Nuclear magnetic resonance and heavy-fermion superconductivity in UBe 13 and related systems

Cheng Tien,D.E Maclaughlin,J.L Smith,W.G Clark,M.D Lan,H.R Ott,Z Fisk

doi:10.1016/0378-4363(85)90422-x

Cheng Tien, D.E Maclaughlin + Show 5 more

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https://doi.org/10.1016/0378-4363(85)90422-x

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Journal: Physica B+C	Publication Date: Dec 1, 1985
Citations: 9	License type: cc-by

Affiliation: University of California, Los Angeles

Abstract

We report the use of the 9Be nuclear magnetic resonance to study heavy-fermion superconductivity in the U 1− x Th x Be 13 alloy system, x = 0 and 0.033. The nuclear spin-lattice relaxation rate 1/T 1, which yields information on thermal excitations in the superconducting state, is found to vary more slowly the temperature than expected for a conventional BCS superconductor with a nonzero energy gap Δ. This indicates an enhanced density of excitations for low energies E ⪡ Δ. At intermediate temperatures 1/ T 1 T 3≃const. for UBe 13, which is consistent with highly anisotropic pairing; the specific temperature dependence suggests lines of zeros of Δ on the Fermi surface. The relaxation data do not agree qualitatively with theories of superconducting pair breaking due to paramagnetic impurities. At low temperatures (T < 0.2 K) 1 T 1T is approximately constant in UBe 13, with a value which decreases with decreasing applied field. This behavior is probably not due to direct relaxation by paramagnetic impurities, for which the temperature dependence would be in the opposite sense. For x= 0.033 1/ T 1 varies less rapidly than T 3. The additional relaxation may be due to filling in the gap by Th-induced pair breaking. The 9Be spectra give no indication of additional broadening due to vortex-lattice inhomogeneity or (for x = 0033) of magnetic ordering at a second transition temperature T c2 below the superconducting transition (at T c1). Our results are ompared to NMR studies of CeCu 2Si 2, and to other experiments in heavy-fermion superconductors.

Highlights

Cerium- and uranium-based intermetallic compounds have recently been discovered [1,2,3] in which very massive (m~ff > 200me) itinerant electrons, or "heavy fermions", are unambiguously involved in the superconducting state
A clear signature of heavyfermion superconductivity is a correspondingly large discontinuity in the specific heat at the superconducting transition temperature Tc, which implies that the heavy fermions themselves participate in the superconducting pairing [4]
This paper reports the results of our Nuclear magnetic resonance (NMR) studies in the superconducting states of the heavyfermion superconductors U1_xTh~Be~3, x = 0 and 0.033. (Data from the normal state will be discussed only as they affect interpretation of superconducting-state results.) Section 2 describes 9Be spin-lattice relaxation in U~ xTh~Be~3, x = 0 and 0.033, and section 3 gives results obtained from 9Be resonance spectra in these materials

Summary

Introduction

Cerium- and uranium-based intermetallic compounds have recently been discovered [1,2,3] in which very massive (m~ff > 200me) itinerant electrons, or "heavy fermions", are unambiguously involved in the superconducting state. The ~lSln data can be fit very well [18] to the activated temperature dependence expected for a BCS superconductor with only slight anisotropy of the gap parameter [9], and the reasonable value ( A ) a v / k B T c = 1.80 --- 0.05 is obtained [18] This comparison indicates that UBe13 is a highly anisotropic superconductor compared to conventional materials. A simple calculation shows, that spindiffusion-limited exponential relaxation would be consistent with the data for impurity concentrations =1000 ppm

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Conclusions