DIPC Seminars

Mario Cuoco CNR-SPIN and Dipartimento di Fisica "E. R. Caianiello", Universita di Salerno, I-84084 Fisciano (Salerno), Italy
Donostia International Physics Center (DIPC).Paseo Manuel de Lardizabal, 4 (nearby the Facultad de Quimica), Donostia
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COMPETING BROKEN SYMMETRIES DUE TO MAGNETISM AND UNCONVENTIONAL SUPERCONDUCTIVITY IN HYBRID STRUCTURES The problem of the interplay between ferromagnetic (FM) and superconducting (SC) long-range order in intrinsic as well as in hybrid structures is nowadays attracting a lot of interest, due to the possibility it offers to explore novel phenomena from a fundamental point of view, and also for its potential technological applications in emerging fields such as spintronics. The two orders come both from a spontaneous symmetry breaking: the SU(2) symmetry for the FM order, and the U(1) symmetry in the case of conventional spin-singlet SC pairing. They are in principle strongly competing. However, if SC pairing and spin polarization are assumed to be generated by unconventional mechanisms, interesting physical aspects can arise due to the interplay of the orbital, spin and spatial symmetry breaking. For instance, a coexisting state of singlet superconductivity and itinerant ferromagnetism can be stabilized within a single carrier system, if the metallic ferromagnetism is driven by a change in the relative mass of electrons with up and down spin polarization, rather than by a Stoner-like instability [1]. Moreover, new possibilities for the coexistence of FM and SC order, like a long-range proximity effect in SC- FM junctions, arise when pairing time correlations lead to an odd-like frequency dependence of the SC order parameter. When extra symmetries other than the U(1) gauge invariance are spontaneously or explicitly broken, the superconducting state can undergo a complex evolution: indeed, time reversal symmetry breaking can lead to inhomogeneous superconducting states and the removal of the crystal inversion symmetry or the presence of interfaces are accompanied by different types of mixed parity pairing. Taking into account these possibilities, we discuss the interplay of different symmetry breakings in artificial materials made of SC and ferromagnetic or nonmagnetic subsystems. In particular, we discuss the modifications induced on a SC spin-triplet chiral state by the simultaneous presence of a translational and time symmetry breaking, explicitly introduced by the presence of an interface and a constant (internal or an external) magnetic field [2]. Then, concerning SC-FM heterostructures, we explore the effects of different mechanisms that yield spin polarization in the ferromagnet. We analyze and compare the proximity behaviour for a SC-FM junction assuming both conventional and unconventional pairing for the superconductor and the possibility of having exchange splitting and spin dependent mass renormalization of up and down spin bands for the itinerant ferromagnet [3]. Looking at the charge and spin transport through such a junction, we show that this system can be used as a probe to get information about the mechanism of ferromagnetism in the first electrode and the order parameter symmetry in the second one [4]. [1] M. Cuoco, P. Gentile, and C. Noce, Phys. Rev. Lett. 91, 197003 (2003). [2] A. Romano, M. Cuoco, C. Noce, P. Gentile, G. Annunziata, Phys. Rev. B, 81, 064513 (2010). [3] M. Cuoco, A. Romano, C. Noce, P. Gentile, Phys. Rev. B 78, 054503 (2008). [4] G. Annunziata, M. Cuoco, C. Noce, A. Romano, P. Gentile, Phys. Rev. B 80, 012503 (2009).