782 research outputs found
Symmetry dependence of phonon lineshapes in superconductors with anisotropic gaps
The temperature dependence below of the lineshape of optical phonons
of different symmetry as seen in Raman scattering is investigated for
superconductors with anisotropic energy gaps. It is shown that the symmetry of
the electron-phonon vertex produces non-trivial couplings to an anisotropic
energy gap which leads to unique changes in the phonon lineshape for phonons of
different symmetry. The phonon lineshape is calculated in detail for
and phonons in a superconductor with pairing
symmetry. The role of satellite peaks generated by the electron-phonon coupling
are also addressed. The theory accounts for the substantial phonon narrowing of
the phonon, while narrowing of the phonon which is
indistinguishable from the normal state is shown, in agreement with recent
measurements on BSCCO.Comment: 15 pages (3 Figures available upon request), Revtex, 1
Quasiparticle interference and the interplay between superconductivity and density wave order in the cuprates
Scanning tunneling spectroscopy (STS) is a useful probe for studying the
cuprates in the superconducting and pseudogap states. Here we present a
theoretical study of the Z-map, defined as the ratio of the local density of
states at positive and negative bias energies, which frequently is used to
analyze STS data. We show how the evolution of the quasiparticle interference
peaks in the Fourier transform Z-map can be understood by considering different
types of impurity scatterers, as well as particle-hole asymmetry in the
underlying bandstructure. We also explore the effects of density wave orders,
and show that the Fourier transform Z-map may be used to both detect and
distinguish between them.Comment: final version published in Phys. Rev.
Fast Times at Ridgemont High? The Effect of Compulsory Schooling Laws on Teenage Births
Research suggests that teenage childbearing adversely affects both the outcomes of the mothers as well as those of their children. We know that low-educated women are more likely to have a teenage birth, but does this imply that policies that increase educational attainment reduce early fertility? This paper investigates whether increasing mandatory educational attainment through compulsory schooling legislation encourages women to delay childbearing. We use variation induced by changes in compulsory schooling laws in both the United States and Norway to estimate the effect in two very different institutional environments. We find evidence that increased compulsory schooling does in fact reduce the incidence of teenage childbearing in both the United States and Norway, and these results are quite robust to various specification checks. Somewhat surprisingly, we also find that the magnitude of these effects is quite similar in the two countries. These results suggest that legislation aimed at improving educational outcomes may have spillover effects onto the fertility decisions of teenagers.
Critical Current Peaks at in Superconductors with Columnar Defects: Recrystalizing the Interstitial Glass
The role of commensurability and the interplay of correlated disorder and
interactions on vortex dynamics in the presence of columnar pins is studied via
molecular dynamics simulations. Simulations of dynamics reveal substantial
caging effects and a non-monotonic dependence of the critical current with
enhancements near integer values of the matching field and
in agreement with experiments on the cuprates. We find qualitative
differences in the phase diagram for small and large values of the matching
field.Comment: 5 pages, 4 figures (3 color
Neutron Scattering and the B_{1g} Phonon in the Cuprates
The momentum dependent lineshape of the out-of-phase oxygen vibration as
measured in recent neutron scattering measurements is investigated. Starting
from a microscopic coupling of the phonon vibration to a local crystal field,
the phonon lineshift and broadening is calculated as a function of transfered
momentum in the superconducting state of YBaCuO. It is shown
that the anisotropy of the density of states, superconducting energy gap, and
the electron-phonon coupling are all crucial in order to explain these
experiments.Comment: new figures and discussio
Comment on "Superconducting gap anisotropy vs. doping level in high-T_c cuprates" by C. Kendziora et al, PRL 77, 727 (1996)
In a recent paper Kendziora et al concluded that the superconducting gap in
overdoped Bi-2212 is isotropic. From data obtained from electronic Raman
scattering measurements, their conclusion was based on the observation that
pair breaking peaks occured at approximately the same frequency in different
scattering geometries and that the normalized scattering intensity at low
energies was strongly depleted. We discuss a different interpretation of the
raw data and present new data which is consistent with a strongly anisotropic
gap with nodes. The spectra can be successfully described by a model for Raman
scattering in a d_{x^{2}-y^{2}} superconductor with spin fluctuations and
impurity scattering included.Comment: 1 page revtex plus 1 postscript figur
Quantum Dynamics of the Hubbard-Holstein Model in Equilibrium and Non-Equilibrium: Application to Pump-Probe Phenomena
The spectral response and physical features of the 2D Hubbard-Holstein model
are calculated both in equilibrium at zero and low chemical dopings, and after
an ultra short powerful light pulse, in undoped systems. At equilibrium and at
strong charge-lattice couplings, the optical conductivity reveals a 3-peak
structure in agreement with experimental observations. After an ultra short
pulse and at nonzero electron-phonon interaction, phonon and spin subsystems
oscillate with the phonon period fs. The decay time of the
phonon oscillations is about 150-200 fs, similar to the relaxation time of the
charge system. We propose a criterion for observing these oscillations in high
compounds: the time span of the pump light pulse has to be
shorter than the phonon oscillation period .Comment: 4 pages, 4 figure
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