E.C. Svensson

Spin and phonon excitations in actinide systems

Abstract Recent Chalk River neutron scattering experiments on antiferromagnetic UN, on ferromagnetic UTe and US, and on non-magnetic UPd 3 , are reviewed. The spin excitations may be sharp or diffuse and the phonons exhibit anomalies reminiscent of mixed-valent compounds. A very detailed study of the dispersion of the sharp collective excitations seen in dhcp UPd 3 has shown that the uranium is in a stable f 2 , J = 4, ground state subject to a conventional crystal field and exchange. Thus, UPd 3 is the canonical 5f system, analogous to dhcp Pr in the 4f series. Although UN is cubic, large anisotropies are found in its critical scattering, and large anisotropy gaps are found in the spin wave spectra of UN and UTe.

Magnetic excitations and lattice vibrations in uranium rock‐salt structure compounds

The inelastic magnetic and lattice vibrational scattering of neutrons from several uranium pnictides (UN, UAs, and USb) and chalcogenides (US, UTe) have been measured in recent years. The character of the low‐temperature magnetic scattering depends on the lattice constant. The compounds with the largest lattice parameters (USb, UTe) exhibit several branches of well defined magnetic excitations. The other compounds exhibit only a broad, featureless frequency response at low temperature, which may be extremely weak, as in new data presented here for US. Nonetheless, intense paramagnetic scattering is observed above the ordering temperature, with a marked 2d anisotropy in all the pnictides. A force‐constant analysis of the phonon dispersion relations for these materials has been carried out. We show here that UTe, US, and UAs have negative values of the elastic constant C12, as has been found in certain mixed valence compounds. As expected from the increase in ionic size, the compressibility increases with t...

Excitations of isolated clusters of magnetic ions

The neutron scattering by isolated clusters of Mn2+ ions in a single‐crystal specimen of KMn0.18Zn0.82F3 has been studied as a function of wave‐vector transfer Q at 4.3 K and as a function of temperature T at Q=2πa−1(0.55,0.55,0.55). At low T we observe peaks corresponding to the ground‐state transitions of isolated pairs and triplets and to two excited‐state transitions of pairs. The frequencies υ of these peaks lead directly to accurate values for the exchange interaction J, e.g., 0.100±0.004 THz and 0.098±0.002 THz from the ground‐state transitions of pairs (υ=2J) and triplets (υ=7J) respectively. Exact calculations of the neutron scattering by isolated pairs, triplets and quartets of Mn2+ ions interacting via nearest‐neighbor antiferromagnetic exchange give a reasonably good description of the principal features of the measurements. There are, however, discrepancies that are probably attributable to the scattering by larger clusters.

On the resonant perturbation of spin waves by impurities

Abstract The resonant perturbation of spin waves caused by zinc impurities in MnF2 has been observed by means of neutron inelastic scattering. A Green-function calculation of the impurity-induced changes in the spin-wave frequencies and the widths of the neutron groups gives reasonable agreement with experiment.

New interpretation of the quasiparticle weight Z(Q) for superfluid 4He

Abstract Glyde and Griffin have recently given a new interpretation of the quasiparticle dispersion relation for superfluid 4He. The low Q phonon peak in S(ovrarr|Q,ω) corresponds to a zero sound mode while the high Q maxon-roton peak is a strongly-renormalized single particle excitation. We propose that Z(Q) should be viewed as the superposition of the weights of zero sound and maxon-roton excitations. We also speculate that the ratio Z(maxon)/Z(roton) may reflect the value of the condensate fraction n0.

Temperature dependence of S(Q) for liquid 4He

It was most appropriate to hold this symposium in a year when two very important helium anniversaries were celebrated — the 40th anniversary of LANDAU’S [1] now famous phonon-roton dispersion relation for the elementary excitations in superfluid 4He (see Fig. 1), and the 30th anniversary of the first experimental observation, using neutron scattering techniques, of rotons by PALEVSKY et al. [2]. Their measurements and others [3–6] which quickly followed, all stimulated by the suggestion of COHEN and FEYNMAN [7] that neutron scattering could be used to directly determine the helium dispersion relation, confirmed that the form proposed by Landau was indeed correct.

Magnetic excitations in Mn‐doped CoF2

The magnetic excitation spectra of CoxMn1−xF2 single crystals having x=0.9 and 0.95 have been studied by neutron inelastic scattering. In both cases there is a branch of strongly perturbed ’’cobalt’’ excitations and a branch of lower frequency ’’manganese’’ excitations whose intensity is very strongly peaked at the zone centre. For x=0.9 (0.95) the zone‐centre frequencies for the Co and Mn modes are 1.46±0.05 THz (1.23±0.02 THz) and 0.67±0.04 THz (0.81±0.02 THz) respectively. For x=0.95, separate shell modes associated with the Co neighbours of Mn impurities have been observed at frequencies ∠2.25 THz near zone boundaries. Low concentration Green function theory, which adequately describes the behavior for low cobalt concentration, does not even qualitatively describe the current results. It predicts flat branches of undamped excitations for the manganese and shell modes whereas both have finite intrinsic widths and the manganese modes exhibit a marked frequency variation. Calculations based on the CPA th...

Anisotropic exchange in antiferromagnetic RbCoF3

Abstract The dispersion relations for the three lowest branches of magnetic excitations in antiferromagnetic RbCoF 3 have been determined by neutron inelastic scattering. A Hamiltonian that includes anisotropic exchange gives a better description of the results than one with only isotropic exchange but is still inadequate.

Two‐Magnon Scattering of Neutrons by MnF2

The inelastic scattering of neutrons by two‐magnon processes has been studied in MnF2 at 4.2°K. The two‐magnon scattering was seen most clearly at the reciprocal lattice point (2π/a, 0, 0) where it took the form of a shoulder on the high‐frequency side of the one‐magnon peak with a well‐defined cut‐off in energy. Similar results were obtained at other wavevectors around this point but sharp peaks due to magnetovibrational scattering were then superposed on the broad two‐magnon scattering distribution. The line shape, the cut‐off in energy, and the intensity calculated from the cross‐section formulas are consistent with experiment.

Resonant perturbation of spin waves in KMn0.99Mg0.01F3 and KMn0.96Zn0.04F3

The perturbation of spin waves in the perovskite antiferromagnet KMnF3 by non‐magnetic Mg and Zn impurities has been studied by neutron inelastic scattering. On the basis of Ising theory, a resonant perturbation is expected at about 1.9 THz, and a marked resonant perturbation center in the range 1.8–1.9 THz is observed for both materials. For KMn0.99Mg0.01F3, single‐peaked lineshapes are observed with a maximum impurity‐induced width Γ of 0.107±0.012 THz. For KMn0.96Zn0.04F3, double‐peaked lineshapes are observed near resonance and the maximum Γ is 0.354±0.01Pd5 THz. The effect of the impurities on the frequencies is small relative to the effect on the widths.