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Spin Liquid State in k -(BEDT-TTF) 2 Cu 2 (CN) 3 Studied by Muon Spin Relaxation Method

Spin Liquid State in k -(BEDT-TTF) 2 Cu 2 (CN) 3 Studied by Muon Spin Relaxation Method. Advanced Meson Science Laboratory, RIKEN. Seiko Ohira. Introduction Muon spin relaxation technique Results. BEDT-TTF (ET). Conducting layer of ET dimers. S. S. S. S. S. S. S. S. Insulating

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Spin Liquid State in k -(BEDT-TTF) 2 Cu 2 (CN) 3 Studied by Muon Spin Relaxation Method

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  1. Spin Liquid State ink-(BEDT-TTF)2Cu2(CN)3Studied by Muon Spin Relaxation Method Advanced Meson Science Laboratory, RIKEN Seiko Ohira • Introduction • Muon spin relaxation technique • Results

  2. BEDT-TTF (ET) Conducting layer of ET dimers S S S S S S S S Insulating layer of X Dimerized ET molecules k-(BEDT-TTF)2X 2D layered structure X : Counter anion top view

  3. Hole is localized on each dimer Distorted triangular lattice with S=1/2 S=1/2 physical pressure Ground state depends on AFM interactions t' / t Mott insulating phase K. Kanoda et al., Physica C 282-287 (1997) 299. K. Kanoda et al., Hyperfine Interact. 104 (1997) 235.

  4. X t’ / t Cu2(CN)3 1.06 No magnetic order? Cu[N(CN)2]Cl 0.75 TN= 27 K ? k-(ET)2Cu2(CN)3 Mott insulator under ambient pressure S=1/2 2D triangular lattice antiferromagnet Strong spin frustration Quantum effect Y. Shimizu et al. PRL 91 (2003) 107001. Magnetic ground state of S=1/2 triangular lattice AFM for Hubbard model...,,Spin liquid?? No experimental reports for other Mott insulators

  5. Previous studies No long-range order down to 32 mK Spin liquid ground state? 1H NMR Y. Shimizu et al. PRL 91 (2003) 107001. 13C NMR Broadening of linewidth at low temperatures Inhomogeneous electron localization? A. Kawamoto et al. PRB 70 (2004) 060510. External field dependence of the broadening Y. Shimizu et al., to be published. Spin state under zero-field ??

  6. e+ 100% spin-polarized m-e decay m+ m+ S=1/2 Lifetime tm=2.2ms Sample e+ Forward counter Backward counter NF(t) -aNB(t) Asymmetry A(t) = NF(t) + aNB(t) Muon spin relaxation (mSR) method

  7. above 1.5 K below 1.5 K No magnetic order down to 20 mK under ZF ZF-mSR time spectra Muon spin relaxation due to nuclear dipole fields Fitting function : A(t) = A e–(D t)2 e–lt

  8. A. Kawamoto et al. PRB 70 (2004) 060510. Additional internal fields corresponding to the broadening are not observed under ZF Y. Shimizu et al., to be published. Internal field A(t) = A e–(Dt)2 e–lt D : distribution width of internal field Static field

  9. Y. Shimizu et al. PRL 91 (2003) 107001. Spin dynamics changes below 4 K? Spin dynamics A(t) = A e–(D t)2 e–lt l : muon spin relaxation rate Dynamical fluctuation

  10. Correction of time spectra T < 1.5K Faster relaxation Sample & Apiezon-N T > 1.5K

  11. ZF-mSR result Weak suppression of the spin fluctuation below 4 K Dilute isolated spins due to randomness of the system contribute to the fluctuations? Quantum fluctuation? (if l saturates below 20 mK) ...now in progress (D : fixed)

  12. m+ Summary and... ZF-mSR study on k-(ET)2Cu2(CN)3 Additional internal fields corresponding to the broadening of the NMR spectra are not detected under ZF Increase in the muon spin relaxation rate below 4 K .....weak interactions of diluted isolated spins due to randomness? Where is muon? D≈ 1.5 G Upper limit : 10-4mB

  13. Acknowledgments Collaborators Y. Shimizu (RIKEN) K. Kanoda (Univ. of Tokyo) G. Saito (Kyoto Univ.) Thanks to: K. Miyagawa (Univ. of Tokyo) T. Itou (Kyoto Univ.) W. Higemoto (JAERI)

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