Giant phonon anomalies in the pseudo-gap phase of TiOCl

P. Lemmens, K.Y. Choi, G. Caimi, L. Degiorgi,
N.N. Kovaleva, A. Seidel, and F.C. Chou,

Phys. Rev. B 70, 134429 (2004).

This figure shows Raman scattering intensity as a function of temperature in (bb) polarization. The lower inset compares (bb) and (aa) polarization. The four most important modes are denoted by alpha to delta. The upper inset shows mode energies on heating the sample. The arrows mark the critical temperatures Tc1, Tc2 and T*.


High energy Raman scattering intensity in (bb), the in-chain polarization, shows maxima at 920 (2J) and 1400 cm-1 (3J) due to magnetic scattering. The inset shows a Ti -displacement assigned to the alpha mode. For clarity only the orbitals dxy along the chain are shown. The dashed lines represent dxz and dyz orbitals.



The spin-1/2 quantum magnet TiOCl is investigated using infrared and Raman spectroscopy. Giant anomalies are found in the temperature dependence of the phonon spectrum, which hint to unusual coupling of the electronic degrees of freedom to the lattice. These anomalies develop over a broad temperature interval, suggesting the presence of an extended fluctuation regime. This defines a pseudo-gap phase, characterized by a local spin-gap. Below 100 K a dimensionality cross-over leads to a dimerized ground state with a global
spin-gap of about 2Dspin = 430K.




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