10,057 research outputs found
Magic identities for conformal four-point integrals
We propose an iterative procedure for constructing classes of off-shell
four-point conformal integrals which are identical. The proof of the identity
is based on the conformal properties of a subintegral common for the whole
class. The simplest example are the so-called `triple scalar box' and `tennis
court' integrals. In this case we also give an independent proof using the
method of Mellin--Barnes representation which can be applied in a similar way
for general off-shell Feynman integrals.Comment: 13 pages, 12 figures. New proof included with neater discussion of
contact terms. Typo correcte
Experimental evidence of s-wave superconductivity in bulk CaC
The temperature dependence of the in-plane magnetic penetration depth,
, has been measured in a c-axis oriented polycrystalline
CaC bulk sample using a high-resolution mutual inductance technique. A
clear exponential behavior of has been observed at low
temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the
standard BCS theory yields Angstroem and
meV. The ratio
gives indication for a conventional weakly coupled superconductor.Comment: To appear in Phys. Rev. Let
Charged excitons in doped extended Hubbard model systems
We show that the charge transfer excitons in a Hubbard model system including
nearest neighbor Coulomb interactions effectively attain some charge in doped
systems and become visible in photoelectron and inverse photoelectron
spectroscopies. This shows that the description of a doped system by an
extended Hubbard model differs substantially from that of a simple Hubbard
model. Longer range Coulomb interactions cause satellites in the one electron
removal and addition spectra and the appearance of spectral weight if the gap
of doped systems at energies corresponding to the excitons of the undoped
systems. The spectral weight of the satellites is proportional to the doping
times the coordination number and therefore is strongly dependent on the
dimension.Comment: 10 pages revtex, 5 figures ps figures adde
Charge ordering in extended Hubbard models: Variational cluster approach
We present a generalization of the recently proposed variational cluster
perturbation theory to extended Hubbard models at half filling with repulsive
nearest neighbor interaction. The method takes into account short-range
correlations correctly by the exact diagonalisation of clusters of finite size,
whereas long-range order beyond the size of the clusters is treated on a
mean-field level. For one dimension, we show that quantum Monte Carlo and
density-matrix renormalization-group results can be reproduced with very good
accuracy. Moreover we apply the method to the two-dimensional extended Hubbard
model on a square lattice. In contrast to the one-dimensional case, a first
order phase transition between spin density wave phase and charge density wave
phase is found as function of the nearest-neighbor interaction at onsite
interactions U>=3t. The single-particle spectral function is calculated for
both the one-dimensional and the two-dimensional system.Comment: 15 pages, 12 figure
Measurement of the Hyperfine Structure and Isotope Shifts of the 3s23p2 3P2 to 3s3p3 3Do3 Transition in Silicon
The hyperfine structure and isotope shifts of the 3s23p2 3P2 to 3s3p3 3Do3
transition in silicon have been measured. The transition at 221.7 nm was
studied by laser induced fluorescence in an atomic Si beam. For 29Si, the
hyperfine A constant for the 3s23p2 3P2 level was determined to be -160.1+-1.3
MHz (1 sigma error), and the A constant for the 3s3p3 3Do3 level is -532.9+-0.6
MHz. This is the first time that these constants were measured. The isotope
shifts (relative to the abundant isotope 28Si) of the transition were
determined to be 1753.3+-1.1 MHz for 29Si and 3359.9+-0.6 MHz for 30Si. This is
an improvement by about two orders of magnitude over a previous measurement.
From these results we are able to predict the hyperfine structure and isotope
shift of the radioactive 31Si atom, which is of interest in building a scalable
quantum computer
InP/Ga0.47In0.53As monolithic, two-junction, three-terminal tandem solar cells
The work presented has focussed on increasing the efficiency of InP-based solar cells through the development of a high-performance InP/Ga(0.47)In(0.53)As two-junction, three-terminal monolithic tandem cell. Such a tandem is particularly suited to space applications where a radiation-hard top cell (i.e., InP) is required. Furthermore, the InP/Ga(0.47)In(0.53)As materials system is lattice matched and offers a top cell/bottom cell bandgap differential (0.60 eV at 300 K) suitable for high tandem cell efficiencies under AMO illumination. A three-terminal configuration was chosen since it allows for independent power collection from each subcell in the monolithic stack, thus minimizing the adverse impact of radiation damage on the overall tandem efficiency. Realistic computer modeling calculations predict an efficiency boost of 7 to 11 percent from the Ga(0.47)In(0.53)As bottom cell under AMO illumination (25 C) for concentration ratios in the 1 to 1000 range. Thus, practical AMO efficiencies of 25 to 32 percent appear possible with the InP/Ga(0.47)In(0.53)As tandem cell. Prototype n/p/n InP/Ga(0.47)In(0.53)As monolithic tandem cells were fabricated and tested successfully. Using an aperture to define the illuminated areas, efficiency measurements performed on a non-optimized device under standard global illumination conditions (25 C) with no antireflection coating (ARC) give 12.2 percent for the InP top cell and 3.2 percent for the Ga(0.47)In(0.53)As bottom cell, yielding an overall tandem efficiency of 15.4 percent. With an ARC, the tandem efficiency could reach approximately 22 percent global and approximately 20 percent AMO. Additional details regarding the performance of individual InP and Ga(0.47)In(0.53)As component cells, fabrication and operation of complete tandem cells and methods for improving the tandem cell performance, are also discussed
Superconductivity in the Cuo Hubbard Model with Long-Range Coulomb Repulsion
A multiband CuO Hubbard model is studied which incorporates long-range (LR)
repulsive Coulomb interactions. In the atomic limit, it is shown that a
charge-transfer from copper to oxygen ions occurs as the strength of the LR
interaction is increased. The regime of phase separation becomes unstable, and
is replaced by a uniform state with doubly occupied oxygens. As the holes
become mobile a superfluid condensate is formed, as suggested by a numerical
analysis of pairing correlation functions and flux quantization. Although most
of the calculations are carried out on one dimensional chains, it isComment: LATEX, 14 pages, 4 figures available as postcript files or hard copy,
preprint ORNL-CCIP/93/1
s-wave Superconductivity Phase Diagram in the Inhomogeneous Two-Dimensional Attractive Hubbard Model
We study s-wave superconductivity in the two-dimensional square lattice
attractive Hubbard Hamiltonian for various inhomogeneous patterns of
interacting sites. Using the Bogoliubov-de Gennes (BdG) mean field
approximation, we obtain the phase diagram for inhomogeneous patterns in which
the on-site attractive interaction U_i between the electrons takes on two
values, U_i=0 and -U/(1-f) (with f the concentration of non-interacting sites)
as a function of average electron occupation per site n, and study the
evolution of the phase diagram as f varies. In certain regions of the phase
diagram, inhomogeneity results in a larger zero temperature average pairing
amplitude (order parameter) and also a higher superconducting (SC) critical
temperature T_c, relative to a uniform system with the same mean interaction
strength (U_i=-U on all sites). These effects are observed for stripes,
checkerboard, and even random patterns of the attractive centers, suggesting
that the pattern of inhomogeneity is unimportant. The phase diagrams also
include regions where superconductivity is obliterated due to the formation of
various charge ordered phases. The enhancement of T_{c} due to inhomogeneity is
robust as long as the electron doping per site n is less than twice the
fraction of interacting sites [2(1-f)] regardless of the pattern. We also show
that for certain inhomogeneous patterns, when n = 2(1-f), increasing
temperature can work against the stability of existing charge ordered phases
for large f and as a result, enhance T_{c}.Comment: 16 pages, 11 figure
Density-functional theory of strongly correlated Fermi gases in elongated harmonic traps
Two-component Fermi gases with tunable repulsive or attractive interactions
inside quasi-one-dimensional (Q1D) harmonic wells may soon become the cleanest
laboratory realizations of strongly correlated Luttiger and Luther-Emery
liquids under confinement. We present a microscopic Kohn-Sham
density-functional theory of these systems, with specific attention to a gas on
the approach to a confinement-induced Feshbach resonance. The theory employs
the one-dimensional Gaudin-Yang model as the reference system and transfers the
appropriate Q1D ground-state correlations to the confined inhomogeneous gas
{\it via} a suitable local-density approximation to the exchange and
correlation energy functional. Quantitative understanding of the role of the
interactions in the bulk shell structure of the axial density profile is
thereby achieved. While repulsive intercomponent interactions depress the
amplitude of the shell structure of the noninteracting gas, attractive
interactions stabilize atomic-density waves through spin pairing. These should
be clearly observable in atomic clouds containing of the order of up to a
hundred atoms.Comment: 13 pages, 9 figures, submitte
Properties of charge density waves in LaBaCuO
We report a comprehensive x-ray scattering study of charge density wave
(stripe) ordering in , for which the
superconducting is greatly suppressed. Strong superlattice reflections
corresponding to static ordering of charge stripes were observed in this
sample. The structural modulation at the lowest temperature was deduced based
on the intensity of over 70 unique superlattice positions surveyed. We found
that the charge order in this sample is described with one-dimensional charge
density waves, which have incommensurate wave-vectors (0.23, 0, 0.5) and (0,
0.23, 0.5) respectively on neighboring planes. The structural
modulation due to the charge density wave order is simply sinusoidal, and no
higher harmonics were observed. Just below the structural transition
temperature, short-range charge density wave correlation appears, which
develops into a large scale charge ordering around 40 K, close to the spin
density wave ordering temperature. However, this charge ordering fails to grow
into a true long range order, and its correlation length saturates at , and slightly decreases below about 15 K, which may be due to the onset
of two-dimensional superconductivity.Comment: 11 pages, 9 figure
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