Alternating Response Of Tunneling In Nano-structures

With the progress of nanofabrication and material growth technologies, it has been possible to fabricate a variety of mesoscopic structures, resulting in extensive investigation of mesoscopic systems. Applying electric field to control and manipulate the transport of mesoscopic structures provides a wide research field for the alternating response of tunneling. In this thesis, we investigate the influence and manipulation on transport of mesoscopic systems under an ac field. Using the nonequilibrium Green function method, we study transport properties of several kinds of mesoscopic systems and make a qualitative analysis of transport conditions (frequency and amplitude of the ac field, temperature, etc.), obtaining some meaningful results. Main results are summarized as follows:(1) We investigate alternating response of the spin current in a quantum dot system coupled to a normal metal electrode. For simplicity, we have neglected the intradot electron-electron Coulomb interaction, and only consider a single level in the quantum dot. An alternating driving voltage and a pumping rotating magnetic field are applied to investigate whether some new phenomenon of spin current can be generated as the driving frequency of the ac field changes. The nonequilibrium Green function technique is used to obtain the general formulas of the time-averaged spin current and its differential. It is found that the spin-flip mechanism induced by pumping rotating magnetic field in the QD leads to the difference of two spin chemical potentials creating a nonequilibrium spin population. For a given rotating frequency, the spin current increases rapidly and shows a series of steps with increasing driving frequency. As the driving frequency is further increasing, the spin current can be significantly enhanced and approaches a stable value. The photon-assisted processes bring about interesting features of spin current. The steps of the spin current are closely related to the photon absorbing and emitting process. These quantum interference features are remarkable at low temperatures. The influence of the gate voltage and temperature on the spin current is examined in detail.(2) We investigate photon-assisted transport through a perfect quantum wire with a side-coupled quantum dot in the presence of an ac field. We find that the electron-photon interaction together with the quantum interference of electron wave function can lead to photon-assisted sideband anti-resonance as the ac field frequency varies, which is then useful for tuning coherence and phases of electrons. Consequently, the coherent absorption and emission of photons can be detected via phase measurement at the sidebands of the main anti-resonance. Zero and limited temperature dependence of the conductance and transmission phase are discussed. Compared to the situation of without the ac field, the ac field leads to a series of photonic sideband valleys atε=εd + nω, that is, an electron absorbs ( n > 0) or emits ( n < 0) photons to become resonant energy level where anti-resonances occur, accompanied with a sudden change in the phase shift due to the quantum interference and the photon-assisted tunneling. The amount of the phase shift varies with the ac field amplitude, despite that the anti-resonance location does not. The conductance amplitude decreases with increasing temperatures and the width of the anti-resonance becomes larger for sufficient low temperatures. The dip structure at the first photonic sideband will develop into a noticeable peak around the anti-resonance level when the temperature increases to 0.4 comparable to the magnitude ofΓ(Γ= 0.49), and this peak is then enhanced with further increasing temperatures. Meanwhile, the dip structures are rapidly smeared out with increasing temperatures, suggesting that phase coherence is destroyed with increasing temperatures. Furthermore, we can manipulate the phase shift of the conductance by changing the frequencyωand magnitudeΔof the ac field.(3) We investigate Andreev reflection (AR) tunneling through a ferromagnet-quantum dot-superconductor (F-QD-S) system in the presence of an external ac field. The intradot spin-flip scattering in the QD is involved. It is shown that the time-averaged AR conductance displays interesting behaviors depending on the external ac field, the intradot spin-flip scattering R , the ratio of the two tunneling coupling strengths and the spin polarization P of the F-lead, respectively. In the presence of the ac field, the AR conductance versus the QD energy level exhibits a series of photonic sideband peaksε=εd±R + nω0, that is, the electron absorbs ( n > 0) or emits ( n < 0) photons to become resonant tunneling through the QD. And the observed behaviors of the AR conductance are a consequence of the competition between the intradot spin-flip scattering and the tunneling coupling strength of the two leads together with the external ac field. In addition, the AR conductance-bias voltage characteristic shows that whenΓf 0≥ΓS0, the AR conductance displays a single-peak resonance and whenΓf 0 <ΓS0, the AR conductance always develops into the double-peak indicating a novel structure.