Friday 31 July 2015 – Jianming Lu- Huge upper critical field of ionic gated MoS2

Jianming Lu reported on the latest experimental studies of superconductivity in transition metal dichalcogenides (TMDs) done in Prof. Ye’s group. The key aspect of these experiments is the use of ionic liquids to create electric double layer transistors, where a sheet of charge is

induced on the surface of the TMD by applying a gate voltage. This gating technique is extremely efficient due to the close (few nanometers) spacing between the charge and the material surface, creating an extremely large gate capacitance. As a result, high carrier densities can be induced in the TMD channel by applying just a few volts.

Prof. Ye’s group used this technique to study the effect of electrical doping on a variety of materials, including superconductors (ZrNCl), metals (Au), semiconductors and semimetals (MoS2 and graphene). This talk focused on Mo-based TMDs.

Ionic gating of MoS2, MoSe2, and MoTe2 revealed superconductivity on the electron-doping side for the first two materials. No superconductivity was reported on the hole-doping side and MoTe2 did not show any superconductivity. MoS2 and MoSe2 showed similar phase diagrams, with the dome-shaped superconducting phase starting at electron density n >  0.6X10^14/cm^2, and the critical temperature increased up to about 10 K for MoS2 and 6.5 K for MoSe2.

All these TMDs were multilayer flakes. However the speaker argued that the doping was mainly affecting the top layer, effectively decoupling it from the remaining layers and making this flake a monolayer superconductor.  The supporting evidence presented was: 1) the angular dependence of the critical magnetic field, showing a cusp for the field direction parallel to the MoS2 plane and 2) the superconducting transition showing a KT tail. This point generated questions from the audience on whether the same effects could be seen if doping and superconductivity were extended to more than one layer. In support to the single-layer argument, Jianming Lu mentioned that they had measured single layer samples and also found superconductivity with similar Tc and Tc dependence on doping. However, the single-layer work is still in progress therefore the data were not included in the presentation.

A striking result was the magnitude of the critical magnetic field in the direction parallel to the flake, larger than 80 T, much larger than the critical field measured for chemical doped bulk MoS2 and far exceeding the Pauli limit. The speaker argued that this large critical field can be explained by orthogonal protection due to Zeeman spin-orbit coupling, aligning the spins in the out-of-plane direction.

These new exciting experimental results show once again that ionic gating is a very powerful technique to uncover the rich physics of low-dimensional materials in the large carrier density regime.

Blogged by Paola Barbara