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Low Energy Quasiparticle Excitations in Cuprates and Iron-Based Superconductors

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Low Energy Quasiparticle Excitations in Cuprates and Iron-Based Superconductors University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 12-2018 Low Energy Quasiparticle Excitations in Cuprates and Iron-based Superconductors Ken Nakatsukasa University of Tennessee, [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Recommended Citation Nakatsukasa, Ken, "Low Energy Quasiparticle Excitations in Cuprates and Iron-based Superconductors. " PhD diss., University of Tennessee, 2018. https://trace.tennessee.edu/utk_graddiss/5300 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Ken Nakatsukasa entitled "Low Energy Quasiparticle Excitations in Cuprates and Iron-based Superconductors." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Physics. Steven Johnston, Major Professor We have read this dissertation and recommend its acceptance: Christian Batista, Elbio Dagotto, David Mandrus Accepted for the Council: Dixie L. Thompson Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) Low Energy Quasiparticle Excitations in Cuprates and Iron-based Superconductors A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Ken Nakatsukasa December 2018 c by Ken Nakatsukasa, 2018 All Rights Reserved. ii Acknowledgments First, I would like to thank everyone, the school officials, the department staff, committee members, professors who helped me overcome the obstacle I faced in graduate school. A particular thanks to my research advisor Dr. Steven Johnston, along with the research group member, Umesh Kumar, Shaouzhi Li, Phil Dee, who were a graduate student and gave me wonderful physics/science discussions, and also to Dr. Yan Wang, the former postdoctoral researcher in our group who game me grateful insights regarding the spin fluctuation theory and electron-phonon calculations using Density Functional Perturbation Theory. Once again, I thank and give my deep gratitude to all the above mentioned. iii Abstract The microscopic properties of the phonon mediated conventional superconductors are well explained by the Bardeen-Cooper-Schrieffer (BCS) theory. However, a comprehensive description of the unconventional superconductivity such as the cuprates and iron-based superconductors is still under considerable debate. One of the theories proposed for explaining the pairing mechanism is the spin fluctuation exchange perhaps playing a leading role in inducing the unconventional superconductivity[1]. These materials are thought to be unconventional because they share certain commonalities that are absent in the conventional superconductors. The materials possess variety of phases upon doping including the superconductivity residing in close proximity to magnetism. They show an anisotropic superconducting order parameter. The transition temperatures tend to be higher than those predicted by the conventional electron-phonon couplings. These observations differ from the conventional superconductors indicating that the electron-phonon interactions may be non-factors or play a minimal role in paring. However, a significant phonon renormalization interpreted from photoemission spectroscopy[2], a pronounced isotope effect in cuprates[3], and the emergence of the replica band due to strong forward scattering phonons[4] raise questions regarding the role of phonons in these magnetic superconductors. McMillan and Rowell have studied the low energy excitations seen from a tunneling experiment to prove the electron-phonon coupling was indeed driving the superconductivity, and thus validated the BCS theory[5]. If the unconventional pairing is explained by the exchange of bosons, then one could use the extended version of the BCS theory such as the Eliashberg theory to describe properties of the superconductivity. In the present analysis, we have investigated the low energy features seen in various unconventional superconductors using a fully self-consistent algorithm. We find that the bosonic excitation picture has iv correctly explained many experimental features, though there were properties. The present analysis has enlightened the picture that the spin fluctuation exchange plays a pivotal role in mediating the pairing; we may further argue that the phonons can play an important role in renormalizing the electronic structure in cuprates; additionally, the forward scattering phonons can coexist with the spin fluctuation exchange to greatly enhance the transition temperature in a selected layered materials. v Table of Contents 1 Overview on Unconventional Superconductivity1 1.1 Early History of the Eliashberg Theory.....................2 1.2 Discovery of Magnetic Superconductors.....................4 2 Superconductivity Due to Bosonic Exchange Interactions8 2.1 Eliashberg Theory of Superconductivity..................... 10 2.2 Single Band, d-wave Superconductors...................... 18 2.2.1 Model................................... 18 2.2.2 Spectral Function............................. 22 2.2.3 Self-Energy, Transition Temperature, and λ .............. 24 2.2.4 Electron Density of States - Single boson model............ 25 2.2.5 Electron Density of States - Two Boson Model............. 26 3 Spin Fluctuation in High Tc Cuprates 30 3.1 Theory of Spin Fluctuation........................... 31 3.2 FLEX Approximations.............................. 37 3.2.1 FLEX: Normal State........................... 38 3.2.2 FLEX: Superconducting State...................... 39 3.3 Results....................................... 45 3.3.1 Tc vs: U .................................. 46 3.3.2 Tc Dependence on the Chemical Potential............... 48 3.3.3 Spin Fluctuation Interaction Vs ..................... 48 3.3.4 Spectral Function and Density of States................ 52 vi 4 Applications 56 4.1 Forward Scattering Phonons in Iron-based Superconductors.......... 57 4.1.1 Model................................... 60 4.1.2 Spectral Function............................. 63 4.1.3 Quasiparticle Interference........................ 66 4.1.4 Sr2VO3FeAs................................ 70 4.2 FLEX plus Phonons............................... 72 4.2.1 Tc vs. (U; λ) with phonons........................ 74 4.2.2 Spectral Function............................ 76 4.3 Competition between the Charge Density Wave and Conventional Supercon- ductivity...................................... 79 4.4 Multi-orbital FLEX Approximations...................... 84 4.4.1 Bilayer Hubbard Model.......................... 90 4.4.2 Pairing with an Incipient Band in a Bilayer System.......... 93 4.5 Concluding Remarks............................... 95 Bibliography 98 Appendices 124 A Vertex Corrections................................ 125 B Fourier Transform between i!n $ τ ....................... 127 C Pad´eApproximants................................ 132 D Anderson Mixing................................. 134 E Relevant Formulas................................ 135 E.1 Optical Conductivity........................... 136 E.2 Specific Heat............................... 137 F Analytic Continuation (Migdal-Eliashberg Theory).............. 137 Vita 140 vii List of Figures 1.1 (a) Neutron scattering data showing the phonon density of states in Pb. (b) α2F (!) extracted from tunneling data.....................3 1.2 Timeline of newly discovered superconductors..................6 2.1 (a) Electron density of states with the retardation effect. (b) Conductance ratio for Bi-2212 in the superconducting state..................9 2.2 (a1) & (a2) Lowest order diagram. (b1) - (b3) Second lowest order diagrams for electron self-energies.............................. 12 2.3 Electron self-energy within ladder approximations................ 13 2.4 (a) Electron self-energy within Migdal approximations. (b) Self-energy diagrams that are included in Migdal approximations.............. 15 2.5 (a) Modeled boson spectrum F (!) for the simulation. (b) Spin polarized neutron scattering intensity at q = (π; π) in YBa2Cu3O6:92.......... 19 2.6 (a) Calcualted spectral function along the nodal direction in the superconduct- ing state. (b) Calculated spectral function along the anti-nodal direction in the superconducting state. (c) & (d) ARPES intensity taken from optimally doped Bi-2212 compounds in the superconducting state along two different directions...................................... 23 2.7 (a) Calculated real part of the electron self-energies in the superconducting state. (b) Tc depenedence on λd. (c) Maximum value of the gap as a function of Tc........................................ 25 2.8 (a) & (b) Electron density of states using a single boson model in the superconducting state for three different λd values................ 26 viii 2.9 (a) & (b1) - (b3) Electron density of states using a two boson model in the superconducitng state for three different λd values. (c) Modeled function F (!) used for the two boson model.......................... 28 3.1 Illustration of dx2−y2 gap symmetry.......................
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