Theory of the spin bath

The quantum dynamics of mesoscopic or macroscopic systems is always complicated by their coupling to many ʹenvironmentalʹ modes. At low T these environmental effects are dominated by localized modes, such as nuclear and paramagnetic spins, and defects (which also dominate the entropy and specific heat). This environment, at low energies, maps onto a ʹspin bathʹ model. This contrasts with ʹoscillator bathʹ models (originated by Feynman and Vernon) which describe delocalized environmental modes such as electrons, phonons, photons, magnons, etc. The couplings to N spin bath modes are independent of N (rather than the ~O1/sqrt(N)) dependence typical of oscillator baths), and often strong. One cannot in general map a spin bath to an oscillator bath (or vice versa), they constitute distinct ʹuniversality classesʹ of quantum environment. We show how the mapping to spin bath models is made, and then discuss several examples in detail, including moving particles, magnetic solitons, nanomagnets, and SQUIDs, coupled to nuclear and paramagnetic spin environments. We then focus on the ʹcentral spinʹ model, which couples a central two-level system to a background spin bath. It is the spin bath analogue of the famous ʹspin-bosonʹ oscillator model, and describes, e.g., the tunnelling dynamics of nanoscopic and mesoscopic magnets and superconductors. We show how to average over (or ʹintegrate outʹ) spin bath modes, using an operator instanton technique, to find the central spin dynamics. The formal manouevres involve four separate averages-each average corresponds physically to a different ʹdecoherenceʹ mechanism acting on the central spin dynamics. Each environmental spin has its own topological ʹspin phaseʹ, which by interacting with the phase of the central system, decoheres it-this can happen even without dissipation. We give analytic results for the central spin correlation functions, under various conditions. We then describe the application of this theory to magnetic and superconducting systems. Particular attention is given to recent work on tunnelling magnetic macromolecules, where the role of the nuclear spin bath in controlling the tunnelling is very clear; we also discuss other magnetic systems in the quantum regime, and the influence of nuclear and paramagnetic spins on flux dynamics in SQUIDs. Finally, we discuss decoherence mechanisms and coherence experiments in superconductors and magnets. We show that a spin bath environment causes decoherence even in the T -> 0 limit. Control of this decoherence will be essential in the effort to construct ʹqubilsʹ for quantum computers.