D.E. MacLaughlin

The Unconventional Superconductivity of Skutterudite PrOs4Sb12: Time-Reversal Symmetry Breaking and Adjacent Field-Induced Quadrupole Ordering

This review presents a summary and evaluation of the experimental properties of unconventional superconductivity in PrOs 4 Sb 12 . After a brief introduction of filled skutterudites, we argue that the normal-state properties of PrOs 4 Sb 12 are quite different from ordinary heavy-fermion superconductors, in the sense that the 4 f -electrons seem to be well localized even at low temperatures where the superconductivity appears. This is reflected in the existence of a field-induced antiferro-quadrupole ordering of Pr 4 f -electrons. It is shown that the 4 f collective excitations can be described as quadrupolar excitons . Their strong temperature dependence in close correlation with the superconductivity suggests a conjecture that the superconducting mechanism may involve quadrupolar-exciton-mediated pairing of some kind. Although the structure of the superconducting order parameter is far from conclusive, outstanding experimental observations reported to date reveal the uniqueness of the unconventional sup...

63Cu NMR and hole depletion in the normal state of yttrium-rich Y1-xPrxBa2Cu3O7

The {sup 63}Cu Knight shift {ital K} and spin-lattice relaxation rate 1/{ital T}{sub 1} have been measured in the superconducting cuprate system Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7}, 0.05{le}{ital x}{le}0.20. With Pr doping {ital K} decreases and develops a temperature dependence at both plane and chain sites. This resembles the behavior of the Cu and Y Knight shifts as well as the bulk susceptibility in oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}. The orbital contribution to {ital K} and the anisotropy of the Cu hyperfine coupling remain essentially unchanged over the entire Pr concentration range. No appreciable direct effect of Pr magnetism on the conduction-band susceptibility was found. Instead, analysis of the bulk susceptibility and NMR data indicate that pair breaking and hole depletion both take part in the suppression of the superconducting transition temperature {ital T}{sub {ital c}}. The temperature dependence of 1/{ital T}{sub 1} for magnetic field parallel to the {ital c} axis is also similar to that for the oxygen-deficient compound. This agreement leads to a consistent picture of the role of antiferromagnetic fluctuations in these materials. An analysis of the data in the framework of the phenomenological theory of Millis, Monien, and Pines is given.

S29iNMR and Hidden Order inURu2Si2

Below T(N) approximately 17 K the 29Si NMR line in URu2Si2 exhibits a previously unobserved field-independent nearly isotropic contribution to the linewidth, which increases to approximately 12 G as T-->0. We argue that this feature does not arise from static freezing of the U-spin magnetization, but is due to coupling between 29Si spins and a hidden order parameter. We discuss time-reversal symmetry-breaking orbital antiferromagnetism and indirect nuclear spin-spin interactions as possible coupling mechanisms. Further NMR experiments and theoretical calculations are suggested.

μSR studies of intercalated HfNCl superconductor

Abstract Muon spin relaxation measurements of the magnetic field penetration depth λ have been performed in a quasi two-dimensional (2-d) superconductor based on HfNCl intercalated with Li and organic molecules. The relaxation rate σ(T→0)∼0.5 μs −1 was obtained with transverse fields applied perpendicular to the aligned HfN planes. Using the relation σ∝λ −2 ∝n s2d /m ∗ (2-d superconducting carrier density/effective mass), we deduced the 2-d Fermi temperature TF2d. The ratio Tc/TF2d for the HfNCl-(Li) system is very close to those in two other quasi-2-d superconductors, YBa2Cu3O7 and (BEDT-TTF)2-X. Compared to the Kosterlitz–Thouless (KT) transition temperature TKT=TF2d/8 for the limit of strong coupling, Tcs of these systems are reduced by a factor of 3–4, indicating the importance of carrier overlapping and/or 3-d coupling.

Pd/Cu Site Interchange and Non-Fermi-Liquid Behavior in UCu{sub 4}Pd

X-ray-absorption fine-structure measurements of the local structure in UCu{sub 4}Pd are described which indicate a probable lattice-disorder origin for non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed, consistent with (24{plus_minus}3){percent} of the Pd atoms occupying nominally Cu sites. A {open_quotes}Kondo disorder{close_quotes} model, based on the effect on the local Kondo temperature T{sub K} of this interchange and some additional bond-length disorder, agrees quantitatively with previous experimental susceptibility data, and therefore also with specific heat and magnetic resonance experiments. {copyright} {ital 1998} {ital The American Physical Society }

Glassy Spin Dynamics in Non-Fermi-Liquid UCu5xPdx, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu5-xPdx, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function G(t) indicates strongly inhomogeneous spin fluctuations and exhibits the scaling G(t,H) = G(t/H(gamma)) expected from glassy dynamics. At 0.05 K gamma(x = 1.0) = 0.35+/-0.1, but gamma(x = 1.5) = 0.7+/-0.1. This is in contrast to inelastic neutron scattering results, which yield gamma = 0.33 for both concentrations. There is no sign of static magnetism approximately greater than 10(-3)(B)/U ion in either material above 0.05 K. Our results strongly suggest that both alloys are quantum spin glasses.

Disorder-driven non-Fermi-liquid behavior in CeRhRuSi2

We report measurements of the bulk magnetic susceptibility and 29 Si nuclear magnetic resonance (NMR) linewidth in the heavy-fermion alloy CeRhRuSi2. The linewidth increases rapidly with decreasing temperature and reaches large values at low temperatures, which strongly suggests the wide distributions of local susceptibilitiesj obtained in disorder-driven theories of non-Fermi-liquid (NFL) behavior. The NMR linewidths agree well with distribution functions P(�) which fit bulk susceptibility and specific heat data. The apparent return to Fermi-liquid behavior observed below 1 K is manifested in the vanishing of P(�) as � → ∞, suggesting the absence of strong magnetic response at low energies. Our results indicate the need for an extension of some current theories of disorder-driven NFL behavior in order to incorporate this low-temperature crossover.

29Si nuclear magnetic resonance and relaxation in the paramagnetic state of CeCu2Si2

Abstract 29Si nuclear magnetic resonance (NMR) and relaxation studies of the unstable-moment compound CeCu2Si2 have been carried out to probe the electronic state of the nearly-trivalent Ce ions. Isotropic and anisotropic NMR shifts indicate effects of both crystalline electric fields (CEF) and low-temperature moment reduction on the Ce ionic states. A relatively large anisotropic shift was observed at low temperatures, and also obtained from a CEF-state calculation in the molecular-field approximation. The hyperfine coupling also appears to be anisotropic, although this is not manifested in the temperature dependence of the shift due to changing CEF-state populations. NMR estimates of the Ce-spin fluctuation time vary with temperature in a manner similar to the neutron quasielastic scattering linewidth at high temperatures, but suggest the onset of spatially coherent fluctuations.

MAGNETIC BEHAVIOR IN LI-DOPED LA2CUO4

We report muon-spin-relaxation studies in the Li-doped cuprates La{sub 2}Cu{sub 1{minus}{ital x}}Li{sub {ital x}}O{sub 4} for {ital x}=0.01, 0.05, 0.10, 0.45, and 0.50. For low Li concentrations ({ital x}{le}0.10) we find a rapid suppression of {ital T}{sub {ital N}} as {ital x} increases, but little change in the magnitude and temperature dependence of the antiferromagnetic order parameter. This indicates that Li doping effectively destroys antiferromagnetism (similar to Sr doping, but different from Zn doping) without strongly affecting either the on-site Cu moments or the shape of the spin-wave excitation spectrum. For high Li concentrations we find that the majority of the sample volume is nonmagnetic, suggesting possible singlet-state formation. {copyright} {ital 1996 The American Physical Society.}

m(+) knight shift measurements in U0.965Th0.035Be13 single crystals

Muon spin rotation ( &mgr;SR) measurements of the temperature dependence of the &mgr;(+) Knight shift in single crystals of U0. 965Th0.035Be13 have been used to study the static spin susceptibility chi(s) below the transition temperatures T(c1) and T(c2). While an abrupt reduction of chi(s) with decreasing temperature is observed below T(c1), chi(s) does not change below T(c2) and remains at a value below the normal-state susceptibility chi(n). In the normal state we find an anomalous anisotropic temperature dependence of the transferred hyperfine coupling between the &mgr;(+) spin and the U 5f electrons.