Anomalous excitation spectra of frustrated quantum antiferromagnets.
Phys Rev Lett 96:5 (2006) 057201
Abstract:
We use series expansions to study the excitation spectra of spin-1/2 antiferromagnets on anisotropic triangular lattices. For the isotropic triangular lattice model (TLM), the high-energy spectra show several anomalous features that differ strongly from linear spin-wave theory (LSWT). Even in the Néel phase, the deviations from LSWT increase sharply with frustration, leading to rotonlike minima at special wave vectors. We argue that these results can be interpreted naturally in a spinon language and provide an explanation for the previously observed anomalous finite-temperature properties of the TLM. In the coupled-chains limit, quantum renormalizations strongly enhance the one-dimensionality of the spectra, in agreement with experiments on Cs2CuCl4.Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnet Cs2CuCl4
ArXiv cond-mat/0601272 (2006)
Abstract:
We report magnetization and specific heat measurements in the 2D frustrated spin-1/2 Heisenberg antiferromagnet Cs2CuCl4 at temperatures down to 0.05 K and high magnetic fields up to 11.5 T applied along a, b and c-axes. The low-field susceptibility chi (T) M/B shows a broad maximum around 2.8 K characteristic of short-range antiferromagnetic correlations and the overall temperature dependence is well described by high temperature series expansion calculations for the partially frustrated triangular lattice with J=4.46 K and J'/J=1/3. At much lower temperatures (< 0.4 K) and in in-plane field (along b and c-axes) several new intermediate-field ordered phases are observed in-between the low-field incommensurate spiral and the high-field saturated ferromagnetic state. The ground state energy extracted from the magnetization curve shows strong zero-point quantum fluctuations in the ground state at low and intermediate fields.On the ordering of Na+ ions in NaxCoO2
AIP CONF PROC 850 (2006) 1213-1214
Abstract:
The influence of electrostatic interactions on the ordering of sodium ions in NaxCoO2 is studied theoretically through Monte-Carlo simulations. For large x small di- or tri-vacancy clusters are stable with respect to isolated Na vacancies. At commensurate fillings these small clusters order in triangular superstructures. These results agree with recent electron diffraction data at x = 1/2 and 3/4. We have performed neutron Laue diffraction experiments at higher x, which confirm the predictions of this simple model. The consequences on the properties of the electronic charges in the Co layers are discussed.Field-induced phase transitions driven by quantum fluctuation in S = 1/2 anisotropic triangular antiferromagnet Cs2 CuBr4
Progress of Theoretical Physics Supplement 159 (2005) 217-221
Abstract:
The field induced magnetic phase transitions of Cs2CuBr 4 were investigated by means of magnetization process and neutron scattering experiments. Cs2CuBr4 should be characterized as S = 1/2 two-dimensional triangular antiferromagnet. Below the ordering temperature TN = 1.4K, the spin structure is the helical incommensurate structure almost within the triangular lattice plane. In the field direction within the triangular lattice plane, Cs2CuBr 4 exhibits the magnetization plateaux at one-third and two-thirds of the saturation magnetization. The spin structure in the one-third plateau phase is found to be almost collinear up-up-down structure which should be stabilized by quantum fluctuation as predicted by the theoretical studies.Spin gaps and magnetic structure of NaxCoO2
ArXiv cond-mat/0510360 (2005)