Quantum spin glasses

In this last chapter, we move beyond the simplest disordered
models considered in Chapter 15, and consider systems
which have magnetically ordered states which are rather more
complicated than those in which the average moments are in a regular
arrangement. In the context of the Ising/rotor
models, such states can be obtained by allowing the exchange
constants to randomly fluctuate
over both negative and positive values.
In particular, we will be interested here in the magnetically-ordered
``spin-glass'' state in which orientation of the spontaneous moment
varies randomly from site to site, with a vanishing average
over sites, [ás^{z}_{i} ñ] = 0
(or [ á**n**_{i} ñ] = 0-the square brackets represent
an average over sites);
such states are clearly special to disordered systems.
For classical spin systems, such ordered states have been
reviewed at length elsewhere [7,22,72].
The structure of the ordered spin-glass phases of quantum models
is very similar, and so this shall not be the focus of our interest here.
Rather, we are interested in the quantum phase transition from
the spin glass to a quantum paramagnet, and the nature of the finite
temperature crossovers in its vicinity, where quantum mechanics plays
a more fundamental role.

The quantum Ising/rotor models of Part 2 also form the
basis of much of our discussion of quantum spin glasses.
However, in parallel, we also consider the appearance
of spin-glass order in the metallic systems of
Chapter 12.
So one of our interests is the transition from a paramagnetic
Fermi liquid to a *spin density glass* state: such a state
is characterized by the analog of the order parameter
for the ordinary spin density wave state, but now the orientation
and magnitude of the amplitude varies randomly in space,
along with random phase offsets.

NEXT ; TABLE OF CONTENTS