221 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
Thermodynamics and tunneling spectroscopy in the pseudogap regime of the boson fermion model
Motivated by the STM experimental data on Bi_2 Sr_2 CaCU_2 O_{8+x} which
indicate the tunneling conductance asymmetry sigma(-V) not equal sigma(V), we
report that such a behavior can be explained in terms of the boson fermion
model. It has been shown in the recent studies, based on various selfconsistent
techniques to capture the many-body effects, that the low energy spectrum of
the boson fermion model is featured by an appearance of the pseudogap at T^* >
T_c. We argue that the pseudogap structure has to exhibit a particle-hole
asymmetry. This asymmetry may eventually depend on the boson concentration.Comment: 4 pages, 2 figures. submitted to Physica
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
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
Continuous unitary transformation approach to pairing interactions in statistical physics
We apply the flow equation method to the study of the fermion systems with pairing interactions which lead
to the BCS instability signalled by the appearance of the off-diagonal order parameter. For this purpose we
rederive the continuous Bogoliubov transformation in a fashion of renormalization group procedure where the
low and high energy sectors are treated subsequently. We further generalize this procedure to the case of
fermions interacting with the discrete boson mode. Andreev-type interactions are responsible for developing
a gap in the excitation spectrum. However, the long-range coherence is destroyed due to strong quantum
fluctuations.Ми застосовуємо метод потокового рiвняння до дослiдження фермiонних систем з взаємодiями парування, що ведуть до БКШ-нестiйкостi, яка виявляється у появi недiагонального параметра порядку. З цiєю метою ми переформульовуємо неперервне перетворення Боголюбова в ренормгрупових термiнах, де низько- i високоенергетичнi сектори розглядаються послiдовно. Ми узагальнюємо цю процедуру на випадок, коли фермiони взаємодiють з дискретною бозонною модою. Взаємодiї типу Андреєва спричинюють появу щiлини у спектрi збуджень, однак далекосяжна кореляцiя руйнується завдяки сильним квантовим флуктуацiям
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
- …