Berlin 2015 – scientific programme
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HL: Fachverband Halbleiterphysik
HL 68: Low-dimensional systems: Molecular conductors (TT with CPP/HL/MA/O)
HL 68.3: Talk
Thursday, March 19, 2015, 10:00–10:15, H 3010
Low-Energy Excitations in the Quantum Spin Liquid κ-(BEDT-TTF)2Cu2(CN)3 — •Andrej Pustogow1, Elena Zhukova2, Boris Gorshunov2, Marko Pinteric3, 4, Silvia Tomic4, John Schlueter5, and Martin Dressel1 — 11. Physikalisches Institut Universität Stuttgart — 2Moscow Institute of Physics and Technology, Russia — 3Faculty of Civil Engineering, Maribor, Slovenia — 4Institut za fiziku, Zagreb, Croatia — 5Argonne National Laboratory, USA
The suppression of long range magnetic order due to geometrical frustration gives rise to the quantum spin liquid state. Theoretical considerations predict enhanced absorption within the Mott gap caused by spinons, which results in a low-frequency power-law behaviour of the optical conductivity, i.e. for ℏωc<J≈250 K. To verify this hypothesis, the optical conductivity of the spin liquid candidate κ-(BEDT-TTF)2Cu2(CN)3 was measured, where the dimerized organic molecules are arranged on a triangular lattice. An extremely wide energy range from radio frequencies up to the near infrared (10−13 eV−1 eV) was covered by dielectric spectroscopy, THz absorption and optical reflectivity measurements. We could indeed identify a power-law behaviour σ(ω)∝ωβ where two distinct exponents β change from 0.9 to 1.7 at low temperatures, with the corresponding crossover scaling with temperature: ℏωc≈ kB T. While our results agree well with (ZnCu)3 (OD)6 (Cl)2, another spin liquid candidate, theory predicts exponents of 2 and 3.33, respectively. Hence, these experimental findings may motivate a refinement of the theoretical framework.