99 research outputs found
Pressure-induced electronic topological transitions in low dimensional superconductors
In the high-Tc cuprates, the unusual dependence of Tc on external pressure
results from the combination of the nonmonotonic dependence of Tc on hole
doping or hole-doping distribution among inequivalent layers, and from an
``intrinsic'' contribution. After reviewing our work on the interplay among Tc,
hole content, and pressure in the bilayered and multilayered cuprate
superconductors, we will discuss how the proximity to an electronic topological
transition (ETT) may give a microscopic justification of the ``intrinsic''
pressure dependence of Tc in the cuprates. As a function of the proximity to an
ETT, we recover a nonmonotonic behaviour of the superconducting gap at T=0,
regardless of the pairing symmetry of the order parameter. This is in agreement
with the trend observed for Tc as a function of pressure and other material
specific quantities in several high-Tc cuprates. In the case of epitaxially
strained cuprate thin films, we argue that an ETT can be driven by a
strain-induced modification of the in-plane band structure, at constant hole
content, at variance with a doping-induced ETT, as is usually assumed. We also
find that an increase of the in-plane anisotropy enhances the effect of
fluctuations above Tc on the normal-state transport properties, which is a
fingerprint of quantum criticality at T=0.Comment: EHPRG Award Lecture, http://www.ehprg.org. To be published in J.
Phys.: Cond. Matte
Superconducting transition temperatures of the elements related to elastic constants
For a given crystal structure, say body-centred-cubic, the many-body
Hamiltonian in which nuclear and electron motions are to be treated from the
outset on the same footing, has parameters, for the elements, which can be
classified as (i) atomic mass M, (ii) atomic number Z, characterizing the
external potential in which electrons move, and (iii) bcc lattice spacing, or
equivalently one can utilize atomic volume, Omega. Since the thermodynamic
quantities can be determined from H, we conclude that Tc, the superconducting
transition temperature, when it is non-zero, may be formally expressed as Tc =
Tc^(M) (Z, Omega). One piece of evidence in support is that, in an atomic
number vs atomic volume graph, the superconducting elements lie in a well
defined region. Two other relevant points are that (a) Tc is related by BCS
theory, though not simply, to the Debye temperature, which in turn is
calculable from the elastic constants C_{11}, C_{12}, and C_{44}, the atomic
weight and the atomic volume, and (b) Tc for five bcc transition metals is
linear in the Cauchy deviation C* = (C_{12} - C_{44})/(C_{12} + C_{44}).
Finally, via elastic constants, mass density and atomic volume, a correlation
between C* and the Debye temperature is established for the five bcc transition
elements.Comment: EPJB, accepte
Multiband superconductors close to a 3D-2D electronic topological transition
Within the two-band model of superconductivity, we study the dependence of
the critical temperature Tc and of the isotope exponent alpha in the proximity
to an electronic topological transition (ETT). The ETT is associated with a
3D-2D crossover of the Fermi surface of one of the two bands: the sigma subband
of the diborides. Our results agree with the observed dependence of Tc on Mg
content in A_{1-x}Mg_xB_2 (A=Al or Sc), where an enhancement of Tc can be
interpreted as due to the proximity to a "shape resonance". Moreover we have
calculated a possible variation of the isotope effect on the superconducting
critical temperature by tuning the chemical potential.Comment: J. Supercond., to appea
Symmetry breaking and restoring under high pressure: the amazing behaviour of the "simple" alkali metals
We argue that an ionic lattice surrounded by a Fermi liquid changes phase
several times under pressure, oscillating between the symmetric phase and a
low-symmetry dimerized structure, as a consequence of Friedel oscillations in
the pair potential. Phase oscillations explain the tendency towards
dimerization which has been recently reported for the light alkali metals under
high pressure. Moreover, a restoring of the symmetric phase is predicted for
such elements at an even higher density.Comment: accepted in Eur. Phys. J.
Linear response theory around a localized impurity in the pseudogap regime of an anisotropic superconductor: precursor pairing vs the d-density-wave scenario
We derive the polarizability of an electron system in (i) the superconducting
phase, with d-wave symmetry, (ii) the pseudogap regime, within the precursor
pairing scenario, and (iii) the d-density-wave (dDW) state, characterized by a
d-wave hidden order parameter, but no pairing. Such a calculation is motivated
by the recent proposals that imaging the effects of an isolated impurity may
distinguish between precursor pairing and dDW order in the pseudogap regime of
the high-Tc superconductors. In all three cases, the wave-vector dependence of
the polarizability is characterized by an azymuthal modulation, consistent with
the d-wave symmetry of the underlying state. However, only the dDW result shows
the fingerprints of nesting, with nesting wave-vector Q=(pi,pi), albeit
imperfect, due to a nonzero value of the hopping ratio t'/t in the band
dispersion relation. As a consequence of nesting, the presence of hole pockets
is also exhibited by the (q,omega) dependence of the retarded polarizability.Comment: accepted in Phys. Rev.
Scaling of the superconducting transition temperature in underdoped high-Tc cuprates with a pseudogap energy: Does this support the anyon model of their superfluidity?
In earlier work, we have been concerned with the scaling properties of some
classes of superconductors, specifically with heavy Fermion materials and with
five bcc transition metals of BCS character. Both of these classes of
superconductors were three-dimensional but here we are concerned solely with
quasi-two-dimensional high-Tc cuprates in the underdoped region of their phase
diagram. A characteristic feature of this part of the phase diagram is the
existence of a pseudogap (pg). We therefore build our approach around the
assumption that kB Tc / E_pg is the basic dimensionless ratio on which to
focus, where the energy E_pg introduced above is a measure of the pseudogap.
Since anyon fractional statistics apply to two-dimensional assemblies, we
expect the fractional statistics parameter allowing `interpolation' between
Fermi-Dirac and Bose-Einstein statistical distribution functions as limiting
cases to play a significant role in determining kB Tc / E_pg and experimental
data are analyzed with this in mind.Comment: Phys. Chem. Liquids, to be publishe
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