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We have measured the temperature dependence of the dielectric constants for singledomain SrTi0 3 crystals. Dielectric anisotropics and deviations from the simple Curie-Weiss law have been observed and are discussed in connection with the structural phase transitions at 110 and 65°K.The structural phase transition at T a~ 110°K in SrTi0 3 is a direct consequence of the condensation of the r 25 optical-phonon mode at the Brillouin-zone boundary. 1 " 4 The dielectric constant of this crystal was reported to obey a simple form, e = C(T-T c )~1 9 with paraelectric Curie temperature T C «35°K. At lower temperatures, an appreciable departure from the Curie-Weiss law was observed, which was ascribed to the quantum effect of ionic polarizability. 5 " 8 Hitherto, the dielectric properties were obscured by the presence of multiple domains when the crystal is cooled through T a . The nonferroelectric structural transitions were generally considered not to affect the dielectric behavior. Recently, it was demonstrated that a uniaxial compressive stress can produce a single-domain crystal at low temperatures. 9,10 We report here dielectric measurements on single-domain crystals produced by this technique.A tetragonal prism with surfaces of (110), (ITO), and (001) and of size 2x2x3 mm 3 was cut from a pure single crystal. Geometries of the crystal axes, electrodes for dielectric measurement, and the direction of compressive stress are shown in Fig. 1, Samples were always cooled from room temperature to liquid-helium temperature under an applied stress higher than 10 kg/ mm 2 and the stress was released during dielectric measurement in order to avoid the pressure dependence of the permittivity. This procedure (a) (b) (c) (d) FIG. 1. Geometries of crystal axes, electrodes, external uniaxial stress (thick arrows) and the direction of c axes in the sample (thin arrows), for measuring (a) e a , (b) € c , (c) € 110 , and (d) efo.Dielectric permittivity was measured by a threeterminal method using a GR model 1615A capacitance bridge at 10 kHz with a field level of 5 V/ cm. Temperature inside the sample cryostat was raised slowly, at the rate of 0.2°C/min.Components of the dielectric permittivity tensor for crystals subjected to the stress treatment were determined as functions of temperature, with results as shown in Fig. 2. In this figure we notice that the dielectric tensor becomes anisotropic below T a9 with e a larger than e c . Furthermore, the dielectric anisotropy in the plane perpendicular to the c axis was found to develop around T b ^65°K 12_14 ; that is, [110] and [1T0] axes I-i-i-i-i-|-i-i-i-i-|-i-i-i-i-|-i-i-i-r 0 50 100 150 200 TEMPERATURE (°K) FIG. 2. Temperature dependence of the dielectric constants in SrTi0 3 for various orientations (see Fig. 1). The insets show the details of the dielectric anisotropics near T a and T b9 respectively. 851

We have studied the superconducting properties of Nd(Ba1−xNdx)2Cu3O7−δ (Nd123, x≊0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O2 in Ar atmosphere. An anomalous peak effect in the magnetization hysteresis (M–B) loop is observed in the Nd123 single crystals as well as in the Nd123 bulk crystals prepared by the oxygen-controlled melt growth (OCMG) method. The critical current density (Jc) of the Nd123 single crystals is 70 600 A/cm2 in 1.0 T at 77 K for the applied field perpendicular to the a-b plane. Uniform flux density distribution with the same shape as the sample is observed in the field perpendicular to the a-b plane by the magneto-optical flux-density observation. Finely dispersed white regions in the dark-field image due to Nd substitutions for Ba are observed in the Nd123 matrix by the transmission electron microscopy. The high Jc value of the Nd123 single crystals in the applied field is explained by the field-induced pinning centers caused by the Nd–Ba substitutions in the Nd123 matrix.

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Antiferromagnetic excitations in single crystals of Pr2 Ce Cu04~, Nd, "Ce"Cu04~, and Sm2 Ce Cu04~were investigated as a function of Ce concentration using Raman scattering at room temperature.The introduction of electrons by Ce doping significantly broadens the twomagnon peak in the B&g mode, indicating that zone-boundary magnons become damped with increasing Ce concentration. The existence of broad spin-pair-excitation spectra in the B,~and B2g modes at x =0.15 suggests that the short-range spin fluctuations persist in these electron-doped superconductors. The energy of the spin-pair-excitation peak in Pr~"Ce Cu04~is not affected by doping up to x=0.2. This is similar in behavior to the Sm& "Ce Cu04~system up to x=0.17.The absence of softening of the short-wavelength magnons in these systems is in marked contrast to the magnon softening behavior in La2 "Sr"Cu04~. The Raman spectra in Pr2 Ce Cu04 make no distinction between the superconducting and the metallic regions. The highly damped magnetic-excitation spectra support the picture that the spin-spin correlation length in the present systems decreases gradually with increasing Ce concentration.

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