Elastic and inelastic X-ray scattering studies of the low dimensional spin-1/2 quantum magnet TiOCl

The ground state for a one dimensional spin 1/2 Heisenberg chain coupled to phonons is a dimerized singlet state known as a "spin-Peierls" state. Currently, the spin-Peierls state is realized in only a handful of known compounds. Even after decades of scientific scrutiny, there is an absence of direct measurements of the lattice dynamics associated with the transition. In this work we present an extensive study of a new one dimensional spin-Peierls compound, TiOC1. The magnetic susceptibility strongly indicates a singlet ground state, with two apparent anomalies observed at T, ,=65 K and Tc2=92 K. Specific heat measurements have been performed and the associated entropy changes quantified. The 65 K transition exhibits a thermal hysteresis, indicative of a first order phase transition. A detailed synchrotron x-ray study of the structure reveals the appearance of superlattice peaks at ( ... ) below 65 K. The intensity of the peaks drop very sharply above T, and a thermal hysteresis is observed which is consistent with a first order phase transition at 65 K. We find that the temperature region between 65 K and 92 K is characterized by a novel incommensurate state. The incommensurate reflections appear at ( ... ). The temperature dependence of the intensity of the incommensurate peaks shows a more gradual onset, with no thermal hysteresis. The incommensurate wavevectors change continuously as a function of temperature and can be analyzed in terms of a mean field theory of phase shifted discommensurations. The observation of the third harmonics enabled a careful characterization of the underlying real space superstructure. We find that all of the observed scattering can be reproduced by a one dimensional long-wavelength modulation of a locally dimerized structure.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Physics.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2007

Author dc:creator

• Abel, Eric T

Advisor dc:contributor.advisor

• Young S. Lee.

• Physics.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/45414

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/45414