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Register a new userInfrared signatures of the spin-Peierls transition in CuGeO3
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Authors
A. Damascelli
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We investigated the infrared reflectivity of several Mg- and Si-substituted CuGeO3 single crystals. The temperature dependent b-axis and c-axis optical response is reported. For T<Tsp we detected the activation of zone-boundary phonons along the b axis of the crystal on the pure sample and for 1% Mg and 0.7% Si concentrations. From a detailed analysis of the phonon parameters the redshift of the B2u mode at 48 cm^-1 is observed and discussed in relation to the soft mode expected to drive the spin-Peierls phase transition in CuGeO3. Moreover, the polarization dependence of a magnetic excitation measured in transmission at 44 cm^-1 has been investigated.
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We present a study of the nearest--neighbor (nn) and next-nearest-neighbor (nnn) exchange constants between magnetic Cu centers of the spin-Peierls material CuGeO3. The dependence of these constants on the lattice parameters (modified e.g. by variation of temperature, pressure or doping) is calculated. Based on the observation that the bond angles are more susceptible than the bond lengths we propose the so-called accordion model for the description of the properties of CuGeO3. We show that the nn exchange constant in the CuO2 ribbon is very sensitive to the presence and position of the side group Ge with respect to this ribbon. The angle between the two basic units the CuO2 ribbon and the GeO3 zig-zag chain is, besides the Cu-O-Cu angle in the ribbon, one of the principal lattice parameters determining the nn exchange in the c direction. The microscopic calculations of different exchange constants and their dependence on the lattice parameters are carried out using different schemes (perturbation theory; exact diagonalization of Cu2O2 clusters; band approach). The results compare favorable with experiment. The influence of Si doping is also calculated, and the reasons of why it is so efficient in suppressing the spin-Peierls phase are discussed. Thus the consistent microscopic picture of the properties of CuGeO3 emerges.
Polarized infrared reflectivity measurements have been performed on single crystals of the spin-Peierls compound alpha-NaV2O5 in the temperature range 20-300 K. Pronounced spectral features associated with the formation of the dimerized phase were detected both in the a- and b-polarizations (perpendicular and parallel to the spin-1/2 chains, respectively). The temperature dependence of a salient spectral line at 718 cm^-1 sharply rising below the transition temperature T_SP obeys a (1-T/T_SP)^(2beta) law with T_SP simeq 34.3$K and beta simeq 0.25. In addition, a continuum signal is observed in the whole temperature range in the a-polarized optical conductivity spectra. In order to interpret these results, calculations of the static dimerization and of the optical conductivity based on a mean-field and a dynamical treatment of the lattice respectively are proposed.
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 Ti3+-Ti3+ dimerization existing already at room temperature. Our results confirm the formation of a metal-metal bond between Ti3+ 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.
Infrared reflectance of alpha-NaV2O5 single crystals in the frequency range from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In addition to phonon modes identification, for the a-polarised spectrum a broad continuum absorption in the range of 1D magnetic excitation energies was found. The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with vanishing intensity in c-polarisation. Activation of new phonons due to the lattice dimerisation were detected below 35K as well as pretransitional structural fluctuations up to 65K.
We investigated the far- and mid-infrared reflectivity (20-6000 wavenumber) of several pure and doped CuGeO3 single crystals. The b-axis and c-axis optical response is presented for different temperatures between 4 K and 300 K. Moreover, a full group theoretical analysis of the lattice vibrational modes of CuGeO3 in the high temperature undistorted phase as well as in the low temperature spin-Peierls phase is reported and compared to the experimental results. We observe the activation of zone boundary phonons along the b axis of the crystal below the spin-Peierls transition temperature.
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