20,212 research outputs found
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
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
Stability analysis of some integrable Euler equations for SO(n)
A family of special cases of the integrable Euler equations on
introduced by Manakov in 1976 is considered. The equilibrium points are found
and their stability is studied. Heteroclinic orbits are constructed that
connect unstable equilibria and are given by the orbits of certain 1-parameter
subgroups of SO(n). The results are complete in the case and incomplete
for .Comment: 15 pages, LaTeX, minor stylistic changes in v
Towards an understanding of hole superconductivity
From the very beginning K. Alex M\"uller emphasized that the materials he and
George Bednorz discovered in 1986 were superconductors. Here I would
like to share with him and others what I believe to be key reason for why
high cuprates as well as all other superconductors are hole
superconductors, which I only came to understand a few months ago. This paper
is dedicated to Alex M\"uller on the occasion of his 90th birthday.Comment: Dedicated to Alex M\"uller on the Occasion of his 90th Birthday.
arXiv admin note: text overlap with arXiv:1703.0977
Impending anterior ischemic optic neuropathy with elements of retinal vein occlusion in a patient on interferon for polycythemia vera.
We describe the course and likely pathophysiology of impending anterior ischemic optic neuropathy (AION) and retinal vein occlusion in a 56-year-old man with polycythemia vera managed with interferon alpha for 2 years. Our patient presented with decreased vision, scintillating scotomata, and floaters. Fundus examination findings and results of a fluorescein angiogram led to the diagnosis of impending AION and retinal vein occlusion. Considering that both polycythemia vera and interferon have possible influences on vascular occlusion and optic disc edema, we stopped interferon treatment and immediately attempted to treat the polycythemia vera empirically with pentoxifylline and any interferon-associated inflammation with prednisone. Our patient experienced complete resolution of fundus abnormalities and return of normal vision within 3 weeks, which may be attributed to our successful treatment of both etiologies. Thus, further study is warranted to elucidate the treatment of both polycythemia vera and interferon-induced impending AION
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
Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory
We examine whether the Mott transition of a half-filled, two-orbital Hubbard
model with unequal bandwidths occurs simultaneously for both bands or whether
it is a two-stage process in which the orbital with narrower bandwith localizes
first (giving rise to an intermediate `orbital-selective' Mott phase). This
question is addressed using both dynamical mean-field theory, and a
representation of fermion operators in terms of slave quantum spins, followed
by a mean-field approximation (similar in spirit to a Gutzwiller
approximation). In the latter approach, the Mott transition is found to be
orbital-selective for all values of the Coulomb exchange (Hund) coupling J when
the bandwidth ratio is small, and only beyond a critical value of J when the
bandwidth ratio is larger. Dynamical mean-field theory partially confirms these
findings, but the intermediate phase at J=0 is found to differ from a
conventional Mott insulator, with spectral weight extending down to arbitrary
low energy. Finally, the orbital-selective Mott phase is found, at
zero-temperature, to be unstable with respect to an inter-orbital
hybridization, and replaced by a state with a large effective mass (and a low
quasiparticle coherence scale) for the narrower band.Comment: Discussion on the effect of hybridization on the OSMT has been
extende
Numerical Study of a Two-Dimensional Quantum Antiferromagnet with Random Ferromagnetic Bonds
A Monte Carlo method for finite-temperature studies of the two-dimensional
quantum Heisenberg antiferromagnet with random ferromagnetic bonds is
presented. The scheme is based on an approximation which allows for an analytic
summation over the realizations of the randomness, thereby significantly
alleviating the ``sign problem'' for this frustrated spin system. The
approximation is shown to be very accurate for ferromagnetic bond
concentrations of up to ten percent. The effects of a low concentration of
ferromagnetic bonds on the antiferromagnetism are discussed.Comment: 11 pages + 5 postscript figures (included), Revtex 3.0, UCSBTH-94-2
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