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Abstract
Superconductivity is often found in a dome around quantum critical points, i.e. 2nd-order quantum phase transitions. Here, we show that an enhancement of superconductivity is avoided at the critical pressure of the charge-density-wave (CDW) state in NbSe$_2$. We present comprehensive high-pressure Hall effect and magnetic susceptibility measurements of the CDW and superconducting state in NbSe$_2$. Initially, the 2nd-order CDW transition is suppressed smoothly but it drops to zero abruptly at PCDW = 4.4 GPa thus indicating a change to 1st order whilstthe superconducting transition temperature Tc rises continuously up to PCDW but is constant above. The putative 1st-order nature of the CDW transition is suggested as the cause for the absence of a superconducting dome at PCDW. Indeed, we show that the suppression of the superconducting state at low pressures is due to the loss of density of states inside the CDW phase whilst the initial suppression of the CDW state is accounted for by the stiffening of the underlying bare phonon mode.
Highlights
The interplay of competing orders is of fundamental and practical interest [1,2,3,4]
We show that the suppression of the superconducting state at low pressures is due to the loss of density of states inside the CDW phase, while the initial suppression of the CDW state is accounted for by the stiffening of the underlying bare phonon mode
We show very clearly that superconductivity is reduced inside the CDW phase because of the loss of density of states
Summary
The interplay of competing orders is of fundamental and practical interest [1,2,3,4]. On a fundamental level, understanding the interplay between ground states provides important insight into the mechanism underlying each ground state and can reveal new phenomena at the border of ordered phases [5,6,7]. A large body of work focuses on the interplay of superconductivity and charge order in cuprate high-temperature superconductors [8]. With both superconductivity and charge-density-wave (CDW) order stabilised by the opening of a gap on (parts of) the Fermi surface, a mutual competition between the two states has been anticipated since early studies [9].
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