Nonlinear interaction enables topological phenomena impossible in linear systems. A paradigm is a nonlinear Thouless pump, where the transport of solitons can be topologically quantized even when band occupation is nonuniform. Such nonlinear quantization traditionally requires a time-periodic Hamiltonian with static nonlinearity and, much as in the linear case, is inherently independent of pumping speed. Instead, we demonstrate a dissipation-induced topological gear switching, where quantized soliton transport can be switched on and off via the adiabatic pumping speed itself. This phenomenon has no counterpart in prior conservative nonlinear pumps, nor in linear non-Hermitian pumps. Crucially, quantization here no longer requires a time-periodic nonlinear Hamiltonian; it stems from a genuinely nonequilibrium mechanism captured by an effective conservative model whose nonlinearity varies aperiodically in time. Remarkably, a quantized nonlinear transport can be induced even when this nonlinear aperiodic driving is such that the system is pumped from the linear to nonlinear regimes. Our results open a route toward nonequilibrium nonlinear topological matter, where topological effect is dynamically reconfigurable via time-varying nonlinearities, with experimental implications for photonic, atomic, or superconducting platforms and beyond.

Since their discovery, the study of magnetic skyrmions has been on the rise. In this paper, we discuss our investigations on the light-induced mechanisms for skyrmion generation in a centrosymmetric triangular magnetic lattice with competing $J_1\text{−}{J}_3$ interactions, and easy-axis anisotropy. We solve the stochastic Landau-Lifshitz-Gilbert equation for the lattice spin dynamics under Laguerre-Gaussian beam irradiation. Numerical results show that skyrmions are nucleated in two thermodynamic regions, each favoring different phases: the ferromagnetic phase and the skyrmion-lattice phase. In the ferromagnetic region, isolated skyrmions are generated mainly through stochastic thermal nucleation. In this regime, higher temperatures and larger beam widths are required to overcome the nucleation barrier. In contrast, in the skyrmion-lattice region, skyrmion nucleation occurs via thermal annealing, where the system relaxes toward its true ground state. These findings establish a comprehensive theoretical framework for optimizing optical control to generate light-induced skyrmionic textures in frustrated magnets.

We take account of the many-body dynamic interactions by including the wave-vector and frequency-dependent exchange-correlation (xc) kernel $f_{\mathrm{xc}}(\mathit{q},\omega )$ in the framework of the dielectric theory of the stopping power of crystals for moving charges. The cases of classical and quantum projectiles are considered. We find that (I) the role of the xc effects in slowing of charges in crystalline solids is more pronounced than it is within the jellium model and (II) for velocities below the stopping maximum, inclusion of xc leads to the improvement of the comparison of the theory with experiment over a range of solid targets. On the other hand, in the high-velocity regime, the role of the dynamic xc proves negligible, which does not contradict experiment, and which we substantiate analytically within the jellium model of the target. The input to our theory is the microscopic dielectric matrix ${\epsilon }_{\mathit{G}{\mathit{G}}^{\prime }}(\mathit{q},\omega )$ within the random phase approximation (i.e., with neglect of $f_{\mathrm{xc}}$), the calculation of which matrix is implemented in the existing solid-state codes.

As a newly classified magnetic phase, altermagnetism integrates characteristics of conventional antiferromagnetism and ferromagnetism, exhibiting zero net magnetization and anisotropic spin-splitting. Two-dimensional altermagnets, in particular, offer significant potential for spintronic transport applications. However, the scarcity of intrinsic two-dimensional altermagnets has substantially impeded related research. Combining symmetry analysis, high-throughput computational screening, and first-principles calculations, we uncover over 300 stable monolayers that exhibit an altermagnetic ground state. These materials display a diverse even-parity anisotropic waveform and layer symmetries and reveal several hitherto unreported layer groups capable of supporting altermagnetism. Furthermore, we demonstrate strain-mediated switching of spin-splitting waveforms, magnetic phase transitions, and reversible electric polarization and anomalous Hall conductivity in these two-dimensional altermagnets. This work enriches the repository of intrinsic altermagnetic monolayers and provides a basis for designing and modulating low-power, high-speed spintronic devices.

Driving quantum materials with coherent light has proven a powerful platform to realize a plethora of interesting phases and transitions, ranging from ferroelectricity to superconductivity and limit cycles in pumped magnonics. In this paper we develop the field theoretical framework to describe nonequilibrium phases that emerge in systems pumped by rapid parametric drives. We consider paradigmatic $\text{O}\left(N\right)$ models that describe the long-wavelength fluctuations of ordering fields in many condensed matter setups. We show that rapid parametric driving of these models can induce an effective pump mechanism in the long-wavelength regime through nonlinear scattering. This induces a nonequilibrium transition into a time-crystalline phase.

The phase transition in copper pyrophosphate, ${\mathrm{Cu}}_2{\mathrm{P}}_2{\mathrm{O}}_7$, has been studied using neutron total scattering and the reverse Monte Carlo method. The results show the growth of disorder of the orientations of the ${\mathrm{PO}}_4$ tetrahedra and of the positions of the copper atoms along the long O–Cu–O linkage in the ${\mathrm{CuO}}_6$ octahedra distorted by the Jahn–Teller effect. These two types of disorder are coupled, and neither show the preference for distinct sites of partial atomic occupancy in the high-temperature phase. The origin of the negative thermal expansion identified in ${\mathrm{Cu}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ can be understood as arising from the ordering of the copper atoms leading to one Cu–O bond expanding more than the other contracts.

Quantum mechanical oscillators operating at frequencies up to the GHz regime have been predicted to support phonon lasing: self-sustained coherent vibrational motion emerging when the effective gain exceeds intrinsic losses. Current phonon-laser proposals face two key limitations, namely, they lack scalability and rely on coupling all oscillators to a common field, which significantly restricts flexibility and prevents selective on-demand phonon lasing at specific locations. Given that numerous applications and theoretical insights naturally emerge from scalable collective systems, addressing these limitations is timely. In this work, we demonstrate how scalable arrays of individually addressable phonon lasers can be generated through local driving in a quantum many-body Ising-like spin chain. We rigorously establish the resonance conditions under which mechanical oscillators transition from thermal motion to sustained coherent self-oscillation. Unlike previous approaches that rely on a common coupling bus, our proposal employs purely local driving, resulting in an inherently modular and scalable architecture ideally suited for integration into large-scale quantum systems. Additionally, our approach enables on-demand lasing of individual mechanical oscillators at specific sites by simply switching the spin-mechanical coupling interaction on and off, provided specific resonance conditions are satisfied. Notably, our phonon laser array continues to operate in the presence of resonance mismatches and naturally exhibits both pairwise self-organized synchronization and global phase locking near resonance. Finally, we outline an experimental implementation within current experimental capabilities.

Tensor network methods leverage the limited entanglement of quantum states to efficiently simulate many-body systems. Alternatively, Clifford circuits provide a framework for handling highly entangled stabilizer states, which have low magic and are thus also classically tractable. Clifford tensor networks combine the benefits of both approaches, exploiting Clifford circuits to reduce the classical complexity of the tensor network description of states, with promising effects on simulation approaches. We study the disentangling power of Clifford transformations acting on tensor networks, with a particular emphasis on entanglement cooling strategies. We identify regimes where exact or heuristic Clifford disentanglers are effective, explain the link between the two approaches, and characterize their breakdown as non-Clifford resources accumulate. Additionally, we prove that, beyond stabilizer settings, no Clifford operation can universally disentangle even a single qubit from an arbitrary non-Clifford rotation. Our results clarify both the capabilities and fundamental limitations of Clifford-based simulation methods.

Electric-field driven magnetization switching in ${\mathrm{FePt}/\mathrm{BaTiO}}_3$ (001) is demonstrated through first-principles calculations. The magnetic easy axis of the FePt layer undergoes a transition from an in-plane to perpendicular direction upon ferroelectric polarization switching, a process sensitively controlled by epitaxial strain ($\eta$) with a threshold value $\eta \approx 2\%$. In this phenomenon, a large interfacial magnetoelectric coupling (${\alpha }_I=3.6\times {10}^{-10} \mathrm{G}{\mathrm{cm}}^2/\mathrm{V}$) is responsible, stemming from the orbital reconstruction. In particular, the redistribution of Pt-$d$ orbital occupancy alters spin-orbit coupling, thereby tuning the competition between magnetic anisotropy ($K_i$) and magnetoelastic energy ($b_1$). Our work clarifies the fundamental physics of strain-engineered magnetoelectricity and suggests a concrete pathway for designing ultralow-power voltage-controlled magnetic memory.

We develop a theory for the plasmon spectrum in dirty superconductors across the entire temperature range. Starting with the microscopic Keldysh $\sigma$ model description, we link the plasmon dispersion $\omega \left(q\right)$ to the optical conductivity $\sigma (\omega ,T)$ of a superconductor, which requires analytical continuation to the lower half plane of complex frequency. This approach reveals a discontinuity at the superconducting transition: a jump in both the real and imaginary parts of $\omega \left(q\right)$ at $T_c$. For any temperature below $T_c$, the plasmon dispersion terminates at a critical wave vector $q_c\left(T\right)$ where plasmons remain undamped, with weakly $T$-dependent $\omega \left[q_c\left(T\right)\right]\approx 2\mathrm{\Delta }\left(0\right)$. Plasmons significantly attenuate only within a narrow 5% window near $T_c$, with the propagating mode recovering at large $q$.

We revisit the U(1) quantum link model in a ladder geometry, finding, by finite-size scaling, that the critical exponent $\nu =1$ and the central charge $c=1/2$ are consistent with the Ising universality class for all phase transitions observed. A blind application of the Harris criterion would suggest that this criticality is lost in the presence of the disorder. It turns out not to be the case. For the disorder affecting ladder's rung hoppings only, we have found that the transitions survive, disappearing only for quite strong disorder. The disorder in the ladder's legs destroys the nonzero mass phase criticality, while the symmetry-protected topological phase for zero mass survives a small disorder. The observed robustness against disorder is explained qualitatively using field-theoretic arguments.

This work investigates the Floquet dynamics of electrons and excitons (particle-hole pairs) in a Dirac material referred to as Kekulé-distorted graphene. Specifically, we examine the role played by a high frequency driving electromagnetic field on the tunneling and blocking by a potential barrier on both the charged single particles as well as the neutral composite particles. We demonstrate that the small effective masses of the electron and hole for the energy spectrum of this Kekulé-distorted graphene leads to practically almost perfect transmission across a symmetric potential barrier for any angle of incidence of impinging excitons. However, this unexpected Klein paradox for excitons does not hold for the single-particle electrons. For individual electrons, tunneling at normal incidence becomes almost perfect for the small energy gap which opens up due to the applied high frequency irradiation. Whenever the gap is increased for larger values of the irradiation parameter, the perfect transmission is noticeably suppressed. The reduced total transmission coefficient at incident angle other than head-on for electrons due to Kekulé distortion is more suppressed in the presence of irradiation. Additionally, we calculate and investigate the exciton binding energy since the quantum tunneling of a bound electron-hole pair across a potential barrier is governed by its mass measured in the center of mass and binding energy of the composite pair. Thus, irradiation with circularly polarized light fundamentally modifies exciton formation, coherence and transport properties, thereby producing unusual topological behaviors. These behaviors are unlike conventional Dirac materials. Possible technical applications of the results arising from our investigation include valleytronics due to the folding of the valleys, thereby making intervalley coupling feasible. Other practical applications include optoelectronics due to Floquet tuning of energy spectrum and transport properties.

We present a comprehensive first-principles investigation of topological phonon band structures in a class of high-symmetry binary compounds of the form $MX$ $(M=\mathrm{Sc}, \mathrm{Y}, \mathrm{La} \text{and} X=\mathrm{Sb}, \mathrm{Bi})$, which crystallize in the face-centered cubic space group $Fm\overline{3}m$. Our phonon dispersion calculations reveal multiple symmetry-protected twofold degeneracies that manifest as nodal rings, nodal chains, and nodal boxes in the Brillouin zone. These topological features are confirmed through the computation of quantized Berry phases. Notably, we identify drumhead-like surface phonon states on the [001] surface, projected along the high-symmetry $\mathrm{\Gamma }$-X-M-$\mathrm{\Gamma }$ path. These flat surface modes are robust against symmetry-preserving perturbations and represent strong topological signatures of the underlying bulk nodal structures. Our findings position these binary $MX$ compounds as promising candidates for realizing phononic topological phases with potential applications in thermal management and surface phononics.

Motivated by the high-temperature superconductivity observed in the bilayer nickelate ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ and the spectroscopic evidences of strong electron correlations in this compound, we address the role of its multiorbital electron correlations by proposing a global phase diagram of a bilayer two-orbital Hubbard model. We find a Mott transition developing at half filling, and identify strong orbital selectivity when the system is at the physical electron count. The orbital selectivity is manifested in the formation of interlayer spin singlets between electrons in the $z^2$ orbitals. These features lead to a strong renormalization of the electronic band structure while sustaining a sizable splitting between the bonding and antibonding $z^2$ bands. The proposed orbital-selective correlations naturally explain a series of features as observed in the angle-resolved photoemission spectroscopy and optical conductivity measurements in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Our results provide a basis to understand both the normal state and the high-temperature superconductivity of multilayer nickelates and thereby elucidate correlated superconductivity in general.

It is well established that causal linear response functions can be found by computing the much simpler imaginary-time-ordered Matsubara functions and performing an analytic continuation. This principle is the basis for much of our understanding of linear response for interacting and disordered systems, via diagrammatic perturbation theory. Similar imaginary-time approaches have recently been introduced for computing nonlinear response functions as well,  as in [Ann. Phys. (Berlin) 536, 2300504 (2024); Phys. Rev. X 11, 041006 (2021)], where the authors analytically continue the Matsubara functions to obtain the Keldysh response functions. In this work, we provide a proof of this connection to all orders in perturbation theory using an equation of motion based approach. We show by induction that causal nonlinear response functions at every order can be obtained from the analytic continuation of an appropriate time-ordered Matsubara function. We demonstrate this connection explicitly for second-order response functions in the Lehmann representation. As a byproduct of our approach, we derive an explicit expression for the Lehmann representation of the $n\mathrm{th}$-order response functions by solving the equations of motion. We also use our result to find an analytic spectral density representation for both causal response functions and Matsubara functions. As an example, we apply our method to derive the nonlinear $A^3$ term in the $SU\left(2\right)$ spin Hall response of an insulator with spin rotation symmetry. Finally, we show how our results lead to a family of generalized sum rules, focusing explicitly on the asymptotic expression for the $n\mathrm{th}$-harmonic-generation rate. Our work opens the door to using imaginary time approaches to study nonlinear response functions in general condensed matter systems.

We theoretically demonstrate a second-order topological insulating phase induced by compensated altermagnetism, while keeping the bulk gap unchanged, in a two-dimensional topological insulator film. By introducing a layer-resolved out-of-plane $d$-wave altermagnetic term with opposite signs on the top and bottom layers, the system preserves $\mathcal{PT}$ symmetry and maintains spin degeneracy in the bulk bands while simultaneously gapping the helical edge states and generating localized corner states. The resulting higher-order phase is characterized by nonzero mirror-graded winding numbers, and an effective edge theory shows that the corner states arise from Dirac mass domain walls. We further determine the phase boundaries analytically and construct the corresponding topological phase diagram, establishing a robust route to higher-order topology without bulk spin splitting.

Anisotropic second harmonic generation (SHG) is of central importance in nonlinear photonics because it enables highly directional light manipulation and advanced polarization-sensitive functionalities. Here, taking two-dimensional (2D) transition metal dichalcogenides as a prototypical platform, we realize extremely anisotropic SHG by progressively lowering structural symmetry through Janus engineering and lateral heterostructuring. This symmetry reduction pathway is exemplified by pristine ${\mathrm{MoS}}_2$, Janus MoSSe to in-plane heterostructures ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$, and MoSSe/WSSe. Along this evolution, the successive breaking of out-of-plane mirror symmetry and in-plane rotational symmetry activates an expanded set of independent susceptibility components, thereby inducing SHG anisotropy both along the same axis and across different axes. Their coexistence in MoSSe/WSSe ultimately drives the SHG response into an extremely anisotropic regime; i.e., the SHG along different incident and polarization angles of light is thoroughly different. Meanwhile, the SHG is strong only along a certain direction while very weak along other directions. We further derive analytical expressions that explicitly reveal how symmetry removal reshapes the polarization-resolved SHG patterns and governs the evolution of SHG anisotropy. By establishing the relationship between the anisotropy parameters and the susceptibility tensor components, the excitation conditions required for realizing extreme anisotropy are identified. Our work establishes Janus lateral heterostructuring as an effective strategy for enhancing SHG anisotropy, providing a promising platform for the rational design of highly directional and polarization-sensitive nonlinear photonic devices.

We develop a theory of polarized photoluminescence (PL) of interface excitons localized at lateral heterojunctions between transition metal dichalcogenide monolayers. We show that the circular selection rules governing interband optical transitions exactly at the band extrema are modified at finite wave vectors. The corresponding wave vector dependent corrections to the optical matrix elements result in a net linear polarization of excitonic PL. We identify two microscopic mechanisms responsible for linear polarization — trigonal warping of the electron and hole dispersions and the energy dependence of the effective masses. Their interplay controls both the magnitude and the angle of the emitted light polarization, with distinct dependences on the crystallographic orientation of the interface. Using a microscopic variational approach, we demonstrate that the degree of linear polarization can reach values exceeding 10% in realistic heterostructures. Furthermore, due to the large built-in dipole moment of interface excitons, their optical response can be tuned by an external in-plane electric field, enabling control over the strength and direction of the polarization.