arXiv subjects

Qing-feng Sun

Publications and source records attributed to Qing-feng Sun.

At least 91 records · Page 5Linked to original sources

arXiv2003

Anomalous Kondo effect of indirectly coupled double quantum dots

We report theoretical investigations of indirectly coupled double quantum dots (QD) side connected to an one-dimensional quantum wire. Due to quantum interference controlled by the parameter $k_F L$, with $k_F$ the Fermi wave number of the wire and $L$ the distance between the two QDs, distinctly different Kondo resonances are predicted depending on the range of $k_F L$. A true bound Kondo states is found while an anomalous Kondo resonance gives rise to both reduction and enhancement of conductance.

cond-mat.mes-hall↗

arXiv2003

Gate-controllable spin-battery

We propose a gate-controllable spin-battery for spin current. The spin-battery consists of a lateral double quantum dot under a uniform magnetic field. A finite DC spin-current is driven out of the device by controlling a set of gate voltages. Spin-current can also be delivered in the absence of charge-current. The proposed device should be realizable using present technology at low temperature.

cond-mat.mes-hall↗

arXiv2003

A spin-cell for spin current

We propose and investigate a spin-cell device which provides the necessary spin-motive force to drive a spin current for future spintronic circuits. Our spin-cell have four basic characteristics: (i) it has two poles so that a spin current flows in from one pole and out from the other pole, this way a complete spin-circuit can be established; (ii) it has a source of energy to drive the spin current; (iii) it maintains spin coherence so that a sizable spin current can be delivered; (iv) it drives a spin current without a charge current. The proposed spin-cell for spin current should be realizable using technologies presently available.

cond-mat.mes-hall↗

arXiv2003

ac Josephson effect in the resonant tunneling through mesoscopic superconducting junctions

We investigate ac Josephson effect in the resonant tunneling through mesoscopic superconducting junctions. In the presence of microwave irradiation, we show that the trajectory of multiple Andreev reflections can be closed by emitting or absorbing photons. Consequently, photon-assisted Andreev states are formed and play the role of carrying supercurrent. On the Shapiro steps, dc component appears when the resonant level is near a series of positions with spacing of half of the microwave frequency. Analytical result is derived in the limit of infinite superconducting gap, based on which new features of ac Josephson effect are revealed.

cond-mat.mes-hall↗

arXiv2003

Spin-current induced electric field

We theoretically predict that a pure steady state spin-current without charge-current can induce an electric field. A formula for the induced electric field is derived and we investigate its characteristics. Conversely, a moving spin is affected by an external electric field and we present a formula for the interaction energy.

cond-mat.mes-hall↗

arXiv2003

Writing Spin in a Quantum Dot with Ferromagnetic and Superconducting Electrodes

We propose an efficient mechanism for the operation of writing spin in a quantum dot, which is an ideal candidate for qubit. The idea is based on the Andreev reflection induced spin polarization (ARISP) in a ferromagnetic / quantum-dot / superconductor system. We find that on the resonance of Andreev reflection, the spin polarization of quantum dot strongly denpends on the magnetization of ferromagnetic electrode, and the sign of the spin polarization is controllable by bias voltage. In the presence of intradot Coulomb interaction, we show that ARISP effect can still survive as long as the charging energy is comparable to the superconducting gap. Detailed conditions and properties of ARISP are also discussed.

cond-mat.mes-hall↗

arXiv2002

Correlated two-electron transport: a principle for a novel charge pump

By considering a correlated two-electron transport process (TET) and using a diagrammatic analysis within the Keldysh nonequilibrium Green's function formalism, we discuss a novel charge pump by which carriers are pumped from a contact with low chemical potential to another contact with a higher potential. The TET process involves two correlated incident electrons scattering and exchanging energy with each other. The process can significantly affect charge current density and it involves high empty states and/or low filled states of the Fermi liquid of the leads.

cond-mat.mes-hall↗

arXiv2002

Andreev-Fano Effect in a Hybrid Normal-Metal / Superconductor Interferometer

We report on a new type of Fano effect, named as Andreev-Fano effect, in a hybrid normal-metal / superconductor (N/S) interferometer embedded with a quantum dot. Compared with the conventional Fano effect, Andreev-Fano effect has some new features related to the characteristics of Andreev reflection. In the linear response regime, the line shape is the square of the conventional Fano shape; while in the nonlinear transport, a sharp resonant structure is superposed on an expanded interference pattern, qualitatively different from the conventional Fano effect. The phase dependence of the hybrid N/S interferometer is also distinguished from those of all-N or all-S interferometers.

cond-mat.mes-hall↗

arXiv2002

Theory of Nonequilibrium Coherent Transport through an Interacting Mesoscopic Region Weakly Coupled to Electrodes

We develop a theory for the nonequilibrium coherent transport through a mesoscopic region, based on the nonequilibrium Green function technique. The theory requires the weak coupling between the central mesoscopic region and the multiple electrodes connected to it, but allows arbitrary hopping and interaction in the central region. An equation determining the nonequilibrium distribution in the central interacting region is derived and plays an important role in the theory. The theory is applied to two special cases for demonstrations, revealing the novel effects associated with the combination of phase coherence, Coulomb interaction, and nonequilibrium distribution.

cond-mat.mes-hall↗

arXiv2002

Microwave-induced pi-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 $E_{0}=\frac{n}{2}\hbar ω(n=\pm 1,\pm 2...)$, where $E_{0}$ is the resonant energy level of the quantum dot and $ω$ 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 $E_{0}=\frac{n}{2}\hbar ω$. The microwave-induced $π$-junction transition is interpreted in the picture of photon-assisted Andreev bound states, which are formed due to multiple photon-assisted Andreev reflection between the two superconductors. Moreover, the main resonance located at $E_{0}=0$ can also be reversed with proper microwave strength and frequency.

cond-mat.mes-hall↗

arXiv2001

Probing Spin States of Coupled Quantum Dots by dc Josephson Current

We propose an idea for probing spin states of two coupled quantum dots (CQD), by the dc Josephson current flowing through them. This theory requires weak coupling between CQD and electrodes, but allows arbitrary inter-dot tunnel coupling, intra- and inter- dot Coulomb interactions. We find that the Coulomb blockade peaks exhibit a non-monotonous dependence on the Zeeman splitting of CQD, which can be understood in terms of the Andreev bound states. More importantly, the supercurrent in the Coulomb blockade valleys may provide the information of the spin states of CQD: for CQD with total electron number N=1,3 (odd), the supercurrent will reverse its sign if CQD becomes a magnetic molecule; for CQD with N=2 (even), the supercurrent will decrease sharply around the transition between the spin singlet and triplet ground states of CQD.

cond-mat.mes-hall↗

arXiv2001

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.

cond-mat.mes-hall↗

arXiv2001

Double quantum dots: interdot interactions, co-tunneling, and Kondo resonances without spin

We show that through an interdot off-site electron correlation in a double quantum-dot (DQD) device, Kondo resonances emerge in the local density of states without the electron spin-degree of freedom. We identify the physical mechanism behind this phenomenon: rather than forming a spin singlet in the device as required in the conventional Kondo physics, we found that exchange of electron position between the two quantum dots, together with the off-site Coulomb interaction, are sufficient to induce the Kondo resonance. Due to peculiar co-tunneling events, the Kondo resonance in one dot may be pinned at the chemical potential of the other one.

cond-mat.mes-hall↗

arXiv2001

Hamiltonian approach to the ac Josephson effect in superconducting-normal hybrid systems

The ac Josephson effect in hybrid systems of a normal mesoscopic conductor coupled to two superconducting (S) leads is investigated theoretically. A general formula of the ac components of time-dependent current is derived which is valid for arbitrary interactions in the normal region. We apply this formula to analyze a S-normal-S system where the normal region is a noninteracting single level quantum dot. We report the physical behavior of time-averaged nonequilibrium distribution of electrons in the quantum dot, the formation of Andreev bound states, and ac components of the time-dependent current. The distribution is found to exhibit a population inversion; and all Andreev bound states between the superconducting gap $Δ$ carry the same amount of current and in the same flow direction. The ac components of time-dependent current show strong oscillatory behavior in marked contrast to the subharmonic gap structure of the average current.

cond-mat.mes-hall↗

arXiv2001

Kondo resonance in a multi-probe quantum dot

We present a theoretical analysis of a possible route for directly detecting Kondo resonances in local density of states (LDOS) of an interacting quantum dot. By very weakly couple a third and/or a fourth lead to a two-probe quantum dot and measuring differential conductance through these extra links, we show that Kondo peaks directly map onto the differential conductance measured from the third link. We analyze the conditions by which this detection of Kondo peaks in LDOS is possible.

cond-mat.mes-hall↗

arXiv2001

Extraordinary Temperature Dependence of the Resonant Andreev Reflection

An extraordinary temperature dependence of the resonant Andreev reflection via discrete energy level in a normal-metal / quantum-dot / superconductor (N-QD-S) system is predicted theoretically by using Green function technique. The width of zero bias conductance peak in N-QD-S is about $\sqrt{Γ_L^2+Γ_R^2}$ and does not exhibit thermal broadening, where $Γ_L$ and $Γ_R$ are the coupling strength between QD and leads. Considering the intra-dot Coulomb interaction, the Coulomb blockade oscillations conducted by Andreev reflection differs dramatically from that in N-QD-N. Instead of thermal broadening, finite temperature induces more resonant peaks around the oscillation peaks of zero temperature. This effect can be applied to determine the coupling strength and QD level spacing in N-QD-S.

cond-mat.mes-hall↗

arXiv2001

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 (QD). By using non-equilibrium 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 $\barσ$ 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.

cond-mat.mes-hall↗

arXiv2001

Asymmetric double quantum dots as a subminiature mesoscopic cell

A subminiature mesoscopic cell, consisting of an asymmetric double quantum dot capacitively coupled to a nearby mesoscopic circuit, is proposed, which can transform disordered noise energy to ordered electric energy. Two schemes, the noise originating from the nearby mesoscopic circuit and from the electromagnetic wave disturbance in external environment, are investigated. We found that the proposed cell can manifest as a good constant current source and the output current may not reach its largest value even if the circuit is shorted.

cond-mat.mes-hall↗