Jungiusstr. 11, Hörsaal AP
SFB 668 - Kolloquium
Dr. M. Cuoco (CNR-Spin Salerno, Fisciano, Italy):
Spin-triplet pairing in superconductor-ferromagnet heterostructure
The singlet superconductor (SSC) and ferromagnet (FM) phases are fundamentally incompatible, as the exchange field of the FM destroys the superconductivity by aligning the anti-parallel spins of the electrons in singlet Cooper pairs . This pair-breaking effect makes homogeneous coexistence of SSC and FM very rare. On the other hand, SSC-FM interfaces can be readily fabricated in artificial heterostructures, and the study of these devices has attracted intense attention . The pair-breaking effect is central to the understanding of these systems, e.g. it causes the spatial oscillation of the SSC correlations in the barrier of a ferromagnetic Josephson junction, which is responsible for the famed 0-pi transition. The FM also suppresses the SSC gap close to the interface and can induce a magnetization in the SSC. Conversely, in order to minimize pair-breaking in the SSC, the magnetization in the FM may be suppressed near to the interface while domains may spontaneously form in a thin FM layer.
In this tak I will generally consider the issue of spin-triplet pairing in superconducting/magnetic heterostructures.
I will firstly discuss the mechanisms for the conversion of spin-singlet into triplet Cooper pairs in singlet based superconducting hybrids. This conversion is generally provided by physical conditions that include the breaking of time-reversal and space-inversion symmetry. Thus, natural systems where to get such effects are i) spin singlet superconductor interfaced to different types of inhomogeneous ferromagnets, ii) superconductors interfaced to uniform ferromagnet where the Rashba-type spin-orbit coupling (RSOC) is the source of singlet-triplet conversion. The magnetic inhomogeneity in SSC/FM hybrids has been demonstrated to yield spin-triplet pairs which are robust against disorder due to their odd symmetry in the time domain .
In the second part of the talk I will consider superconductors with an intrinsic spin triplet component , marked by unconventional interactions for the pairing formation, by focusing on two remarkable effects: i) the spin-orbital coupling emerging at the interface with an itinerant ferromagnet , ii) the occurrence of magnetic Andreev states at their edge if the system allows for singlet pairing in a subdominant channel .
In a TSC-FM heterostructure, the orientation of the FM moment relative to the TSC vector order parameter is a crucial variable that controls the physical behavior. In addition to the pair breaking, spin-flip reflection processes at the interface with the FM scatter the triplet Cooper pairs between the spin up and down condensates, setting up an effective Josephson-like coupling between them.  The pair-breaking and spin Josephson coupling both make significant contributions to the free energy of a TSC-FM junction through the proximity effect, interface electronic reconstruction, and the variation of the TSC gap. Although these contributions depend upon the direction of the FM's exchange field, the two effects do not necessarily act constructively. For a single-component p-wave TSC, we find that the variation of the gap controls the orientation of the FM's moment via the change in condensation energy. The stable configuration is either parallel or perpendicular to the TSC vector order parameter, depending on the alignment of the TSC gap with respect to the interface, thus evidencing a unique form of spin-orbital coupling. The competing orbital components of the chiral px+ipy state generate a non unique behavior and a first-order transition from the perpendicular to the parallel configuration as the FM exchange field is increased. When the interface is imperfect or spin active the scenario is different and other processes can play the decisive role in setting the magnetic profile .
Concerning the surface states of TSC, novel magnetic effects can occur if triplet and singlet pairing get mixed and have a non-uniform spatial profile. I will show that a) a subdominant in-phase s-wave superconducting order can exist near the edge of the sample; b) the in-phase s-wave component gives a non-unitary superconducting state at the boundary; c) as a result, the bound states are spin-polarized, leading to a finite surface magnetization; d) spin current flows along the interface in this regime; e) surface charge currents exhibit anomalous dependence on the magnetization .