Nanocomposites and nanomaterials

Observation of spin-selective collective excitations of the condensates relative phase in proximitized singlet superconductor ‒ half-metallic manganite nanostructures
Krivoruchko V.N., Tarenkov V.Yu.
Donetsk Institute for Physics and Engineering of the NAS of Ukraine, 
Prospect Nauki, 46, Kyiv-03028, Ukraine. 
E-mail: krivoruc@gmail.com

Proximitized materials based on conventional superconductor/half-metallic ferromagnet (SC/hmF) heterostructures are promising materials with spin-triplet superconductivity. Their multi-band superconducting state is characterized by the superposition of singlet and triplet Cooper pairs condensates. In a multi-band SC, collective excitations of a relative phase of the condensates (Leggett modes) are allowed. Thus, the question about possible connection of Leggett-like collective excitations in SC/hmF proximity materials with the spin conservation law is a burning question. 
In this report, we present the results of experimental investigations by the point-contact (Andreev) spectroscopy of the Leggett modes in proximity induced three-band superconducting state of the s-wave SC/hmF nanocomposite, MgB₂/(La,Sr)MnO₃. Two types of point contacts (PC) have been used: the nanocomposite - hmF and the nanocomposite - nonmagnetic metal PCs. Small amplitude spin-dependent equidistant oscillations against the background of the gap structure were observed in the conductance characteristics of both types of high-quality Andreev junctions. The distinctive feature between the PCs conductance is the period of oscillations, it is doubled for contacts with a nonmagnetic metal in comparison to the nanocomposite - hmF PCs. We attribute the spin-selective conductance oscillations to an emission/absorption of nonequilibrium spin-selective Leggett excitations – the collective fluctuation of phase difference between Cooper pair condensates. To our best knowledge, the obtained experimental data are the first observation of Leggett excitations in proximity induced mixture of s- and p- Cooper pair condensates.