Quantum Transport Through Open Systems
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Quantum Transport Through Open Systems by Jinshan Wu B.Sc., The Beijing Normal University, 1999 M.Sc., The Beijing Normal University, 2002 M.Sc., The University Of British Columbia, 2006 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in The Faculty of Graduate Studies (Physics) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April, 2011 c Jinshan Wu 2011 Abstract To study transport properties, one needs to investigate the system of inter- est when coupled to biased external baths. This requires solving a master equation for this open quantum system. Obtaining this solution is very chal- lenging, especially for large systems. This limits applications of the theories of open quantum systems, especially insofar as studies of transport in large quantum systems, of interest in condensed matter, is concerned. In this thesis, I propose three efficient methods to solve the Redfield equation | an example of such a master equation. The first is an open- system Kubo formula, valid in the limit of weak bias. The second is a solu- tion in terms of Green's functions, based on a BBGKY (Bogoliubov{Born{ Green{Kirkwood{Yvon)-like hierarchy. In the third, the Redfield equation is mapped to a generalized Fokker-Planck equation using the coherent-state representation. All three methods, but especially the latter two, have much better efficiency than direct methods such as numerical integration of the Redfield equation via the Runge-Kutta method. For a central system with a d-dimensional Hilbert space, the direct methods have complexity of d3, while that of the latter two methods is on the order of polynomials of log d. The first method, besides converting the task of solving the Redfield equation to solving the much easier Schr¨odinger'sequation, also provides an even more important conceptual lesson: the standard Kubo formula is not applicable to open systems. Besides these general methodologies, I also investigate transport proper- ties of spin systems using the framework of the Redfield equation and with direct methods. Normal energy and spin transport is found for integrable but interacting systems. This conflicts with the well-known conjecture linking anomalous conductivity to integrability, and it also contradicts the relation- ship, suggested by some, between gapped systems (Jz > Jxy) and normal spin conductivity. I propose a new conjecture, linking anomalous transport to the existence of a mapping of the problem to one for non-interacting particles. ii Preface The work presented in Chapters 2 and 6 are a continuation of the work done for my M.Sc. thesis. This work is reported in the manuscript: \Heat transport in quantum spin chains: the relevance of integrability", Jinshan Wu and Mona Berciu, submitted to Physical Review B and posted as arXiv:1003.1559. The work presented in Chapter 3 is reported in Jinshan Wu and Mona Berciu 2010 Europhysics Letters 92 30003. The first part of the work presented in Chapter 4 is reported in Jinshan Wu 2010 New J. Phys. 12 083042. The second part, which is on the second- order BBGKY-like method, is currently under preparation for publication. Original work reported in other chapters will be prepared for publication in the near future. All the original work in this thesis was carried out by JW who also developed the conception, scope and methods of this research, with various degrees of consultation and editing support from MB. iii Table of Contents Abstract ................................. ii Preface .................................. iii Table of Contents ............................ iv List of Figures .............................. vii Acknowledgements ........................... ix Dedication ................................ x Chapter 1 Introduction ....................... 1 1.1 Two major theoretical approaches . 2 1.2 A big challenge for the system-baths scenario . 5 1.3 Our methods and their applications . 7 1.3.1 Linear response theory . 7 1.3.2 The BBGKY-like hierarchy . 8 1.3.3 The GFPE in the coherent-state representation . 9 1.3.4 Thermal conductance of spin chains . 9 1.4 Summary . 11 Chapter 2 The projector technique and the Redfield equation 12 2.1 Outline . 12 2.2 The effective equation of motion for open systems . 12 2.2.1 The projector operator technique . 14 2.2.2 An alternative derivation . 19 2.2.3 QME in terms of creation and annihilation operators 20 2.3 Relaxation towards thermal equilibrium . 22 2.3.1 One-site open system . 22 2.3.2 Two-site open system . 24 2.4 Various other extensions . 28 iv Table of Contents 2.5 Evolution towards a NESS . 30 2.5.1 NESSs from the RE . 31 2.5.2 NESSs from the LOLE and the MQME . 34 2.5.3 The general Lindblad form . 37 2.6 Major challenge for large systems . 38 Chapter 3 Linear Response Theory: Kubo formula for open systems ................................ 40 3.1 Introduction . 40 3.2 General Linear Response Theory . 41 3.3 Limitations of the standard Kubo formula . 43 3.4 OKF: the Kubo formula for open finite-size systems . 47 3.5 Comparison of the two OKFs . 50 3.6 Summary and discussion . 53 Chapter 4 BBGKY-like hierarchy for the Redfield equation 55 4.1 Introduction . 55 4.2 Derivation of the BBGKY-like hierarchy . 56 4.3 Non-equilibrium Wick's theorem . 59 4.4 Truncation of the hierarchy . 62 4.4.1 Method 1: equilibrium substitution . 62 4.4.2 Method 2: first-level cluster expansion . 67 4.4.3 Method 2: second-level cluster expansion . 72 4.5 Conclusion and discussion . 78 Chapter 5 Solving the Redfield equation using coherent-state representations ........................... 79 5.1 Introduction . 79 5.2 The model and its effective equation of motion . 81 5.3 Exact solution for non-interacting systems . 85 5.4 Interacting systems: BBGKY-like hierarchy derived from the GFPE . 87 5.5 Approximate numerical solution for interacting systems . 90 5.6 Solving the LOLE via the GFPE method . 94 5.7 Conclusions . 96 Chapter 6 Application: heat transport in spin chains .... 97 6.1 Introduction . 97 6.2 The model and its Redfield equation . 99 6.3 Numerical methods . 100 v Table of Contents 6.4 Definitions of the thermal current and local temperatures . 101 6.5 Results . 103 6.6 Conclusions . 111 Chapter 7 Conclusions and discussions ............. 112 Bibliography ............................... 117 Appendix A Perturbational decomposition of the operators m^ 128 A.1 Expanded in eigenbasis of HS ................. 128 A.2 Perturbational expansion starting from non-interacting sys- tems . 129 Appendix B Second-order cluster expansion: G1, G2 ..... 134 B.1 Second-order cluster expansion . 134 B.2 Second-order equations of G1 and G2 ............. 135 Appendix C Estimation of convergence of the BBGKY-like methods ................................ 141 E;(1) C.1 ∆1 from method 1 . 141 C;(1) C.2 ∆1 from method 2 . 145 Appendix D Coherent states and coherent-state representa- tion ................................... 148 D.1 Coherent states for systems with a single mode . 148 D.2 Coherent states for multi-mode systems . 151 Appendix E FPE and Langevin equation ............ 153 E.1 Langevin equation for standard FPEs . 153 E.2 Analytical solution for the Ornstein-Uhlenbeck process . 155 E.3 Stochastic difference equation for truncated GFPEs . 155 Appendix F Solving the RE and LOLE of bosonic systems with BBGKY-like hierarchy ................... 157 F.1 Exact solution on non-interacting systems . 158 F.2 Approximate solution on interacting systems . 159 vi List of Figures 1.1 Sketch of a typical setup of heat conduction. 10 2.1 Sketch of a one-site system coupled to a heat bath. 23 2.2 Sketch of a two-site system coupled to a heat bath. 24 2.3 Sketch of a two-site system coupled to two heat baths. 27 2.4 Difference between non-equilibrium and equilibrium states . 33 2.5 Stationary states from the local-operator Lindblad equation and the multi-mode quantum master equation compared against those from the Redfield equation . 36 3.1 Difference between ρ(1) (1) and ρ(2) (1) for different values of δ ................................. 51 3.2 Difference between ρ(1) (1) and ρ(2) (1) for different values of λ ................................ 52 3.3 Difference between ρ(1) (1) and ρ(2) (1) for different values of V0 ................................ 53 E;(1) Ex 4.1 g1 compared with g1 for interacting systems at non- equilibrium . 66 C;(1) Ex 4.2 g1 compared with g1 for non-equilibrium interacting sys- tems . 71 C;(2) Ex 4.3 g1 compared with g1 for non-equilibrium interacting sys- tems . 74 C;(2) Ex 4.4 g2 is compared with g2 for non-equilibrium interacting systems . 75 4.5 Particle currents calculated from the second-, first- and the zeroth-order forms are compared . 76 4.6 Heat currents calculated from the second-, first- and the zeroth- order forms are compared . 77 5.1 Analytical solution compared against the BBGKY numerical solution . 88 vii List of Figures 5.2 The numerical simulation of the generalized Fokker-Planck equation for interacting systems . 93 5.3 A similar application to the local-operator Lindblad equation 96 6.1 Typical temperature profiles for N = 10 spin chains . 104 6.2 Current v.s. system size . 105 6.3 Normalized profiles . 106 6.4 Typical temperature profile for the cases of small Jz and small Bz ................................. 107 6.5 Temperature profiles on Ising spin chains and fermionic chains 109 7.1 A sketch of an exactly solvable decoherence model and its variation . 114 viii Acknowledgements It is a pleasure to thank those who made this thesis possible. I owe my deepest gratitude to my supervisor, Dr. Mona Berciu. This thesis would not have been possible without her great efforts to educate and to help me in so many ways.