4,066 research outputs found
Exact Ground State of the 2D Hubbard Model at Half Filling for
We solve analytically the square lattice Hubbard model for even
at half filling and weak coupling by a new approach. The exact ground state
wave function provides an intriguing and appealing picture of the
antiferromagnetic order. Like at strong coupling, the ground state has total
momentum and transforms as an wave for even and as a
wave otherwise.Comment: 4 pages, typos in equation 5 correcte
Repulsion-Sustained Supercurrent and Flux Quantization in Rings of Symmetric Hubbard Clusters
We test the response to a threading magnetic field of rings of 5-site
-symmetric repulsive Hubbard clusters connected by weak intercell
links; each 5-site unit has the topology of a CuO cluster and a repulsive
interaction is included on every site. In a numerical study of the three-unit
ring with 8 particles, we take advantage of a novel exact-diagonalization
technique which can be generally applied to many-fermion problems. For O-O
hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu
links bound pair propagation is hindered by symmetry. The results agree with
W=0 pairing theory.Comment: 4 pages, 2 figure
"Spin-Disentangled" Exact Diagonalization of Repulsive Hubbard Systems: Superconducting Pair Propagation
By a novel exact diagonalization technique we show that bound pairs propagate
between repulsive Hubbard clusters in a superconducting fashion. The size of
the matrices that must be handled depends on the number of fermion
configurations {\em per spin}, which is of the order of the square root of the
overall size of the Hilbert space. We use CuO units connected by weak O-O
links to model interplanar coupling and c-axis superconductivity in Cuprates.
The numerical evidence on CuO and CuO prompts a new
analytic scheme describing the propagation of bound pairs and also the
superconducting flux quantization in a 3-d geometry.Comment: 5 pages, 3 figure
Three-Body and One-Body Channels of the Auger Core-Valence-Valence decay: Simplified Approach
We propose a computationally simple model of Auger and APECS line shapes from
open-band solids. Part of the intensity comes from the decay of unscreened
core-holes and is obtained by the two-body Green's function ,
as in the case of filled bands. The rest of the intensity arises from screened
core-holes and is derived using a variational description of the relaxed ground
state; this involves the two-holes-one-electron propagator , which
also contains one-hole contributions. For many transition metals, the two-hole
Green's function can be well described by the Ladder
Approximation, but the three-body Green's function poses serious further
problems. To calculate , treating electrons and holes on equal
footing, we propose a practical approach to sum the series to all orders. We
achieve that by formally rewriting the problem in terms of a fictitious
three-body interaction. Our method grants non-negative densities of states,
explains the apparent negative-U behavior of the spectra of early transition
metals and interpolates well between weak and strong coupling, as we
demonstrate by test model calculations.Comment: AMS-LaTeX file, 23 pages, 8 eps and 3 ps figures embedded in the text
with epsfig.sty and float.sty, submitted to Phys. Rev.
Quantum Mechanics without Waves: a Generalization of Classical Statistical Mechanics
We generalize classical statistical mechanics to describe the kinematics and
the dynamics of systems whose variables are constrained by a single quantum
postulate (discreteness of the spectrum of values of at least one variable of
the theory). This is possible provided we adopt Feynman's suggestion of
dropping the assumption that the probability for an event must always be a
positive number. This approach has the advantage of allowing a reformulation of
quantum theory in phase space without introducing the unphysical concept of
probability amplitudes, together with all the problems concerning their
ambiguous properties.Comment: 25 pages, Late
Organizational Culture and Reform: The Case of the European Commission under Jacques Santer
Europeanization; European Commission; institutionalisation; institutions; administrative adaptation
Bouncing transient currents and SQUID-like voltage in nano devices at half filling
Nanorings asymmetrically connected to wires show different kinds of quantum
interference phenomena under sudden excitations and in steady current
conditions. Here we contrast the transient current caused by an abrupt bias to
the magnetic effects at constant current. A repulsive impurity can cause charge
build-up in one of the arms and reverse current spikes.
Moreover, it can cause transitions from laminar current flow to vortices, and
also change the chirality of the vortex. The magnetic behavior of these devices
is also very peculiar. Those nano-circuits which consist of an odd number of
atoms behave in a fundamentally different manner compared to those which
consist of an even number of atoms. The circuits having an odd number of sites
connected to long enough symmetric wires are diamagnetic; they display
half-fluxon periodicity induced by many-body symmetry even in the absence of
electron-phonon and electron-electron interactions. In principle one can
operate a new kind of quantum interference device without superconductors.
Since there is no gap and no critical temperature, one predicts qualitatively
the same behavior at and above room temperature, although with a reduced
current. The circuits with even site numbers, on the other hand, are
paramagnetic.Comment: 7 pages, 10 figures, accepted by Phys. Rev.
Magnetization Transfer by a Quantum Ring Device
We show that a tight-binding model device consisting of a laterally connected
ring at half filling in a tangent time-dependent magnetic field can in
principle be designed to pump a purely spin current. The process exploits the
spin-orbit interaction in the ring. This behavior is understood analytically
and found to be robust with respect to temperature and small deviations from
half filling.Comment: 4 figures, 1 typo correcte
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