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TT: Fachverband Tiefe Temperaturen

TT 28: Correlated Electrons: Low-dimensional Systems - Materials 2

TT 28.4: Talk

Wednesday, March 28, 2012, 15:45–16:00, H 0104

Evidence for a Kosterlitz-Thouless transition in the quasi-2D square-lattice compound Pb₂VO(PO₄)₂ — •Tobias Förster, Jörg Sichelschmidt, Enrique Kaul, Christoph Geibel, and Frank Steglich — Max Planck Institute f. Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany

The layered vanadium-oxide-bis(phosphates) AA’VO(PO₄)₂ (AA’=Pb₂, SrZn, BaCd) present an interesting class of quasi-2D compounds. The crystal structure is dominated by layers of VO₅-pyramids and PO₄-tetrahedra forming a S=1/2-square lattice of V⁴⁺ ions. In contrast to the majority of square lattice compounds these vanadium systems present a significant amount of frustration, because the ferromagnetic exchange along the sides of the squares is of the same order of magnitude as the antiferromagnetic exchange along the diagonals. Therefore one finds a good agreement of the bulk properties with the S=1/2 Heisenberg J₁-J₂-model [1].
In such 2D S=1/2 square-lattices quantum Monte Carlo simulations suggest that a small easy-plane anisotropy results in a crossover from Heisenberg to XY behavior [2]. With Electron Spin Resonance (ESR) measurements on single crystals of Pb₂VO(PO₄)₂ we show that this compound indeed presents an easy-plane anisotropy and that the spin dynamics measured via the ESR linewidth is well described by the occurrence of a Kosterlitz-Thouless transition at a temperature slightly below T_N.

[1] E. E. Kaul et al. J. Magn. Magn. Mater. 272-76, 922 (2004)

[2] A. Cuccoli et al. Phys. Rev. Lett. 90, 167205 (2003)