Guojun Jin

Negative tunnel magnetoresistance and spin transport in ferromagnetic graphene junctions

We study the tunnel magnetoresistance (TMR) and spin transport in ferromagnetic graphene junctions composed of ferromagnetic graphene (FG) and normal graphene (NG) layers. It is found that the TMR in the FG/NG/FG junction oscillates from positive to negative values with respect to the chemical potential adjusted by the gate voltage in the barrier region when the Fermi level is low enough. Particularly, the conventionally defined TMR in the FG/FG/FG junction oscillates periodically from a positive to negative value with increasing the barrier height at any Fermi level. The spin polarization of the current through the FG/FG/FG junction also has an oscillating behavior with increasing barrier height, whose oscillating amplitude can be modulated by the exchange splitting in the ferromagnetic graphene.

Stretching-enhanced ballistic thermal conductance in graphene nanoribbons

We combine the nonequilibrium Greens function method with the elastic theory to investigate the ballistic thermal transport in graphene nanoribbons under homogeneous uniaxial stretching applied in the longitudinal direction. Remarkable enhanced thermal conductance is found for the samples with width in the appropriate range. The enhancement ratios could be up to 17% and 36% for the 5 nm width zigzag and armchair graphene nanoribbons, respectively. This enhancement effect results from a lot of dispersive phonon modes which are converged to the low-frequency region. In addition, the transverse shrinkage induced by the Poisson effect is beneficial for enhancing thermal conductance, while the transverse stretching has only a modest modification on thermal conductance. It is also observed that for 2.6 nm width nanoribbons the power-law temperature dependence, Tβ, of thermal conductance at low temperatures is independent of strain, with β=1 below a critical temperature about 10 K and β2 at 20 K ≤T≤ 50 K, which differ markedly from β=1.5 in the two-dimensional case. Moreover, above the critical width 11 nm, the armchair nanoribbon displays higher phonon thermal conductance than the same-width zigzag nanoribbon.

Optically and thermally manipulated spin transport through a quantum dot

We study the associated effects of polarized light and temperature bias on the charge and spin transport through a semiconductor quantum dot connected to two ferromagnetic electrodes. A spin-dependent thermoelectric current is generated in such a system, and a totally pure spin current can be obtained without an accompanying charge current. Furthermore, the sign reversal of tunnel magnetoresistance is found, which is induced by temperature difference and Rabi frequency.

Exotic electronic properties in Thue–Morse graphene superlattices

To show the specific behavior of Dirac fermions in a quasi-periodic structure, we investigate the electronic properties in a deterministic Thue-Morse graphene superlattice. Our main findings include the following. (i) Unlike conventional Schrödinger electrons, quasi-periodic features such as the striking self-similarity and trifurcation in the transmission spectrum can be manifested only at oblique incidence. (ii) In the vicinity of the usual Dirac point, extra Dirac points emerge; their locations are dependent merely on the second generation of the Thue-Morse structure and the number is double that in the periodic graphene superlattice. (iii) A classification is given about the wavefunctions in the Thue-Morse structure which are transformed from the critical states into extended ones at the Dirac points. (iv) The electrons can transmit perfectly at the extra Dirac points, and such a collimation supplies a convenient way to experimentally detect the numbers and the locations of the extra Dirac points. These exotic electronic properties in the aperiodic graphene superlattices may facilitate some applications in graphene-based electronics.

Bipolar-unipolar transition in thermospin transport through a graphene-based transistor

Based on the mean-field Hubbard model, we study the thermally driven spin-polarized transport through a local-gated magnetic zigzag graphene nanoribbon by using the nonequilibrium Green’s function method. The spin currents are tuned by the source temperature, the temperature bias, and the gate voltage. We find this transistor exhibits a transition from the bipolar to unipolar spin transport under associated modulations of thermal bias and gate voltage. It is argued that the result originates from the band selective rule related to parity conservation of wave functions in quantum tunneling. We also find the thermal magnetoresistance of the ribbon between the ferromagnetic excited state and antiferromagnetic ground state could reach up to 105% under a small local gate voltage. This proposed device provides possibility for bettering control of the spin freedom of electrons in graphene materials.

Effect of a tilted electric field on the magnetoexciton ground state in a semiconductor quantum dot

The variational approach within the effective mass approximation is used to investigate the effect of a tilted electric field on the energy and wave function of a magnetoexciton in a cylindrical quantum dot with a finite thickness. Calculations are performed for parameters of a typical GaAs quantum dot. We reveal the dependence of the ground-state binding energy of the magnetoexciton on the magnitude and orientation of the applied electric field. It is found that in weak magnetic fields, the electric field direction can strongly influence the magnetoexciton binding energy and thus give rise to a measurable Stark shift. However, in very strong magnetic fields, the binding energy is almost independent of electric field orientation. In addition, we discuss the competition between the tilted electric field and the magnetic field and find that the configuration of the applied electric and magnetic fields can cause either the redshift or blueshift of the exciton energy.

Phase effects in the enhanced transmission through compound subwavelength rectangular hole arrays

We study the enhanced transmission through compound subwavelength rectangular hole arrays in perfect electronically conductive films. It is shown that phase effects become important in the enhanced transmission when extra rectangular holes are added per period. There are two types of resonance mechanisms due to the phase effects: one is the localized waveguide mode phase resonance and the other is the surface mode phase resonance. The localized waveguide mode phase resonances, which are related to the number of holes per period and their relative positions, split the waveguide resonance peak into several peaks. The surface mode phase resonances, which appear evidently only for some special arrangements of holes in a unit cell, lead to the suppression of some surface mode resonance peaks. Our approach is advantageous to modulate the enhanced transmission in metallic hole arrays in the terahertz or microwave frequency regime.

Tunneling transport in a graphene-based ferromagnet/insulator/d-wave superconductor junction

We study the electronic transport in a graphene-based ferromagnet/insulator/d-wave superconductor (F/I/S) junction by use of the Dirac-Bogoliubov-de Gennes equation. The effects of spin polarization in the F region, barrier strength in the I region, and Fermi wave vector mismatch between the F and S regions are taken into account. It is found that the differential conductance and shot noise are strongly modulated by these parameters and display different features compared with other junctions. One interesting finding is that, at zero bias voltage and the maximum orientation angle of the superconductive gap α=π/4, the conductance, shot noise and Fano factor are only controlled by one parameter, i.e. the spin polarization, irrespective of all the other parameters. This universal feature could be applied to measure the magnitude of the spin polarization induced in graphene.

Thermally driven spin transport through a transverse-biased zigzag-edge graphene nanoribbon

In the framework of the Landauer-Büttiker formalism, we investigate coherent spin transport through a transverse-biased magnetic zigzag-edge graphene nanoribbon, with a temperature difference applied between the source and the drain. It is shown that a critical source temperature is needed to generate a spin-polarized current due to the presence of a forbidden transport gap. The magnitude of the obtained spin polarization exceeds 90% in a wide range of source temperatures, and its polarization direction could be changed by reversing the transverse electric field. We also find that, at fixed temperature difference, the spin-polarized current undergoes a transition from increasing to decreasing as the source temperature rises, which is attributed to the competition between the excited energy of electrons and the relative temperature difference. Moreover, by modulating the transverse electric field, the source temperature and the width of the ribbon, we can control the device to work well for generating a highly spin-polarized current.

Strain-induced 0–π transition in a zigzag graphene nanoribbon Josephson junction

We study theoretically the supercurrent through a superconductor/ferromagnetic zigzag graphene nanoribbon/superconductor junction by the Matsubara Green function method. The transformation of the supercurrent between the 0 and π states is remarkably realized in this Josephson junction by the combination of the uniaxial strain and gate-controlled barrier potential. Such strain-induced 0–π transition is found to result from the dependence of the effective Fermi velocity on the uniaxial strain in the graphene nanoribbon modulated by mechanical approaches.