arXiv subjects
Publications and source records attributed to C. M. Varma.
Recent angular-resolved photoemission experiments on high-temperature superconductors are consistent with a phenomenological description of the normal state of these materials as Marginal Fermi Liquids. The experiments also provide constraints on microscopic theories
We develop a variational mean-field theory of the ferromagnetic transition in compounds like Lanthanum-Manganite within the framework of the Double-Exchange Model supplemented by modest disorder. We obtain analytical expressions for the transition temperature, its variation with the valence electron-density and its decrease with disorder. We derive an expression for the conductivity for both the paramagnetic and the ferromangetic metallic phases, and study its dependence on the temperature and magnetic field. A simple relation between the resistivity in the ferromagnetic phase and the spontaneous magnetization is found. Our results are in a good agreement with the experimental data on transition temperatures and resistivity in the manganite compounds with relatively small disorder. We comment on the effects of increased disorder.
A theory for the phenomena observed in Copper-Oxide based high temperature superconducting materials derives an elusive time-reversal and rotational symmetry breaking order parameter for the observed pseudogap phase ending at a quantum-critical point near the composition for the highest $T_c$. An experiment is proposed to observe such a symmetry breaking. It is shown that Angle-resolved Photoemission yields a current density which is different for left and right circularly polarized photons. The magnitude of the effect and its momentum dependence is estimated. Barring the presence of domains of the predicted phase an asymmetry of about 0.1 is predicted at low temperatures in moderately underdoped samples.
We solve the problem of a magnetic impurity in a $d_{x^2-y^2}$-wave superconductor by a variational method. A moment is found to exist in the superconducting state only if the Kondo-temperature in the normal state is larger than the maximum in the superconducting gap-function. If a moment exists in the superconducting state, and provided spin-orbit coupling is non-zero, it induces a time-reversal breaking superconducting state locally around the inpurity which is a linear combination of the $d_{x^2-y^2}$ and $d_{xy}$ states. The current pattern around the impurity in this state are evaluated.
A systematic solution of a general model for Copper-Oxides reveals a line of transitions $T = T_p (x)$ for $x$, the doping away from half-filling less than a critical value, to a phase with broken time reversal and rotational symmetry but with translational symmetry preserved. The single-particle spectrum in this phase is calculated to have a gap of $d_{x^2 - y^2}$ symmetry. The properties in this phase are compared to the properties of the so called ''pseudo-gap phase'' in the Copper-Oxides. The fluctuations towards this phase promote superconductivity which is either of the $d_{x^2-y^2}$ or the generalised s-symmetry depending on whether the projected density of states at the chemical potential is larger in the $d_{x^2-y^2}$ channel, as in the hole-doped copper-oxides, or in the s-wave channel.
The recent discovery of heavy-Fermion properties in Lithium Vanadate and the enormous difference in its properties from the properties of Lithium Titanate as well as of the manganite compounds raise some puzzling questions about strongly correlated Fermions. These are disscussed as well as a solution to the puzzles provided.
We calculate the corrections to the conductivity and compressibility of a disordered metal when the mean free path is smaller than the screening length. Such a condition is shown to be realized for low densities and large disorder. Analysis of the stability of the metallic state reveals a transition to the insulating state in two-dimensions.
We present analytic results for the polarizabilities, $χ({\bf Q},ω)$, in the charge, spin, and current channels for two-dimensional s- and d-wave superconductors for large momentum transfer. A collective mode in the charge channel is predicted to exist for extremum vectors of the Fermi-surface with energy below twice the maximum superconducting gap. Such modes are directly observable through inelastic x-ray or electron scattering. Scattering of single particle excitations by these collective modes leads to several unusual features in the single particle spectrum in the superconducting state which are seen in angle resolved photoemission experiments.
A model of copper-oxygen bonding and anti-bonding bands with the most general two-body interactions allowable by symmetry is considered. The model has a continuous transition as a function of hole-density x and temperature T to a phase in which a current circulates in each unit cell. This phase preserves the translational symmetry of the lattice while breaking time-reversal invariance and the four-fold rotational symmetry. The product of time-reversal and four-fold rotation is preserved. The circulating current phase terminates at a critical point at $x=x_c, T=0$. In the quantum-critical region about this point the logarithm of the frequency of the current fluctuations scales with their momentum. The microscopic basis for the marginal Fermi-liquid phenemenology and the observed long wavelength transport anomalies near $x=x_c$ are derived from such fluctuations. The symmetry of the current fluctuations is such that the associated magnetic field fluctuations are absent at oxygen sites and have the correct form to explain the anomalous copper nuclear relaxation rate. Cross-overs to the Fermi-liquid phase on either side of $x_c$ and the role of disorder are briefly considered. The current fluctuations promote superconductive instability with a propensity towards ``D-wave" symmetry or ``extended S-wave"symmetry depending on details of the band-structure.
Any singular deviation from Landau Fermi-liquid theory appears to lead, for arbitrarily small concentration of impurities coupling to a non-conserved quantity, to a vanishing density of states at the chemical potential and infinite resistivity as temperature approaches zero. Applications to copper-oxide metals including the temperature dependence of the anisotropy in resistivity, and to other cases of non Fermi-liquids are discussed.
We calculate the electronic polarizability in the superconducting state near extremum vectors ${\vec Q}_0$ of the Fermi surface. A pole appears in the polarizability at frequencies $ω$ near the superconducting gap $2Δ$ which leads to a sharp peak at $ω$ just below $2 Δ$ in the lattice vibrations near ${\vec Q}_0$. A second order transition to a charge density wave state below the superconducting transition is shown to be unlikely. The results are compared with the recent inelastic neutron scattering measurements in compounds, $ANi_2B_2C$, $A=Y$ or $Lu$.
The flux-line lattice in the compound $ErNi_2B_2C$, which has a tendency to ferromagnetic order in the a-b plane is studied with external magnetic field direction close to the c-axis. We show the existence of an instability where the direction of flux-lines spontaneously tilts away from that of the applied field near the onset of ferromagnetic order. The enhanced fluctuations in the flux lattice and the square flux lattice recently observed are explained and further experiments suggested.
The anomalous part of the NMR relaxation rate of coppernuclei in the normal state of copper-oxide metals is calculated using the orbital magnetic parts of the fluctuations derived in a recent theory to explain the long wavelength transport anomalies. Oxygen and Yttrium reside on lattice sites at which the anomalous contribution is absent at all hole densities. The frequency, momentum dependence, and the form-factor of the fluctuations is predicted,which is verifiable by inelastic neutron scattering experiments.
It is argued that a spontaneous vortex phase may exist in the recently discovered compound $ErNi_2B_2C$ at temperature below 2.3K. The consequences of this proposal are discussed. In particular the magnetic response of the system are studied both above and below 2.3K and further experiments proposed.
The paramagnetic insulator to ferromagnetic metal transition in Lanthanum manganites and the associated magnetoresistive phenomena is treated by considering the localization due to random hopping induced by slowly fluctuating spin configurations and electron-electron interactions. The transition temperature and its variation with composition is derived. The primary effect of the magnetic field on transport is to alter the localization length, an effect which is enhanced as the magnetic susceptibility increases. Expressions for the conductivity, its variation with magnetic field, and its connection with magnetic susceptibility in the paramagnetic phase are given and can be tested with further experiments.