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Oxford University Press eBooks

Strongly correlated bosons and fermions in optical lattices

Antoine Georges, Thierry Giamarchi

Published Nov 15, 2012• 33 citations
Publisher

Abstract

This chapter gives an introduction to the physics of interacting quantum systems, both bosonic and fermionic. Section 1.2 introduces the physics of quantum particles in periodic lattices. Section 1.3 examines, for the case of bosons, how the combined effects of lattice and interaction can turn the system into an insulator, the so-called Mott insulator, and discusses the corresponding physics. Section 1.4 discusses what happens when the system is one-dimensional. Section 1.5 moves on to the case of fermions. It discusses first Fermi-liquid theory and the concept of quasi-particles, Landau’s description of the low-energy excitations of interacting fermion systems. Section 1.6 shows how, like the method for bosons, the combination of a lattice and strong interactions can turn a Fermi liquid into a Mott insulator. Section 1.7 looks at the properties of one-dimensional fermions, shows how Fermi liquid theory fails because low-energy excitations are now collective modes instead of quasi-particles, and examines the corresponding physics for both the conducting and insulating phases. Finally, Section 1.8 draws some conclusions and give some perspectives.

DOI
10.1093/acprof:oso/9780199661886.003.0001

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This chapter gives an introduction to the physics of interacting quantum systems, both bosonic and fermionic. Section 1.2 introduces the physics of quantum particles in periodic lattices. Section 1.3 examines, for the case of bosons, how the combined effects of lattice and interaction can turn the system into an insulator, the so-called Mott insulator, and discusses the corresponding physics. Section 1.4 discusses what happens when the system is one-dimensional. Section 1.5 moves on to the case of fermions. It discusses first Fermi-liquid theory and the concept of quasi-particles, Landau’s description of the low-energy excitations of interacting fermion systems. Section 1.6 shows how, like the method for bosons, the combination of a lattice and strong interactions can turn a Fermi liquid into a Mott insulator. Section 1.7 looks at the properties of one-dimensional fermions, shows how Fermi liquid theory fails because low-energy excitations are now collective modes instead of quasi-particles, and examines the corresponding physics for both the conducting and insulating phases. Finally, Section 1.8 draws some conclusions and give some perspectives.
This chapter gives an introduction to the physics of interacting quantum systems, both bosonic and fermionic. Section 1.2 introduces the physics of quantum particles in periodic lattices. Section 1.3 examines, for the case of bosons, how the combined effects of lattice and interaction can turn the system into an insulator, the so-called Mott insulator, and discusses the corresponding physics. Section 1.4 discusses what happens when the system is one-dimensional. Section 1.5 moves on to the case of fermions. It discusses first Fermi-liquid theory and the concept of quasi-particles, Landau’s description of the low-energy excitations of interacting fermion systems. Section 1.6 shows how, like the method for bosons, the combination of a lattice and strong interactions can turn a Fermi liquid into a Mott insulator. Section 1.7 looks at the properties of one-dimensional fermions, shows how Fermi liquid theory fails because low-energy excitations are now collective modes instead of quasi-particles, and examines the corresponding physics for both the conducting and insulating phases. Finally, Section 1.8 draws some conclusions and give some perspectives.
I describe in these notes the physical properties of one dimensional interacting quantum particles. In one dimension the combined effects of interactions and quantum fluctuations lead to a radically new physics quite different from the one existing in the higher dimensional world. Although the general physics and concepts are presented, I focuss in these notes on the properties of interacting bosons, with a special emphasis on cold atomic physics in optical lattices. The method of bosonization used to tackle such problems is presented. It is then used to solve two fundamental problems. The first one is the action of a periodic potential, leading to a superfluid to (Mott)-Insulator transition. The second is the action of a random potential that transforms the superfluid in phase localized by disorder, the Bose glass. Some discussion of other interesting extensions of these studies is given.
Abstract. I describe in these notes the physical properties of one dimensional interacting quantum particles. In one dimension the combined effects of interactions and quantum fluctuations lead to a radically new physics quite different from the one existing in the higher dimensional world. Although the general physics and concepts are presented, I focuss in these notes on the properties of interacting bosons, with a special emphasis on cold atomic physics in optical lattices. The method of bosonization used to tackle such problems is presented. It is then used to solve two fundamental problems. The first one is the action of a periodic potential, leading to a superfluid to (Mott)-Insulator transition. The second is the action of a random potential that transforms the superfluid in phase localized by disorder, the Bose glass. Some discussion of other interesting extensions of these studies is given. INTRODUCTION AND CHOICE OF CONTENTS One-dimensional systems of interacting particles are particularly fascinating both from a theoretical and experimental point of view. Such systems have been extensively investigated theoretically for more than 40 years now. They are wonderful systems in which interactions play a very special role and whose physics is drastically different
This thesis covers most of my work in the field of ultracold atoms loaded in optical lattices. It makes a route through the physics of cold atoms in periodic potentials starting from the simple noninteracting system and going into the many-body physics that describes the strongly correlated Mott insulator regime. Even though this thesis is a theoretical work all the chapters are linked either with experiments already done or with ongoing experimental efforts. This thesis can be divided into four different parts. The first part comprises chapters 1 to 3. In these chapters, after a brief introduction to the field of optical lattices I review the fundamental aspects pertaining to the physics of systems in periodic potentials. The second part deals with the superfluid weakly interacting regime where standard mean field techniques can be applied. This is covered in chapters 4 and 5. Specifically, chapter 4 introduces the discrete nonlinear Schroedinger equation (DNLSE) and uses it to model some experiments. In chapter 5 I go one step further and include the small quantum fluctuations neglected in the DNLSE by studying quadratic approximations of the Bose-Hubbard Hamiltonian. Chapters 6 to 8 can be grouped as the third part of the thesis. In them I adopt an effective action formalism, the so called two particle irreducible effective action (2PI) together with the closed time path (CTP) formalism to study far-from-equilibrium dynamics. The many-body techniques discussed in these chapters systematically include higher order quantum corrections, not included in the quadratic approximations of the Hamiltonian, which we show are crucial for a correct description of the quantum dynamics outside the very weakly interacting regime. Finally, chapter 9 to 11 are devoted to study the Mott insulator phase. In these chapters using perturbation theory I study the Mott insulator ground state and its excitation spectrum, the response of the system to Bragg spectroscopy, and propose a mechanism to correct for the residual quantum coherences inherent to the Mott insulator ground state. Even though small these are not ideal for the use of neutral atoms in optical lattice as a tool for quantum computation.
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.
Abstract A strongly interacting Bose gas in an optical lattice is studied using a hard‐core interaction. Two different approaches are introduced, one is based on a spin‐1/2 Fermi gas with attractive interaction, the other one on a functional integral with an additional constraint (slave‐boson approach). The relation between fermions and hard‐core bosons is briefly discussed for the case of a one‐dimensional Bose gas. For a three‐dimensional gas we identify the order parameter of the Bose‐Einstein condensate through a Hubbard‐Stratonovich transformation and treat the corresponding theories within a mean‐field approximation and with Gaussian fluctuations. This allows us to evaluate the phase diagram, including the Bose‐Einstein condensate and the Mott insulator, the density‐density correlation function, the static structure factor, and the quasiparticle excitation spectrum. The role of quantum and thermal fluctuations are studied in detail for both approaches, where we find good agreement with the Gross‐Pitaevskii equation and with the Bogoliubov approach in the dilute regime. In the dense regime, which is characterized by the phase transition between the Bose‐Einstein condensate and the Mott insulator, we discuss a renormalized Gross‐Pitaevskii equation. This equation can describe the macroscopic wave function of the Bose‐Einstein condensate in the dilute regime as well as close to the transition to the Mott insulator. Finally, we compare the results of the attractive spin‐1/2 Fermi gas and those of the slave‐boson approach and find good agreement for all physical quantities.