4,037 research outputs found
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
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
On the Impact of Helium Content on the RR Lyrae Distance Scale
Indexación: Scopus.We constructed new sets of He-enhanced (Y = 0.30, Y = 0.40) nonlinear, time-dependent convective hydrodynamical models of RR Lyrae (RRL) stars covering a broad range in metal abundances (Z = 0.0001-0.02). The increase in He content from the canonical value (Y = 0.245) to Y = 0.30-0.40 causes a simultaneous increase in stellar luminosity and in pulsation period. To investigate the dependence of the RRL distance scale on the He abundance, we computed new optical (RI) and near-infrared (JHK) Period-luminosity-metallicity-helium relations. Interestingly enough, the increase in He content causes a minimal change in the coefficients of both period and metallicity terms, since canonical and He-enhanced models obey similar PLZ relations. On the contrary, the classical B-And V-band mean magnitude metallicity relations and the R-band PLZ relation display a significant dependence on the He content. The He-enhanced models are, at fixed metal content, 0.2-0.5 mag brighter than canonical ones. This variation is only marginally affected by evolutionary effects. The quoted distance diagnostics once calibrated with trigonometric parallaxes (Gaia) will provide the opportunity to estimate the He content of field and cluster RRLs. Moreover, the use of either spectroscopic or photometric metal abundances will pave the way to new empirical constraints on the universality of the helium-To-metal enrichment ratio in old (t10 Gyr) stellar tracers. © 2018. The American Astronomical Society. All rights reserved.https://iopscience.iop.org/article/10.3847/2041-8213/aada1
Galactic Globular Cluster Stars: from Theory to Observation
We use evolutionary calculations presented in a recent paper (Cassisi et al.
1998) to predict B,V,I magnitudes for stars in galactic globulars. The effect
of the adopted mixing length on stellar magnitudes and colors is discussed,
showing that the uncertainty on such a theoretical parameter prevents the use
of MS stars as bona fide theoretical standard candles. However, comparison with
Hipparcos data for field subdwarfs discloses a substantial agreement between
theory and observation. Present predictions concerning the magnitude of TO and
of HB stars are compared with similar results appeared in the recent
literature. We present and discuss a theoretical calibration of the difference
in magnitude between HB and TO as evaluated with or without element
sedimentation. Finally we use theoretical HB magnitudes to best fit the CM
diagram of M68 and M5, taken as representative of metal poor and intermediate
metallicity galactic globulars, deriving an age of 111.0 Gyr and
101.0 Gyr, respectively, for the adopted chemical compositions, plus an
additional uncertainty of 1.4 Gyr if the uncertainty on the chemical
composition is taken into account. This result is discussed on the basis of
current evaluations concerning cluster ages and distance moduli.Comment: 8 pages, 13 postscript figures, 6 postscript tables To be published
on Astronomy & Astrophysics Supplement Serie
Temperature dependence of the optical spectral weight in the cuprates: Role of electron correlations
We compare calculations based on the Dynamical Mean-Field Theory of the
Hubbard model with the infrared spectral weight of
LaSrCuO and other cuprates. Without using fitting parameters we
show that most of the anomalies found in with respect to normal
metals, including the existence of two different energy scales for the doping-
and the -dependence of , can be ascribed to strong correlation
effects.Comment: 4 pages, 3 figures. Minor corrections, corrected some typos and added
reference
Noncommutative geometry and physics: a review of selected recent results
This review is based on two lectures given at the 2000 TMR school in Torino.
We discuss two main themes: i) Moyal-type deformations of gauge theories, as
emerging from M-theory and open string theories, and ii) the noncommutative
geometry of finite groups, with the explicit example of Z_2, and its
application to Kaluza-Klein gauge theories on discrete internal spaces.Comment: Based on lectures given at the TMR School on contemporary string
theory and brane physics, Jan 26- Feb 2, 2000, Torino, Italy. To be published
in Class. Quant. Grav. 17 (2000). 3 ref.s added, typos corrected, formula on
exterior product of n left-invariant one-forms corrected, small changes in
the Sect. on integratio
Charge and spin inhomogeneity as a key to the physics of the high Tc cuprates
We present a coherent scenario for the physics of cuprate superconductors,
which is based on a charge-driven inhomogeneity, i.e. the ``stripe phase''. We
show that spin and charge critical fluctuations near the stripe instability of
strongly correlated electron systems provide an effective interaction between
the quasiparticles, which is strongly momentum, frequency, temperature and
doping dependent. This accounts for the various phenomena occurring in the
overdoped, optimally and underdoped regimes both for the normal and the
superconductive phase.Comment: 6 pages, 1 enclosed figure, proceedings of LT2
- …