We introduce a large class of models exhibiting robust ergodicity breaking in quantum dynamics. Our work is inspired by recent discussions of “topologically robust Hilbert space fragmentation” but massively generalizes in two directions: First from states describable as “loop-soups” to a broader class of states reminiscent of string-nets and sponges and, second, from models restricted to square or cubic lattices to models defined on arbitrary lattices and graphs, including those without translation invariance. Our constructions leverage a recently proposed group-theory framework [Balasubramanian et al., Phys. Rev. X 14, 021034 (2024)] and identify a host of new phenomena arising from the interplay of “group-model dynamics” and lattice structure. We make crisp connections to gauge theories, and our construction generalizes Kitaev's quantum double to infinite groups.

Perturbation theory with respect to the kinetic energy of the heavy component of a two-component quantum system is introduced. An effective Hamiltonian that is accurate to second order in the inverse heavy mass is derived. It contains a new form of kinetic energy operator with a Hermitian mass tensor and a complex-valued vector potential. All of the potentials in the effective Hamiltonian can be expressed in terms of covariant derivatives and a resolvent operator. The most salient application of the theory is to systems of electrons and nuclei. The accuracy of the theory is verified numerically in a model diatomic molecule and analytically in a vibronic coupling model.

Using a tight-binding model along with the mean-field Hubbard method, we investigate the effect of twisting angle on the magnetic properties of twisted bilayer graphene (tBLG) quantum dots (QDs) with triangular shape and zigzag edges. We consider such QDs in two configurations: when their initial untwisted structure is a perfect AA- or AB-stacked BLG, referred to as AA- or AB-like dots. We find that AA-like dots exhibit an antiferromagnetic spin polarization for small twist angles, which transits to a ferromagnetic spin polarization beyond a critical twisting angle ${\theta }_c$. Our analysis shows that ${\theta }_c$ decreases as the dot size increases, obeying a criterion, according to which once the maximum energy difference between electron and hole edge states (in the single-particle picture) is less than $(U/{\gamma }_0)t_0$, the spin-polarized energy levels are aligned ferromagnetically $[U$ is the Hubbard parameter and ${\gamma }_0$ $\left(t_0\right)$ the graphene intralayer (interlayer) hopping]. Unlike AA-like dots, AB-like dots exhibit finite magnetization for any twist angle. Furthermore, in the ferromagnetic polarization state, the ground net spin for both dot configurations agrees with the prediction from Lieb's theorem.

$D{0}_{19}\text{−}\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$ is a noncollinear antiferromagnet characterized by a unique ferroic ordering of cluster magnetic octupoles, which has attracted significant attention due to its strong ferromagneticlike responses. In this study, we study the magnetic properties of a $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$/NiFe bilayer. We characterize the damping constant of NiFe grown on an epitaxial $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn} \left(01\overline{1}0\right)$ layer, with kagome planes oriented perpendicular to the sample surface. The spin-mixing conductance of $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$/MgO (0.35 nm)/NiFe is calculated to be $g_{\mathrm{eff}}^{\uparrow \downarrow }=3.6\pm 0.6\times {10}^{18}{\mathrm{m}}^{-2}$ when the magnetization of NiFe is directed out of the kagome plane of $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$, and $g_{\mathrm{eff}}^{\uparrow \downarrow }=1.5\pm 0.1\times {10}^{18}{\mathrm{m}}^{-2}$ when the magnetization of NiFe aligned within the kagome plane. The relatively large spin-mixing conductance cannot be solely attributed to the pure spin current injection from NiFe to $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$ due to spin pumping; rather, it results from the coupled spin dynamics between the magnetic moments of NiFe and $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$, mediated by exchange interaction at the interface. Moreover, the anisotropy in the Gilbert damping constant of NiFe is likely due to the exchange spring effect in the $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$. This insight is crucial for the development of antiferromagnetic spintronic devices.

Recent studies show different results on the magnetic orders of monolayer $1T\text{−}\mathrm{CrT}{\mathrm{e}}_2$, an emerging two-dimensional (2D) magnetic material with a high magnetic transition temperature. Herein, based on first-principles calculations and Monte Carlo simulations, we find $1T\text{−}\mathrm{CrT}{\mathrm{e}}_2$ can host diverse magnetic orders, ranging from collinear antiferromagnetic states to helimagnetic states with single or multiple incommensurate propagation vectors, which can be sensitively tuned by strain and interlayer coupling effects in van der Waals heterostructures. Some of these magnetic orders can host $Z_6$ topological domain-wall structures that are rarely seen in 2D magnets. While compressive biaxial strain tends to induce unconventional antiferromagnetic states, increasing tensile biaxial strain generally stabilizes $1T\text{−}\mathrm{CrT}{\mathrm{e}}_2$ in ferromagnetic orders with single-ion magnetic anisotropy energies affected by its substrates. The highly susceptible magnetic order of monolayer $1T\text{−}\mathrm{CrT}{\mathrm{e}}_2$ originates from the synergic effects between the triangular Cr lattice that can mediate magnetic frustration and the itinerant electrons that can lead to long-range magnetic couplings and significant charge transfer with substrates.

This study provides insights into the static and dynamic quantum properties of the trinuclear high-spin iron(III) molecular complex $\left[{\mathrm{Fe}}_3{\mathrm{Cl}}_3\left({\mathrm{saltag}}^{\mathrm{Br}}\right){\left(\mathrm{py}\right)}_6\right]{\mathrm{ClO}}_4$, abbreviated as ${\mathrm{Fe}}_3$. Using exact diagonalization of a spin-$\frac{5}{2}$ Heisenberg triangle in a magnetic field, we model the corresponding quantum behavior of the molecular compound ${\mathrm{Fe}}_3$. Our rigorous analysis employs various key metrics to explore the rich quantum behavior of this molecular compound. At sufficiently low temperatures, the bipartite negativity reveals that the pairwise entanglement between any pair of iron(III) magnetic ions of the molecular complex ${\mathrm{Fe}}_3$ can be significantly enhanced by a small magnetic field. This enhancement is followed by unconventional steplike changes characterized by a sequence of plateaus and sudden downturns as the magnetic field further increases. A qualitatively similar behavior is also observed in the genuine tripartite entanglement among all three iron(III) magnetic ions in the trinuclear complex ${\mathrm{Fe}}_3$. Notably, the bipartite and tripartite entanglements persist in the molecular complex ${\mathrm{Fe}}_3$ up to moderate temperatures of approximately 30 and 70 K, respectively. Additionally, we demonstrate the achievement of quantum-enhanced sensitivity by initializing the molecular complex ${\mathrm{Fe}}_3$ in Dicke states. Finally, we investigate a quantum sensing protocol by applying a local magnetic field specifically to one iron(III) magnetic ion of the molecular compound ${\mathrm{Fe}}_3$ and performing readout sequentially on one of the two remaining iron(III) magnetic ions.

The two-dimensional Hubbard model is widely believed to capture the key ingredients of high-temperature superconductivity in cuprate materials. Here we report a constrained path quantum Monte Carlo study of the square-lattice extended Hubbard model with on-site Coulomb repulsion $U$ and nearest-neighbor (NN) electron attraction $V$. Upon doping $\delta$ = 0.125, we find that NN electron attraction $V$ significantly promotes an exotic spin-triplet ($p$-wave) pairing, with the $p$-wave pairing correlations growing stronger as $V$ increases. Meanwhile, the $d_{x^2-y^2}$-wave ($d$-wave) pairing in the intermediate coupling regime shows an insignificant response to the increase of $V$. The spin density wave order, which is observed near half filling in the particle-hole channel, is also almost unaffected by the NN electron attraction $V$, reinforcing the established relationship between $d$-wave superconductivity (SC) and spin order. As the doping becomes heavier (i.e., $\delta$ increases), the dominant region of $p$-wave pairing expands, further suppressing the presence of $d$-wave pairing. Our paper suggests the $p$-wave pairing region can be induced and further broadened by the NN electron attraction $V$, offering a feasible mechanism to realize $p$-wave SC in realistic cuprate materials.

Krylov complexity has recently been proposed as a quantum probe of chaos. The Krylov exponent characterizing the exponential growth of Krylov complexity is conjectured to upper-bound the Lyapunov exponent. We compute the Krylov and the Lyapunov exponents in the Sachdev-Ye-Kitaev model and in some of its deformations. We do this analysis both at infinite and finite temperatures, in models where the number of fermionic interactions is both finite and infinite. We consider deformations that interpolate between two regions of near-maximal chaos and deformations that become nearly integrable at low temperatures. In all cases, we find that the Krylov exponent upper-bounds the Lyapunov one. However, we find that while the Lyapunov exponent can have nonmonotonic behavior as a function of temperature, in all studied examples the Krylov exponent behaves monotonically. For instance, we find models where the Lyapunov exponent goes to zero at low temperatures, while the Krylov exponent saturates to its maximal bound. We speculate on the possibility that this monotonicity might be a generic feature of the Krylov exponent in quantum systems evolving under unitary evolution.

The properties and experimental identification of quantum spin liquids (QSLs) remain an important topic with many fundamental questions. Here, we explore the dynamics of a single charge dopant (hole) in a $t-J_1-J_2$ model on a square lattice, which realizes a gapless ${\mathbb{Z}}_2$ QSL at half filling. Using a field theory approach based on the parton construction, which includes an infinite number of scatterings between the low-energy quasiparticle excitations of the QSL via a self-consistent Born approximation, we calculate both the equilibrium and nonequilibrium properties of the hole for weak and strong interactions. Quasiparticle branches as well as stringlike excitations of the holon are identified, and we furthermore explore the time-dependent spreading of a hole throughout the QSL after it has been injected at a given lattice site. The final ballistic expansion speed is shown to exhibit a nonmonotonic behavior as a quantum phase transition between an antiferromagnetic and the QSL phase is crossed, which is caused by a qualitative change in the fundamental kinematics of the interactions between the hole and the surrounding spins. Our results demonstrate how charge dopants can be used as a quantum probe for QSLs and are directly relevant to optical lattice experiments with single site resolution.

Using finite temperature Raman spectroscopy, we investigate the electron-phonon interactions (EPI) and phonon-phonon scattering dynamics in the Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ in different frequency regimes. Strong softening of the Raman shifts below 200 K is observed for almost all the phonon modes with a marked deviation from the standard anharmonic behavior. The experimentally observed Raman linewidth seems to be captured well by a combination of EPI, relevant at low temperature (LT) and phonon-phonon scattering, which is predominant at high temperatures (HT), leading to an observable minima in the thermal evolution of the linewidth. While this feature is most prominently observed in the highest-frequency Raman mode ($\sim 196{\mathrm{cm}}^{-1}$), its intensity gradually diminishes as the Raman frequency decreases. Computation of the electronic contribution to the phonon linewidth, for both the high and low frequency modes, from the phonon self-energy shows that it qualitatively mimics the experimental observations. It is found that phonon-induced interband scattering results in the presence of a maxima in phonon linewidth that crucially depends on the finiteness of the chemical potential.

The non-Hermitian skin effect, anomalous localization of an extensive number of eigenstates induced by nonreciprocal dissipation, plays a pivotal role in non-Hermitian topology and significantly influences the open quantum dynamics. However, its genuinely quantum characterization in many-body systems has yet to be developed. Here, we elucidate that the skin effect manifests itself as multifractality in the many-body Hilbert space. This multifractality does not accompany the single-particle skin effect and is therefore intrinsic to the many-body skin effect. Furthermore, we demonstrate that the many-body skin effect coexists with spectral statistics of random matrices, in contrast to multifractality associated with many-body localization, which necessitates the absence of ergodicity. We also illustrate multifractality caused by the Liouvillian skin effect in Markovian open quantum systems. Our paper establishes a defining characterization of the non-Hermitian skin effect and uncovers a fundamental relationship between multifractality and ergodicity in open quantum many-body systems.

The discovery of collinear symmetric-compensated altermagnets (AM) with intrinsic spin splitting provides a route toward energy-efficient and ultrafast device applications. Here, using first-principle calculations and symmetry analysis, we construct a series of AM $\mathrm{C}{\mathrm{r}}_2\mathrm{S}X$ ($X$ = O, S, Se) monolayers and explore the spin splitting in $\mathrm{C}{\mathrm{r}}_2\mathrm{S}X$ multilayer. A spin-layer coupling design principle for predicting electronic spin behavior in odd and even layers is mapped out based on the comprehensive analysis of spin group symmetry. The spin-splitting behavior related to the ${\mathcal{M}}_{\mathcal{z}}\mathcal{U}t$, ${\mathcal{M}}_{\mathcal{z}}$ and ${\mathcal{M}}_L$ symmetries in AM multilayers can be significantly modulated by magnetic orders, crystal symmetry, and external perpendicular gate field ($E_{\mathcal{z}}$). Due to the spin-compensated bands of sublayers linked by overall ${\mathcal{M}}_{\mathcal{z}}$ and interlayers ${\mathcal{M}}_L$ symmetries, the $\mathrm{C}{\mathrm{r}}_2{\mathrm{S}}_2$ odd-layer exhibits the unique coexistence of spin splitting and spin degeneracy at high symmetry paths and in the X or Y valley, respectively. Furthermore, owing to the higher priority of overall ${\mathcal{M}}_L$ symmetry compared to interlayers ${\mathcal{M}}_L$ symmetry in AM even layers, the spin-layer coupling of AM multilayers shows strong odd- and even-layer dependence. Our work not only offers a new direction for manipulating spin splitting but also greatly enriches the research on AM monolayer and multilayer.

We study the asymmetric simple exclusion process with nondiagonal boundary terms under a specific constraint. A symmetric chiral basis is constructed and a special string solution of the Bethe ansatz equations corresponding to the steady state is presented. Using the coordinate Bethe ansatz method, we derive a concise expression for the steady state. The current and density profile in the steady state are also studied.

The realization of two-dimensional multiferroics offers significant potential for nanoscale device functionality. However, type-I two-dimensional multiferroics with strong magnetoelectric coupling, enabling electric field control of spin, remain scarce. In this study, using density functional theory and Monte Carlo simulations, we predict that the niobium phthalocyanine covalent organic framework (NbPc COF) monolayer exhibits type-I multiferroic behavior, with a ferroelectric transition occurring above room temperature. Remarkably, the strong magnetoelectric coupling in NbPc COF monolayer arises from the same origin of magnetism and ferroelectricity. Our findings offer flexible pathways for the design and development of organic nanoscale multiferroic devices with broad applications.

The formation of metallic nanofilaments bridging two electrodes across an insulator is a mechanism for resistive switching. Examples of such phenomena include atomic synapses, which constitute a distinct class of memristive devices the behavior of which is closely tied to the properties of the filament. Until recently, experimental investigation of the low-temperature regime and quantum transport effects has been limited. However, with growing interest in understanding the true impacts of the filament on device conductance, comprehending quantum effects has become crucial for quantum neuromorphic hardware. We discuss quantum transport resulting from filamentary switching in a narrow region where the continuous approximation of the contact is not valid, and only a few atoms are involved. In this scenario, the filament can be represented by a graph depicting the adjacency of atoms and the overlap between atomic orbitals. Using the theory of quantum graphs with locally diffusive node scattering, we calculate the scattering amplitude of charge carriers on this graph and explore the interplay between filamentary formation and quantum transport effects.

We report the nonthermal melting dynamics of charge density wave (CDW) induced by intense terahertz (THz) pulse in a thin film of transition-metal dichalcogenide, $3R\text{−}\mathrm{T}{\mathrm{a}}_{1+x}\mathrm{S}{\mathrm{e}}_2$. With increasing the pump THz fluence, a progressive softening of the CDW amplitude mode appears, accompanied by rapid increase of the mode damping. This behavior is well reproduced by time-dependent Ginzburg-Landau (GL) model analysis, manifesting the dynamical suppression of the GL potential and thus the CDW order. The injected energy needed for the melting of the CDW order is one order of magnitude smaller than the energy required to heat the sample to the CDW transition temperature, indicating the nonthermal melting of the CDW order induced by the THz pulse excitation. The observed highly efficient melting of the CDW order and the subsequent appearance of long-lived prethermalized state demonstrate that THz pulse is an efficient tool for the nonthermal control of CDW order through the mode-selective excitation of the electron-phonon subsystem responsible for the CDW formation.

We investigate the electronic structure of graphene monolayers subjected to patterned dielectric superlattices (SLs). Through a capacitance model approach, we simulate realistic devices capable of imposing periodic potentials on graphene. By means of both tight-binding and continuum models, we analyze the electronic structure across varied patterning geometries, including triangular, kagome, and square configurations. We explicitly explore the influence of device parameters such as the SL potential strength, geometry, and periodicity on the electronic properties of graphene. By introducing a long-range Coulomb interaction, we found an emergent periodic potential strong enough to open a mass gap, thereby generating a Chern band. In this paper, we highlight the robustness and versatility of patterned dielectric SLs for band engineering in graphene systems.