Bjorn Fak

Dynamics of liquid 4He in vycor

Using inelastic neutron scattering, we have observed well-defined phonon-roton ( p-r) excitations in superfluid 4He in Vycor over a wide wave-vector range, 0.3</=Q</=2.15 A(-1). The p-r energies and lifetimes at all temperatures are the same as in bulk liquid 4He. However, the weight of the single p-r component does not scale with the superfluid fraction rho(S)(T)/rho as it does in the bulk. In particular, we observe a p-r excitation above T(c) = 1.952 K, where rho(s)(T) = 0 in Vycor. This suggests, if the p-r excitation intensity scales with the Bose condensate, that there is a separation of the Bose-Einstein condensation temperature and the superfluid transition temperature T(c) of 4He in Vycor.

Precision determination of S(Q, ω) of liquid 4He as a function of temperature

Abstract We report high-resolution high-precision measurements of S ( Q , ω ) of liquid 4 He at SVP at a range of temperatures above and below T λ , covering the wave vector range from 0.3 to 2.1A -1 . The temperature dependence is seen to depend sensitively on Q , the phonon remaining a well-defined excitation above T λ , while the maxon and roton excitations change abruptly at T λ , leaving no sharp peak at temperatures above.

Coherent diffuse neutron scattering from the anion-excess fluorite (Sr, Y)Cl2.03

The anion-excess fluorite (Sr,Y)Cl2.03 has been studied by coherent diffuse neutron scattering techniques. At ambient temperature the disorder is found to be static within instrumental resolution. The parameters of defect cluster models have been fitted directly to the data. Excess chlorine ions are found to aggregate into cuboctahedral clusters whose ionic coordinates agree with those calculated from a simple hard sphere model. At elevated temperatures the scattering exhibits quasi-elastic energy broadening indicating the dynamic nature of the disorder. Data obtained at 1050 K show that the cuboctahedral clusters have broken up, the scattering being reminiscent of that obtained from pure fluorites in the fast-ion phase. It is possible to account for the high temperature scattering in terms of snapshot models of the diffusing anions and their associated relaxation fields.

Anomalous Spin Response in the Non-Centrosymmetric Metal CePt3Si

Non-centrosymmetric metals with tetragonal structure are characterized by an antisymmetric Rashba-type spin–orbit interaction, which has been predicted to give rise to anomalous spin susceptibility. Our polarized inelastic neutron scattering measurements in the normal phase of the heavy-fermion superconductor CePt3 Si confirm this prediction. We find that for a finite wave vector along the crystallographic a-direction the spin susceptibility χaa is different from χbb by a factor of two. The effect is even larger than in theoretical calculations based on the Hubbard model, and we speculate that this is due to the strong spin–orbit coupling of rare-earth heavy fermions.

Spin excitations in the two-dimensional strongly coupled dimer system malachite

The mineral malachite, Cu2(OD)2CO3, has a quantum spin-liquid ground state and no long-range magnetic order down to at least T=0.4 K. Inelastic neutron scattering measurements show that the excitation spectrum consists of dispersive gapped singlet-triplet excitations, characteristic of spin-1/2 dimer-forming Heisenberg antiferromagnets. We identify a new two-dimensional dimerized coupling scheme with strong interdimer coupling J/J1~0.3 that places malachite between strongly coupled alternating chains, square lattice antiferromagnets, and infinite-legged ladders. The geometry of the interaction scheme resembles the staggered dimer lattice, which may allow unconventional quantum criticality.

Diffuse neutron scattering from anion-excess strontium chloride

Abstract The defect structure and diffusional processes have been studied in the anion-excess fluorite ( Sr , Y ) Cl 2.03 by diffuse neutron scattering techniques. Static cuboctahedral clusters found at ambient temperature break up at temperatures below 1050 K, where the anion disorder is highly dynamic. The anion diffusion rate is faster than that for pure SrCl 2 at the same temperature.