5,527 research outputs found
Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
We present an resonating valence bond (RVB) theory of superconductivity for
the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show
that these calculations are consistent with the observed phase diagram of the
half-filled layered organic superconductors, such as the beta, beta', kappa and
lambda phases of (BEDT-TTF)_2X [bis(ethylenedithio)tetrathiafulvalene] and
(BETS)_2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order
transition from a Mott insulator to a d_{x^2-y^2} superconductor with a small
superfluid stiffness and a pseudogap with d_{x^2-y^2} symmetry. The
Mott--Hubbard transition can be driven either by increasing the on-site Coulomb
repulsion, U, or by changing the anisotropy of the two hopping integrals, t'/t.
Our results suggest that the ratio t'/t plays an important role in determining
the phase diagram of the organic superconductors.Comment: 4 pages, 3 figur
Temperature Dependence of the Magnetic Susceptibility for Triangular-Lattice Antiferromagnets with spatially anisotropic exchange constants
We present the temperature dependence of the uniform susceptibility of
spin-half quantum antiferromagnets on spatially anisotropic
triangular-lattices, using high temperature series expansions. We consider a
model with two exchange constants, and on a lattice that
interpolates between the limits of a square-lattice (), a
triangular-lattice (), and decoupled linear chains (). In all
cases, the susceptibility which has a Curie-Weiss behavior at high
temperatures, rolls over and begins to decrease below a peak temperature,
. Scaling the exchange constants to get the same peak temperature, shows
that the susceptibilities for the square-lattice and linear chain limits have
similar magnitudes near the peak. Maximum deviation arises near the
triangular-lattice limit, where frustration leads to much smaller
susceptibility and with a flatter temperature dependence. We compare our
results to the inorganic materials CsCuCl and CsCuBr and to a
number of organic molecular crystals. We find that the former (CsCuCl
and CsCuBr) are weakly frustrated and their exchange parameters
determined through the temperature dependence of the susceptibility are in
agreement with neutron-scattering measurements. In contrast, the organic
materials are strongly frustrated with exchange parameters near the isotropic
triangular-lattice limit.Comment: 10 pages, 9 figures and 1 table, revised versio
Evidence from satellite altimetry for small-scale convection in the mantle
Small scale convection can be defined as that part of the mantle circulation in which upwellings and downwellings can occur beneath the lithosphere within the interiors of plates, in contrast to the large scale flow associated with plate motions where upwellings and downwellings occur at ridges and trenches. The two scales of convection will interact so that the form of the small scale convection will depend on how it arises within the large scale flow. Observations based on GEOS-3 and SEASAT altimetry suggest that small scale convection occurs in at least two different ways
Temperature dependence of the interlayer magnetoresistance of quasi-one-dimensional Fermi liquids at the magic angles
The interlayer magnetoresistance of a quasi-one-dimensional Fermi liquid is
considered for the case of a magnetic field that is rotated within the plane
perpendicular to the most-conducting direction. Within semi-classical transport
theory dips in the magnetoresistance occur at integer amgic angles only when
the electronic dispersion parallel to the chains is nonlinear. If the field
direction is fixed at one of the magic angles and the temperature is varied the
resulting variation of the scattering rate can lead to a non-monotonic
variation of the interlayer magnetoresistance with temperature. Although the
model considered here gives a good description of some of the properties of the
Bechgaard salts, (TMTSF)2PF6 for pressures less than 8kbar and (TMTSF)2ClO4 it
gives a poor description of their properties when the field is parallel to the
layers and of the intralayer transport.Comment: 10pages, RevTeX + epsf, 3 figure
Ginzburg-Landau theory of phase transitions in quasi-one-dimensional systems
A wide range of quasi-one-dimensional materials, consisting of weakly coupled
chains, undergo three-dimensional phase transitions that can be described by a
complex order parameter. A Ginzburg-Landau theory is derived for such a
transition. It is shown that intrachain fluctuations in the order parameter
play a crucial role and must be treated exactly. The effect of these
fluctuations is determined by a single dimensionless parameter. The
three-dimensional transition temperature, the associated specific heat jump,
coherence lengths, and width of the critical region, are computed assuming that
the single chain Ginzburg-Landau coefficients are independent of temperature.
The width of the critical region, estimated from the Ginzburg criterion, is
virtually parameter independent, being about 5-8 per cent of the transition
temperature. To appear in {\it Physical Review B,} March 1, 1995.Comment: 15 pages, RevTeX, 5 figures in uuencoded compressed tar file
Superconducting Pairing Symmetries in Anisotropic Triangular Quantum Antiferromagnets
Motivated by the recent discovery of a low temperature spin liquid phase in
layered organic compound -(ET)Cu(CN) which becomes a
superconductor under pressure, we examine the phase transition of Mott
insulating and superconducting (SC) states in a Hubbard-Heisenberg model on an
anisotropic triangular lattice. We use a renormalized mean field theory to
study the Gutzwiller projected BCS wavefucntions. The half filled electron
system is a Mott insulator at large on-site repulsion , and is a
superconductor at a moderate . The symmetry of the SC state depends on the
anisotropy, and is gapful with symmetry near the
isotropic limit and is gapless with symmetry at small anisotropy
ratio.Comment: 6 pages, 5 figure
Dimerization structures on the metallic and semiconducting fullerene tubules with half-filled electrons
Possible dimerization patterns and electronic structures in fullerene tubules
as the one-dimensional pi-conjugated systems are studied with the extended
Su-Schrieffer-Heeger model. We assume various lattice geometries, including
helical and nonhelical tubules. The model is solved for the half-filling case
of -electrons. (1) When the undimerized systems do not have a gap, the
Kekule structures prone to occur. The energy gap is of the order of the room
temperatures at most and metallic properties would be expected. (2) If the
undimerized systems have a large gap (about 1eV), the most stable structures
are the chain-like distortions where the direction of the arranged
trans-polyacetylene chains is along almost the tubular axis. The electronic
structures are ofsemiconductors due to the large gap.Comment: submitted to Phys. Rev. B, pages 15, figures 1
Excitation spectra and ground state properties of the layered spin-1/2 frustrated antiferromagnets Cs_2CuCl_4 and Cs_2CuBr_4
We use series expansion methods to study ground- and excited-state properties
in the helically ordered phase of spin-1/2 frustrated antiferromagnets on an
anisotropic triangular lattice. We calculate the ground state energy, ordering
wavevector, sublattice magnetization and one-magnon excitation spectrum for
parameters relevant to Cs_2CuCl_4 and Cs_2CuBr_4. Both materials are modeled in
terms of a Heisenberg model with spatially anisotropic exchange constants; for
Cs_2CuCl_4 we also take into account the additional Dzyaloshinskii-Moriya (DM)
interaction. We compare our results for Cs_2CuCl_4 with unpolarized neutron
scattering experiments and find good agreement. In particular, the large
quantum renormalizations of the one-magnon dispersion are well accounted for in
our analysis, and inclusion of the DM interaction brings the theoretical
predictions for the ordering wavevector and the magnon dispersion closer to the
experimental results.Comment: 10 pages, 8 figure
Universal subgap optical conductivity in quasi-one-dimensional Peierls systems
Quasi-one-dimensional Peierls systems with quantum and thermal lattice
fluctuations can be modeled by a Dirac-type equation with a Gaussian-correlated
off-diagonal disorder. A powerful new method gives the exact disorder-averaged
Green function used to compute the optical conductivity. The strong subgap tail
of the conductivity has a universal scaling form. The frequency and temperature
dependence of the calculated spectrum agrees with experiments on KCP(Br) and
trans-polyacetylene.Comment: 11 pages (+ 3 figures), LATEX (REVTEX 3.0
A plasma wakefield acceleration experiment using CLARA beam
We propose a Plasma Accelerator Research Station (PARS) based at proposed FEL
test facility CLARA (Compact Linear Accelerator for Research and Applications)
at Daresbury Laboratory. The idea is to use the relativistic electron beam from
CLARA, to investigate some key issues in electron beam transport and in
electron beam driven plasma wakefield acceleration, e.g. high gradient plasma
wakefield excitation driven by a relativistic electron bunch, two bunch
experiment for CLARA beam energy doubling, high transformer ratio, long bunch
self-modulation and some other advanced beam dynamics issues. This paper
presents the feasibility studies of electron beam transport to meet the
requirements for beam driven wakefield acceleration and presents the plasma
wakefield simulation results based on CLARA beam parameters. Other possible
experiments which can be conducted at the PARS beam line are also discussed
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