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C M Varma

Publications and source records attributed to C M Varma.

12 recordsLinked to original sources
PubMed2006

Helicity order: hidden order parameter in URu2Si2.

We propose that the "hidden order parameter" in URu2Si2 is a helicity order that must arise if the Pomeranchuk criteria for the spin-antisymmetric Landau parameters with respect to the stability of a Fermi liquid state are violated. In a simple model, we calculate the specific heat, the linear and nonlinear magnetic susceptibilities, and the change of transition temperature in a magnetic field with such an order parameter, and obtain quantitative agreement with experiments in terms of two parameters extracted from the data. The peculiar temperature dependence of the NMR linewidth and the nature of the loss of excitations in the ordered phase seen by neutron scattering are also explained, and experiments are suggested to directly confirm the proposed order parameter.

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PubMed2005

Undressing the Kondo effect near the antiferromagnetic quantum critical point.

The problem of a spin-1/2 magnetic impurity near an antiferromagnetic transition of the host lattice is shown to transform to a multichannel problem. A variety of fixed points is discovered asymptotically near the antiferromagnetic critical point. Among these is a new variety of stable fixed point of a multichannel Kondo problem which does not require channel isotropy. At this point Kondo screening disappears but coupling to spin fluctuations remains. In addition to its intrinsic interest, the problem is an essential ingredient in the problem of quantum critical points in heavy fermions.

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PubMed2003

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.

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PubMed2003

Proposal to determine the spectrum of pairing glue in high-temperature superconductors.

We propose a method for an analysis of the angle-resolved photoemission data in two-dimensional anisotropic superconductors which directly yields the spectral function of the bosons mediating Cooper pairing. The method includes a self-consistency check for the validity of the approximations made in the analysis. We explicitly describe the experimental data needed for implementing the proposed procedure.

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PubMed2002

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.

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PubMed2002

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.

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PubMed2002

Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high-Tc superconductor.

A change in 'symmetry' is often observed when matter undergoes a phase transition-the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field transition as seen in other superconductors. Rather, there is a region in the phase diagram above the superconducting transition temperature Tc (where phase coherence and superconductivity begin) but below a characteristic temperature T* where a 'pseudogap' appears in the spectrum of electronic excitations. It is therefore important to establish if T* is just a cross-over temperature arising from fluctuations in the order parameter that will establish superconductivity at Tc (refs 3, 4), or if it marks a phase transition where symmetry is spontaneously broken. Here we report that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right-circularly polarized photons. This shows that time-reversal symmetry is spontaneously broken below T*, which therefore corresponds to a phase transition.

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PubMed2001

Effective Lorentz force due to small-sngle impurity scattering: magnetotransport in high- Tc superconductors.

We show that a scattering rate which varies with angle around the Fermi surface has the same effect as a periodic Lorentz force on magnetotransport coefficients. This effect, together with the marginal Fermi liquid inelastic scattering rate, gives a quantitative explanation of the temperature dependence and the magnitude of the observed Hall effect and magnetoresistance with just the measured zero-field resistivity as input.

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PubMed2001

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.

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