Superconductivity in the t-J model is studied by extending the recently introduced extremely correlated fermi liquid theory. Exact equations for the Greens functions are obtained by generalizing Gorkovs equations to include extremely strong local rep
ulsion between electrons of opposite spin. These equation are expanded in a parameter $lambda$ representing the fraction of double occupancy, and the lowest order equations are further simplified near $T_c$, resulting in an approximate integral equation for the superconducting gap. The condition for $T_c$ is studied using a model spectral function embodying a reduced quasiparticle weight $Z$ near half-filling, yielding an approximate analytical formula for $T_c$. This formula is evaluated using parameters representative of single layer High-$T_c$ systems. In a narrow range of electron densities that is necessarily separated from the Mott-Hubbard insulator at half filling, we find superconductivity with a typical $T_c$$sim$$10^2$K.
Planar normal state resistivity data taken from three families of cuprate superconductors are compared with theoretical calculations from the recent extremely correlated Fermi liquid theory (ECFL). The two hole doped cuprate materials $LSCO$ and $BSL
CO$ and the electron doped material $LCCO$ have yielded rich data sets at several densities $delta$ and temperatures T, thereby enabling a systematic comparison with theory. The recent ECFL resistivity calculations for the highly correlated $t$-$t$-$J$ model by us give the resistivity for a wide set of model parameters. After using X-ray diffraction and angle resolved photoemission data to fix parameters appearing in the theoretical resistivity, only one parameter, the magnitude of the hopping $t$, remains undetermined. For each data set, the slope of the experimental resistivity at a single temperature-density point is sufficient to determine $t$, and hence the resistivity on absolute scale at all remaining densities and temperatures. This procedure is shown to give a fair account of the entire data.
Three Fermion sumrules for interacting systems are derived at T=0, involving the number expectation $bar{N}(mu)$, canonical chemical potentials $mu(m)$, a logarithmic time derivative of the Greens function $gamma_{vec{k} sigma}$ and the static Greens
function. In essence we establish at zero temperature the sumrules linking: $$ bar{N}(mu) leftrightarrow sum_{m} Theta(mu- mu(m)) leftrightarrow sum_{vec{k},sigma} Thetaleft(gamma_{vec{k} sigma}right) leftrightarrow sum_{vec{k},sigma} Thetaleft(G_sigma(vec{k},0)right). $$ Connecting them across leads to the Luttinger and Ward sumrule, originally proved perturbatively for Fermi liquids. Our sumrules are nonperturbative in character and valid in a considerably broader setting that additionally includes non-canonical Fermions and Tomonaga-Luttinger models. Generalizations are given for singlet-paired superconductors, where one of the sumrules requires a testable assumption of particle-hole symmetry at all couplings. The sumrules are found by requiring a continuous evolution from the Fermi gas, and by assuming a monotonic increase of $mu(m)$ with particle number m. At finite T a pseudo-Fermi surface, accessible to angle resolved photoemission, is defined using the zero crossings of the first frequency moment of a weighted spectral function.
Low energy properties of the metallic state of the 2-dimensional tJ model are presented at various densities and temperatures for second neighbor hopping t, with signs that are negative or positive corresponding to hole or electron doping. The calcul
ation employs a closed set of equations for the Greens functions obtained from the extremely correlated Fermi liquid theory. These equations, when used in $d=infty$ reproduce most of the known low energies features of the $U=infty$ Hubbard model. In 2-dimensions we are able to study the variations due to the superexchange J. The resulting Dyson self energy is found to be momentum dependent as expected. The density and temperature dependent quasiparticle weight, decay rate and the peak spectral heights over the Brillouin zone are calculated. We also calculate the resistivity, Hall conductivity and cotangent of the Hall angle in experimentally relevant units. These display significant thermal sensitivity for density n >~ 0.8, signifying an effective Fermi-liquid temperature scale which is two or three orders of magnitude below the bare bandwidth. Flipping the sign of the hopping t, i.e. studying hole versus electron doping, is found to induce a change in curvature of the temperature dependent resistivity from convex to concave at low temperatures. Our results provide a natural route for understanding the observed difference in the temperature dependent resistivity of strongly correlated electron-doped and hole-doped matter.
The t-J model is studied using a novel and rigorous mapping of the Gutzwiller projected electrons, in terms of canonical electrons. The mapping has considerable similarity to the Dyson-Maleev transformation relating spin operators to canonical Bosons
. This representation gives rise to a non Hermitean quantum theory, characterized by minimal redundancies. A path integral representation of the canonical theory is given. Using it, the salient results of the extremely correlated Fermi liquid (ECFL) theory, including the previously found Schwinger equations of motion, are easily rederived. Further a transparent physical interpretation of the previously introduced auxiliary Greens functions and the caparison factor is obtained. The low energy electron spectral function in this theory with a strong intrinsic asymmetry, is summarized in terms of a few expansion coefficients. These include an important emergent energy scale $Delta_0$ that shrinks to zero on approaching the insulating state, thereby making it difficult to access the underlying low energy Fermi liquid behavior. The scaled low frequency ECFL spectral function is related simply to the Fano line shape. The resulting energy dispersion (EDC or MDC) is a hybrid of a massive and a massless Dirac spectrum $ E^*_Qsim gamma, Q- sqrt{Gamma_0^2 + Q^2} $, where the vanishing of $Q$, a momentum like variable, locates the kink. Therefore the quasiparticle velocity interpolates between $(gamma mp 1)$ over a width $Gamma_0$ on the two sides of $Q=0$. The resulting kink strongly resembles a prominent low energy feature seen in angle resolved photoemission spectra (ARPES) of cuprate materials. We also propose novel ways of analyzing the ARPES data to isolate the predicted asymmetry between particle and hole excitations.
We apply the recently developed extremely correlated Fermi liquid theory to the Anderson impurity model, in the extreme correlation limit. We develop an expansion in a parameter lambda, related to n_d, the average occupation of the localized orbital,
and find analytic expressions for the Greens functions to O(lambda^2). These yield the impurity spectral function and also the self-energy Sigma(omega) in terms of the two self energies of the ECFL formalism. The imaginary parts of the latter, have roughly symmetric low energy behaviour (~ omega^2), as predicted by Fermi Liquid theory. However, the inferred impurity self energy Sigma(omega) develops asymmetric corrections near n_d ~ 1, leading in turn to a strongly asymmetric impurity spectral function with a skew towards the occupied states. Within this approximation the Friedel sum rule is satisfied but we overestimate the quasiparticle weight z relative to the known exact results, resulting in an over broadening of the Kondo peak. Upon scaling the frequency by the quasiparticle weight z, the spectrum is found to be in reasonable agreement with numerical renormalization group results over a wide range of densities.
We study a recently proposed quantum integrable model defined on a lattice with N sites, with Fermions or Bosons populating each site, as a close relative of the well known spin-1/2 Gaudin model. This model has 2N arbitrary parameters, a linear depen
dence on an interaction type parameter x, and can be solved exactly. It has N known constants of motion that are linear in x. We display further constants of motion with higher Fermion content, that are are linearly independent of the known conservation laws. Our main result is that despite the existence of these higher conservation laws, the model has only N functionally independent conservation laws. Therefore we propose that N can be viewed as the number of degrees of freedom, in parallel to the classical definition of integrability.
We present a software package DiracQ, for use in quantum many-body Physics. It is designed for helping with typical algebraic manipulations that arise in quantum Condensed Matter Physics and Nuclear Physics problems, and also in some subareas of Chem
istry. DiracQ is invoked within a Mathematica session, and extends the symbolic capabilities of Mathematica by building in standard commutation and anticommutation rules for several objects relevant in many-body Physics. It enables the user to carry out computations such as evaluating the commutators of arbitrary combinations of spin, Bose and Fermi operators defined on a discrete lattice, or the position and momentum operators in the continuum. Some examples from popular systems, such as the Hubbard model, are provided to illustrate the capabilities of the package.
We present detailed results from a recent microscopic theory of extremely correlated Fermi liquids, applied to the t-J model in two dimensions. We use typical sets of band parameters relevant to the cuprate superconductors. The second order theory in
the parameter lambda is argued to be quantitatively valid in the overdoped regime for 0 < n < 0.75 (n is the particle density). The calculation involves the self consistent solution of equations for an auxiliary Fermi liquid type Greens function and an adaptive spectral weight, or caparison factor, described in recent papers by Shastry (Refs. (1) and (5)). We present the numerical results at low as well as high T at various low to intermediate densities in the normal phase with emphasis placed on features that are experimentally accessible. We display the momentum space occupation function m(k), various energy dispersions locating the peaks of spectral functions, the optical conductivity, relaxation rates for quasiparticles, and the electronic spectral functions along various directions in the Brillouin zone, and with typical additional elastic scattering. The line-shapes have an asymmetric shape and a broad background that is seen in experiments near and beyond optimal hole doping, and validate approximate recent rece
We present the detailed formalism of the extremely correlated Fermi liquid theory, developed for treating the physics of the t-J model. We start from the exact Schwinger equation of motion for the Greens function for projected electrons, and develop
a systematic expansion in a parameter lambda, relating to the double occupancy. The resulting Greens function has a canonical part arising from an effective Hamiltonian of the auxiliary electrons, and a caparison part, playing the role of a frequency dependent adaptive spectral weight. This adaptive weight balances the requirement at low omega, of the invariance of the Fermi volume, and at high omega, of decaying as c_0/(i omega), with a correlation depleted c_0 <1. The effective Hamiltonian H_{eff} describing the auxiliary Fermions is given a natural interpretation with an effective interaction V_{eff} containing both the exchange J(ij), and the hopping parameters t(ij). It is made Hermitian by adding suitable terms that ultimately vanish, in the symmetrized theory developed in this paper. Simple but important shift invariances of the t-J model are noted with respect to translating its parameters uniformly. These play a crucial role in constraining the form of V_{eff} and also provide checks for further approximations. The auxiliary and physical Greens function satisfy two sum rules, and the Lagrange multipliers for these are identified. A complete set of expressions for the Greens functions to second order in lambda is given, satisfying various invariances. A systematic iterative procedure for higher order approximations is detailed. A superconducting instability of the theory is noted at the simplest level with a high transition temperature.
