# Residual Entropy and Spin Fractionalizations in the Mixed-Spin Kitaev Model

## TL;DR

This study explores the unique ground-state and thermal properties of the mixed-spin Kitaev model, revealing macroscopic degeneracy, spin fractionalization, and the influence of smaller spins on thermodynamics.

## Contribution

It introduces local symmetries in the mixed-spin Kitaev model, demonstrating macroscopic degeneracy and spin fractionalization, which differ from conventional models.

## Key findings

- Presence of macroscopic degeneracy in ground states
- Double peak structure in specific heat at finite temperatures
- Entropy plateau indicating spin fractionalization

## Abstract

We investigate ground-state and finite temperature properties of the mixed-spin $(s, S)$ Kitaev model. When one of spins is half-integer and the other is integer, we introduce two kinds of local symmetries, which results in a macroscopic degeneracy in each energy level. Applying the exact diagonalization to several clusters with $(s, S)=(1/2, 1)$, we confirm the presence of this large degeneracy in the ground states, in contrast to the conventional Kitaev models. By means of the thermal pure quantum state technique, we calculate the specific heat, entropy, and spin-spin correlations in the system. We find that in the mixed-spin Kitaev model with $(s, S)=(1/2, 1)$, at least, the double peak structure appears in the specific heat and the plateau in the entropy at intermediate temperatures, indicating the existence of the spin fractionalization. Deducing the entropy in the mixed-spin system with $s, S\le 2$ systematically, we clarify that the smaller spin-$s$ is responsible for the thermodynamic properties at higher temperatures.