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High-resolution x-ray diffraction measurements have been performed between 10 and 300 K on several single crystals of KTa 1-x Nb x O 3 with x≤0.157. Contrary to the published phase diagram, no deviation from cubic symmetry is seen for x≤0.060. Instead, a low-temperature minimum in the cubic unit cell volume is observed at a temperature T min which increases monotonically for 0.012≤x≤0.060. Data for x=0.157, however, are consistent with the previously established sequence of structural transitions from cubic to tetragonal to orthorhombic to rhombohedral symmetries

The effect of pump-wave misalignment on the oscillation spectra of a semilinear photorefractive oscillator is studied numerically and compared with the results of experiments performed with ${\text{BaTiO}}_{3}:\text{Co}$ and ${\text{KNbO}}_{3}:\text{Ag},\text{Fe}$ crystals.

Stretched exponential relaxation of stress-induced quadrupolar order is found in K 0.989 Li 0.011 TaO 3 at temperatures T

Neutron-scattering and resistivity experiments on single crystals of ${\mathrm{La}}_{1.95}$${\mathrm{Ba}}_{0.05}$${\mathrm{CuO}}_{4}$ are described. On warming to as high as 250 K, the correlation length does not change, and the temperature dependence of the generalized susceptibility \ensuremath{\chi}(Q,\ensuremath{\omega}) is due solely to the temperature-dependence of the local response. We show that the associated relaxation rate in the metallic regime is \ensuremath{\Elzxh}\ensuremath{\Gamma}\ensuremath{\simeq}1/2${\mathit{k}}_{\mathit{B}}$T, in accord with the marginal-Fermi-liquid hypothesis, and relate our results to nuclear spin-lattice relaxation data.

Transient light-induced absorption in nominally undoped and Fe-doped ${\text{KNbO}}_{3}$ crystals is observed in the visible and infrared spectral ranges after single pulse illumination with $\ensuremath{\lambda}=532\text{ }\text{nm}$. For nominally undoped ${\text{KNbO}}_{3}$ the decay of the light-induced absorption in a single step can be explained by incoherent hopping transport of optically generated small bound ${\text{O}}^{\ensuremath{-}}$ hole and small free ${\text{Nb}}^{4+}$ electron polarons and their mutual recombination. Iron doping causes an additional slow decay component and, remarkably, accelerates the initial decay process. A consistent model for the formation, hopping, and recombination paths of hole and electron polarons is deduced from the experimental data set for both nominally undoped and Fe-doped ${\text{KNbO}}_{3}$. The decrease in the polaron hopping-transport length in Fe-doped samples is attributed to the increased number densities of optically generated hole polarons by additional one-quantum excitations.

An inelastic neutron scattering study of KTa{sub 1{minus}{ital x}}Nb{sub {ital x}}O{sub 3} ({ital x}=0.012) (KTN) shows that there is a marked softening of the zone-center optic mode with decreasing temperature. This mode reaches a nonzero minimum energy of {approx}0.9 meV at {ital T}=20 K and renormalizes to higher energies as the temperature is further lowered, in agreement with light-scattering results on the same sample. In addition, x-ray diffraction does not reveal any structural phase transition in this temperature range. A recent light-scattering study on a {ital x}=0.009 sample suggests that KTN has a dipolar glass phase with a transition temperature of 3 K. The present observation of incomplete softening is similar to the soft-mode behavior characteristic of other mixed systems and provides further evidence of the dipolar glass properties of KTN.

the two nonequivalent 2b and 2d sites of the I4 ¯ structure, but Yb 3+ and Gd 3+ ions are found preferentially in the 2b site. Optical spectroscopy at low 5K temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F7/2n and 2 F5/2n Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the 2b and 2d sites. As a novel uniaxial laser host for Yb 3+ , NaGdWO42 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb 3+ -laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the 1014– 1079 nm spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces 120 fs long pulses at 1037.5 nm with an average power of 360 mW at 97 MHz repetition rate. Using uncoated samples of Yb: NaGdWO42 at normal incidence in simple two-mirror cavities, output powers as high as 1.45 W and slope efficiencies as high as 51% are achieved with different diode laser pump sources.

High-energy inelastic neutron scattering is used to resolve spin waves in La/sub 2/CuO/sub 4/. The corresponding long-wavelength velocity is 0.85 +- 0.03 eV A, well above previous bounds set by thermal neutron scattering. Within experimental error, conventional spin-wave theory gives an excellent description of the magnetic dynamics for momentum and energy transfers below 0.15 A/sup -1/ and 0.1 eV, respectively. Doping with Ba changes the magnetic dynamics; the data indicate a combination of spin-wave softening and damping effects.

Li, substituting for K in KTa${\mathrm{O}}_{3}$, creates a local electric dipole, due to its off-center position with respect to the cubic site. We have studied such crystals with different amounts of Li (and in a few cases also doped with Nb, substituting for Ta) by nuclear magnetic resonance, dielectric relaxation, pyroelectricity, ultrasound, and birefringence methods. Birefringence and dielectric susceptibility results show that collective effects between the Li dipoles occur below a fairly well-defined concentration-dependent temperature of the order of 50 K, but nuclear magnetic and dielectric relaxation indicate the absence of criticality at the onset of these effects. These collective effects are related to those arising in spin-glasses. We discuss the data in the light of theoretical models and computer simulations of systems of randomly interacting moments, which predict an apparent condensation into a system of metastable clusters without long-range order.

Cubic and tetragonal ${\mathrm{BaTiO}}_{3}$ and orthorhombic ${\mathrm{KNbO}}_{3}$ have been studied by Raman scattering, using an iodine filter to eliminate elastically scattered light. In addition to the usual phonon features, such as the soft E mode in tetragonal ${\mathrm{BaTiO}}_{3}$ and the soft ${B}_{2}$ mode in orthorhombic ${\mathrm{KNbO}}_{3}$, central components have been observed in both materials for the first time. These components have the linewidths, line shapes, and thermal properties of relaxation modes, and symmetry properties consistent with an eight-site order-disorder model of the successive phase transitions. Additionally, the data provide direct evidence that clusters of precursor order are present in cubic ${\mathrm{BaTiO}}_{3}$ and the model provides estimates of the temperatures at which these clusters are present in both ${\mathrm{BaTiO}}_{3}$ and ${\mathrm{KNbO}}_{3}$. These values are in good agreement with the results of linear birefringence and refractive-index measurements.

The three symmetry components of the Raman spectra in paraelectric ${\mathrm{KTa}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Nb}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (KTN) mixed crystals have been measured as a function of temperature (${T}_{c}$800 K) and for several niobium concentrations (x=0.008, x=0.012, x=0.02, x=0.09, x=0.21, and x=0.36). The results are analyzed in the framework of an extended shell model by assuming an anisotropic and nonlinear polarizability of the oxygen ion. This intraionic anharmonicity is indicated to be primarily responsible for the strong second-order Raman effect. Conclusions on the leading mechanism of the phase transitions in the KTN system are discussed.

Mesure de l'indice de refraction et de la birefringence lineaire pour x=0,008, 0,012 et 0,02. Observation de transitions «sales» vers un ordre a longue distance. En accord avec les etudes anterieures, les donnees optiques s'expliquent sur la base d'un modele de verre dipolaire cooperatif. Ceci implique un ordre local dans une region de Curie finie d'une transition de phase diffuse, suivi par une reorientation collective des microdomaines geles, induite par les deformations

Nuclear magnetic resonance and dielectric susceptibility data are reported in KTa${\mathrm{O}}_{3}$: Li which show, for the first time, ordering of random-site electric dipoles in a nonpolar lattice.

Sharp first-order ferroelectric phase transitions as indicated by sharp peaks of the dielectric permittivity arise in ${\mathrm{K}}_{0.937}$${\mathrm{Li}}^{0.063}$${\mathrm{TaO}}_{3}$ upon cooling or heating in electric fields 75lEl300 kV/m. They cancel the random dipolar fields due to frozen-in off-center ${\mathrm{Li}}_{+}$ ions, which destroy the phase transition and give rise to low-temperature domain states if E=0. Sharp zero-field transitions with a dispersionless peak of \ensuremath{\epsilon}' at ${\mathrm{T}}_{\mathrm{c}}^{+}$=104.5 K, which accurately follows the Landau-Devonshire theory at T${\mathrm{T}}_{\mathrm{c}}^{+}$, are observed after field cooling with E75 kV/m.

In the preceding Comment Fontana /ital et/ /ital al/. make three assertions basedon experimental facts they believe to be in contradiction with our results, andthen describe results and conclusions of their own recent Raman studies. Inparticular, they show spectra which they believe indicate that there is acoupled relaxation-soft phonon mode of /ital B//sub 2/ symmetry inorthorhombic KNbO/sub 3/. In our Reply we attempt to answer their threeobjections in some detail, and show why we do not believe their objections tobe valid. Additionally we describe the differences between their Ramanmeasurements and ours, which show clearly that there is no evidence of a/ital B//sub 2/ relaxation mode in orthorhombic KNbO/sub 3/, anddiscuss why we think such difference may have occurred.