Regensburg 2013 – scientific programme
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TT: Fachverband Tiefe Temperaturen
TT 55: Correlated Electrons: Low-Dimensional Systems -Materials 2
TT 55.9: Talk
Thursday, March 14, 2013, 11:45–12:00, H21
Lattice effects in the 2D valence-bond-solid Mott insulator EtMe3P[Pd(dmit)2]2 — •Rudra Sekhar Manna1, Reizo Kato2, and Michael Lang1 — 1Physikalisches Institut, Goethe-University Frankfurt (M), SFB/TR 49, D-60438 Frankfurt (M), Germany — 2RIKEN, Wako, Saitama 351-0198, Japan
EtMe3P[Pd(dmit)2]2 is a quasi-twodimensional valence-bond-solid (VBS) Mott insulator where Pd(dmit)2 molecules form dimers in the conducting layer, arranged in a triangular lattice with transfer integrals t and t′ (t′/ t = 0.87) and an average inter-dimer exchange coupling J/kB ≈ 250 K. Despite this strong spin frustration the system reveals a low-temperature transition into a spin-gapped Mott insulating phase. Here we present results of the uniaxial expansion coefficients αi on single crystals of EtMe3P[Pd(dmit)2]2 for temperatures 1.4 K ≤ T ≤ 200 K and magnetic fields B ≤ 8 T. We find a sharp and well-pronounced λ-type thermal expansion anomaly at 24.1 K corresponding to the second-order phase transition into the low-T VBS phase. The discontinuity in αi at TVBS reveals a pronounced in-plane (c-axis) vs out-of-plane (b-axis) anisotropy. Strongly anisotropic expansivities also characterize the state T > TVBS. While the in-plane c-axis shows a large positive expansivity, as often found in soft organic materials, the out-of-plane b-axis is dominated by a large negative contribution which sets in rather abruptly around 40 K. Measurements in magnetic fields up to 8 T were found to have no effect on the VBS phase transition, i.e., there are no indications of a field-induced melting of the VBS state at this field level.