This thesis studies two modern mean field theories of the t-J model for high Tc superconductors, namely the Slave-Boson Mean Field Theory and the Spinon-Dopon Mixing Mean Field Theory, from the view point of the traditional Fermi Liquid language. This study mainly focuses on whether the two mean field theories are consistent with Fermi Liquid behaviour in the underdoped regime of the superconducting and pseudogap phases of the t-J model. Historically, experimental results indicate that the superconducting phase can be well described by the conventional Fermi liquid-like quasiparticle picture; while the pseudogap phase exhibits non-trivial non-Fermi Liquid properties. This work aims at interpreting the pseudogap phase in a generalized Fermi Liquid framework and explaining why the simple quasiparticle picture works so well in the superconducting phase even in this strongly correlated superconducting system. Within the Gaussian approximation, we formally derived the Landau interaction in Fermi liquid theory, which serves as the key to classify the behaviour of the quasiparticles in different phases. It is found that the Slave-Boson Mean Field Theory describes the superconducting to the pseudogap phase transition as a transition with an interesting Landau parameter F1s changing from 1+F1s~x to 1+F1s=0, reflecting the occurrence of a quasiparticle instability. A new phase is proposed and its experimental consequences are discussed. On the other hand, we also find that the Spinon-Dopon Mixing Mean Field Theory can be understood in a conventional Fermi Liquid framework; even though quasiparticles in the theory carry larger charges. The issue of superfluid density is also clarified.
This thesis studies two modern mean field theories of the t-J model for high Tc superconductors, namely the Slave-Boson Mean Field Theory and the Spinon-Dopon Mixing Mean Field Theory, from the view point of the traditional Fermi Liquid language. This study mainly focuses on whether the two mean field theories are consistent with Fermi Liquid behaviour in the underdoped regime of the superconducting and pseudogap phases of the t-J model. Historically, experimental results indicate that the superconducting phase can be well described by the conventional Fermi liquid-like quasiparticle picture; while the pseudogap phase exhibits non-trivial non-Fermi Liquid properties. This work aims at interpreting the pseudogap phase in a generalized Fermi Liquid framework and explaining why the simple quasiparticle picture works so well in the superconducting phase even in this strongly correlated superconducting system. Within the Gaussian approximation, we formally derived the Landau interaction in Fermi liquid theory, which serves as the key to classify the behaviour of the quasiparticles in different phases. It is found that the Slave-Boson Mean Field Theory describes the superconducting to the pseudogap phase transition as a transition with an interesting Landau parameter F1s changing from 1+F1s~x to 1+F1s=0, reflecting the occurrence of a quasiparticle instability. A new phase is proposed and its experimental consequences are discussed. On the other hand, we also find that the Spinon-Dopon Mixing Mean Field Theory can be understood in a conventional Fermi Liquid framework; even though quasiparticles in the theory carry larger charges. The issue of superfluid density is also clarified.
The Gaussian fluctuations in slave-boson mean-field theory of the $t\text{\ensuremath{-}}J$ model are analyzed in this paper in the low-doping regime where the superconducting to normal (pseudogap phase) transition is driven by vanishing of bose-condensation amplitude. By eliminating the boson and constraint fields exactly in the linear-response regime, we show that the Gaussian theory describes an unconventional Fermi liquid superconductor where the superconducting to normal transition is a new type of phase transition characterized by a change of Landau parameter from ${F}_{1}>\ensuremath{-}1$ in the superconducting phase to ${F}_{1}=\ensuremath{-}1$ in the pseudo-gap phase, corresponding to a state with spin-charge separation. The consequences of this proposal are discussed.
The U(1) slave-boson mean-field theory of the t - J model has been used by many authors as a starting point for the theoretical analysis of high - Tc cuprate superconductors. With suitable refinements, the theory can explain a lot of qualitative features of cuprates. However, the theory does not produce a correct description of the low-energy quasi-particle properties in the under-doped regime. There exist several possible schemes to resolve this difficulty, among them, one generalizes the U(1) theory to SU(2) formulation and the other introduces confinement between spinons and holons. In this thesis, we generalize the Landau transport equations of the U(1) slave-boson mean-field theory of the t - J model to the SU(2) formulation. We first generalize the SU(2) slave-boson mean-field theory to the time-dependent regime. After eliminating the boson and constraint fields exactly in the linear-response regime at zero-temperature, we obtain a set of transport equations for quasi-particles, which has a similar structure as the Landau transport equations for Fermi-liquid superconductors, with all the Landau parameters explicitly given except that besides the usual effective interaction in the particle-hole channel in Fermi-liquid theory, we find an effective interaction in the particle-particle channel, which reflects the SU(2) symmetry between particles and holes. The transport equation is SU(2) invariant at half-filling. Around q⃗ = 0, the electron density couples only to the transverse part of the external field and the quasi-particle charge is still of order x. Besides the SU(2) formulation approach, we also investigate spinon-holon confinement in the U(1) slave-boson theory. The optical and thermal conduc-tivities of U (1) slave-boson theory with confinement are calculated. For optical conductivity, we consider only the contributions from static impurities. The qualitative behavior of our results agrees with experimental results. However, to explain the experiments quantitatively, we need to study further the con-tributions from phonons. We found a phenomenological effective Hamiltonian for the quasi-particle Green's functions in the U(1) slave-boson theory with spinon-hoion confinement. Starting from this effective Hamiltonian, we exam-ine the thermal conductivity within the theory. In particular, we compare the thermal conductivity for four different situations: (i) the ordinary d-wave su-perconducting state with four Dirac Fermi points; (ii) the normal metal state with a half-pocket Fermi surface; (iii) the spinon-holon confinement supercon-ducting state with a half-pocket Fermi surface; (iv) BCS superconducting state with a half-pocket Fermi surface.
The Gaussian fluctuations in slave-boson mean-field theory of t−J model is analyzed in this paper in the low-doping regime where the superconducting to normal (pseudo-gap phase) transition is driven by vanishing of bose-condensation amplitude. By eliminating the boson and constraint fields exactly in the linear response regime we show that the Gaussian theory describes a Fermi liquid superconductor where the superconducting to normal transition is actually a spin-charge separation transition characterized by a change of Landau parameter from F1>−1 in the superconducting phase to F1=−1 in the pseudo-gap phase. Consequences of this proposal are discussed.
This thesis is organized in two independent parts, in which I study two different aspects of high Tc superconductivity. The first part begins with an introduction aimed to briefly introduce some relevant experimental and theoretical works performed in recent years, that have helped us to think about cuprates the way we do now. Afterwards, I introduce Landau Fermi liquid theory in a standard text book way. The question of validity of Fermi liquid theory in 2-d is then raised and investigated by searching for singularities in Landau's /-function. I show that the interaction function between two quasiparticles whose momenta approach each other near a curved point of the Fermi surface, contains a 1-d singularity not strong enough to change the Fermi liquid behavior. On the other hand, inflection points provide 2-d singularities that have to be taken seriously in Fermi liquid considerations. I then introduce nearly antiferromagnetic Fermi liquid theory (NAFL), which is a phenomenological theory proposed to describe high Tc systems. I mainly focus on the self-consistency of the theory in calculations. I criticize the theory on the basis of overlooking the vertex corrections in the strong coupling calculations of the transition temperature Tc. I calculate the first vertex correction for an optimally doped system and show that it is of the same order of magnitude as the bare vertex. Migdal's theorem is therefore not valid and Eliashberg formalism is not applicable to this situation. The same conclusion is obtained even after inclusion of the quasiparticle residue Z to the calculation. The sign of the vertex correction is then considered. I show that the positive sign of the vertex correction for the optimally doped system requires a phase transition of some sort as the doping is decreased. Part II of the thesis is devoted to the vortex lattice properties of high Tc superconductors. I establish a method to study vortex lattice properties of d-wave superconductors based on a generalization of the London model. The method has the advantage of simplicity as well as having very few free parameters (one at most) compared to other methods. The generalized London free energy is obtained from an s-d mixing Ginsburg-Landau free energy and also from the microscopic theory of Gorkov. The generalized London equation is found to be analytic at high temperatures. At very low temperatures however nonanalyticities arise as a result of the nodes on the superconducting gap. I then present the results of our calculations of some measurable quantities, such as the vortex lattice geometry and the effective penetration depth (as denned in μSR experiments). Comparison between our results and different experimental data is then performed. Especially our prediction for the magnetic field dependence of the effective penetration depth at T = 0, which is recently observed in //SR experiments with excellent agreement, is discussed.
In this paper we generalize slave-boson mean-field theory for $t\ensuremath{-}J$ model to the time-dependent regime, and derive transport equations for $t\ensuremath{-}J$ model, both in the normal and superconducting states. By eliminating the boson and constraint fields exactly at zero temperature in the linear-response regime we obtain a set of transport equations for physical electrons which have the same form as Landau transport equations for normal Fermi liquid and Fermi-liquid superconductor, respectively, with all Landau parameters explicitly given. Our theory can be viewed as a refined version of U(1) Gauge theory where all lattice effects are retained and strong correlation effects are reflected as strong Fermi-liquid interactions in the transport equation. Some experimental consequences are discussed.
The principal purpose of this topical review is to single out some of the universal features of high-temperature (high-Tc) and related strongly-correlated systems, which can be compared with experiment in a quantitative manner. The description starts with the concept of exchange-interaction-mediated (real-space) pairing, combined with strong correlations among narrow band electrons, for which the reference state is that of the Mott–Hubbard insulator. The physical discussion of concrete properties relies on variational approach, starting from generalized renormalized mean-field theory (RMFT) in the form of statistically-consistent Gutzwiller approximation (SGA), and its subsequent generalization, i.e., a systematic Diagrammatic Expansion of the Variational (Gutzwiller-type) Wave Function (DE-GWF). The solution leads to the two energy scales, one involving unconventional quasiparticles close to the Fermi energy, and the other reflecting the fully correlated state, involving electrons deeper below the Fermi surface. Those two regimes are separated by a kink in the dispersion relation, which is observed in photoemission. As a result, one obtains both the doping dependent properties in the correlated state, and effective renormalized quasiparticles. The reviewed ground-state characteristics for high-Tc systems encompass high-Tc superconductivity, nematicity, charge- (and pair-) density-wave effects, as well as non-BCS kinetic energy gain in the paired state. The calculated dynamic properties are: the universal Fermi velocity, the Fermi wave-vector, the effective mass enhancement, the pseudogap; and the d-wave gap magnitude. We discuss that, within the variational approach, the minimal realistic model is represented by the so-called t-J-U Hamiltonian. Inadequacy of the t-J and Hubbard models, particularly of their RMFT versions, is discussed explicitly. For heavy fermion systems, modeled by the Anderson lattice model and with the DE-GWF approach, we discuss the phase diagram, encompassing superconducting, Kondo insulating, ferro- and anti-ferromagnetic states. The superconducting state is then a two-d-wave gap system. If the orbital degeneracy of f-electrons is included, the coexistent ferromagnetic- (spin–triplet) superconducting phases appear and match those observed for UGe2 in a semiquantitative manner. Finally, in the second part, we generalize our approach to the collective spin and charge fluctuations in high-Tc systems, starting from variational approach, defining the saddle-point state, and combining it with 1/N expansion. The present scheme differs essentially from that starting from the saddle-point Hartree–Fock approximation, and incorporating the fluctuations in the random phase approximation (RPA). The spectrum of collective spin and charge excitations is determined for the Hubbard and t-J-U models, and subsequently compared quantitatively with recent experiments. The Appendices provide formal details to make this review self-contained.