Yao-Ming Shi

Spin-dependent Andreev reflection tunneling through a quantum dot with intradot spin-flip scattering

We study Andreev reflection tunneling through a ferromagnet\char21{}quantum dot\char21{}superconductor system. The intradot spin-flip interaction is considered. By using the nonequilibrium Green function method, an expression for the linear Andreev reflection conductance is derived at zero temperature. It is found that competition between the intradot spin-flip scattering and the tunneling coupling to the leads dominates the resonant behaviors of the Andreev reflection conductance versus the gate voltage. A weak spin-flip scattering leads to a single-peak resonance. However, with the spin-flip scattering strength increasing, the Andreev reflection conductance will develop into a double-peak resonance indicating a novel structure in the conductance tunneling spectrum. Besides, the influences of the spin-dependent tunneling couplings, the Fermi velocity matching condition, and the spin polarization of the ferromagnet on the conductance are examined in detail.

Spin-dependent transmission through a mesoscopic ring with a quantum gate

Abstract We investigate the spin-dependent transmission through an Aharonov–Bohm ring with a quantum gate that is tuned by an uniform external magnetic field. The formula of spin-dependent transmission coefficient at zero temperature is obtained as a function of the flux, the magnetic field and Fermi energy in term of quantum waveguide theory. We found that for some special Fermi energies, spin-state electrons are driven into a perfect transmissive state or reflective state, which is not affected by the flux, when Zeeman energy of electron moving in the stub coincides with a level of the isolated stub. As Zeeman energy crosses the level of the stub, Aharonov–Bohm oscillations of spin-state conductance have no abrupt change of phase by π and are in phase. The effect of the magnetic field on transmission behavior of spin-state electrons are examined.

ANTIRESONANCES MODULATED BY MAGNETIC FLUX IN AN AHARONOV-BOHM RING

Abstract With or without an inserted quantum dot or an attached side resonator, the transmission behavior of an Aharonov–Bohm (AB) ring is investigated in the framework of quantum waveguide theory in the complex-energy plane. The calculated results show that the AB ring possesses antiresonances in the conductance and several kinds of transmission poles in the complex-energy plane. Piercing the ring, magnetic flux causes a series of crossovers among zero-pole pairs, zero-double-pole pairs, peak-double-pole pairs, and peak-pole pairs. With the inserted quantum dot or attached side resonator in one of its arms, the ring produces complicated behaviors in conductance: some of the poles are split into more than two parts. A kind of deformed Fano resonance is found in the conductance of the AB ring with the attached resonator.

Andreev reflection resonant tunneling through a precessing spin

Abstract We investigated Andreev reflection (AR) resonant tunneling through a precessing spin which is coupled to a normal metallic lead and a superconducting lead. The formula of the AR conductance at zero temperature is obtained as a function of chemical potential and azimuthal angle of the spin precessing by using the nonequilibrium Green function method. It is found that as the local spin precesses in a weak external magnetic field at Larmor frequency ω l , the AR tunneling conductance exhibits an oscillation at the frequency 2 ω l alone. The amplitude of AR conductance oscillation enhances with spin–flip tunneling coupling increasing. The study also shows that spin–orbit interaction in tunneling barriers is crucial for the oscillations of AR conductance. The effect of local spin precessing and spin–flip tunneling coupling caused by spin–orbit interaction on the resonant behavior of the AR conductance is examined.