Spin-Peierls transition in TiOCl

Abstract

Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2 quantum magnet TiOCl shows that the phase transition at T_{c2} = 90 K corresponds to a lowering of the lattice symmetry. Below T_{c1} = 66 K a twofold superstructure develops, that indicates the formation of spin-singlet pairs via direct exchange between neighboring Ti atoms, while the role of superexchange is found to be negligible. TiOCl thus is identified as a spin-Peierls system of pure 1D chains of atoms. The first-order character of the transition at T_{c1} is explained by the competition between the structurally deformed state below T_{c2} and the spin-Peierls state below T_{c1}.Comment: Phys. Rev. B (Rapid Communications) in pres

Electronic structure of the spin-12quantum magnet TiOCl

“…7 shows LDA+U AFM dispersions for the dimerized lowtemperature phase. 8 We note that such fluctuation effects have been observed in charge Peierls systems well above the actual transition temperature. 27 However, as is evident from the figure, the effect of (fluctuating) dimerization results in band doubling but is otherwise rather small and hence cannot explain the phenomenology of the ARPES data.…”

“…5 Correspondingly, the sudden drop of the susceptibility at T c1 = 67 K to almost zero and a kink anomaly at T c2 = 91 K have been discussed in terms of an unusual first-order spin-Peierls transition and possibly some kind of precursor transition, respectively. While the dimerized nature of the ground state indeed could be proven by x-ray diffraction, 8 the results from magnetic resonance, 9,10 Raman 11 and infrared spectroscopy, 12 and specific heat measurements 13 point to the importance of strong spin and/or orbital fluctuations in the hightemperature phase up to 130 K. In partial contradiction, there is recent evidence from cluster calculations in connection with polarization-dependent optical data that the orbital degrees of freedom are actually quenched. 14 Based on Ginzburg-Landau arguments it was concluded that the frustration of the interchain interactions in the bilayers give rise to incommensurate order in the intermediate phase, which commensurately locks in below T c1 .…”

Incommensurate interactions and nonconventional spin-Peierls transition in TiOBr

“…1 The spin-Peierls state is now well established by the temperature dependence of the magnetic susceptibility (χ m ), that is zero below the phase transition at T c1 = 67 K, the observation by NMR of two independent Ti atoms below T c1 , the two-fold crystallographic superstructure below T c1 and electronic band-structure calculations. 1,2,3,4 The atomic displacements in the superstructure as well as the calculated band structure, with the single valence electron of Ti 3+ occupying the d xy orbital, indicate that the spin-Peierls state is formed on the chains of Ti atoms parallel to b via direct exchange interactions. 1,3,4 Although the properties of the low-temperature phase of TiOCl are those of a true spin-Peierls system, TiOCl is not a conventional spin-Peierls compound, because the phase transition at T c1 is first-order.…”

“…1,3,4 Although the properties of the low-temperature phase of TiOCl are those of a true spin-Peierls system, TiOCl is not a conventional spin-Peierls compound, because the phase transition at T c1 is first-order. The temperature dependencies of χ m , electron spin resonance (ESR), nuclear magnetic resonance (NMR), specific heat (C p ) and x-ray diffraction have shown that a second-order phase transition occurs at T c2 = 91 K. 1,2,3,5,6 The 1D character of the magnetic interactions was also supported by the temperature dependencies of optical reflectivity and angle-resolved photoelectron spectroscopy (ARPES), 7,8,9 although it was suggested that on cooling from room temperature, a crossover from two-dimensional (2D) towards 1D interactions occurs. 7,10 The nature of the state above T c1 is not understood yet.…”

Symmetry disquisition on theTiOXphase diagram(X=Br,Cl)Fausti1, Lummen2, Angelescu3 et al. 2007Phys. Rev. BSelf Cite264271View full textAdd to dashboardCite

“…At low temperature ͑T c1 ͒, TiOX shows a first-order phase transition to a dimerized nonmagnetic state, discussed in terms of a spin-Peierls state. 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.…”