Nodeless Versus Nodal Scenarios of Possible Triplet Superconductivity in the Quasi-One-Dimensional Layered Conductor Li$_{0.9}$Mo$_6$O$_{17}$

Abstract

We consider the problem of the orbital upper critical magnetic field, parallel to the most conducting axis of a quasi-one-dimensional layered superconductor. It is shown that superconductivity can be destroyed through orbital effects at fields much higher than the so-called Clogston-Chandrasekhar paramagnetic limiting field, $H_p$, provided that superconducting pairing of electrons are of a triplet nature. We demonstrate that the superconducting state of the quasi-one-dimensional layered conductor, $\mathrm{Li_{0.9}Mo_6O_{17}}$, is well described by the suggested theory. To this end, we consider two competing scenarios: 1: a superconducting order parameter without zeros on the Fermi surface, and 2: one with zeros on the Fermi surface - both are shown to lead to destruction of superconductivity at a magnetic field, $H^x_{c_2}$, five times higher than $H_p$. With recent experimental measurements on the $\mathrm{Li_{0.9}Mo_6O_{17}}$ favoring the nodeless order parameter, we present a strong argument supporting triplet pairing in this compound.