Thermal fractionalization of quantum spins in a Kitaev model: $T$-linear specific heat and coherent transport of Majorana fermions

TL;DR

This study uses quantum Monte Carlo simulations to explore finite-temperature properties of a Kitaev honeycomb model, revealing thermal fractionalization of spins into Majorana fermions, with distinctive specific heat and transport behaviors.

Contribution

It demonstrates the thermal fractionalization process in the Kitaev model and links it to observable thermodynamic and transport phenomena, highlighting the role of Majorana fermions.

Findings

Entropy is released at two distinct temperature scales.

Intermediate temperature range shows T-linear specific heat.

Coherent Majorana fermion transport occurs between crossovers.

Abstract

Finite-temperature ($T$) properties of a Kitaev model defined on a honeycomb lattice are investigated by a quantum Monte Carlo simulation, from the viewpoint of fractionalization of quantum $S=1/2$ spins into two types of Majorana fermions, itinerant and localized. In this system, the entropy is released successively at two well-separated $T$ scales, as a clear indication of the thermal fractionalization. We show that the high-$T$ crossover, which is driven by itinerant Majorana fermions, is closely related with the development of nearest-neighbor spin correlations. On the other hand, the low-$T$ crossover originates in thermal fluctuations of fluxes composed of localized Majorana fermions, by which the spectrum of itinerant Majorana fermions is significantly disturbed. As a consequence, in the intermediate-$T$ range between the two crossovers, the system exhibits $T$-linear behavior in the specific heat and coherent transport of Majorana fermions, which are unexpected for the Dirac semimetallic spectrum in the low-$T$ limit. We also show that the flux fluctuations tend to open an energy gap in the Majorana spectrum near the gapless-gapped phase boundary. Our results indicate that the fractionalization is experimentally observable in the specific heat, spin correlations, and transport properties.