We have studied the collective short wavelength dynamics in deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayers by inelastic neutron scattering. The corresponding dispersion relation $\hslash \omega (Q)$ is presented for the gel and the fluid phase of this model system. The temperature dependence of the inelastic excitations indicates a phase coexistence between the two phases over a broad range and leads to a different assignment of excitations from that reported in a preceding inelastic x-ray scattering study [Phys. Rev. Lett. 86, 740 (2001)]. As a consequence, we find that the minimum in the dispersion relation is actually deeper in the gel than in the fluid phase. Finally, we can clearly identify an additional nondispersive (optical) mode predicted by molecular dynamics simulations [Phys. Rev. Lett. 87, 238101 (2001)]. DOI: http://dx.doi.org/10.1103/PhysRevLett.93.108107 © 2004 The American Physical Society

Neutron scattering in strong magnetic fields is used to show the spin resonance in superconducting ${\mathrm{CeCoIn}}_5$ ($T_c=2.3\mathrm{K}$) is a doublet. The underdamped resonance ($\hslash \Gamma =0.069\pm 0.019\mathrm{meV}$) Zeeman splits into two modes at $E_{\pm }=\hslash {\Omega }_0\pm \alpha {\mu }_B{\mu }_{0}H$ with $\alpha =0.96\pm 0.05$. A linear extrapolation of the lower peak reaches zero energy at $11.2\pm 0.5\mathrm{T}$, near the critical field for the incommensurate “$Q$ phase.” Kenzelmann et al. [Science 321, 1652 (2008)] This, taken with the integrated weight and polarization of the low-energy mode ($E_{-}$), indicates that the $Q$ phase can be interpreted as a Bose condensate of spin excitons.

The diffusion coefficient of liquid aluminium has been obtained through coherent quasielastic neutron scattering. The resulting values agree well with calculations derived from the viscosity applying the Stokes-Einstein relation and with ab initio calculations. The temperature dependence of the diffusion constant displays an Arrhenius-type behavior with a single activation energy of 274 meV, consistent with results obtained by utilizing embedded atom method potentials.

Spin fluctuations are reported near the magnetic field—driven quantum critical point in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. On cooling, ferromagnetic fluctuations evolve into incommensurate correlations located at ${\mathbf{q}}_{\mathbf{0}}=\pm \left(\delta ,\delta \right)$, with $\delta =0.14\pm 0.04\mathrm{r}.\mathrm{l}.\mathrm{u}.$ At low temperatures, an in-plane magnetic field induces a sharp intradoublet resonant excitation at an energy $E_0=g{\mu }_B{\mu }_{0}H$ with $g=3.8\pm 0.2$. The intensity is localized at the zone center, indicating precession of spin density extending $\xi =6\pm 2\AA$ beyond the $4f$ site.

The fundamental excitations in an antiferromagnetic chain of spins-1/2 are spinons, de-confined fractional quasiparticles that when combined in pairs, form a triplet excitation continuum. In an Ising-like spin chain the continuum is gapped and the ground state is Néel ordered. Here, we report high resolution neutron scattering experiments, which reveal how a magnetic field closes this gap and drives the spin chains in Yb₂Pt₂Pb to a critical, disordered Luttinger-liquid state. In Yb₂Pt₂Pb the effective spins-1/2 describe the dynamics of large, Ising-like Yb magnetic moments, ensuring that the measured excitations are exclusively longitudinal, which we find to be well described by time-dependent density matrix renormalization group calculations. The inter-chain coupling leads to the confinement of spinons, a condensed matter analog of quark confinement in quantum chromodynamics. Insensitive to transverse fluctuations, our measurements show how a gapless, dispersive longitudinal mode arises from confinement and evolves with magnetic order.