262 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
Flow equation approach to the linear response theory of superconductors
We apply the flow equation method for studying the current-current response
function of electron systems with the pairing instability. To illustrate the
specific scheme in which the flow equation procedure determines the
two-particle Green's functions we reproduce the standard response kernel of the
BCS superconductor. We next generalize this non-perturbative treatment
considering the pairing field fluctuations. Our study indicates that the
residual diamagnetic behavior detected above the transition temperature in the
cuprate superconductors can originate from the noncondensed preformed pairs.Comment: 12 pages, 4 figure
Unconventional particle-hole mixing in the systems with strong superconducting fluctuations
Development of the STM and ARPES spectroscopies enabled to reach the
resolution level sufficient for detecting the particle-hole entanglement in
superconducting materials. On a quantitative level one can characterize such
entanglement in terms of the, so called, Bogoliubov angle which determines to
what extent the particles and holes constitute the spatially or momentum
resolved excitation spectra. In classical superconductors, where the phase
transition is related to formation of the Cooper pairs almost simultaneously
accompanied by onset of their long-range phase coherence, the Bogoliubov angle
is slanted all the way up to the critical temperature Tc. In the high
temperature superconductors and in superfluid ultracold fermion atoms near the
Feshbach resonance the situation is different because of the preformed pairs
which exist above Tc albeit loosing coherence due to the strong quantum
fluctuations. We discuss a generic temperature dependence of the Bogoliubov
angle in such pseudogap state indicating a novel, non-BCS behavior. For
quantitative analysis we use a two-component model describing the pairs
coexisting with single fermions and study their mutual feedback effects by the
selfconsistent procedure originating from the renormalization group approach.Comment: 4 pages, 4 figure
Pairing of bosons in the condensed state of the boson-fermion model
A two component model of negative U centers coupled with the Fermi sea of
itinerant fermions is discussed in connection with high-temperature
superconductivity of cuprates, and superfluidity of atomic fermions. We examine
the phase transition and the condensed state of this boson-fermion model (BFM)
beyond the ordinary mean-field approximation in two and three dimensions. No
pairing of fermions and no condensation are found in two-dimensions for any
symmetry of the order parameter. The expansion in the strength of the order
parameter near the transition yields no linear homogeneous term in the
Ginzburg-Landau-Gor'kov equation and a zero upper critical field in
any-dimensional BFM, which indicates that previous mean-field discussions of
the model are flawed. Normal and anomalous Green's functions are obtained
diagrammatically and analytically in the condensed state of a simplest version
of 3D BFM. A pairing of bosons analogous to the Cooper pairing of fermions is
found. There are three coupled condensates in the model, described by the
off-diagonal single-particle boson, pair-fermion and pair-boson fields. These
results negate the common wisdom that the boson-fermion model is adequately
described by the BCS theory at weak coupling.Comment: 7 pages, 4 figure
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