arXiv subjects

Kang-Le Cai

Publications and source records attributed to Kang-Le Cai.

4 recordsLinked to original sources

arXiv2026

Entanglement negativity in decohered topological states

We investigate universal entanglement signatures of mixed-state phases obtained by decohering pure-state topological order (TO), focusing on topological corrections to logarithmic entanglement negativity and mutual information: topological entanglement negativity (TEN) and topological mutual information (TMI). For Abelian TOs under decoherence, we develop a replica field-theory framework based on a doubled-state construction that relates TEN and TMI to the quantum dimensions of domain-wall defects between decoherence-induced topological boundary conditions, yielding general expressions in the strong-decoherence regime. We further compute TEN and TMI exactly for decohered $G$-graded string-net states, including cases with non-Abelian anyons. We interpret the results within the strong one-form-symmetry framework for mixed-state TOs: TMI probes the total quantum dimension of the emergent premodular anyon theory, whereas TEN detects only its modular part.

cond-mat.str-el↗

arXiv2025

Disorder operators in two-dimensional Fermi and non-Fermi liquids through multidimensional bosonization

Disorder operators are a type of non-local observables for quantum many-body systems, measuring the fluctuations of symmetry charges inside a region. It has been shown that disorder operators can reveal global aspects of many-body states that are otherwise difficult to access through local measurements. We study the disorder operator for U(1) (charge or spin) symmetry in two-dimensional Fermi and non-Fermi liquid states, using the multidimensional bosonization formalism. For a region $A$, the logarithm of the charge disorder parameter in a Fermi liquid with isotropic interactions scales asymptotically as $l_A\ln l_A$, with $l_A$ being the linear size of the region $A$. We calculate the proportionality coefficient in terms of Landau parameters of the Fermi liquid theory. We then study models of the Fermi surface coupled to gapless bosonic fields realizing non-Fermi liquid states. In a simple spinless model, where the fermion density is coupled to a critical scalar, we find that at the quantum critical point the scaling behavior of the charge disorder operators is drastically modified to $l_A \ln^2 l_A$. We also consider the composite Fermi liquid state and argue that the charge disorder operator scales as $l_A$.

cond-mat.str-el↗

arXiv2025

Universal contributions to charge fluctuations in spin chains at finite temperature

At finite temperature, conserved charges undergo thermal fluctuations in a quantum many-body system in the grand canonical ensemble. The full structure of the fluctuations of the total U(1) charge $Q$ can be succinctly captured by the generating function $G(θ)=\left\langle e^{i θQ}\right\rangle$. For a 1D translation-invariant spin chain, in the thermodynamic limit the magnitude $|G(θ)|$ scales with the system size $L$ as $\ln |G(θ)|=-α(θ)L+γ(θ)$, where $γ(θ)$ is the scale-invariant contribution and may encode universal information about the underlying system. In this work we investigate the behavior and physical meaning of $γ(θ)$ when the system is periodic. We find that $γ(θ)$ only takes nonzero values at isolated points of $θ$, which is $θ=π$ for all our examples. In two exemplary lattice systems we show that $γ(π)$ takes quantized values when the U(1) symmetry exhibits a specific type of 't Hooft anomaly with other symmetries. In other cases, we investigate how $γ(θ)$ depends on microscopic conditions (such as the filling factor) in field theory and exactly solvable lattice models.

cond-mat.str-el↗

arXiv2024

Corner Charge Fluctuations and Many-Body Quantum Geometry

In many-body systems with U(1) global symmetry, the charge fluctuations in a subregion reveal important insights into entanglement and other global properties. For subregions with sharp corners, bipartite fluctuations have been predicted to exhibit a universal shape dependence on the corner angle in certain quantum phases and transitions, characterized by a "universal angle function" and a "universal coefficient." However, we demonstrate that this simple formula is insufficient for charge insulators, including composite fermi liquids. In these systems, the corner contribution may depend on the corner angle, subregion orientation, and other microscopic details. We provide an infinite series representation of the corner term, introducing orientation-resolved universal angle functions with their non-universal coefficients. In the small-angle limit or under orientation averaging, the remaining terms' coefficients are fully determined by the many-body quantum metric, which, while not universal, adheres to both a universal topological lower bound and an energetic upper bound. We also clarify the conditions for bound saturation in (anisotropic) Landau levels, leveraging the generalized Kohn theorem and holomorphic properties of many-body wavefunctions. We find that a broad class of fractional quantum Hall wavefunctions, including unprojected parton states and composite-fermion Fermi sea wavefunctions, saturates the bounds.

cond-mat.str-el↗