Effects of disorder and hydrostatic pressure on charge density wave and superconductivity in 2H−TaS2

Abstract

We report the comparative effects of disorder and hydrostatic pressure on charge density wave (CDW) and superconductivity (SC) in $2H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ by measuring electrical resistivity and ac magnetic susceptibility. For the crystals in the clean limit (low disorder level), CDW ground state is suppressed completely at a critical pressure ${P}_{\mathrm{c}}\ensuremath{\sim}6.24(5)\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ where a dome-shaped pressure dependence of superconducting transition temperature ${T}_{\mathrm{c}}$(P) appears with a maximum value of ${{T}_{\mathrm{c}}}^{\mathrm{max}}\ensuremath{\sim}9.15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, indicating strong competitions between CDW and SC. The temperature exponent $n$ of low-temperature resistivity data decreases from \ensuremath{\sim}3.36 at ambient pressure (AP) to \ensuremath{\sim}1.29(2) at ${P}_{\mathrm{c}}$ and then retains a saturated value \ensuremath{\sim}2.10(4) when the pressure is higher than 7.5 GPa; accordingly, the quadratic temperature coefficient of normal-state resistivity A peaks out just at ${P}_{\mathrm{c}}$ with an enhancement by nearly one order in magnitude. These features strongly manifest that the enhanced critical CDW fluctuations near ${P}_{\mathrm{c}}$ are possible important glues for superconducting pairings. High-pressure magnetic susceptibility indicates that superconducting shielding volume increases with pressure and retains a nearly constant value above ${P}_{\mathrm{c}}$, which evidences that the enhancement of ${T}_{\mathrm{c}}$(P) is accompanied by the expense of CDW. For those crystals in dirty limit (high disorder level), there is no clear CDW phase transition in resistivity; the pressure dependence of ${T}_{\mathrm{c}}$(P) and $n$ broadens up and becomes less apparent in comparison with the clean crystals. Our results suggest that disorder scattering and the melting of CDW are two factors affecting SC, and the melting of CDW dominates the change of ${T}_{\mathrm{c}}$ below ${P}_{\mathrm{c}}$; the enhancement of ${T}_{\mathrm{c}}$(P) is associated with the suppression of CDW by pressure and the increase in the density of states at Fermi level; however, after the CDW collapse, superconducting pairing strength is strongly weakened by impurity scattering above ${P}_{\mathrm{c}}$ according to Anderson's theorem.