arXiv subjects

Xi Dai

Publications and source records attributed to Xi Dai.

At least 19 recordsLinked to original sources

arXiv2026

Gauge-covariant magnetic Bloch sums for general multiorbital Hofstadter models

We formulate a unified treatment of the Hofstadter problem for general two-dimensional multiorbital Peierls tight-binding Hamiltonians using a gauge-covariant magnetic Bloch-sum basis. The construction retains the full Bravais geometry, arbitrary intracell orbital positions, and the original hopping table, so lattice geometry, orbital embedding, hopping range, and orbital content can all be handled within the same framework. At rational flux $\Phi/\Phi_0=p/q$, commuting magnetic translations reduce the Peierls Hamiltonian to minimal $qN_{\rm orb}\times qN_{\rm orb}$ blocks. Their dimension depends only on the flux through the primitive cell, even when the fractional orbital coordinates are irrational. Electromagnetic gauge transformations act by unitary conjugation within the construction and do not alter the required magnetic supercell. We derive an explicit sparse matrix in an oblique Landau gauge and establish the associated band counting, spectral redundancy, Chern-number formulation, magnetic spatial constraints, and flux periodicity. Numerical examples include elementary lattices, topological and flat-band models, and a spinful 22-band Wannier Hamiltonian of monolayer $\mathrm{MoS}_2$, demonstrating a direct interface with first-principles electronic-structure calculations. As a complementary representation, we also derive exact generalized Harper equations from the same hopping data and relate them to the finite magnetic-Bloch blocks.

cond-mat.mes-hall↗

arXiv2026

Ising Dirac fermions across a topological phase transition

Dirac fermions have attracted significant interest due to their relativistic dispersions and close connections to topological physics, yet they are generally expected to be gapped in two-dimensional systems with strong Ising spin orbit coupling, making their realization in such materials an outstanding challenge. Here we report the emergence of six fold degenerate Dirac fermions in an Ising moire system across a quantum spin Hall transition in twisted WSe2. In a 3.65 degree device, we observe a quantum spin Hall phase at high electric fields with nearly quantized resistance h/(2e2), and a Dirac semimetal phase over a broad range of electric fields near zero field. Magnetotransport measurements of the Dirac phase exhibit a half-integer Landau fan sequence, characteristic of Dirac fermions, with six-fold degeneracy on the hole-doped side and two fold degeneracy on the electron-doped side. Temperature dependence shows weakly metallic behavior consistent with a semimetallic state. Our twist-angle-dependent transport measurements map out a complete phase diagram and identify a critical twist angle of 3.3 degree, establishing the phase boundary between the quantum spin Hall and Dirac semimetal regimes. Our work establishes a new route to realizing Dirac fermions in strongly spin orbit coupled moire systems through a topological phase transition, providing a promising platform for high mobility spintronics.

cond-mat.mes-hall↗

arXiv2026

Boundary-Robust Transmission Asymmetry as a Topological Signature in Open Floquet Lattices

We identify a boundary-robust topological signature of open Floquet lattices: although nonadiabatic boundaries strongly reshape the transmission lineshape, the integrated left--right transmission asymmetry saturates to a plateau set by the bulk Floquet winding number. Its origin is a deep-bulk branch-population principle: in the long-sample limit, each propagating Floquet--Bloch branch is generically populated with unit weight, since true Floquet bound states are nongeneric. The robust observable is therefore the cumulative transmission imbalance rather than the boundary-sensitive transmission profile. We propose direct detection by cold-atom transmission spectroscopy. For electronic transport, the same asymmetry admits contact-model-dependent electrical readouts: a coherent Floquet--Landauer--B\"uttiker interpretation predicts a near-$2ef$ response in weak SAW devices, whereas a blocking-factor post-processing yields a qualitatively different signal.

cond-mat.mes-hall↗

arXiv2026

Geometric Spin Degeneracy in Spin-Orbit-Free Compensated Magnets

Compensated magnets with vanishing net magnetization can exhibit both pronounced spin splitting and unconventional band degeneracies. In altermagnets, such degeneracies are enforced by crystal and magnetic symmetries. In compensated ferrimagnets, however, they may arise even in the absence of the corresponding symmetry protection, raising a fundamental question about the origin of spin degeneracy in spin-orbit-free magnetic systems. Here, we develop a theoretical framework for spin-orbit-free compensated magnets in which spin degeneracies are protected by geometric constraints rather than by spin symmetry. We show that zero net magnetization imposes a strong condition for the emergence of nodes formed by formally spin-degenerate bands, even when no conventional spin symmetry is present. Our analysis, applicable in the weak-interaction regime, identifies a general mechanism for spin degeneracy beyond group-theoretical protection. The framework accounts for the unconventional spin degeneracies recently reported in compensated ferrimagnets and provides a unified description of band degeneracies across a broad class of magnetic phases with negligible spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2026

Strongly Correlated Superconductivity in Twisted Bilayer Graphene: a Gutzwiller Study

We study strongly correlated superconductivity in magic-angle twisted bilayer graphene (MATBG) using a variational Gutzwiller wavefunction $\ket{\Psi_G} = \prod_{\vb{R}} \hat{P}_{\vb{R}} \ket{\Phi_0}$, where the Gutzwiller projector $\hat{P}_{\vb{R}}$ is allowed to break charge U(1) symmetry to accommodate superconducting (SC) order. The ground state energy is evaluated via the \textit{Gutzwiller Approximation} applied to an 8-band model consisting of correlated $f$-orbitals and uncorrelated $c$-orbitals, with interactions including onsite Coulomb repulsion $U$, phonon-mediated anti-Hund's coupling $\hat{H}_{J_A}$, and intra-orbital Hund's coupling $\hat{H}_{J_H}$. At filling $\nu = 2.5$, we map out the phase diagram as a function of $U$ and $J_A$, and reveal a strongly correlated SC (SC-SC) phase dominates at large $U$, wherethe strong on-site interaction U strongly suppress the $f$-orbital charge fluctuations while maintaining finite pairing order and a sizeable quasiparticle weight Z, distinguishing it from a conventional Mott insulator. For a range of $J_{\rm A}$, SC-SC transitions to FL as $U$ decreases, until the weakly correlated BCS-like SC (BCS-SC) re-enters as $U \to 0$. We further identify a novel small Fermi liquid (sFL) state with effective Fermi surface formed by $c$-orbitals, which is essentially different with the normal Fermi liquid. Interestingly, in the intermediate- ($U \lesssim 40$ meV) and large-$U$ ($U \gtrsim 40$ meV) regimes, the conventional FL and the sFL are the lowest-energy normal phases, respectively, potentially serve as the parent states of the SC-SC phase. These results illuminate the interplay between strong correlations and unconventional pairing in MATBG, and establish a versatile Gutzwiller framework applicable to other strongly correlated superconductors.

cond-mat.str-el↗

arXiv2026

Symmetry-Indicated Time-Reversal-Doubled Axion Insulators

The axion insulator exhibits a topological magnetoelectric effect characterized by an axion angle $\theta=\pi$, while the time-reversal-doubled axion insulator (T-DAXI) can be viewed as two copies of an axion insulator related by time-reversal symmetry. In this work, we show that a topological crystalline insulator with nonsymmorphic glide or screw symmetry hosts the T-DAXI phase. The spin-resolved topology of the T-DAXI phase is guaranteed by the nonsymmorphic symmetry invariant $\delta_g=1$ or $\delta_s=1$ in certain spin directions. In this phase, the partial axion angles are quantized to $\pi$, and the gapped surfaces realize half-quantized quantum spin Hall states. By applying an external magnetic field along the $z$ direction, electrons with opposite spins accumulate on opposite $(001)$ surfaces, producing a topological spin polarization in real space. When the magnetic field is time-periodic, this leads to an alternating spin current detectable in experiment. Using $\mathrm{\textit{ab initio}}$ calculations, we demonstrate that mixed bismuth monohalides Bi4Br3I and Bi4BrI3 realize the nonsymmorphic T-DAXI with $\delta_g=\delta_s=1$. Our findings not only reveal the symmetry-enforced T-DAXIs in nonsymmorphic topological crystalline insulators, but also introduce the spin magnetoelectric effect as a novel topological spin response.

cond-mat.mtrl-sci↗

arXiv2026

Scattering-state theory of open Floquet lattices: transfer matrices, branch openness, and robust asymmetry

We establish a scattering-state theory for open one-dimensional Floquet lattices based on a frequency-domain transfer-matrix formulation. For real quasienergy, the conjugate-symplectic structure of the transfer matrix separates bulk Floquet--Bloch modes into propagating and evanescent sectors, enabling a consistent treatment of interface matching and the shrinking-window smoothing required for long-sample transport. By tracking how incoming states populate deep-bulk propagating branches, we define branch-resolved weights $p_{\mu\alpha}$ and total branch weights $p_\mu$. We prove that $p_\mu$ equals the escape probability of a wave packet initialized on the corresponding branch. In the open geometries considered here, true bound trapping of propagating branches is nongeneric, yielding $p_\mu=1$ for generic parameters. This generic openness implies that long-sample transport is governed by deep-bulk branch populations rather than by boundary-sensitive interference. Consequently, the integrated left--right transmission asymmetry reduces to the net chirality, and hence the winding contribution, of an isolated Floquet band. The robust topological observable is therefore the accumulated asymmetry plateau, not the detailed transmission line shape, which remains strongly reshaped by nonadiabatic boundaries. A spatially adiabatic boundary serves only as a transparent benchmark for resolving the branch structure, not as the origin of the topological response.

cond-mat.mes-hall↗

arXiv2026

Majorana Zero Modes and Topological Nature in Bi2Ta3S6-family Superconductors

In this work, we report that Bi2Ta3S6-family superconductors exhibit nontrivial band topology. They possess a natural quantum-well structure consisting of alternating stacks of TaS2 and honeycomb Bi layers, which contribute superconducting and topological properties, respectively. Symmetry-based indicators $(\mathbb{Z}_4;\mathbb{Z}_{2}\mathbb{Z}_{2}\mathbb{Z}_{2})=(2;000)$ reveal that the topological nature arises entirely from the Bi layers, which belong to a quantum spin Hall phase characterized by a $p_x-p_y$ model on a honeycomb lattice. The topological zigzag (ZZ) and armchair (AC) edge states are obtained. Using VASP2KP, the in-plane $g$ factors of these topological edge states are computed from the ab initio calculations: $g_{x/y}^{\mathrm{ZZ}}=2.07/1.60$ and $g_{x/y}^{\mathrm{AC}}=0.50/0.06$. The strong anisotropy of the edge-state $g$ factors allows us to explore Majorana zero modes in the Bi monolayer on a superconductor, which can be obtained by exfoliation or molecular beam epitaxy. The relaxed structures of the Bi2Ta3Se6, Bi2Nb3S6 and Bi2Nb3Se6 are obtained. Their superconducting transition temperature $T_c$ are estimated based on the electron-phonon coupling and the McMillan formula. Furthermore, using the experimental superconducting gap $\Delta$ and the computed $g$ factors, we obtain the phase diagram, which shows that the in-plane field $B_y>2.62\mathrm{ T}$ can generate corner Majorana zero modes in the Bi monolayer of the superconductor Bi2Ta3S6. A similar paradigm also applies to the Bi2Ta3S6 bulk with the emergence of Majorana hinge states. These natural quantum-well superconductors therefore offer ideal platforms for exploring topological superconductivity and Majorana zero modes.

cond-mat.supr-con↗

arXiv2025

Emergence of Topological Electron Crystals in Bilayer Graphene--Mott Insulator Heterostructures

The interplay between strong electron correlation and band topology offers a playground for discovering exotic quantum phases. Here, we predict the emergence of topological electron crystals in a charge-transfer bilayer graphene-Mott insulator heterostructure. In this system, interlayer charge transfer induces a charge-neutral electron-hole bilayer with strong mass asymmetry. While the extreme dilute limit favors a classical triangular dipolar Wigner crystal, we show that increasing the carrier density triggers a critical competition between the Coulomb interaction and the underlying topological band structure of bilayer graphene. This interplay destabilizes the triangular dipolar Wigner crystal and instead stabilizes intrinsic quantum electron crystals with spontaneously formed honeycomb and kagome geometries. Crucially, these new phases can host distinct topological responses including the quantum anomalous and quantum spin Hall effects, which inherit the nonlocal quantum geometry of the bilayer graphene wave functions. Our results establish this artificial heterostructure as a highly tunable platform for mimicking two-dimensional topological solids in a single device.

cond-mat.mes-hall↗

arXiv2025

Interplay of ferromagnetism, nematicity and Fermi surface nesting in kagome flat band

Recent experiment on Fe-doped CoSn has uncovered a series of correlated phases upon hole doping of the kagome flat bands. Among the phases observed, a nematic phase with a six- to two-fold rotation symmetry breaking is found to prevail over a wide doping and temperature range. Motivated by these observations, we investigate the interaction-driven phases realized in a kagome model with partially filled, weakly dispersing flat bands. Density-density interactions up to second-nearest neighbors are considered. We identify a close competition between ferromagnetic and nematic phases in our self-consistent Hartree-Fock calculations: while on-site interaction favors ferromagnetism, the sizable inter-sublattice interactions stabilize nematicity over a wide doping window. Competition from translational-symmetry-breaking phases is also considered. Overall, our results show that nematicity is a generic outcome of partially filled kagome flat bands and establish a minimal framework for understanding correlated flat-band phases.

cond-mat.str-el↗

arXiv2025

Electromagnetic responses of bilayer excitonic insulators: from exciton London equations to dipole and inverse dipole Hall effects

We develop a microscopic theory of the linear electromagnetic response of bilayer excitonic insulators relevant to electron-hole double-layer systems. Using a self-consistent Hartree-Fock description of the excitonic ground state and time-dependent Hartree-Fock for its dynamics, we compute the collective mode spectrum and the full first-order response to layer-symmetric (charge) and layer-antisymmetric (exciton) gauge fields. At zero magnetic field, we find that two gapped plasmon modes dominate the long-wavelength charge response, while the exciton channel is governed by a linearly dispersing phase (Goldstone) mode. From the Goldstone-dominated kernel, we derive London-like equations for the exciton condensate. The nondissipative acceleration under a layer-antisymmetric electric field provides a direct signature of exciton superfluidity; in contrast, a normal exciton fluid shows a Drude-like dissipative response. In a perpendicular magnetic field, the Goldstone mode develops a magnetic-roton minimum that signals an instability toward a finite-momentum stripe-ordered excitonic insulator. In addition, the field couples charge and exciton motion, giving rise to dipole and inverse dipole Hall effects in which a charge (exciton) bias induces a transverse exciton (charge) current. In the condensate, these mixed Hall conductances approach finite values in the $\omega\to0$ linear-response limit. Our findings provide concrete targets for microwave and transport probes of bilayer exciton superfluidity.

cond-mat.mes-hall↗

arXiv2025

CMPhysBench: A Benchmark for Evaluating Large Language Models in Condensed Matter Physics

We introduce CMPhysBench, designed to assess the proficiency of Large Language Models (LLMs) in Condensed Matter Physics, as a novel Benchmark. CMPhysBench is composed of more than 520 graduate-level meticulously curated questions covering both representative subfields and foundational theoretical frameworks of condensed matter physics, such as magnetism, superconductivity, strongly correlated systems, etc. To ensure a deep understanding of the problem-solving process,we focus exclusively on calculation problems, requiring LLMs to independently generate comprehensive solutions. Meanwhile, leveraging tree-based representations of expressions, we introduce the Scalable Expression Edit Distance (SEED) score, which provides fine-grained (non-binary) partial credit and yields a more accurate assessment of similarity between prediction and ground-truth. Our results show that even the best models, Grok-4, reach only 36 average SEED score and 28% accuracy on CMPhysBench, underscoring a significant capability gap, especially for this practical and frontier domain relative to traditional physics. The code anddataset are publicly available at https://github.com/CMPhysBench/CMPhysBench.

cs.LG↗

arXiv2025

Spin-polarized triplet excitonic insulators in Ta3X8 (X=I, Br) monolayers

Bose-Einstein condensation of spin-polarized triplet excitons can give rise to an intriguing spin supercurrent, enabling experimental detection of exciton condensation. In this work, we predict that Ta3X8 (X=I, Br) ferromagnetic monolayers are spin-polarized triplet excitonic insulators (EIs), based on the systematic first-principles GW calculations coupled with the Bethe-Salpeter equation (GW+BSE). The single-particle calculations of spin-polarized band structures reveal that these monolayers are bipolar magnetic semiconductors, where the highest valence band and the lowest conduction band possess opposite spin polarization. The two low-energy bands, primarily originating from Ta $d_{z^2}$ orbitals, are almost flat. The same-orbital parity and opposite-spin natures of the band-edge states effectively suppress dielectric screening, promoting the emergence of the EI state. The GW+BSE calculations reveal that the binding energy of the lowest-energy exciton is 1.499 eV for Ta3I8 monolayer and 1.986 eV for Ta3Br8 monolayer. Since both values exceed the respective GW band gaps, these results indicate a strong excitonic instability in these monolayers. A wavefunction analysis confirms that the lowest-energy exciton is a tightly bound Frenkel-like state, exhibiting a spin-polarized triplet nature with $S_z=1$. Our findings establish a valuable material platform for investigating spin-polarized triplet EIs, offering promising potential for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Towards an experimental implementation of entanglement harvesting in superconducting circuits: effect of detector gap variation on entanglement harvesting

Motivated by the prospect of experimental implementations of entanglement harvesting in superconducting circuits, we propose a model of variable-gap particle detector that aims to bridge some of the gaps between Unruh-DeWitt (UDW) models and realistic implementations. Using parameters tailored to potential experimental setups, we investigate entanglement harvesting in both spacelike-separated and causally connected scenarios. Our findings reveal that while variations in the energy gap reduce the ability to harvest entanglement for spacelike-separated detectors, detectors in causal contact can still become entangled through their interaction with the field. Notably, our analysis shows that (due to the derivative coupling nature of the model) even for causally connected detectors, the entanglement primarily originates from the field's correlations. This demonstrates the potential for genuine entanglement harvesting in the lab and opens the door to near-future entanglement harvesting experiments in superconducting circuits.

quant-ph↗

arXiv2025

Detecting genuine multipartite entanglement in multi-qubit devices with restricted measurements

Detecting genuine multipartite entanglement (GME) is a state-characterization task that benchmarks coherence and experimental control in quantum systems. Existing GME tests often require joint measurements on many qubits, posing challenges for systems like time-bin encoded qubits and microwave photons from superconducting circuits, where qubit connectivity is limited or measurement noise grows with the number of jointly measured qubits. Here we introduce versatile GME and $k$-inseparability criteria applicable to any state, which only require measuring $O(n^2)$ out of $2^n$ (at most) $m$-body stabilizers of $n$-qubit target graph states, with $m$ upper-bounded by twice the graph's maximum degree. For cluster or ring-graph states, only constant-weight stabilizers are needed. Using semidefinite programming (and sometimes graph-local complementations), we can reduce the number or weight of required stabilizers. Analytical and numerical results show that our criteria are noise-robust and may infer state infidelity from certified $k$-inseparability in microwave photonic graph states generated under realistic conditions.

quant-ph↗

arXiv2025

Realizing a topological diode effect on the surface of a topological Kondo insulator

Introducing the concept of topology into material science has sparked a revolution from classic electronic and optoelectronic devices to topological quantum devices. The latter has potential for transferring energy and information with unprecedented efficiency. Here, we demonstrate a topological diode effect on the surface of a three-dimensional material, SmB6, a candidate topological Kondo insulator. The diode effect is evidenced by pronounced rectification and photogalvanic effects under electromagnetic modulation and radiation at radio frequency. Our experimental results and modeling suggest that these prominent effects are intimately tied to the spatially inhomogeneous formation of topological surface states (TSS) at the intermediate temperature. This work provides a manner of breaking the mirror symmetry (in addition to the inversion symmetry), resulting in the formation of pn-junctions between puddles of metallic TSS. This effect paves the way for efficient current rectifiers or energy-harvesting devices working down to radio frequency range at low temperature, which could be extended to high temperatures using other topological insulators with large bulk gap.

cond-mat.str-el↗

arXiv2024

Two-dimensional moir\'{e} phonon polaritons

Phonon polaritons (PhPs) are hybrid light-matter modes. We investigate them in two-dimensional (2D) materials with twisted moir\'{e} structures, revealing that the moir\'{e} potential creates a new class of `moir\'{e} PhPs'. These exhibit a fundamental spectral reconstruction into multiple branches and, crucially, electromagnetic wavefunctions that are nano-patterned by the superlattice. Through numerical simulations based on realistic lattice models, we confirm the existence of these intriguing modes. The inherent nanoscale structuring produces a robust, spatially varying near-field response, establishing moir\'{e} superlattices as a platform for engineering light-matter interactions.

cond-mat.mes-hall↗

arXiv2024

Universal Moir\'e-Model-Building Method without Fitting: Application to Twisted MoTe$_2$ and WSe$_2$

We develop a comprehensive method to construct analytical continuum models for moir\'e systems directly from first-principle calculations without any parameter fitting. The core idea of this method is to interpret the terms in the continuum model as a basis, allowing us to determine model parameters as coefficients of this basis through Gram-Schmidt orthogonalization. We apply our method to twisted MoTe$_2$ and WSe$_2$ with twist angles ranging from 2.13$^\circ$ to 3.89$^\circ$, producing continuum models that exhibit excellent agreement with both energy bands and wavefunctions obtained from first-principles calculations. We further propose a strategy to integrate out the higher-energy degrees of freedom to reduce the number of the parameters in the model without sacrificing the accuracy for low-energy bands. Our findings reveal that decreasing twist angles typically need an increasing number of harmonics in the moir\'e potentials to accurately replicate first-principles results. We provide parameter values for all derived continuum models, facilitating further robust many-body calculations. Our approach is general and applicable to any commensurate moir\'e materials accessible by first-principles calculations.

cond-mat.mes-hall↗