R. Kadono

Positive muon Knight shift and spin relaxation in heavy fermion superconductors UPt3 and UBe13

Abstract We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductors UBe 13 and UPt 3 . In both materials we find that the muon Knight shift is unchanged in the superconducting state, consistent with odd-parity pairing (such as p-wave). The magnetic field penetration depths in UPt 3 and UBe 13 are extremely long, greater than 10000 A. We find no evidence of a magnetic transition in UBe 13 below 10 K.

Probing magnetism and superconductivity of high-Tc systems with positive muons

Abstract Recent results from our μSR experiments on high- T c systems are presented. We describe studies of static magnetic order in hole-doped and electron-doped systems, correlations between T c and n s / m * (carrier density / effective mass) in hole-doped superconducting cuprates, the temperature dependence of the penetration depth in c-axis oriented sintered ceramic of Y Ba 2 Cu 3 O 7 , as well as the detection of flux pinning phenomena by μSR.

Static magnetic order in electron-superconductor related compounds

Abstract Using the technique of muon spin relaxation, we have shown that the parent compounds Ln 2 CuO 4- y where [ Ln =( Nd , Sm or Pr )] of the new electron-superconductors exhibit static magnetic order below 300 K. No static magnetic order with any appreciable moment was observed above 4 K in superconducting samples of Nd 2- χ Ce χ CuO 4− y for χ > .155. Measurements on a single crystal of Nd 2 CuO 4− y show magnetic order with a sharp onset at 250 K, followed by a number of changes in the ordered spin direction at T ∼ 65 K and T ∼ 33 K.

Magnetic ordering in YBa2Cu3Ox studied by μ+ spin rotation☆

YBa2Cu3Ox is known to be antiferromagnetic (AFM) for x ≲ 6.45 but the details of its sublattice magnetization(s) are still controversial. A zero-field μ+SR investigation of the x- and T-dependence of the local fields at several muon sites reveals sharp changes at 10K and 80K for an oriented powder sample with x ≈ 6.0, but other x ≈ 6.0 samples do not show the same features. The direction of the local field at one μ+ site is shown to be along the ĉ-axis. For x < 6.0 the samples still show local spin freezing up to 300K, but coherent AFM order is lost even at 3K.

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe13, URu2Si2 and U6Fe. The magnetic field penetration depth in the superconducting state of UBe13 is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowTc. A sharp increase in the precession frequency is seen, starting atTc. This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U6Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atTc=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenTc and the relaxation rate σ αns/m* (carrier density/effective mass) of hole-doped high-Tc cuprate superconductors (La2, Sr3)CuO4, YBa2Cu3O7 (Y1−xPrx)Ba2Cu3O7, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)2Cu(NCS)2.

Resolved nuclear hyperfine structure of muonium in CuCl by means of muon level-crossing resonance

Datailed muon level-crossing resonance measurements of Mu1 and Mu11 centres in single crystals of CuCl are presented. The hyperfine and nuclear hyperfine parameters of the closest two shells of nuclei are remarkably similar for the two centres, indicating that both are located at the same tetrahedral interstitial site with four Cu nearest neighbours and six Cl next-nearest neighbours. About 30% of the total unpaired-electron spin density is located on the muon, about 60% on the four nearest neighbours and the rest on the six next-nearest neighbours, with nothing observable for any other shell.

Coexistence of superconductivity and magnetic order in YBa2Cu3Ox

Abstract Resistivity, susceptibility and μ + SR measurements have been used to investigate the ( x , T ) phase diagram of YBa 2 Cu 3 O x . Carefully annealed samples are antiferromagnetic (AFM) insulators for 6.0 ≲ x ≲ 6.35 and nonmagnetic superconductors (SC) for 6.5 ≲ x ≲ 7.0; SC coexists with AFM or spin glass (SG) static magnetic order at low temperatures for 6.38 ≲ x ≲ 6.48. A sample with x = 6.462 is SC below 25 K and simultaneously magnetic below 10 K; the internal diamagnetism of the SC state is progressively reduced as local spin freezing sets in at low temperatures.

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd2−xCexCuO4−y and a large single crystal of Nd2CuO4−y. We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La2−xSrxCuO4−y, although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd2CuO4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.

μ+Sr studies of flux mobility in the mixed state of Bi2Sr2CaCu2O8

Abstract Muon Spin Rotation (μ + SR) studies of flux vortex structure in single crystal, crushed crystal, thick film, and sintered specimens of Bi 2 Sr 2 CaCu 2 O 8 are reported. Field-Cooled & Zero-Field-Cooled measurements indicate that the flux vortices in the mixed state of each material are highly mobile at temperatures above a pinning/freezing temperature T p c . Measurements on the thick film sample with H ext =10, 4.1, 2 and 1kG show that T p decreases with increasing field strength.