The emergence of Majorana bound states in finite length superconductor-semiconductor hybrid systems has been predicted to occur in the form of oscillatory energy levels with parity crossings around zero energy. Each zero-energy crossing is expected to produce a quantized zero-bias conductance peak but several studies have reported conductance peaks pinned at zero energy over a range of Zeeman fields, whose origin, however, is not clear. In this work, we consider superconducting systems with spin-orbit coupling under a Zeeman field and demonstrate that non-Hermitian effects, due to coupling to ferromagnet leads, induce zero-energy pinning of Majorana and trivial Andreev bound states. We find that this zero-energy pinning effect occurs due to the formation of non-Hermitian spectral degeneracies known as exceptional points, whose emergence can be controlled by the interplay of non-Hermiticity, the applied Zeeman field, and chemical potentials. Moreover, depending on the non-Hermitian spatial profile, we find that non-Hermiticity changes the single point Hermitian topological phase transition into a flattened zero energy line bounded by exceptional points from multiple low energy levels. This seemingly innocent change notably enables a gap closing well below the Hermitian topological phase transition, which can be in principle simpler to achieve. Furthermore, we reveal that the energy gaps separating Majorana and trivial Andreev bound states from the quasicontinuum remain robust for the values that give rise to the zero-energy pinning effect. While reasonable values of non-Hermiticity can be indeed beneficial, very strong non-Hermitian effects can be detrimental as it might destroy superconductivity. Our findings can be therefore useful for understanding the zero-energy pinning of trivial and topological states in Majorana devices.

Graphene has significantly influenced advanced photonics, due to its exceptional ability to confine light at atomic scales. However, the presence of isotropic Dirac cones (DCs) in graphene restricts its capability to support hyperbolic surface plasmon polaritons (SPPs) propagation without specific patterning treatments. Additionally, the plasmon frequency in graphene is highly sensitive to the Fermi level, making it unstable under external perturbations. In this study, we present a dual anisotropic Dirac cone (DADC) model that addresses these limitations. We demonstrate that this DADC exhibits highly in-plane anisotropic plasmons and extensive hyperbolic regions capable of supporting hyperbolic SPPs. By maintaining the Fermi level between the Dirac points of the two DCs, we ensure that the plasmon frequency remains independent of Fermi level. Furthermore, we identify ω-graphene, a graphene allotrope, as a potential material for this model. Our first-principles calculations revealed that ω-graphene exhibits significantly anisotropic plasmons, with a maximal frequency of 1.68 eV along the $x$ direction and 0.128 eV along the $y$ direction, accompanied by a low-loss hyperbolic region spanning from 0.48 to 1.16 eV. Notably, the plasmon frequency remains stable despite variations in Fermi energy within an experimentally attainable region. The highly directional propagation of SPPs in the hyperbolic regions was also confirmed using Maxwell's equations. These findings introduce a compelling candidate for SPPs devices and open exotic avenues for designing and investigating natural hyperbolic surfaces.

Phonon resonance scattering caused by point defects has long been believed to be able to induce an abnormal temperature dependence of thermal conductivity κ that significantly deviates from the prediction of Rayleigh scattering. However, a rigorous demonstration and microscopic origin of this behavior are still lacking. Here, using an ab initio Green's function approach, we show that three types of point defects cause phonon resonance scattering in cubic BN, among which only the nitrogen vacancy indeed produces an unusual nonmonotonic temperature dependence of κ. This abnormal behavior arises only when the phonon resonance is sufficiently strong and occurs at relatively low phonon frequencies, which overwhelms the intrinsic scattering of low-frequency phonons that otherwise would produce a monotonic temperature dependence of κ. Our work provides deep insights into the phonon resonance caused by point defects and will benefit the manipulation of κ by defect engineering, particularly for doping scenarios.

Topological superconductors are appealing building blocks for robust and reliable quantum information processing. Most platforms for engineering topological superconductivity rely on a combination of superconductors, materials with intrinsic strong spin-orbit coupling, and external magnetic fields, detrimental for superconductivity. We propose a setup where a conventional Josephson junction is linked via a magnetic-textured barrier. Antiferromagnetic and ferromagnetic insulators with periodically arranged domains are compatible with our proposal, which does not require intrinsic spin-orbit or external magnetic fields. We find that the topological phase depends on the magnitude and period of the barrier magnetization. The superconducting phase controls the topological transition, which could be detected as a sharp suppression of the supercurrent across the junction.

The polaron in magnetic materials can carry not only charge, but also spin information. Therefore, the polaron dynamics in magnetic material is directly correlated with the spin dynamics. In this work, taking two-dimensional (2D) ${\mathrm{CoCl}}_2$ as a prototypical system, it is found that the 2D transition metal dihalide is an ideal platform for small polaron formation. At zero temperature, three distinct structures of small polarons, with their charges mainly localized on one, two, and three Co atoms, have been identified and designated as polarons I, II, and III, respectively. Polarons II and III are stable at 300 K, and their site-to-site hopping involves the transition to each other. Notable in-plane spin canting is observed when polaron II is formed. Moreover, the photoexcited charge carrier lifetime for the spin-minority channel is distinctly reduced by the spin-polarized polaron state in the band gap, making it four orders of magnitude shorter than the spin-majority channel. The existence of a small polaron in 2D magnetic materials provides a different stratagem to design 2D spintronic and polaronic devices.

We present a theoretical investigation of the stochastic dynamics of a damped particle in a tilted periodic potential with a double well per period. By applying the matrix continued fraction technique to the Fokker-Planck equation in conjunction with the full counting statistics and master equation approaches, we determine the rates of specific processes contributing to the system's overall dynamics. At low temperatures, the system can exhibit one running state and two distinct locked metastable states. We focus primarily on two aspects: the dynamics of positional jumps, which are rare thermally induced particle jumps over potential maxima, and their impact on the overall velocity noise; and the retrapping process, involving the transition from the running to the locked metastable states. We demonstrate the existence of fractional (in units of $2\pi$) positional slips that differ qualitatively from conventional $2\pi$ jumps observed in single-well systems. Fractional positional slips significantly influence the system dynamics even in regimes dominated by dichotomous-like switching between running and locked states. Furthermore, we introduce a simple master equation approach that proves effective in analyzing various stages of the retrapping process. Interestingly, our analysis shows that even for a system featuring a well-developed double-well periodic potential, there exists a broad parameter range where the stochastic dynamics can be accurately described by an effective single-well periodic model. The techniques introduced here allow for valuable insights into the complex behavior of the system, offering avenues for understanding and controlling its steady-state and transient dynamics, which go beyond or can be complementary to direct stochastic simulations.

Mechanisms of spin/charge interconversion like the spin Hall effect can be used to generate and detect spin currents at timescales ranging from dc to subpicosecond. In the dc regime, the best candidates are Rashba interfaces showing the inverse Edelstein effect, with ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ presenting a record conversion efficiency. At picosecond timescales, devices relying on inverse spin Hall effect are efficient THz emitters, but this is less clear for Rashba-Edelstein systems. Here we study the conversion of angular momentum into charge at different timescales in the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. We show that while the effect is exceptionally large in the dc regime, it is reduced by more than four orders of magnitude at the picosecond timescale. This loss of efficiency is discussed in terms of specificities linked to the Rashba state at the interface between these two large band-gap insulators. This has strong implications for the spintronic THz emitters and our work underlines the salient features required for their optimal operation.

A generalized approach derived from Bloch's equation of motion of nuclear magnetic moments is presented to model the frequency, magnetic field, spin density, and temperature dependencies in the electromagnetic permeability tensor for materials with magnetic resonances. The resulting tensor model predicts characteristic polarization signatures which can be observed, for example, in Mueller matrix element spectra measured. When augmented with thermodynamic considerations and suitable Hamiltonian description of the magnetic eigenvalue spectrum, important parameters such as density, spectral amplitude distribution, relaxation time constants, and geometrical orientation parameters of the magnetic moments can be obtained from comparing the generalized model approach to experimental data. We demonstrate our approach by comparing model calculations with full Mueller matrix element spectra measured at an oblique angle of incidence in the terahertz spectral range, across electron spin resonance quintuplet transitions observed in wurtzite-structure GaN doped with iron. Our model correctly predicts the complexity of the polarization signatures observed in the 15 independent elements of the normalized Mueller matrix for both positive and negative magnetic fields and will become useful for future analysis of frequency and magnetic field-dependent magnetic resonance measurements.

Current-driven DW motion in a ferrimagnetic (FiM) nanowire through spin transfer torques (STTs) is explained by the energy-work principle. An adiabatic STT (a-STT) can be incorporated into the Lagrangian as an energy functional and tends to twist DW planes. DWs in homogeneous FiM nanowires can resist an a-STT from moving below a critical value proportional to the maximal transverse anisotropic field. Second, a nonadiabatic STT (na-STT) cannot be included in the Lagrangian and can enter the spin dynamics through the Rayleigh functional. A static DW cannot exist under a na-STT such that a DW in a homogeneous FiM nanowire must propagate under an arbitrarily small na-STT. Third, STTs do positive work on a DW which must compensate by energy dissipated by moving spins inside the DW through damping. Below the Walker breakdown current, the na-STT does positive work and the a-STT does no work. Above the Walker breakdown, a DW starts to precess around the wire axis during its propagation along the axis. Whether the a-STT does a negative or a positive work depends on the direction of the DW precession, while the na-STT still does a positive work. Last, a DW velocity formula is obtained, which agrees with simulations both below and above the Walker breakdown current. In the vicinity of the angular momentum compensation point, the precession frequency of a DW reaches its maximum, and the DW structure is distorted. The DW distortion and spin wave emission modify the DW motion, which deviates from its linear dependence on current density. This theory explains well the observed DW mobility near the angular momentum compensation point of FiM nanowires and resolves the puzzle of the unphysical negative na-STT problem.

In this work, we show that surface anisotropy in nanomagnets induces a nutational motion of their magnetization at various frequencies, the lowest of which can be described by the macrospin model whose dynamics is governed by an effective energy potential. We derive analytical expressions for the precession and nutation frequencies and amplitudes as functions of the size of the nanomagnet and its atomistic parameters, such as the exchange coupling and the onsite anisotropy. Our analytical model predicts a reduction of the precession frequency with increased surface anisotropy. We also simulate the dynamics of the corresponding atomistic many-spin system and compare the results with the effective model. We thereby show that the first nutation mode induced by the finite size and surface anisotropy occurs at a frequency that is four times larger than the precession frequency, thus lending itself to a relatively easy detection by standard experiments of magnetic resonance.

We study the effect of disorder in systems having a nontrivial Euler class. As these recently proposed multigap topological phases come about by braiding non-Abelian charged band nodes residing between different bands to induce stable pairs within isolated band subspaces, novel properties may be expected. Namely, a modified stability and critical phases under the unbraiding to metals can arise when the disorder preserves the underlying $C_2\mathcal{T}$ or $\mathcal{P}\mathcal{T}$ symmetry on average. Employing elaborate numerical computations, we verify the robustness of associated topology by evaluating the changes in the average densities of states and conductivities for different types of disorders. Upon performing a scaling analysis around the corresponding quantum critical points, we retrieve a universality for the localization length exponent of $\nu =1.4\pm 0.1$ for Euler-protected phases, relating to two-dimensional percolation models. We generically find that quenched disorder drives Euler semimetals into critical metallic phases. Finally, we show that magnetic disorder can also induce topological transitions to quantum anomalous Hall plaquettes with local Chern numbers determined by the initial value of the Euler invariant.

We investigate the nonequilibrium dynamics of two multiband boundary-obstructed topological systems (BOTSs) and a higher-order topological superconductor (HOTSC). The singular behavior exhibited by the dynamical free energy indicates the presence of dynamical quantum phase transitions (DQPTs) within these systems. By studying the nonequilibrium dynamics in the BOTS featuring even-parity Cooper pairing, we find that there are DQPTs in this BOTS via quenching from different topological phase regions. The analysis of the energy spectra shows that two previously considered identical topological phases are actually different. Based on this result, we identify that all DQPTs in the BOTS featuring even-parity Cooper pairing result from topology-changing quenches. Moreover, we also study DQPTs in another BOTS featuring odd-parity Cooper pairing, and we obtain similar results to those from the BOTS featuring even-parity Cooper pairing. The appearance of DQPTs in different BOTSs reveals that the occurrence of DQPTs is related to the topology of BOTSs. In addition, we adopt the same method to investigate DQPT in HOTSC for comparison. The numerical results show that DQPT always takes place for quenches between topological and trivial phases. This is completely different from the results of BOTSs, for which not all topology-changing quenches are accompanied by DQPTs. The study of DQPTs unveils the apparent difference between BOTS and HOTSC, and thus our study provides an alternative approach to revealing the topological nature of novel higher-order topological systems, which does not rely on the specific definitions of topological invariants.

A giant narrowing of the energy gap in the nonlinear optical crystal $\mathrm{ZnGe}{\mathrm{P}}_2$ (ZGP) after the incorporation of manganese atoms into the chalcopyrite crystal lattice has been detected. The (Zn,Mn)$\mathrm{Ge}{\mathrm{P}}_2$ (ZMGP) thin films grown by solid-phase endotaxy on ZGP (001) substrate share the tetrahedrally coordinated structure of chalcopyrite. The measured fine crystal structure and fundamental optical properties point to a semiconductor substance. However, other properties, such as electrical conductivity, current transport across the energy barrier ZMGP/ZGP, and photoemission of holes from the ferromagnetic ZMGP layer, claim that this substance is a metal. We investigated this unusual combination of properties in ZMGP using comprehensive measurements, including detailed chemical composition at the surface and in depth, and obtained direct experimental evidence for half-metallicity (HM) in the Mn-rich ZMGP material. Existing theories agree with these experimental proofs obtained. We discuss possible emitters of spin-polarized holes on these HM films operating at room temperature in fully diamondlike structures.

${\mathrm{EuCd}}_2{\mathrm{P}}_2$, a layered phosphide crystal, exhibits a nonconventional magnetoresistive trend that deviates from typical material behavior. This unique characteristic prompts further intriguing and complex investigation. Herein, we utilize critical exponent analysis, angle-resolved photoemission spectroscopy (ARPES), and density functional theory to uncover physical insights into ${\mathrm{EuCd}}_2{\mathrm{P}}_2$. Magnetization analysis reveals a persistent ferromagnetic phase above the antiferromagnetic phase, extending to higher temperatures at high magnetic fields. Critical behavior analysis of $H\parallel ab$ reveals critical exponents $\beta =$ 0.2432(2), $\gamma =$ 0.883(2), and $\delta =$ 4.63(1), aligning with a three-dimensional tricritical mean-field model coupled with long-range interaction ($\sigma$ = 1.53). The $H\text{−}T$ phase diagram establishes a tricritical point ($H$ = 620 Oe, $T$ = 18.3 K), signifying diverse spin interactions and magnetic phases. Temperature-dependent ARPES discloses significant modifications in the electronic band spectrum of ${\mathrm{EuCd}}_2{\mathrm{P}}_2$ during the spin ordering transitions, characterized by a substantial band gap alteration and valence band splitting. Remarkably, the semiconducting properties of ${\mathrm{EuCd}}_2{\mathrm{P}}_2$ remain unaffected throughout these transitions. First-principles simulations unveil a near-degenerate coexistence of layer-antiferromagnetic and -ferromagnetic phases, significant valence band modifications in CdP due to distinct proximity effects induced by the Eu lattice's spin-magnetic moments, and the half-metallic phases in ferromagnetic ${\mathrm{EuCd}}_2{\mathrm{P}}_2$ crystals. These findings advance our understanding of magnetic and electronic properties in ${\mathrm{EuCd}}_2{\mathrm{P}}_2$.

We report on the origin of spin freezing in the disordered fluorite ${\mathrm{Ho}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. The system is investigated by low-temperature heat capacity as well as by dc and ac magnetization. While the system does not show a long-range magnetic order down to at least $T=280$ mK, we observe signatures of slow spin dynamics, magnetic-field-induced relaxation processes at relatively high temperatures, and a spin-frozen state below $T=0.6$ K. Our results suggest that similar to the canonical spin-ice systems ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ [J. Phys.: Condens. Matter 16, S635 (2004)] and ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ [Nature (London) 413, 48 (2001)], spin freezing in ${\mathrm{Ho}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ is preceded by two slow spin-relaxation processes; the first forms a field-induced region extending to at least 18 K and the second is rather field independent and appears at $T_{\mathrm{g}2}=1$ K [J. Ramón, Ph.D. thesis, Universidade de São Paulo (2020)].

We report the magnetic, magnetoelastic, and magnetotransport properties of single crystals of polar magnets $R\mathrm{AuGe}$ $(R=\mathrm{Dy}, \mathrm{Ho}, \text{and} \mathrm{Gd})$, grown by Au-Ge self-flux. Magnetization and magnetostriction measurements reveal multistep metamagnetic transitions for the $c$-axis magnetic field $(H\parallel c)$ for DyAuGe and HoAuGe, suggesting magnetic frustration in the triangular lattice of $R$ ions. The magnetic phase diagrams have clarified a close connection between the magnetoelastic property and the emergence of the intermediate metamagentic phase. The magnetic-field dependence of the resistivity and Hall resistivity reveal the semimetallic transport dominated by hole-type carriers, consistent with the behavior in a nonmagnetic analog YAuGe. We also identify a signature of an anomalous Hall effect (AHE) proportional to the field-induced magnetization in $R=\mathrm{Dy}$, Ho, and Gd. GdAuGe shows magnetic and transport behavior as reported in a previous study using Bi-flux grown single crystals, while the self-flux grown crystal shows larger magnetoresistance $(\sim 345\%, \text{at} 1.8 \mathrm{K} \text{and} 9 \mathrm{T})$ due to higher hole-type carrier mobility $[\sim 6400{\mathrm{cm}}^2/\left(\mathrm{V}\mathrm{s}\right)]$. Using the two-band model analysis considering the mobility change during the magnetization process, we extract the anomalous Hall conductivity: $\sim 1200$ and $\sim 530$ S/cm for $R=\mathrm{Dy} \text{and} \mathrm{Ho}$, respectively, at 1.8 K with 9 T for $H\parallel c$. The magnitude of conductivity suggests a contribution of intrinsic origin, possibly related to the Berry curvature in the electron bands induced by the time-reversal symmetry breaking and the polar lattice.

We study quantum criticality in the infinite range transverse-field Ising model. We find subtle differences with respect to the well-known single-site mean-field theory, especially in terms of gap, entanglement and quantum criticality. The calculations are based on numerical diagonalization of Hamiltonians with up to a few thousand spins. This is made possible by the enhanced symmetries of the model, which divide the Hamiltonian into many block-diagonal sectors. The finite temperature phase diagram and the characteristic jump in heat capacity closely resemble the behavior in mean-field theory. However, unlike mean-field theory where excitations are always gapped, the excitation gap in the infinite range model goes to zero from both the paramagnetic side and from the ferromagnetic side on approach to the quantum critical point. Also, contrary to mean-field theory, at the quantum critical point the quantum Fisher information becomes large, implying long-range multipartite entanglement. We find that the main role of temperature is to shift statistical weights from one conserved sector to another. However, low energy excitations in each sector arise only near the quantum critical point implying that low energy quantum fluctuations can arise only in the vicinity of the quantum critical field where they can persist up to temperatures of order the exchange constant.

Density oscillations in quantum fluids can reveal their fundamental characteristic features. In this work, we study the density oscillation of incompressible fractional quantum Hall (FQH) fluids created by flux insertion. For the model Laughlin state, we find that the complex oscillations seen in various density profiles in real space can be universally captured by a simple damped oscillator model in the occupation-number space. It requires only two independent fitting parameters or characteristic length scales: the decay length and the oscillation wave number. Realistic Coulomb quasiholes can be viewed as Laughlin quasiholes dressed by magnetorotons, which can be modeled by a generalized damped oscillator model. Our work reveals the fundamental connections between the oscillations seen in various aspects of FQH fluids such as in the density of quasiholes, the edge, and the pair correlation function. The presented model is useful for the study of quasihole sizes for their control and braiding in experiments and large-scale numerical computation of variational energies.

With the emergence of rich topological phases in artificial photonic crystals (PCs), numerous peculiar physical phenomena occur within domain walls (DWs) constructed by PCs with different topological phases. These phenomena include robust topological edge states (TESs), strongly localized topological corner states (TCSs), the quantum spin Hall effect, the quantum valley Hall effect, filling anomalies, and fractional charges at corners and edges. In this paper, we propose triangular lattice PCs composed of six triangle-shaped silicon rods to form two pairs of interconvertible DWs under two types of perturbations. We show that topological phase transitions occur with perturbations affecting 3 out of 6 rods. The first perturbation opens a band gap at the Γ point, supporting a pair of pseudospin helical edge states. The second perturbation opens another band gap at the K point, supporting valley edge states while preserving the initial topological band gap. We find that all designed topological PCs exhibit Wannier centers partially deviating from the center, causing different filling anomalies, thereby marking PCs with different topological indices corresponding to distinct higher-order topological phases. The results from mode charge distribution, calculated by integrating the local density of states (LDOS), are consistent with the Wannier center position analysis. By analyzing the LDOS under different boundary conditions at the corners and edges, we uncover significant disparities in the distribution of TES and TCS. Intriguingly, under certain conditions, the corner states can hybridize with edge states or even with bulk states. The rich topological physics presented not only provides insights into topological phases but also opens avenues for engineering topological states with potential applications.