K. Kojima

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.

Spatial Redistribution of Oxygen Ions in Oxide Resistance Switching Device after Forming Process

The change in the spatial distribution of oxygen ions after an initial voltage application called the forming process was investigated for oxide resistance switching devices by secondary ion mass spectrometry mapping. To track the motion of oxygen ions, tracer 18O ions were implanted in a planar Pt/CuO/Pt device. We found clear evidence for the oxygen reduction in the conductive bridge structure formed between two electrodes. In addition, the oxygen ions in the bridge structure drift to the anode, implying the oxygen diffusion (migration) induced by high electric field and/or current density. We discuss those results in terms of a filament model.

Unconventional superconductivity in Sr2RuO4

Abstract We have performed ZF-μSR measurements of Sr2RuO4 and have observed a spontaneous magnetic field below Tc. This observation, combined with other symmetry considerations demonstrates that superconductivity in Sr2RuO4 is of ‘p-wave’ (odd-parity) type. Our transverse field μSR measurements in the mixed state with B=150 G || c clearly indicate the presence of a square flux lattice. We have analysed the field distribution in terms of the penetration depth, coherence length and a Fermi surface anisotropy term and found λ(T=0)=1900xa0A. λ(T→0) is quite flat, consistent with a superconducting gap with no nodes as expected for the E u (k x ± i k y ) state.

μSR studies of the kagomé antiferromagnet (H3O)Fe3(OH)6(SO4)2

Abstract The Heisenberg kagome antiferromagnet is a frustrated system that provides a simple model for the study of strongly fluctuating magnets with a highly degenerate ground state. Hydronium jarosite, (H3O)Fe3(OH)6(SO4)2 possesses a kagome lattice of Fe3+ ions that behave as S= 5 2 Heisenberg moments coupled through strong antiferromagnetic exchange. DC susceptibility measurements reveal a spin-glass-like transition at Tf≅15xa0K. We have performed muon-spin relaxation (μSR) measurements on this material to elucidate the nature of this transition. Well above Tf, the muon depolarisation adopts a simple exponential form, becoming a stretched exponential with exponent β approaching 0.5 as T→Tf. On cooling further, β drops further to 1 3 , and the depolarisation adopts a dynamic Kubo–Toyabe form which may be decoupled by applied longitudinal fields of the order of 5xa0kG. We estimate the upper limit of the static component of the moment to be 3.4xa0μB per iron ion, compared with a value of ≅5.92xa0μB expected for high-spin Fe3+.

Spin frustration in Y(Sc)Mn2

The relaxation rate of muon polarization of Y0.97Sc0.03Mn2 with a magnetic lattice composed of corner‐shared tetrahedrons shows a broad peak around 2.5 K indicating a random spin freezing at low temperatures. The substitution by nonmagnetic Al atoms for 10% Mn atoms raises the spin freezing temperature to 45 K. The present work shows clearly that due to the strong geometrical frustration, the ground state of the system is singlet tetrahedrons mixed with frozen paramagnetic tetrahedrons.

Spin-Peierls and spin-glass phases in pure and doped CuGeO3: a μSR study

Abstract We have conducted μSR measurements in pure and Zn-doped CuGeO 3 . The pure compound does not have static magnetic moments between 3 and 100 K, in agreement with its earlier proposed spin-Peierls structure. In Cu 0.96 Zn 0.04 GeO 3 , we found a spin-glass-like freezing of a small portion of Cu spins, presumably near the chain-cutting Zn atoms, occurring gradually between 3.5 and 6 K.

Is Y(Sc)Mn2 really a quantum spin liquid

Abstract Spin fluctuations in frustrated Y 0.97 Sc 0.03 Mn 2 have been studied by μ + SR. The relaxation rate increases monotonically with decreasing temperature and is not appreciably changed by the longitudinal field, in contrast with that in Y 0.97 Sc 0.03 (Mn 0.9 Al 0.1 ) 2 . These results imply fluctuations in a quantum spin liquid.

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.

Magnetism of CePt2Sn2

Abstract CePt 2 Sn 2 has the tetragonal P4/nmm structure. Polycrystalline (pcr) material possesses in addition a small mono-clinic distortion and shows a magnetic transition below 1xa0K, while no such transition is seen in purely tetragonal single-crystalline (scr) samples. Our μSR, specific heat and magnetization data are consistent with a spin-glass-like (sgl) state below 0.45xa0K in scr and long-range order (lro) below 0.85xa0K in pcr samples. μSR demonstrates clearly that the Ce 3+ spin ensemble remains dynamic down to lowest temperatures (0.02xa0K) in the sgl state, but reaches the usual quasi–static limit in the lro state. These results are discussed in terms of geometrical magnetic frustration.

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.