Giant phonon softening in the pseudogap phase of the quantum spin systemTiOCl

Abstract

Giant anomalies are found in the temperature dependence of Raman-active phonons in the quantum magnet TiOCl, suggesting the presence of an extended fluctuation regime. This regime coincides with a pseudogap phase identified in earlier NMR experiments. The observed large, local spin-gap is proposed to origin from coupled spin/lattice fluctuations. Below 100 K, in the long-range crystallographically distorted phase, a dimerized ground state with a smaller, global spin-gap of about 2⌬ spin Ϸ 430 K exists. This transition also marks a dimensionality cross-over of the system.

Symmetry disquisition on theTiOXphase diagram(X=Br,Cl)Fausti1, Lummen2, Angelescu3 et al. 2007Phys. Rev. B274271View full textAdd to dashboardCite

“…2͑a͔͒. In addition to these expected modes, TiOCl displays a broad peak in the c͑bb͒c polarization, centered at around 160 cm −1 at 300 K; a similar feature is observed in TiOBr as a broad background in the low frequency region at 100 K. As discussed for TiOCl, 13 these modes are thought to be due to pretransitional fluctuations. Upon decreasing the temperature, this "peaked" background first softens, resulting in a broad mode at T c2 ͓see Fig.…”

“…Afterward, through the analysis of Raman spectra, the crystal symmetry in the low temperature phases will be discussed, and in the final part, a comparison with the isostructural VOCl will be helpful to shed some light on the origin of the anomalous high energy scattering reported for TiOCl and TiOBr. 13,19 A. High temperature phase…”

“…Through pressure-dependence measurements of the magnetic susceptibility, the role of magnonphonon coupling in determining the complex phase diagram of TiOX is discussed. Finally, via a comparison with the isostructural compound VOCl, the previously reported 13,19 high energy scattering is revisited, ruling out a possible interpretation in terms of magnon excitations.…”

“…6,9,10 Between this low temperature spin-Peierls phase ͑SP͒ and the one-dimensional antiferromagnet in the high temperature ͑HT͒ phase, various experimental evidences 4,[11][12][13] showed the existence of an intermediate phase, whose nature and origin is still debated. The temperature region of the intermediate phase is different for the two compounds considered in this work: for TiOBr, T c1 = 28 K and T c2 = 48 K, while for TiOCl, T c1 = 67 K and T c2 = 91 K. To summarize the properties reported so far, the intermediate phase ͑T c1 Ͻ T c2 ͒ exhibits a gapped magnetic excitation spectrum, 4 anomalous broadening of the phonon modes in Raman and IR spectra, 9,13 and features of a periodicity incommensurate with the lattice. [14][15][16][17] Moreover, the presence of a pressure induced metal to insulator transition has been recently suggested for TiOCl.…”

Electronic structure of the spin-12quantum magnet TiOCl

“…11,12 This indicates strong coupling of the lattice to the electronic (spin or orbital) degrees of freedom. Indeed, LDA+U calculations for frozen lattice distortions corresponding to the relevant phonon modes have shown that the orbital ground state may switch from 3d xy to 3d xz,yz for sufficiently large amplitude.…”