C.V. Stager

STATIC CRITICAL BEHAVIOR OF THE SPIN-FREEZING TRANSITION IN THE GEOMETRICALLY FRUSTRATED PYROCHLORE ANTIFERROMAGNET Y2MO2O7

Some frustrated pyrochlore antiferromagnets, such as Y{sub 2}Mo{sub 2}O{sub 7}, show a spin-freezing transition and magnetic irreversibilities below a temperature T{sub f} similar to what is observed in {ital randomly} frustrated spin-glasses. We present results of nonlinear dc magnetization measurements on Y{sub 2}Mo{sub 2}O{sub 7} that provide strong evidence that there is an underlying thermodynamic phase transition at T{sub f}, which is characterized by critical exponents {gamma}{approx}2.8 and {beta}{approx}0.8. These values are typical of those found in random spin-glasses, despite the fact that the level of random disorder in Y{sub 2}Mo{sub 2}O{sub 7} is immeasurably small. {copyright} {ital 1997} {ital The American Physical Society}

Far-infrared properties of ab -plane oriented YBa 2 Cu 3 O 7 − δ

Polycrystalline samples of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ with a variety of surface treatments show differences in absolute reflectance and width of phonon lines. Samples that are not polished and are measured immediately after annealing have largely grains with the c axis normal to the surface. Such oriented samples show a gaplike depression of conductivity in the far infrared that sets in below the superconducting transition temperature but no true gap. Phonon lines at 195 cm/sup -1/ and at 155 cm/sup -1/ narrow in the superconducting state, in analogy with the effect of the electron-phonon interaction in Bardeen-Cooper-Schrieffer superconductors. In the normal state the background conductivity is Drudelike with a plasma frequency of 0.75 eV and a relaxation rate of 200 cm/sup -1/. The extrapolated far-infrared conductivity agrees with the measured dc conductivity.

Synthesis, characterization and superconducting properties of Pb2Sr2(Y/Ca)Cu3O9−δ single crystals

Abstract Single crystals of Pb 2 Sr 2 (Y/Ca)Cu 3 O 9-δ have been grown from a PbO-NaCl flux. The absence of CuO flux enables the crystals to be separated non-destructively from the melt using an organic solvent. This method yields high quality, untwinned, reasonably sized crystals with a sharp transition temperature of 84 K. The crystals exhibit an orthorhombic structure with lattice parameters; deduced from a Guinier camera analysis, a = 5.383 (1) A , b = 5.423 (1) A and c = 15.765 (2) A . The crystals are considerably thicker along the c -direction than typical crystals of YBa 2 Cu 3 O 7-δ and Bi 2 Sr 2 CaCu 2 O 8 which facilitates measurements of anisotropic properties. As an example we present a measurement of the component of the resistivity in the ab -plane, and along the c -axis.

Infrared properties of YBa2Cu3O7-δ

Abstract We review recent experimental data on the infrared properties of YBa 2 Cu 3 O 7-δ ceramics and single crystals. Kramers-Kronig analysis shows that the normal state far-infrared optical conductivity is dominated by a Drude response from free carriers with a plasma frequency of the order of 1.0 eV and a relaxation rate of 0.37 eV (300 cm -1 at 100 K. A strong mid-infrared band centered between 0.2 and 0.5 eV is seen in all the samples. Below the superconducting transition temperatures the far-infrared conductivity is depressed below 400 cm -1 but this is not a true energy gap since there remains, even in the best oriented crystal samples, a residual absorption.

Nonlinear susceptibility measurements at the spin‐glass transition of the pyrochlore antiferromagnet Y2Mo2O7

We have measured the magnetic field and temperature dependence of the nonlinear dc susceptibility, χnl, of the frustrated pyrochlore antiferromagnet Y2Mo2O7 close to and above the temperature, Tg∼22 K, where this material exhibits irreversible (spin‐glass like) magnetic behavior. Our results suggest that the observed irreversible magnetic properties in this material are due to a thermodynamic spin‐glass transition signaled by a divergence of the nonlinear magnetic susceptibility coefficient χ3∼(T/Tg−1)−γ at Tg with γ=3.3±0.5. χnl shows two power‐law behavior at Tg, χnl∼H2/δ, with δ∼2.8 for H<1000 Oe and δ∼4.1 for H≳2000 Oe. These values for γ and δ, as well as the crossover behavior of χnl at Tg, is consistent what is found in common disordered Heisenberg spin glass materials.

Magnetic and crystal phase transitions in KNiCl3

Neutron scattering measurements revealed that even a seemingly perfect crystal of KNiCl3 at room temperature may segregate into two phases with different lattice distortions in the basal ab plane at temperatures below T≊270 K. In phase A, distortion leads to an increase of the unit cell size by a factor of 3, giving rise to nuclear Bragg peaks of type (h/3,h/3,l). In phase B the distortion gives rise to Bragg peaks at (3h/4,0,l), with integer h and l. Two different magnetic structures have been observed with TN=12.5 and 8.6 K in phases A and B, respectively. Magnetization measurements confirm the values of TN found by neutron scattering for the two magnetic structures.

Ferromagnetic order in U2NiSi3

Abstract Neutron scattering and magnetization measurements have been performed on a single crystal of U2NiSi3. Development of long-range ordered ferromagnetic domains, instead of spin glass freezing, was observed below 30 K. The ordered moment is (0.6 ±0.1)μB per uranium atom and lies perpendicular to the hexagonal axis.

Growth and characterization of superconducting single crystals of layered 1212 PbSrYCaCu oxide

High quality single crystals of Pb1−ySr2Y1−xCaxCu2+yOz have been grown by spontaneous nucleation from a self-flux and by a semiceramic method from partially molten material. Crystals up to 2×2×1 mm3 have tetragonal symmetry with 3.819 < a < 3.826 A and 11.823 < c < 11.851 A. Some of the crystals are partially superconducting “as grown”, others become superconducting after annealing in oxygen with Tonsetc = 40–65 K. These differences are probably caused by inhomogeneous distribution of oxygen and/or calcium in the crystal lattice. The crystals have been characterized by EDAX, EPMA and Guinier powder X-ray diffraction and their uniformity, as well as the influence of annealing on their superconducting properties are discussed.

Far-infrared ab plane reflectance and transmission of Bi2Sr2CaCu2O8

Abstract We review recent experimental data on the infrared thin film transmission and single crystal reflection spectra of Bi 2 Sr 2 CaCu 2 O 8 (T c ∼ 85 K). Both show that the low frequency response can be described by a Drude absorption with a relaxation rate for the carriers of the order of 100 cm −1 . This places the materials in the clean limit and no superconducting gap structure is evident. A temperature independent midinfrared absorption is present with a sharp onset at 300 cm −1 .

Infrared studies of textured ceramic high-Tc superconductors

The surfaces of ceramic samples of high-temperature superconductors can have a high degree of crystalline orientation, depending of the way in which they are prepared. (In particular it is essential that the sample not be polished in an attempt to “improve” the optical quality.) The surfaces of these textured ceramic samples consist of small crystallites, with about 80% orientation of the crystallographic c axis normal to the surface. The infrared and optical properties of these samples have been measured. We observe that the frequency-dependent conductivity can be decomposed into the sum of a far-infrared Drude part (which at zero frequency equals the ordinary dc conductivity) and a broad, temperature-independent mid-infrared band. The mid-infrared band has about four times the oscillator strength of the low-frequency Drude part.