83,331 research outputs found
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.
Correcting 100 years of misunderstanding: electric fields in superconductors, hole superconductivity, and the Meissner effect
From the outset of superconductivity research it was assumed that no
electrostatic fields could exist inside superconductors, and this assumption
was incorporated into conventional London electrodynamics. Yet the London
brothers themselves initially (in 1935) had proposed an electrodynamic theory
of superconductors that allowed for static electric fields in their interior,
which they unfortunately discarded a year later. I argue that the Meissner
effect in superconductors necessitates the existence of an electrostatic field
in their interior, originating in the expulsion of negative charge from the
interior to the surface when a metal becomes superconducting. The theory of
hole superconductivity predicts this physics, and associated with it a
macroscopic spin current in the ground state of superconductors ("Spin Meissner
effect"), qualitatively different from what is predicted by conventional
BCS-London theory. A new London-like electrodynamic description of
superconductors is proposed to describe this physics. Within this theory
superconductivity is driven by lowering of quantum kinetic energy, the fact
that the Coulomb repulsion strongly depends on the character of the charge
carriers, namely whether electron- or hole-like, and the spin-orbit
interaction. The electron-phonon interaction does not play a significant role,
yet the existence of an isotope effect in many superconductors is easily
understood. In the strong coupling regime the theory appears to favor local
charge inhomogeneity. The theory is proposed to apply to all superconducting
materials, from the elements to the high cuprates and pnictides, is
highly falsifiable, and explains a wide variety of experimental observations.Comment: Proceedings of the conference "Quantum phenomena in complex matter
2011 - Stripes 2011", Rome, 10 July -16 July 2011, to be published in J.
Supercond. Nov. Mag
Meissner effect, Spin Meissner effect and charge expulsion in superconductors
The Meissner effect and the Spin Meissner effect are the spontaneous
generation of charge and spin current respectively near the surface of a metal
making a transition to the superconducting state. The Meissner effect is well
known but, I argue, not explained by the conventional theory, the Spin Meissner
effect has yet to be detected. I propose that both effects take place in all
superconductors, the first one in the presence of an applied magnetostatic
field, the second one even in the absence of applied external fields. Both
effects can be understood under the assumption that electrons expand their
orbits and thereby lower their quantum kinetic energy in the transition to
superconductivity. Associated with this process, the metal expels negative
charge from the interior to the surface and an electric field is generated in
the interior. The resulting charge current can be understood as arising from
the magnetic Lorentz force on radially outgoing electrons, and the resulting
spin current can be understood as arising from a spin Hall effect originating
in the Rashba-like coupling of the electron magnetic moment to the internal
electric field. The associated electrodynamics is qualitatively different from
London electrodynamics, yet can be described by a small modification of the
conventional London equations. The stability of the superconducting state and
its macroscopic phase coherence hinge on the fact that the orbital angular
momentum of the carriers of the spin current is found to be exactly ,
indicating a topological origin. The simplicity and universality of our theory
argue for its validity, and the occurrence of superconductivity in many classes
of materials can be understood within our theory.Comment: Submitted to SLAFES XX Proceeding
Quasiparticle undressing in a dynamic Hubbard model: exact diagonalization study
Dynamic Hubbard models have been proposed as extensions of the conventional
Hubbard model to describe the orbital relaxation that occurs upon double
occupancy of an atomic orbital. These models give rise to pairing of holes and
superconductivity in certain parameter ranges. Here we explore the changes in
carrier effective mass and quasiparticle weight and in one- and two-particle
spectral functions that occur in a dynamic Hubbard model upon pairing, by exact
diagonalization of small systems. It is found that pairing is associated with
lowering of effective mass and increase of quasiparticle weight, manifested in
transfer of spectral weight from high to low frequencies in one- and
two-particle spectral functions. This 'undressing' phenomenology resembles
observations in transport, photoemission and optical experiments in high T_c
cuprates. This behavior is contrasted with that of a conventional electron-hole
symmetric Holstein-like model with attractive on-site interaction, where
pairing is associated with 'dressing' instead of 'undressing'
Electromotive forces and the Meissner effect puzzle
In a voltaic cell, positive (negative) ions flow from the low (high)
potential electrode to the high (low) potential electrode, driven by an
`electromotive force' which points in opposite direction and overcomes the
electric force. Similarly in a superconductor charge flows in direction
opposite to that dictated by the Faraday electric field as the magnetic field
is expelled in the Meissner effect. The puzzle is the same in both cases: what
drives electric charges against electromagnetic forces? I propose that the
answer is also the same in both cases: kinetic energy lowering, or `quantum
pressure'
Invariants of 2+1 Quantum Gravity
In [1,2] we established and discussed the algebra of observables for 2+1
gravity at both the classical and quantum level. Here our treatment broadens
and extends previous results to any genus with a systematic discussion of
the centre of the algebra. The reduction of the number of independent
observables to is treated in detail with a precise
classification for and .Comment: 10 pages, plain TEX, no figures, DFTT 46/9
Account of Operations in the Boulogne Area
Account by Lieutenant-Colonel J.E. Anderson, Officer Commanding, North Shore Regiment, Given to Historical Officer, 27 September 1944
A Key and Annotated List of the Scutelleroidea of Michigan (Hemiptera)
Excerpt: Although Hussey (1922) compiled a list of the Hemiptera of Berrien County, and Stoner (1922) contributed a list of the Scutelleroidea of the Douglas Lake region, no publications have dealt with Michigan Scutelleroidea on a state-wide basis. However, collections in the Entomology Museum of Michigan State University (MSU), East Lansing, and in the Museum of Zoology of the University of Michigan (UMMZ), Ann Arbor, indicate that collecting has been extensive throughout the state (Fig. 1). The key and annotated list are based on material I identified in these two collection
Cognitive radio Over fibre: potential advantages for spectrum management
The paper explores some of the advantages that radio over fibre technologies can bring to wireless networks when combined with cognitive radio techniques
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