Weak-magnetism phenomena in heavy-fermion superconductors: selected  SR studies

Abstract

The behavior of the so-called weak moment antiferromagnetic states, observed in the heavy-fermion superconductors UPt 3 and URu 2 Si 2 , is discussed in view of recent µSR results obtained as function of control parameters like chemical substitution and external pressure. In UPt 3 , the Pd substitution for Pt reveals the dynamical character of the weak moment order. On the other hand, µSR measurements performed on samples in which Th substitutes U suggest that crystallographic disorder on the magnetic sites deeply affects the fluctuation timescale. In URu 2 Si 2 , a phase separation between the so-called hidden order state, present at ambient pressure, and an antiferromagnetic state, occurring under pressure, is observed. In view of the pressure-temperature phase diagram obtained by µSR, it is deduced that the respective order parameters have different symmetries.

Group theory and octupolar order inURu2Si2

“…Let us recall the hydrostatic pressure -temperature phase diagram obtained from highpressure µSR experiments [6] (a phase diagram of the same shape results from our mean field theory, see Fig. 5, left).…”

“…A recent µSR study [6] finds that the symmetry of hidden order is different from A 2u (Q) which is the symmetry of the high-pressure antiferromagnetic phase (the same structure was ascribed to the supposed "micromagnetism" of URu 2 Si 2 , which is now understood to be extrinsic). The present experimental status is that the intrinsic low-pressure behavior of URu 2 Si 2 is purely non-magnetic.…”

“…As we are going to describe, theoretical progress has long been held up by the ambiguity of experimental findings on apparently heterogeneous samples. However, crucial recent experiments [6,7] allow to infer what the equilibrium properties of ideal samples of URu 2 Si 2 would be.…”

“…6). There are two thermodynamic phases, a non-magnetic phase with ψ = 0 and m = 0, and the antiferromagnetic phase with m = 0 and ψ = 0.…”

Hydrostaticity and hidden order: effects of experimental conditions on the temperature–pressure phase diagram of URu2Si2

“…Evidence for the heterogeneous coexistence of the HO and AFM states below 15 kbar was suggested from 29 Si NMR measurements [16,17] and µSR measurements indicated that the AFM volume fraction emerged at ≈ 5 kbar (extrapolated to T = 0 K) [18,19]. This general pressure-dependence was corroborated by measurements of dilation under pressure [20], while measurements of ac magnetic susceptibility demonstrated the suppression of the SC state at approximately 4 kbar, at the T = 0 K onset of the HO-AFM transition [21,22].…”