J. van Duijn

Quantized antiferromagnetic spin waves in the molecular Heisenberg ring CsFe8

We report on inelastic neutron-scattering (INS) measurements on the molecular spin ring

Absence of low temperature anomaly in the Debye-Waller factor of solid He-4

{\text{CsFe}}_{8}

From cooperative paramagnetism to Néel order in Y2Ru2O7: Neutron scattering measurements

, in which eight spin-5/2 Fe(III) ions are coupled by nearest-neighbor antiferromagnetic Heisenberg interaction. We have recorded INS data on a nondeuterated powder sample up to high energies at the time-of-flight spectrometers FOCUS at PSI and MARI at ISIS, which clearly show the excitation of spin waves in the ring. Due to the small number of spin sites, the spin-wave dispersion relation is not continuous but quantized. Furthermore, the system exhibits a gap between the ground state and the first excited state. We have modeled our data using exact diagonalization of a Heisenberg-exchange Hamiltonian together with a small single-ion anisotropy term. Due to the molecules symmetry, only two parameters

Inhomogeneous Level Splitting in Pr2 xBixRu2O7

J

Inhomogeneous level splitting in Pr

and

_{2-x}

D

Bi

are needed to obtain excellent agreement with the data. The results can be well described within the framework of the rotational-band model as well as antiferromagnetic spin-wave theories.

_x

The mean-square atomic displacement in hcp-phase solid {sup 4}He has been measured in crystals with a molar volume of 21.3 cm{sup 3}. It is temperature independent from 1 K to 140 mK, with no evidence for an anomaly in the vicinity of the proposed supersolid transition. The mean-square displacement is different for in-plane motions (0.122{+-}0.001 A{sup 2}) and out-of-plane motions (0.150{+-}0.001 A{sup 2})

Ru

J. van Duijn, 2, 3 N. Hur, J. W. Taylor, Y. Qiu, Q. Z. Huang, S.-W. Cheong, C. Broholm, 5 and T. G. Perring Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. Dept. Quim. Inorgan., Fac. Quim., Unversidad Complutense de Madrid, Madrid, E-28040, Spain. Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Dated: February 1, 2008)

_2

We report that Bi doping drives Pr 2-x BixRu2O7 from an antiferromagnetic insulator (x = 0) to a metallic paramagnet (x approximately 1) with a broad low T maximum in C/T. Neutron scattering reveals local low energy spin excitations (variant Plancks omega approximately 1 meV) with a spectrum that is unaffected by heating to k(B)T >> variant Plancks omega. We show that a continuous distribution of splittings of the non-Kramers Pr3+ ground-state doublet such as might result from various types of lattice strain can account for all the data.