Anisotropic temperature dependence of the magnetic-field penetration in superconductingUPt3

Abstract

The anisotropy and temperature dependence of the magnetic-field penetration in superconducting UPt3 have been measured by muon spin relaxation. The extrapolated zero-temperature values for the penetration depths parallel and perpendicular to the c axis are Xw =7070 ± 30 A and X± =7820 ± 30 A, respectively. The temperature dependences of Xw and X± are different and can both be accounted for by a superconducting gap function with a line of nodes in the basal plane and axial point nodes.PACS numbers: 74.70.Tx, 74.60.Ec, 76.75.+i There are many experiments which give indirect evidence for anisotropic superconductivity in heavy-fermion systems. These fall into two categories, namely, measurements of transport coefficients or specific heat at low temperatures (70, 12 and measurements showing transitions between different superconducting states. 3,4 Experiments of the first type have shown that the temperature dependence of bulk properties of heavy-fermion superconductors follow power laws at low temperatures as opposed to the exponentially activated behavior characteristic of conventional superconductors. The absence of activated T dependence implies nodes in the gap functions, which can be due to either unconventional pairing in a clean sample or conventional pairing in the presence of magnetic impurities. 5 In spite of the great activity 5 on heavy-fermion superconductivity, the only experiment which by itself resolves this ambiguity in favor of unconventional pairing is a transverse ultrasound measurement on UPt3, 2 where the attenuation was found to rise (from 7=0) by Aa~~T and Aa~~T 3 for sound propagating in the basal plane with polarization parallel and perpendicular to the basal plane, respectively. For conventional gapless superconductors, Aa~ T 2 , independent of polarization direction. Ultrasound attenuation is a nonequilibrium property, and a prerequisite for its interpretation is a detailed understanding of the mean free path for thermally excited quasiparticles. In the present paper, we describe the first demonstration that a static property, namely, the magnetic penetration depth X, of a heavyfermion superconductor has an anisotropic T dependence. Previous measurements of X for several heavyfermion systems showed that X increased from its 7=0 value in proportion to T 2 , but did not address the issue of anisotropy. 6,7 The technique used here, muon spin re-laxation (n + SR), is unique in that it both gives absolute values for X and allows anisotropy to be measured in a single experimental run for a single face of the sample, with no mechanical disturbance of the apparatus. Furthermore, the 4.2-MeV muons used in this experiment penetrate ~ 70 jim into UPt3 and thus probe bulk superconducting material. The important results of our experiment are (i) that X(T=0) is consistent with the London formula where the effective mass is the renormalized (heavy) mass, and (ii) that the T dependence of X is anisotropic in a manner which implies that the superconducting gap function has a line of nodes in the basal p...

Topological Weyl superconductor to diffusive thermal Hall metal crossover in theBphase ofUPt3

“…22 , magnetic field penetration depth 23 and thermal conductivity 24 suggest the existence of nodal excitations with density of states (DOS) D( ) ∝ . The linear in energy DOS is also supported by the magnetic field dependence of the specific heat C ∝ √ H 25 .…”

Space-Group Approach to the Nodal Structure of the Superconducting Order Parameter in UPt3

“…Recent experimental investigations reveal interesting features of the superconducting order parameter (SOP) in UPt 3 . The transport and thermal properties of heavy-fermion superconductors were explained in terms of the anisotropic order parameter with line (lines) of nodes [1,2].…”

Muon spin relaxation in the heavy fermion system UPt3

“…We therefore conclude that the spin susceptibility is unchanged below Tc· This is in agreement with agreement with previous work. 13 Plotting the field dependence of the increase in tbe relaxation rate below Tc [inset of Fig. 2(a)], we see that there is a reasonably smootb decrease in u with increasing applied field.…”