Orateurs
Description
Superconducting transition metal dichalcogenides in the few- and monolayer limit, such
as NbSe2, can sustain high in-plane magnetic fields, due to their Ising (or valley Zeeman)
spin-orbit coupling (ISOC), which pins the internal spin axis of Cooper pairs out-of-plane. The
interplay of the Ising field and the in-plane magnetic field has been predicted to give rise to
non-conventional superconducting phases such as odd-parity equal-spin triplet pairs
(ESTPs), the orbital FFLO (Fulde-Ferrell-Larkon-Ovchinnikov) phase, pair density waves,
striped phases etc. [1]
Using van der Waals tunnel junctions, we perform spectroscopy of superconducting
NbSe2 flakes, of thicknesses ranging from 2--25 monolayers, measuring the quasiparticle
density of states as a function of applied in-plane magnetic field up to 33T, the first
spectroscopy measurements on TMDs at these fields. In flakes up to ~15 monolayers thick,
we find that the density of states is well-described by a single band superconductor. In these
thin samples, the magnetic field acts primarily on the spin (vs orbital) degree of freedom of
the electrons, and superconductivity is further protected by ISOC. We extract the
superconducting energy gap as a function of the applied magnetic field from our tunnelling
data. In bilayer NbSe2, close to the critical field (up to 30T, much larger than the Pauli limit),
superconductivity appears to be even more robust than expected if only ISOC is considered.
Our data are well-explained by the above-mentioned equal-spin triplet pairs. These ESTPs
are revealed by the magnetic field, which also couples them to the dominant singlet order
parameter [2, 3].
We have also fabricated a new generation of devices, featuring multiple tunnel
junctions to few-layer NbSe2, forming SIS’ (superconductor-insulator-superconductor)
Josephson junctions. We’ll briefly discuss non-local and out-of-equilibrium phenomena as
well as possible emergent magnetism from these devices.