Observation of a roton collective mode in a two-dimensional Fermi liquid.

Understanding the dynamics of correlated many-body quantum systems is a challenge for modern physics. Owing to the simplicity of their Hamiltonians, (4)He (bosons) and (3)He (fermions) have served as model systems for strongly interacting quantum fluids, with substantial efforts devoted to their understanding. An important milestone was the direct observation of the collective phonon-roton mode in liquid (4)He by neutron scattering, verifying Landau's prediction and his fruitful concept of elementary excitations. In a Fermi system, collective density fluctuations (known as 'zero-sound' in (3)He, and 'plasmons' in charged systems) and incoherent particle-hole excitations are observed. At small wavevectors and energies, both types of excitation are described by Landau's theory of Fermi liquids. At higher wavevectors, the collective mode enters the particle-hole band, where it is strongly damped. The dynamics of Fermi liquids at high wavevectors was thus believed to be essentially incoherent. Here we report inelastic neutron scattering measurements of a monolayer of liquid (3)He, observing a roton-like excitation. We find that the collective density mode reappears as a well defined excitation at momentum transfers larger than twice the Fermi momentum. We thus observe unexpected collective behaviour of a Fermi many-body system in the regime beyond the scope of Landau's theory. A satisfactory interpretation of the measured spectra is obtained using a dynamic many-body theory.

Morphology of thin nanocomposite films of asymmetric diblock copolymer and magnetite nanoparticles.

Thin self-assembled nanocomposite films of an asymmetric diblock copolymer and nanoparticles are fabricated. The morphologies of the films of the diblock copolymer poly(styrene-block-n-butyl methacrylate), P(Sd-b-BMA), with different volume fractions of large magnetite Fe(3)O(4) nanoparticles are studied before and after annealing. Neutron reflectometry reveals remarkable evidence that confining asymmetric copolymer to a limit of two layers forces the film, after the annealing, to form a mixed cylindrical-lamellar two-layer structure. This complex morphology is very stable and is preserved after the incorporation of nanoparticles up to 10% volume fraction. The other striking result is that the monodispersed nanoparticles with affinity to the polystyrene (PS) domain and with a size of 10 nm, which is close to the size of the PS chains, are assembled by the diblock copolymer matrix, so the distribution of the nanoparticles is reduced solely to the PS domain of the film. Our studies demonstrate that for asymmetric block copolymers in thin film geometry the self-assembly is strongly influenced by the interfacial and surface energies of the blocks and substrate.

Membrane-induced folding and structure of membrane-bound annexin A1 N-terminal peptides: implications for annexin-induced membrane aggregation.

Annexins constitute a family of calcium-dependent membrane-binding proteins and can be classified into two groups, depending on the length of the N-terminal domain unique for each individual annexin. The N-terminal domain of annexin A1 can adopt an alpha-helical conformation and has been implicated in mediating the membrane aggregation behavior of this protein. Although the calcium-independent interaction of the annexin A1 N-terminal domain has been known for some time, there was no structural information about the membrane interaction of this secondary membrane-binding site of annexin A1. This study used circular dichroism spectroscopy to show that a rat annexin A1 N-terminal peptide possesses random coil structure in aqueous buffer but an alpha-helical structure in the presence of small unilamellar vesicles. The binding of peptides to membranes was confirmed by surface pressure (Langmuir film balance) measurements using phosphatidylcholine/phosphatidylserine monolayers, which show a significant increase after injection of rat annexin A1 N-terminal peptides. Lamellar neutron diffraction with human and rat annexin A1 N-terminal peptides reveals an intercalation of the helical peptides with the phospholipid bilayer, with the helix axis lying parallel to the surface of membrane. Our findings confirm that phospholipid membranes assist the folding of the N-terminal peptides into alpha-helical structures and that this conformation enables favorable direct interactions with the membrane. The results are consistent with the hypothesis that the N-terminal domain of annexin A1 can serve as a secondary membrane binding site in the process of membrane aggregation by providing a peripheral membrane anchor.