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TT: Fachverband Tiefe Temperaturen

TT 87: Low-Dimensional Systems: Molecular Conductors (jointly with CPP, HL, MA, O)

TT 87.3: Vortrag

Donnerstag, 19. März 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 Pinteric^{3, 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 ℏω_c<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)∝ω^β where two distinct exponents β change from 0.9 to 1.7 at low temperatures, with the corresponding crossover scaling with temperature: ℏω_c≈ k_B 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.