We theoretically show that the carrier transport in a monolayer ${\text{Fe}}_3{\text{GeTe}}_2$ experiences a transition between anomalous and spin Hall effects when the spin polarization of disorders switches between out-of-plane and in-plane. These Hall effects are allowed when the magnetization is polarized in-plane, breaking the ${\mathcal{C}}_3$ rotation symmetry. The transition originates from the selection rule of spin scattering, the strong spin-orbit coupling, and the van Hove singularities near the Fermi surface. The scattering selection rule tolerates the sign change of the disorder spin, which provides a convenient method to detect the switching of antiferromagnetic insulators regardless of the interfacial roughness in a heterostructure. This provides a convenient platform for the study of 2D spintronics through various van der Waals heterostructures.

We elucidate the role of the $R$-site magnetic moment in structural, electrical transport, and magnetization in three pyrochlore iridates; ${\mathrm{Dy}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7, {\left({\mathrm{Dy}}_{0.5}{\mathrm{Gd}}_{0.5}\right)}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, and ${\mathrm{Gd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, where ${\mathrm{Dy}}^{3+}$ has a higher magnetic moment than ${\mathrm{Gd}}^{3+}$. We report a semiconducting to Mott insulating transition when the compound transforms from a paramagnetic to an antiferromagnetic all-in/all-out state at temperature $T_N$, attributed to ${\mathrm{Ir}}^{4+}$ ordering. In ${\mathrm{Dy}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, the temperature-dependent irreversible magnetization $M_{\mathrm{irr}}$ data crossed over from negative values at a low magnetic field to positive values at high field levels below the temperature $T_N$, indicating magnetostriction arising from the coupling of the ordered moment with the lattice. Raman spectroscopy of the $T_{2g}^4$ stretching and $A_{1g}$ bending modes around $T_N$ reveals the spin-phonon coupling in these compounds. Specifically, in ${\mathrm{Dy}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, the full width at half maximum (FWHM) of Raman modes remains constant near $T_N$, suggesting magnetostriction in addition to spin-phonon coupling. However, in ${\left({\mathrm{Dy}}_{0.5}{\mathrm{Gd}}_{0.5}\right)}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Gd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, where ${\mathrm{Gd}}^{3+}$ substitutes for ${\mathrm{Dy}}^{3+}$, no evidence of magnetostriction is observed in either magnetization or Raman data, although an increase in the FWHM of Raman modes near $T_N$ suggests only the presence of weak spin-phonon coupling. Furthermore, the temperature-dependent lattice constant confirms the presence of magnetostriction in ${\mathrm{Dy}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

Excitation by a femtosecond laser pulse is an efficient method to control various ultrafast spin manipulation processes, e.g., ultrafast demagnetization, remagnetization, and damping. Thus, a comprehensive study of magnetization dynamics over a broad timescale by varying the excitation energy can provide a solid foundation of integrating these processes within a single system. Here, time-resolved magneto-optical Kerr effect magnetometry is used to investigate the evolution of magnetization dynamics from femtosecond- to nanosecond timescale in ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ thin films. The correlation between ultrafast demagnetization time and fast remagnetization time with the laser pump fluence is demonstrated experimentally and is validated by the phenomenological three-temperature modeling involving different coupling parameters and specific heat of electron-, lattice-, and spin baths. Significant enhancement of the Gilbert damping parameter and a redshift in the precessional frequency are observed for uniform Kittel mode and perpendicular standing spin-wave modes with increasing excitation energy. Direct correlation between demagnetization time and Gilbert damping parameter indicates spin-flip scattering as the responsible mechanism for the ultrafast demagnetization and intraband conductivity-like contribution to the damping process. This extensive study develops an in-depth understanding about the role of laser fluence on the ultrafast spin dynamics. Our findings will be beneficial for the construction of ultrahigh-speed spintronics devices which rely on the heat-assisted modification of the dynamic properties in ferromagnetic thin films.

In ferromagnetic systems lacking inversion symmetry, an applied electric field can control the ferromagnetic order parameters through the spin-orbit torque. The prototypical example is a bilayer heterostructure composed of a ferromagnet and a heavy metal that acts as a spin current source. In addition to such bilayers, spin-orbit coupling can mediate spin-orbit torques in ferromagnets that lack bulk inversion symmetry. A recently discovered example is the two-dimensional monolayer ferromagnet ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. In this paper, we use first-principles calculations to study the spin-orbit torque and ensuing magnetic dynamics in this material. By expanding the torque versus magnetization direction as a series of vector spherical harmonics, we find that higher order terms (up to $\ell =4$) are significant and play important roles in the magnetic dynamics. They give rise to deterministic, magnetic field-free electrical switching of perpendicular magnetization.

ABC-stacked multilayer graphene (ABC-MLG) exhibits topological surface flat bands with a divergent density of states, leading to many-body instabilities at charge neutrality. Here, we explore electronic ordering within a mean-field approach with full generic treatment of all spin-isotropic, two-site charge density and spin interactions up to next-nearest-neighbor (NNN) sites. We find that surface superconductivity and magnetism are significantly enhanced over bulk values. We find spin-singlet $s$ wave and unconventional NNN bond spin-triplet $f$ wave to be the dominant superconducting pairing symmetries, both with a full energy gap. By establishing the existence of ferromagnetic intrasublattice interaction ($J_2<0$) we conclude that the $f$-wave state is favored in ABC-MLG, in sharp contrast to bulk ABC-graphite where chiral $d$- or $p$-wave states, together with $s$-wave states, display stronger ordering tendencies albeit not achievable at charge neutrality. We trace this distinctive surface behavior to the strong sublattice polarization of the surface flat bands. We also find competing ferrimagnetic order, fully consistent with density functional theory (DFT) calculations. The magnetic order interpolates between sublattice ferromagnetism and antiferromagnetism, but only with the ratio of the sublattice magnetic moments ($R$) being insensitive to the DFT exchange correlation functional. We finally establish the full phase diagram by constraining the interactions to the $R$ value identified by DFT. We find $f$-wave superconductivity being favored for all weak to moderately strong couplings $J_2$ and as long as $J_2$ is a sufficiently large part of the full interaction mix. Gating ABC-MLG away from charge neutrality further enhances the $f$-wave state over the ferrimagnetic state, establishing ABC-MLG as a strong candidate for $f$-wave superconductivity.

In this work, we map the phase diagrams of one-dimensional quasiperiodic models using artificial neural networks. We observe that the multiclass classifier precisely distinguishes the various phases, namely the delocalized, multifractal, and localized phases, when trained on the eigenstates of the long-range Aubry-André Harper (LRH) model. Additionally, when this trained multilayer perceptron is fed with the eigenstates of the Aubry-André Harper (AAH) model, it identifies various phases with reasonable accuracy. We examine the resulting phase diagrams produced using a single disorder realization and demonstrate that they are consistent with those obtained from the conventional method of fractal dimension analysis. Interestingly, when the neural network is trained using the eigenstates of the AAH model, the resulting phase diagrams for the LRH model are less exemplary than those previously obtained. Further, we study binary classification by training the neural network on the probability density corresponding to the delocalized and localized eigenstates of the AAH model. We are able to pinpoint the critical transition point by examining the metric “accuracy” for the central eigenstate. The effectiveness of the binary classifier in identifying a previously unknown multifractal phase is then evaluated by applying it to the LRH model.

The Quantum Hall Bilayers (QHB) at filling factor $\nu =1$ represents a competition between Bose-Einstein condensation (BEC) at small distances between layers and fermionic condensation, whose influence grows with distance and results in two separate Fermi liquid states for the underlying quasiparticles at very large (or infinite) distances. The question that can be raised is whether, at intermediate distances between layers, a distinct phase exists or if a singular transition occurs, with the possibility that this happens at infinite distances. Here, using a dipole representation for fermionic quasiparticles, we find support for the latter scenario: Within a large and relevant range of distances, BEC condensation, identified as Cooper $s$-wave pairing of dipole quasiparticles, prevails over both Cooper $p$-wave pairing and $s$-wave excitonic pairing of the same quasiparticles.

We investigate optically induced magnetization in Floquet-Weyl semimetals generated by irradiation of a circularly polarized continuous-wave laser from the group II-V narrow gap semiconductor ${\mathrm{Zn}}_3{\mathrm{As}}_2$ in a theoretical manner. Here, this trivial and nonmagnetic crystal is driven by the laser with a nearly resonant frequency with a band gap to generate two types of Floquet-Weyl semimetal phases composed of different spin states. These two phases host nontrivial two-dimensional surface states pinned to the respective pairs of the Weyl points. By numerically evaluating the laser-induced transient carrier dynamics, it is found that both spins are distributed in an uneven manner on the corresponding surface states due to significantly different excitation probabilities caused by the circularly polarized laser with the nearly resonant frequency. It is likely that such spin-polarized surface states produce surface magnetization and furthermore the inverse Faraday effect also contributes almost as much as the spin magnetization. To be more specific, excited carriers with high density of the order of ${10}^{21}{\mathrm{cm}}^{-3}$ are generated by the laser with electric field strength of a few MV/cm to result in the surface magnetization that becomes asymptotically constant with respect to time, around 1 mT. The magnitude and the direction of it depend sharply on both the intensity and frequency of the driving laser, which would be detected by virtue of the magneto-optic Kerr effect.

The shift current is part of the second-order optical response of materials with a close connection to topology. Here we report a sign inversion in the band-edge shift photoconductivity of the Haldane model when the system undergoes a topological phase transition. This result is obtained following two complementary schemes. On one hand, we derive an analytical expression for the band-edge shift current in a two-band tight-binding model showing that the sign reversal is driven by the mass term. On the other hand, we perform a numerical evaluation on a continuum version of the Haldane model. This approach allows us to include off-diagonal matrix elements of the position operator, which are discarded in tight-binding models but can contribute significantly to the shift current. Explicit evaluation of the shift current shows that while the model predictions remain accurate in the deep tight-binding regime, significant deviations arise for shallow potential landscapes. Notably, the sign reversal across the topological phase transition is observed in all regimes, implying it is a robust effect that could be observable in a wide range of topological insulators.

We investigate how the breaking of charge conjugation symmetry $\mathcal{C}$ impacts on the dynamics of a half-filled fermionic lattice system after global quenches. We show that, when the initial state is insulating and the $\mathcal{C}$ symmetry is broken nonlocally by a constant magnetic flux, local observables, and correlations behave as if the symmetry were unbroken for a time interval proportional to the system size $L$. In particular, the local particle density of a quenched dimerized insulator remains pinned to $1/2$ in each lattice site for an extensively long time, while it starts to significantly fluctuate only afterwards. Due to its qualitative resemblance to the sudden arrival of rapidly rising ocean waves, we dub this phenomenon the “tsunami effect.” Notably, it occurs even though the chiral symmetry is dynamically broken right after the quench. Furthermore, we identify a way to quantify the amount of symmetry breaking in the quantum state, showing that in insulators perturbed by a flux, it is exponentially suppressed as a function of the system size, while it is only algebraically suppressed in metals and in insulators with locally broken $\mathcal{C}$ symmetry. The robustness of the tsunami effect to weak disorder and interactions is demonstrated, and possible experimental realizations are proposed.

We present an exact analytical solution for quantum strong long-range models in the canonical ensemble by extending the classical solution proposed in Campa et al. [J. Phys. A: Math. Gen. 36, 6897 (2003)]. Specifically, we utilize the equivalence between generalized Dicke models and interacting quantum models as a generalization of the Hubbard-Stratonovich transformation. To demonstrate our method, we apply it to the Ising chain in transverse field and discuss its potential application to other models, such as the Fermi-Hubbard model, combined short- and long-range models, and models with antiferromagnetic interactions. Our findings indicate that the critical behavior of a model is independent of the range of interactions, within the strong long-range regime, and the dimensionality of the model. Moreover, we show that the order-parameter expression is equivalent to that provided by mean-field theory, thus confirming the exactness of the latter. Finally, we examine the algebraic decay of correlations and characterize its dependence on the range of interactions in the full phase diagram.

We carry out a sign-problem-free quantum Monte Carlo calculation of a bilayer model with a repulsive intralayer Hubbard interaction and a ferromagnetic interlayer interaction. The latter breaks the global $SU\left(2\right)$ spin rotational symmetry but preserves a $U\left(2\right)\times U\left(2\right)$ invariance under mixing of same-spin electrons between layers. We show that despite the difference in symmetry, the bilayer model exhibits the same qualitative features found in the single-layer Hubbard model. These include stripe phases, whose nature is sensitive to the presence of next-nearest-neighbor hopping, a maximum in the Knight shift that moves to lower temperatures with increasing hole doping, and lack of evidence for intralayer $d$-wave superconductivity. Instead, we find a superconducting phase, coexisting with stripes, whose critical temperature traces a dome as a function of doping and is due to interlayer spin-polarized pairing that is induced by the ferromagnetic interaction.

In this paper, we systematically investigate impurity-induced bound states in one-dimensional non-Hermitian systems. By establishing the relationship between bound-state energy and the requisite impurity potential, we conveniently construct an impurity potential diagram corresponding to point gaps. This diagram indicates both the minimal impurity potential required to generate bound states within each point gap and the distribution of bound states across these point gaps for a given impurity potential. From this, we reveal that a finite impurity potential is required to generate bound states in the absence of Bloch saddle points; otherwise, even a negligible impurity potential can yield bound states. Additionally, we show that bound states in point gaps with nonzero spectral winding numbers are sensitive to boundary conditions and abruptly shift to the edges upon opening the boundaries, signifying the bulk-boundary correspondence in point-gap topology.

Enhancing robustness of topological orders against perturbations is one of the main goals in topological quantum computing. Since the kinetic of excitations is in conflict with the robustness of topological orders, any mechanism that reduces the mobility of excitations will be in favor of robustness. A strategy in this direction is adding frustration to topological systems. In this paper we consider a frustrated toric code on a kagome lattice, and show that although increasing the strength of perturbation reduces the topological order of the system, it cannot destroy it completely. Our frustrated toric code is indeed a quantum loop gas model with string tension and pressure which their competition leads to a partially topological phase (PTP) in which the excitations are restricted to move in particular sublattices. In this phase the ground state is a product of many copies of fluctuating loop states corresponding to quasi-one-dimensional ladders. By defining a nonlocal matrix order parameter and studying the behavior of ground-state global entanglement, we distinguish the PTP from the standard topological phase. The partial mobility of excitations in our system is reminiscent of fracton codes with restricted mobility, and therefore our results propose an alternative way for making such a restriction in three dimensions.

In this paper, we consider the possibility and conditions for pair photoemission, whereby two incident photons emit pairs of electrons from a candidate material, as a method to measure and visualize electronic correlations. As opposed to double photoemission—where a single photon precipitates the ejection of a pair of electrons via a subsequent electron energy loss scattering process—we show that pair photoemission need not be limited to interference between initial photoelectrons and valence electrons and moreover can occur without the energy penalty of two work functions. This enables detection of pairs of electrons at high-energy resolution that may be correlated in the same quantum many-body states.

In this paper, an effective scattering-potential approach is proposed for treating interface-roughness scattering of drifting electrons within a superlattice. By utilizing established effective scattering potentials, a quasi-one-dimensional generalized Boltzmann transport equation is developed after a self-consistent internal scattering force is included. This generalized Boltzmann transport equation is solved exactly afterwords beyond the relaxation-time approximation. Furthermore, the dependence of the resulting steady-state current on interface-roughness parameters is analyzed at various temperatures and dc electric-field strengths by utilizing the calculated nonequilibrium electron occupation function. Meanwhile, the microscopic mechanism behind non-ohmic transport behavior is revealed by analyzing numerically the computed nonequilibrium electron occupation function and its dependence on interface roughness parameters as well as the dc electric field strength.

Geometrical frustration in correlated systems can give rise to a plethora of ordered states and intriguing phases. Here, we theoretically analyze vertex-sharing frustrated Kagome lattices of Josephson junctions and identify various classical and quantum phases. The frustration is provided by periodically arranged 0- and $\pi$-Josephson junctions. In the frustrated regime, the macroscopic phases are composed of different patterns of vortices/antivortices penetrating each basic element of the Kagome lattice, i.e., a superconducting triangle interrupted by three Josephson junctions. We obtain that numerous topological constraints, related to the flux quantization in any hexagon loop, lead to highly anisotropic and long-range interaction between well separated vortices/antivortices. Considering this interaction and a possibility of macroscopic tunneling between vortex and antivortex in single superconducting triangles, we derive an effective Ising-type spin Hamiltonian with strongly anisotropic long-range interaction. In the classically frustrated regime, we numerically calculate the temperature-dependent spatially averaged spin polarization $\overline{m}\left(T\right)$ characterizing the crossover between the ordered and disordered vortex/antivortex states. In the coherent quantum regime, we analyze the lifting of the degeneracy of the ground state and the appearance of the highly entangled states.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are considered highly promising platforms for next-generation optoelectronic devices. Owing to its atomically thin structure, device performance is strongly impacted by a minute amount of defects. Although defects are usually considered to be disturbing, defect engineering has become an important strategy to control and design new properties of 2D materials. Here, we produce single S vacancies in a monolayer of ${\mathrm{MoS}}_2$ on Au(111). Using a combination of scanning tunneling and atomic force microscopy, we show that these defects are negatively charged and give rise to a Kondo resonance, revealing the presence of an unpaired electron spin exchange coupled to the metal substrate. The strength of the exchange coupling depends on the density of states at the Fermi level, which is modulated by the moiré structure of the ${\mathrm{MoS}}_2$ lattice and the Au(111) substrate. In the absence of direct hybridization of ${\mathrm{MoS}}_2$ with the metal substrate, the S vacancy remains charge neutral. Our results suggest that defect engineering may be used to induce and tune magnetic properties of otherwise nonmagnetic materials.