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Journal of Physics A Mathematical and General

A Langevin approach to fermion and quantum spin correlation functions

John R. Klauder

Published Jul 11, 1983• 111 citations
Publisher

Abstract

By using spin-coherent states, the author shows that correlation functions for fermions or quantum spins follow from solutions to Langevin equations associated with a functional integral representation of the partition function. His method is applicable to any number of dimensions, may also be combined with boson variables, and is suitable for computer simulations.

DOI
10.1088/0305-4470/16/10/001

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By using spin-coherent states, the author shows that correlation functions for fermions or quantum spins follow from solutions to Langevin equations associated with a functional integral representation of the partition function. His method is applicable to any number of dimensions, may also be combined with boson variables, and is suitable for computer simulations.
The authors derive a functional integral representation for the partition function of the transverse Ising model. The spin operators are replaced by fermion operators and special care is taken to achieve complete equivalence of the spin model and the fermion model. They discuss several approximations for the functional integral and show that in a recent paper on spin glasses, where a similar method was used, the original spin model and the fermion model, which was obtained by replacing the spin operators by fermion operators, are not equivalent and have different critical temperatures.
The author establishes relationships between Bose, Fermi and spin-1/2 systems on lattices of arbitrary dimension and arbitrary but finite size which are defined to live in the (common) state space of the boson system. The underlying systems display an essential affinity by being described by precisely the same form of Hamiltonians, which are the polynomial functions of canonical operator variables: boson, fermion or spin-1/2 variables, respectively. For large enough, densely populated finite volume lattices it entails the computation of partition and correlation functions for lattice fermions in terms of bosons only. Albeit an approximation, its accuracy increases as the continuum limit is approached.
Functional-integral techniques are used to study correlated fermion models of popular interest: the prototypical Hubbard models. Recent progress has been made in the functional-integral approach to such correlated Fermi systems by introducing a spin-space reference frame that :fluctuates in space and time, Introducing fermions with varying quantization a.xis on the lattice allows the functional-integral representation to assume a uniform approximation over the entire range of the interaction strength U, reproducing the expected mean fields at the saddlepoints along with gapless excitation spectrums. First, the framework necessary for this study is developed: second quantization and the second quantized models of correlated fermions that are to be examined are briefly reviewed. Next, an introduction to first and second quantized functional integrals is followed by basic functional-integral techniques employed to study quantum many-particle systems. Once the background necessary to investigate this approach has been given, the :fluctuating spin-space reference frame is described and is examined in the two-site problem, where an exact diagonalization may be performed. A functional-integral over one-vertex polygonal paths is used to approximate the partition function in a fixed reference frame orientation. This functional-integral representation is studied in the low-to-intermediate and high temperature regimes. A classification of the quantization a.xis configurations, based on classical spin waves, is motivated and presented.
Functional integral representation of the partition functionThe customary approach to nonrelativistic many-body theory is to proceed with the method of second quantization begun in the first chapter.There is another approach, the method of functional integrals, which we shall follow here.Of course, what can be done in one formalism can always be done in another.Nevertheless, functional integrals seem to be the method of choice for most elementary particle theorists these days, and they seem to lend themselves more readily to nonperturbative phenomena such as tunneling, instantons, lattice gauge theory, etc.For gauge theories they are practically indispensable.However, there is a certain amount of formalism that must be developed before we can start to discuss physical applications.In this chapter, we shall derive the functional integral representation of the partition function for interacting relativistic non-gauge field theories.As a check on the formalism, as well as to obtain some feeling for how functional integrals work, we shall then rederive some well-known results on relativistic ideal gases for bosons and fermions.
In a first part we give a brief presentation of general Fock space setting to describe quantum field theory. Bosons are quantum particles with integer spin and have symmetric wave functions; Fermions are quantum particles with half-integer spin and are represented by anti-symmetric wave functions. The functional setting is given by a symmetric or anti-symmetric tensor product of Hilbert spaces. We describe these spaces and transformations between these spaces. We shall follow the references [23, 42]. Coherent states are defined by translating the vacuum states with the Weyl operators. This is easily done here for bosons. We shall see in the next chapter how to deal with fermions. In a second part we give an interesting application of bosonic coherent states to the study of the classical limit as ℏ↘0\hbar \searrow 0ℏ↘0 of non-relativistic boson systems with two body interaction in the neighborhood of a solution of the classical system (here the Hartree equation). The classical limit corresponds here to the mean-field limit as the number of particles goes to infinity. As we have done for finite systems, we use here Hepp’s method, which is a linearization procedure of the quantum Hamiltonian around the classical field. The fluctuations around this solution are controlled by a purely quadratic Hamiltonian. In a series of several important papers [114–116] Ginibre and Velo have proven an asymptotic expansion and remainder estimates for these quantum fluctuations. Finally, following the paper [225] one can show that, in the limit ℏ→0\hbar \rightarrow 0ℏ→0, the marginal distribution of the time-evolved coherent states tends in trace-norm to the projector onto the solution of the classical fields equation (Hartree equation) with a uniform remainder estimates in time.