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Predicted electric field near small superconducting ellipsoids
We predict the existence of large electric fields near the surface of
superconducting bodies of ellipsoidal shape of dimensions comparable to the
penetration depth. The electric field is quadrupolar in nature with significant
corrections from higher order multipoles. Prolate (oblate) superconducting
ellipsoids are predicted to exhibit fields consistent with negative (positive)
quadrupole moments, reflecting the fundamental charge asymmetry of matter.Comment: To be published in Phys.Rev.Let
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
Quantum Monte Carlo and exact diagonalization study of a dynamic Hubbard model
A one-dimensional model of electrons locally coupled to spin-1/2 degrees of
freedom is studied by numerical techniques. The model is one in the class of
that describe the relaxation of an atomic orbital
upon double electron occupancy due to electron-electron interactions. We study
the parameter regime where pairing occurs in this model by exact
diagonalization of small clusters. World line quantum Monte Carlo simulations
support the results of exact diagonalization for larger systems and show that
kinetic energy is lowered when pairing occurs. The qualitative physics of this
model and others in its class, obtained through approximate analytic
calculations, is that superconductivity occurs through hole undressing even in
parameter regimes where the effective on-site interaction is strongly
repulsive. Our numerical results confirm the expected qualitative behavior, and
show that pairing will occur in a substantially larger parameter regime than
predicted by the approximate low energy effective Hamiltonian.Comment: Some changes made in response to referees comments. To be published
in Phys.Rev.
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