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510 results on '"Tokiwa, Y."'

167. High-field phase diagram of the heavy-fermion metal YbRh2Si2

Author

Gegenwart, P, primary, Tokiwa, Y, additional, Westerkamp, T, additional, Weickert, F, additional, Custers, J, additional, Ferstl, J, additional, Krellner, C, additional, Geibel, C, additional, Kerschl, P, additional, Müller, K-H, additional, and Steglich, F, additional

Published
2006
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178. Magnetic properties of U2RhGa8and U2FeGa8

Author

Ikeda, S, primary, Okubo, T, additional, Inada, Y, additional, Tokiwa, Y, additional, Kaneko, K, additional, Matsuda, T D, additional, Yamamoto, E, additional, Haga, Y, additional, and Onuki, Y, additional

Published
2003
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180. Interplay Of Magnetism And Superconductivity In Cecoin5.

Author

Movshovich, R., Tokiwa, Y., Ronning, F., Bianchi, A., Capan, C., Young, B. L., Urbano, R. R., Curro, N. J., Park, T., Thompson, J. D., Bauer, E., and Sarrao, J. L.

Abstract

CeCoIn5 is a heavy fermion superconductor which appears to be straddling the boundary between the superconducting and magnetic ground states. At the superconducting critical field Hc2 this material displays NFL behavior in transport and thermodynamic properties, pointing at a Quantum Critical Point (QCP) at Hc2, and hinting at the presence of magnetic fluctuations, probably due to an AFM order superseded by the superconductivity. In the High-Field-Low-Temperature (HFLT) corner of the superconducting phase of CeCoIn5, within 20% off Hc2, an additional phase appears within the superconducting phase, and the normal-to-superconducting transition itself becomes first order. This behavior is consistent with a strong Pauli limited superconductivity, and the low temperature high field phase being an inhomogeneous superconducting FFLO phase. Recent NMR experiments, however, point to a long range magnetic order within HFLT state. Experiments on CeRhIn5 under pressure show magnetic field induced AFM order within the superconducting phase, with some similarities to the phase diagram of CeCoIn5. Could the HFLT phase transition be due to magnetic order? Importantly, the HFLT phase does not extend into the normal state above Hc2. We need a picture of a magnetism ˵attracted″ to superconductivity to explain the data on the HFLT phase in CeCoIn5. [ABSTRACT FROM AUTHOR]

Published
2009
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186. Spin liquid close to a quantum critical point in Na4Ir3O8.

Author

Singh, Yogesh, Tokiwa, Y., J. Dong, and Gegenwart, P.

Subjects
*SPIN waves, *QUANTUM theory, *LIQUIDS, *THERMAL conductivity, *POLYCRYSTALS
Abstract

Na4Ir3O8 is a candidate material for a three-dimensional quantum spin liquid on the hyperkagome lattice. We present thermodynamic measurements of heat capacity C and thermal conductivity k on high-quality polycrystalline samples of Na4Ir3O8 down to T = 500 and 75 mK, respectively. Absence of long-range magnetic order down to T = 75 mK strongly supports claims of a spin-liquid ground state. The constant magnetic susceptibility / below T ≈ 25 K and the presence of a small but finite linear-T term in C(T) suggest the presence of gapless spin excitations. Additionally, the magnetic Griineisen ratio shows a divergence as T → 0 K and a scaling behavior, which clearly demonstrates that Na4Ir3O8 is situated close to a zero-field QCP [ABSTRACT FROM AUTHOR]

Published
2013
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192. Zero-Field Quantum Critical Point in CeCoIn5.

Author

Tokiwa, Y., Bauer, E. D., and Gegenwart, P.

Subjects
*FERMIONS, *GRUNEISEN equation of state, *MAGNETIC fields, *SUPERCONDUCTORS, *QUASIPARTICLES
Abstract

Quantum criticality in the normal and superconducting states of the heavy-fermion metal CeCoIn5 is studied by measurements of the magnetic Grüneisen ratio ΓH and specific heat in different field orientations and temperatures down to 50 mK. A universal temperature over magnetic field scaling of ΓH in the normal state indicates a hidden quantum critical point at zero field. Within the superconducting state, the quasiparticle entropy at constant temperature increases upon reducing the field towards zero, providing additional evidence for zero-field quantum criticality. [ABSTRACT FROM AUTHOR]

Published
2013
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193. Unique phase diagram with narrow superconducting dome in EuFe2(As1-xPx)2 due to Eu2+ local magnetic moments.

Author

Tokiwa, Y., Hübner, S.-H., Beck, O., Jeevan, H. S., and Gegenwart, P.

Subjects
*SUPERCONDUCTING composites, *EUROPIUM compounds, *ELECTRICAL resistivity, *HYDROSTATIC pressure, *SUPERCONDUCTIVITY
Abstract

The interplay between superconductivity and Eu2+ magnetic moments in EuFe2(As1-xPx)2 is studied with electrical resistivity measurements under hydrostatic pressure on x = 0.13 and x = 0.18 single crystals. We can map hydrostatic pressure to chemical pressure x and show that superconductivity is confined to a very narrow range 0.18 ⩽x ⩽ 0.23 in the phase diagram, beyond which ferromagnetic (FM) Eu ordering suppresses superconductivity. The change from antiferro- to FM Eu ordering at the latter concentration coincides with a Lifshitz transition and the complete depression of iron magnetic order. [ABSTRACT FROM AUTHOR]

Published
2012
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194. Quasiparticle Entropy in the High-Field Superconducting Phase of CeCoIn5.

Author

Tokiwa, Y., Bauer, E. D., and Gegenwart, P.

Subjects
*QUASIPARTICLES, *ENTROPY, *SUPERCONDUCTORS, *FERMIONS, *TEMPERATURE effect, *SUPERCONDUCTIVITY, *PHASE transitions
Abstract

The heavy-fermion superconductor CeCoIn5 displays an additional transition within its superconduct-ing (SC) state, whose nature is characterized by high-precision studies of the isothermal field dependence of the entropy, derived from combined specific heat and magnetocaloric effect measurements at temperatures T ≥ 100 mK and fields H ≤ 12 T aligned along different directions. For any of these conditions, we do not observe an additional entropy contribution upon tuning at constant temperature by magnetic field from the homogeneous SC into the presumed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) SC state. By contrast, for H || [100] a reduction of entropy was found that quantitatively agrees with the expectation for spin-density-wave order without FFLO superconductivity. Our data exclude the formation of a FFLO state in CeCoIn5 for out-of-plane field directions, where no spin-density-wave order exists. [ABSTRACT FROM AUTHOR]

Published
2012
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195. Quantum criticality near the upper critical field of Ce2PdIn8.

Author

Tokiwa, Y. and Gegenwart, P.

Subjects
*MAGNETIC fields, *SUPERCONDUCTORS, *TEMPERATURE, *ELECTRONICS, *LOGARITHMIC functions
Abstract

We report low-temperature specific-heat measurements in magnetic fields up to 12 T applied parallel and perpendicular to the tetragonal c axis of the heavy fermion superconductor Ce2PdIn8. In contrast to its quasi-two-dimensional (2D) relative CeCoIn5, the system displays an almost isotropic upper critical field. While there is no indication for a high-field and low-temperature phase in Ce2PdIn8, the data suggest a smeared weak first-order superconducting transition close to Hc2 ≈ 2 T. The normal-state electronic specific-heat coefficient displays logarithmically divergent behavior, comparable to CeCoIn5 and in agreement with 2D quantum criticality of spin-density-wave type. [ABSTRACT FROM AUTHOR]

Published
2011
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198. UV mutagenesis of Cupriavidus necator for extracellular production of ( R)-3-hydroxybutyric acid.

Author

Ugwu, C. U., Tokiwa, Y., Aoyagi, H., Uchiyama, H., and Tanaka, H.

Subjects
*MUTAGENESIS, *ULTRAVIOLET radiation, *ACETATES, *DEHYDROGENASES, *ESTERS, *MUTAGENS
Abstract

Aim: Ultraviolet (UV) mutagenesis was carried out to obtain mutant strains of Cupriavidus necator that could produce ( R)-3-hydroxybutyric acid [( R)-3-HB] in the culture supernatant. Methods and Results: C. necator (formerly known as Ralstonia eutropha) was subjected to UV radiation to generate mutants that are capable of producing ( R)-3-HB in the culture supernatant. Results indicated that UV mutagen disrupted the phbB ( phbB knock-out) and thus, promoted production of ( R)-3-HB in mutant strains. Inclusion of acetoacetate esters (carbonyl compounds) in the culture broth led to increased production of ( R)-3-HB. Thus, acetoacetyl-CoA (an intermediate of the PHB synthetic pathway) might have been converted to acetoacetate, which in the presence of ( R)-3-HB dehydrogenase and NADPH/NADP+, resulted in extracellular production of ( R)-3-HB. Conclusions: UV mutagenesis proved to be a satisfactory method in generating interesting mutants for extracellular production of ( R)-3-HB. Extracellular production of ( R)-3-HB upon addition of acetoacetate esters would suggest a likely ( R)-3-HB biosynthetic pathway in C. necator. Significance and Impact of the Study: Mutants obtained in this study are very useful for production of ( R)-3-HB. For the first time, the production of ( R)-3-HB by C. necator via acetoacetate is reported. [ABSTRACT FROM AUTHOR]

Published
2008
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199. Enzymatic synthesis of arbutin undecylenic acid ester and its inhibitory effect on mushroom tyrosinase.

Author

Tokiwa, Y., Kitagawa, M., and Raku, T.

Subjects
PHENOL oxidase, TYROSINE, ENZYMES, CATECHOL, HYDROXYLATION, OXIDATION
Abstract

A novel tyrosinase inhibitor, an arbutin derivative having undecylenic acid at the 6-position of its glucose moiety, was enzymatically synthesized. Its inhibitory activity was studied in vitro by using catechol and phenol as substrates. The IC50 value of the arbutin ester on tyrosinase using catechol (4 × 10−4 M) was 1% of that when arbutin (4 × 10−2 M) was used. Using phenol, IC50 of the arbutin ester (3 × 10−4 M) as substrate was 10% of that of arbutin (3 × 10−3 M). These results suggest that the arbutin ester inhibits the latter part of the tyrosinase reaction, which consists of hydroxylation and oxidation. [ABSTRACT FROM AUTHOR]

Published
2007
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