Akira Oosawa

Publications (10)0 Total impact

• Article: Microscopic phase separation in triangular-lattice quantum spin magnet kappa-(BEDT-TTF)2Cu2(CN)3 probed by muon spin relaxation

  Saori Nakajima, Takao Suzuki, Yasuyuki Ishii, Kazuki Ohishi, Isao Watanabe, Takayuki Goto, Akira Oosawa, Naoki Yoneyama, Norio Kobayashi, Francis L. Pratt, Takahiko Sasaki
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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.
  04/2012;
• Source

  Available from: ArXiv

  Article: Intermediate Phase IV in Structural Phase Transitions of TlCoCl$_3$

  Yoichi Nishiwaki, Akira Oosawa, Tetsuya Kato, Takumi Hasegawa, Haruhiko Kuroe, Kazuhisa Kakurai
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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;
• Source

  Available from: arxiv.org

  Article: \mu SR-detection of the soft-mode toward the exotic magnetic ground state in bond-disordered quantum spin system IPA-Cu(Cl$_{0.35}$Br$_{0.65}$)$_3$

  Takayuki Goto, Takao Suzuki, Keishi Kanada, Takehiro Saito, Akira Oosawa, Isao Watanabe, Hirotaka Manaka
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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 (mSR) 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. Comment: 19 pages, 4 figures, accepted as a publication in PRB
  07/2008;
• Source

  Available from: ArXiv

  Article: Magnetic field-induced one-magnon Raman scattering in the magnon Bose-Einstein condensation phase of TlCuCl$_{3}$

  Haruhiko Kuroe, Kouhei Kusakabe, Akira Oosawa, Tomoyuki Sekine, Fumiko Yamada, Hidekazu Tanaka, Masashige Matsumoto
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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
  03/2008;
• Source

  Available from: ArXiv

  Article: The effect of randomness on the quantum spin system Tl$_{1-x}$K$_x$CuCl$_3$ with ${\boldmath $x$}$ = 0.44 studied by the Zero-field Muon-Spin-Relaxation (ZF-$\mu$SR) method

  Takao Suzuki, Fumiko Yamada, Isao Watanabe, Takayuki Goto, Akira Oosawa, Hidekazu Tanaka
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  ABSTRACT: Zero-field muon-spin-relaxation (ZF-{\boldmath $\mu$}SR) measurements were carried out down to 80 mK on the randomness bond system Tl$_{1-x}$K$_x$CuCl$_3$ with $x$ = 0.44. Time spectra are well fitted by the stretched exponential function $\exp (-\lambda t)^{\beta}$. The muon spin relaxation rate $\lambda$ increases rapidly with decreasing temperature, and $\beta$ tends to 0.5 at 80 mK. The divergent increase of $\lambda$ suggests the critical slowing down of the frequency of the Cu-3{\it d} spin fluctuations toward a spin frozen state below 80 mK, and the root-exponential-like behavior of the time spectrum indicates that the origin of the relaxation is possibly the spatially-fixed fluctuating dilute moments. Comment: Accepted on J. Phys. Soc. Jpn
  05/2007;
• Source

  Available from: arxiv.org

  Article: 1H-NMR Study on the Magnetic Order in the Mixture of Two Spin Gap Systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3

  Keishi Kanada, Takehiro Saito, Akira Oosawa, Takayuki Goto, Takao Suzuki
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  ABSTRACT: The antiferromagnetic ordering in the solid-solution of the two spin-gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3 has been investigated by 1H-NMR. The sample with the Cl-content ratio x=0.85 showed a clear splitting in spectra below TN=13.5 K, where the spin-lattice relaxation rate T1-1 showed a diverging behavior. The critical exponent of the temperature dependence of the hyperfine field is found to be 0.33. Comment: 11pages, 4 figures
  04/2007;
• Source

  Available from: arxiv.org

  Article: Impurity-induced magnetic order in the mixture of two spin gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3

  Keishi Kanada, Takehiro Saito, Akira Oosawa, Takayuki Goto, Takao Suzuki, Hirotaka Manaka
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  ABSTRACT: The ground state of the solid solution of the two spin gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3 has been investigated by 1H-NMR. The existence of a magnetic ordering in the sample with the Cl-content x=0.85 was clearly demonstrated by a drastic splitting in a resonance line at low temperatures below TN=13.5K. The observed NMR spectra in the ordered state was qualitatively consistent with the simple antiferromagnetic state. Comment: QuBS2006
  10/2006;
• Source

  Available from: arxiv.org

  Article: Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3

  Kenji Goto, Masashi Fujisawa, Hidekazu Tanaka, Yoshiya Uwatoko, Akira Oosawa, Toyotaka Osakabe, Kazuhisa Kakurai
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  ABSTRACT: Magnetization and neutron elastic scattering measurements under a hydrostatic pressure were performed on KCuCl3, which is a three-dimensionally coupled spin dimer system with a gapped ground state. It was found that an intradimer interaction decreases with increasing pressure, while the sum of interdimer interactions increases. This leads to the shrinkage of spin gap. A quantum phase transition from a gapped state to an antiferromagnetic state occurs at Pc ? 8.2 kbar. For P > P c, magnetic Bragg reflections were observed at reciprocal lattice points equivalent to those for the lowest magnetic excitation at zero pressure. This confirms that the spin gap decreases and closes under applied pressure. Comment: 7 pages, 10 figures, submitted to J. Phys. Soc. Jpn
  02/2006;
• Source

  Available from: arxiv.org

  Article: High-field Magnetic Torque Measurements in (CH$_3$)$_2$CHNH$_3$CuCl$_3$

  Takehiro Saito, Takahiko Sasaki, Takao Suzuki, Akira Oosawa, Takayuki Goto, Satoshi Awaji, Kazuo Watanabe, Norio Kobayashi
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  ABSTRACT: High-field magnetic torque measurements were carried out in the spin gap system (CH$_3$)$_2$CHNH$_3$CuCl$_3$. It was observed that the magnitude of the magnetic torque $\tau$ is almost zero until the critical field and then increases rapidly, which indicates the existence of the magnetic quantum phase transition from the spin-gap phase to the field-induced magnetic ordered phase. From the temperature dependence of the magnetic torque $\tau$, the cusp-like minimum indicative of the Bose-Einstein condensation (BEC) of magnons was observed for $H \perp$ C-plane, while the kink-anomaly was observed for $H \perp$ A-plane at the field-induced transition temperature. It was found that this different behavior between $H \perp$ A-plane and $H \perp$ C-plane can be interpreted as the breaking of the magnon BEC picture due to the rotational symmetry breaking. The additional anomaly was also observed in the field-induced magnetic ordered phase at the intermediate angle of the applied magnetic field in the A-plane and discussed in terms of the spin-flop transition, the spin-reorientation transition and the spin-lattice correlations. In order to investigate the spin-lattice correlations in (CH$_3$)$_2$CHNH$_3$CuCl$_3$, we performed the magnetostriction measurements and observed that the magnetostriction appears by vanishing of the spin gap.
  01/2006;
• Article: Random bond effect in the quantum spin system (Tl_ {1-x} K_ {x}) CuCl_ {3}

  Akira Oosawa, Hidekazu Tanaka
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  ABSTRACT: The effect of exchange bond randomness on the ground state and the field-induced magnetic ordering was investigated through magnetization measurements in the spin-1/2 mixed quantum spin system (Tl1-xKx)CuCl3 for x<0.36. Both parent compounds TlCuCl3 and KCuCl3 are coupled spin dimer systems, which have the singlet ground state with excitation gaps Δ/kB=7.7 K and 31 K, respectively. Due to bond randomness, the singlet ground state turns into the magnetic state with finite susceptibility, nevertheless, the excitation gap remains. Field-induced magnetic ordering, which can be described by the Bose condensation of excited triplets, magnons, was observed as in the parent systems. The phase transition temperature is suppressed by the bond randomness. This behavior may be attributed to the localization effect.
  Phys. Rev. B. 05/2002; 65(18).