X-ray and neutron scattering

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.