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 $T_{c}$ 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