Symmetry disquisition on theTiOXphase diagram(X=Br,Cl) (original) (raw)

Symmetry disquisition on the TiOX phase diagram

2000

The sequence of phase transitions and the symmetry of in particular the low temperature in- commensurate and spin-Peierls phases of the quasi one-dimensional inorganic spin-Peierls system TiOX (X=Br and Cl) have been studied using inelastic light scattering experiments. The anoma- lous first-order character of the transition to the spin-Peierls phase is found to be a consequence of the different symmetries

Incommensurate spin Luttinger liquid phase in a model for the spin-Peierls materials TiOBr and TiOCl

2011

In the present work we aim to characterize the lattice configurations and the magnetic behavior in the incommensurate phase of spin-Peierls systems. This phase emerges when the magnetic exchange interaction is coupled to the distortions of an underlying triangular lattice and has its experimental realization in the quasi-one dimensional compound family TiOX (X = Cl, Br). With a simple model of spin-1/2 chains inserted in a planar triangular geometry which couples them elastically, we are able to obtain the uniform-incommensurate and incommensurate-dimerized phase transitions seen in these compounds. Moreover, we follow the evolution of the wave-vector of the distortions with temperature inside the incommensurate phase. Finally, we predict gapless spin excitations for the intermediate phase of TiOX compounds along with incommensurate spin-spin correlations, offering to experimentalists a candidate to observe an exotic Luttinger liquid-like behavior.

X-ray scattering study of the spin-Peierls transition and soft phonon behavior in TiOCl

Physical Review B, 2007

We have studied the S = 1/2 quasi-one-dimensional antiferromagnet TiOCl using single crystal x-ray diffraction and inelastic x-ray scattering techniques. The Ti ions form staggered spin chains which dimerize below Tc1 = 66 K and have an incommensurate lattice distortion between Tc1 and Tc2 = 92 K. Based on our measurements of the intensities, wave vectors, and harmonics of the incommensurate superlattice peaks, we construct a model for the incommensurate modulation. The results are in good agreement with a soliton lattice model, though some quantitative discrepancies exist near Tc2. The behavior of the phonons has been studied using inelastic x-ray scattering with ∼ 2 meV energy resolution. For the first time, a zone boundary phonon which softens at the spin-Peierls temperature TSP has been observed. Our results show reasonably good quantitative agreement with the Cross-Fisher theory for the phonon dynamics at wave vectors near the zone boundary and temperatures near TSP . However, not all aspects of the data can be described, such as the strong overdamping of the soft mode above TSP . Overall, our results show that TiOCl is a good realization of a spin-Peierls system, where the phonon softening allows us to identify the transition temperature as TSP = Tc2 = 92 K.

Incommensurate to commensurate phase transition in a new spin-Peierls system TiOBr

Physica B-condensed Matter, 2006

We report here on the observation of superlattice reflections in the new quasi-one-dimensional spin system TiOBr by electron and synchrotron radiation X-ray diffractions. We discussed on crystal symmetry and temperature dependence of superlattice reflections. Superlattice reflections can be observed at (h,k+12,0) below Tc1 and (h±δh,k±δk,0) above Tc1; (h+k=2n). We discussed the origin of two phase transitions, Tc1(=27K) and Tc2(=47K) in

High-pressure magnetic and structural properties of TiOX (X= Cl, Br)

We report high-pressure diffraction and magnetization measurements to demonstrate that the partial collapse of electronic gap at high-pressure insulator to metal transition reported in TiOCl (C. Kuntscher et al. Phys. Rev. B 74 184402 (2006).) corresponds to a Ti 3+ -Ti 3+ dimerization at room temperature within the space group P2 1 /m. The shortest Ti-Ti distance is comparable to that of the Ti metal, but a Peierls-like distortion prevents a metallic behaviour.

Optical phonons in spin - Peierls compound

Journal of Physics: Condensed Matter, 1998

We measured the room temperature far-infrared reflectivity and Raman scattering spectra of NaV 2 O 5 single crystals. The frequencies of infrared active modes are obtained by Kramers-Kronig analysis of reflectivity data. The assignation of the observed modes is given according to the lattice dynamical calculation based on the valence shell model. According to the factor group analysis of the P 2 1 mn space group, which assumes the existence of the V 4+ and V 5+ chains, the 15 A 1 and 7 B 1 modes can be expected in both ir and Raman scattering spectra from the (001) plane. Only eight Raman and six infrared modes of A 1 symmetry are clearly seen. In the case of B 1 symmetry, three B 1 modes are observed both in the Raman and in the ir reflectivity spectra. The frequencies of these ir and Raman modes differ significantly. Because of this, we concluded that the space group of the NaV 2 O 5 crystal structure cannot be P 2 1 mn (non-centrosymmetric), but the space group which includes the mutual exclusion between Raman and infrared activity (centrosymmetric space group). We have shown that the appropriate space group is P mmn, for which we found our experimental spectra in complete agreement with factor-group analysis. This means that V atoms are indistinguishable in the unit cell and in a mixed-valence state.

Spin-Peierls transition in the S=1/2 compound TiPO_{4} featuring large intrachain coupling

Physical Review B, 2011

We investigated the magnetic and structural properties of the quasi-one-dimensional 3d 1 quantum chain system TiPO 4 (J ∼ 965 K) by magnetic susceptibility, heat capacity, electron spin resonance, x-ray diffraction, and nuclear magnetic resonance (NMR) measurements, and by density functional theory (DFT) calculations. TiPO 4 undergoes two magnetostructural phase transitions, one at 111 K and the other at 74 K. Below 74 K, NMR detects two different 31 P signals and the magnetic susceptibility vanishes, while DFT calculations evidence a bond alternation of the Ti-Ti distances within each chain. Thus, the 74 K phase transition is a spin-Peierls transition which evolves from an incommensurate phase existing between 111 and 74 K.

Theoretical study of the magnetism in the incommensurate phase of

2012

Going beyond a recently proposed microscopic model [1] for the incommensurate transition in the spin-Peierls TiOX (X=Cl, Br) compounds, in the present work we start by studying the thermodynamics of the model with XY spins and adiabatic phonons. We find that the system enters in an incommensurate phase by a first order transition at a low temperature Tc1. At a higher temperature Tc2 a continuous transition to a uniform phase is found. Furthermore, we study the magnetism in the incommensurate phase by Density Matrix Renormalization Group (DMRG) calculations on a 1D Heisenberg model where the exchange is modulated by the incommensurate atomic position pattern. When the wave vector q of the modulation is near π, we find local magnetized zones (LMZ) in which spins abandon their singlets as a result of the domain walls induced by the modulated distortion. When q moves far away enough from π, the LMZ disappear and the system develops incommensurate magnetic correlations induced by the str...

Enhanced Dimerization of TiOCl under Pressure: Spin-Peierls to Peierls Transition

Physical Review Letters, 2009

We report high-pressure x-ray diffraction and magnetization measurements combined with ab-initio calculations to demonstrate that the high-pressure optical and transport transitions recently reported in TiOCl, correspond in fact to an enhanced Ti 3+ -Ti 3+ dimerization existing already at room temperature. Our results confirm the formation of a metal-metal bond between Ti 3+ ions along the b-axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.