Hans Bill

Room Temperature Persistent Spectral Hole Burning in Sm-Doped SrFCl1/2Br1/2 Mixed Crystals

Two-photon persistent hole burning is observed at T = 296 K in 5D1–7F0 and 5D0–7F0 transitions of Sm2+ ions in substitutionally disordered SrFCl1/2 single crystals, showing the ratio of inhomogeneous to homogeneous linewidth of 20 ÷ 25. The minimum hole widths of 3.5 cm-1 and 2.6 cm-1, respectively, were determined for these transitions. In fact, the applicability of persistent spectral hole burning, known as a low-temperature high-resolution spectroscopic method and a potential basis for the use of a new–spectral–dimension in optical data storage and processing, is demonstrated for the first time at room temperature in inorganic materials.

Raman spectra of single crystal CuO

Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm−1 (Ag, 344cm−1 (Bg) and 629cm−1 (Bg). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm−1. The temperature dependence of the linewidth of the Ag mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.

Lattice dynamics and the diffuse phase transition of lithium sulphide investigated by coherent neutron scattering

Elastic neutron diffraction of Li2S, measured as a function of temperature, shows the onset of a diffuse phase transition near 900 K to a highly conducting state. Inelastic neutron scattering has been used to investigate the harmonic lattice dynamics of Li2S at 15 K. A shell model has been successfully fitted to the data. The dynamical properties are computed within the harmonic approximation and compared with experimental data.

Color centers, associated rare-earth ions and the origin of coloration in natural fluorites

Natural colored fluorites were studied by means of optical absorption and electron paramagnetic resonance (EPR). Complex centers involving rare-earth ions and/or oxygen give rise to the various colors observed. These include yttrium-associated F centers (blue), coexisting yttrium and cerium-associated F centers (yellowish-green), the (YO2) center (rose) and the O3−molecule ion (yellow). Divalent rare-earth ions also contribute to the colorations, as for instance Sm3+ (green fluorites), or they are at the origin of strong fluorescence observed (Eu2+). Strong irradiation of the crystals with ionizing radiation leads to coagulation of color centers, and to precipitation of metallic calcium colloids. There is probably no simple relation connecting the coloration and the growth process of the crystal. Thermal stability studies, however, have allowed to partially classify the colors as being of primary or secondary origin.

Europium doped BaMgF4, an EPR and optical investigation

Eu2+ was introduced into pure and oxygen codoped BaMgF4 single crystals. A detailed EPR study of this ion (S=7/2) was realized on both types of systems. The result is that only one spectrum was observed involving a strong crystal field. The associated site symmetry of the impurity is Cs. It occupies very closely a Barium lattice site as was established by correlating the EPR results with those of a refined X-ray structure analysis on a Ba0.8Eu0.2MgF4 single crystal realized in our laboratory. The oxygen codoped crystals exhibited this same Eu2+ EPR spectrum (the only one). Optical emission and excitation experiments were performed between 13 000 and 53 000 cm−1. The results due to the Eu2+ impurity are given and discussed qualitatively within the 4f7⇔4f65d1 scheme.

Experimental Raman scattering investigation of phonon anharmonicity effects in

Experiments in which the Raman linewidth was measured as a function of temperature (7-1183 K) and pressure (0-400 bar) were performed on the (111) and (100) planes of single crystals of the cubic anti-fluorite . The temperature dependence of the lattice constant was determined by x-ray diffraction (11-295 K). From these results and published Brillouin scattering data for this host, the volume thermal expansion coefficient as a function of temperature was obtained as well as the isothermal compressibility and the isothermal Raman mode Gruneisen parameter. Using the thermodynamic approach within the quasi-harmonic approximation, we show that below 400 K the volume effects describe well the temperature dependence of the Raman linewidth whereas above this temperature there are direct anharmonic effects appearing. Above approximately 850 K new Raman lines appear that are and E polarized.

EPR and optical spectroscopy of Yb3+ ions in CaF2 and SrF2

Abstract The Yb 3+ paramagnetic center of the trigonal symmetry (“oxygen” paramagnetic center T 2 ) in CaF 2 and SrF 2 single crystals is studied by EPR and optical spectroscopy. The Stark level energies of the Yb 3+ multiplets are established from absorption, luminescence and excitation luminescence spectra and the crystal field parameters are calculated.

Growth of single crystals, thermal dependency of lattice parameters and Raman scattering in the Nd2-xCexCuO4-δ system

Abstract We report on the growth of Nd 2- x Ce x CuO 4- δ single crystals (0 x 2 O flux. Free separated crystals with maximum size of 5x8x0.15 nm 3 have been obtained. Magnetic AC susceptibility measurements show a sharp superconducting transition at temperatures up to 23 K. The temperature dependence of the lattice parameters has been measured by means of X-ray powder diffraction between 10 K ( a=3.9413(3) A , c=12.0290(18) A ) and 290 K ( a=3.9482(3) A , c=12.0590(18) A ). Room temperature Raman spectra reveal a new band at 320 cm -1 which is not observed in Nd 2 CuO 4 . Raman spectra of crystals with T c ranging from 7 to 22 K show a systematic intensity change of the broad band at 590 cm -1 .

High-temperature spectral hole burning on samarium(II) in single crystals of the lead fluorohalide structure family and in thin films of calcium fluoride

When modern spectral hole burning applications for high-density information storage under noncryogenic temperatures are envisioned, it is necessary to develop new frequency-selective photoactive materials for this purpose. Mixed compounds of the PbFCI family, doped with samarium (II) ions, exhibit promising and true room-temperature hole burning capabilities. We investigate this class of systems (and related ones) by combining material synthesis and high-resolution spectroscopy. Whole groups of isomorphous crystals were synthesized with varying degrees of halide anion and/or cation substitutions. Thin films of fluoride-based materials were made in a laboratory-built molecular beam epitaxy system. An extended x-ray study, differential thermal analysis, luminescence, and Raman measurements allowed the characterization of the materials. Formal models were developed for both the inhomogeneous zero-phonon optical line shapes of the Samarium (II) and the time evolution of hole burning.

Unusual behavior of the Gd ESR in single crystals of GdyY1−yBa2Cu3O6+x with x=0.1−0.8 and y=0.03−0.06: Evidence for magnetic interaction in the superconductors

The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of GdyY1−yBa2Cu3O6−x has been measured. In the insulating compound, with x∼0.1, and the superdconducting materials with 30 K < Tc < 80 K, the measurements were performed at X-band, 9.3 GHz, and Kα-band, 36 GHz, over a large temperature range above Tc. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual Tc of 70 K. The crystal field parameters are D = 3B02 = 1307 MHz, B04 = 3.014 MHz and B44 = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 0.005. These values change only slightly in the superconducting crystals.