Two-dimensional transition metal dichalcogenides with strong spin-orbit interactions and valley-dependentBerry curvature effects have attracted tremendous recent interests 1-7 . Although novel single-particle and excitonic phenomena related to spin-valley coupling have been extensively studied 1,3-6 , effects of spin-momentum locking on collective quantum phenomena remain unexplored. Here we report an observation of superconducting monolayer NbSe2 with an inplane upper critical field over six times of the Pauli paramagnetic limit by magnetotransport measurements. The effect can be understood in terms of the competing Zeeman effect and large intrinsic spin-orbit interactions in non-centrosymmetric NbSe2 monolayers, where the electronic spin is locked to the out-of-plane direction. Our results provide a strong evidence of unconventional Ising pairing protected by spin-momentum locking and open up a new avenue for studies of noncentrosymmetric superconductivity with unique spin and valley degrees of freedom in the exact two-dimensional limit.Monolayer transition metal dichalcogenide (TMD) of the hexagonal structure consists of a layer of transition metal atoms sandwiched between two layers of chalcogen atoms in the trigonal prismatic structure 8 (Fig. 1a). It possesses out-of-plane mirror symmetry and in-plane inversion asymmetry. The presence of the transition metal also gives rise to large spin-orbit interactions (SOIs). The mirror symmetry restricts the crystal field ( ⃗) to the plane. The SOIs split the spin states at finite momentum ⃗⃗ in the absence of inversion symmetry. They manifest as an effective magnetic field along the direction of ⃗⃗ × ⃗ , which is out-of-plane for the restricted two-dimensional (2D) motion of electrons in the plane. The electronic spin is thus oriented in the out-of-plane direction and in opposite directions for electrons of opposite momenta 1-3 (Fig. 1a). Such spinmomentum locking is destroyed in the bulk where inversion symmetry and spin degeneracy are restored 1,2,7 (Fig. 1b). Novel valley-and spin-dependent phenomena including optical orientation of the valley polarization 3,4 and the valley Hall effect 5 arisen from spin-momentum locking have been recently demonstrated in group-VI TMD

The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field B(c2) far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced B(c2) is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field.

We describe experimental signatures of Majorana fermion edge states, which form at the interface between a superconductor and the surface of a topological insulator. If a lead couples to the Majorana fermions through electron tunneling, the Majorana fermions induce resonant Andreev reflections from the lead to the grounded superconductor. The linear tunneling conductance is 0 (2e(2)/h) if there is an even (odd) number of vortices in the superconductor. Similar resonance occurs for tunneling into the zero mode in the vortex core. We also study the current and noise of a two-lead device.