Study On The Spin Polarization-dependent Transport Properties Of Graphene Superconducting Junctions

Graphene,a single layer of carbon atoms,has been attracting considerable interests for the past decade,which is characterized by not only the linear dispersion with zero density of states at the Fermi level but also the two nonequivalent valleys in the two-dimensional hexagonal Brillouin zone.Although an undoping graphene is intrinsically a gapless semiconductor and not an superconductor(SC)or ferromagnet(FM),it was experimentally demonstrated that superconductivity and ferromagnetism can be induced in graphene via the proximity effect.And thus,in this paper,we extend the Blonder-Tinkham-Klapwijk(BTK)theory to study the quantum transport properties in the graphene-based d-wave or p-wave SC hybrid structure.Firstly,we theoretically investigate the graphene-based FM/insulator(I)/p-wave SC hybrid structure,where an anomalous zero bias conductance peak(ZBCP)is exhibited.Note that the I region refers to a high energy barrier potential.With the exchange field h increased above the Fermi energyE_F(h/E _F?1),the zero-bias conductance dips(ZBCDs)for both thep_x-andp_y-wave situations develop into ZBCPs,which are just contrary to the situation ath/E _F?1.Furthermore,with the enhancement of barrier strengthc,the conversion from the ZBCD to ZBCP is displayed in the region ofh/E _F?1for thep_x-wave,whereas the sharp ZBCP remains all the time at any h for thep_y-wave.More interestingly,the zero-bias conductance for thep_x-wave keeps unchanged upon increasingc,while for thep_y-wave exhibits periodical modulation ofc.Secondly,we theoretically investigate the graphene-based FM/I/d-wave SC hybrid structure.It is found that in the regime ofh?E_F,the ZBCP width narrows gradually withcincreased from 0 top/2,while for the regionh?E_F,the ZBCP width gets a non-monotonous enhancement.It is shown that the whole ZBCP in the thin I limit displays the periodic variation withc,whose corresponding phases forh?E _Fandh?E _Fare exactly opposite to each other.Most interestingly,in the context of a finite width of the insulating region,a splitting ZBCP,two subpeaks separated by a ZBCD,is exhibited forh?E _F,which is an singular feature compared to the thin-barrier limit.Thirdly,we theoretically present an anomalous splitting ZBCP in a graphen-based FM/Rashba spin-orbit coupling(RSOC)/I/d-wave SC hybrid structure.It is shown that with increasing exchange energy fromh/E _F=0,the ZBCP starts to turn into a splitting one with a ZBCD sandwiched between two subpeaks,while in the absence of RSOC,the anomalous splitting ZBCP can not be found.These peculiar features are ascribed to novel spin-triplet Andreev reflection in the context of the RSOC,characteristic by the anisotropic d-wave pair symmetry combined with the relativistic nodal fermions.More crucial is that the spin-triplet superconducting correlations can be modulated by the Fermi level.These results pave the way to a new class of tunable,high-temperature superconducting spintronic devices based on large-scale graphene.