Competition between phonon mediated superconductivity and carriers localization in field-effect doped molybdenum dichalcogenides

TL;DR

This study uses advanced first-principles calculations to investigate superconductivity in doped molybdenum dichalcogenides, revealing that carrier localization and disorder, rather than charge density waves, limit the critical temperature.

Contribution

It provides a comprehensive theoretical analysis accounting for sample thickness and anharmonicity, clarifying the pairing mechanism and the cause of Tc saturation in these materials.

Findings

Charge density waves are ruled out as the cause of Tc saturation.

Electron-phonon coupling is suppressed by an order of magnitude, aligning with experimental data.

Carrier localization and disorder explain the Tc saturation phenomenon.

Abstract

Superconductivity occurs in electrochemically doped molybdenum dichalcogenides samples thicker than four layers. While the critical temperature (Tc) strongly depends on the field effect geometry (single or double gate) and on the sample (MoS2 or MoSe2), Tc always saturates at high doping. The pairing mechanism and the complicate dependence of Tc on doping, samples and field-effect geometry are currently not understood. Previous theoretical works assumed homogeneous doping of a single layer and attributed the Tc saturation to a charge density wave instability, however the calculated values of the electron-phonon coupling in the harmonic approximation were one order of magnitude larger than the experimental estimates based on transport data. Here, by performing fully relativistic first principles calculations accounting for the sample thickness, the field-effect geometry and anharmonicity, we rule out the occurrence of charge density waves in the experimental doping range and demonstrate a suppression of one order of magnitude in the electron-phonon coupling, now in excellent agreement with transport data. By solving the anisotropic Migdal-Eliashberg equations, we explain the behaviour of Tc in different systems and geometries. From an analysis of mobility data, we propose that the Tc saturation is due to carriers localization and disorder.