Spin Waves in the Ground State of Co3V2O8 & Disordered Liquid Crystals

Mehmet Ramazanoglu (McMaster University),

In the first part of my talk, I will focus on the inelastic neutron scattering study of cobalt vanadate, Co3V2O8. Inelastic neutron scattering measurements were performed on a single crystal of Co3V2O8. In this material, magnetic cobalt ions reside on distinct spine and cross-tie sites within kagomé staircase planes. This system displays a rich magnetic phase diagram from incommensurate to commensurate antiferromagnetic phases starting at TN=11 K . Finally, the phase diagram culminates in a ferromagnetic ground state below TC~6 K. We have studied the low-lying magnetic excitations in this phase within the kagomé plane. Despite the complexity of the system at higher temperatures, linear spin-wave theory describes most of the quantitative detail of the inelastic neutron measurements. Our results show two spin-wave branches, the higher energy of which displays finite spin-wave lifetimes well below TC and our theoretical calculations show an interesting negligible magnetic exchange coupling for two adjacent spine-spine sites [1].

In the second part, my focus will move towards randomly disordered liquid crystal systems. Liquid Crystals (LCs) are found in many different phases. The most well-known, basic ones are Isotropic (I), Nematic (N), and Smectic-A (SmA). LCs show a rich variety of phase transitions between these phases. This makes them very interesting materials to study the basics of phase transitions and related topics. In the low symmetry phases, LCs show positional and directional order. X-ray diffraction techniques are used to probe the instantaneous positional correlations in these phases. Random forces produce distortions on phase transitions in nature. It has been found that aerosils posses a quenched randomness in the mixture of LC+aerosil samples destroying long-range order (LRO) in the SmA phase. This can be modeled as a Random Field problem. To study these phase transitions with aerosil dispersion carries the opportunity to probe the effect of induced random disorder on a positional phase transition, which can be 2nd or 1st order simply by selecting the correct LC [2,3]. Besides, a reentrant system can be produced with a mixture of appropriate LCs, therefore we were also able to study the random disorder effects on the reentrant nematic 6OCB:8OCB+aerosil mixture [4]. The later subject is mainly investigated using the experimental results of the previous measurements. A two-component (thermal and static fluctuations) line shape analysis is developed to define the phases in all temperature ranges.

[1] Spin Waves in the Ferromagnetic Ground State of the Kagomé Staircase System Co3V2O8, M. Ramazanoglu, C.P. Adams, J.P. Clancy, A.J. Berlinsky, Z. Yamani, , R. Szymczak, H. Szymczak, J. Fink-Finowicki and B.D. Gaulin (arxiv:0804.1929)

[2] M. Ramazanoglu, S. Larochelle, R.J. Birgeneau, Effects of disorder on a Smectic A-Nematic phase transition, PRE 75, 061705 (2007)

[3] M. K. Ramazanoglu, P. S. Clegg, R. J. Birgeneau, C. W. Garland, M. E. Neubert, and J. M. Kim, Phys. Rev. E 69, 061706 (2004).

[4] M. Ramazanoglu, S. Larochelle, R.J. Birgeneau, Effects of disorder on a Smectic A-Nematic phase transition, PRE 77, 031702 (2008)

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