970 research outputs found
Gap and pseudogap evolution within the charge-ordering scenario for superconducting cuprates
We describe the spectral properties of underdoped cuprates as resulting from
a momentum-dependent pseudogap in the normal state spectrum. Such a model
accounts, within a BCS approach, for the doping dependence of the critical
temperature and for the two-parameter leading-edge shift observed in the
cuprates. By introducing a phenomenological temperature dependence of the
pseudogap, which finds a natural interpretation within the stripe
quantum-critical-point scenario for high-T_c superconductors, we reproduce also
the T_c-T^* bifurcation near optimum doping. Finally, we briefly discuss the
different role of the gap and the pseudogap in determining the spectral and
thermodynamical properties of the model at low temperatures.Comment: 13 pages (EPY style), 7 enclosed figures, to appear on Eur. Phys. J.
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
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
The physics of the stripe quantum critical point in the superconducting cuprates
We elaborate on several observable consequences of the Quantum-Critical-Point
scenario. In particular we show that the strong k-dependent scattering of the
quasiparticles with the quasi-critical charge and spin fluctuations reproduces
the main features of the low-energy spectral weights and of the observed Fermi
surfaces. In the underdoped cuprates the attractive k-dependent charge
scattering drives the formation of the pseudogap at the M points below the
crossover temperature T^*. In this context we discuss models for pseudogap
formation with relevant scattering in the particle-particle and particle-hole
channels. The experimental consequences for the pair-fluctuation and for the
pseudogap behavior are investigated.Comment: 4 pages, 2 enclosed figures, Proceedings of M2S-HTS
Charge-fluctuation contribution to the Raman response in superconducting cuprates
We calculate the Raman response contribution due to collective modes, finding
a strong dependence on the photon polarizations and on the characteristic
wavevectors of the modes. We compare our results with recent Raman spectroscopy
experiments in underdoped cuprates, and
, where anomalous low-energy peaks are
observed, which soften upon lowering the temperature. We show that the specific
dependence on doping and on photon polarizations of these peaks is only
compatible with charge collective excitations at finite wavelength.Comment: 5 pages, 3 figure
Effective medium theory for superconducting layers: A systematic analysis including space correlation effects
We investigate the effects of mesoscopic inhomogeneities on the
metal-superconductor transition occurring in several two-dimensional electron
systems. Specifically, as a model of systems with mesoscopic inhomogeneities,
we consider a random-resistor network, which we solve both with an exact
numerical approach and by the effective medium theory. We find that the width
of the transition in these two-dimensional superconductors is mainly ruled by
disorder rather than by fluctuations. We also find that "tail" features in
resistivity curves of interfaces between LaAlO3 or LaTiO3 and SrTiO3 can arise
from a bimodal distribution of mesoscopic local Tc's and/or substantial space
correlations between the mesoscopic domains.Comment: 12 pages, 10 figure
Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates
We consider the lightly doped cuprates YCaBaCuO
and LaSrCuO (with ,0.04), where the presence of a
fluctuating nematic state has often been proposed as a precursor of the stripe
(or, more generically, charge-density wave) phase, which sets in at higher
doping. We phenomenologically assume a quantum critical character for the
longitudinal and transverse nematic, and for the charge-ordering fluctuations,
and investigate the effects of these fluctuations in Raman spectra. We find
that the longitudinal nematic fluctuations peaked at zero transferred momentum
account well for the anomalous Raman absorption observed in these systems in
the channel, while the absence of such effect in the channel
may be due to the overall suppression of Raman response at low frequencies,
associated with the pseudogap. While in YCaBaCuO the
low-frequency lineshape is fully accounted by longitudinal nematic collective
modes alone, in LaSrCuO also charge-ordering modes with finite
characteristic wavevector are needed to reproduce the shoulders observed in the
Raman response. This different involvement of the nearly critical modes in the
two materials suggests a different evolution of the nematic state at very low
doping into the nearly charge-ordered state at higher doping.Comment: 12 pages with 10 figures, to appear in Phys. Rev. B 201
Extended paraconductivity regime in underdoped cuprates
We reconsider transport experiments in strongly anisotropic superconducting
cuprates and we find that universal Aslamazov-Larkin (AL) paraconductivity in
two dimensions is surprisingly robust even in the underdoped regime below the
pseudogap crossover temperature T^*. We also establish that the underlying
normal state resistivity in the pseudogap phase is (almost) linear in
temperature, with all the deviations being quantitatively accounted by AL
paraconductivity. The disappearence of paraconductivity is governed by the
disappearence of gaussian pair fluctuations at an energy scale related to T^*.Comment: 5 pages and 2 figure
Tumor cellular and microenvironmental cues controlling invadopodia formation
During the metastatic progression, invading cells might achieve degradation and subsequent invasion into the extracellular matrix (ECM) and the underlying vasculature using invadopodia, F-actin-based and force-supporting protrusive membrane structures, operating focalized proteolysis. Their formation is a dynamic process requiring the combined and synergistic activity of ECM-modifying proteins with cellular receptors, and the interplay with factors from the tumor microenvironment (TME). Significant advances have been made in understanding how invadopodia are assembled and how they progress in degradative protrusions, as well as their disassembly, and the cooperation between cellular signals and ECM conditions governing invadopodia formation and activity, holding promise to translation into the identification of molecular targets for therapeutic interventions. These findings have revealed the existence of biochemical and mechanical interactions not only between the actin cores of invadopodia and specific intracellular structures, including the cell nucleus, the microtubular network, and vesicular trafficking players, but also with elements of the TME, such as stromal cells, ECM components, mechanical forces, and metabolic conditions. These interactions reflect the complexity and intricate regulation of invadopodia and suggest that many aspects of their formation and function remain to be determined. In this review, we will provide a brief description of invadopodia and tackle the most recent findings on their regulation by cellular signaling as well as by inputs from the TME. The identification and interplay between these inputs will offer a deeper mechanistic understanding of cell invasion during the metastatic process and will help the development of more effective therapeutic strategies
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