1,185 research outputs found
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
Dynamical charge density waves rule the phase diagram of cuprates
In the last few years charge density waves (CDWs) have been ubiquitously
observed in high-temperature superconducting cuprates and are now the most
investigated among the competing orders in the still hot debate on these
systems. A wealth of new experimental data raise several fundamental issues
that challenge the various theoretical proposals. Here, we account for the
complex experimental temperature vs. doping phase diagram and we provide a
coherent scenario explaining why different CDW onset curves are observed by
different experimental probes and seem to extrapolate at zero temperature into
seemingly different quantum critical points (QCPs) in the intermediate and
overdoped region. We also account for the pseudogap and its onset temperature
T*(p) on the basis of dynamically fluctuating CDWs. The nearly singular
anisotropic scattering mediated by these fluctuations also account for the
rapid changes of the Hall number seen in experiments and provides the first
necessary step for a possible Fermi surface reconstruction fully establishing
at lower doping. Finally we show that phase fluctuations of the CDWs, which are
enhanced in the presence of strong correlations near the Mott insulating phase,
naturally account for the disappearance of the CDWs at low doping with yet
another QCP.Comment: 13 pages, 7 figure
Stripe ordering and two-gap model for underdoped cuprates
The evidence of edge-gaps around the M-points in the metallic state of
underdoped cuprates has triggered a very active debate on their origin. We
first consider the possibility that this spectroscopic feature results from a
quasi-static charge ordering taking place in the underdoped regime. It comes
out that to explain the coexistence of gaps and arcs on the Fermi surface the
charge modulation should be in an eggbox form. In the lack of evidences for
that, we then investigate the local pairing induced by charge-stripe
fluctuations. A proper description of the strong anisotropy of both the
interactions and the Fermi velocities requires a two-gap model for pairing. We
find that a gap due to incoherent pairing forms near the M-points, while
coherence is established by the stiffness of the pairing near the nodal points.
The model allows for a continuos evolution from a pure BCS pairing (over- and
optimally doped regime) to a mixed boson-fermion model (heavily underdoped
regime).Comment: 4 pages, Proceedings of M2S-HTS
Spectral properties of incommensurate charge-density wave systems
The concept of frustrated phase separation is applied to investigate its
consequences for the electronic structure of the high T_c cuprates. The
resulting incommensurate charge density wave (CDW) scattering is most effective
in creating local gaps in k-space when the scattering vector connects states
with equal energy. Starting from an open Fermi surface we find that the
resulting CDW is oriented along the (10)- and (or) (01)-direction which allows
for a purely one-dimensional or a two-dimensional ``eggbox type'' charge
modulation. In both cases the van Hove singularities are substantially
enhanced, and the spectral weight of Fermi surface states near the M-points,
tends to be suppressed. Remarkably, a leading edge gap arises near these
points, which, in the eggbox case, leaves finite arcs of the Fermi surface
gapless. We discuss our results with repect to possible consequences for
photoemission experiments
Advanced Strain-Isolation-Pad Material with Bonded Fibrous Construction
The feasibility of utilizing air lay and liquid lay felt deposition techniques to fabricate strain isolation pad (SIP) materials for the Space Shuttle Orbiter was demonstrated. These materials were developed as candidate replacements for the present needled felt SIP used between the ceramic tiles and the aluminum skin on the undersurface of the Orbiter. The SIP materials that were developed consisted of high temperature aramid fibers deposited by controlled fluid (air or liquid) carriers to form low density unbonded felts. The deposited felts were then bonded at the fiber intersections with a small amount of high temperature polyimide resin. This type of bonded felt construction can potentially eliminate two of the problems associated with the present SIP, viz., transmittal of localized stresses into the tiles and load history dependent mechanical response. However, further work is needed to achieve adequate through thickness tensile strength in the bonded felts
Spectroscopic evidences of quantum critical charge fluctuations in cuprates
We calculate the optical conductivity in a clean system of quasiparticles
coupled to charge-ordering collective modes. The absorption induced by these
modes may produce an anomalous frequency and temperature dependence of
low-energy optical absorption in some cuprates. However, the coupling with
lattice degrees of freedom introduces a non-universal energy scale leading to
scaling violation in low-temperature optical conductivity.Comment: Proceedings of M2S 2006. To appear in Physica
Optical excitation of phase modes in strongly disordered superconductors
According to the Goldstone theorem the breaking of a continuous U(1) symmetry
comes along with the existence of low-energy collective modes. In the context
of superconductivity these excitations are related to the phase of the
superconducting (SC) order parameter and for clean systems are optically
inactive. Here we show that for strongly disordered superconductors phase modes
acquire a dipole moment and appear as a subgap spectral feature in the optical
conductivity. This finding is obtained with both a gauge-invariant random-phase
approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as
with a prototypical bosonic model for disordered superconductors. In the
strongly disordered regime, where the system displays an effective granularity
of the SC properties, the optically active dipoles are linked to the isolated
SC islands, offering a new perspective for realizing microwave optical devices
Electronic polymers and soft-matter-like broken symmetries in underdoped cuprates
Empirical evidence in heavy fermion, pnictide, and other systems suggests
that unconventional superconductivity appears associated to some form of
real-space electronic order. For the cuprates, despite several proposals, the
emergence of order in the phase diagram between the commensurate
antiferromagnetic state and the superconducting state is not well understood.
Here we show that in this regime doped holes assemble in "electronic polymers."
Within a Monte Carlo study we find, that in clean systems by lowering the
temperature the polymer melt condenses first in a smectic state and then in a
Wigner crystal both with the addition of inversion symmetry breaking. Disorder
blurs the positional order leaving a robust inversion symmetry breaking and a
nematic order, accompanied by vector chiral spin order and with the persistence
of a thermodynamic transition. Such electronic phases, whose properties are
reminiscent of soft-matter physics, produce charge and spin responses in good
accord with experiments.Comment: 10 pages, 4 figures plus supplementary informatio
Linear and non-linear current response in disordered d-wave superconductors
We present a detailed theoretical investigation of the linear and non-linear
optical response in a model system for a disordered d-wave superconductor,
showing that for both quantities the gap symmetry considerably changes the
paradigm of the optical response based on the conventional s-wave case. For
what concerns the linear response our findings agree with previous work showing
that in strongly-disordered d-wave superconductors a large fraction of
uncondensed spectral weight survives below Tc, making the optical absorption
around the gap-frequency scale almost unchanged with respect to the normal
state. Our numerical results are in excellent quantitative agreement with
experiments in overdoped cuprates. In the non-linear regime we focus on the
third-harmonic generation (THG), finding that, as already established for the
s-wave case, in general a large THG is triggered by disorder-activated
paramagnetic processes. However, in the d-wave case the BCS response is
monotonously increasing in frequency, loosing any signature of THG enhancement
when the THz pump frequency matches the gap maximum , a
hallmark of previous experiments in conventional s-wave superconductors. Our
findings, along with the mild polarization dependence of the response, provides
an explanation for recent THG measurements in cuprates, setting the framework
for the theoretical understanding of non-linear effects in unconventional
cuprates.Comment: 16 pages, 9 figure
Adiabatic transition from a BCS superconductor to a Fermi liquid and phase dynamics
We investigate the physics of an adiabatic transition from a BCS
superconductor to a Fermi liquid for an exponentially slow decreasing pairing
interaction. In particular, we show that the metal keeps memory of the parent
BCS state so it is possible to reverse the dynamics and go back to the original
state similarly to a spin/photon echo experiment. Moreover, we study the
evolution of the order parameter phase phi in transforming the BCS
superconductor to a conventional metal. Since the global phase is the conjugate
variable of the density we explicitly show how to use the dynamics of phi
together with gauge invariance to build up the non-interacting chemical
potential away from particle-hole symmetry. We further analyze the role of phi
in restoring the gauge invariant current response when the non-interacting
Fermi liquid is approached starting from a BCS superconductor in the presence
of an external vector field.Comment: 13 pages, 12 figure
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