Pressure up to 2 GPa turns 6R-TaS2 superconductivity from nodal to nodeless, muon-spin data show

Researchers at the Paul Scherrer Institute (PSI) in Switzerland, led by Zurab Guguchia, studied the layered transition metal dichalcogenide 6R-TaS2 with muon-spin rotation, magnetotransport and hydrostatic pressure up to 2 GPa. The work, described on nature.com and published in Nature Communications, identifies a nodal superconducting state with low superfluid density at ambient pressure nature.comnature.comnature.comOpen source ↗. Under pressure the pairing becomes nodeless nature.comnature.comnature.comOpen source ↗.

Material and open questions

6R-TaS2 is a bulk heterostructure of alternating trigonal prismatic (1H) and octahedral (1T) layers. Earlier studies found a charge density wave (CDW) transition at about 305 K and a bulk superconducting transition at about 2.6 K, determined from magnetization measurements nature.comnature.comnature.comOpen source ↗. Superconductivity was proposed to reside mainly in the 1H layers, separated by insulating 1T layers in the CDW state nature.comnature.comnature.comOpen source ↗.

Below T* of about 40 K the material shows pronounced magnetoresistance and a nonlinear Hall effect (NHE), which had been interpreted as a sign of hidden magnetism nature.comnature.comnature.comOpen source ↗. The gap symmetry, the superfluid density, the origin of the NHE and its relation to the CDW remained undetermined nature.comnature.comnature.comOpen source ↗.

What pressure changed

Muon-spin rotation detected no spontaneous magnetic order below T*. According to the authors, this rules out magnetism as the origin of the NHE nature.comnature.comnature.comOpen source ↗.

At pressures up to 2 GPa the superfluid density rises markedly, in correlation with the superconducting transition temperature. The nodal pairing shifts to a nodeless state, and the CDW onset is reduced by half nature.comnature.comnature.comOpen source ↗. The NHE is fully suppressed and the magnetoresistance drops by 50% within just 0.2 GPa, which the authors take as evidence of the fragility of the state with the NHE nature.comnature.comnature.comOpen source ↗.

The authors conclude that superconductivity, charge order and the nonlinear Hall effect compete in 6R-TaS2, driven by weakened interlayer coupling and shared electronic states nature.comnature.comnature.comOpen source ↗. In the PSI team's account, compression brings the superconducting layers closer together and lets electrons localized in the insulating layers take part in superconductivity; they also report a sevenfold increase in the number of electrons involved.