Variational studies of dressed quasiparticles’ properties and their interactions with external potentials by S. Hadi Ebrahimnejad Rahbari B.Sc., Tabriz University, 2004 M.Sc., Sharif University of Technology, 2007 M.Sc., The University of British Columbia, 2009 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Physics) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) December 2014 c S. Hadi Ebrahimnejad Rahbari 2014 Abstract In this thesis, we investigated the spectral properties of polaronic quasiparti- cles resulting from the coupling of a charge carrier to the bosonic excitations of ordered environments. Holstein polarons, describing an electron locally coupled to dispersionless phonons, and spin polarons in hole-doped antifer- romagnets are considered. The Green’s function of the Holstein polaron in a lattice with various kinds of impurities is calculated using the momentum average approximation. The main finding is that the scenario where the mere effect of the coupling to phonons is to enhance the electron’s effective mass is incomplete as the phonons are found to also renormalize the impurity potential the polaron interacts with. This formalism is applied first to the case of a single impurity and the range of parameters where the polaron’s ground state is localized is identified. The lifetime of the polaron due to scattering from weak but extended disorder is then studied and it is shown that the renormalization of the disorder potential leads to deviation of the strong coupling results from Fermi’s golden rule’s predictions. Motivated by the hole-doped cuprate superconductors, the motion of a hole in a two-dimensional Ising antiferromagnet and its binding to an attractive impurity is studied next, based on a variational scheme that allows for configurations with a certain maximum number of spin flips. The role of the magnetic sublattices in determining the symmetry of the resulting bound states is discussed. Next, a more realistic model describing a hole in a CuO2 layer which retains the O explicitly, is considered. By neglecting the fluctuations of the Cu spins and using a variational principle similar to that of the previous chapter, a semi-analytic solution for the Green’s function of the hole in an infinite 2D lattice is constructed. The resulting quasiparticle dispersion shows the proper ground state and other features observed in experiments. The lack of importance of the background spin fluctuations is justified based on the hole’s ability to move on the O sublattice without disturbing the Cu spins. Finally, the model is generalized to gauge the importance of other relevant O orbitals. ii Preface A version of the work discussed in chapter 2 is published as “H. • Ebrahimnejad and M. Berciu, Physical Review B 85, 165117 (2012)”. It is a development of a previous work by Prof. M. Berciu which was published in Ref. [1]. The work discussed in chapter 3 is published as “H. Ebrahimnejad and • M. Berciu, Physical Review B 86, 205109 (2012)”. The work discussed in chapter 4 is published as “H. Ebrahimnejad and • M. Berciu, Physical Review B 88, 104410 (2013)”. The variational method used here was suggested before by M. Berciu in Ref. [2]. The work discussed in chapter 5 is published as “H. Ebrahimnejad, G. • A. Sawatzky and M. Berciu, Nature Physics 10, 951-955 (2014)”. A more detailed manuscript is under review for publication in another journal. The model we used here was introduced in Ref. [3]. I carried out analytical and numerical work for all four projects and wrote the first draft of the manuscript for the first three, which were then revised by M. Berciu before submission. The preparation of the draft for the paper published in Nature Physics as well as calculations for the three-magnon case are done by M. Berciu. iii Table of Contents Abstract ................................. ii Preface .................................. iii Table of Contents ............................ iv List of Figures .............................. vii Acknowledgements ........................... xv 1 Introduction ............................. 1 1.1 Hamiltonian for lattice polarons . 3 1.1.1 Polarons in systems with other forms of electron-boson coupling ......................... 6 1.2 Studying spectral properties using Green’s function formalism 7 1.3 Green’s function of the Holstein model and its variational calculation ............................ 8 1.4 Overview of thesis and further reading . 9 2 Trapping of three-dimensional Holstein polarons by various impurities ............................... 12 2.1 Introduction ........................... 12 2.2 Momentum average approximation for inhomogeneous sys- tems................................ 13 2.3 Polaron near a single impurity . 21 2.3.1 Impurity changing the on-site energy . 22 2.3.2 Impurity changing the electron-phonon coupling . 29 2.3.3 Isotopeimpurity . 31 2.4 Summaryandconclusions . 35 iv Table of Contents 3 A perturbational study of the lifetime of a Holstein polaron in the presence of weak disorder ................ 38 3.1 Introduction ........................... 38 3.2 Themodelanditssolution . 39 3.3 Results .............................. 42 3.3.1 Weak electron-phonon coupling . 45 3.3.2 Strong electron-phonon coupling . 47 3.4 Summaryandconclusions . 52 4 Binding carriers to a non-magnetic impurity in a two-dimensional square Ising anti-ferromagnet .................. 55 4.1 Introduction ........................... 55 4.2 TheModel ............................ 57 4.3 Propagation of the hole in the clean system . 59 4.4 Theeffectoftheimpurity . 66 4.4.1 Quasiparticle and impurity on the same sublattice . 67 4.4.2 Quasiparticle and impurity on different sublattices . 71 4.5 Summaryandconclusions . 76 5 Accurate variational solution for a hole doped in a CuO2 layer .................................. 79 5.1 Introduction ........................... 79 5.2 Three-band model for a cuprate layer and its t-J analogue . 80 5.2.1 Doping the CuO2 layer with a hole . 84 5.3 Green’sfunctions ........................ 87 5.3.1 One-magnon approximation . 88 5.3.2 Two-magnon approximation . 93 5.4 Results .............................. 94 5.4.1 Dispersion relations . 94 5.4.2 Effect of various terms in the Hamiltonian . 97 5.5 Quasiparticle weight . 99 5.5.1 ComparisonwithARPES . 99 5.6 Spin polaron vs the Zhang-Rice singlet . 102 5.7 From three-band to five-band model: effect of other in-plane 2p orbitals ............................105 5.8 Conclusion ............................109 6 Summary and development ...................110 6.1 Summaryofthiswork . 110 6.2 Outlook and further developments . 112 v Table of Contents Bibliography ...............................114 Appendices A MA0 solution for the clean system ...............121 B IMA1 ..................................123 0 (n) nm C MA solution for Fk,si(ω) and W (ω) .............126 D Disorder self-energy in Average T-matrix approximation 128 E The equations of motion for G0,R(ω) ..............129 F Derivation of ARPES intensity .................131 G Solving for Gα,β(k,ω) in the five-band model .........135 vi List of Figures d 2.1 Diagrammatic expansion for (a) the disorder GF Gij(ω) (bold 0 red line); (b) the polaron GF in a clean system Gij(˜ω) (double thin line), and (c) the “instantaneous” approximation for the d polaron GF in a disordered system, Gij(˜ω) (double bold red line). The thin black lines depict free electron propagators, the wriggly lines correspond to phonons, and scattering on the disorder potential is depicted the dashed lines ending with circles. See text for more details. 16 2.2 (color online) (a) Diagrammatic expansion for the full MA solution Gij(ω) (thick dashed blue line) in terms of the clean polaron Green’s function (double thin line) and scattering on the renormalized disorder potential ǫl∗(ω), depicted by dashed lines ended with squares. (b) Diagrammatic expansion of ǫl∗(ω). For more details, see text. 19 2.3 LDOS at the impurity site ρ(0,ω) in units of 1/t, vs. the 0 energy ω/t, for (a) MA with lc = 0 for U = 1.8, 1.9, 1.95 and 2.0. The dashed line shows the DOS for the clean system, times 100; (b) Bound polaron peak in MA0 at U/t = 2, and cutoffs in the renormalized potential lc = 0, 1, 2. Panels (c) and (d) are the same as (a) and (b), respectively, but using MA1. Panel (e) shows the DMC results from Ref. [4], for the same parameters as (a) and (b). Other parameters are 3 Ω = 2t, λ = 0.8,η/t = 10− . ................... 24 2.4 Phase diagram separating the regime where the polaron is mobile (below the line) and trapped (above the line). The effective coupling is λ = g2/(6tΩ), and the threshold trapping potential Uc is shown for several values of Ω/t. The MA results (filled symbols) compare well with the DMC results of Ref.[4](emptysymbols). 25 vii List of Figures 2.5 (color online) Effective value of the impurity attraction U ∗/U, extracted from the scaling EB∗ /t∗ = f(U ∗/t∗), for λ = 0.5, 1.5 and Ω = 3t. In order to have the best fit to the data, we plot each curve starting from slightly larger U/t than the corresponding Uc/t given in Fig. (2.4). For more details, see text. ................................ 26 2.6 (color online) Real part of the additional on-site MA0 disorder potential v0(0,ω) over a wide energy range, for U = 2t, Ω = 2t, λ = 0.8 and two values of η. ................. 28 2.7 (color online) Phase diagram separating the regime where the polaron is delocalized (below the line) and trapped (above the line), as a function of the difference between the impu- rity and the bulk electron-phonon coupling, g g. Symbols d − show MA0 results, while the dashed lines correspond to the instantaneous approximation. 30 2.8 For large λ, in the clean system (dashed red line) the first polaron band is separated by an energy gap from the next features in the spectrum (here, the band associated with the second bound state).