Special Condensed-Matter Seminar:" "

Thu, 19/09/2019 - 10:00 to 11:30

Danciger B building, Seminar room
Lecturer: Dr. Angelo DiBarnardo
Title:Superspintronics: from its origin to the first oxide devices.
The interaction between materials with radically different properties can lead to the
emergence of tantalising physical phenomena. A classic example of such an interaction is that
occurring at a superconductor/ferromagnet (S/F) interface, where unconventional – odd
frequency spin-triplet – superconducting states can arise which support a net spin-
polarization [1]. Spin-triplet superconducting states have raised great interest for the
development of superconducting spintronics (“superspintronics) devices which can operate
with low energy dissipation [2].
The spin-polarization of these states stems from the fact that, in the presence of an
inhomogeneous magnetisation in the F, the Cooper pairs injected from S into F form a spin-
triplet state in which the electron spins are parallel with a pair wavefunction that is odd in
symmetry with respect to exchange of time coordinates [1]. Experimental evidence for such
spin-triplet states has been demonstrated first indirectly via transition temperature (T c )
measurements of S/F 1 /F 2 spin-valves [3-4] and long-ranged coupling in S/F/S Josephson
junctions [5-7], and then directly through scanning tunnelling microscopy and low-energy
muon spectroscopy techniques [8-9].
In this talk, after giving a general introduction to the field of superconducting spintronics
and reviewing some of my earlier work focusing on finding spectroscopic evidence for spin-
triplets in metallic S/F systems [8-9], I will present the motivations of my most recent work
aimed at extending the functionalities of superconducting spintronic devices through the
usage of oxide S/F materials [10-11]. In particular, I will discuss a novel form of exchange
coupling arising between layers of the ferromagnetic insulator (FI) Pr 0.8 Ca 0.2 MnO 3
sandwiching a thin layer of the S YBa 2 Cu 3 O 7-δ (YBCO). The results obtained demonstrate
evidence for a novel coupling between the FIs mediated by quasiparticle excitation states at
the Fermi surface of the nodal (d-wave) S YBCO [11]. Such nodal exchange coupling gives
rise to a colossal variation in T c (up to 2 K) between the parallel and antiparallel
magnetization-alignment of the FIs, which occurs over length-scale larger than 100 times the
c-axis YBCO coherence length (≈0.2-0.3 nm), thus defying classical proximity-effect theories
developed by de Gennes for similar FI/S/FI systems with a s-wave S material.
[1] F.S. Bergeret et al., Rev. Mod. Phys. 77, 1321 (2005).
[2] J. Linder et al., Nat. Phys. 11, 307 (2015).
[3] P. V. Leksin et al., Phys. Rev. Lett. 109, 057005 (2012).
[4] X. L. Wang et al., Phys. Rev. B 89, 140508(R) (2014).
[5] T. Khaire et al., Phys. Rev. Lett. 104, 137002 (2010).
[6] J.W.A. Robinson et al., Phys. Rev. Lett. 104, 207001, 458 (2010).
[7] J.W.A. Robinson et al., Science 329, 59 (2010).
[8] A. Di Bernardo et al., Nat. Comm. 6, 8053 (2015).
[9] A. Di Bernardo et al., Phys. Rev. X 5, 041021 (2015).
[10] A. Di Bernardo et al., Nat. Comm. 8, 14024 (2017).
[11] A. Di Bernardo et al., Nat. Mater. (accepted for publication, 2019).