61 research outputs found
Influence of the pair coherence on the charge tunneling through a quantum dot connected to a superconducting lead
We analyze the charge transport through a single level quantum dot coupled to
a normal (N) and superconducting (S) leads where the electron pairs exist
either as the coherent (for temperatures below T_c) or incoherent objects (in a
region T_c < T < T*). This situation can be achieved in practice if one uses
the high T_c superconducting material where various precursor effects have been
observed upon approaching from above. Without restricting to any
particular microscopic mechanism we investigate some qualitative changes of the
nonequilibrium charge current caused by the electron pair coherence.Comment: 7 pages, 9 figure
Meservey-Tedrow-Fulde effect in a quantum dot embedded between metallic and superconducting electrodes
Magnetic field applied to the quantum dot coupled between one metallic and
one superconducting electrode can produce a similar effect as has been
experimentally observed by Meservey, Tedrow and Fulde [Phys. Rev. Lett. 25,
1270 (1970)] for the planar normal metal -- superconductor junctions. We
investigate the tunneling current and show that indeed the square root
singularities of differential conductance exhibit the Zeeman splitting near the
gap edge features V = +/- Delta/e. Since magnetic field affects also the in-gap
states of quantum dot it furthermore imposes a hyperfine structure on the
anomalous (subgap) Andreev current which has a crucial importance for a
signature of the Kondo resonance.Comment: 7 pages, 8 figure
Spin-Orbit Coupling and Symmetry of the Order Parameter in Strontium Ruthenate
Determination of the orbital symmetry of a state in spin triplet
SrRuO superconductor is a challenge of considerable importance. Most of
the experiments show that the chiral state of the type
is realized and remains stable on lowering the temperature. Here we have
studied the stability of various superconducting states of SrRuO in the
presence of spin-orbit coupling.
Numerically we found that the chiral state is never the minimum energy. Alone
among the five states studied it has and is
therefore not affected to linear order in the coupling parameter . We
found that stability of the chiral state requires spin dependent pairing
interactions. This imposes strong constraint on the pairing mechanism.Comment: 4 pages, 4 figure
Thermoelectric phenomena in a quantum dot asymmetrically coupled to external leads
We study thermoelectric phenomena in a system consisting of strongly
correlated quantum dot coupled to external leads in the Kondo regime. We
calculate linear and nonlinear electrical and thermal conductance and
thermopower of the quantum dot and discuss the role of asymmetry in the
couplings to external electrodes. In the linear regime electrical and thermal
conductances are modified, while thermopower remains unchanged. In the
nonlinear regime the Kondo resonance in differential conductance develops at
non-zero source-drain voltage, which has important consequences on
thermoelectric properties of the system and the thermopower starts to depend on
the asymmetry. We also discuss Wiedemann-Franz relation, thermoelectric figure
of merit and validity of the Mott formula for thermopower.Comment: 6 pages, 7 figure
Magnetic field induced rotation of the d-vector in the spin triplet superconductor SrRuO
In zero magnetic field the superconductor SrRuO is believed to have a
chiral spin triplet pairing state in which the gap function d-vector is aligned
along the crystal c-axis. Using a phenomenological but orbital specific
description of the spin dependent electron-electron attraction and a realistic
quantitative account of the electronic structure in the normal state we analyze
the orientation of the spin triplet Cooper pair d-vector in response to an
external c-axis magnetic field. We show that for suitable values of the model
parameters a c-axis field of only 20 mT is able to cause a reorientation phase
transition of the d-vector from along to the plane, in agreement with
recent experiments.Comment: 6 pages, 7 figures. Submitted Phys Rev
Interlayer Coupling and p-wave Pairing in Strontium Ruthenate
On the basis of a three orbital model and an effective attractive interaction
between electrons we investigate the possible superconducting states, with
and -wave internal symmetry, of SrRuO. For an orbital dependent
interaction which acts between in plane and out of plane nearest neighbour
Ruthenium atoms we find a state for which the gap in the quasi-particle spectra
has a line node on the and sheets of the Fermi Surface, but
it is complex with no nodes on the -sheet. We show that this state is
consistent with all the available experimental data. In particular, we present
the results of our calculations of the specific heat and penetration depth as
functions of the temperature.Comment: 4 pages, 5 figure
Extension of Frohlich's method to 4-fermion interactions
Higher order terms of the transformed electron-phonon Hamiltonian, obtained
by performing the Frohlich's transformation, are investigated. The influence of
terms discarded by Frohlich (in particular those proportional to the third
power of electron-phonon coupling) on the effective Hamiltonian is examined. To
this end a second Frohlich-type transformation is performed, which yields,
among others, an effective 4-electron interaction. This interaction is reduced
to a form admitting solution of thermodynamics. The form of the coupling of the
4-electron interaction is found. By applying standard approximations, it is
shown that this interaction is attractive with interaction coupling given by -
D_{k_F}^6 / \omega_{k_F}^5, where D_{k} is electron-phonon coupling,
\omega_{k}$ is phonon energy and k_F is Fermi momentum.
The form of higher order terms of the original Frohlich-transformed H_{e-ph}
are also found, up to terms proportional to the 6-th power of the coupling,
that is up to those, which yield the effective 4-electron interactions.Comment: REVTeX4, 25 pages; major changes: added section and appendix about
the form of 4-fermion interaction coupling, typos correcte
Effect of disorder on superconductivity in the boson-fermion model
We study how a randomness of either boson or fermion site energies affects
the superconducting phase of the boson fermion model. We find that, contrary to
what is expected for s-wave superconductors, the non-magnetic disorder is
detrimental to the s-wave superconductivity. However, depending in which
subsystem the disorder is located, we can observe different channels being
affected. Weak disorder of the fermion subsystem is responsible mainly for
renormalization of the single particle density of states while disorder in the
boson subsystem directly leads to fluctuation of the strength of the effective
pairing between fermions.Comment: 7 pages, 6 figures. Physical Review B (accepted for publication
Density expansion for transport coefficients: Long-wavelength versus Fermi surface nonanalyticities
The expansion of the conductivity in 2-d quantum Lorentz models in terms of
the scatterer density n is considered. We show that nonanalyticities in the
density expansion due to scattering processes with small and large momentum
transfers, respectively, have different functional forms. Some of the latter
are not logarithmic, but rather of power-law nature, in sharp contrast to the
3-d case. In a 2-d model with point-like scatterers we find that the leading
nonanalytic correction to the Boltzmann conductivity, apart from the frequency
dependent weak-localization term, is of order n^{3/2}.Comment: 4 pp., REVTeX, epsf, 3 eps figs, final version as publishe
Real space inhomogeneities in high temperature superconductors: the perspective of two-component model
The two-component model of high temperature superconductors in its real space
version has been solved using Bogoliubov-de Gennes equations. The disorder in
the electron and boson subsystem has been taken into account. It strongly
modifies the superconducting properties and leads to local variations of the
gap parameter and density of states. The assumption that the impurities mainly
modify boson energies offers natural explanation of the puzzling positive
correlation between the positions of impurities and the values of the order
parameter found in the scanning tunnelling microscopy experiments.Comment: 19 pages, IOPP style include
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