Superconducting Pairs with Extreme Uniaxial Anisotropy inURu2Si2

Abstract

We report magnetic field orientation-dependent measurements of the superconducting upper critical field in high quality single crystals of URu(2)Si(2) and find the effective g factor estimated from the Pauli limit to agree remarkably well with that found in quantum oscillation experiments, both quantitatively and in the extreme anisotropy (≈10(3)) of the spin susceptibility. Rather than a strictly itinerant or purely local f-electron picture being applicable, the latter suggests the quasiparticles subject to pairing in URu(2)Si(2) to be "composite heavy fermions" formed from bound states between conduction electrons and local moments with a protected Ising behavior. Non-Kramers doublet local magnetic degrees of freedom suggested by the extreme anisotropy favor a local pairing mechanism.

Hidden order behaviour in URu2Si2(A critical review of the status of hidden order in 2014)

“…Finally, we treat the debate over the localised description that relies on the experimental realisation of a strong Isinglike anisotropy in URu 2 Si 2 that even occurs in dilute Th 1−x U x Ru 2 Si 2 . The huge Ising anisotropy of quasiparticles in URu 2 Si 2 has been emphasised as a hallmark of localised 5f character [13,60]. Notwithstanding, here we will illustrate that an itinerant DFT description can also explain the Ising anisotropy.…”

“…Specifically CEF excitations archetypical of localised 5f levels could not be detected (see [6] for a discussion). However, recent QO experiments reported evidence for localised 5f character [13]. For one of the QO branches a g-factor anisotropy (g c /g a ) exceeding 30 was estimated.…”

“…Such Ising anisotropy would conceivably support the picture of localised 5f states in a CEF. Conversely, for bandlike electrons, in general, a g-factor of two with little anisotropy is expected and would definitely seem to disqualify itinerant 5f behaviour [13].…”

“…Initial de Haas-van Alphen (dH-vA) and Shubnikov-de Haas (S-dH) data have been available onwards from pioneering work in 1999 [76] onwards with efforts continuing into the present [13]. The quantum oscillation (QO) experiments are limited to very low temperatures at significant ranges of applied magnetic field.…”

Hidden order in URu2Si2

“…However such multipolar order can not naturally account for the emergence of heavy Ising quasiparticles, a signature feature of URu 2 Si 2 that has been probed by two distinct experiments. 2,[32][33][34] The essential point here is that conventional quasiparticles have half-integer spin and are magnetically isotropic; they thus lack the essential Ising protection required by observation. In addition optical and tunnelling probes [35][36][37][38][39] indicate that the hybridization in URu 2 Si 2 develops abruptly at T HO and is thus associated with a global broken symmetry; 22,26,40,41 this is to be contrasted with the usual situation in heavy fermion materials where it is simply a crossover.…”