Thermodynamics in a split Hilbert space: Quantum impurity at the edge of the Heisenberg chain

TL;DR

This paper investigates the thermodynamic behavior of a spin-1/2 Heisenberg chain with an edge impurity, revealing four distinct impurity phases and providing an analytic framework that explains entropy undershoots through Hilbert space fractionalization.

Contribution

It introduces a comprehensive analytic model for impurity thermodynamics in the Heisenberg chain, accounting for boundary-bound modes and Hilbert space fractionalization across all phases.

Findings

Identifies four impurity phases with distinct entropy behaviors.

Provides closed-form expressions for impurity free energy and entropy.

Validates analytic results with tensor network and exact diagonalization methods.

Abstract

We study the isotropic spin-$\frac{1}{2}$ Heisenberg chain with a single edge-coupled impurity of arbitrary exchange strength $J$. The model exhibits four impurity phases. For antiferromagnetic couplings ($J>0$): a \textit{Kondo phase} at weak $J$, where the impurity is screened by many-body excitations and the impurity entropy decreases monotonically from $\ln 2$ at $T \to \infty$ to $0$ at $T\to 0$; and an \textit{antiferromagnetic bound-mode (ABM) phase} at strong $J$, where the impurity screened by an exponentially localized bound mode drives $S_{\mathrm{imp}}(T)$ nonmonotonically, with undershoots below zero at intermediate temperatures, while tending to $\ln 2$ as $T \to \infty$ and to $0$ as $T \to 0$. For ferromagnetic couplings ($J<0$): a local-moment (LM) phase at weak $|J|$, where the impurity remains unscreened with $S_{\mathrm{imp}}\to \ln 2$ as $T \to 0$ but exhibits shallow undershoots at intermediate scales; and a \textit{ferromagnetic bound-mode (FBM) phase} at strong $|J|$, where $S_{\mathrm{imp}}=\ln 2$ in both UV and IR limits, yet develops an intermediate-temperature undershoot. We provide an analytic understanding of this behavior, showing that the undershoots originate from the fractionalization of the Hilbert space into several towers of states: for antiferromagnetic couplings this occurs only at strong $J$, driven by boundary-localized bound modes, while for ferromagnetic couplings undershoots occur for all $J<0$, becoming deeper with increasing $|J|$ and vanishing as $J\to 0^{-}$. These bound modes screen the impurity. Incorporating the bound modes and edge states provides a complete analytic understanding of this phenomenon and yields closed expressions for the impurity contribution to free energy and entropy that are valid across all phases. These are checked and found to be in excellent agreement with tensor network and exact diagonalization results.