Using van der Waals tunnel junctions, we perform spectroscopy of superconducting $mathrm{NbSe_2}$ 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. In flakes up to $approx$ 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 Ising spin-orbit coupling (ISOC), which pins Cooper pair spins out-of-plane. The superconducting energy gap, extracted from our tunnelling spectra, decreases as a function of the applied magnetic field. However, in bilayer $mathrm{NbSe_2}$, 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. This can be explained by a predicted subdominant triplet component of the order parameter, coupled to the dominant singlet component at finite field. This equal-spin, odd-parity triplet state arises from the non-colinearity between the applied magnetic field and the Ising field.
تحميل البحث