J. Merrin

Time-reversal symmetry-breaking superconductivity in Sr2RuO4

We report muon spin relaxation measurements on the superconductor Sr2RuO4 that reveal the spontaneous appearance of an internal magnetic field below the transition temperature: the appearance of such a field indicates that the superconducting state in this material is characterized by the breaking of time-reversal symmetry. These results, combined with other symmetry considerations, suggest that superconductivity in Sr2RuO4 is of p-wave (odd-parity) type, analogous to superfluid 3He.Although the properties of most superconducting materials are well described by the theory of Bardeen, Cooper and Schrieffer (BCS), considerable effort has been devoted to the search for exotic superconducting systems in which BCS theory does not apply. The transition to the superconducting state in conventional BCS superconductors involves the breaking of gauge symmetry only, whereby the wavefunction describing the Cooper pairs—the paired electron states responsible for superconductivity—adopt a definite phase. In contrast, a signature of an unconventional superconducting state is the breaking of additional symmetries, which can lead to anisotropic pairing (such as the ‘d-wave’ symmetry observed in the copper oxide superconductors) and the presence of multiple superconducting phases (as seen in UPt3 and analogous behaviour in superfluid 3He; refs 3–5). Here we report muon spin-relaxation measurements on the superconductor Sr2RuO4 that reveal the spontaneous appearance of an internal magnetic field below the transition temperature: the appearance of such a field indicates that the superconducting state in this material is characterized by the breaking of time-reversal symmetry. These results, combined with other symmetry considerations, suggest that superconductivity in Sr2RuO4 is of ‘p-wave’ (odd-parity) type, analogous to superfluid 3He.

Reduction of Ordered Moment and Néel Temperature of Quasi-One-Dimensional Antiferromagnets Sr2CuO3 and Ca2CuO3

We report elastic neutron diffraction and muon spin relaxation ({mu}SR) measurements of the quasi-one-dimensional antiferromagnets Sr{sub 2}CuO{sub 3} and Ca{sub 2}CuO{sub 3}, which have extraordinarily reduced T{sub N}/J ratios. We observe almost resolution-limited antiferromagnetic Bragg reflections in Sr{sub 2}CuO{sub 3} and obtain a reduced ordered moment size of {approximately}0.06{mu}{sub B}. We find that the ratio of ordered moment size {mu}(Ca{sub 2}CuO{sub 3})/{mu}(Sr{sub 2}CuO{sub 3})=1.5(1) roughly scales with their N{acute e}el temperatures, which suggests that the ordered moment size of quasi-one-dimensional antiferromagnets decreases continuously in the limit of vanishing interchain interactions. {copyright} {ital 1997} {ital The American Physical Society}

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.

Muon spin relaxation measurements of magnetic-field penetration depth in Ba8Si46

Abstract Muon spin relaxation (μSR) and high-field magnetization measurements have been performed in Ba8Si46, a superconductor with T c =8 K having a silicon clathrate three-dimensional cage structure. The upper critical field H c2 =6.5 T , determined from the magnetization, indicates a short coherence length ξ=72 A . The magnetic field penetration depth λ(T→0)=4000 A was obtained from the muon spin relaxation rate σ(T→0)∼0.46 μs −1 observed in transverse fields of 0.1–1 T. These values of ξ and λ indicate that this system is a strongly type-II superconductor with a high value of κ=56. We deduce the effective Fermi temperature TF and compare the present system with other superconducting systems in a plot of Tc versus TF.

Superconductivity and magnetism in UPt3

We describe muon spin rotation/relaxation (μSR) measurements of UPt3. In zero applied field, we find that a spontaneous field appears below the lower superconducting transition and is visible when the initial muon polarization is both along theĉ axis and in the basal plane. In addition, we have measured the muon Knight shift for fields in both primary directions. We find that the Knight shift decreases within the superconducting state for fields alongĉ, as well as for a field of 1 T in the basal plane.

Magnetic field penetration depth in single crystal La1.85Sr0.15CuO4 and Nd1.85Ce0.15CuO4

Abstract We have measured the magnetic field penetration depth in single crystals of La 1.85 Sr 0.15 CuO 4 . Nd 1.85 Ce 0.15 CuO 4 using the technique of muon spin rotation. In La 1.85 Sr 0.15 CuO 4 and dependence of λ ( T → 0), characteristic of a d-wave pairing state. The absolute value of the penetration depth in electron-doped Nd 1.85 Ce 0.15 CuO 4 is comparable to that in the hole-doped La 1.85 Sr 0.15 CuO 4 .

Spin fluctuations and magnetic order in the heavy fermion compound CePt2Sn2

We present zero and longitudinal field μ SR measurements of single crystal and polycrystalline specimens of the heavy fermion compound CePt2Sn2. Above 1 K the behaviour of the two samples is indistinguishable; the muon 1/T_1 increases with decreasing temperature until 25 K when it plateaus. The 1/T_1 relaxation rate differs strongly for the two cases below \sim\,0.8\ K. At 0.1 K a rate of about 20 μ s-1 is seen in the polycrystal while in the single crystal it is only about 5 μ s-1. Even more revealing is the fact that longitudinal field decoupling spectra at very low temperatures demonstrate an essentially static spin system to be present in the polycrystalline material while the single crystal shows definite dynamic spin properties. We conclude that, in the presence of the distortion, long range magnetic order occurs below 0.9 K while in tetragonal symmetry long range order is suppressed (probably due to frustration) and spin fluctuations remain for T\rightarrow0.

Magnetic order in La2-x Bax CuO4 and La1.6-x Nd0.4 Srx CuO4 for x=0.125

We present μSR experiments on La2-x Bax CuO4 and La1.6-x Nd0.4 Srx CuO4 for x=0.125. Both of these materials order magnetically with TN\approx30\ K, while a superconducting sample of La1.4 Nd0.4 Sr0.2 CuO4 showed no evidence for static copper moments. Our results support the conclusion that the so‐called “1/8” anomaly in La2-x Bax CuO4 is a result of static (pinned) charge segregation.

Exponential field distribution in Sr(Cu1−xZnx)2O3

Abstract We discuss the use of the Kubo Golden rule (KGR) in the derivation of new muon polarization functions for arbitrary static field distributions in both zero (ZF) and longitudinal (LF) applied fields. ZF/LF muon spin relaxation measurements in spin-ladder system Sr(Cu1−xZnx)2O3 are reported, with observations of dilute moment freezing for x≲0.008 and dense moment freezing for x≳0.008. In neither the dilute nor the dense case do the data conform to classic muon polarization functions. Using the KGR we derive new muon polarization functions to deal with both of these cases. The dilute case assumes dilute moments with a dense moment background, while the dense case assumes an exponential distribution of fields. Simulation of this system is consistent with an exponential distribution of fields.

Spin freezing and ordering in CaV4O9, CaV3O7 and CaV2O5

Abstract We have performed muon spin relaxation measurements of CaV 4 O 9 , CaV 3 O 7 and CaV 2 O 5 . The sublattice magnetization in the antiferromagnetic state of CaV 3 O 7 agrees well with spin wave theory. In CaV 4 O 9 and CaV 2 O 5 we observe spin freezing below 15 and 50 K, respectively, which indicates that a substantial fraction of the vanadium magnetic moment is not in a spin-singlet state at low temperatures in either material.