# Low temperature properties in the Bilayer Kitaev model

## TL;DR

This paper investigates the low-temperature quantum phases of the bilayer Kitaev model with interlayer Heisenberg interactions, revealing a phase transition between quantum spin liquid and dimer states, and connecting different Kitaev QSL states.

## Contribution

It provides the first detailed analysis of the bilayer Kitaev model's phase diagram using exact diagonalization and thermal pure quantum methods, highlighting quantum phase transitions and state connections.

## Key findings

- Existence of a quantum phase transition between Kitaev QSL and dimer states with antiferromagnetic coupling.
- No singularity observed with ferromagnetic coupling, indicating adiabatic connection between different Kitaev QSL states.
- Thermodynamic properties support the phase transition and state connection findings.

## Abstract

The ground state of the bilayer Kitaev model with the Heisenberg-type interlayer exchange interaction is investigated by means of the exact diagonalization. Calculating the ground-state energy, local quantity defined on each plaquette, and dynamical spin structure factor, we obtain results suggesting the existence of a quantum phase transition between the Kitaev quantum spin liquid (QSL) and dimer singlet states when the interlayer coupling is antiferromagnetic. On the other hand, increasing the ferromagnetic interlayer coupling, there exists no singularity in the physical quantities, suggesting that the $S=1/2$ Kitaev QSL state realized in each layer adiabatically connects to another QSL state realized in the $S=1$ Kitaev model. Thermodynamic properties are also studied by means of the thermal pure quantum state method.