This article was submitted to Optics and Photonics, a section of the journal Frontiers in Physics

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We have studied theoretically the properties of electrical current and tunnel magnetoresistance (TMR) through a serially connected double quantum dot (DQD) sandwiched between two ferromagnetic leads by using the nonequilibrium Green’s function technique. We consider that each of the DQD couples to one mode of the Majorana bound states (MBSs) formed at the ends of a topological superconductor nanowire with spin-dependent coupling strength. By adjusting the sign of the spin polarization of dot–MBS coupling strength and the arrangement of magnetic moments of the two leads, the currents’ magnitude can be effectively enhanced or suppressed. Under some conditions, a negative TMR emerges which is useful in detection of the MBSs, a research subject currently under extensive investigations. Moreover, the amplitude of the TMR can be adjusted in a large regime by variation of several system parameters, such as direct hybridization strength between the MBSs or the dots and the positions of the dots’ energy levels. Such tunable currents and TMR may also find use in high-efficiency spintronic devices or information processes.

Electronic transport through structures composing of quantum dots (QDs) hybridized with a topological superconductor nanowire (TSNW) hosting Majorana bound states (MBSs) [

Some previous theoretical and experimental work has shown that the MBSs will affect the electronic transport processes through QD-based devices in a significant way [

It is known that the zero-energy MBSs exert remarkable effects on the electrical conductance and current around the zero-bias regimes. But under such a condition, the amplitude and changes of the above quantities are usually small and hard to be detected. In fact, the quantity of tunnel magnetoresistance (TMR) [

Schematic plot of the DQD coupled to ferromagnetic leads with coupling strength

The Hamiltonian of the studied system shown in

In this section, we present our numerical results for the spin-dependent currents and TMR. We choose the leads’ bandwidth

Electronic current

With the results in

Spin-dependent and total currents for parallel configuration in A–C, and antiparallel one in D–F with

As is seen from

TMR for positive

Total currents in parallel configuration

TMR for different values of

In summary, we have studied the spin-polarized currents and TMR in a DQD coupled to both ferromagnetic leads and MBSs formed at the ends of a topological superconductor nanowire. Our calculation results show that the currents through the system can be effectively adjusted in terms of the spin polarization of either ferromagnetic leads or coupling strength between the dots and the MBSs. When the two spin polarizations are the same in sign, the currents’ amplitude in the antiparallel configuration can be larger than that in the parallel one, which results in an obvious negative TMR that can be used for detecting the existence of the MBSs. If the two spin polarizations are different in sign, however, then the TMR is positive and can be further enhanced by adjusting system’s parameters. Such a result is useful in designing high-efficiency spintronic devices. The negative or positive TMR is robust against variations of the overlapping between the MBSs, the tunnel coupling between the two dots, or even the difference between the dots’ energy levels.

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

W-GM and L-WT contributed the ideas equally and performed the numerical calculations. L-WT derived the formulae in the paper and wrote the original manuscript.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 11772287 and 11572277).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.