Tsung-han Lin

Andreev reflection through a quantum dot coupled with two ferromagnets and a superconductor

We study the Andreev reflection (AR) in a three-terminal mesoscopic hybrid system, in which two ferromagnets (F 1 and F 2 ) are coupled to a superconductor (S) through a quantum dot. By using nonequilibrium Green function,we derive a general current formula which allows arbitrary spin polarizations, magnetization orientations, and bias voltages in F 1 and F 2 . The formula is applied to study both zero bias conductance and finite bias current. The current conducted by crossed AR involving F 1 , F 2 , and S is particularly unusual, in which an electron with spin σ incident from one of the ferromagnets picks up another electron with spin σ from the other one, both enter S and form a Cooper pair. Several special cases are investigated to reveal the properties of AR in this system.

Excess Kondo Resonance in a Quantum Dot Device with Normal and Superconducting Leads: The Physics of Andreev-Normal Co-tunneling

We report on a novel Kondo phenomenon of interacting quantum dots coupled asymmetrically to a normal and a superconducting lead. The effects of intradot Coulomb interaction and Andreev tunneling give rise to Andreev bound resonances. As a result, a new type of co-tunneling process, which we term Andreev-normal co-tunneling, is predicted. At low temperatures, coherent superposition of these co-tunneling processes induces a Kondo effect in which Cooper pairs directly participate formation of a spin singlet, leading to four Kondo resonance peaks in the local density of states, and enhancing the tunneling current.

Photon-assisted Fano resonance and corresponding shot noise in a quantum dot

We have studied the Fano resonance in photon-assisted transport through a quantum dot. Both the coherent current and the spectral density of shot noise have been calculated. It is predicted that the shape of the Fano profile will also appear in satellite peaks. It is found that the variations of Fano profiles with the strengths of nonresonant transmissions are not synchronous in absorption and emission sidebands. The effect of interference on photon-assisted pumped current has also been investigated. We further predict the current and spectral density of shot noise as a periodic function of the phase, which exhibits an intrinsic property of resonant and nonresonant channels in the structures.

Nonlinear transport theory for hybrid normal-superconducting devices

We report a theory for analyzing nonlinear DC transport properties of mesoscopic or nanoscopic normalsuperconducting ~NS! systems. Special attention was paid such that our theory satisfies gauge invariance. At the nonlinear regime, our theory allows the investigation of a number of important problems: for NS hybrid systems we have derived the general nonlinear current-voltage characteristics in terms of the scattering Green’s function, the second-order nonlinear conductance at the weakly nonlinear regime, and nonequilibrium charge pileup in the device that defines the electrochemical capacitance coefficients.

Nonequilibrium transport through a quantum dot weakly coupled to Luttinger liquids

We study the nonequlibrium transport through a quantum dot weakly coupled to Luttinger liquids (LL). A general current expression is derived by using nonequilibrium Green function method. Then a special case of the dot with only a single energy level is discussed. As a function of the dots energy level, we find that the current as well as differential conductance is strongly renormalized by the interaction in the LL leads. In comparison with the system with Fermi liquid (FL) leads, the current is suppressed, consistent with the suppression of the electron tunneling density of states of the LL; and the outset of the resonant tunneling is shifted to higher bias voltages. Besides, the linear conductance obtained by Furusaki using master equation can be reproduced from our result.

Microwave-induced π-junction transition in a superconductor/quantum dot/superconductor structure

Using the nonequilibrium Green function, we show that microwave irradiation can reverse the supercurrent flowing through a superconductor/quantum dot/superconductor structure. In contrast with the conventional sideband effect in normal-metal/quantum dot/normal-metal junctions, the photon-assisted structures appear near

Probing spin states of coupled quantum dots by a dc Josephson current

{E}_{0}=(n/2)\ensuremath{\Elzxh}\ensuremath{\omega}(n=\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}),

Extraordinary temperature dependence of the resonant Andreev reflection

where

ac Josephson effect in resonant tunneling through mesoscopic superconducting junctions

{E}_{0}

Submergence of the sidebands in the photon-assisted tunneling through a quantum dot weakly coupled to Luttinger liquid leads

is the resonant energy level of the quantum dot and \ensuremath{\omega} is the frequency of microwave field. Each photon-assisted structure is composed of a negative and a positive peak, with an abrupt jump from the negative peak to the positive peak around