18,404 research outputs found
Procedure to prepare transparent silica gels
This invention relates to the production of silica gels and in particular to a process for the preparation of silica gels which can be used as a crystal growth medium that simulates the convectionless environment of space to produce structurally perfect crystals. Modern utilizations of substances in electronics, such as radio transmitters and high frequency microphones, often require single crystals with controlled purity and structural perfection. The near convectionless environment of silica gel suppresses nucleation, thereby reducing the competitive nature of crystal growth. This competition limits the size and perfection of the crystal; and it is obviously desirable to suppress nucleation until, ideally, only one crystal grows in a predetermined location. A silica gel is not a completely convectionless environment like outer space, but is the closest known environment to that of outer space that can be created on Earth
Odd Parity and Line Nodes in Heavy Fermion Superconductors
Group theory arguments have demonstrated that a general odd parity order
parameter cannot have line nodes in the presence of spin-orbit coupling. In
this paper, it is shown that these arguments do not hold on the
zone face of a hexagonal close packed lattice. In particular, three of the six
odd parity representations vanish identically on this face. This has potential
relevance to the heavy fermion superconductor .Comment: 5 pages, revte
Extraction of the Electron Self-Energy from Angle Resolved Photoemission Data: Application to Bi2212
The self-energy , the fundamental function which
describes the effects of many-body interactions on an electron in a solid, is
usually difficult to obtain directly from experimental data. In this paper, we
show that by making certain reasonable assumptions, the self-energy can be
directly determined from angle resolved photoemission data. We demonstrate this
method on data for the high temperature superconductor
(Bi2212) in the normal, superconducting, and pseudogap phases.Comment: expanded version (6 pages), to be published, Phys Rev B (1 Sept 99
Local Density of States and Angle-Resolved Photoemission Spectral Function of an Inhomogeneous D-wave Superconductor
Nanoscale inhomogeneity seems to be a central feature of the d-wave
superconductivity in the cuprates. Such a feature can strongly affect the local
density of states (LDOS) and the spectral weight functions. Within the
Bogoliubov-de Gennes formalism we examine various inhomogeneous configurations
of the superconducting order parameter to see which ones better agree with the
experimental data. Nanoscale large amplitude oscillations in the order
parameter seem to fit the LDOS data for the underdoped cuprates. The
one-particle spectral function for a general inhomogeneous configuration
exhibits a coherent peak in the nodal direction. In contrast, the spectral
function in the antinodal region is easily rendered incoherent by the
inhomogeneity. This throws new light on the dichotomy between the nodal and
antinodal quasiparticles in the underdoped cuprates.Comment: 5 pages, 9 pictures. Phys. Rev. B (in press
The Strong Levinson Theorem for the Dirac Equation
We consider the Dirac equation in one space dimension in the presence of a
symmetric potential well. We connect the scattering phase shifts at E=+m and
E=-m to the number of states that have left the positive energy continuum or
joined the negative energy continuum respectively as the potential is turned on
from zero.Comment: Submitted to Physical Review Letter
Phenomenology of Photoemission Lineshapes of High Tc Superconductors
We introduce a simple phenomenological form for the self-energy which allows
us to extract important information from angle resolved photoemission data on
the high Tc superconductor Bi2212. First, we find a rapid suppression of the
single particle scattering rate below Tc for all doping levels. Second, we find
that in the overdoped materials the gap Delta at all k-points on the Fermi
surface has significant temperature dependence and vanishes near Tc. In
contrast, in the underdoped samples such behavior is found only at k-points
close to the diagonal. Near (pi,0), Delta is essentially T-independent in the
underdoped samples. The filling-in of the pseudogap with increasing T is
described by a broadening proportional to T-Tc, which is naturally explained by
pairing correlations above Tc.Comment: 4 pages, revtex, 3 encapsulated postscript figure
The Temperature Evolution of the Spectral Peak in High Temperature Superconductors
Recent photoemission data in the high temperature cuprate superconductor
Bi2212 have been interpreted in terms of a sharp spectral peak with a
temperature independent lifetime, whose weight strongly decreases upon heating.
By a detailed analysis of the data, we are able to extract the temperature
dependence of the electron self-energy, and demonstrate that this intepretation
is misleading. Rather, the spectral peak loses its integrity above Tc due to a
large reduction in the electron lifetime.Comment: 5 pages, revtex, 4 encapsulated postscript figure
HST Imaging in the Chandra Deep Field South: II. WFPC2 Observations of an X-Ray Flux-Limited Sample from the 1 Msec Chandra Catalog
We present HST/WFPC2 observations of a well-defined sample of 40 X-ray
sources with X-ray fluxes above the detection threshold of the full 1 Msec
Chandra Deep Field South (CDFS). The sensitivity and spatial resolution of our
HST observations are sufficient to detect the optical counterparts of 37 of the
X-ray sources, yielding information on their morphologies and environments. In
this paper we extend the results obtained in our previous study on the 300 ks
CDFS X-ray data (Schreier et al. 2001, Paper I). Specifically, we show that the
optical counterparts to the X-ray sources are divided into two distinct
populations: 1) an optically faint group with relatively blue colors, similar
to the faint blue field galaxy population, and 2) an optically brighter group,
including resolved galaxies with average colors significantly redder than the
corresponding bright field galaxy population. The brighter objects comprise a
wide range of types, including early and late type galaxies, starbursts, and
AGN. By contrast, we show that the faint blue X-ray population are most
consistent with being predominantly Type 2 AGN of low to moderate luminosity,
located at higher redshifts (z ~ 1 - 2). This conclusion is supported by
luminosity function models of the various classes of objects. Hence, the
combination of deep X-ray data with the high spatial resolution of HST are for
the first time allowing us to probe the faint end of the AGN luminosity
function at cosmologically interesting redshifts.Comment: AASTEX-Latex, 25 pages, 4 postscript figures, 9 jpg figures. Accepted
by the Astrophysical Journal. Full-size postscript images and figures,
included in the preprint, are available from:
http://www.stsci.edu/~koekemoe/papers/cdfs-hst
Transport implications of Fermi arcs in the pseudogap phase of the cuprates
We derive the fermionic contribution to the longitudinal and Hall
conductivities within a Kubo formalism, using a phenomenological Greens
function which has been previously developed to describe photoemission data in
the pseudogap phase of the cuprates. We find that the in-plane electrical and
thermal conductivities are metallic-like, showing a universal limit behavior
characteristic of a d-wave spectrum as the scattering rate goes to zero. In
contrast, the c-axis resistivity and the Hall number are insulating-like, being
divergent in the same limit. The relation of these results to transport data in
the pseudogap phase is discussed.Comment: 3 pages, 2 figure
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