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Oxford University Press eBooks

SPIN 1/2 CHAINS

Thierry Giamarchi

Published Dec 18, 2003• 0 citations
Publisher

Abstract

This chapter examines the question of spin systems, the simplest realization of Luttinger liquids. The peculiarities of one dimension make the solution of this problem quite surprising and different from what our intuition expects based on a classical vision of the spins. The spin 1/2 chain and its physical properties are discussed, along with the use of one-dimensional interacting fermions to solve the spin chain problem. The ideal spin 1/2 chain can be perturbed in many ways. Each of these perturbations can lead to some quite rich physics. This chapter considers the simplest of them, perturbations that apply to strictly one-dimensional chains, and examines coupling between chains.

DOI
10.1093/acprof:oso/9780198525004.003.0006

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•Jan 1, 2015•0 citations
This chapter examines the question of spin systems, the simplest realization of Luttinger liquids. The peculiarities of one dimension make the solution of this problem quite surprising and different from what our intuition expects based on a classical vision of the spins. The spin 1/2 chain and its physical properties are discussed, along with the use of one-dimensional interacting fermions to solve the spin chain problem. The ideal spin 1/2 chain can be perturbed in many ways. Each of these perturbations can lead to some quite rich physics. This chapter considers the simplest of them, perturbations that apply to strictly one-dimensional chains, and examines coupling between chains.
We present in this thesis a series of studies on the physical properties of some one dimensional systems. In particular we study the low energy properties of various spin chains and a junction of Luttinger wires. For spin chains we specifically look at the role of perturbations like frustrating interactions and dimerisation in a nearest neighbour chain and the formation of magnetisation plateaus in two kinds of models; one purely theoretical and the other motivated by experiments. In our second subject of interest we study using a renormalisation group analysis the effect of spin dependent scattering at a junction of Luttinger wires. We look at the physical effects caused by the interplay of electronic interactions in the wires and the scattering processes at the junction. The thesis begins with an introductory chapter which gives a brief glimpse of the ideas and techniques used in the specific problems that we have worked on. Our work on these problems is then described in detail in chapters 25. We now present a brief summary of each of those chapters. In the second chapter we look at the ground state phase diagram of the mixed-spin sawtooth chain, i.e a system where the spins along the baseline are allowed to be different from the spins on the vertices. The spins S1 along the baseline interact with a coupling strength J1(> 0). The coupling of the spins on the vertex (S2) to the baseline spins has a strength J2. We study the phase diagram as a function of J2/J1 [1]. The model exhibits a rich variety of phases which we study using spinwave theory, exact diagonalisation and a semi-numerical perturbation theory leading to an effective Hamiltonian. The spinwave theory predicts a transition from a spiral state to a ferrimagnetic state at J2S2/2J1S1 = 1 as J2/J1 is increased. The spectrum has two branches one of which is gapless and dispersionless (at the linear order) in the spiral phase. This arises because of the infinite degeneracy of classical ground states in that phase. Numerically, we study the system using exact diagonalisation of up to 12 unit cells and S1 = 1 and S2 =1/2. We look at the variation of ground state energy, gap to the lowest excitations, and the relevant spin correlation functions in the model. This unearths a richer phase diagram than the spinwave calculation. Apart from revealing a possibility of the presence of more than one kind of spiral phases, numerical results tell us about a very interesting phase for small J2. The spin correlation function (for the spin1/2s) in this region have a property that the nextnearest-neighbour correlations are much larger than the nearest neighbour correlations. We call this phase the NNNAFM (nextnearest neighbour antiferromagnet) phase and provide an understanding of this phase by deriving an effective Hamiltonian between the spin1/2s. We also show the existence of macroscopic magnetisation jumps in the model when one looks at the system close to saturation fields. The third chapter is concerned with the formation of magnetisation plateaus in two different spin…
The bosonization technique is useful for solving the interacting problem and for extracting a certain number of properties such as power law behavior of the correlation functions. However, this derivation is, strictly speaking, based on a linearization of the spectrum close to the Fermi energy. One can thus wonder what will remain of the results of the previous chapter when it is impossible to stay confined close to the Fermi level, for example, when the interactions become strong. In order to answer this question, it is necessary to rederive the bosonization formulas, in a more phenomenological but more general way. This chapter demonstrates that a similar concept as Fermi liquids exists in one dimension. This concept, called Luttinger liquid, encompasses most of the physics of simple one-dimensional systems and will be the starting point for the study more complex situations. The concept of Luttinger liquids is explained using very low level techniques.
This chapter discusses the technique of bosonization for studying systems of interacting fermions in one dimension. It reviews the low-energy properties of Fermi and Luttinger liquids and presents some of the relations between bosonic and fermionic operators in one dimension. The chapter explores these relations to calculate the correlation functions and the renormalization group properties of various operators for a system of spinless fermions. It explains the methods of bosonization to study the Heisenberg antiferromagnetic spin 1/2 chain, the Hubbard model in one dimension and transport in clean quantum wires and in the presence of isolated impurities. The basic idea of bosonization is that there are certain objects which can be calculated either in a fermionic theory or in a bosonic theory, and the two calculations give the same answer. The chapter also discusses another application of bosonization, which is to study transport, in particular the DC conductivity in one-dimensional wires of interacting fermions.
This chapter looks at less usual realizations of Luttinger liquids than the ones coming from fermionic systems. The first realization is provided by interacting bosons. Indeed, in one dimension it is impossible to decouple completely the statistics from the interactions, and one- and quasi-one-dimensional bosons are also described by Luttinger liquids. The discussion shows how the techniques used to tackle one dimension can be used in higher dimensions as well. An important class of problems examined in this chapter is the case of impurities coupled with a three-dimensional electron gas. Josephson junction arrays and Bose condensates are also considered, along with X-ray, Kondo, and multi-channel Kondo problems. Applications of bosonization to the study of quantum impurities in three-dimensional Fermi liquids are analyzed.
The theoretical description of interacting fermions in one spatial dimension is simplified by the fact that the low-energy spectrum of noninteracting fermions is identical to the one for a harmonic chain. This allows us to describe interacting fermions as a system of coupled oscillators. The exact solution of the Tomonaga–Luttinger model is discussed and results for the momentum distribution and spectral functions are presented. The general Luttinger liquid phenomenology is briefly discussed and various attempts at the experimental verification of the theoretical predictions are examined. Extended defects in semiconductors might also be candidate systems.
Interacting one-dimensional electron systems are generally referred to as "Luttinger liquids", after the effective low-energy theory in which spin and charge behave as separate degrees of freedom with independent energy scales. The "spin-incoherent Luttinger liquid" describes a finite-temperature regime that is realized when the temperature is very small relative to the Fermi energy, but larger than the characteristic spin energy scale, and it is realized for instance in the strongly interacting Hubbard chain (with U → ∞). Similar physics can take place in the ground-state, when a Luttinger Liquid is coupled to a spin bath, which effectively introduces a "spin temperature" through its entanglement with the spin degree of freedom. We show that the spin-incoherent state can be exactly written as a factorized wave-function, with a spin wave-function that can be described within a valence bond formalism. This enables us to calculate exact expressions for the momentum distribution function and the entanglement entropy. This picture holds not only for two antiferromagnetically coupled t-J chains, but also for the t-J-Kondo chain with strongly interacting conduction electrons. In chapter 3 we argue that this theory is quite universal and may describe a family of problems that could be dubbed "spin-incoherent".