426 research outputs found
Interaction versus dimerization in one-dimensional Fermi systems
In order to study the effect of interaction and lattice distortion on quantum
coherence in one-dimensional Fermi systems, we calculate the ground state
energy and the phase sensitivity of a ring of interacting spinless fermions on
a dimerized lattice. Our numerical DMRG (Density Matrix Renormalization Group)
studies, in which we keep up to 1000 states for systems of about 100 sites, are
supplemented by analytical considerations using bosonization techniques. We
find a delocalized phase for an attractive interaction, which differs from that
obtained for random lattice distortions. The extension of this delocalized
phase depends strongly on the dimerization induced modification of the
interaction. Taking into account the harmonic lattice energy, we find a
dimerized ground state for a repulsive interaction only. The dimerization is
suppressed at half filling, when the correlation gap becomes large.Comment: EPJ-style, 8 pages including 12 figures, to be published in EPJ
Persistent current induced by magnetic impurities
We calculate the average persistent current in a normal conducting,
mesoscopic ring in the diffusive regime. In the presence of magnetic
impurities, a contribution to the persistent current is identified, which is
related to fluctuations in the electron spin density. Assuming a spin-flip
scattering rate which is comparable to the Thouless energy E_c and low
temperature, this new contribution to the persistent current is of the order
, which is considerably larger than the persistent
current induced by the electron-electron interaction.Comment: 19 pages, 7 figures, accepted by Z. Phys.
Comment on "Magnetic response of Disordered Metallic Rings: Large Contributions of Far Levels"
Comment on cond-mat/0205390; PRL 90, 026805 (2003
Persistent Currents versus Phase Breaking in Mesoscopic Metallic Samples
Persistent currents in mesoscopic normal metal rings represent, even a decade
after their first experimental observation, a challenge to both, theorists and
experimentalists. After giving a brief review of the existing -- experimental
and theoretical -- results, we concentrate on the (proposed) relationship of
the size of the persistent current to the phase breaking rate. In particular,
we consider effects induced by noise, scattering at two-level systems, and
magnetic impurities.Comment: accepted by JLT
Multi-terminal far-from-equilibrium thermoelectric nano-devices in the Kondo regime
The quest for good thermoelectric materials and/or high-efficiency
thermoelectric devices is of primary importance from theoretical and practical
points of view. Low-dimensional structures with quantum dots or molecules are
promising candidates to achieve the goal. Interactions between electrons,
far-from-equilibrium conditions and strongly non-linear transport are important
factors affecting the usefulness of the devices. This paper analyses the
thermoelectric power of a two-terminal quantum dot under large thermal and voltage biases as well as the performance of the three-terminal
system as a heat engine. To properly characterise the non-linear effects under
these conditions, two different Seebeck coefficients are introduced,
generalizing the linear response expression. The direct calculations of
thermally induced electric and heat currents show, in agreement with recent
work, that the efficiency of the thermoelectric heat engine as measured by the
delivered power is maximal far from equilibrium. Moreover, the strong Coulomb
interactions between electrons on the quantum dot are found to diminish the
efficiency at maximum power and the maximal value of the delivered power, both
in the Kondo regime and outside of it.Comment: 29 pages, 8 figure
Non-linear conductivity of charge-density-wave systems
We consider the problem of sliding motion of a charge-density-wave subject to
static disorder within an elastic medium model. Starting with a
field-theoretical formulation, which allows exact disorder averaging, we
propose a self-consistent approximation scheme to obtain results beyond the
standard large-velocity expansion. Explicit calculations are carried out in
three spatial dimensions. For the conductivity, we find a strong-coupling
regime at electrical fields just above the pinning threshold. Phase and
velocity correlation functions scale differently from the high-field regime,
and static phase correlations converge to the pinned-phase behaviour. The
sliding charge-density-wave is accompanied by narrow-band noise.Comment: 16 pages, 6 figure
Quantum Coherence in an Exactly Solvable One-dimensional Model with Defects
Using the Quantum Inverse Scattering Method we construct an integrable
Heisenberg-XXZ-model, or equivalently a model for spinless fermions with
nearest-neighbour interaction, with defects. Each defect involves three sites
with a fine tuning between nearest-neighbour and next-nearest-neighbour terms.
We investigate the finite size corrections to the ground state energy and its
dependence on an external flux as a function of a parameter ,
characterizing the strength of the defects. For intermediate values of ,
both quantities become very small, although the ground state wavefunction
remains extended.Comment: accepted by Europhys. Lett., latex, 7 pages. A postscript version
including the figures is available at:
http://www.physik.uni-augsburg.de/theo2/Publications
Thermoelectric properties of the misfit cobaltate CaCoO
The layered misfit cobaltate CaCoO, also known as
CaCoO[CoO], is a promising p-type thermoelectric oxide.
Employing density functional theory, we study its electronic structure and
determine, on the basis of Boltzmann theory within the constant-relaxation-time
approximation, the thermoelectric transport coefficients. The dependence on
strain and temperature is determined. In particular, we find that the
-component of the thermopower is strongly enhanced, while the
-component is strongly reduced, when applying 2% tensile strain. A similar
anisotropy is also found in the power factor. The temperature dependence of the
conductivity in the - plane is found to be rather weak above 200 K, which
clearly indicates that the experimentally observed transport properties are
dominated by inhomogeneities arising during sample growth, i.e., are not
intrinsic.Comment: 12 pages (preprint style), 5 figure
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