C. M. Varma

Magnetodielectric Effects from Spin Fluctuations in Isostructural Ferromagnetic and Antiferromagnetic Systems

We report on the effects of spin fluctuations, magnetic ordering, and external magnetic field on the dielectric constant of the ferromagnet SeCuO3, and the antiferromagnet TeCuO3. A model based on the coupling between uniform polarization and the q-dependent spin-spin correlation function is presented to explain the different behaviors for these isostructural compounds. The large magnetocapacitance near the transition temperature in the ferromagnet SeCuO3 suggests routes to enhancing the magnetodielectric response for practical applications.

Detection and implications of a time-reversal breaking state in underdoped cuprates.

We present general symmetry considerations on how a time-reversal breaking state may be detected by angle-resolved photoemission using circularly polarized photons as has been proposed earlier. Results of recent experiments utilizing the proposal in underdoped cuprates are analyzed and found to be consistent in their symmetry and magnitude with a theory of the copper oxides. These experiments if correct, together with evidence of a quantum critical point and marginal Fermi-liquid properties near optimum doping, suggest that the essentials of a valid microscopic theory of the phenomena in the cuprates may have been found.

The underdoped phase of cuprate superconductors

The Richness and variety of phenomena observed in condensed-matter physics continues to surprise and confound, and no family of materials demonstrates this better than the cuprates. These are ceramic compounds in which elements such as lanthanum and strontium, or yttrium and barium, are sandwiched between layers of copper and oxygen atoms. For instance, the material La2CuO4 is an insulator, but when some of the lanthanum atoms are replaced by strontium, it loses all resistance to electric current and becomes a superconductor. This was essentially the breakthrough made by Georg Bedmorz and Alex Muller in 1986. The temperature below which this doped cuprate becomes superconducting varies with the concentration of strontium, and reaches a maximum of 38 K when the composition of the ceramic material is La1.85Sr0.15CuO4.

Spontaneous Vortex Phase Discovered

It is argued that a spontaneous vortex phase probably exists in the recently discovered compound ErNi{sub 2}B{sub 2}C at temperatures below 2.3K. The consequences of this proposal are discussed. In particular, the magnetic response of the system is studied both above and below 2.3K, and further experiments are proposed. {copyright} {ital 1997} {ital The American Physical Society}

METAL-INSULATOR TRANSITION OF DISORDERED INTERACTING ELECTRONS

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.

Dynamics of some constrained lattices.

We consider the dynamics of lattices which have constrained constitutive units flexible in only their mutual orientations. A continuum description is derived through which it is shown that the models have zero shear velocity, free-particle-like internal rotational modes and volume decreasing linearly with temperature. The relevance of models to a range of problems is pointed out.

Singular Effects of Impurities near the Ferromagnetic Quantum-Critical Point

Systematic theoretical results for the effects of a dilute concentration of magnetic impurities on the thermodynamic and transport properties in the region around the quantum critical point of a ferromagnetic transition are obtained. In the quasiclassical regime, the dynamical spin fluctuations enhance the Kondo temperature. This energy scale decreases rapidly in the quantum fluctuation regime, where the properties are those of a line of critical points of the multichannel Kondo problem with the number of channels increasing as the critical point is approached, except at unattainably low temperatures where a single channel wins out.

Only Fermi Liquids Are Metals

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.

TILTING INSTABILITY AND OTHER ANOMALIES IN THE FLUX LATTICE IN SOME MAGNETIC SUPERCONDUCTORS

The flux-line lattice in the compound ErNi{sub 2}B{sub 2}C, which has a tendency to ferromagnetic order in the a-bplane, is studied with the external magnetic field direction close to the caxis. We show the existence of an instability, where the direction of flux lines spontaneously tilts away from that of the applied field as a prelude to the onset of ferromagnetic order and the predicted spontaneous vortex phase. This tilting instability is accompanied by a decreasing longitudinal correlation length of the flux lattice, as observed. {copyright} {ital 1997} {ital The American Physical Society}

Coulomb effects at the metal–insulator transition of disordered electrons

Abstract At low enough density, the screening length of a disordered interacting electron gas may become larger than its mean free path. Calculation of the conductivity in such a case shows a strong tendency towards the insulating state in two dimensions. The compressibility being proportional to the inverse screening length is expected to vanish as the transition to the insulating state is approached.