Wei-Jiang Gong

Detection of a Majorana fermion zero mode by a T-shaped quantum-dot structure

We investigate the electron transport through the T-shaped quantum-dot (QD) structure theoretically, by coupling a Majorana zero mode to the terminal QD. It is found that in the double-QD configuration, the presence of the Majorana zero mode can efficiently dissolve the antiresonance point in the conductance spectrum while inducing a conductance peak to appear at the same energy position. In the case of asymmetric QD-lead coupling, such a valley-to-peak transition induced by the Majorana zero mode still exists. Next, we observe in the multi-QD case that at the zero-bias limit, the conductance values are always the same as the double-QD result, independent of the parity of the QD number. We believe that all these results can be helpful for understanding the properties of Majorana bound states.

Antiresonance and bound states in the continuum in electron transport through parallel-coupled quantum-dot structures

In this paper we make a theoretical study of electron transport through a multi-quantum-dot system, in which the peripheral quantum dots of a one-dimensional chain are embodied in the two arms of an Aharonov-Bohm interferometer. It is found that, in the absence of magnetic flux, all the even molecule states of odd-numbered quantum-dot structures decouple from the leads and in even-numbered quantum-dot systems all the odd molecule states decouple from the leads, which indicates the formation of remarkable bound states in the continuum. Meanwhile, what is interesting is that apparent antiresonance occurs in electron transport through this structure, the positions of which are accordant with all even (odd) eigenenergies of the sub-molecule of the even (odd)-numbered quantum dots without the peripheral dots. All these results are efficiently modified by the presence of magnetic flux through this system.

Odd-even effect of transport through a chain of Majorana bound states in a T-shaped junction

We investigate the transport characteristics of a chain of Majorana bound states (MBSs) in a T-shaped junction, where two normal leads are coupled to the same terminate MBS. We find the apparent odd-even effect in the transport process. Namely, when the MBS number is odd, the conductance spectrum exhibits a peak in the zero-bias limit. Besides, the shot noise Fano factor in the zero-bias limit and the conductance maximum are related by equation with . Otherwise, in the case of even-numbered MBSs, at the zero-bias limit, the conductance encounters its zero value, and the relation between F0 and changes as . Further investigation shows that these two kinds of relations are caused by the different interplay mechanisms between the crossed Andreev reflection and the local Andreev reflection. In addition, it is observed that the fluctuation of the inter-MBS couplings contributes little to the transport results. We ascertain that these results are helpful for understanding the MBS signature in transport spectra.

Thermoelectric Properties in a Parallel Double Quantum Dot Structure Modulated by the Fano Interferences

We discuss the thermoelectric properties assisted by the Fano effect of a parallel double quantum dot (QD) structure. By adjusting the couplings between the QDs and leads, we facilitate the nonresonant and resonant channels for the Fano interference. It is found that at low temperature, Fano lineshapes appear in the electronic and thermal conductance spectra, which can also be reversed by an applied local magnetic flux with its phase factor � ¼ � . And, the Fano effect contributes significantly to the enhancement of thermoelectric efficiency. However, at the same temperature, the thermoelectric effect in the case of � ¼ � is much more apparent, compared with the case of zero magnetic flux. And, with the temperature increase, the thermoelectric effect in the case of � ¼ � seems to be more robust. Using the concept of Feynman path, we analyze the difference between the quantum interferences in the cases of � ¼ 0 and � . It is seen that in the absence of magnetic flux the Fano interference originates from the quantum interference among infinite-order Feynman paths, but it occurs only between two lowest-order Feynman paths when � ¼ � . The increase of temperature inevitably destroys the electron coherent transmission in each paths. So, in the case of zero magnetic field, the thermoelectric effect assisted by the Fano interference is easy to weaken by a little increase of temperature.

Andreev Reflection in a T-Shaped Double-Quantum-Dot Structure Induced by Majorana Bound States

The Andreev reflection in a T-shaped double-quantum-dot structure induced by Majorana bound states is theoretically investigated. It is found that due to the Fano effect, the Andreev conductance spectrum presents a well-defined insulating band in the low-bias region. By discussing the influences of quantum dot levels, interdot coupling, and the coupling between the quantum dot and the Majorana bound state, the properties of the insulating band are analyzed in detail. We believe that all the results can help the researchers to understand the nontrivial role of the Majorana bound states in manipulating the Andreev reflection.

Coulomb-modified equilibrium and nonequilibrium properties in a double quantum dot Aharonov-Bohm-Fano interference device

In this paper, the Coulomb-induced changes of electronic transport through a double quantum dot (QD) Aharonov-Bohm-Fano interferometer are discussed. For the linear-transport case, the variation of Coulomb interaction in the reference-channel QD can remarkably modify the sign of the Fano parameter, which leads to the change in Fano interference, including the increase or decrease in the symmetry of the Fano lineshape, as well as the inversion of the Fano lineshape. When both the QD levels are adjustable, the Coulomb-induced splitting of the reference channel induces the destruction of Fano interference; whereas two blurry Fano lineshapes may appear in the conductance spectra when the many-body effect in the resonant-channel QD is also considered. Interestingly, in the absence of magnetic field, when the different-strength electron interactions make one pair of QD levels of different channels the same, the corresponding resonant state becomes a vacuum state. As for the nonlinear electron transport, the Fano lineshapes emerge in the differential conductance spectra when the resonant-channel QD level is adjusted to the vicinity of the chemical potential of either lead, except for the case where it encounters the reference-channel QD level. The presented Coulomb interactions also play a nontrivial role in the appearance of the negative differential conductance

Rashba-induced transverse pure spin currents in a four-terminal quantum dot ring

Abstract By applying a local Rashba spin–orbit interaction to an individual quantum dot of a four-terminal four-quantum-dot ring and introducing a finite bias between the longitudinal terminals, we theoretically investigate the charge and spin currents in the transverse terminals. It is found that when the quantum dot levels are separate from the chemical potentials of the transverse terminals, notable pure spin currents appear in the transverse terminals with the same amplitude but opposite polarization directions. In addition, the polarization directions of such pure spin currents can be inverted by altering the structure parameters, i.e., the magnetic flux, the bias voltage, and the values of quantum dot levels with respect to the chemical potentials of the transverse terminals.

Decoupling and antiresonance in electronic transport through a quantum dot chain embodied in an Aharonov–Bohm interferometer

Abstract Electronic transport through a quantum dot chain embodied in an Aharonov–Bohm interferometer is theoretically investigated. In such a system, we find that when the quantum dot chain is symmetrically placed some of its molecular states decouple from the leads. Namely, in the absence of magnetic flux all odd molecular states decouple from the leads, but all even molecular states decouple from the leads when an appropriate magnetic flux is introduced. Interestingly, the antiresonance position in the electron transport spectrum is independent of the change of the decoupled molecular states. When incorporating the many-body effect by only considering the Hubbard term and truncating the motion equation of the Green functions to the second-order, we show that the emergence of decoupling gives rise to the apparent destruction of electron-hole symmetry. By adjusting the magnetic flux through either subring, some molecular states decouple from one lead but still couple to the other, and then some new antiresonances occur.

Fano effect and bound state in continuum in electron transport through an armchair graphene nanoribbon with line defect

Electron transport properties in an armchair graphene nanoribbon are theoretically investigated by considering the presence of line defect. It is found that the line defect causes the abundant Fano effects and bound state in continuum (BIC) in the electron transport process, which are tightly dependent on the width of the nanoribbon. By plotting the spectra of the density of electron states of the line defect, we see that the line defect induces some localized quantum states around the Dirac point and that the different localizations of these states lead to these two kinds of transport results. Next, the Fano effect and BIC phenomenon are detailedly described via the analysis about the influence of the structure parameters. According to the numerical results, we propose such a structure to be a promising candidate for graphene nanoswitch.PACS: 81.05.Uw, 71.55.-i, 73.23.-b, 73.25.+i

Line-defect–induced Fano interference in an armchair graphene nanoribbon

Electron transport through a metallic armchair graphene nanoribbon is theoretically investigated by considering the presence of line defect. The line defect is formed by the staggered stacking of the pentagons and heptagons. Our calculation results show that the line defect mainly destroys the electron transport in the conduction-band region by inducing the abundant Fano effects in the electron transport process. Moreover, the properties of the Fano effects are tightly dependent on the width M of the nanoribbon, and the results of are completely different from those of M > 17. The spectra of the density of electron states illustrate that the line defect induces some localized quantum states, and that the different localizations of these states lead to the distinct transport results. By analyzing the influence of the structure parameters, the Fano effects are described in detail. All the results demonstrate that such a structure can be a promising candidate for electron manipulation in graphene nanoribbon.