J. D. Axe

Coexistence of, and Competition between, Superconductivity and Charge-Stripe Order in La{sub 1.6-{ital x}}Nd{sub 0.4}Sr{sub {ital x}}CuO{sub 4}

Previously we have presented evidence for stripe order of holes and spins in La{sub 1.6-x}Nd{sub 0.4}Sr{sub x}CuO{sub 4} with x=0.12. Here we show, via neutron diffraction measurements of magnetic scattering, that similar order occurs in crystals with x=0.15 and 0.20. Zero-field-cooled magnetization measurements show that all three compositions are also superconducting, with the superconducting transition temperature increasing as the low-temperature staggered magnetization decreases.These results directly demonstrate an intimate connection between stripe correlations and superconductivity. {copyright} {ital 1997} {ital The American Physical Society}

IMPACT OF CHARGE ORDERING ON MAGNETIC CORRELATIONS IN PEROVSKITE (BI, CA)MNO3

Single crystalline Bi{sub 1-x}Ca{sub x}MnO{sub 3} (0.74{le}x{le}0.82) were studied with neutron scattering, electron diffraction, and bulk magnetic measurement. We discovered dynamic ferromagnetic spin correlations at high temperatures, which are replaced by antiferromagnetic spin fluctuations at a concomitant charge ordering and structural transition. Our results indicate that thermal-activated hopping of the Jahn-Teller active e{sub g} electrons in these insulating materials, nevertheless, induces ferromagnetic interaction through the double-exchange mechanism. It is the ordering of these charges which competes with the double-exchange ferromagnetic metallic state. {copyright} {ital 1997} {ital The American Physical Society}

A neutron diffraction study of a single crystal of YBa2Cu3O7-δ

Abstract A single crystal neutron diffraction study of a superconducting sample (Tc=∼90K) of YBa2Cu3O7-δ was performed at 295K. Despite complications due to the small sample size (∼0.3×0.2×0.1mm3, about 10-2 of optimal) and twinning, a satisfactory refinement of the structure has been carried out based on an orthorhombic unit cell (a=3.810, b=3.883, c =11.674 A ). The structure parameters are similar to those found for ceramic powders, indicating that well-annealed, oxygen-site ordered single crystals can be prepared. Anisotropic Debye-Waller factors (U11 =U22 ≠U33) for all atoms are derived for the first time and compared with isotropic values found in previous studies. The results are not consistent with an alternative structure recently proposed by Reller, Bednorz, and Muller.

Orientational disorder and the intermolecular potential of FCC C60

Careful diffraction studies reveal that the orientational distribution of C60 molecules in disordered high-temperature FCC fullerene deviates significantly from random. These studies show that the molecular orientational preferences anticipate the low temperature orientationally ordered phase, help explain the orientational glass behavior noted in this material and provide quantitative estimates of the intermolecular orientational forces at work.

X-ray search for charge-density-wave satellites in single-crystal Ba1-xKxBiO3.

Two electrochemically grown single crystals of Ba[sub 1[minus][ital x]]K[sub [ital x]]BiO[sub 3] were investigated on the insertion line X25 at the National Synchrotron Light Source: one in the middle of the semiconducting regime ([ital x]=0.23) studied at 300 K and one near the metal-semiconductor, or cubic-orthorhombic, boundary ([ital x]=0.35) studied at 300 and 11 K. The proposed charge-density-wave satellites, seen along [l angle]110[r angle] directions with electron diffraction, have been attributed previously to electron-beam irradiation effects even though their positions seem to vary smoothly with K content. A careful x-ray search in all equivalent [l angle]110[r angle] directions of the pseudocubic reciprocal lattice revealed no satellite structure in either of the samples. We therefore conclude that the reported electron-diffraction results are indeed induced through electron-beam effects and discuss possible interpretations of our observed absence of these satellites in the as-grown crystals and their dependence on electron-beam irradiation.

Structural phase transformations and high-T/sub c/ superconductivity

There is an intimate if imprecisely understood connection between high Tc superconductivity and crystal structure instability. Most of the highest Tc superconductors (Tc ≈20K) discovered in the past 20 years have the so-called A15-type structure, which is marginally stable in the sense that many of the A15 compounds spontaneously distort at low temperatures into a lower symmetry structure. These distortions are driven by unstable low frequency phonon modes. It is believed that these “soft” phonon modes enhance the attractive interaction between electrons and thus tend to increase Tc. On the other hand (the argument goes) phase transformation which lower the symmetry introduce structural gaps at the Fermi surface of metals which compete with superconductivity and tend to decrease Tc.

Ammonium Ion Reorientation in NH4 + β-Alumina

Neutron inelastic scattering techniques have been used to study the motion of NH4 + ions in NH4 + β-Alumina. The results strongly suggest jump rotational reorientation of the NH4 + ion between equivalent configurations. The translational diffusion coefficient of the NH4 + ion appears to be less than 2.5 × 10−6 cm2/sec at 473 K.