284 research outputs found
Spin-fermion mappings for even Hamiltonian operators
We revisit the Jordan-Wigner transformation, showing that --rather than a
non-local isomorphism between different fermionic and spin Hamiltonian
operators-- it can be viewed in terms of local identities relating different
realizations of projection operators. The construction works for arbitrary
dimension of the ambient lattice, as well as of the on-site vector space,
generalizing Jordan-Wigner's result. It provides direct mapping of local
quantum spin problems into local fermionic problems (and viceversa), under the
(rather physical) requirement that the latter are described by Hamiltonian's
which are even products of fermionic operators. As an application, we
specialize to mappings between constrained-fermions models and spin 1 models on
chains, obtaining in particular some new integrable spin Hamiltonian, and the
corresponding ground state energies.Comment: 7 pages, ReVTeX file, no figure
Finite-temperature properties of the Hubbard chain with bond-charge interaction
We investigate the one-dimensional Hubbard model with an additional
bond-charge interaction, recently considered in the description of compounds
that exhibit strong 1D features above the temperature of ordered phases. The
partition function of the model is exactly calculated for a value of the
bond-charge coupling; the behavior of the specific heat and spin susceptibility
as a function of temperature is derived at arbitrary filling, and particularly
discussed across the occurring metal-insulator transition. The results show
that the bond-charge terms weaken the spin excitations of the system.Comment: 5 pages, 3 eps figure
Switching the sign of Josephson current through Aharonov-Bohm interferometry
We investigate the DC Josephson effect in a superconductor-normal
metal-superconductor junction where the normal region consists of a ballistic
ring. We show that a fully controllable -junction can be realized through
the electro-magnetostatic Aharonov-Bohm effect in the ring. The sign and the
magnitude of the supercurrent can be tuned by varying the magnetic flux and the
gate voltage applied to one arm, around suitable values. The implementation in
a realistic set-up is discussed.Comment: 4 pages, 3 figure
Electron tunneling into a quantum wire in the Fabry-Perot regime
We study a gated quantum wire contacted to source and drain electrodes in the
Fabry-Perot regime. The wire is also coupled to a third terminal (tip), and we
allow for an asymmetry of the tip tunneling amplitudes of right and left moving
electrons. We analyze configurations where the tip acts as an electron injector
or as a voltage-probe, and show that the transport properties of this
three-terminal set-up exhibit very rich physical behavior. For a
non-interacting wire we find that a tip in the voltage-probe configuration
affects the source-drain transport in different ways, namely by suppressing the
conductance, by modulating the Fabry-Perot oscillations, and by reducing their
visibility. The combined effect of electron electron interaction and finite
length of the wire, accounted for by the inhomogeneous Luttinger liquid model,
leads to significantly modified predictions as compared to models based on
infinite wires. We show that when the tip injects electrons asymmetrically the
charge fractionalization induced by interaction cannot be inferred from the
asymmetry of the currents flowing in source and drain. Nevertheless interaction
effects are visible as oscillations in the non-linear tip-source and tip-drain
conductances. Important differences with respect to a two-terminal set-up
emerge, suggesting new strategies for the experimental investigation of
Luttinger liquid behavior.Comment: 27 pages, 10 figure
Detection of Tiny Mechanical Motion by Means of the Ratchet Effect
We propose a position detection scheme for a nanoelectromechanical resonator
based on the ratchet effect. This scheme has an advantage of being a dc
measurement. We consider a three-junction SQUID where a part of the
superconducting loop can perform mechanical motion. The response of the ratchet
to a dc current is sensitive to the position of the resonator and the effect
can be further enhanced by biasing the SQUID with an ac current. We discuss the
feasibility of the proposed scheme in existing experimental setups.Comment: 8 pages, 9 figure
Strongly Interacting Luttinger Liquid and Superconductivity in an Exactly Solvable Model
A new family of exactly solvable one dimensional models with a hard-core
repulsive potential is solved by the Bethe Ansatz for an arbitrary hard-core
radius. The exact ground state phase diagrams in a plane 'electron density -
on-site interaction' have been studied for several values of a hard-core
radius. It is shown that superconducting phase and strongly interacting
Luttinger liquid state are coexisted at a high electron density and unusually
high value of repulsive on-site Coulomb interaction.Comment: 4 pages, 2 figures, RevTe
Band and filling controlled transitions in exactly solved electronic models
We describe a general method to study the ground state phase diagram of
electronic models on chains whose extended Hubbard hamiltonian is formed by a
generalized permutator plus a band-controlling term. The method, based on the
appropriate interpretation of Sutherland's species, yields under described
conditions a reduction of the effective Hilbert space. In particular, we derive
the phase diagrams of two new models; the first one exhibits a band-controlled
insulator-superconductor transition at half-filling for the unusually high
value ; the second one is characterized by a filling-controlled
metal-insulator transition between two finite regions of the diagram.Comment: 5 pages, REVTEX, 2 eps figure
Adiabatic Magnetization of Superconductors as a High-Performance Cooling Mechanism
The adiabatic magnetization of a superconductor is a cooling principle
proposed in the 30s, which has never been exploited up to now. Here we present
a detailed dynamic description of the effect, computing the achievable final
temperatures as well as the process timescales for different superconductors in
various regimes. We show that, although in the experimental conditions explored
so far the method is in fact inefficient, a suitable choice of initial
temperatures and metals can lead to unexpectedly large cooling effect, even in
the presence of dissipative phenomena. Our results suggest that this principle
can be re-envisaged today as a performing refrigeration method to access the
microK regime in nanodevices.Comment: 4 pages, 3 color figure
Nanotransformation and current fluctuations in exciton condensate junctions
We analyze the nonlinear transport properties of a bilayer exciton condensate
that is contacted by four metallic leads by calculating the full counting
statistics of electron transport for arbitrary system parameters. Despite its
formal similarity to a superconductor the transport properties of the exciton
condensate turn out to be completely different. We recover the generic features
of exciton condensates such as counterpropagating currents driven by excitonic
Andreev reflections and make predictions for nonlinear transconductance between
the layers as well as for the current (cross)correlations and generalized
Johnson-Nyquist relationships. Finally, we explore the possibility of
connecting another mesoscopic system (in our case a quantum point contact) to
the bottom layer of the exciton condensate and show how the excitonic Andreev
reflections can be used for transforming voltage at the nanoscale.Comment: 5 pages, 4 figures, accepted by PR
Correlation length and the scaling parameter in the Renormalization Group
The basic procedure of renormalization group theory is used to split the free energy into a Kadanoff block formation part, and a renormalized block-block interaction part. The study of this redistribution as a function of the scaling parameter s shows that there is a stationarity value s* of s, which turns out to have the same critical behavior as the correlation length. It is suggested that s* can be used as an appropriate measure and definition of the correlation length, even for noncritical regions. The calculation of s* is thereby performed explicitly for the Gaussian, and numerically for the S4 model. A sharp separation between noncorrelated and correlated regimes is also found for the Gaussian model, well above the critical temperature. For the S4 model, the results suggest that ξ is characterized by a high-temperature Gaussian branch and by a genuine S4 branch at low temperatures, connected by a "plateau" in the intermediate region
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