Berlin 2012 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 45: Correlated Electrons: Poster Session

TT 45.71: Poster

Thursday, March 29, 2012, 15:00–19:00, Poster B

Searching for the Power-Law Conductivity Inside the Mott Gap in κ-(ET)₂Cu₂(CN)₃ — •Sebastian Elsässer¹, John A. Schlueter², and Martin Dressel¹ — ¹1. Physikalisches Institut, Universität Stuttgart, Germany — ²Argonne Nat. Lab., U.S.A.

The organic charge-transfer salt κ-(ET)₂Cu₂(CN)₃, which is a Mott insulator, has recently gained interest as possible realisation of the spin-liquid state in two dimensions. This phenomenon is due to spin frustration in the nearly isotropic triangular lattice where spin fluctuations are enhanced and do not order even at the lowest temperatures despite comparable strong antiferromagnetic interactions [1]. Also, spin-liquids support exotic excitations like spinons. Although spinons do not carry charge and thus can not interact with the electromagnetic field directly, they couple to the induced gauge field, leading to a contribution to optical conductivity inside the charge gap. For low frequencies a power-law conductivity σ ₁ ∝ ω ^3.33 for ℏ ω > k_B T and σ ₁ ∝ ω ² for ℏ ω < k_B T is predicted [2]. To verify this on a quantitative basis, we performed polarisation dependent optical investigations ranging from 23 - 8000 cm⁻¹ at temperatures from 300 K - 12.6 K. We find that a power-law is applicable over a wide range inside the Mott gap, however, the exponent clearly deviates from the proposed values as it is actually much lower. The temperature-dependent crossover could not be observed as well. Upon cooling, a saturation of the power-law is observed.
Y. Shimizu et al., Phys. Rev. Lett. 91, 107001 (2003)
T.-K. Ng and P. A. Lee, Phys. Rev. Lett. 99, 156402 (2007)