Detecting a true quantum pump effect

Even though quantum pumping is a very promising field, it has unfortunately not been unambiguously experimentally detected. The reason being that in the experiments the rectification effect overshadows the pumped current. One of the better known ways to detect it is by using the magnetic field symmetry properties of the rectified and pumped currents. The rectified currents are symmetric with respect to magnetic field reversal while the pumped currents do not possess any definite symmetry with respect to field reversal. This feature has been exploited in some recent works. In this work we look beyond this magnetic field symmetry properties and provide examples wherein the nature or magnitudes of the pumped and rectified currents are exactly opposite enabling an effective distinction between the two.Comment: 8 pages, 8 figures, accepted for publication in European Physical Journal

Strain designed Josephson π\pi junction qubits with topological insulators

A Josephson qubit is designed via the application of a tensile strain to a topological insulator surface sandwiched between two s-wave superconductors. The strain applied leads to a shift in the Dirac point without changing the pre-existing conducting states, on the surface of a topological insulator. Strain applied can be tuned to form a $\pi$ junction in such a structure. Combining two such junctions in a ring architecture leads to the ground state of the ring being in doubly degenerate state- the "0" and "1" states of a qubit. A qubit designed this way is quite easily controlled via the tunable strain applied. We report on the conditions necessary to design such a qubit. Finally the operating time of a single qubit phase gate is derived.Comment: 8 pages and 8 figures with supplementary materia

Can dephasing generate non-local spin correlations?

By examining the full counting statistics of a non adiabatic pure spin pump with particular emphasis on the second and third moments, it is shown that incoherent or sequential transport, in contrast to coherent transport, can change non-local spin shot noise cross-correlations from being anti-correlated to being completely correlated, a truly counterintuitive result. The third moment on the other hand is shown to be much more resilient and its nature remains unaltered in incoherent transport regime. However, phenomenologically including dephasing modifies this picture as both Shot noise and more so the third moment are non-trivially affected. In fact non-local spin correlations are completely positive for maximal dephasing.Comment: arXiv admin note: substantial text overlap with arXiv:0808.363

Playing a true Parrondo's game with a three state coin on a quantum walk

Playing a Parrondo's game with a qutrit is the subject of this paper. We show that a true quantum Parrondo's game can be played with a 3 state coin(qutrit) in a 1D quantum walk in contrast to the fact that playing a true Parrondo's game with a 2 state coin(qubit) in 1D quantum walk fails in the asymptotic limits.Comment: 7 pages, 9 figures, accepted for publication in EPL (Euro Physics Letters

Crossed Andreev reflection as a probe for the pairing symmetry of Ferromagnetic Superconductors

The coexistence of superconductivity and ferromagnetism has brought about the phenomena of ferromagnetic superconductors. The theory needed to understand the compatibility of such antagonistic phenomena cannot be built until the pairing symmetry of such superconductors is correctly identified. The proper and unambiguous identification of the pairing symmetry of such superconductors is the subject of this paper. This work shows that crossed Andreev reflection can be a very effective tool in order to identify the pairing symmetry of these superconductors.Comment: 5 pages, 1 figure, 1 table. Accepted for publication in Phys. Rev. B(Rapid Communication

Role of helical edge modes in the chiral quantum anomalous Hall state

Although indications are that a single chiral quantum anomalous Hall(QAH) edge mode might have been experimentally detected. There have been very many recent experiments which conjecture that a single chiral QAH edge mode always materializes along with a pair of quasi-helical quantum spin Hall (QSH) edge modes. The reason for this seems to lie in the origin of QAH edge modes. These evolve from QSH edge modes via suppression of one of the spin edge modes by application of a ferromagnet or magnetic impurity. In this work we deal with a substantial 'What If ?' question- in case the QSH edge modes, from which these QAH edge modes evolve, are not topologically protected then the QAH edge modes wont be topologically protected too and thus unfit for use in any applications. Further, as a corollary one can also ask if the topological protection of QSH edge modes does not carry over during the evolution process to QAH edge modes then again our 'What if?' scenario becomes apparent. The "how" of the resolution of this 'What if?' conundrum is the main objective of our work. We show in similar set-ups affected by disorder and inelastic scattering, transport via trivial QAH edge mode leads to quantization of Hall resistance and not that via topological QAH edge modes. This perhaps begs a substantial reinterpretation of those experiments which purported to find signatures of chiral(topological) QAH edge modes albeit in conjunction with quasi helical QSH edge modes.Comment: 16 pages, 8 figures, accepted for publication in Scientific Report

Characterizing a high spin magnetic impurity via Andreev reflection spectroscopy

The ground state properties of a high spin magnetic impurity and its interaction with an electronic spin are probed via Andreev reflection. We see that through the charge and spin conductance one can effectively estimate the interaction strength, the ground state spin and magnetic moment of any high spin magnetic impurity. We show how a high spin magnetic impurity at the junction between a normal metal and superconductor can contribute to superconducting spintronics applications. Particularly, while spin conductance is absent below the gap for Ferromagnet-Insulator-Superconductor junctions we show that in the case of a Normal metal-High spin magnetic impurity-Normal Metal-Insulator-Superconductor (NMNIS) junction it is present. Further, it is seen that pure spin conduction can exist without any accompanying charge conduction in the NMNIS junction.Comment: 19 pages, 13 Figures, accepted for publication in The European Physical Journal