10,995 research outputs found
Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates
Experimental evidence indicates that the superconducting transition in high
cuprates is an 'undressing' transition. Microscopic mechanisms giving
rise to this physics were discussed in the first paper of this series. Here we
discuss the calculation of the single particle Green's function and spectral
function for Hamiltonians describing undressing transitions in the normal and
superconducting states. A single parameter, , describes the strength
of the undressing process and drives the transition to superconductivity. In
the normal state, the spectral function evolves from predominantly incoherent
to partly coherent as the hole concentration increases. In the superconducting
state, the 'normal' Green's function acquires a contribution from the anomalous
Green's function when is non-zero; the resulting contribution to
the spectral function is for hole extraction and for hole
injection. It is proposed that these results explain the observation of sharp
quasiparticle states in the superconducting state of cuprates along the
direction and their absence along the direction.Comment: figures have been condensed in fewer pages for easier readin
EDIN design study alternate space shuttle booster replacement concepts. Volume 2: Design simulation results
Historical weight estimating relationships were developed for the liquid rocket booster (LRB) using Saturn technology, and modified as required to support the EDIN05 study. Mission performance was computed using February 1975 shuttle configuration groundrules to allow reasonable comparison of the existing shuttle with the EDIN05 designs. The launch trajectory was constrained to pass through both the RTLS/AOA and main engine cut-off points. Performance analysis was based on a point design trajectory model which optimized initial tilt rate and exo-atmospheric pitch profile. A gravity turn was employed during the boost phase in place of the shuttle angle-of-attack profile. Engine throttling add/or shutdown was used to constrain dynamic pressure and/or longitudinal acceleration where necessary
Superconductivity from Undressing
Photoemission experiments in high cuprates indicate that quasiparticles
are heavily 'dressed' in the normal state, particularly in the low doping
regime. Furthermore these experiments show that a gradual undressing occurs
both in the normal state as the system is doped and the carrier concentration
increases, as well as at fixed carrier concentration as the temperature is
lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be
simply understood with the single assumption that the quasiparticle dressing is
a function of the local carrier concentration. Microscopic Hamiltonians
describing this physics are discussed. The undressing process manifests itself
in both the one-particle and two-particle Green's functions, hence leads to
observable consequences in photoemission and optical experiments respectively.
An essential consequence of this phenomenology is that the microscopic
Hamiltonians describing it break electron-hole symmetry: these Hamiltonians
predict that superconductivity will only occur for carriers with hole-like
character, as proposed in the theory of hole superconductivity
Forced Symmetry Breaking from SO(3) to SO(2) for Rotating Waves on the Sphere
We consider a small SO(2)-equivariant perturbation of a reaction-diffusion
system on the sphere, which is equivariant with respect to the group SO(3) of
all rigid rotations. We consider a normally hyperbolic SO(3)-group orbit of a
rotating wave on the sphere that persists to a normally hyperbolic
SO(2)-invariant manifold . We investigate the effects of this
forced symmetry breaking by studying the perturbed dynamics induced on
by the above reaction-diffusion system. We prove that depending
on the frequency vectors of the rotating waves that form the relative
equilibrium SO(3)u_{0}, these rotating waves will give SO(2)-orbits of rotating
waves or SO(2)-orbits of modulated rotating waves (if some transversality
conditions hold). The orbital stability of these solutions is established as
well. Our main tools are the orbit space reduction, Poincare map and implicit
function theorem
Quantum Transition between an Antiferromagnetic Mott Insulator and Superconductor in Two Dimensions
We consider a Hubbard model on a square lattice with an additional
interaction, , which depends upon the square of a near-neighbor hopping. At
half-filling and a constant value of the Hubbard repulsion, increasing the
strength of the interaction drives the system from an antiferromagnetic
Mott insulator to a superconductor. This conclusion is reached
on the basis of zero temperature quantum Monte Carlo simulations on lattice
sizes up to .Comment: 4 pages (latex) and 4 postscript figure
R-parity Conserving Supersymmetry, Neutrino Mass and Neutrinoless Double Beta Decay
We consider contributions of R-parity conserving softly broken supersymmetry
(SUSY) to neutrinoless double beta (\znbb) decay via the (B-L)-violating
sneutrino mass term. The latter is a generic ingredient of any weak-scale SUSY
model with a Majorana neutrino mass. The new R-parity conserving SUSY
contributions to \znbb are realized at the level of box diagrams. We derive
the effective Lagrangian describing the SUSY-box mechanism of \znbb-decay and
the corresponding nuclear matrix elements. The 1-loop sneutrino contribution to
the Majorana neutrino mass is also derived.
Given the data on the \znbb-decay half-life of Ge and the neutrino
mass we obtain constraints on the (B-L)-violating sneutrino mass. These
constraints leave room for accelerator searches for certain manifestations of
the 2nd and 3rd generation (B-L)-violating sneutrino mass term, but are most
probably too tight for first generation (B-L)-violating sneutrino masses to be
searched for directly.Comment: LATEX, 29 pages + 4 (uuencoded) figures appende
Superconductivity in an exactly solvable Hubbard model with bond-charge interaction
The Hubbard model with an additional bond-charge interaction is solved
exactly in one dimension for the case where is the hopping amplitude.
In this case the number of doubly occupied sites is conserved. In the sector
with no double occupations the model reduces to the Hubbard model.
In arbitrary dimensions the qualitative form of the phase diagram is obtained.
It is shown that for moderate Hubbard interactions the model has
superconducting ground states.Comment: Revtex, 14 pages, 1 figure (uuencoded compressed tar-file
Loop algorithms for quantum simulations of fermion models on lattices
Two cluster algorithms, based on constructing and flipping loops, are
presented for worldline quantum Monte Carlo simulations of fermions and are
tested on the one-dimensional repulsive Hubbard model. We call these algorithms
the loop-flip and loop-exchange algorithms. For these two algorithms and the
standard worldline algorithm, we calculated the autocorrelation times for
various physical quantities and found that the ordinary worldline algorithm,
which uses only local moves, suffers from very long correlation times that
makes not only the estimate of the error difficult but also the estimate of the
average values themselves difficult. These difficulties are especially severe
in the low-temperature, large- regime. In contrast, we find that new
algorithms, when used alone or in combinations with themselves and the standard
algorithm, can have significantly smaller autocorrelation times, in some cases
being smaller by three orders of magnitude. The new algorithms, which use
non-local moves, are discussed from the point of view of a general prescription
for developing cluster algorithms. The loop-flip algorithm is also shown to be
ergodic and to belong to the grand canonical ensemble. Extensions to other
models and higher dimensions is briefly discussed.Comment: 36 pages, RevTex ver.
Stability and convergence in discrete convex monotone dynamical systems
We study the stable behaviour of discrete dynamical systems where the map is
convex and monotone with respect to the standard positive cone. The notion of
tangential stability for fixed points and periodic points is introduced, which
is weaker than Lyapunov stability. Among others we show that the set of
tangentially stable fixed points is isomorphic to a convex inf-semilattice, and
a criterion is given for the existence of a unique tangentially stable fixed
point. We also show that periods of tangentially stable periodic points are
orders of permutations on letters, where is the dimension of the
underlying space, and a sufficient condition for global convergence to periodic
orbits is presented.Comment: 36 pages, 1 fugur
The Low-Energy Fixed Points of Random Quantum Spin Chains
The one-dimensional isotropic quantum Heisenberg spin systems with random
couplings and random spin sizes are investigated using a real-space
renormalization group scheme. It is demonstrated that these systems belong to a
universality class of disordered spin systems, characterized by weakly coupled
large effective spins. In this large-spin phase the uniform magnetic
susceptibility diverges as 1/T with a non-universal Curie constant at low
temperatures T, while the specific heat vanishes as T^delta |ln T| for T->0.
For broad range of initial distributions of couplings and spin sizes the
distribution functions approach a single fixed-point form, where delta \approx
0.44. For some singular initial distributions, however, fixed-point
distributions have non-universal values of delta, suggesting that there is a
line of fixed points.Comment: 19 pages, REVTeX, 13 figure
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