Phase Dependent Energy Levels of Bound States and D.C. Josephson Current in Unconventional Superconductor / Ferromagnetic Insulator / Unconventional Superconductor Junctions

Abstract

Properties of the DC Josephson effects in unconventional-superconductors under the presence of an exchange field in the insulator are studied theoretically. The current-phase relation of the DC Josephson current I(ϕ) and the temperature endence of the maximum Josephson current I C (T ) are calculated for various pairing symmetries and crystal orientations as a function of the exchange interaction strength (I m ). The influences of the spin-orbit coupling are also discussed. The phase dependent energy levels of quasiparticle bound states are sensitive to the magnitude of I m , orientaions of the junctions, and especially parities of two supercondctors. When the two superconductors have d-wave symmetries, I C (T ) shows non-monotonous behavior on the magnitude of I m , and accordingly the Josephson junction changes between "0-junction" and "π-junction". On the other hand, when the two superconductors have different parities, e.g. swave and p-wave, the current is largely due to SU(2) symmetry in the space in the absence of I m . However, the Josephson current rapidly recovers with the increase of I m because of the breakdown in SU(2) symmetry. These anomalous features can be used as a criteria for the identification of the pairing symmetries in novel superconductors. KEYWORDS: Josephson effect, ferromagnetic insulator, unconventional superconductor, zero-energy states dep Josephson suppressed spin §1. IntroductionNowadays, stimulated by the discovery of so-called colossal magneto-resistance (CMR) in Mn oxides, trials to realize new functional devices using oxides compounds have been made vigorously. In particular, quasiparticle properties of spin-polarized tunneling between unconventional superconductor / ferromagnet have been widely studied in many papers. 1-9) On the other hand, the DC Josephson current in superconductor / ferromagnet (ferromagnetic insulator) / superconductor (S/F i/S) junctions have been studied for several cases. 10-15) As regards the current phase relation of the DC Josephson effect I(ϕ), an explicit formula of the I(ϕ) in S/F i/S junction was presented with conventional s-wave superconductors, using the bound state formed at the interface. 16,17) It was clarified that with the increase of the magnitude of the exchange interaction in F i, the junction changes from "0-junction" to "π-junction". 18-22) As regards unconventional supreconductor junctions, Millis et al. discussed the generalized boudary condition for magnetic active interfaces. 23) Geshkenbein and Larkin discussed the existence of π junction between singlet and triplet superconductor junctions. 24) However, in unconventional superconductors, we must pay attention to bound states formation due to the presence of the internal phase of the pair potentials. [25][26][27][28][29] The existence of the bound states at interfaces of unconventional superconductors arouses distinctive properties in charge transports which are not expected for conven-1152 tional superconductor junctions. One of the most drastic effects is the formation of the ...

Theory of charge transport in diffusive normal metal/unconventional singlet superconductor contacts

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