Nie Yi-Hang

Spin-dependent thermoelectric transport through double quantum dots

We study the thermoelectric transport through a double-quantum-dot system with spin-dependent interdot coupling and ferromagnetic electrodes by means of the non-equilibrium Greens function in the linear response regime. It is found that the thermoelectric coefficients are strongly dependent on the splitting of the interdot coupling, the relative magnetic configurations, and the spin polarization of leads. In particular, the thermoelectric efficiency can reach a considerable value in the parallel configuration when the effective interdot coupling and the tunnel coupling between the quantum dots and the leads for the spin-down electrons are small. Moreover, the thermoelectric efficiency increases with the intradot Coulomb interaction increasing and can reach very high values at appropriate temperatures. In the presence of the magnetic field, the spin accumulation in the leads strongly suppresses the thermoelectric efficiency, and a pure spin thermopower can be obtained.

Transport through artificial single-molecule magnets: Spin-pair state sequential tunneling and Kondo effects

The transport properties of an artificial single-molecule magnet based on a CdTe quantum dot doped with a single Mn+2 ion (S=5/2) are investigated by the non-equilibrium Green function method. We consider a minimal model where the Mn—hole exchange coupling is strongly anisotropic so that spin-flip is suppressed and the impurity spin S and a hole spin s entering the quantum dot are coupled into spin pair states with (2S+1) sublevels. In the sequential tunneling regime, the differential conductance exhibits (2S+1) possible peaks, corresponding to resonance tunneling via (2S+1) sublevels. At low temperature, Kondo physics dominates transport and (2S+1) Kondo peaks occur in the local density of states and conductance. These peaks originate from the spin-singlet state formed by the holes in the leads and on the dot via higher-order processes and are related to the parallel and antiparallel spin pair states.

Electronic transport through a periodic array of quantum-dot rings

Using the tight-binding approximation and the transfer matrix method, this paper studies the electronic transport properties through a periodic array of quantum-dot (QD) rings threaded by a magnetic flux. It demonstrates that the even-odd parity of the QD number in a single ring and the number of the QD rings in the array play a crucial role in the electron transmission. For a single QD ring, the resonance and antiresonance transmission depend not only on the applied magnetic flux but also on the difference between the number of QDs on the two arms of the ring. For an array of QD rings, the transmission properties are related not only to the even–odd parity of the number N0 of QDs in the single ring but also to the even–odd parity of the ring number N in the array. When the incident electron energy is aligned with the site energy, for the array of N rings with N0 = odd the antiresonance transmission cannot occur but the resonance transmission may occur and the transmission spectrum has N resonance peaks (N − 1 resonance peaks) in a period for N = odd (for N = even). For the array of N rings with N0 = even the transmission properties depend on the flux threading the ring and the QD number on one arm of the ring. These results may be helpful in designing QD devices.

Phonon-Assisted Spin Current in Single Molecular Magnet Junctions

Owing to the bistable character of the single molecular magnet (SMM), it can generate 100% spin-polarized currents even connected with normal (N) leads. In this work, we study the phonon-assisted spin current in N-SMM-N systems. We mainly focus on the interplay of SMMs bistable character and electron-phonon coupling. It is found that when SMM is trapped in one of the lowest bistable states, it can generate phonon-assisted spin-polarized currents. At the up-spin transport channel, it is accompanied by a phonon-assisted up-spin current, while at the down-spin transport channel, it is accompanied by a phonon-assisted down-spin current.

Field-assisted electron transport through a symmetric double-well structure with spin?orbit coupling and the Fano-resonance induced spin filtering

We have investigated theoretically the field-driven electron-transport through a double-quantum-well semiconductor-heterostructure with spin?orbit coupling. The numerical results demonstrate that the transmission spectra are divided into two sets due to the bound-state level-splitting and each set contains two asymmetric resonance peaks which may be selectively suppressed by changing the difference in phase between two driving fields. When the phase difference changes from 0 to ?, the dip of asymmetric resonance shifts from one side of resonance peak to the other side and the asymmetric Fano resonance degenerates into the symmetric Breit?Wigner resonance at a critical value of phase difference. Within a given range of incident electron energy, the spin polarization of transmission current is completely governed by the phase difference which may be used to realize the tunable spin filtering.

Larmor Precession and Tunnelling Time of a Non-Relativistic Neutral Spin-1/2 Particle Through an Arbitrary Potential Barrier

We investigate the Larmor precession of a non-relativistic neutral spin-1/2 particle in a uniform constant magnetic field confined to the region of a one-dimensional arbitrary potential barrier. With the help of a general spin coherent state it is explicitly shown that the spin precession time is equal to the dwell time. For the special case of a symmetric rectangular potential barrier, the precession time is also in agreement with the transmission time. We present a numerical estimation of the precession time showing an apparent superluminal tunnelling.

Macroscopic Quantum Coherence in Magnetic Molecular Clusters

The oscillation of tunnel splitting in Fe8 molecular clusters is obtained as a function of magnetic field applied along the hard axis by means of the instanton method with both semiclassical treatment and the effective potential field description of the quantum spin system. The theoretical splittings of the instanton method are compared with the numerical result by diagonalization of spin Hamiltonian operators and experimental observations. By taking the appropriate parameters, our theoretical formula yields a result the same as the experimental observation.

Multi-step shot noise spectrum induced by a local large spin*

We use non-equilibrium Greens function method to analyze the shot noise spectrum of artificial single molecular magnets (ASMM) model in the strong spin–orbit coupling limit in sequential tunneling regime, mainly focusing on the effects of local large spin. In the linear response regime, the shot noise shows 2S + 1 peaks and is strongly spin-dependent. In the nonlinear response regime, one can observe 2S + 1 steps in shot noise and Fano factor. In these steps one can see the significant enhancement effect due to the spin-dependent multi-channel process of local large spin, which reduces electron correlations.

Electron resonance-transmission through a driven quantum well with spin–orbit coupling

We have studied the spin-dependent electron transmission through a quantum well driven by both dipole-type and homogeneous oscillating fields. The numerical evaluations show that Dresselhaus spin–orbit coupling induces the splitting of asymmetric Fano-type resonance peaks in the conductivity, in which the dipole modulation and the homogeneous modulation are equivalent. Therefore, we predict that the dipole-type oscillation, which is more practical in the experimental setup, can be used to realize the tunable spin filters by adjusting the field oscillation-frequency and the amplitude as well.

Bifurcation of a periodic instanton and quantum-classical transition in the biaxial nano-ferromagnet with a magnetic field along hard axis

Crossover from classical to quantum regimes of the barrier transition rate in a biaxial ferromagnetic magnet with a magnetic field applied along hard anisotropy axis is investigated. We show that the type of action-temperature diagrams can be determined by counting the number of bifurcation points. The model possesses not only the known type I and II, but also the interesting type III and IV of transition which do not occur in general.