Enhanced negative nonlocal conductance in an interacting quantum dot connected to two ferromagnetic leads and one superconducting lead

In this paper, we investigate the electronic transport properties of a quantum dot (QD) connected to two ferromagnetic leads and one superconductor lead in the Kondo regime by means of the finite-$U$ slave boson mean field approach and nonequilibrium Green function technique. In this three-terminal hybrid nano-device, we will focus our attention on the joint effects of the Konod correlation, superconducting proximity pairing, and spin polarization of leads. It is found that: the superconducting proximity effect will suppress the linear local conductance (LLC) stemming from the weakened Kondo peak, and when its coupling $\Gamma_s$ is bigger than the tunnel-coupling $\Gamma$ of two normal leads, the linear cross conductance (LCC) becomes negative in the Kondo region; for antiparallel configuration, increasing spin polarization further suppresses LLC but enhances LCC, i.e. causing larger negative values of LCC, since it is benefit for emergence of cross Andreev reflection; On the contrary, for parallel configuration, with increasing spin polarization, the LLC descends and greatly widens with the appearance of shoulders, and eventually splits into four peaks, and meanwhile the LCC reduces relatively rapidly to the normal conductance.Comment: 9 pages, 6 figure

Enhanced current noise correlations in a Coulomb-Majorana device

Majorana bound states (MBSs) nested in a topological nanowire are predicted to manifest nonlocal correlations in the presence of a finite energy splitting between the MBSs. However, the signal of the nonlocal correlations has not yet been detected in experiments. A possible reason is that the energy splitting is too weak and seriously affected by many system parameters. Here we investigate the charging energy induced nonlocal correlations in a hybrid device of MBSs and quantum dots. The nanowire that hosts the MBSs is assumed in proximity to a mesoscopic superconducting island with a finite charging energy. Each end of the nanowire is coupled to one lead via a quantum dot with resonant levels. With a floating superconducting island, the devices shows a negative differential conductance and giant super-Poissonian shot noise, due to the interplay between the nonlocality of the MBSs and dynamical Coulomb blockade effect. When the island is strongly coupled to a bulk superconductor, the current cross correlations at small lead chemical potentials are negative by tuning the dot energy levels. In contrast, the cross correlation is always positive in a non-Majorana setup. This difference may provide a signature for the existence of the MBSs.Comment: 11 pages, 10 figure

Charge transfer statistics and entanglement in normal-quantum dot-superconductor hybrid structures

We analyze the full counting statistics (FCS) of a single-site quantum dot coupled to multiple metallic electrodes in the normal state and a superconductor for arbitrary transmission. We present an analytical solution of the problem taking into account the full energy dependence of the transmission coefficient. We identify two transport processes as sources of entanglement between the current carriers by observing positive cross current correlations. Furthermore, we consider ferromagnetic electrodes and show how they can be used as detectors in experiments violating the Bell-Clauser-Horne-Shimony-Holt inequality.Comment: 6 pages, 3 figures, typos fixed, references adde

Correlation effects in transport through quantum dot systems

Wydział FizykiPrzedstawiona praca doktorska dotyczy teoretycznych badań transportu ładunkowego i spinowego przez układy sprzężonych kropek kwantowych. Rozważane struktury wykazują wiele interesujących własności fizycznych, wynikających z różnych efektów i korelacji kwantowych. Dysertacja skupiona jest na różnych efektach korelacji i ich wpływie na transport przez układy kropek kwantowych. W szczególności przebadano efekty kwantowej interferencji odpowiedzialne za pojawianie się tzw. stanów ciemnych w układach kropek kwantowych o trójkątnej geometrii. Ponadto, rozważony został transport przez tzw. rozdzielacze par Coopera oparte na kropkach kwantowych. Efekt bliskości nadprzewodnika indukuje parujące korelacje w rozważanym układzie, co prowadzi do powstawania tzw. stanów związanych Andriejewa. Transport przez te stany został kompleksowo przeanalizowany ze szczególnym naciskiem na badanie wzajemnych korelacji prądu w celu optymalizacji parametrów układu sprzyjających wysokiej skuteczności rozdzielania.The PhD thesis concerns theoretical studies of charge and spin transport through the systems consisting of coupled quantum dots attached to external leads. The considered structures exhibit many interesting physical properties, not observed in bulk materials, resulting from various quantum effects and correlations, as well as different geometrical arrangements. The focus of this dissertation is on various correlation effects and their influence on transport through quantum dot systems. In particular, the quantum interference effects in triangular quantum dots responsible for the appearance of dark states are extensively studied. Moreover, an important class of considered systems are quantum dot based Cooper pair splitters. The proximity of superconductor induces pairing correlations in quantum dots and gives rise to the formation of Andreev bound states for energies smaller than the superconducting energy gap. The Andreev transport is analyzed with an emphasis on the current cross-correlations in order to optimize the parameters for obtaining high Cooper pair splitting efficiency.The financial support from the Polish National Science Centre awarded through Grant Theoretical studies of transport in magnetic nanostructures No. DEC-2013/10/E/ST3/00213 is acknowledge