Layered transition-metal dichalcogenides that host coexisting charge-density wave (CDW) and superconducting orders provide ideal systems for exploring the effects of dimensionality on correlated electronic phases. Dimensionality has a profound effect on both superconductivity and CDW instabilities. Here we report a substantial enhancement of the superconducting $T_c$ to 3.4 K for $2H\text{−}{\mathrm{TaS}}_2$ in the monolayer limit, compared to 0.8 K in the bulk. In addition, the transport signature of a CDW phase transition vanishes in the two-dimensional limit. In our analysis of electronic and vibrational properties of this material, we show that a reduction of the CDW amplitude results in a substantial increase of the density of states at the Fermi energy, which can boost $T_c$ by an amount similar to that seen in experiment. Our results indicate competition between CDW order and superconductivity in ultrathin $2H\text{−}{\mathrm{TaS}}_2$ down to the monolayer limit, providing insight toward understanding correlated electronic phases in reduced dimensions.

• Received 26 October 2017
• Revised 18 June 2018

DOI:https://doi.org/10.1103/PhysRevB.98.035203