High-press. NMR　　　　　 Frustration　　　　 Mott-Insulator 　　　 Spin liquid　　　　 Orbital order

■ Quantum spin liquid and her neighborhoods

Quantum spin liquid in two dimensions has been longed as a new state of matter since P.W. Anderson originally proposed on antiferromagnetic triangular lattice. Extensive theoretical calculations based on a Heisenberg model, however, supported long-range magnetic order. Experimentally, the spin liquid has been found only for one-dimensional system. A breakthrough is the discovery of spin liquid in the organic Mott insulator κ-(BEDT-TTF)2Cu2(CN)3 having triangular lattice, located on the border of Mott transition [1,2]. Despite the antiferromagnetic exchange of 250 K, no indication of magnetic order was observed down to 20 mK in the NMR spectrum and relaxation rates.
Although new candidates of spin liquid have been found recently, κ-(BEDT-TTF)2Cu2(CN)3 is unique in that Mott transition and superconducting phase appears by applying hydrostatic pressure [3] and uniaxial strains [4]. Tc under uniaxial strains is much higher than that under hydrostatic pressure, which implies significant spin frustration effect on Tc. The symmetry of Cooper pairs can be intriguing as observed in the depressed Knight shift and the T3 dependence of 1/T1 [5].
The spin and charge excitations in the spin liquid are almost gapless, as observed by specific heat [6], μSR [7], NMR [2], and optical conductivity [8]. The facts imply that the spin liquid is realized near the magnetic quantum critical point as well as the Mott critical point.

References

  • [1] Y. Shimizu, K. Miyagawa, K. Kanoda, M. Maesato, and G. Saito,
    "Spin liquid state in an organic Mott insulator with a triangular lattice"
    Physical Review Letters, 91, 107001 (2003).

  • [2] Y. Shimizu, K. Miyagawa, K. Kanoda, M. Maesato, and G. Saito,
    "Emergence of inhomogeneous moments from spin liquid in the triangular-lattice Mott insulator κ-(ET)2Cu2(CN)3"
    Physical Review B, 73, R140407 (2006).

  • [3] K. Kurosaki, Y. Shimizu, K. Miyagawa, K. Kanoda, and G. Saito,
    "Mott Transition from a Spin Liquid to a Fermi Liquid in the Spin-Frustrated Organic Conductor κ-(ET)2Cu2(CN)3"
    Physical Review Letters, 95, 177001 (2005).

  • [4] Y. Shimizu, M. Maesato, G. Saito,
    "Uniaxial Strain effects on the Mott and superconducting transitions in κ-(ET)2Cu2(CN)3"
    J. Phys. Soc. Jpn. In Press.

  • [5] S. Yamashita, Y. Nakazawa, M. Oguni, Y. Oshima, H. Nojiri, Y. Shimizu, K. Miyagawa, K. Kanoda,
    "Thermodynamic properties of a spin-1/2 spin-liquid in a κ-type organic salt"
    Nature Physics 4, 459 (2008).

  • [6] F.L. Pratt, P.J. Baker, S.J. Blundell, T. Lancaster, S. Ohira-Kawamura, C. Baines, Y. Shimizu, K. Kanoda and I. Watanabe,
    "Uncovering the magnetic and non-magnetic phases of a quantum spin liquid",
    Nature, 471, 612 (2011).

  • [7] Y. Shimizu, H. Kasahara, A. Furuta, K. Miyagawa, K. Kanoda, M. Maesato, and G. Saito,
    "Pressure-induced superconductivity and Mott transition in spin-liquid κ-(ET)2Cu2(CN)3 probed by 13C NMR"
    Physical Review B, 81, 224508 (2010).

  • [8] I. Kezsmarki, Y. Shimizu, G. Mihaly, Y. Tokura, K. Kanoda, and G. Saito,
    "Depressed charge gap in the triangular-lattice Mott insulator κ-(ET)2Cu2(CN)3"
    Physical Review B, 74, R201101 (2006).


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