1,015 research outputs found

    Specific Heat and Sound Velocity Distinguish the Relevant Competing Phase in the Pseudogap Region of High Temperature Superconductors

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    A great step forward towards the understanding of high temperature superconductors are the variety of experimental results which have led to the wide-spread acceptance of the idea that a phase with a broken symmetry competes with superconductivity in the under-doped region, often called the pseudo-gap region. There are a plethora of suggested phases. The idea, that a broken symmetry phase competes with superconductivity makes thermodynamic sense only if the energy gained due to it is comparable to that gained through the superconducting transition in their co-existence region. Extraordinarily, however, no specific heat signature of a phase transition has been identified at the pseudo-gap temperature TT^*. We use the recent highly accurate sound-velocity measurements and the best available specific heat measurements in YBa2_2Cu3_3O6+δ_{6+\delta} to show that phase transitions to the universality class of the loop-current ordered state with free-energy reduction similar to the measured superconducting condensation are consistent with the sound velocity and with lack of identifiable observation in the specific heat. We also compare the measured specific heat with some more usual transitions and show that transitions with such symmetry classes can easily be shown by existing specific heat measurements to have energy reduction due to them less than 1/20 the superconducting condensation energy

    Ordered Loop Current States in Bilayer Graphene

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    While single-layer graphene shows extraordinary phenomena which are stable against electronic interactions, the non-interacting state of bilayer graphene is unstable to infinitesimal interactions leading to one of many possible exotic states. Indeed a gapped state is found in experiments but none of the states proposed so far can provide full accounts of its properties. Here we show that a magnetoelectric (ME) state is consistent with the experimental observations. This state breaks time-reversal symmetry through a pair of spontaneously generated current loops in each layer, and has odd-parity with respect to the two layers. We also suggest further experiments to check whether the ME state is indeed the gapped state found in experiments.Comment: 8 pages, 10 figure

    Theory of melting of glasses

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    Glassy matter like crystals resists change in shape. Therefore a theory for their continuous melting should show how the shear elastic constant μ\mu goes to zero. Since viscosity is the long wave-length low frequency limit of shear correlations, the same theory should give phenomena like the Volger-Fulcher dependence of the viscosity on temperature near the transition. A continuum model interrupted randomly by asymmetric rigid defects with orientational degrees of freedom is considered. Such defects are orthogonal to the continuum excitations, and are required to be imprisoned by rotational motion of the nearby atoms of the continuum. The defects interact with an angle dependent μ/r3\mu/r^3 potential. A renormalization group for the elastic constants, and the fugacity of the defects in 3D is constructed. The principal results are that there is a scale-invariant reduction of μ\mu as a function of length at any temperature T<T0T < T_0, above which it is 0 macrosopically but has a finite correlation length ξ(T)\xi(T) which diverges as TT0T \to T_0. Viscosity is shown to be proportional to ξ2(T)\xi^2(T) and has the Vogel-Fulcher form. The specific heat is ξ3(T)\propto \xi^{-3}(T). As TT0T \to T_0, the Kauzman temperature from above, the configuration entropy of the liquid is exhausted. The theory also gives the ``fragility" of glasses in terms of their T0/μT_0/\mu.Comment: Exact solution of RG equations with same answers as before and a few typos correcte
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