Structural properties in Sr0.61a0.39Nb2O6 in the temperature range 10 K to 500 K investigated by high-resolution neutron powder diffraction and specific heat measurements

We report high-resolution neutron powder diffraction on Sr0.61Ba0.39Nb2O6, SBN61, in the temperature range 15-500 K. The results indicate that the low-temperature anomalies (T<100K) observed in the dielectric dispersion are due to small changes in the incommensurate modulation of the NbO6-octahedra, as no structural phase transition of the average structure was observed. This interpretation is supported by specific heat measurements, which show no latent heat, but a glass-like behavior at low temperatures. Furthermore we find that the structural changes connected with the ferroelectric phase transition at Tc approx. 350K start already at 200K, explaining the anisotropic thermal expansion in the temperature range 200-300K observed in a recent x-ray diffraction study.Comment: Accepted by PRB (2006

Spin-resolved photoemission from Xe-adlayers on Pd(111)

Kessler B, Müller N, Schmiedeskamp B, Vogt B, Heinzmann U. Spin-resolved photoemission from Xe-adlayers on Pd(111). Physica Scripta. 1990;41(6):953-957.Xe-adsorbates on Pd(111) in the coverage region from one layer up to a Xe(111) crystal with first layer Xe-Xe distances comparable to bulk Xe are studied by spin-resolved photoemission. Normally incident circularly polarized synchrotron radiation from BESSY (Berlin) is used and normally emitted electrons are analyzed. No dispersion with photon energy can be found for a system of two Xe-layers. The spin-polarization component measured along the direction of light points to a changing symmetry of the final states. Their symmetry is atom-like for a one-layer system and bulk-like for two and more layers. The initial state symmetry is examined by excitation and decay of excitonic states and is found to be the same for two Xe-layers as for one single Xe-layer. The degeneracy of the valence levels connected with the high symmetry of the [Gamma]-point in bulk Xe can be observed for a system of much more than 5 Xe-layers thickness

Intensity and wavelength dependence of the photoconductivity in Cr-doped Sr

We examine the light-induced charge transport properties of a series of chromium-doped $\rm{Sr}_{0.61}\rm{Ba}_{0.39}\rm{Nb}_2\rm{O}_6$ single crystals by measurements of the optical absorption and the electric conductivity. By comparing the wavelength dependence of the specific photoconductivity and the optical absorption we show that both effects stem from the same center. The intensity dependence of the photoconductivity shows the applicability of a one-center charge transport model for high doping concentrations, while for low doping concentrations a more sophisticated model is needed. The validity of a one-center model is exemplarily verified for a crystal doped with 0.51 mol % Cr over a wide intensity range using a holographic method. The product of mobility and recombination time of photoexcited electrons is deduced from the specific photoconductivity

Evidence of random electric fields in the relaxor-ferroelectric

We present strong evidence of the existence of quenched random electric fields in relaxor ferroelectrics by investigating the aging in ferroelectric hysteresis loops under light illumination. The temporal behaviour of ferroelectric domains under external electric fields and light illumination is measured in cerium-doped strontium-barium-niobate crystals using quasistatic surface charge detection. The aging, caused by electric pinning centres which hinder domain wall motion, leads to a decrease in domain wall mobility. Raising the sample conductivity by illumination eliminates the pinning centres and fully restores the polarization dynamics

Trap-recharging waves versus damped, forced charge-density oscillations in hexagonal silicon carbide

Resonant excitation of space-charge waves (SCW) by means of an oscillating light pattern has been investigated in hexagonal silicon carbide with 4H and 6H stacking sequence, respectively. The experimental data set can be well explained by the existence of trap recharging waves for the 4H sample and allows to determine the product of mobility and lifetime μτ=(5.7   ±   0.6) × 10 -7 cm 2/V, and the effective trap density N eff =(8.0 ±   1.0) × 10 13 cm -3 , respectively. The data set of the 6H polytype indicates a comparably smaller effective trap density, but an unambiguous assignment to the existence of trap recharging waves fails. Taking into account the general classification of material parameters which provides the existence for SCW, the particular case of damped, forced charge-density oscillations can be concluded. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200742.65.-k Nonlinear optics, 42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials, 71.45.Lr Charge-density-wave systems,