X-ray and neutron scattering

Abstract:

Single crystals of ANb(2)O(6) (A = Nil Co, Fe, Mn) niobates have been grown by the floating-zone technique for the first time. The crystals were melt-grown from stoichiometric mixtures of the starting oxides. The orientation of the crystallographic axes relative to the crystal rod axis was found to be influenced by the size of the molten zone. Magnetization measurements as a function of temperature and applied field show that the magnetic properties are consistent with those found in previous studies. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

Large, high-quality crystals of undoped and Zn-doped CuO have been prepared successfully by the floating-zone technique. Formation Of Cu2O during the crystal growth of CuO has been eliminated under optimal growth conditions for the first time. The size of the grown crystals is typically 5-7 cm length and 8-9 turn in diameter, although with Zn doping the single crystal grains are found to form less readily. The Neel temperature is found to decrease with increasing Zn doping. Magnetic hysteresis measurements did not show any irreversible behaviour in either high purity ZnxCu1-xO crystals or polycrystalline CuO. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

The crystal structures of KHCO3 and KDCO3 at 14 K were determined with the single-crystal neutron-diffraction technique. There is no evidence for proton/deuteron disorder. The diffraction pattern of KHCO3 reveals rods of intensity separate from the Bragg peaks. These rods are not observed for KDCO3. They are attributed to diffraction by the sublattice of protons forming regular arrays of double slits parallel to the dimer planes. They reveal quantum coherence in two dimensions for the proton dynamics. It is shown that quantum statistics for fermions impose quantum entanglement and strict separation of the H dynamics from the lattice. The main decoherence mechanism for the vibrational states is thus canceled out. The vibrational ground state is a superposition of fully entangled macroscopic wave functions with long-lived quantum coherence.

Abstract:

We report measurements and analysis of the magnetic critical phenomena in a single crystal of La0.875Sr0.125MnO3. The critical exponents associated with the ferromagnetic transition have been determined from ac susceptibility and dc magnetization data. Techniques like the Kouvel-Fischer plots, the modified Arrots plots, and the critical isotherm analysis were used for this purpose. The values of the exponents γ, β, and δ obtained are found to match very well with those predicted for the three-dimensional Heisenberg model. Our results are consistent with recent numerical calculations and suggest that though the double exchange interaction is driven by the motion of conduction electrons, the effective magnetic interaction near the transition is renormalized to a short range one. © 2003 The American Physical Society.