Charge and spin transport through a ferromagnet/insulator/unconventional superconductor junction

Abstract

We analyze the charge and spin transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. For the ferromagnetic side we assume that ferromagnetism may be driven by an unequal mass renormalization of oppositely polarized carriers, i.e. a spin bandwidth asymmetry, and/or by a rigid splitting of up-and down-spin electron bands, as in a standard Stoner ferromagnet, whereas the superconducting side is assumed to exhibit a d-wave symmetry of the order parameter, which can be pure or accompanied by a minority component breaking time-reversal symmetry. Several remarkable features in the charge conductance arise in this kind of junction, providing useful information about the mechanism of ferromagnetism in the ferromagnetic electrode, as well as of the order parameter symmetry in the superconducting one. In particular, we show that when a time-reversal symmetry breaking superconductor is considered, the use of the two kinds of ferromagnet mentioned above represents a valuable tool to discriminate between the different superconducting mixed states. We also explain how this junction may mimic a switch able to turn on and off a spin current, leaving the charge conductance unchanged, and we show that for a wide range of insulating barrier strengths, a spin bandwidth asymmetry ferromagnet may support a spin current larger than a standard Stoner one.

Charge transport of graphene ferromagnetic-insulator-superconductor junction with pairing state of broken time reversal symmetry

“…[1][2][3][4][5][6] Unlike the usual proximity effect between a superconductor and a normal metal, a superconductor in contact with a ferromagnetic is subject to time reversal symmetry breaking which splits the up and down spin components of the spin-singlet Cooper pairs. 1,7 In these junctions there are two kinds of electrons, respectively, responsible for the magnetism and superconductivity. One is the localized electrons forming a ferromagnetic background in the metal, and the other one is the conduction electrons forming the Cooper pairs in superconductor.…”

“…[24][25][26] The splitting of the zero-bias conductance peak (ZBCP) at low temperatures observed experimentally in YBCO interpreted as a support for the admixture of an imaginary pair potential component to the subdominant d x 2 −y 2 -wave symmetry, leading to a broken time reversal symmetry (BTRS), and is consistent with both the d x 2 −y 2 + is, or d x 2 −y 2 + id x y paring symmetry. 6,7,[26][27][28][29] The peak splitting reflects the fact that the zero energy states are shifted by a positive and negative amount due to Doppler shift of a finite vector potential. It is possible for a secondary component to be induced wherever the subdominant order parameter varies spatially in regions close to surface.…”

Ferromagnetic features of zero-bias conductance peaks in a ferromagnet/insulator/superconductor junction

“…Recently, Annunziata, et al analyzed the charge and spin transport in ferromagnet/insulator/superconductor(F/I/S) junctions with taking account above mentioned the spin-dependent bandwidth asymmetry ferromagnet (SBAF) making the effective masses have different values in ferromagnet [23][24][25]. They clarified that from the knowledge of the critical transmission angle the measurement of the effective mass difference among the particles could be possible [23].…”

“…In this paper, a formulation of the tunneling conductance for charge and spin currents in ferromagnet/ferromagnetic-insulator/superconductor (F/FI/S) junctions will be presented by taking the effective mass difference leading the spin-band asymmetry between ↑-and ↓-spin particles in ferromagnet [23][24][25] into our previous theory [14]. Although the formulation can be used for singlet and S z = 0 triplet superconductors, we will study a chiral p-wave state.…”

“…In there, it has been also shown that the F/I/S junction is an effective probe to investigate the mechanism of ferromagnetism and the pairing symmetry of unconventional superconductor. Furthermore, it is suggested that the F/I/S junction can be useful as switching device using the spin current in the case of the symmetry of superconductor is conventional s-wave case [24] and be possible as a spin-filtering device [25]. They have studied on F/I/S junctions with several types of superconducting symmetries as the conventional swave, unconventional d x 2 −y 2 -wave, and the time reversal symmetry broken d x 2 −y 2 + is or d x 2 −y 2 + d xy states.…”