T. Yanagisawa

University of California, San Diego, San Diego, California, United States

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Publications (74)101.54 Total impact

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    ABSTRACT: Positive-muon ($\mu^+$) Knight shifts have been measured in the paramagnetic states of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloys, where $x =$ 0, 0.25, 0.45, 0.50, 0.55, 0.75, and 1.00. In Pr-substituted NdOs$_4$Sb$_{12}$ ($x \le$ 0.75), but not in NdOs$_4$Sb$_{12}$, Clogston-Jaccarino plots of $\mu^+$ Knight shift~$K$ versus magnetic susceptibility~$\chi$ exhibit an anomalous saturation of $K(\chi)$ at $\sim-$0.5% for large susceptibilities (low temperatures), indicating a reduction of the coupling strength between $\mu^+$ spins and $4f$ paramagnetism for temperatures $\lesssim$ 15~K. We speculate that itinerant Pr$^{3+}$ quadrupolar excitations, invoked to mediate the superconducting Cooper-pair interaction, might modify the $\mu^+$-$4f$ ion indirect spin-spin interaction.
    06/2014;
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    ABSTRACT: Muon spin rotation and relaxation ($\mu$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloy series. In the ferromagnetic end compound NdOs$_4$Sb$_{12}$ the spontaneous local field at positive-muon ($\mu^+$) sites below the ordering temperature $T_C$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd$^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For 0.45 $\le x \le$ 0.75, $\mu^+$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at $\mu^+$ sites below $T_C$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd$^{3+}$ moments, and speculate that the Nd$^{3+}$-$\mu^+$ interaction is suppressed and disordered by Pr doping. In an $x$ = 0.25 sample, which is superconducting below $T_c$ = 1.3 K, there is no sign of "spin freezing" (static Nd$^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic $\mu^+$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The $\mu^+$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below $T_c$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For 0.25 $\le x \le$ 0.75, the $\mu$SR data provide evidence against phase separation; superconductivity and Nd$^{3+}$ magnetism coexist on the atomic scale.
    04/2014;
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    ABSTRACT: Muon spin rotation and relaxation (μSR) experiments have been carried out to characterize magnetic and superconducting ground states in the Pr1-xNdxOs4Sb12 alloy series. In the ferromagnetic end compound NdOs4Sb12 the spontaneous local field at positive-muon (μ+) sites below the ordering temperature TC is greater than expected from dipolar coupling to ferromagnetically aligned Nd3+ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For 0.45≤x≤0.75, μ+ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at μ+ sites below TC are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd3+ moments, and speculate that the Nd3+-μ+ interaction is suppressed and disordered by Pr doping. In an x =0.25 sample, which is superconducting below Tc=1.3 K, there is no sign of "spin freezing" (static Nd3+ magnetism), ordered or disordered, down to 25 mK. Dynamic μ+ spin relaxation is strong, indicating significant Nd-moment fluctuations. The μ+ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below Tc, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For 0.25≤x≤0.75, the μSR data provide evidence against phase separation; superconductivity and Nd3+ magnetism coexist on the atomic scale.
    03/2014; 89(14).
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    ABSTRACT: We have measured the elastic constant (C11-C12)/2 in URu2Si2 by means of high-frequency ultrasonic measurements in pulsed magnetic fields H || [001] up to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction of (C11-C12)/2 that appears only in the temperature and magnetic field region in which URu2Si2 exhibits a heavy-electron state and hidden-order. This change in (C11-C12)/2 appears to be a response of the 5f-electrons to an orthorhombic and volume conservative strain field \epsilon_xx-\epsilon_yy with {\Gamma}3-symmetry. This lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized f electrons with the conduction electrons, and is probably linked to the hidden-order parameter of this compound.
    Physical Review B 11/2013; · 3.66 Impact Factor
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    ABSTRACT: The filled skutterudite compound, PrOs4Sb12, displays unconventional superconductivity at a relatively low critical temperature Tc=1.85K. The underlying physics behind this heavy fermion superconducting compound is unknown. To gain better insight into this phenomenon, we studied the effect of ferromagnetism on the unconventional superconductivity of PrOs4Sb12 by using neodymium substituted samples of Pr1-xNdxOs4Sb12. The heat capacities of the samples with x = 1, 0.75, 0.5, and 0.25 were measured using finite heat pulse relaxation calorimetry in a cryocooler system from 11K to 300K. The electronic specific heat coefficient γ of NdOs4Sb12 (estimated from the contributions from conduction electrons, Debye and Einstein lattice modes, CEF effects of rare earth atoms) is found to be ˜60 mJ/K^2-mol, which is much smaller than the previously estimated value γ˜520 mJ/K^2-mol (by using the low temperature specific heat data), but is still relatively large compared to values for simple metals. In addition, the high temperature specific heat of all four samples are approximately the same.
    03/2013;
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    ABSTRACT: The pseudo ternary system Pr1-xNdxOs4Sb12 has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC) and quantum critical behavior of PrOs4Sb12 [1]. SC in this system disappears near a critical concentration xcr,1˜0.58 and FM appears above xcr,2˜0.33 [1,2]. The new μSR measurements have been performed on samples with x = 0.25, 0.75, and 1. For x = 1 and 0.75, the estimated frozen moments agree with the Nd^3+ CEF ground state moment. For x = 0.25, neither time reversal symmetry breaking nor evidence of freezing of Nd^3+ spins was observed in zero-field μSR measurements, the behavior of which is very different than what is observed for x = 0.45-0.55 [2]. In the SC state, an unexpected linear T dependence of the Gaussian relaxation rate was also found in the transverse field μSR data for x = 0.25, which is different than the plateau in PrOs4Sb12 below 1.3K [3]. [1] Ho, et al., PRB 83, 024511 (2011).[2] Ho, et al., 2010 APS March Meeting, A38.00005 (2010). [3] MacLaughlin et al., PRL 105, 019701 (2010).
    03/2013;
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    ABSTRACT: We have measured the electrical resistivity of clathrate Ce3Pd20Ge6 under high pressure up to 3.86 GPa in order to investigate pressure effects on two successive phase transitions: O02-type ferroquadrupole order on Ce-8c site at TQ = 1.2 K and antiferromagnetic order on Ce-4a site at TN = 0.75 K. With applying pressure, TQ stays nearly constant at low pressures and gradually decreases above 2 GPa, while TN first increases and then decreases above 2.5 GPa. From these results, critical pressures, where TN and TQ become 0, are roughly estimated to be around 5-6 GPa. In addition, we found that the magnetic resistivity, which is obtained by subtracting the resistivity of La3Pd20Ge6 from that of Ce3Pd20Ge6, exhibits -InT dependence in two different temperature ranges, suggesting presence of two different Kondo temperatures originated in the two crystallographic Ce-sites.
    Journal of Physics Conference Series 12/2012; 391(1):2019-.
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    ABSTRACT: We have studied anomalous upper critical field Bc2(T) of a single crystal of UBe13 by DC magnetization measurements down to 0.14 K and up to 8 T. We have observed a broad anomaly and a peak effect below Bc2 in magnetization curves. Although a possibility of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been proposed as an origin of the unusual upper critical field by previous works, our results have shown that thermal equilibrium magnetization curves reveal no step-like behavior, which could be observed at 1st-order phase transition from a superconducting mixed state to the FFLO state. We have also investigated anisotropy of Bc2 for B || <001> and B || <110>, and found that Bc2 for B || <001< is larger than Bc2 for B || <110> below ~ 0.5 K. These results might indicate that the anomalous upward curvature of Bc2 in UBe13 is related to anisotropy of Bc2 rather than the FFLO state.
    Journal of Physics Conference Series 12/2012; 391(1):2065-.
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    ABSTRACT: Ultrasonic investigations of the elastic constants of non-5f compound ThRu2Si2 were performed in order to highlight possible quadrupolar response in the elastic constants of heavy fermion compound URu2Si2, by using the ThRu2Si2 for backgrounds to subtract the phonon contributions in the elastic constants of URu2Si2. We confirmed that the all-elastic constants of ThRu2Si2 show no elastic anomaly, i.e., increasing monotonically and saturating toward low temperatures, which can be understood regular phonon contributions for the solid states. The background-subtracted (C11-C12)/2 of URu2Si2 keeps decreasing with decreasing temperatures from 300 K, while other share modes C44 and C66 show little change. These contrasts suggest that the Γ3-type quadrupole moment O22 will be still dominant and active in URu2Si2 at low temperatures.
    Journal of Physics Conference Series 12/2012; 391(1):2079-.
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    ABSTRACT: We have performed ultrasonic measurements on single-crystalline URu2Si2 with pulsed magnetic fields, in order to check for possible lattice instabilities due to the hybridized state and the hidden-order state of this compound. The elastic constant (C11-C12)/2, which is associated with a response to the {\Gamma}3-type symmetry-breaking (orthorhombic) strain field, shows a three-step increase at H > 35 T for H || c at low temperatures, where successive meta-magnetic transitions are observed in the magnetization. We discovered a new fact that the absolute change of the softening of (C11-C12)/2 in the temperature dependence is quantitatively recovered at the suppression of hybridized-electronic state and the hidden order in high-magnetic field for H \perp c associated with the successive transitions. The present results suggest that the {\Gamma}3-type lattice instability, is related to both the emergence of the hybridized electronic state and the hidden-order parameter of URu2Si2. On the other hand, magnetic fields H || [100] and [110] enhance the softening of (C11-C12)/2 in the hidden order phase, while no step-like anomaly is observed up to 68.7 T. We discuss the limitation of the localized-electron picture for describing these features of URu2Si2 by examination of a crystalline electric field model in terms of mean-field theory.
    Journal of the Physical Society of Japan 11/2012; 82(1). · 2.09 Impact Factor
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    ABSTRACT: We have performed precise dc magnetization measurements for a single crystal of UBe_{13} down to 0.14 K, up to 80 kOe. We observed a magnetic anomaly in the superconducting (SC) mixed state at a field, named H_{Mag}^{*} (∼26  kOe, at 0.14 K), implying that UBe_{13} has a magnetically unusual SC state. We studied the magnetization curves of UBe_{13}, assuming that the H_{Mag}^{*} anomaly originates from (1) and unusual SC diamagnetic response, or (2) a peculiarity of the normal-state magnetization due to vortices in the SC mixed state. The origin of the H_{Mag}^{*} anomaly is discussed.
    Physical Review Letters 11/2012; 109(21):217001. · 7.73 Impact Factor
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    ABSTRACT: The effects of Ni substitution on Ce(Cu1-xNix)2Si2 have been studied by specific heat and electrical resistivity measurements. The specific heat measurement has revealed that the enhanced quantum fluctuations around an antiferromagnetic quantum critical point are markedly suppressed by Ni substitution, and that the Fermi liquid state recovers in the Nirich region (x > 0.12). The characteristic T-linear dependence of the resistivity has been observed at approximately x ~ 0.10 together with a sign of superconductivity. The variation of n in the form of rho - rho0 = aT^n against Tmax^1, at which the resistivity peaks, coincides with the case of high-pressure experiments on pure CeCu2Si2. The anomalous T-linear behavior appears to occur in the crossover region from the Kondo regime to the valence fluctuation regime rather than in the conventional antiferromagnetic quantum critical region.
    Journal of the Physical Society of Japan 06/2012; 81(8). · 2.09 Impact Factor
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    ABSTRACT: The pseudo ternary system Pr1-xNdxOs4Sb12 has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC), the high field ordered phase (HFOP), and quantum critical behavior [1], that was observed in PrOs4Sb12. SC in this system disappears near the Nd concentration x ˜ 0.58. Between x ˜ 0.33 and ˜ 0.58, weak FM, confirmed by the μSR experiments [2], was found to coexist with SC. In order to further inspect the possible quantum critical behavior, a power-law analysis of the temperature dependence of the electrical resistivity data was performed. Upon suppression of SC, for samples in the range 0.33 < x < 0.58, the power-law exponent decreases from ˜ 1.8 toward 1 in the temperature region below 2.5,, resembling non-Fermi liquid behavior. Detailed T-x, H-x, and H-T phase diagrams for various x will be discussed. [4pt] [1] Ho, et. al., PRB 83, 024511 (2011).[0pt] [2] Ho, et. al., 2010 APS March Meeting, A38.00005 (2010).
    02/2012;
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    ABSTRACT: The compound NdOs4Sb12 is a mean-field ferromagnet at about 1 K. As inferred from specific heat measurements below 10 K, the electronic specific heat coefficient of this compound is very large (˜ 520 mJ/mol-K^2). Intriguingly, a recent ultrasonic measurement shows that this compound has double ultrasonic dispersions at ˜15 K and 40 K. We have used our newly developed relaxation calorimeter to measure the specific heat of NdOs4Sb12 in the temperature range 11 K to 300 K. In this presentation, we will describe the experimental setup used for the finite pulse relaxation calorimetry in a crycooler and the results of our measurements on the NdOs4Sb12 sample.
    02/2012;
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    ABSTRACT: We have performed low-temperature specific-heat measurements in magnetic fields for a single crystal UBe13. It has been observed that our sample exhibits a superconducting transition at an intermediate temperature between previously reported values for two variant samples called H type and L type. The specific heat C(T) of our sample shows a T^3 behavior in the temperature range below 0.7 Tc, which is similar to the behavior of the H-type sample, suggesting the existence of point nodes in the superconducting gap function. We have obtained the upper-critical-field curves Hc2 for the [001], [110], and [111] crystal axes, which show no anisotropy at least down to the lowest measured temperature of 0.5 K. We have also derived the Maki parameter kappa2, and it has been revealed that the kappa2 steeply decreases isotropically upon cooling just below Tc. Paramagnetic effects and the symmetry of Cooper pairing of UBe13 are discussed.
    Journal of the Physical Society of Japan 06/2011; 80. · 2.09 Impact Factor
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    ABSTRACT: The study of the Pr1-xNdxOs4Sb12 series has been carried out in order to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC), the high field ordered phase (HFOP), and quantum critical behavior in PrOs4Sb12 [1, 2, 3]. Two critical concentrations xcr,1 ˜0.58 and xcr,2 ˜0.33 were previously identified in this system [2]: SC disappears near xcr,1 and weak FM extends into the SC region for xcr,2< x < xcr,1 [3]. In order to further examine the possible quantum critical behavior, a power-law analysis of the temperature dependence of the electrical resistivity data is performed. Upon suppression of SC, for samples of xcr,2< x < xcr,1, the power-law exponent decreases from ˜ 1.8 toward 1 in the temperature region below 2.5,, resembling non-Fermi liquid behavior. [4pt] [1] Ho, et. al., Physica B 403, 1038 (2008).[0pt] [2] Ho, et. al., arXiv:1008.5198v1 (2010).[0pt] [3] Ho, et. al., 2010 APS March Meeting, A38.00005 (2010).
    03/2011;
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    ABSTRACT: The filled skutterudite compounds CeOs4Sb12, PrOs4Sb12, and NdOs4Sb12 are respectively a 1 K antiferromagnetic (AFM) Kondo insulator, a 1.85 K unconventional superconductor, and a 1 K mean-field type ferromagnet (FM), suggesting that superconductivity in PrOs4Sb12 may result from proximity to AFM and FM quantum-critical points. Fermi-surface measurements of NdOs4Sb12 and CeOs4Sb12 could therefore give insights into the pairing mechanism. We have used a MHz skin-depth technique to observe Shubnikov-de Haas oscillations (SdHos) in single crystals of these materials at fields of up to 60 T. In CeOs4Sb12 for H // [001], a previously-unobserved semimetal-to-metal transition was detected at 25 T; above this, a series of SdHos with a frequency of 1700 T and mCR 3.6 me emerge. For H // [011] in NdOs4Sb12, a single series of SdHos, frequency 874 T, was found. These may correspond to the beta band in PrOs4Sb12, but with a much smaller mCR 1.5 me. Research at CSU-Fresno is supported by RC CCSA #7669 and the start-up fund; at NHMFL by DOE, NSF, and FL.; at UCSD by NSF#0802478 and US DOE DE FG02-04ER46105; at Hokkaido U by MEXT, Japan.
    03/2011;
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    ABSTRACT: We have studied the effects of pressure on the incommensurate (IC)-commensurate (C) antiferromagnetic (AFM) phase transition of UPd2Si2 by measuring the electrical resistivity up to 4.21 GPa. We observed that the low-temperature C-AFM (type-I) phase expands and prevails over the IC-AFM phase as pressure is increased. The obtained pressure-temperature phase diagram strongly suggests the existence of a novel bicritical point, more specifically, the so-called Lifshitz point at Pc ~ 3.4 GPa, where magnetic modulation continuously changes from IC to C.
    Journal of Physics Conference Series 02/2011; 273(1):012032.
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    ABSTRACT: Ultrasonic measurements with frequencies varying between 73 and 160 MHz were performed for the LaRu4As12 single crystal. This As-based filled skutterudite compound is a BCS-type superconductor with Tsc as high as 10.3 K. In order to examine a presence of off-center rattling motion in LaRu4As12, we measured elastic constants CL111 = (C11 + 2C12 + 4C44)/3 and CT111 = (C11 – C12 + 4C44)/3, which are related to longitudinal and transverse sound waves propagating along the [111] direction, respectively. No ultrasonic dispersion was found in the whole temperature range from 1.4 to 300 K.
    Journal of Physics Conference Series 02/2011; 273(1):012065.
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    ABSTRACT: Static dc magnetization measurements in the superconducting mixed state of a single crystal UBe13 along the [110] axis were performed by means of a capacitance Faraday-force method down to 0.24 K. Below the upper critical field Bc2, not only a peak effect but also an additional broad anomaly have been observed in magnetization curves. We report superconducting phase diagram of UBe13 obtained by our magnetization measurements, including these anomalies.
    Journal of Physics Conference Series 02/2011; 273(1):012084.

Publication Stats

312 Citations
101.54 Total Impact Points

Institutions

  • 2008–2014
    • University of California, San Diego
      • Department of Physics
      San Diego, California, United States
  • 2009–2012
    • Hokkaido University
      • • Graduate School of Science
      • • Department of Physics
      Sapporo-shi, Hokkaido, Japan
  • 2007
    • Applied Physical Sciences
      Groton, Connecticut, United States
  • 2003–2007
    • Niigata University
      • Graduate School of Science and Technology
      Niahi-niigata, Niigata, Japan