The thermal conductivity of the spin-Peierls ͑SP͒ compound CuGeO 3 was measured in magnetic fields up to 16 T. Above the SP transition, the heat transport due to spin excitations causes a peak at ϳ22 K, while below the transition the spin excitations rapidly diminish and the heat transport is dominated by phonons; however, the main scattering process of the phonons is with spin excitations, which demonstrates itself in an unusual peak in at ϳ5.5 K. This low-temperature peak is strongly suppressed with magnetic fields in excess of 12.5 T. RAPID COMMUNICATIONS R2914PRB 58 YOICHI ANDO et al. RAPID COMMUNICATIONS R2916PRB 58 YOICHI ANDO et al.

We report the first operation of an optical parametric oscillator in a chalcopyrite crystal, AgGaS2. Tuning from 1.4 to 4.0 μm is demonstrated for 1.06-μm Nd:yttrium aluminum garnet pumping. The potential tuning range extends to the 12-μm transparency limit of the crystal.

No abstract

We have continuously tuned between 7 and 15 μm by mixing the output of a LiNbO3 parametric oscillator in the chalcopyrite AgGaSe2. We have doubled a CO2 laser with 2.7% efficiency which agrees very well with the expected efficiency and verifies the high optical quality of the 1.53-cm-long AgGaSe2 crystal. The measured transparency range, indices of refraction, and nonlinear coefficient of d36 = (3.7 ± 0.6) × 10−11 m/V show that AgGaSe2 is a useful infrared nonlinear material phase matchable over the entire 3–18-μm infrared region.

The optical fluorescence of nominally pure MnF 2 has been studied at low temperatures. It is found that all of the fluorescence, except the very weak intrinsic fluorescence, is induced by small concentrations of cation impurities in the samples. By selectively doping crystals, the characteristic structure for Mg 2+ , Zn 2+ , and Ca 2+ impurities has been identified. In each case, the fluorescence consists of one or two sharp electronic transitions (exciton lines) shifted to a lower energy relative to the intrinsic exciton line, a magnon sideband associated with each exciton line, and a broad band. The exciton lines are identified by polarization and Zeeman experiments. The magnon sidebands are identified by their positions, shapes, and Zeeman behaviors. Each sideband is associated with a particular exciton line by comparing the temperature-dependent lifetimes and intensities of the individual lines. All of these experiments, especially the lifetime and intensity results, strongly suggest that the fluorescence originates at Mn 2+ ions which are perturbed by nearby impurity ions. The excited energy level of the perturbed Mn 2+ ion can be shifted to a lower energy, thereby creating a trap for the freely propagating intrinsic excitons. As a result, the intensities and inverse lifetimes of the fluorescence exhibit an activation behavior; that is, they vary as e^l kT , where A is the depth of the trap. The data are accounted for quantitatively by this model, and values are obtained for the various parameters that describe the energy transfer between the perturbed and unperturbed ions. Finally, it is shown that this model provides an alternative interpretation for the band shift previously discussed by other workers.