Akira Oosawa

Sophia University, Tokyo, Tokyo-to, Japan

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Publications (88)159.87 Total impact

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    ABSTRACT: The ground state of the quantum spin system kappa-(BEDT-TTF)2Cu2(CN)3 in which antiferromagnetically-interacting S=1/2 spins are located on a nearly equilateral triangular lattice attracts considerable interest both from experimental and theoretical aspects, because a simple antiferromagnetic order may be inhibited because of the geometrical frustration and hence an exotic ground state is expected. Furthermore, recent two reports on the ground state of this system have made it further intriguing by showing completely controversial results; one indicates the gapless state and the other gapped. By utilizing microscopic probe of muSR, we have investigated its spin dynamics below 0.1 K, unveiling its microscopically phase separated ground state at zero field.
    Journal of the Physical Society of Japan 04/2012; 81(6). · 1.48 Impact Factor
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    ABSTRACT: The successive magnetic phase transitions of the distorted-triangular-lattice antiferromagnet RbFeBr3 have been investigated by single-crystal neutron diffraction measurements. The intermediate magnetic phase (TN2 = 2.0 K< T < TN1 = 5.6 K) appears as a partially disordered (PD) spin structure. An inclusive understanding of a distortion model of this crystal with the symmetry P63cm, which enables the spontaneous electric polarization and the PD spin structure with a weak ferromagnetic moment, was established.
    Journal of the Physical Society of Japan 08/2011; 80. · 1.48 Impact Factor
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    ABSTRACT: NMR measurements were carried out on the impurity doped quantum spin system TlCu1-xMgxCl3 with nominal concentration x = 0.03 and 0.01, in which the magnetic phase transition is expected at 3.6 K and 2.2 K, respectively. For the sample x = 0.03, the peaks of NMR spectra split clearly below T = 3.8 K. However, for the sample x = 0.01, no peak splitting was observed down to T = 1.52 K. This result supports the idea of coherent spin fluctuation in the impurity-induced ordered state as suggested by Suzuki et al. [2009 J. Phys. Soc. Jpn. 78 074705.].
    Journal of Physics Conference Series 07/2011; 302(1).
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    ABSTRACT: Tris(beta-diketonato)ruthenium(III) complexes of mononuclear [Ru(acac)3] and binuclear [{Ru(acac)2}2(mu-OEt)2] are spin-1/2 quantum spin magnets. In both system, Ru and surrounding O6 atoms consist of octahedral geometry, and in the binuclear complex, two (acetylacetonato) ruthenium complexes are bridged by two oxygen atoms of ethoxyl groups, making up a dimer with Ru-Ru bond. The electronic state of ruthenium is easily controlled by changing the methyl groups on the acetylacetone to the different substituents such as electronegative trifluoromethyl group. We expect to introduce a new degree of freedom, a charge fluctuation into the dimer spin system. In this study, we investigate the spin state of monomer and dimer quantum spin systems by means of 13C, 19F, 101Ru-NMR at low temperatures. In the former [Ru(acac)3], we observed Curie-like temperature dependence in the Knight shift down to 1.5 K, indicating that the system was simply in the paramagnetic state. The dimer system [{Ru(acac)2}2(mu-OEt)2] on the other hand showed the NMR signal with very long T1 at zero shift position at low temperatures, indicating that the system was the spin singlet state. In order to investigate the effect of substitution, we also report on results of mer-[Ru(ehfa)3] and [Ru(fhea)3], for which methyl groups are substituted by trifluoromethyl groups.
    Journal of Physics Conference Series 01/2011; 302(1).
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    ABSTRACT: Solid solution of the two spin gap systems (CH3)2CHNH3CuX3 (X=Cl, Br) was reported from results of macroscopic measurements to be gapless when fraction of Cl content x is in between 0.44 and 0.87, and otherwise remains gapped. We have investigated by μSR the ground state of Cl-rich gapped region x=0.95 and 0.88 to find that they are microscopically phase separated into two components, magnetic tiny islands and singlet sea surrounding them. With decreasing temperature, the characteristic frequency of spin fluctuation in islands showed a significant slow down. However, the slow down ceased around 1 K, below which the spin fluctuation spectrum did not change down to 15 mK. These results indicate that unlike the Br-rich system x < 0.44 where appearance of an exotic ground state has been argued, the Cl-rich system x > 0.87 persists to be paramagnetic without showing a long-range order. This difference is considered to be originated in the robustness of Haldane gap against disorder.
    Journal of Physics Conference Series 02/2010; 200(2):022011.
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    ABSTRACT: Zero- and longitudinal-field muon-spin-relaxation (ZF- and LF-muSR) measurements were carried out down to T=20 mK on the slightly doped quantum spin system TlCu1-xMgxCl3 with x=0.0047, in which a magnetic phase transition is observed at 0.70 K by specific heat measurement. LF-muSR measurements above and below the phase transition temperature revealed that the impurity-induced magnetic moments of the Cu-3d spins slowly fluctuate at a frequency of 0.74 MHz at T=20 mK, far below the magnetic phase transition temperature. It is possible that the impurity-induced magnetic moments are in an exotic state where they fluctuate while preserving their wave vector and relative phase, i.e., a coherently fluctuating state.
    Journal of the Physical Society of Japan 07/2009; 78. · 1.48 Impact Factor
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    ABSTRACT: C63/65u - and C35/37l-NMR experiments were performed to investigate triplet localization in the S=1/2 dimer compound NH4CuCl3 , which shows magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. In C63/65u-NMR experiments, signal from only the singlet Cu site was observed, because that from the triplet Cu site was invisible due to the strong spin fluctuation of onsite 3d spins. We found that the temperature dependence of the shift of C63/65u-NMR spectra at the singlet Cu site deviated from that of macroscopic magnetization below T=6K . This deviation is interpreted as the triplet localization in this system. From the C35/37l-NMR experiments at the 1/4-plateau phase, we found the two different temperature dependences of Cl shift, namely, the temperature dependence of one deviates below T=6K from that of the macroscopic magnetization as observed in the C63/65u-NMR experiments, whereas the other corresponds well with that of the macroscopic magnetization in the entire experimental temperature region. We interpreted these dependences as reflecting the transferred hyperfine field at the Cl site located at a singlet site and at a triplet site, respectively. This result also indicates that the triplets are localized at low temperatures. C63/65u-NMR experiments performed at high magnetic fields between the one-quarter and three-quarters magnetization plateaus have revealed that the two differently oriented dimers in the unit cell are equally occupied by triplets, the fact of which limits the theoretical model on the periodic structure of the localized triplets.
    Physical review. B, Condensed matter 05/2009; 79(17). · 3.66 Impact Factor
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    ABSTRACT: The spin-lattice relaxation rate $T_1^{-1}$ of $^1$H-NMR has been measured in (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$ with $x=0$ and 0.35, in order to investigate the microscopic magnetism of systems. Previous macroscopic magnetization and specific heat measurements suggested that these two exist in a singlet-dimer phase. The temperature dependence of $T_1^{-1}$ in an $x=0$ system decreased exponentially toward zero, indicating microscopic evidence of the gapped singlet ground state, which is consistent with the macroscopic experiments. At the same time, in the $x=0.35$ system, $T_1^{-1}$ showed a sharp peak structure at around 7.5 K though no splitting of $^1$H-NMR spectra indicative of the magnetic ordering was observed. We discuss the observed sharp peak structure in the $x=0.35$ system with the soft mode toward the exotic magnetic ground state suggested by the recent $\mu$SR experiments.
    04/2009;
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    ABSTRACT: Muon-spin-relaxation (μSR) measurements were carried out on the randomness-introduced quantum spin system Tl1-xKxCuCl3. Zero-field μSR measurement results reveal that the rapid increase of the μSR rate λ with decreasing temperature in the case of x=0.58. From the result of longitudinal-field μSR measurements, a finite relaxation remains at 4000G, which means that Cu-3d spins fluctuate down to 20mK although the slowing down of the frequency of the spin fluctuation occurs. These results suggest that there is a quite possibility of the existence of an ordered or spin frozen phase in a neighboring concentration of x around 0.58.
    Physica B Condensed Matter 04/2009; 404(5):590-593. · 1.28 Impact Factor
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    ABSTRACT: Measurements of the macroscopic properties have indicated that the bond-disordered spin-gap system (CH3)2CHNH3–Cu(ClxBr1−x)3 is gapless when x is between 0.44 and 0.87. Using muon spin relaxation to investigate the microscopic properties of a sample with x=0.35, we observed a dynamical spin fluctuation, whose characteristic frequency decreased with decreasing temperature, indicating a magnetic ground state.
    Physica B Condensed Matter 04/2009; 404(5):594-596. · 1.28 Impact Factor
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    ABSTRACT: (CH3)2CHNH3CuCl3 is one of the best laboratory models for the S = 1/2 spin ladder comprising ferromagnetic rungs and antiferromagnetic legs. We have determined the phase boundary between the low-field spin liquid and the field-induced ordered phase of this compound to 100 mK in temperature by means of specific-heat and magnetocaloric-effect measurements for two crystal orientations, with the magnetic field perpendicular to either the so-called B or C plane. The critical field of antiferromagnetic ordering shows power-law dependences on temperature, indicative of the Bose-Einstein condensation of spin triplets due to closing of the energy gap by the field.
    Journal of Physics Conference Series 03/2009; 150(4):042217.
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    ABSTRACT: Raman spectra in TlCuCl3 under the magnetic field up to 12 T are reported. Above the critical magnetic field of the quantum phase transition to the magnon Bose-Einstein condensation phase, the one-magnon Raman peak is observed. Above 10 T, the one-magnon energy approaches the energy of the Ag phonon (3.1 meV) and the anticrossing effect is observed. By using the Green-function theory with three adjustable parameters, we estimate the amplitude of the attractive magnon-phonon interaction as 0.043 meV. The consistency between the present results and those of previous works is discussed.
    Journal of Physics Conference Series 03/2009; 150(4):042104.
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    ABSTRACT: The spin-lattice relaxation rate T1-1 of 1H-NMR has been measured in (CH3)2CHNH3Cu(ClxBr1-x)3 with x = 0 and 0.35, in order to investigate their microscopic magnetism. Previous macroscopic magnetization and specific heat measurements suggested that these two exist in a singlet-dimer phase. The temperature dependence of T1-1 in an x = 0 system decreased exponentially toward zero, providing microscopic evidence of a gapped singlet ground state, which is consistent with results of macroscopic experiments. At the same time, in the x = 0.35 system, T1-1 showed a sharp peak structure at approximately 7.5 K but no splitting of 1H-NMR spectra, indicative of magnetic ordering. We discuss the observed sharp peak structure in the x = 0.35 system with the soft mode toward the exotic magnetic ground state suggested by recent muSR experiments.
    Journal of the Physical Society of Japan 01/2009; 78. · 1.48 Impact Factor
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    ABSTRACT: The structural changes of TlCoCl3 around phase IV (68 < T < 75 K) were studied by single-crystal neutron diffraction measurement. The present investigation revealed that a small but definite Bragg peak at (1/6, 1/6, 2) appeared only in phase IV. A precursor phenomenon of the phase V (T < 68 K) structure in the phase IV temperatures was also identified as the coexistence of a slightly broadened (1/4, 1/4, 2) Bragg peak. We confirmed that intermediate phase IV is a stable, nontransient, single phase with 2\sqrt{3}a × 2\sqrt{3}a × c unit cells. A possible sequence of structural phase transitions was proposed.
    Journal of the Physical Society of Japan 12/2008; 77(12). · 1.48 Impact Factor
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    ABSTRACT: Powder and single-crystal neutron diffraction measurements were performed on RbCoBr3. The magnetic phase transitions of this compound are well explained by the Ising spin model for a distorted triangular lattice. Magnetic peaks of powder diffraction patterns at 34 and 20 K closely fit a partial disordered phase and a ferrimagnetic phase, respectively. The temperature dependences of the (h,h,1) scan profile and magnetic Bragg peak heights at (1,1,1), (1/3,1/3,1), and (2/3,2/3,1) were measured for a single-crystal sample. We conclude that the ferrimagnetic phase only exists below 30.5 K. The partial disordered phase is identified between 30.5 (=TN2) and 37.2 K (=TN1). We found a characteristic linear increase in the (1,1,1) peak height below TN2 with decreasing temperature. These results show that a lattice distortion couples to the spin system. Numerical simulations are also performed to estimate magnetic interaction parameters.
    Journal of the Physical Society of Japan 10/2008; 77(10):4703-. · 1.48 Impact Factor
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    ABSTRACT: The structural changes of TlCoCl$_3$ around phase IV (68 K $<T<$ 75 K) were studied by single-crystal neutron diffraction measurement. The present investigation revealed that a small but definite Bragg peak at (${1/6} {1/6} 2$) appeared only in phase IV. A precursor phenomenon of the phase V ($T<$ 68 K) structure in the phase IV temperatures was also identified as the coexistence of a slightly broadened (${1/4} {1/4} 2$) Bragg peak. We confirmed that intermediate phase IV is a stable, nontransient, single phase with $2\sqrt{3}a\times 2\sqrt{3}a\times c$ unit cells. A possible sequence of structural phase transitions was proposed.
    09/2008;
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    ABSTRACT: The spin-gap system with a bond-disorder (CH3)2CHNH3-Cu(ClxBr1-x)3 is reported by experiments on macroscopic quantities of the uniform susceptibility and the specific heat to show a magnetic order at low temperatures (when the value of x is within the limited region between 0.44 and 0.87) and otherwise a gapped behavior. We have investigated microscopically the ground state of the sample with x=0.35 by muon spin relaxation (muSR) to observe that the frequency spectrum of the spin fluctuation tends to concentrate around zero energy as lowering temperatures. This behavior was interpreted as the soft mode toward the possible magnetic phase transition at absolute zero.
    Physical review. B, Condensed matter 08/2008; 78(5). · 3.66 Impact Factor
  • Physical review. B, Condensed matter 04/2008; 77(13). · 3.66 Impact Factor
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    ABSTRACT: To date, there has been no evidence for the macroscopic structural phase transition to the low temperature tetragonal structure (LTT) with a space group P42/ncm in high-TC cuprate of rare earth-free La2-xSrxCuO4 (LSCO). By investigating Cu-NMR on single crystals, we have found that spatially incoherent LTT structure emerges below 50 K in the sample with x=0.12. This incoherent structure is considered to play a key role for the slight depression of the superconductivity around x=1/8.
    Journal of Physics Conference Series 03/2008;
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    ABSTRACT: We report the observation of the $A_{\rm g}$-symmetric one-magnon Raman peak in the magnon Bose-Einstein condensation phase of TlCuCl$_{3}$. Its Raman shift traces the one-magnon energy at the magnetic $\Gamma$ point, and its intensity is proportional to the squared transverse magnetization. The appearance of the one-magnon Raman scattering originates from the exchange magnon Raman process and reflects the change of the magnetic-state symmetry. Using the bond-operator representation, we theoretically clarify the Raman selection rules, being consistent with the experimental results. Comment: 6 pages, 4 figures
    Physical review. B, Condensed matter 03/2008; · 3.66 Impact Factor

Publication Stats

1k Citations
159.87 Total Impact Points

Institutions

  • 2006–2010
    • Sophia University
      • • Faculty of Science and Technology
      • • Division of Physics
      Tokyo, Tokyo-to, Japan
    • University of Cologne
      • II. Institute of Physics
      Köln, North Rhine-Westphalia, Germany
  • 2003
    • Karl-Franzens-Universität Graz
      • Fachbereich Theoretische Physik
      Graz, Styria, Austria
  • 1998–2003
    • Tokyo Institute of Technology
      • Department of Physics
      Edo, Tōkyō, Japan