Transport through a double quantum dot in the sequential- and co- tunneling regimes

We study transport through a double quantum dot, both in the sequential tunneling and cotunneling regimes. Using a master equation approach, we find that, in the sequential tunneling regime, the differential conductance $G$ as a function of the bias voltage $\Delta\mu$ has a number of satellite peaks with respect to the main peak of the Coulomb blockade diamond. The position of these peaks is related to the interdot tunnel splitting and the singlet-triplet splitting. We find satellite peaks with both {\em positive} and {\em negative} values of differential conductance for realistic parameter regimes. Relating our theory to a microscopic (Hund-Mulliken) model for the double dot, we find a temperature regime for which the Hubbard ratio (=tunnel coupling over on-site Coulomb repulsion) can be extracted from $G(\Delta\mu)$ in the cotunneling regime. In addition, we consider a combined effect of cotunneling and sequential tunneling, which leads to new peaks (dips) in $G(\Delta\mu)$ inside the Coulomb blockade diamond below some temperature scales, which we specify.Comment: 16 pages, 10 figure

Revealing the magnetic proximity effect in EuS/Al bilayers through superconducting tunneling spectroscopy

A ferromagnetic insulator attached to a superconductor is known to induce an exchange splitting of the Bardeen-Cooper-Schrieffer (BCS) singularity by a magnitude proportional to the magnetization, and penetrating into the superconductor to a depth comparable with the superconducting coherence length. We study this long-range magnetic proximity effect in EuS/Al bilayers and find that the exchange splitting of the BCS peaks is present already in the unpolarized state of the ferromagnetic insulator (EuS), and is being further enhanced when magnetizing the sample by a magnetic field. The measurement data taken at the lowest temperatures feature a high contrast which has allowed us to relate the line shape of the split BCS conductance peaks to the characteristic magnetic domain structure of the EuS layer in the unpolarized state. These results pave the way to engineering triplet superconducting correlations at domain walls in EuS/Al bilayers. Furthermore, the hard gap and clear splitting observed in our tunneling spectroscopy measurements indicate that EuS/Al bilayers are excellent candidates for substituting strong magnetic fields in experiments studying Majorana bound states.Comment: 9 pages, 4 color figure

Proposal for a phonon laser utilizing quantum-dot spin states

We propose a nanoscale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators. © 2013 American Physical Society.A. K. acknowledges financial support from the SPINMET Project (FP7-PEOPLE-2009-IRSES). The work was supported by the ONR, DOE-BES DESC0004890, and NSF-ECCS. X. Hu also acknowledges support by the U.S. ARO and NSF-PIF. The work of V. N. G. was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02.Peer Reviewe

Theory of Spin Hall Magnetoresistance from a Microscopic Perspective

We present a theory of the spin Hall magnetoresistance of metals in contact with magnetic insulators. We express the spin-mixing conductances, which govern the phenomenology of the effect, in terms of the microscopic parameters of the interface and the spin-spin correlation functions of the local moments on the surface of the magnetic insulator. The magnetic field and temperature dependence of the spin-mixing conductances leads to a rich behaviour of the resistance due to an interplay between the Hanle effect and spin mixing at the interface. Our theory provides a useful tool for understanding the experiments on heavy metals in contact with magnetic insulators of different kinds, and it predicts striking behaviours of magnetoresistance.Comment: 8 pages, four figure

Phase-Controllable Nonlocal Spin Polarization in Proximitized Nanowires

We study the magnetic and superconducting proximity effects in a semiconducting nanowire (NW) attached to superconducting leads and a ferromagnetic insulator (FI). We show that a sizable equilibrium spin polarization arises in the NW due to the interplay between the superconducting correlations and the exchange field in the FI. The resulting magnetization has a nonlocal contribution that spreads in the NW over the superconducting coherence length and is opposite in sign to the local spin polarization induced by the magnetic proximity effect in the normal state. For a Josephson-junction setup, we show that the nonlocal magnetization can be controlled by the superconducting phase bias across the junction. Our findings are relevant for the implementation of Majorana bound states in state-of-the-art hybrid structures.Comment: 12 pages, 5 figure

SiGe quantum dots for fast hole spin Rabi oscillations

We report on hole g-factor measurements in three terminal SiGe self-assembled quantum dot devices with a top gate electrode positioned very close to the nanostructure. Measurements of both the perpendicular as well as the parallel g-factor reveal significant changes for a small modulation of the top gate voltage. From the observed modulations we estimate that, for realistic experimental conditions, hole spins can be electrically manipulated with Rabi frequencies in the order of 100MHz. This work emphasises the potential of hole-based nano-devices for efficient spin manipulation by means of the g-tensor modulation technique

Phonon-induced decay of the electron spin in quantum dots

We study spin relaxation and decoherence in a GaAs quantum dot due to spin-orbit interaction. We derive an effective Hamiltonian which couples the electron spin to phonons or any other fluctuation of the dot potential. We show that the spin decoherence time $T_2$ is as large as the spin relaxation time $T_1$, under realistic conditions. For the Dresselhaus and Rashba spin-orbit couplings, we find that, in leading order, the effective magnetic field can have only fluctuations transverse to the applied magnetic field. As a result, $T_2=2T_1$ for arbitrarily large Zeeman splittings, in contrast to the naively expected case $T_2\ll T_1$. We show that the spin decay is drastically suppressed for certain magnetic field directions and values of the Rashba coupling constant. Finally, for the spin coupling to acoustic phonons, we show that $T_2=2T_1$ for all spin-orbit mechanisms in leading order in the electron-phonon interaction.Comment: 5 pages, 1 figur

The readout of the fullerene-based quantum computing by a scanning tunneling microscope

We consider to detect the electron spin of a doped atom, i.e., a nitrogen or a phosphorus, caged in a fullerene by currently available technique of the scanning tunneling microscope (STM), which actually corresponds to the readout of a qubit in the fullerene-based quantum computing. Under the conditions of polarized STM current and Coulomb blockade, we investigate the tunneling matrix elements involving the exchange coupling between the tunneling polarized electrons and the encapsulated polarized electron, and calculate the variation of the tunneling current with respect to different orientations of the encapsulated electron spin. The experimental feasibility of our scheme is discussed under the consideration of some imperfect factors.Comment: RevTex file, 3 figures. To appear in New Journal of Physic

Nucleon Resonance Structure from CLAS and CLAS12 Experiments

The recent results on the photo- and electrocouplings extraction from the reaction of one- and two-pion photo- and electroproduction off protons in the resonances region are presented. The production of two charged pions is of particular importance for evaluation of the photocouplings for the Δ(1620)12 −, Δ(1700)32 −