The light-induced femtosecond dynamics of the sublattice magnetizations in the antiferromagnetically ordered phase of the semiconductor $\alpha \text{−}\mathrm{MnTe}$ is investigated theoretically as a function of an external driving field. The electromagnetic field is coupled to optical modes and the concomitant atomic displacements modulate the Heisenberg exchange couplings. We derive the equations of motion for the time-dependent sublattice magnetization in spin wave theory and analyze the contributions from the driven magnon modes. The antiferromagnetic order parameter exhibits coherent longitudinal oscillations determined by the external driving frequency which decay due to dephasing. Including a phenomenological dissipative term to mimic spin-lattice relaxation processes leads to relaxation back to thermal equilibrium. We provide approximate analytic solutions of the resulting differential equations which allow us to understand the effect of the driving light pulse on the amplitude, frequency, and lifetime of the coherent spin dynamics.

Long-range entanglement in quantum spin liquids (QSLs) leads to novel low-energy excitations with fractionalized quantum numbers and (in two dimensions) statistics. Experimental detection and manipulation of these excitations present a challenge particularly in view of diverse candidate magnets. A promising probe of fractionalization is their coupling to phonons. Here, we present Raman scattering results for the $S=1/2$ honeycomb iridate ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$, a candidate Kitaev QSL with fractionalized Majorana fermions and Ising flux excitations. We observe anomalous low-temperature frequency shift and linewidth broadening of the Raman intensities in addition to a broad magnetic continuum, both of which, as we derive, are naturally attributed to the phonon decaying into itinerant Majoranas. The dynamic Raman susceptibility marks a crossover from the QSL to a thermal paramagnet at $\sim 120$ K. The phonon anomalies below this temperature demonstrate a strong phonon-Majorana coupling. These results provide evidence of spin fractionalization in ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$.

Time series measurements [G. Shaw et al., Phys. Rev. B 85, 174517 (2012); B. Bag et al., Sci. Rep. 7, 5531 (2017)] in $2H\text{−}{\mathrm{NbS}}_2$ crystal had unraveled a drive-induced transition wherein the critical current $\left(I_c\right)$ changes from a low to a high $I_c$ jammed vortex state, via a negative differential resistance (NDR) transition. Here, using multiple current-voltage $\left(I\text{−}V\right)$ measurement cycles, we explore the statistical nature of observing the NDR (or a quasi-NDR in reversing $I$ measurements) transition in the free-flux-flow (FF) regime in a single crystal of $2H\text{−}{\mathrm{NbS}}_2$. Prior to the occurrence of the NDR transition, the pristine full $I\text{−}V$ curve exhibits a featureless smooth depinning from the low $I_c$ state. With subsequent current cycling, the NDR transition appears in the $I\text{−}V$ curve. Post-NDR, the full $I\text{−}V$ curve is seen to be noisy with depinning commencing from the higher $I_c$ state. The probability of observing the NDR transition always remains finite for a vortex state created with either fast or slow rate of magnetic field, $\dot{B}$. The probability of observing the NDR transition in the FF regime is found to systematically increase with magnetic field $\left(B\right)$ in weak collective pinning regime. In the strong pinning regime, the said probability becomes field independent. Retaining of a nonzero probability for the occurrence of the NDR transition under all conditions, the observed new data shows that the $I\text{−}V$ branch with higher $I_c$ is the more stable compared to the lower $I_c$ branch. We show that the higher $I_c$ state, generated via the NDR transition, is unique and cannot be accessed via any conventional route, in particular, by preparing the static vortex state with a different thermomagnetic history. While the $I\text{−}V$ curves do not distinguish between zero field cooled (ZFC) and field cooled (FC) modes of preparing the vortex state, the probability for observing an NDR transition has different $B$ dependences for the vortex matter prepared in the ZFC and FC modes. We find that the NDR transition occurs in a high dissipation regime, where the flow resistivity is well above the theoretical value expected in the FF regime. We understand our results on the basis of a rapid drop in vortex viscosity at high drives in $2H\text{−}{\mathrm{NbS}}_2$, which triggers a rapid increase in the vortex velocity and reorganization in the moving vortex matter leading to a dynamical unstable vortex flow. This dynamical instability leads to the NDR transition into a high entropy vortex state with high $I_c$.

Topological characterization of non-Hermitian band structures demands more than a straightforward generalization of the Hermitian cases. Even for one-dimensional tight-binding models with nonreciprocal hopping, the appearance of point gaps and the skin effect leads to the breakdown of the usual bulk-boundary correspondence. Luckily, the correspondence can be resurrected by introducing a winding number for the generalized Brillouin zone for systems with an even number of bands and chiral symmetry. Here, we analyze the topological phases of a nonreciprocal hopping model on the stub lattice, where one of the three bands remains flat. Due to the lack of chiral symmetry, the biorthogonal Zak phase is no longer quantized, invalidating the winding number as a topological index. Instead, we show that a $Z_2$ invariant can be defined from Majorana's stellar representation of the eigenstates on the Bloch sphere. The parity of the total azimuthal winding of the entire Majorana constellation correctly predicts the appearance of edge states between the bulk gaps. We further show that the system is not a square-root topological insulator, despite the fact that its parent Hamiltonian can be block diagonalized and related to a sawtooth lattice model. The analysis presented here may be generalized to understand other non-Hermitian systems with multiple bands.

We investigate thermally driven transport of heat and charge in a superconducting single-electron transistor by means of a real-time diagrammatic transport theory. Our theoretical approach allows us to account for strong Coulomb interactions and arbitrary nonequilibrium conditions while performing a systematic expansion in the tunnel coupling. We find that a temperature bias across the system gives rise to finite heat and charge currents close to the particle-hole symmetric point which depend both on the gate voltage as well as on the phase difference between the superconducting reservoirs. The finite thermoelectric effect arises due to level renormalization from virtual tunneling processes. Furthermore, we find that the phase bias can give rise to finite charge currents even in the presence of an inversion-symmetric temperature bias.

Based on the tight-binding formalism, we study the effect of side potential on the spin- and valley-related electronic property of $2\mathrm{D}$ honeycomb lattices with intrinsic spin-orbit coupling, such as silicene and germanene. The side potential is composed of potential field and exchange field applied on the boundaries of the zigzag nanoribbon. It is found that the side potential could greatly affect the helical edge states with different spin indices and the spin and valley are locked to each other. By adjusting the side potential and ribbon width, the system shows a quantum spin-valley Hall effect, valley-polarized quantum spin Hall effect, and spin-polarized quantum anomalous Hall effect. Due to the side potential and the coupling of edge states in the narrow ribbon, a band gap could be opened for specific spin and the time-reversal symmetry could be broken, leading to a spin-polarized quantum anomalous Hall phase. Various kinds of spin-valley polarized edge states are formed at the two boundaries. Furthermore, the spin-valley polarized insulating states can be used to realize a perfect spin-valley switch.

Novel quantum phenomena, including high-temperature superconductivity, topological properties, and charge/spin density waves, appear in low-dimensional conductive materials. It is possible to artificially create low-dimensional systems by fabricating ultrathin films, quantum wires, or quantum dots with flat interfaces. We show that extraordinary metal-insulator transitions that oscillate depending on the scale occur in ${\mathrm{CaRuO}}_3$ films with a thickness of around several unit cells. We grow high-crystalline ${\mathrm{CaRuO}}_3$ ultrathin films, whose surface roughness is controlled at 199 pm, by molecular beam epitaxy. We observe that resistivity oscillates with a “magic” thickness of 25 Å, and changes by three and nine orders of magnitude at room temperature and at low temperature, respectively. These changes are much larger than quantum size effects. We also confirm the same periodicity with photoelectron spectroscopy by etching the ultrathin film. Considering the large energy, periodicity, and anisotropy, we suggest that the oscillating transitions originate from the commensurability of Mott insulation triggered by Peierls instability.

Machine learning opens new avenues for modeling correlated materials. Quantum embedding approaches, such as dynamical mean-field theory (DMFT), provide corrections to first-principles calculations for strongly correlated materials, which are poorly described at lower levels of theory. Such embedding approaches are computationally demanding on classical computing architectures, and hence remain restricted to small systems, which limits the scope of applicability without exceptional computational resources. Here we outline a data-driven machine-learning process for solving the Anderson impurity model (AIM)—the central component of DMFT calculations. The key advance is the use of an ensemble error-correction approach to generate fast and accurate solutions of AIM. An example calculation of the Mott transition using DMFT in the single band Hubbard model is given as an example of the technique, and is validated against the most accurate available method. This approach is called data-driven dynamical mean-field theory (${\mathbf{d}}^3\mathbf{MFT}$).

Both radical-lanthanide exchange coupling and crystal-field tunnel splitting are responsible for the ground to first-excited Kramers doublet energy gap $\mathrm{\Delta }{E}_{\text{KD}}$ in an Ising $4f$ single-ion magnet coupled to a noninnocent ligand. However, these two mechanisms cannot be easily disentangled via spectroscopy alone because their interplay results in a single absorption line. Here we present a microscopic theory for the continuous measurement of the dissipative Landau-Zener transitions recently discussed for a ${\mathrm{TbPc}}_2$ spin transistor [Phys. Rev. Lett. 118, 257701 (2017)] and show how this quantum transport experiment can be used to disentangle the magnetic coupling and the tunnel splitting contributions to $\mathrm{\Delta }{E}_{\text{KD}}$. In addition, our model elucidates the microscopic origin of the decoherent spin tunneling observed in the ${\mathrm{TbPc}}_2$ spin transistor, by explicitly exposing the role played by phonon-mediated spin relaxation and sequential electron transport through the noninnocent ligand, in a time-dependent magnetic field.

The recent observations of nonlinear Hall effect in time-reversal symmetry protected systems and on the surface of three-dimensional topological insulators due to an in-plane magnetic field have attracted immense experimental and theoretical investigations in two-dimensional transition metal dichalcogenides and Weyl semimetals. The origin of this type of second order effect has been attributed to the emergence of a Berry curvature dipole, which requires a low-symmetry environment. Here, we propose a mechanism for generating such a second order nonlinear Hall effect in Kane-Mele two-dimensional topological insulators due to spatial and time reversal symmetry breaking in the presence of Zeeman and Rashba couplings. By actively tuning the energy gaps with external electromagnetic fields we also demonstrate that the nonlinear Hall effect shows remarkable signatures of topological phase transitions existing in the considered two-dimensional systems.

We introduce an entanglement entropy analysis to quantitatively identify the confinement and deconfinement of the spinons in the spin excitations of quantum magnets. Our proposal is implemented by the parton construction of a honeycomb-lattice antiferromagnet exhibiting high-energy anomalous spectra. To obtain the quasiparticles of spin excitations for entanglement entropy calculations, we develop an effective Hamiltonian using the random phase approximation. We elaborate quantitatively the deconfinement-to-confinement transition of spinons in the anomalous spectra with the increase of the Hubbard interaction, indicating the avoided fractionalization of magnons in the strong interaction regime. Meanwhile, the Higgs mode at the ${\mathrm{\Gamma }}^{\prime }$ point is fractionalized into four degenerate spinons, although it appears as a sharp well-defined peak in the spectra. Our work extends our understanding of the deconfinement of the spinon and its coexistence with the magnon in quantum magnets.

We use the density matrix renormalization group method to study a one-dimensional chain with Peierls electron-phonon coupling, which describes the modulation of the electron hopping by lattice distortions. We demonstrate that this system is stable against phase separation in the dilute density limit. We only find phase separation numerically for large couplings for which the linear approximation for the electron-phonon coupling becomes invalid; this behavior can be stabilized in a narrow sliver of the physical parameter space if the dispersion of the phonons is carefully tuned. These results indicate that in the dilute electron density limit, Peierls bipolaron liquids are generically stable, unlike in other models of electron-phonon coupling. We show that this behavior extends to finite carrier concentrations of up to quarter filling. This stability of low-density, light-mass bipolaron liquids in the Peierls model opens a path to high-$T_c$ superconductivity based on a bipolaronic mechanism, in higher dimensions.

We propose an effect called information constraint which is characterized by the existence of different decay rates of signal strengths propagating along opposite directions. It is an intrinsic property of a type of open quantum systems, which does not rely on boundary conditions. We define the value of information constraint $\left(I_C\right)$ as the ratio of different decay rates and derive the analytical representation of $I_C$ for general quadratic Lindbladian systems. Based on information constraint, we can provide a simple and elegant explanation of chiral and helical damping, and get the local maximum points of relative particle number for the periodical boundary system, consistent with numerical calculations. Inspired by information constraint, we propose and prove the correspondence between edge modes and damping modes. A new damping mode called Dirac damping is constructed, and chiral/helical damping can be regarded as a special case of Dirac damping.

Mechanical topological insulators are well understood for linear and weakly nonlinear systems, however traditional analysis methods break down for strongly nonlinear systems since linear methods cannot be applied in that case. We study one such system in the form of a one-dimensional mechanical analog of the Su-Schrieffer-Heeger interface model with strong nonlinearity of the cubic form. The frequency-energy dependence of the nonlinear bulk modes and topologically insulated mode is explored using numerical continuation of the system's nonlinear normal modes (NNMs), and the linear stability of the NNMs is investigated using Floquet multipliers and Krein signature analysis. We find that the nonlinear topological lattice supports a family of topologically insulated NNMs that are parametrized by the total energy of the system and are stable within a range of frequencies. Next, it is shown that empirical calculations of the geometric Zak phase can define an energy threshold to predict the excitability of the nonlinear topological mode, and that this threshold coincides with the energy at which the topological NNM intersects the linear bulk spectrum. These predictions are validated with numerical simulations of the nonlinear topological system. These results are also tested for parametric perturbations that preserve and break chirality in the system. Thus, we provide a method for analyzing and predicting the existence of topologically insulated modes in a strongly nonlinear lattice based on the physical observable of band topology.

Topological magnetism typically appears in noncentrosymmetric compounds or compounds with geometric frustration. Here, we report the effective tuning of magnetism in centrosymmetric tetragonal ${\mathrm{Mn}}_{2-x}{\mathrm{Zn}}_x\mathrm{Sb}$ by Zn substitution. The magnetism is found to be closely coupled to the transport properties, giving rise to a very large topological Hall effect with fine-tuning of Zn content, which even persists to high temperature $(\sim 250\mathrm{K})$. The further magnetoentropic analysis suggests that the topological Hall effect is possibly associated with topological magnetism. Our finding suggests ${\mathrm{Mn}}_{2-x}{\mathrm{Zn}}_x\mathrm{Sb}$ is a candidate material for a centrosymmetric tetragonal topological magnetic system, offering opportunities for studying and tuning spin textures and developing near room temperature spin-based devices.

Spontaneous polarization in displacive ferroelectric oxides originates from the separation of positive charge centers from negative charge centers, which is induced by the off-center displacements of cations and anions in unit cells. For hugely strained ferroelectric oxides, understanding the correlation between the off-center displacements and the strain level is a prerequisite for understanding the polarization behavior. In the present work, the off-center displacements of atoms in nanometer columnar ${\mathrm{PbTiO}}_3$ under a strong tensile strain of 13% along the $c$ -axis direction are quantified by quantitative high-resolution transmission electron microscopy. The measured off-center displacement ${\mathrm{\Delta }}_{\mathrm{Ti}-\mathrm{O}}$ (0.052 nm) is about 60% larger than the value of unstrained bulk ${\mathrm{PbTiO}}_3$. The experimental results are confirmed by first-principle calculations, leading to a polarization of $122\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$, providing the basis for understanding the enhanced spontaneous polarization of highly strained displacive ferroelectric oxides.

Weyl semimetals (WSMs) have recently stimulated intensive interest because they exhibit fascinating physical properties and also promise exciting technological applications. So far, however, the few confirmed magnetic WSMs generally have a large number of Weyl points either located away from the Fermi level $\left(E_F\right)$ or shrouded by nontopological Fermi surface pockets. Based on first-principles density functional theory calculations, we establish cubic double perovskite ${\mathrm{BaEuNiOsO}}_6$ to be a high Curie temperature $\left(T_c\right)$ ferromagnetic (FM) WSM with magnetization along the [111] direction, just two pairs of Weyl points at the $E_F$ and $T_c=325$ K. The strong FM is attributed to the strong FM Ni $3d$-Eu $4f$-Os $5d$ coupling induced by the substitution of half of Ba atoms with Eu atoms in double perovskite ${\mathrm{Ba}}_2{\mathrm{NiOsO}}_6$. Moreover, the momentum separation of one Weyl point pair is large, thus giving rise to not only a long (001) surface Fermi arc but also large anomalous Hall conductivity. Intriguingly, as a unique physical result of bulk ${\mathrm{BaEuNiOsO}}_6$ being a FM WSM, the (111) ${\mathrm{BaEuNiOsO}}_6$ monolayer (ML) superlattice $\left({\mathrm{BaEuNiOsO}}_6\right)/{\left({\mathrm{BaTiO}}_3\right)}_{10}$ is found to be a high-temperature ($T_c=210$ K) Chern insulator with a large band gap of $\sim 90$ meV. Therefore, cubic double perovskite ${\mathrm{BaEuNiOsO}}_6$ will provide a superior high-temperature material platform for exploring fundamental physics of Weyl fermions, and its (111) ML superlattices will offer a high-temperature magnetic topological insulator for studying exotic quantum phenomena such as the quantum anomalous Hall effect.

Nematic phase transitions in high-temperature superconductors have a strong impact on the electronic properties of these systems. $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$, with an established nematic transition around 137 K induced by magnetic fluctuations, and $\mathrm{Ba}{\mathrm{Ni}}_2{\mathrm{As}}_2$, a nonmagnetic analog of $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$ with a structural transition in the same temperature range, share a common tetragonal aristotype crystal structure with space-group type $I4/mmm$. In contrast to $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$ where collinear stripe magnetic order is found for the orthorhombic low-$T$ phase, a unidirectional charge density wave together with (distorted) zigzag chains are observed for the triclinic low-$T$ phase of $\mathrm{Ba}{\mathrm{Ni}}_2{\mathrm{As}}_2$. Here we show that between the high- and low-$T$ phases of $\mathrm{Ba}{(\mathrm{Ni},\mathrm{Co})}_2{(\mathrm{As},\mathrm{P})}_2$ an additional phase with broken fourfold symmetry and $d_{xz}$ orbital order exists which is a promising candidate for charge/orbital-fluctuation-induced nematicity. Moreover, our data suggest that the enhanced $T_{\mathrm{c}}$ found for $\mathrm{Ba}{(\mathrm{Ni},\mathrm{Co})}_2{(\mathrm{As},\mathrm{P})}_2$ for higher Co or P substitution levels might result from suppression of the (distorted) zigzag chains by reducing the contribution of the $d_{xy}$ orbitals.

Recent high-pressure experiments discovered abnormal double-dome superconductivities in the newly synthesized kagome materials ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$, which also host abundant emergent quantum phenomena such as charge density wave (CDW), anomalous Hall effect, nontrivial topological property, and so on. In this work, by using first-principles electronic structure calculations, we studied the CDW state, superconductivity, and topological property in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ under pressures $(< 50 \mathrm{GPa})$. Based on the electron-phonon coupling theory, our calculated superconducting $T_{\text{c}}\mathrm{s}$ are consistent with the observed ones in the second superconducting dome at high pressure, but are much higher than the measured values at low pressure. The further calculations including the Hubbard $U$ indicate that with modest electron-electron correlation the magnetism on the V atoms exists at low pressure and diminishes gradually at high pressure. We thus propose that the experimentally observed superconductivity in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ at ambient/low pressures may still belong to the conventional Bardeen-Cooper-Schrieffer (BCS) type, but is partially suppressed by the V magnetism, while the superconductivity under high pressure is fully conventional without invoking the magnetism. We also predict that there are a second weak CDW state and topological phase transitions in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ under pressures. Our theoretical assertion calls for future experimental examination.

In this paper, we study a model for monolayer molybdenum disulfide, including the intravalley and intervalley electron-electron interaction. We solve the model at a self-consistent mean-field level and get three solutions: $L_0$, $L_{+}$, and $L_{-}$. As for $L_0$, the spin polarizations are opposite at $\mathbf{K}$ and ${\mathbf{K}}^{\prime }$ valleys and the total magnetization is zero. $L_{\pm }$ describes two degenerate spin-polarized states, and the directions of polarization are opposite for the states of $L_{+}$ and $L_{-}$. Based on these results, the ground state can be deduced to be spin polarized in domains in which their particular states can be randomly described by $L_{+}$ or $L_{-}$. Therefore, a zero net magnetization is induced for zero external magnetic field $\mathbf{B}$, but a global ferromagnetic ground state for a nonzero $\mathbf{B}$. We estimate the size of domains as several nanometers. With the increase of the chemical potential, the ground state changes between $L_0$ and $L_{\pm }$, indicating first-order phase transitions at the borders, which is coincident with the observation of photoluminescence experiments in the absence of the external magnetic field [J. G. Roch, et al., Phys. Rev. Lett. 124, 187602 (2020)].