Wataru Higemoto

Time-reversal symmetry-breaking superconductivity in heavy-fermion PrOs4Sb12 detected by muon-spin relaxation.

We report on muon-spin relaxation measurements of the 4f(2)-based heavy-fermion superconductor filled-skutterudite Pr(Os4Sb12. The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature, providing unambiguous evidence for the breaking of time-reversal symmetry in the superconducting state. A discussion is made on which of the spin or orbital component of Cooper pairs carries a nonzero momentum.

Li(Zn,Mn)As as a new generation ferromagnet based on a I–II–V semiconductor

In a prototypical ferromagnet (Ga,Mn)As based on a III-V semiconductor, substitution of divalent Mn atoms into trivalent Ga sites leads to severely limited chemical solubility and metastable specimens available only as thin films. The doping of hole carriers via (Ga,Mn) substitution also prohibits electron doping. To overcome these difficulties, Masek et al. theoretically proposed systems based on a I-II-V semiconductor LiZnAs, where isovalent (Zn,Mn) substitution is decoupled from carrier doping with excess/deficient Li concentrations. Here we show successful synthesis of Li(1+y)(Zn(1-x)Mn(x))As in bulk materials. Ferromagnetism with a critical temperature of up to 50 K is observed in nominally Li-excess (y=0.05-0.2) compounds with Mn concentrations of x=0.02-0.15, which have p-type metallic carriers. This is presumably due to excess Li in substitutional Zn sites. Semiconducting LiZnAs, ferromagnetic Li(Zn,Mn)As, antiferromagnetic LiMnAs, and superconducting LiFeAs systems share square lattice As layers, which may enable development of novel junction devices in the future.

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...

Magnetic Properties in Phase IV of Ce0.7La0.3B6 Studied by Muon Spin Relaxation

The magnetic properties of Ce 0.7 La 0.3 B 6 single crystal have been studied by muon spin relaxation (µSR). The absence of a clear muon spin precession under zero external field in phase IV indicates that no commensurate long-range magnetic order is present in this phase. Moreover, the time spectra under a longitudinal field strongly suggests that there are two sources of local fields exerting coherently on muons in phase IV. It was inferred from the muon Knight shift under a transverse field that the direction of the effective internal magnetic field probed by muon exhibits a marked change between phase III and IV.

Evidence for Appearance of an Internal Field in the Ordered State of CeRu2Al10 by µ+SR

We report zero-field µ + SR measurements made at J-PARC on CeRu 2 Al 10 . Below the phase transition temperature T 0 ∼27 K, a clear modulation of µ + polarization due to an internal field is observed, indicating that magnetic ordering takes place. Compared with the internal field expected from the effective magnetic moment in the disordered state, the observed internal field at the µ + site is quite small. This indicates that the ordered Ce magnetic moment is small (∼10 -2 µ B ) and/or the nearest pair of Ce magnetic moments are mutually anti-parallel. The T -dependence of the internal field is somewhat peculiar, suggesting that another ordering is taking place as well.

Hidden order and weak antiferromagnetism in URu2Si2

Abstract The origin and nature of the low-temperature phase appearing below 17.5 K (≡T 0 ) in URu2Si2 is still not fully understood. Since the weak antiferromagnetic state (WAF; μord∼0.02–0.04μB) developing below around T0 is not simply reconciled with a large reduction in the magnetic entropy (∼0.2R ln 2) , the existence of some hidden order parameter has been argued, where WAF is considered to be of a secondary nature. The paper reviews recent development of microscopic characterization of WAF based on neutron scattering and 29 Si NMR under high pressure. It is demonstrated that, at least, for pressures ranging from ∼0.3 to ∼0.83 GPa , there are spatially competing two different ordered states in the crystal: a normal-moment (∼0.25μB/U) antiferromagnetic state and some unknown, non-magnetically ordered state. Simple extrapolation to ambient pressure suggests that WAF originates from one percent volume fraction of this antiferromagnetic phase. We also discuss the unusual magnetic properties at ambient pressure, presenting our latest results of zero-field and longitudinal-field μSR experiments.

Magnetic properties of the diamond chain compound Cu3(CO3)2(OH)2

Azurite (Cu 3 (CO 3 ) 2 (OH) 2 ) is a model substance of a diamond spin chain, one of frustrated antiferromagnetic quantum spin chains. The high field magnetization up to 60 T, magnetic susceptibility, magnetic entropy, muon spin relaxation and 1 H-NMR of azurite have been measured using single crystals. A distinct 1/3 magnetization plateau is confirmed to be present in the magnetization curve. Two resonance peaks of 1 H-NMR. were observed. By analyzing temperature dependencies of their hyperfine shift. we found two different local susceptibilities which correspond to susceptibility of dimer and monomer spins, respectively. New field induced phase transition was found to occur at a critical field where the magnetic plateau onsets. A short range magnetic ordering developed in a peculiar two-stage process, which was revealed by temperature dependence of the magnetic specific heat and susceptibility.

Evolution of local magnetic state in SmRu4P12 probed by muon spin relaxation

We performed muon spin relaxation measurements on randomly aligned single-crystalline samples of SmRu 4 P 12 down to 0.02 K to investigate static and dynamical magnetic properties in low fields. Th...

µSR Studies on Heavy Fermion Superconductors CeIrIn5 and CeCoIn5

We report that the muon spin relaxation rate under zero-external field in the heavy-fermion compound Ce Tr In 5 ( Tr =Ir, Co) is independent of temperature down to 20 mK, demonstrating the absence of magnetism due to long-range magnetic order and/or to the breakdown of time-reversal symmetry in the superconducting state. The magnetic penetration depth deduced by the muon spin relaxation rate under a transverse field is λ∼6700 A for CeIrIn 5 at 402 G and ∼5500 A for CeCoIn 5 at 3 kG.

Cu spin dynamics in the overdoped regime of La2-xSrxCu1-yZnyO4 probed by muon spin relaxation

Muon-spin-relaxation measurements have been performed for partially Zn-substituted