Observation of Macroscopic Structural Fluctuations in bcc Solid 4He

We report neutron diffraction studies of low density bcc and hcp solid 4He. In the bcc phase, we observed a continuous dynamical behaviour involving macroscopic structural changes of the solid. The dynamical behaviour takes place in a cell full of solid, and therefore represents a solidsolid transformation. The structural changes are consistent with a gradual rotation of macroscopic grains separated by low angle grain boundaries. We suggest that these changes are triggered by random momentary vibrations of the experimental system. An analysis of Laue diffraction patterns indicates that in some cases these structural changes, once initiated by a momentary impulse, seem to proceed at a constant rate over times approaching an hour. The energy associated with these macroscopic changes appears to be on the order of kT. Under similar conditions (temperature and pressure), these effects were absent in the hcp phase.Comment: 14 pages, 6 figure, accepted for PR

Coupled CDW and SDW Fluctuations as an Origin of Anomalous Properties of Ferromagnetic Superconductor UGe_2

It is shown that anomalous properties of UGe_2 can be understood in a unified way on the basis of a single assumption that the superconductivity is mediated by the coupled SDW and CDW fluctuations induced by the imperfect nesting of the Fermi surface with majority spins at T=T_x(P) deep in the ferromagnetic phase. Excess growth of uniform magnetization is shown to develop in the temperature range T<T_x(P) as a mode-coupling effect of coupled growth of SDW and CDW orderings, which has been observed by two different types of experiments. The coupled CDW and SDW fluctuations are shown to be essentially ferromagnetic spin fluctuations which induce a spin-triplet p-wave attraction. These fluctuations consist of two modes, spin and charge fluctuations with large momentum transfer of the nesting vector. An anomalous temperature dependence of the upper critical field H_c2(T) such as crossing of H_c2(T) at P=11.4 kbar and P=13.5 kbar, can be understood by the strong-coupling-superconductivity formalism. Temperature dependence of the lattice specific heat including a large shoulder near T_x is also explained quite well as an effect of a kind of Kohn anomaly associated with coupled SDW-CDW transition.Comment: (12 pages, 10 eps figures) submitted to J. Phys. Soc. Jp

"FERROMAGNÉTISME PLAN DANS (AsO4Co)2Ba"

Dans (AsO4Co)2Ba les ions cobalt sont disposés dans des plans séparés de 8 Å suivant une structure en nid d'abeille de 2,7 Å de côté. Une forte énergie d'anisotropie maintient les moments dans les plans. La loi d'aimantation dans les champs forts est compatible avec l'approximation du champ moléculaire proportionnel à l'aimantation. Au-dessous de 5 °K on observe une structure ferrimagnétique intermédiaire avec σ = σs/3 jusqu'à 5 000 Oe. Nous avons calculé, dans l'hypothèse colinéaire, les différentes structures magnétiques en minimisant l'énergie interne.In (AsO4Co)2Ba the cobaltous ions are disposed in honeycomb lattices of 2.7 Å separated by 8 Å from each other. A strong anisotropy energy maintains the spins in the planes. In high magnetic fields the magnetization law is well accounted by a molecular field proportionnal to magnetization. Below 5 °K there is an intermediate ferrimagnetic structure with σ = σs/3 up to 5 000 Oe. We have calculated, with a colinear hypothesis, the internal energy of all the different possible structures