3,731 research outputs found
Josephson Effect through an isotropic magnetic molecule
We investigate the Josephson effect through a molecular quantum dot magnet
connected to superconducting leads. The molecule contains a magnetic atom,
whose spin is assumed to be isotropic. It is coupled to the electron spin on
the dot via exchange coupling. Using the numerical renormalization group method
we calculate the Andreev levels and the supercurrent and examine intertwined
effect of the exchange coupling, Kondo correlation, and superconductivity on
the current. Exchange coupling typically suppresses the Kondo correlation so
that the system undergoes a phase transition from 0 to state as the
modulus of exchange coupling increases. Antiferromagnetic coupling is found to
drive exotic transitions: the reentrance to the state for a small
superconducting gap and the restoration of 0 state for large antiferromagnetic
exchange coupling. We suggest that the asymmetric dependence of supercurrent on
the exchange coupling could be used as to detect its sign in experiments
Phonon-mediated negative differential conductance in molecular quantum dots
Transport through a single molecular conductor is considered, showing
negative differential conductance behavior associated with phonon-mediated
electron tunneling processes. This theoretical work is motivated by a recent
experiment by Leroy et al. using a carbon nanotube contacted by an STM tip
[Nature {\bf 432}, 371 (2004)], where negative differential conductance of the
breathing mode phonon side peaks could be observed. A peculiarity of this
system is that the tunneling couplings which inject electrons and those which
collect them on the substrate are highly asymmetrical. A quantum dot model is
used, coupling a single electronic level to a local phonon, forming polaron
levels. A "half-shuttle" mechanism is also introduced. A quantum kinetic
formulation allows to derive rate equations. Assuming asymmetric tunneling
rates, and in the absence of the half-shuttle coupling, negative differential
conductance is obtained for a wide range of parameters. A detailed explanation
of this phenomenon is provided, showing that NDC is maximal for intermediate
electron-phonon coupling. In addition, in absence of a gate, the "floating"
level results in two distinct lengths for the current plateaus, related to the
capacitive couplings at the two junctions. It is shown that the "half-shuttle"
mechanism tends to reinforce the negative differential regions, but it cannot
trigger this behavior on its own
Luttinger liquid of trimers in Fermi gases with unequal masses
We investigate one dimensional attractive Fermi gases in spin-dependent
optical lattices. We show that three-body bound states - "trimers" - exist as
soon as the two tunneling rates are different. We calculate the binding energy
and the effective mass of a single trimer. We then show numerically that for
finite and commensurate densities an energy gap
appears, implying that the gas is a one-component Luttinger liquid of trimers
with suppressed superfluid ordering. The boundaries of this novel phase are
given. We discuss experimental situations to test our predictions.Comment: 4 pages, 4 figures. Final accepted versio
Extension of the osp(m|n)~ so(m-n) Correspondence to the Infinite-Dimensional Chiral Spinors and Self Dual Tensors
The spinor representations of the orthosymplectic Lie superalgebras osp(m|n)
are considered and constructed. These are infinite-dimensional irreducible
representations, of which the superdimension coincides with the dimension of
the spinor representation of so(m-n). Next, we consider the self dual tensor
representations of osp(m|n) and their generalizations: these are also
infinite-dimensional and correspond to the highest irreducible component of the
power of the spinor representation. We determine the character of
these representations, and deduce a superdimension formula. From this, it
follows that also for these representations the osp(m|n)~ so(m-n)
correspondence holds
Nanoscale studies of domain wall motion in epitaxial ferroelectric thin films
Atomic force microscopy was used to investigate ferroelectric switching and
nanoscale domain dynamics in epitaxial PbZr0.2Ti0.8O3 thin films. Measurements
of the writing time dependence of domain size reveal a two-step process in
which nucleation is followed by radial domain growth. During this growth, the
domain wall velocity exhibits a v ~ exp[-(1/E)^mu] dependence on the electric
field, characteristic of a creep process. The domain wall motion was analyzed
both in the context of stochastic nucleation in a periodic potential as well as
the canonical creep motion of an elastic manifold in a disorder potential. The
dimensionality of the films suggests that disorder is at the origin of the
observed domain wall creep. To investigate the effects of changing the disorder
in the films, defects were introduced during crystal growth (a-axis inclusions)
or by heavy ion irradiation, producing films with planar and columnar defects,
respectively. The presence of these defects was found to significantly decrease
the creep exponent mu, from 0.62 - 0.69 to 0.38 - 0.5 in the irradiated films
and 0.19 - 0.31 in the films containing a-axis inclusions.Comment: 13 pages, 15 figures, to be published in J. App. Phys. special issue
on ferroelectric
Electronic, optical and thermal properties of the hexagonal and fcc Ge2Sb2Te5 chalcogenide from first-principle calculations
We present a comprehensive computational study on the properties of
face-centered cubic and hexagonal chalcogenide Ge2Sb2Te5. We calculate the
electronic structure using density functional theory (DFT); the obtained
density of states (DOS) compares favorably with experiments, also looking
suitable for transport analysis. Optical constants including refraction index
and absorption coefficient capture major experimental features, aside from an
energy shift owed to an underestimate of the band gap that is typical of DFT
calculations. We also compute the phonon DOS for the hexagonal phase, obtaining
a speed of sound and thermal conductivity in good agreement with the
experimental lattice contribution. The calculated heat capacity reaches ~ 1.4 x
106 J/(m3 K) at high temperature, in agreement with experimental data, and
provides insight into the low-temperature range (< 150 K), where data are
unavailable.Comment: 19 pages, 8 figure
Evidence for Diverging Barriers in the Disordered Vortex Solid in the K,Ba BiO3 Superconducting Oxide
International audienceVortex dynamics has been investigated in the cubic K, Ba BiO3 superconductor using ac susceptibility measurements on a large frequency range 0.03 Hz , v , 60 kHz . Power law diverging barriers have been obtained on both sides of the order-disorder transition line. The m exponent remains close to 5 2 (elastic creep value) in some part of the disordered phase and finally decreases at high temperature and/or high field, in good agreement with the recent plastic collective creep theory [J. Kierfeld, H. Nordborg, and V. M. Vinokur, Phys. Rev. Lett., 85, 4948 (2000)]
Multimer formation in 1D two-component gases and trimer phase in the asymmetric attractive Hubbard model
We consider two-component one-dimensional quantum gases at special imbalanced
commensurabilities which lead to the formation of multimer (multi-particle
bound-states) as the dominant order parameter. Luttinger liquid theory supports
a mode-locking mechanism in which mass (or velocity) asymmetry is identified as
the key ingredient to stabilize such states. While the scenario is valid both
in the continuum and on a lattice, the effects of umklapp terms relevant for
densities commensurate with the lattice spacing are also mentioned. These ideas
are illustrated and confronted with the physics of the asymmetric
(mass-imbalanced) fermionic Hubbard model with attractive interactions and
densities such that a trimer phase can be stabilized. Phase diagrams are
computed using density-matrix renormalization group techniques, showing the
important role of the total density in achieving the novel phase. The effective
physics of the trimer gas is as well studied. Lastly, the effect of a parabolic
confinement and the emergence of a crystal phase of trimers are briefly
addressed. This model has connections with the physics of imbalanced
two-component fermionic gases and Bose-Fermi mixtures as the latter gives a
good phenomenological description of the numerics in the strong-coupling
regime.Comment: 17 pages, 15 figure
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