1,015 research outputs found
Specific Heat and Sound Velocity Distinguish the Relevant Competing Phase in the Pseudogap Region of High Temperature Superconductors
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 . We use the recent highly accurate
sound-velocity measurements and the best available specific heat measurements
in YBaCuO 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
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
Glassy matter like crystals resists change in shape. Therefore a theory for
their continuous melting should show how the shear elastic constant 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
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 as a function of length at any
temperature , above which it is 0 macrosopically but has a finite
correlation length which diverges as . Viscosity is shown
to be proportional to and has the Vogel-Fulcher form. The specific
heat is . As , 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 .Comment: Exact solution of RG equations with same answers as before and a few
typos correcte
- …