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.

We report results on the electrodynamic response of TiOCl, a low-dimensional spin-1 2 quantum magnet that shows a spin gap formation for TϽT c1 ϭ67 K. The Fano-like shape of a few selected infrared active phonons suggests an interaction between lattice vibrations and a continuum of low-frequency ͑spin͒ excitations. The temperature dependence of the phonon mode parameters extends over a broad temperature range well above T c1 , indicating the presence of an extended fluctuation regime. In the temperature interval between 200 K and T c1 , there is a progressive dimensionality crossover ͑from two to one͒, as well as a spectral weight shift from low towards high frequencies. This allows us to identify a characteristic energy scale of about 430 K, ascribed to a pseudo-spin-gap.

We present the optical properties of Na 0.7 CoO 2 single crystals, measured over a broad spectral range as a function of temperature (T ). The capability to cover the energy range from the farinfrared up to the ultraviolet allows us to perform reliable Kramers-Kronig transformation, in order to obtain the absorption spectrum (i.e., the complex optical conductivity). To the complex optical conductivity we apply the generalized Drude model, extracting the frequency dependence of the scattering rate (Γ) and effective mass (m * ) of the itinerant charge carriers. We find that Γ(ω) ∼ ω at low temperatures and for ω > T . This suggests that Na 0.7 CoO 2 is at the verge of a spin-density-wave metallic phase.

Cu 3 T eO 6 is a modest frustrated S = 1/2 spin system, which undergoes an anti-ferromagnetic transition at T N ∼ 61 K. The anti-ferromagnetic spin alignment in Cu 3 T eO 6 below T N is supposed to induce a magneto-elastic strain of the lattice. The complete absorption spectrum of Cu 3 T eO 6 is obtained through Kramers-Kronig transformation of the optical reflectivity, measured from the far-infrared up to the ultraviolet spectral range as a function of temperature (T ). Below T * ∼ 50 K, we find a new mode at 208 cm −1 . The spectral weight associated to this additional mode increases as ∝ (T * − T ) 1/2 with decreasing T below T * . The implication of the optical findings will be discussed in relation to the magnetic phase transition at T N .

Angle-resolved photoemission ͑ARPES͒ and optical measurements were performed on single crystal samples of LiCu 2 O 2 , an antiferromagnetic S =1/2 spin-chain compound. The ARPES spectra show several dispersive branches associated with hybrid copper-oxygen states. The occurrence of the valence band maximum halfway between the center and the edge of the Brillouin zone, and the complex spectral line shapes are not reproduced by the existing calculations of the electronic structure. We suggest that they can be interpreted within a one-dimensional scenario of strongly correlated antiferromagnetic insulators. The combination of ARPES and optics allows us to estimate the magnitude of the charge-transfer gap ͑⌬ = 1.95 eV͒. Moreover, the temperature-dependent optical conductivity bears signatures of the three different magnetic phases of this material.