Thermodynamic anomalies in open quantum systems: Strong coupling effects in the isotropic XY model

TL;DR

This paper investigates thermodynamic anomalies in an exactly solvable open quantum system modeled by a one-dimensional isotropic XY spin chain, revealing negative thermodynamic quantities and quantum phase transitions due to strong coupling effects.

Contribution

It introduces a detailed analysis of strong coupling effects on thermodynamics in the XY model, highlighting anomalous behaviors and quantum phase transition phenomena.

Findings

Observation of negative entropy and specific heat

Identification of negative susceptibilities at certain parameters

Detection of ground state changes akin to quantum phase transitions

Abstract

The exactly solvable model of a one dimensional isotropic XY spin chain is employed to study the thermodynamics of open systems. For this purpose the chain is subdivided into two parts, one part is considered as the system while the rest as the environment or bath. The equilibrium properties of the system display several anomalous aspects such as negative entropies, negative specific heat, negative susceptibilities in dependence of temperature and coupling strength between system and bath. The statistical mechanics of this system is studied in terms of a reduced density matrix. At zero temperature and for a certain parameter values we observe a change of the ground state, a situation akin to a quantum phase transition.