Frustrated lattice close to spin liquid

Frustrated lattice

The quasi-two-dimensional organic conductors of the BEDT-TTF family crystallize in different phases, labeled by Greek letters. The k-phase compounds draw particular attention since the molecules form dimers within the plane.

Due to the rather strong intradimer coupling a dimer can be considered as a single site with one charge (hole). The system can then be treated as half-filled with Mott physics being relevant. According to the crystallographic arrangement, the overlap between the dimers forms a triangular lattice with coupling t and t’. Since each dimer can also carry one spin, the system is close to frustration when t » t’. While k-(BEDT-TTF)2Cu[N(CN)2]Cl is a Mott insulator with magnetic order below 40 K, k-(BEDT-TTF)2Cu2(CN)3 does not order despite the appreciable coupling of J = 250 K among the spins. Hence it is proposed to be a spin liquid. The optical properties between both compounds are rather different. k-(BEDT-TTF)2Cu[N(CN)2]Cl shows a clear Mott-gap, while for k-(BEDT-TTF)2Cu2(CN)3 a power-law behavior is found all the way to high frequencies and temperatures.

Fig 9: The temperature dependence of the in-plane dc resistivity of κ -(BEDT-TTF)2Cu2(CN)3 (bold blue line) and κ-(BEDT-TTF)2Cu[N(CN)2]Cl (light red line) evidences an insulating behavior. (b) The low-temperature optical conductivity (κ-CN: T=13 K; κ-Cl: T=20 K) does not show a Mott gap, but also reveals important differences between both compounds.

Various methods (NMR, Raman and IR spectroscopy) clearly rule out any charge disproportionation on the dimers. The vibrational lines do not split or shift appreciably. In the case of k-(BEDT-TTF)2Hg(SCN)Cl2 a clear splitting of the vibrational features is observed at the charge order transition TCO = 30 K.

Fig 10: Temperature evolution of the out-of-plane optical conductivity of κ-(BEDT-TTF)2Cu[N(CN)2]Br, κ-(BEDT-TTF)2Cu[N(CN)2]Cl, and κ-(BEDT-TTF)2Cu2(CN)3, measured at the small side of crystals with the electric field polarized perpendicular to the BEDT-TTF planes. The dominant vibrational mode ν27(b1u)of BEDT-TTF molecules is a very sensitive local probe of charge per molecule, namely its frequency depends on the valency (charge) of the molecule. In the case of the three κ-compounds no significant shifts in its frequency are observed with cooling.


Literature:

  1. K. Sedlmeier, S. Elsässer, D. Neubauer, R. Beyer, D. Wu, T. Ivek, S. Tomić, J. A. Schlueter, M. Dressel
    Absence of charge order in the dimerized κ-phase BEDT-TTF salts
    Phys. Rev. B 86, 245103 (2012).
  2. S. Elsässer, D. Wu, M. Dressel, J.A. Schlueter
    Power-law dependence of the optical conductivity observed in the quantum spin-liquid compound κ-(BEDT-TTF)2Cu2(CN)3
    Phys. Rev. B 86, 155150 (2012).