Crossovers in spin-boson and central spin models

TL;DR

This paper explores how adding localized spin modes to the environment alters the decoherence crossovers in spin-boson models, with implications for magnetic insulators and tunneling nanomagnets.

Contribution

It introduces the concept of a spin bath affecting decoherence crossovers and analyzes its impact on quantum-classical transition in tunneling systems.

Findings

Low T crossover shows very low decoherence in the presence of a spin bath.

Spin bath broadens the temperature range of the quantum-classical crossover.

Application to magnetic insulators and tunneling nanomagnets demonstrates practical relevance.

Abstract

We discuss how the crossovers in models like spin-boson model are changed by adding the coupling of the central spin to localised modes- the latter modelled as a 'spin bath'. These modes contain most of the environmental entropy and energy at low T in solid-state systems. We find that the low T crossover between oscillator bath and spin bath dominated decoherence, occurring as one reduces the energy scale of the central spin, is characterised by very low decoherence- we show how this works out in practise in magnetic insulators. We then reconsider the standard quantum-classical crossover in the dynamics of a tunneling system, including both spin and oscillator baths. It is found that the general effect of the spin bath is to broaden the crossover in temperature between the quantum and classical activated regimes. The example of tunneling nanomagnets is used to illustrate this.