2,642 research outputs found
Free Differential Algebras: Their Use in Field Theory and Dual Formulation
The gauging of free differential algebras (FDA's) produces gauge field
theories containing antisymmetric tensors. The FDA's extend the Cartan-Maurer
equations of ordinary Lie algebras by incorporating p-form potentials (). We study here the algebra of FDA transformations. To every p-form in the
FDA we associate an extended Lie derivative generating a corresponding
``gauge" transformation. The field theory based on the FDA is invariant under
these new transformations. This gives geometrical meaning to the antisymmetric
tensors. The algebra of Lie derivatives is shown to close and provides the dual
formulation of FDA's.Comment: 10 pages, latex, no figures. Talk presented at the 4-th Colloquium on
"Quantum Groups and Integrable Sysytems", Prague, June 199
Non-linear optical effects and third-harmonic generation in superconductors: Cooper-pairs vs Higgs mode contribution
The recent observation of a transmitted Thz pulse oscillating at three times
the frequency of the incident light paves the way to a new protocol to access
resonant excitations in a superconductor. Here we show that this non-linear
optical process is dominated by light-induced excitation of Cooper pairs, in
analogy with a standard Raman experiment. The collective amplitude (Higgs)
fluctuations of the superconducting order parameter give in general a smaller
contribution, unless one designs the experiment by combining properly the light
polarization with the lattice symmetry.Comment: Slightly revised introduction, to appear on Phys. Rev. B. as Rapid
Communicatio
Spectroscopic and thermodynamic properties in a four-band model for pnictides
In this paper we provide a comprehesive analysis of different properties of
pnictides both in the normal and superconducting state, with a particular focus
on the optimally-doped BaKFeAs system. We show that, by
using the band dispersions experimentally measured by ARPES, a four-band
Eliashberg model in the intermediate-coupling regime can account for both the
measured hierarchy of the gaps and for several spectroscopic and thermodynamic
signatures of low-energy renormalization. These include the kinks in the band
dispersion and the effective masses determined via specific-heat and
superfluid-density measurements. We also show that, although an
intermediate-coupling Eliashberg approach is needed to account for the
magnitude of the gaps, the temperature behavior of the thermodynamic quantities
does not show in this regime a significant deviation with respect to
weak-coupling BCS calculations. This can explain the apparent success of
two-band BCS fits of experimental data reported often in the literature.Comment: 12 pages, 6 figures, final versio
On the application of Mattis-Bardeen theory in strongly disordered superconductors
The low energy optical conductivity of conventional superconductors is
usually well described by Mattis-Bardeen (MB) theory which predicts the onset
of absorption above an energy corresponding to twice the superconducing (SC)
gap parameter Delta. Recent experiments on strongly disordered superconductors
have challenged the application of the MB formulas due to the occurrence of
additional spectral weight at low energies below 2Delta. Here we identify three
crucial items which have to be included in the analysis of optical-conductivity
data for these systems: (a) the correct identification of the optical threshold
in the Mattis-Bardeen theory, and its relation with the gap value extracted
from the measured density of states, (b) the gauge-invariant evaluation of the
current-current response function, needed to account for the optical absorption
by SC collective modes, and (c) the inclusion into the MB formula of the energy
dependence of the density of states present already above Tc. By computing the
optical conductvity in the disordered attractive Hubbard model we analyze the
relevance of all these items, and we provide a compelling scheme for the
analysis and interpretation of the optical data in real materials.Comment: 11 pages, 6 figure
Quasi-particle dephasing time in disordered d-wave superconductors
We evaluate the low-temperature cutoff for quantum interference 1/tf induced
in a d-wave superconductor by the diffusion enhanced quasiparticle interactions
in the presence of disorder. We carry out our analysis in the framework of the
non-linear sigma-model which allows a direct calculation of 1/tf, as the mass
of the transverse modes of the theory. Only the triplet amplitude in the
particle-hole channel and the Cooper amplitude with is pairing symmetry
contribute to 1/tf. We discuss the possible relevance of our results to the
present disagreement between thermal transport data in cuprates and the
localization theory for d-wave quasiparticles
Doping dependence of the vortex-core energy in bilayer films of cuprates
The energy needed to create a vortex core is the basic ingredient to address
the physics of thermal vortex fluctuations in underdoped cuprates. Here we
theoretically investigate its role on the occurrence of the
Beresinskii-Kosterlitz-Thouless transition in a bilayer film with
inhomogeneity. From the comparison with recent measurements of the penetration
depth in two-unit cell thin films of
YCaBaCuO_{7-\d} (YBCO) by Hetel et al. [Nat. Phys.
3, 700 (2007)] we can extract the value of the vortex-core energy , and
show that scales linearly with at low doping.Comment: 4pages, 3 figures. References added, final versio
Theory of fluctuation conductivity from interband pairing in pnictide superconductors
We derive the effective action for superconducting fluctuations in a
four-band model for pnictides, discussing the emergence of a single critical
mode out of a dominant interband pairing mechanism. We then apply our model to
calculate the paraconductivity in two-dimensional and layered three-dimensional
systems, and compare our results with recent resistivity measurements in
SmFeAsOFComment: 4 pages, 1 figure; final versio
Polarization dependence of the third-harmonic generation in multiband superconductors
In a superconductor the third-harmonic generation (THG) of a strong THz pulse
is enhanced below Tc by the resonant excitation of lattice-modulated charge
fluctuations (LCF), which modulate the response according to the polarization
of the field. Here we compute the THG within a multiband model for the
prototype NbN superconductor. We show that the non-resonant contribution coming
from the instantaneous electronic response and the finite width of the pulse
significantly suppress the polarization dependence of the signal, challenging
its observation in real systems.Comment: Final version, as publishe
Coherence length in superconductors from weak to strong coupling
We study the evolution of the superconducting coherence length from
weak to strong coupling, both within a s-wave and a d-wave lattice model. We
show that the identification of with the Cooper-pair size
in the weak-coupling regime is meaningful only for a fully-gapped (e.g.,
s-wave) superconductor. Instead in the d-wave superconductor, where
diverges, we show that is properly defined as the
characteristic length scale for the correlation function of the modulus of the
superconducting order parameter. The strong-coupling regime is quite
intriguing, since the interplay between particle-particle and particle-hole
channel is no more negligible. In the case of s-wave pairing, which allows for
an analytical treatment, we show that is of order of the lattice
spacing at finite densities. In the diluted regime diverges, recovering
the behavior of the coherence length of a weakly interacting effective bosonic
system. Similar results are expected to hold for d-wave superconductors.Comment: 11 pages, 5 figures. Two appendices and new references adde
Unconventional Hall effect in pnictides from interband interactions
We calculate the Hall transport in a multiband systems with a dominant
interband interaction between carriers having electron and hole character. We
show that this situation gives rise to an unconventional scenario, beyond the
Boltzmann theory, where the quasiparticle currents dressed by vertex
corrections acquire the character of the majority carriers. This leads to a
larger (positive or negative) Hall coefficient than what expected on the basis
of the carrier balance, with a marked temperature dependence. Our results
explain the puzzling measurements in pnictides and they provide a more general
framework for transport properties in multiband materials.Comment: 5 pages, 2 figure
- …