We present a functional-based approach to compute thermal expectation values for actions expressed in the $G\text{−}\mathrm{\Sigma }$ formalism, applicable to any time sequence ordering. Utilizing this framework, we analyze the linear response to an electric field in various Sachdev-Ye-Kitaev (SYK) chains. We consider the SYK chain where each dot is a complex $q/2$-body interacting SYK model, and we allow for $r/2$-body nearest-neighbor hopping where $r=\kappa q$. We find exact analytical expressions in the large-$q$ limit for conductivities across all temperatures at leading order in $1/q$ for three cases, namely, $\kappa =\{1/2,1,2\}$. When $\kappa =\{1/2,1\}$, we observe linear-in-temperature $T$ resistivities at low temperatures, indicative of strange metal behavior. Conversely, when $\kappa =2$, the resistivity diverges as a power law at low temperatures, namely, as $1/T^2$, resembling insulating behavior. As $T$ increases, there is a crossover to Fermi liquid behavior ($\sim T^2$) at the minimum resistivity. Beyond this, another crossover occurs to strange metal behavior ($\sim T$). In comparison to previous linear-in-$T$ results in the literature, we also show that the resistivity behavior exists even below the MIR bound, indicating a true strange metal instead of a bad metal. In particular, we find for the $\kappa =2$ case a smooth crossover from an insulating phase to a Fermi liquid behavior to a true strange metal and eventually becoming a bad metal as temperature increases. We extend and generalize previously known results on resistivities to all temperatures, do a comparative analysis across the three models where we highlight the universal features and invoke scaling arguments to create a physical picture out of our analyses. Remarkably, we find a universal maximum DC conductivity across all three models when the hopping coupling strength becomes large.

We consider families of invertible many-body quantum states in $d$ spatial dimensions that are parameterized over some parameter space $X$. The space of such families is expected to have topologically distinct sectors classified by the cohomology group ${\mathrm{H}}^{d+2}(X;\mathbb{Z})$. These topological sectors are distinguished by a topological invariant built from a generalization of the Berry phase, called the higher Berry phase. In the previous work [Phys. Rev. B 109, 115152 (2024)], we introduced a generalized inner product for three one-dimensional many-body quantum states (“triple inner product”). The higher Berry phase for one-dimensional invertible states can be introduced through the triple inner product and furthermore the topological invariant, which takes its value in ${\mathrm{H}}^3(X;\mathbb{Z})$, can be extracted. In this paper, we introduce an inner product of four two-dimensional invertible quantum many-body states. We use it to measure the topological nontriviality of parameterized families of 2d invertible states. In particular, we define a topological invariant of such families that takes values in ${\mathrm{H}}^4(X;\mathbb{Z})$. Our formalism uses projected entangled pair states (PEPS). We also construct a specific example of nontrivial parameterized families of 2d invertible states parameterized over $\mathbb{R}{P}^4$ and demonstrate the use of our formula. Applications for symmetry-protected topological phases are also discussed.

We demonstrate that (001) grown ${\mathrm{Cd}}_3{\mathrm{As}}_2$ thin films with a superlattice-patterned gate can potentially realize the moiré Bernevig-Hughes-Zhang model. Our calculations identify the parametrization region necessary to achieve topological flat minibands with a $C_{4z}$ symmetric and a $C_{6z}$ symmetric potential. Additionally, we show that a spin-polarized state can serve as the minimal platform for hosting the moiré-induced quantum anomalous Hall effect, supported by Hartree-Fock interaction kernel analysis and self-consistent mean-field calculations.

Recently, Floquet topological matter has attracted significant attention for its potential to reveal novel topological phases inaccessible in static systems. In this paper, we investigate the effect of a time-periodic driving on the five-dimensional (5D) normal insulators. We show that the time-periodic driving can induce a topological phase transition from a TP (time reversal combined with space inversion) symmetry-preserving normal insulator to a 5D Floquet topological semimetal with emergent Yang monopoles characterized by the second Chern number. Additionally, we show that this time-periodic driving can also lead to a topological phase transition from a TP symmetry-breaking normal insulator to a 5D Floquet topological semimetal with a Hopf link formed by Weyl surfaces. Besides, further increasing the strength of the time-periodic driving, the two 5D Floquet topological semimetal phases are transformed into the 5D Floquet Chern insulators. Our paper is helpful for future studies in higher-dimensional Floquet systems.

We theoretically find that the second-order topological insulator, i.e., corner states, can be engineered by coupling two copies of two-dimensional ${\mathbb{Z}}_2$ topological insulators with opposite spin helicities. As concrete examples, we utilize Kane-Mele models (i.e., graphene with intrinsic spin-orbit coupling) to realize the corner states by setting the respective graphenes as ${\mathbb{Z}}_2$ topological insulators with opposite intrinsic spin-orbit couplings. To exhibit its universality, we generalize our findings to other representative ${\mathbb{Z}}_2$ topological insulators, e.g., the Bernevig-Hughes-Zhang model. An effective model is presented to reveal the physical origin of the corner states. We further show that the corner states can also be designed in other topological systems, e.g., by coupling quantum anomalous Hall systems with opposite Chern numbers. Our work suggests that interlayer coupling can be treated as a simple and efficient strategy to drive two-dimensional lower-order topological insulators to the higher-order ones.

Antiferromagnetic spintronics exhibits ultrahigh operational speed and stability in a magnetic field, holding promise for the realization of next-generation ultra-high-speed magnetic storage. However, theoretical exploration of the electronic transport properties of antiferromagnetic-multiferroic tunnel junction (AMFTJ) devices remains largely unexplored. Here, we design an antiferromagnet/ferroelectric barrier/antiferromagnet van der Waals heterojunction, renamed vdW AMFTJ, using a bilayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{In}}_2{\mathrm{Se}}_3/\mathrm{bilayer}{\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT-2L/IS/MBT-2L) as the prototype. Based on first-principles calculations using the nonequilibrium Green's function method combined with density functional theory, we theoretically investigate the spin-resolved electronic transport properties of this AMFTJ. By manipulating the various possible magnetization directions of the multilayer antiferromagnetic ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and the ferroelectric polarization direction of the ${\mathrm{In}}_2{\mathrm{Se}}_3$ within the junction, 16 distinct nonvolatile resistance states can be revealed and manipulated by applying external biaxial strain and bias voltage. We predict maximum tunneling magnetoresistance (electroresistance) values of $3.79\times {10}^4\%$ ($2.41\times {10}^5\%$) in the equilibrium state, which can increase up to $5.01\times {10}^5\%$ ($4.97\times {10}^5\%$) under external bias voltage. Furthermore, the perfect spin-filtering effect is also present in our AMFTJ. Our results highlight the tremendous potential of the MBT-2L/IS/MBT-2L vdW AMFTJ in nonvolatile memory, expanding the application avenues for antiferromagnetic spintronic devices.

In this work, we propose a mechanism of inducing stable nodal superconductivity for multiple-flat-band systems. This mechanism is based on the degenerate flat band nature, so that not only intra-eigenband pairing, but also inter-eigenband pairing has a significant influence on the superconductivity properties. Based on the Bogoliubov-de Gennes formalism of the heavy fermion model for the twisted bilayer graphene as an example, we show that although the nodal nematic Euler obstructed pairing has higher energy around the nodal points compared to the fully gapped chiral $d$-wave pairing in the momentum space, the inter-eigenband pairing can lower the energy in other momenta away from the nodes, so that the nematic Euler obstructed pairing is energetically favored for its overall condensation energy. This type of mechanism is particular to multiple-flat-band systems with no Fermi surfaces.

We study the stroboscopic nonequilibrium quantum dynamics of periodically kicked Hamiltonians involving homogeneous central-spin interactions. The system exhibits a strong fragmentation of Hilbert space into four-dimensional Floquet-Krylov subspaces, which oscillate between two disjointed two-dimensional subspaces and thus break the discrete time-translation symmetry of the system. Our analytical and numerical analyses reveal that fully polarized states of the satellite spins exhibit fragmentations that are stable against perturbations and have high overlap with Floquet eigenstates of atypically low bipartite entanglement entropy (scar states). Motivated by the breaking of discrete time translation symmetry by Floquet-Krylov subspaces, we introduce a novel type of time crystal that we call a “subspace time crystal.” We present evidence of robust time-crystalline behavior in the form of a period doubling of the total magnetization of fully polarized satellite spin states that persists over long timescales. We compute nonequilibrium phase diagrams with respect to a magnetic field, coupling terms, and pulse error for various interaction types, including Heisenberg, Ising, XXZ, and XX. We also discuss possible experimental realizations of scar time crystals in color center, quantum dot, and rare-earth ion platforms.

The topological flat band close to the Fermi level serves as a promising platform for realizing unique and intriguing phenomena. In theory, such a flat band can appear in certain geometrically frustrated lattices under stringent conditions, and then generally can be destroyed by the hybridization with other bands and the application of external fields. Consequently, the realistic materials that host topological flat bands are very rare. Via first-principles calculations, we investigate the electronic structure of the monolayer MOF PTC-Fe, which host a kagome lattice and has been experimentally synthesized. The monolayer PTC-Fe is ferromagnetic in its ground state. Remarkably, around the Fermi level, a flat band with almost divergent density of state occurs in the down-spin channel. Moreover, the flat band is robust against band hybridization that it crosses with two other bands, forming two concentric critical nodal lines. The symmetry protection of the robust flat band, the optical responses, and the intrinsic Hall conductivities of the monolayer PTC-Fe are investigated. Our results highlight the 2D kagome MOF PTC-Fe as a promising candidate material to study the interaction between flat band and novel topological properties.

We study the size of two-body bound states and Cooper pairs within a multiband Hubbard model that features time-reversal symmetry and uniform pairing on a generic lattice. Our analysis involves (i) an exact calculation of the localization tensor to determine the size of lowest-lying two-body bound state in vacuum, and (ii) an evaluation of the analogous tensor to estimate the average size of Cooper pairs within the mean-field Bardeen-Cooper-Schrieffer to Bose-Einstein condensate (BCS-BEC) crossover theory at zero temperature. Beyond the conventional intraband contribution that depends on Bloch bands, we show that pair size also has a geometric contribution governed by the quantum-metric tensor of the Bloch states and their band-resolved quantum-metric tensors. As a concrete example, we investigate the pyrochlore-Hubbard model numerically and demonstrate that, while the pair size diverges in the weakly interacting BCS regime of dispersive bands, it remains finite and relatively small in the flat-band regime, even for infinitesimal interaction, perfectly matching the exact two-body result in the dilute limit.

We report the observation of current-modulated magnetoresistance hysteresis below the superconducting transition temperature in the kagome superconductor $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$. This highly tunable hysteresis behavior is confined to the superconducting state and vanishes when superconductivity is fully suppressed, directly linking magnetoresistance hysteresis to the superconducting order in $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$. Additionally, the superconducting diode effect driven by a small magnetic field is observed, indicating the enhanced electronic magnetochiral anisotropy by the chiral domain-wall scattering. Our findings position $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$ as a promising platform for exploring nontrivial physical phenomena, including unconventional pairing mechanisms and topological superconductivity.

Understanding the nature of liquid structure and its evolution under different conditions is a major challenge in condensed-matter physics and materials science. Here, we report a pressure-induced structure change spanning a wide pressure range in liquid-state lithium hydride (LiH) by first-principles molecular dynamic simulations. This behavior can be described as a continuous crossover from low pressure liquid with $\mathrm{L}{\mathrm{i}}^{+}\text{−}{\mathrm{H}}^{-}$ duality symmetry to a high pressure one with broken duality symmetry. The thermodynamic quantities such as heat capacity and ionic transport properties such as diffusivity are also saliently impacted. It is important to stress that such behavior is first predicted for this category of materials, which is ubiquitous in the universe as well as in industry applications. Last, a comprehensive high-pressure high-temperature phase diagram of LiH is constructed, which embodies rich physics in this previously thought simple ionic compound.

The non-Hermitian skin effect (NHSE) characterized by the tendency of bulk states to concentrate toward the boundary is one of the most important and widespread phenomena within non-Hermitian Hamiltonian systems. Due to their topological origin, skin modes are usually robust and difficult to regulate. In our work, we engineered magnetic-field-like on-site and randomlike on-site potentials, and we found nonmonotonic and monotonic suppression patterns on skin modes. These suppression patterns represent characteristic transitions as the on-site potential shifts from order to disorder. Relying solely on on-site potential energy engineering, we develop a general strategy to control suppression of skin modes. By integrating with the scaling theory, we have extended the concept of controllable suppression of NHSEs to higher-dimensional systems.

Dual-unitary quantum circuits can provide analytic spatiotemporal correlation functions of local operators from transfer matrices, enriching our understanding of quantum dynamics with exact solutions. Nevertheless, a full understanding is still lacking as the case of a nondiagonalizable transfer matrix with exceptional points has barely been explored. In this paper, we give an inverse approach for constructing dual-unitary quantum circuits with exceptional points in the transfer matrices by establishing relations between transfer matrices and local unitaries. As a consequence of the coalesce of eigenvectors, the correlation functions exhibit a polynomial modified exponential decay, which is significantly different from pure exponential decay, especially at early stages. Moreover, we point out that the Hamiltonian evolution of a kicked $XXZ$ spin chain can be approximately mapped to a dual-unitary circuit with exceptional points by Trotter decomposition. Finally, we investigate the dynamics approaching and at exceptional points, showing that behaviors of correlation functions are distinct by $Z$ transformation.

Quantum Hall plateaus at quarter fillings occur in GaAs wide quantum wells, hole-doped GaAs, and bilayer graphene. However, the interactions favoring incompressible states over compressible composite-Fermi liquids at such fillings are not well understood. We devise a method for the computation of the trial energies for Haldane pseudopotentials via Monte Carlo sampling. Applying it to the quarter-filled lowest Landau level, we find that tuning the third and fifth pseudopotential values can stabilize the anti-Pfaffian, Moore-Read, and $f$-wave states. The smallest deviations from pure Coulomb interactions are required by anti-Pfaffian, whose presence is indicated by daughter states in recent experiments of bilayer graphene at $\nu =\frac{3}{4}$.

The contraction of tensor networks is a central task in the application of tensor network methods to the study of quantum and classical many-body systems. In this paper, we investigate the impact of gauge degrees of freedom in the virtual indices of the tensor network on the contraction process, specifically focusing on boundary matrix product state methods for contracting two-dimensional tensor networks. We show that the gauge transformation can affect the entanglement structures of the eigenstates of the transfer matrix and change how the physical information is encoded in the eigenstates, which can influence the accuracy of the numerical simulation. We demonstrate this effect by looking at two different examples. First, we focus on the local gauge transformation, and analyze its effect by viewing it as an imaginary-time evolution governed by a diagonal Hamiltonian. As a specific example, we perform a numerical analysis in the classical Ising model on the square lattice. Second, we go beyond the scope of local gauge transformations and study the antiferromagnetic Ising model on the triangular lattice. The partition function of this model has two tensor network representations connected by a nonlocal gauge transformation, resulting in distinct numerical performances in the boundary matrix product state calculation.

In this paper, we introduce a methodology that employs machine learning, specifically convolutional neural networks (CNNs), to discern the disorder landscape in the parameters of the disordered extended Hubbard model underlying the semiconductor quantum dot spin qubit architectures. Our approach, which we demonstrate with numerically simulated devices, takes advantage of currently used experimentally obtainable charge stability diagrams from neighboring quantum dot pairs, enabling the CNN to accurately identify disorder in each parameter of the extended Hubbard model. Remarkably, our CNN can process site-specific disorder in Hubbard parameters, including variations in hopping constants, on-site potentials (gate voltages), and both intrasite and intersite Coulomb terms. This advancement facilitates the prediction of spatially dependent disorder across all parameters simultaneously with high accuracy ($R^2>0.994$) and fewer parameter constraints, marking a significant improvement over previous methods that were focused only on analyzing on-site potentials at low coupling. Furthermore, our approach allows for the tuning of five or more quantum dots at a time, effectively addressing the often-overlooked issue of crosstalk. Not only does our method streamline the tuning process, potentially enabling fully automated adjustments, but it also introduces a “no trust” verification method to rigorously validate the neural network's predictions. Ultimately, this paper aims to lay the groundwork for generalizing our method to tackle a broad spectrum of physical problems.

We investigate the transmission properties of a junction between a normal metal and a $p$-wave magnet in the ballistic regime. We introduce a two-dimensional square lattice that confirms the $p$-wave magnet criteria. The $\alpha$ vector of the magnet breaks the inversion symmetry of the parabolic dispersion, shifting them in $k$ space. These shifts alter the propagation direction of fermions passing through the junction interface. Depending on spin orientation, the transmission process exhibits anisotropic, angle-dependent behavior. We also demonstrate a mirror symmetry between fermions with opposite spin directions, leading to the emergence of a transverse spin current that flows parallel to the interface. Additionally, we show that the $\alpha$ vector acts as a source for the dynamics of the spin-density wave and observe the formation of an indirect gap in the conductance of the junction. Our findings highlight the unique transmission characteristics and spin transport phenomena in normal metal/$p$-wave magnet junctions, paving the way for potential applications in spintronic devices.

We report transport measurements of infinite-layer cuprate ${\mathrm{Sr}}_{1-x}{\mathrm{Eu}}_x{\mathrm{CuO}}_{2+y}$ films with controlled electron (by trivalent europium) and hole (by interstitial apical oxygen) carriers grown on ${\mathrm{SrTiO}}_3$(001) with molecular beam epitaxy. An unusual enhancement of superconductivity upon the moderate coexistence of electron and hole carriers in the films is found, which spans over the whole superconducting phase diagram and becomes more prominent in the underdoped regime. The superconductivity exhibits a two-dimensional nature with a thickness of approximately 5.2 nm, irrespective of the varying carriers, confirmed by angle-resolved magnetoresistance measurements and the Berezinskii-Kosterlitz-Thouless transition. Nevertheless, the coexisting electron and hole carriers enlarge the thermal activation energy of vortex motion that deviates obviously from the usual logarithmic evolution with the magnetic field. Our results offer a promising perspective to understand and enhance the high-temperature superconductivity in cuprates.