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.

The semihydrogenated bilayer graphene (${\mathrm{C}}_2{\mathrm{C}}_2\mathrm{H}$), which is a bilayer graphene semihydrogenated on one graphene layer, can be considered as the merging of a monolayer graphene and a semihydrogenated graphene (${\mathrm{C}}_2\mathrm{H}$) through the interlayer bonds. Based on this picture, and an approximation to the low-energy band of ${\mathrm{C}}_2\mathrm{H}$, we construct the low-energy effective model of ${\mathrm{C}}_2{\mathrm{C}}_2\mathrm{H}$. In both the nonmagnetic state and the magnetic ordered state, the low-energy effective model of ${\mathrm{C}}_2{\mathrm{C}}_2\mathrm{H}$ is equivalent to the model of ${\mathrm{C}}_2\mathrm{H}$. Similar to ${\mathrm{C}}_2\mathrm{H}$, the Berry curvature distributions in the low-energy bands of ${\mathrm{C}}_2{\mathrm{C}}_2\mathrm{H}$ concentrate close to the two valleys of the hexagonal Brillouin zone and have opposite signs at the two valleys. Because of the close connection between the Berry curvature distribution and the optical selection rule, the optical excitations under circularly polarized light are valley polarized in both the nonmagnetic state and the magnetic state. In the magnetic ordered state, the optical excitations are also spin resolved, and are therefore spin and valley coupled. By tuning the resonant energies of the two spin channels via a vertical electric field, the low-energy optical excitations under circularly polarized light can be switched to any one of the four spin-valley polarization channels, indicating that ${\mathrm{C}}_2{\mathrm{C}}_2\mathrm{H}$ is a two-dimensional magnetic material with promising spin- and valley-resolved optoelectronic applications.

One of the major advances of quantum technology is the engineering of complex quantum channels in lattice systems, which paves the way for a variety of nonequilibrium phenomena. For a boundary driven lattice with such engineered quantum channels, the analysis of the full counting statistics of current across boundaries has received limited attention. In this work, we consider a boundary driven free-fermionic lattice with carefully engineered correlated gain and loss channels and obtain the cumulant generating function of the steady-state particle current. We also discuss the limit for simplifying the correlated gain-loss channel to a local gain-loss channel and obtain the average current and its fluctuation in such cases. Generally, in the presence of gain-loss, the current statistics are different at the two ends of the lattice. Hence, for both local and correlated gain-loss, we devise the conditions for which the statistics can coincide, giving rise to a $\mathcal{PT}$ symmetric balanced gain-loss scenario. A striking difference between the correlated gain-loss and their local counterpart is the emergence of nonreciprocity in the system and we observe that it has a dramatic impact in the current as well as fluctuations. Our work therefore provides interesting insights about the importance of engineered dissipators in boundary driven systems.

Multiferroic altermagnets, which integrate ferroelectricity with altermagnetism, provide a promising platform for realizing unconventional magnetoelectric coupling. However, the realization of multiple multiferroic orders in such systems remains largely unexplored. Here, we uncover electrically reconfigurable altermagnetism in three-dimensional (3D) van der Waals (vdW) crystals assembled from orthogonally stacked ferromagnetic (FM) transition metal oxytrihalide nanowires. Taking ${\mathrm{MoOBr}}_3$ as an example, first-principles calculations identify a ferroelectric (FE) ground state and four metastable polar variants—ferrielectric (FiE) plus three symmetry-inequivalent antiferroelectric (AFE) configurations—created by Mo off-center movement along the nanowire axis. All five phases retain altermagnetic (AM)-like nonrelativistic spin splitting characteristics, while spinorbit coupling (SOC) merely endows each with a distinct magnetic-anisotropy landscape. Crucially, controlled switching among the FE, AFE, and FiE states concurrently reshapes the momentum-resolved spin splitting and rotates the Néel vector by up to ${90}^{\circ }$, revealing a strong coupling between electric polarization and AM order. Additionally, uniaxial strain breaks the $C_{4x}$ symmetry protection, inducing a piezomagnetic response that transforms the compensated AM state into a weakly ferrimagnetic (FiM) state while simultaneously modulating the FE polarization magnitude. This synergy and coupling of multiple multiferroic orders hold potential candidates for developing nonvolatile memories, reconfigurable logic devices, and energy-efficient mechanospintronics.

The parity anomaly of a single massless Dirac fermion in $(2+1)$ dimensions produces a half-quantized Hall conductance, a phenomenon recently confirmed on the surface of three-dimensional (3D) ${\mathbb{Z}}_2$ topological insulators. Whether and how this anomaly generalizes to 3D topological phases carrying an integer winding number $W$ has remained an open question. Here, we address this problem for 3D chiral topological systems in the Altland-Zirnbauer symmetry classes AIII and DIII. By constructing minimal lattice models, we show that the winding number fixes the net chirality of the boundary Dirac or Majorana cones. By introducing a time-reversal-breaking surface mass to provide the necessary high-energy regularization, we explicitly expose their intrinsic parity anomaly. Through a layer-resolved Chern-number analysis, we establish a macroscopic topological magnitude law, $|{\mathcal{C}}_{\mathrm{surf}}|=|W|/2$, revealing that the shared chirality enforces a constructive accumulation of the fractional topological charges. We further demonstrate that this amplified anomaly coefficient is unambiguously readable in transport: a four-terminal setup in class AIII yields a quantized transmission asymmetry $\left|\mathrm{\Delta }T\right|=\left|W\right|/2$, while a six-terminal thermal Hall measurement in class DIII topological superconductors gives $|{\kappa }_{xy}^{\mathrm{surf}}|=|W|/4{\kappa }_0$. Notably, this thermal signature remains strictly robust against 3D Anderson disorder. Our results establish that the bulk winding number universally controls the macroscopic surface parity-anomaly coefficient in 3D chiral topological phases, providing concrete transport protocols for its detection in artificial metamaterial platforms and topological superconductor candidates.