexcitation induced spectral change does not exhibit significant decay up to the longest measured time delay 210 ps. The result suggests that the intense near-infrared pump drives the system from a superconducting state with a uniform Josephson coupling to a metastable superconducting phase with modulated Josephson coupling strengths below $T_c$.

We study the nonequilibrium dynamics in the pump-probe spectroscopy of excitonic insulators using the spinless two-orbital model with phonon degrees of freedom in the time-dependent mean-field approximation. We introduce the pulse light as a time-dependent vector potential via the Peierls phase in the Hamiltonian. We find that, in the Bose-Einstein condensation regime where the normal state is semiconducting, the excitonic order is suppressed when the frequency of the pulse light is slightly larger than the band gap, while the order is enhanced when the frequency of the pulse is much larger than the band gap. We moreover find that the excitonic order is completely destroyed in the former situation if the intensity of the pulse is sufficiently strong. In the BCS regime where the normal state is semimetallic, we find that the excitonic order is always suppressed, irrespective of the frequency of the pulse light. The quasiparticle band structure and optical conductivity spectrum after the pumping are also calculated for the instantaneous states.

The possibility of an inhomogeneous charge density wave phase is investigated in a system of two coupled electron and hole monolayers separated by a hexagonal boron nitride insulating layer. The charge-density-wave state is induced through the assumption of negative compressibility of electron/hole gases in a Coulomb drag configuration between the electron and hole sheets. Under equilibrium conditions, we derive analytical expressions for the density oscillation along the zigzag and armchair directions. We find that the density modulation not only depends on the sign of the compressibility but also on the anisotropy of the low-energy bands. Our results are applicable to any two-dimensional system with anisotropic parabolic bands, characterized by different effective masses. For equal effective masses, i.e., isotropic energy bands, our results agree with Hroblak et al. [Phys. Rev. B 96, 075422 (2017)]. Our numerical results are applied to phosphorene.