306 research outputs found
Superconductivity in the Two-Dimensional - Model at Low Hole Doping
By combining a generalized Lanczos scheme with the variational Monte Carlo
method we can optimize the short- and long-range properties of the groundstate
separately. This allows us to measure the long-range order of the groundstate
of the - model as a function of the coupling constant , and identify
a region of finite d-wave superconducting long-range order. With a lattice size
of 50 sites we can reliably examine hole densities down to 0.16.Comment: 12 pages and 4 PostScript figures, ReVTeX 3.0, ETH-TH/94-1
Ferromagnetism in the two dimensional t-t' Hubbard model at the Van Hove density
Using an improved version of the projection quantum Monte Carlo technique, we
study the square-lattice Hubbard model with nearest-neighbor hopping t and
next-nearest-neighbor hopping t', by simulation of lattices with up to 20 X 20
sites. For a given R=2t'/t, we consider that filling which leads to a singular
density of states of the noninteracting problem. For repulsive interactions, we
find an itinerant ferromagnet (antiferromagnet) for R=0.94 (R=0.2). This is
consistent with the prediction of the T-matrix approximation, which sums the
most singular set of diagrams.Comment: 10 pages, RevTeX 3.0 + a single postscript file with all figure
Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
This paper introduces a new modular Fish Bone Active Camber morphing wing with novel 3D printed skin panels. These skin panels are printed using two different Thermoplastic Polyurethane (TPU) formulations: a soft, high strain formulation for the deformable membrane of the skin, reinforced with a stiffer formulation for the stringers and mounting tabs. Additionally, this is the first FishBAC device designed to be modular in its installation and actuation. Therefore, all components can be removed and replaced for maintenance purposes without having to remove or disassemble other parts. A 1 m span, 0.27 m chord morphing wing with a 25% chord FishBAC was built and tested mechanically and in a low-speed wind tunnel. Results show that the new design is capable of achieving the same large changes in airfoil lift coefficient (approximate ΔCL≈0.55) with a low drag penalty seen in previous FishBAC work, but with a much simpler, practical and modular design. Additionally, the device shows a change in the pitching moment coefficient of ΔCM≈0.1, which shows the potential that the FishBAC has as a control surface
From antiferromagnetism to d-wave superconductivity in the 2D t-J model
We have found that the two dimensional t-J model, for the physical parameter
range J/t = 0.4 reproduces the main experimental qualitative features of
High-Tc copper oxide superconductors: d-wave superconducting correlations are
strongly enhanced upon small doping and clear evidence of off diagonal long
range order is found at the optimal doping \delta ~ 0.15. On the other hand
antiferromagnetic long range order, clearly present at zero hole doping, is
suppressed at small hole density with clear absence of antiferromagnetism at
\delta >~ 0.1.Comment: 4 pages, 5 figure
d-Wave Pairing Correlation in the Two-Dimensional t-J Model
The pair-pair correlation function of the two-dimensional t-J model is
studied by using the power-Lanczos method and an assumption of monotonic
behavior. In comparison with the results of the ideal Fermi gas, we conclude
that the 2D t-J model does not have long range d-wave superconducting
correlation in the interesting parameter range of . Implications
of this result will also be discussed.Comment: 4 pages, 6 figures, accepted by PR
Staggered-vorticity correlations in a lightly doped t-J model: a variational approach
We report staggered vorticity correlations of current in the d-wave
variational wave function for the lightly-doped t-J model. Such correlations
are explained from the SU(2) symmetry relating d-wave and staggered-flux
mean-field phases. The correlation functions computed by the variational Monte
Carlo method suggest that pairs are formed of holes circulating in opposite
directions.Comment: ReVTeX, 4 pages, 3 figure
Vortex Charging Effect in a Chiral -Wave Superconductor
Quasiparticle states around a single vortex in a -wave
superconductor are studied on the basis of the Bogoliubov-de Gennes (BdG)
theory, where both charge and current screenings are taken into account. Due to
the violation of time reversal symmetry, there are two types of vortices which
are distinguished by their winding orientations relative to the angular
momentum of the chiral Cooper pair. The BdG solution shows that the charges of
the two types of vortices are quite different, reflecting the rotating Cooper
pair of the -wave paring state.Comment: 10 pages, 5 figures, revtex, to be published in Phys. Rev.
Singlet Stripe Phases in the planar t-J Model
The energies of singlet stripe phases in which a plane is broken up into spin
liquid ladders by lines of holes, is examined. If the holes were static then
patterns containing spin liquids with a finite spin gap are favored. The case
of dynamic holes is treated by assembling t-J ladders oriented perpendicular to
the stripes. For a wide region around the hole-hole
correlations in a single ladder are found to be predominantly charge density
wave type but an attraction between hole pairs on adjacent ladders leads to a
stripe phase. A quantum mechanical melting of the hole lines at smaller
values leads to a Bose condensate of hole pairs, i.e. a superconducting phase.Comment: 5 pages, uuencoded compressed PostScript file including 5 figures,
ETH-TH/942
Phase transition in the one-dimensional Kondo lattice model with attractive electron-electron interaction
The one-dimensional Kondo lattice model with attractive interaction among the
conduction electrons is analyzed in the case of half-filling. It is shown that
there are three distinct phases depending on the coupling constants of the
model. Two phases have a spin and charge gap. While one shows a clear
separation of the spin and charge excitation spectrum the other phase may be
characterized as a band insulator type where both excitations are due to
two-particle states. The third phase is gapless in both channels and has quasi
long-range order in the spin and charge density wave correlation. In this phase
the spin and charge excitations have again a clearly separated spectrum. For
the analysis we discuss first two limiting cases. Then a density matrix
renormalization group calculation on finite systems is applied to determine the
phase diagram and the correlation functions in the gapped and gapless phase for
general couplding constants.Comment: 9 pages, 7 Postscript figures, REVTe
Pair breaking by impurities in the two-dimensional t-J model
Pair breaking mechanisms by impurities are investigated in the
two-dimensional t-J model by exact diagonalization techniques. Analysis of
binding energies, pairing correlations, dynamical spin and pair
susceptibilities shows that non-magnetic impurities are more effective in
suppressing pairing than magnetic ones in agreement with experimental studies
of Zn- and Ni- substituted High-Tc superconductors.Comment: 4 pages, Revtex v3.0, 4 figures uuencoded, ask for hardcopies at
[email protected] A missleading statement in the introduction was correcte
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