1,416 research outputs found
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
Seed particle formation for silicate dust condensation by SiO nucleation
Clustering of the abundant SiO molecules has been discussed as a possible
mechanism of seed particle formation for silicate dust in stellar outflows with
an oxygen rich element mixture. Previous results indicated that condensation
temperatures based on this mechanism are significant lower than what is really
observed. This negative result strongly rests on experimental data on vapour
pressure of SiO. New determinations show the older data to be seriously in
error. Here we aim to check with improved data the possibility that SiO
nucleation triggers the cosmic silicate dust formation. First we present
results of our measurements of vapour pressure of solid SiO. Second, we use the
improved vapour pressure data to re-calibrate existing experimental data on SiO
nucleation from the literature. Third, we use the re-calibrated data on SiO
nucleation in a simple model for dust-driven winds to determine the
condensation temperature of silicate in stellar outflows from AGB stars. We
show that onset of nucleation under circumstellar conditions commences at
higher temperature than was previously found. Calculated condensation
temperatures are still by about 100 K lower than observed ones, but this may be
due to the greenhouse effect of silicate dust temperatures. The assumption that
the onset of silicate dust formation in late-type M stars is triggered by
cluster formation of SiO is compatible with dust condensation temperatures
derived from IR observations.Comment: 11 pages, 11 figure
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
Resonant modes in strain-induced graphene superlattices
We study tunneling across a strain-induced superlattice in graphene. In
studying the effect of applied strain on the low-lying Dirac-like spectrum,
both a shift of the Dirac points in reciprocal space, and a deformation of the
Dirac cones is explicitly considered. The latter corresponds to an anisotropic,
possibly non-uniform, Fermi velocity. Along with the modes with unit
transmission usually found across a single barrier, we analytically find
additional resonant modes when considering a periodic structure of several
strain-induced barriers. We also study the band-like spectrum of bound states,
as a function of conserved energy and transverse momentum. Such a
strain-induced superlattice may thus effectively work as a mode filter for
transport in graphene
The structure of trailing vortices generated by model rotor blades
Hot-wire anemometry to analyze the structure and geometry of rotary wing trailing vortices is studied. Tests cover a range of aspect ratios and blade twist. For all configurations, measured vortex strength correlates well with maximum blade-bound circulation. Measurements of wake geometry are in agreement with classical data for high-aspect ratios. The detailed vortex structure is similar to that found for fixed wings and consists of four well defined regions--a viscous core, a turbulent mixing region, a merging region, and an inviscid outer region. A single set of empirical formulas for the entire set of test data is described
- ā¦