N. Rosov

Counterion associative behavior with flexible polyelectrolytes

At low ionic strength, organic counterions dress a flexible charged polymer as measured directly by small-angle neutron scattering and neutron spin-echo spectroscopy. This dressed state, quantified by the concentration dependence of the static correlation length, illustrates the polymer-counterion coupled nature on the nanometer length scale. The counterions, made visible by selective hydrogen and deuterium labeling, undress from the polymeric template by addition of sodium chloride. The addition of this electrolyte leads to two effects: increased Debye electrostatic screening and decoupled organic counterion-polymer correlations. Neutron spin-echo spectroscopy measures a slowing down of the effective diffusion coefficient of the labeled counterions at the length scale of 8 nm, the static correlation length, indicating the nanosecond counterion dynamics mimics the polymer. These experiments, performed with semidilute solutions of tetramethylammonium poly(styrene sulfonate) [(h-TMA(+)) d-PSS], apply to relevant biopolymers including single and double stranded DNA and unfolded proteins, which undergo orchestrated dynamics of counterions and chain segments to fold, unfold, and assemble.

Neutron scattering studies of the vortex lattice in niobium and R123 superconductors (invited)

The magnetic flux lattice undergoes a melting transition not only in high‐Tc oxide superconductors, but also in conventional superconductors, as recently observed in superconducting niobium films. Small‐angle neutron scattering was used to investigate the properties of the magnetic flux lattice in a large, high‐quality single crystal of niobium. The small London penetration depth of niobium gives a large magnetic scattering signal, and the use of a high‐quality single crystal eliminates other unwanted scattering (from twin boundaries, voids, etc.). The signal‐to‐noise ratio is therefore improved by several orders of magnitude over the best available measurements of high‐Tc oxide superconductors. A sixfold hexagonal pattern of peaks is observed in the mixed state (Hc1<H<Hc2) at all temperatures. These peaks are resolution limited below the irreversibility line; above it, the width in the transverse direction increases with temperature due to the vortex dynamics. Close to Hc2, the radial widths of the peaks...

In-plane spin structure studies on MnCl2 graphite intercalation compounds

Abstract The in-plane spin order of MnCl 2 GIC with a mixture of stage-1 and stage-2 has been studied in the temperature range between 63 mK and 30 K by means of elastic neutron scattering. The magnetic Bragg reflections are observed below 7.5 K at the in-plane wavevector Q ∥ = { k i } and { a ∗ ± k i } where a ∗ is the where a ∗ is the in-plane reciprocal lattice vector of MnCl 2 layers and k i ( i = 1, 2) is the reciprocal lattice vector of the in-plane spin structure: ¦ a ∗ ¦ = 1.965 A −1 . This incommensurate in-plane spin structure occurs as a result of competition between ferromagnetic nearest neighbor (NN) intraplanar exchange interaction, and antiferromagnetic second and third NN interactions.

The structure of R-NiF3, and synthesis, and magnetism of new R3 MIINiIVF6 (M = Fe, Co, Cu, Zn), and MIINiIIF4 (M = Co, Cu)

Abstract Neutron diffraction, at 2 K, of R -NiF 3 indicates the formulation approaches Ni II Ni IV F 6 , with Ni II − F = 1.959(3) and Ni IV − F = 1.811(3) A, but 295 K data allow for only a slight increase in any Ni III . Relatives have been precipitated from liquid anhydrous HF, at ≤ 20 °C, by adding K 2 NiF 6 to M(SbF 6 ) 2 (M = Co, Cu, Zn) or M(AsF 6 ) 2 (M = Fe). CuNiF 6 like NiNiF 6 is metastable and loses F 2 easily, above 40 °C. CuNiF 6 is reduced by Xe or C 3 F 6 at −20 °C; CoNiF 6 by H 2 at 350 °C, each giving pseudo-rutile MNiF 4 . Magnetic data indicate the dominant formulation is M II Ni IV F 6 (Ni(IV) low spin d 6 ) with field dependence in CoNiF 6 (≤ 220 K) and FeNiF 6 (≤ 295 K).

Vortex dynamics and melting in niobium

Abstract Small angle neutron scattering has been used to investigate the vortex scattering in a single crystal of niobium. Below the irreversibility line resolution-limited Bragg peaks are observed, indicating that a crystalline vortex lattice with long range order exists. Above the irreversibility line intrinsic transverse widths develop, while close to Hc2 intrinsic radial widths also develop. Nevertheless the basic six-fold symmetry of scattering is observed throughout the vortex phase, indicating that a correlated flux fluid exists in the reversible regime.