Dresden 2009 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 42: Correlated Electrons: Low-dimensional Systems - Models 2

TT 42.2: Talk

Thursday, March 26, 2009, 14:15–14:30, HSZ 301

Even-odd Effects in Short Antiferromagnetic Heisenberg Chains — •A. Machens¹, O. Waldmann¹, I. Schneider², and S. Eggert² — ¹Physikalisches Institut, Universität Freiburg, D-79104 Freiburg, Germany — ²Department of Physics, University of Kaiserslautern, D-67663 Kaiserslautern, Germany

We have recently studied the magnetic properties of Cr₆ and Cr₇ molecular horseshoes by magnetometry and inelastic neutron scattering [1,2]. These molecules could be identified as chains of six or seven s = 3/2 spins, respectively, with antiferromagnetic nearest-neighbor Heisenberg exchange interactions. The analysis of the energy spectra has revealed a striking difference for even and odd chains: For the odd chain the energy E(S) of the lowest state for each total spin S closely follows E(S) ∝ S(S+1), while for the even chain a strong renormalization is found at small values of S. Numerical exact diagonalization of the Heisenberg Hamiltonian for chains of different lengths N and spins s has confirmed this difference. We will present a more general understanding of this phenomenon by symmetry considerations, application of valence bond techniques and comparison with an effective Hamiltonian. This Hamiltonian will be modified depending on the couplings and symmetries of the chains.

[1] S. T. Ochsenbein et al., Europhys. Lett. 79, 17003 (2007).

[2] S. T. Ochsenbein, et al., Chem. Eur. J. 14, 5144 (2008).