One-sided Competition of Charge-Density-Wave order and Superconductivity in NbSe2

The interplay of incompatible ground states is a key concept in physics and central to many materials classes from monolayer graphene and transition metal dichalcogenides to cuparte high-temperature superconductors. Tuning the balance between these ground states reveals competition, coexisitence, and cooperation - sometimes even within the same system. Here, we present comprehensive high-pressure Hall effect and magnetic susceptibility measurements of the charge-density-wave (CDW) and superconducting state in NbSe2. We show that superconductivity is enhanced upon suppressing CDW order at high pressures whilst CDW order is insensitive to a suppression of superconductivity in high magnetic fields. With a realistic model calculation we show that the suppression of the CDW state at high pressures is described by the stiffening of the underlying bare phonon modes, whilst the suppression of the superconducting state at low pressures is due to the loss of density of states inside the CDW phase. We conclude that the competition between the two states is one-sided in NbSe2. Our work clarifies the interplay of superconductivity and CDW order in prototypical NbSe2 and highlights routes to optimise superconductivity in materials with competing order.