Continuous-time quantum Monte Carlo refers to a class of algorithms designed to sample the thermal distribution of a quantum Hamiltonian through exact expansions of the Boltzmann exponential in terms of stochastic trajectories which are periodic in imaginary time. Here, we show that for (sign-problem-free) quantum many-body systems with discrete degrees of freedom—such as spins on a lattice—this sampling can be done in a rejection-free manner using transition path sampling (TPS). The key idea is to converge the trajectory ensemble through updates where one individual degree of freedom is modified across all time while the remaining unaltered ones provide a time-dependent background. The ensuing single-body dynamics provides a way to generate trajectory updates exactly, allowing one to obtain the target ensemble efficiently via rejection-free TPS. We demonstrate our method on the transverse field Ising model in one and two dimensions, and on the quantum triangular plaquette (or Newman-Moore) model. We show that despite large autocorrelation times, our method is able to efficiently recover the respective quantum phase transition of each model. We also discuss the connection to rare event sampling in continuous-time Markov dynamics.

Magnetic skyrmions are nanoscale spin textures whose thermal stability originates from the nontrivial topology in nature. Recently, a plethora of topological spin textures have been theoretically predicted or experimentally observed, enriching the diversity of the skyrmionic family. In this work, we theoretically demonstrate the stabilities of various topological spin textures against homochiral states in chiral magnets, including chiral bobbers, dipole strings, and skyrmion tubes. They can be effectively classified by the associated topological Hall signals. Multiple transition paths are found among these textures, mediated by Bloch-point singularities, and the topological protection property here can be manifested by a finite energy barrier with the saddle point corresponding to the Bloch-point creation/destruction. By carefully modulating the local property of a surface, such as interfacial Dzyaloshinskii-Moriya interaction induced by breaking the structural symmetry, the energy landscape of a magnetic system can be tailored decisively. Significantly, the proposed scenario also enables the manipulation of stabilities and transition barriers of these textures, even accompanied by the discovery of ground-state chiral bobbers. This study may raise great expectations on the coexistence of topological spin textures as spintronics-based information carriers for future applications.

We report a study on low-temperature heat transport of Kitaev magnet ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, with the heat current and magnetic fields along the honeycomb spin layer (the $ab$ plane). The zero-field thermal conductivity of ${\kappa }_{xx}^a$ and ${\kappa }_{xx}^{a*}$ display similar temperature dependence and small difference in their magnitudes, whereas their magnetic field (parallel to the heat current) dependence are quite different and are related to the field-induced magnetic transitions. The ${\kappa }_{xx}^a\left(B\right)$ data for $B\parallel a$ at very low temperatures have an anomaly at 10.25–10.5 T, which reveals an unexplored magnetic transition. The planar thermal Hall conductivity ${\kappa }_{xy}^a$ and ${\kappa }_{xy}^{a*}$ show very weak signals at low fields and rather large values with sign change at high fields. This may point to a possible magnetic structure transition or the change of the magnon band topology that induces a radical change of magnon Berry curvature distribution before entering the spin polarized state. These results put clear constraints on the high-field phase and the theoretical models for ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$.

Magnetic skyrmions in two-dimensional lattice attract considerable interest for their fundamental physics, which, however, suffers from the deleterious skyrmion Hall effect. So far, a number of theoretical proposals have been made to conquer this challenge, but all focused on antiferromagnetic skyrmions and antiskyrmions. Here, we alternatively show by solving the Landau-Lifshitz-Gilbert equation and symmetry analysis that the mixed Bloch-Néel type magnetic skyrmions could be presented in two-dimensional lattice. Importantly, such skyrmions could exhibit a near zero skyrmion Hall angle, thereby suppressing the skyrmion Hall effect. Using first-principles and Monte Carlo simulations, we further demonstrate that such mixed skyrmion can be stabilized in a real material of monolayer ${\mathrm{FeB}}_6$. Through strain engineering, the near zero Hall angle is obtained for the skyrmions in monolayer ${\mathrm{FeB}}_6$, which is validated by the spin dynamic simulations. Especially for the case under magnetic field of 300 mT, the presence of skyrmion Hall effect is absolutely prohibited. These explored phenomena and insights greatly enrich the two-dimensional magnetic skyrmion research.

We employ quadratic-response Kubo formulas to investigate the nonlinear magnetotransport in bilayers composed of a topological insulator and a magnetic insulator, and predict both unidirectional magnetoresistance and nonlinear planar Hall effects driven by interfacial disorder and spin-orbit scattering. These effects exhibit strong dependencies on the Fermi energy relative to the strength of the exchange interaction between the spins of Dirac electrons and the interfacial magnetization. In particular, as the Fermi energy becomes comparable to the exchange energy, the nonlinear magnetotransport coefficients can be greatly amplified and their dependencies on the magnetization orientation deviate significantly from conventional sinusoidal behavior. These findings may not only deepen our understanding of the origin of nonlinear magnetotransport in magnetic topological systems but also open new pathways to probe the Fermi and exchange energies via transport measurements.

Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient spintronic devices. Here, we experimentally investigate the coherent spin dynamics after ultrashort laser excitation by performing time-resolved magneto-optical Kerr effect experiments with thin ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ films. We provide a detailed study of the magnetic field and pump fluence dependence of the coherent precessional dynamics after optical excitation with femtosecond laser pulses. We show that the coherent precession lifetime increases with the strength of the applied magnetic field, which cannot be understood in terms of viscous Gilbert damping of the coherent magnons. Instead, it can be explained by nonlinear magnon interactions and by the change in the fraction of incoherent magnons. This interpretation is in agreement with the observed trends of the coherent magnon amplitude and lifetime as a function of the applied exciting laser fluence. Our results provide insight into the magnetization relaxation processes in ferromagnetic thin films, which are of great importance for further spintronic applications.

The widespread use of the noninteracting ground state as the initial state for the digital quantum simulation of the Fermi-Hubbard model is largely due to the scarcity of alternative easy-to-prepare approximations to the exact ground state in the literature. Exploiting the fact that the spin-$\frac{1}{2}$ Heisenberg model is the effective low-energy theory of the Fermi-Hubbard model at half-filling in the strongly interacting limit, here we propose a three-step deterministic quantum routine to prepare an educated guess of the ground state of the Fermi-Hubbard model through a shallow circuit suitable for near-term quantum hardware. First, the ground state of the Heisenberg model is initialized via a hybrid variational method using an ansatz that explores only the correct symmetry subspace. Second, a general method is devised to convert a multi-spin-$\frac{1}{2}$ wave function into its fermionic version. Third, taking inspiration from the Baeriswyl ansatz, a constant-depth single-parameter layer that adds doublon-holon pairs is applied to this fermionic state. Numerical simulations on chains and ladders with up to 12 sites confirm the improvement over the noninteracting ground state of the overlap with the exact ground state for the intermediate values of the interaction strength at which quantum simulation is found to be most relevant. More broadly, the general scheme to convert a multi-spin-$\frac{1}{2}$ state into a half-filled fermionic state may bridge the gap between quantum spin models and lattice models of correlated fermions in the realm of digital quantum simulation.

${\mathrm{TaCo}}_2{\mathrm{Te}}_2$ is recently reported to be an air-stable, high mobility van der Waals material with probable magnetic order. Here we investigate the scaling behavior of its magnetoresistance. We measured both the longitudinal (${\rho }_{xx}$) and Hall (${\rho }_{xy}$) magnetoresistivities of ${\mathrm{TaCo}}_2{\mathrm{Te}}_2$ crystals in magnetic fields parallel to the $c$ axis and found that the magnetoresistance violates the Kohler's rule $\mathrm{MR}\sim f[H/{\rho }_0]$ while obeying the extended Kohler's rule $\mathrm{MR}\sim f[H/\left(n_T{\rho }_0\right)]$, where $\mathrm{MR}\sim [{\rho }_{xx}\left(H\right)-{\rho }_0]/{\rho }_0$, $H$ is the magnetic field, $n_{\mathrm{T}}$ is a thermal factor, and ${\rho }_{xx}\left(H\right)$ and ${\rho }_0$ are the resistivities at $H$ and zero field, respectively. While deviating from those of the densities of electrons ($n_{\mathrm{e}}$) and holes ($n_{\mathrm{h}}$) obtained from the two-band model analysis of the magnetoconductivities, the temperature dependence of $n_{\mathrm{T}}$ is close to that of the Hall carrier densities $n_{\mathrm{H}}$ calculated from the slopes of ${\rho }_{xy}\left(H\right)$ curves at low magnetic fields, providing a different way to obtain the thermal factor in the extended Kohler's rule.

This paper introduces complex dynamics methods to study dynamical quantum phase transitions in the one- and two-dimensional quantum three-state Potts model. The quench involves switching off an infinite transverse field. The time-dependent Loschmidt echo is evaluated by an exact renormalization group (RG) transformation in the complex plane where the thermal Boltzmann factor is along the positive real axis, and the quantum time evolution is along the unit circle. One of the characteristics of the complex dynamics constituted by repeated applications of RG is the Julia set, which determines the phase transitions. We show that special boundary conditions can alter the nature of the transitions, and verify the claim for the one-dimensional system by transfer matrix calculations. In two dimensions, there are alternating symmetry-breaking and restoring transitions, both of which are first order, despite the criticality of the Curie point. In addition, there are finer structures because of the fractal nature of the Julia set. Our approach can be extended to multi-variable problems, higher dimensions, and approximate RG transformations expressed as rational functions.

Crystal symmetries play an essential role in band structures of non-Hermitian Hamiltonians. In this paper, we propose a non-Hermitian skin effect (NHSE) enforced by nonsymmorphic symmetries. We show that the NHSE inevitably occurs if a two-dimensional non-Hermitian system satisfies conditions derived from the nonsymmorphic symmetry of the doubled Hermitian Hamiltonian. This NHSE occurs in symmetry classes with and without time-reversal symmetry. The NHSE enforced by nonsymmorphic symmetries always occurs simultaneously with the closing of the point gap at zero energy. We also show that such a NHSE can occur in specific three-dimensional space groups with nonsymmorphic symmetries.

X-ray Raman scattering (XRS) of the A-site ordered perovskite $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ was measured under high pressure up to 13 GPa. Core excitations such as Cu $\underline{2p}3d$ and Cu $\underline{2p}4p$, where the underline denotes the core hole, were detected under excitations around the Cu K absorption edge. Unoccupied electronic states were investigated with changing pressure through the XRS. The Cu ion in $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ showed mixed valence $({\mathrm{Cu}}^{+1} \mathrm{and} {\mathrm{Cu}}^{+2})$ at atmospheric pressure: a divalent component was prevailing, with an increasing monovalent feature at increasing pressure. An electronic state was also revealed between these states at high pressure, without evidence of a structural phase transition from x-ray diffraction study.

To characterize in detail the charge density wave (CDW) transition of $1T\text{−}{\mathrm{VSe}}_2$, its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, muon spectroscopy, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering, and state-of-the-art first-principles density functional theory calculations. Resonant elastic x-ray scattering does not show any resonant enhancement at either V or Se, indicating that the CDW peak at the $K$ edges describes a purely structural modulation of the electronic ordering. ARPES experiments identify (i) a pseudogap at $T>T{}_{\mathrm{CDW}}$, which leads to a depletion of the density of states in the ML-M'L' plane at $T<T{}_{\mathrm{CDW}}$, and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon $\left({\omega }_1\right)$ with propagation vector (0.25 0 −0.3) r.l.u. We show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependent EPI with a small contribution from nesting. The pressure dependence of the ${\omega }_1$ soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic C2/m phase. The wide set of experimental data is well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals corrections in the exchange-correlation functional. The comprehensive description of the electronic and dynamical properties of ${\mathrm{VSe}}_2$ reported here adds important pieces of information to the understanding of the electronic modulations in the family of transition-metal dichalcogenides.

We study the relation between the quantum geometry of wave functions and the Landau level (LL) spectrum of two-band Hamiltonians with a quadratic band crossing point (QBCP) in two-dimensions. By investigating the influence of interband coupling parameters on the wave function geometry of general QBCPs, we demonstrate that the interband coupling parameters can be entirely determined by the projected elliptic image of the wave functions on the Bloch sphere, which can be characterized by three parameters, i.e., the major $d_1$ and minor $d_2$ diameters of the ellipse, and one angular parameter $\varphi$ describing the orientation of the ellipse. These parameters govern the geometric properties of the system such as the Berry phase and modified LL spectra. Explicitly, by comparing the LL spectra of two quadratic band models with and without interband couplings, we show that the product of $d_1$ and $d_2$ determines the constant shift in LL energy while their ratio governs the initial LL energies near a QBCP. Also, by examining the influence of the rotation and time-reversal symmetries on the wave function geometry, we construct a minimal continuum model, which exhibits various wave function geometries. We calculate the LL spectra of this model and discuss how interband couplings give LL structure for dispersive bands as well as nearly flat bands.

We present an interacting theory of a $\text{U}\left(1\right)$ gauge boson with a quadratic dispersion relation, which we call the “nonlinear Lifshitz photon theory.” The Lifshitz photon is a three-dimensional generalization of the Tkachenko mode in rotating superfluids. Starting from the Wigner crystal of charged particles coupled to a dynamical $\text{U}\left(1\right)$ gauge field, after integrating out gapped degrees of freedom, we arrive at the Lagrangian for the nonlinear Lifshitz photon. The symmetries of the theory include a global $\text{U}\left(1\right)$ 1-form symmetry and nonlinearly realized “magnetic” translation and rotation symmetries. The interaction terms in the theory lead to the decay of the Lifshitz photon, the rate of which we estimate. We show that the Wilson loop, which plays the role of the order parameter of the spontaneous breaking of the 1-form global symmetry, deviates from the perimeter law by an additional logarithmic factor. We explore potential connections to other condensed matter systems, with a particular focus on quantum spin ice and ferromagnets. Finally, we generalize our theory to higher dimensions.

We study Berry connection polarizability (BCP) induced electric polarization and the third-order Hall (TOH) effect in a two-dimensional electron/hole gas (2DEG/2DHG) with Rashba-Dresselhaus (RD) spin-orbit couplings in III-V semiconductor heterostructures. The electric polarization decreases with increase of the Fermi energy and is responsive to the electric field orientation in the presence of RD spin-orbit couplings for both systems. We determine the BCP-induced TOH conductivity $\left({\chi }_{\perp }^{\text{I}}\right)$ along with the TOH conductivity associated with the band velocity $\left({\chi }_{\perp }^{\text{II}}\right)$. We find that the presence of an infinitesimal amount of Dresselhaus coupling in addition to the dominant Rashba coupling results in finite TOH responses. These conductivities vanish when the field is aligned with and/or orthogonal to the symmetry lines $k_x\pm k_y=0$ in both the systems. For typical system parameters in a 2DEG with $k$-linear RD interactions, the magnitude of ${\chi }_{\perp }^{\text{I}}$ is smaller than that of ${\chi }_{\perp }^{\text{II}}$. On the other hand, when both the SO couplings are comparable, ${\chi }_{\perp }^{\text{I}}$ shows a notable increase in magnitude, owing to the distinctive characteristics of BCP. The TOH conductivity of 2DEG remains unchanged when Rashba and Dresselhaus spin-orbit couplings are exchanged. For 2DHG with $k$-cubic RD interactions, ${\chi }_{\perp }^{\text{I},h}$ exhibits a larger magnitude compared to ${\chi }_{\perp }^{\text{II},h}$. Unlike the electron case, the BCP-induced ${\chi }_{\perp }^{\text{I},h}$ alters under the exchange of spin-orbit coupling parameters, whereas ${\chi }_{\perp }^{\text{II},h}$ remains the same.

Electronic cloaking of confined states is an exotic phenomenon in bilayer graphene and phosphorene owing to the chirality mismatch between electrons and holes in an electrostatic barrier. Anti-Klein tunneling or almost perfect reflection, another exotic phenomenon in bilayer graphene, is also the manifestation of the chirality mismatch between electrons and holes in an electrostatic barrier. Anti-Klein tunneling can be destroyed by the multiplication of electrostatic barriers. Here, we show that the multiplication of electrostatic barriers in phosphorene along the zigzag direction does not destroy the electronic cloaking of confined states. The so-called persistent electronic cloaking is manifested as null transmission at normal incidence. By reducing the width of the barriers, transmission takes place at normal incidence due to the overlapping of the electron states of the quantum wells, not to the destruction of the electronic cloaking. In addition, the multiplication of the electrostatic barriers gives rise to $n-1$ folding for electron states and $n$ folding for hole states. We also study the electronic transport through multiple electrostatic barriers along the armchair direction, finding transmission at normal incidence, $n-1$ folding for electron states, and $n$ folding for hole states. Moreover, we find perfect transmission at normal incidence or Klein tunneling by closing the phosphorene band gap and appropriately rescaling the structural parameters of the electrostatic barriers.

Calculating the thermal conductivity of heterostructures with multiple layers presents a significant challenge for state-of-the-art ab initio methods. In this study we introduce an efficient neural-network force field (NNFF) to explore the thermal transport characteristics of van der Waals heterostructures based on PtSTe, using both the phonon Boltzmann transport equation and molecular dynamics (MD) simulations. Besides demonstrating a remarkable level of agreement with both theoretical and experimental data, our predictions reveal that heterogeneous combinations like $\mathrm{PtSTe}\text{−}{\mathrm{PtTe}}_2$ display a notable reduction in thermal conductivity at room temperature, primarily due to broken out-of-plane symmetries and the presence of weak van der Waals interactions. Furthermore, our study highlights the superiority of MD simulations with NNFFs in capturing higher-order anharmonic phonon properties. This is demonstrated through the analysis of the temperature-dependent thermal conductivity curves of PtSTe-based van der Waals heterostructures and advances our understanding of phonon transport in those materials.

We propose a novel way of modulating exceptional topology by implementing Floquet engineering in non-Hermitian (NH) systems. We introduce Floquet exceptional topological insulator which results from shining circularly polarized light on a three-dimensional NH topological insulator i.e. NH generalization of Floquet topological insulator. The light-matter interaction facilitates the quantum phases of matter to exhibit a novel phenomenon, where the point gap in the bulk hosts surface states. These doubly degenerate surface states, which are localized at opposite surfaces, fill the point gap in the complex eigenspectrum isotropically, thus showcasing a single-sheet structure. In the presence of an external magnetic field, the system exhibits exceptional points at the surface and the order of them can be modulated using light. We also introduce Floquet driving of an unconventional three-dimensional NH topological insulator that has two Fermi points and transition between single-sheet to double-sheet can be achieved by tuning system parameters. The existence of the Floquet biorthogonal Chern number which is connected to the three-dimensional winding number and spectral winding number shows that the momentum slices exhibit skin effect, even though the system as a whole does not. We employ wave dynamics evolution to illustrate the consequence of NH surface skin effect.

Since the discovery of superconductivity in cuprate materials, the minimal ingredients for high-$T_c$ superconductivity have been an outstanding puzzle. Motivated by the recently discovered nickelate bilayer superconductor ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under pressure, we study a minimal bilayer model, in which, as in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, interlayer and intralayer magnetic interactions but no interlayer hopping are present: A mixed-dimensional (mixD) $t\text{−}J$ model. In the setting of a mixD ladder, we show that the system exhibits a crossover associated with a Feshbach resonance: From a closed-channel-dominated regime of tightly bound bosonic pairs of holes to an open-channel-dominated regime of spatially more extended Cooper pairs. The crossover can be tuned by varying doping, or by a nearest-neighbor Coulomb repulsion $V$ that we include in our model. Using density matrix renormalization group simulations and analytical descriptions of both regimes, we find that the ground state is a Luther-Emery liquid, competing with a density wave of tetraparton plaquettes at commensurate filling $\delta =0.5$ at large repulsion, and exhibits a pairing dome where binding is facilitated by doping. Our observations can be understood in terms of pairs of correlated spinon-chargon excitations constituting the open channel, which are subject to attractive interactions mediated by the closed channel of tightly bound chargon-chargon pairs. When the closed channel is lowered in energy by doping or tuning $V$, a Feshbach resonance is realized, associated with a dome in the binding energy. Our predictions can be directly tested in state-of-the art quantum simulators, and we argue that the pairing mechanism we describe may be realized in the nickelate bilayer superconductor ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$.

Superlattices of correlated oxides have been used to explore interface effects and to achieve additional control over the physical properties of individual constituents. In this work, we present a first-principles perspective of the strain and thickness effects in ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3/{\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (LSMO/YBCO) superlattices. Our findings indicate that the presence of epitaxial strain and LSMO leads to a reduction of buckling parameters of the interface ${\mathrm{CuO}}_2$ planes, as well as the transfer of electrons from LSMO to YBCO. In addition, the change in $\mathrm{Cu}3d$ valence is slightly dependent on the LSMO layer thickness. More interestingly, the in-plane ferromagnetic ground state within the ${\mathrm{CuO}}_2$ planes near the interface is induced due to the local moments centered at the copper atoms. These local moments are decoupled from the charge transfer and, according to our calculations, appear mainly due to the Mn $3d$–O $2p$–Cu $3d$ hybridization being restricted to the interface region. The induced net magnetic ordering in interface copper atoms may have implications in the control of the superconducting state in the LSMO/YBCO superlattices.