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The interface between ultrathin deuterated poly(methyl methacrylate) films on polystyrene substrates is subject to a dispersive force driven instability consisting of thermally excited capillary waves. We have studied this example of liquid/liquid spinodal dewetting, using specular and off-specular neutron reflection measurements to characterize the length scale and growth rate of the instability of the buried, polymer interfaces.

The phenomenon of superconductivity continues to be of considerable scientific and practical interest. Underlying this phenomenon is the formation of electron pairs, which in conventional superconductors do not rotate about their centre of mass (‘s -wave’ pairing; refs 1, 2). This contrasts with the situation in high-temperature superconductors, where the electrons in a pair are believed to have two units of relative angular momentum (‘d -wave’ pairing; ref. 3 and references therein). Here we report small-angle neutron-scattering measurements of magnetic flux lines in the perovskite superconductor Sr2RuO4 (ref. 4), which is a candidate for another unconventional paired electron state—‘p -wave’ pairing, which has one unit of angular momentum5,6,7. We find that the magnetic flux lines form a square lattice over a wide range of fields and temperatures, which is the result predicted by a recent theory8,9 of p -wave superconductivity in Sr2RuO4. This theory also indicates that only a fraction of the electrons are strongly paired and that the orientation of the square flux lattice relative to the crystal lattice will determine which parts of the three-sheet Fermi surface of this material are responsible for superconductivity. Our results suggest that superconductivity resides mainly on the ‘γ’ sheet9.

We report the results of a preliminary investigation by specular neutron reflectivity of adsorption and wetting of alkane + perfluoroalkane binary liquid mixtures in the vicinity of the liquid/liquid critical point against a chemically modified (alkane-like) silicon wall. The results suggest the existence of a surprisingly long-ranged profile , even some degrees Kelvin above the upper critical point, and enable us to detect a wetting layer and its liquid/liquid interface below the critical point. The technique is capable of determining the composition, uniformity, and thickness of the wetting layer.

A neutron small-angle scattering study of the flux-line lattice in heavily twinned YBa2Cu3O72x is presented. It is found that the diffraction signal disappears at temperatures well below Tc , associated with a melting of the flux lattice. The shape of the melting line is consistent with both a Lindemann criterion and the scaling expected for a vortex-glass transition with the superconducting parameters from the three-dimensional XY model. The influence of twin planes on the structure of the vortex lattice and its melting is studied by applying the field at different angles to the c axis. The results are compared with recent specific heat measurements on similar crystals. @S0163-1829~98!03022-7#

Neutron reflectivity measurements were made on electroactive films of [Os(bipy) 2 (PVP) 10 Cl] + spin coated onto Au electrodes. Solvent content and film swelling were monitored as functions of the solution pH and counterion (perchlorate vs. p-toluenesulfonate). Film swelling was much greater near the polymer/solution interface than within the interior of the film. Furthermore, this spatial variation in film swelling was more pronounced in p-toluenesulfonate media than in perchlorate media. Structural information is presented here in the form of spatial distribution of polymer and solvent within the film; these profiles could not have been obtained directly from purely electrochemical measurements.

High resolution, neutron small angle scattering studies have been performed to investigate the low field $(Bl250\mathrm{mT})$ region of the phase diagram for the vortex lattice in the superconducting state of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$. The data present clear evidence for a vortex lattice reorientation transition from a state with the diagonal of the rhombic unit cell along a $[110]$ direction to a $[100]$ direction. Above this transition the lattice distorts under the influence of the applied field until the apex angle becomes constant at some higher field. For $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$ a square lattice configuration is formed. These experiments confirm qualitatively many of the predictions of a general model based on nonlocal corrections to the London model as applied to this and similar materials.

In situ the swelling of thin polystyrene (PSd) films by toluene is investigated with specular and off-specular neutron reflectivity. Due to the very high time resolution of 30 s, the initial stages of swelling as well as the fast swelling are detected. Followed by a slow swelling and final stages, in total four different regimes are identified until equilibrium with respect to a homogenously swollen PSd film is reached. Film instabilities are avoided in the absence of any motor movements in the time-of-flight mode of the neutron reflectometer D17.

A detailed study has been made of a transition in the flux lattice of the superconductor ${\mathrm{Bi}}_{2.15}$${\mathrm{Sr}}_{1.85}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ using the technique of muon-spin rotation. The results are in excellent agreement with a recent theoretical analysis on the low-field phase diagram of highly anisotropic superconductors. In particular, the important role of electromagnetic interactions between vortex segments is demonstrated. The form of the phase boundary suggests that the transition may be identified with a simultaneous melting and decoupling of the vortices in adjacent layers.

A systematic muon-spin-rotation ({mu}{sup +}SR) study is presented of the temperature dependence of the London penetration depth in sintered powder samples of the YBa{sub 2}Cu{sub 3}O{sub {ital x}} system and related compounds. The in-plane penetration depth {lambda}{sub {ital a}{ital b}} is estimated from the {mu}{sup +}SR depolarization rate of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}, YBa{sub 2}Cu{sub 4}O{sub 8}, and a series of samples of the YBa{sub 2}Cu{sub 3}O{sub {ital x}} family, respectively. It is found that not only the low-temperature value {lambda}{sub {ital a}{ital b}}(0), but also the temperature behavior {lambda}{sub {ital a}{ital b}}({ital T}) is specific to each compound. The form of {lambda}{sub {ital a}{ital b}}({ital T}) can be well characterized by a simple power law. In particular, the YBa{sub 2}Cu{sub 3}O{sub {ital x}} family shows a systematic variation of the form of {lambda}{sub {ital a}{ital b}}({ital T}) with the oxygen content {ital x} which points to a varying coupling strength, whereas {lambda}{sub {ital a}{ital b}}(0) as a function of {ital x} suggests a positive charge transfer into the CuO{sub 2} planes with increasing oxygen doping. Furthermore, our data is consistent with an empirical ansatz which has been proposed in the framework of a Bose-gas picture of high-temperature superconductivity.more » As a consequence, the pressure and the isotope coefficients can be extracted from the {mu}{sup +}SR depolarization rate and compared to direct measurements of these quantities, showing good agreement. Moreover, in the Bose-gas picture the variation of {lambda}{sub {ital a}{ital b}}({ital T}) in the YBa{sub 2}Cu{sub 3}O{sub {ital x}} family may be interpreted as a crossover from a dense (high-{ital T}{sub {ital c}}) to a dilute (low-{ital T}{sub {ital c}}) system of weakly interacting local pairs.« less

The switching kinetics of thin thermo-responsive hydrogel films of poly(monomethoxy-diethyleneglycol-acrylate) (PMDEGA) are investigated. Homogeneous and smooth PMDEGA films with a thickness of 35.9 nm are prepared on silicon substrates by spin coating. As probed with white light interferometry, PMDEGA films with a thickness of 35.9 nm exhibit a phase transition temperature of the lower critical solution temperature (LCST) type of 40 degrees C. In situ neutron reflectivity is performed to investigate the thermo-responsive behavior of these PMDEGA hydrogel films in response to a sudden thermal stimulus in deuterated water vapor atmosphere. The collapse transition proceeds in a complex way which can be seen as three steps. The first step is the shrinkage of the initially swollen film by a release of water. In the second step the thickness remains constant with water molecules embedded in the film. In the third step, perhaps due to a conformational rearrangement of the collapsed PMDEGA chains, water is reabsorbed from the vapor atmosphere, thereby giving rise to a relaxation process. Both the shrinkage and relaxation processes can be described by a simple model of hydrogel deswelling.

Neutron reflectivity was used to study the interface between the semiconducting polymer poly(9,9-dioctylfluorene) (PFO) and the insulating polymer poly(methyl methacrylate) (PMMA). The PFO/PMMA interfacial width was measured in the nematic and crystalline phases of the PFO, both with the PMMA on top of the PFO and vice versa. These interfaces are broad compared to atomic length scales, with measured interfacial widths in the range from 10 to 20 A. We found that the interfacial width was independent of both the chosen geometry and the thermal processing history. The equilibrium interfacial width only depended on temperature, with the width in the nematic phase of the PFO being broader than in the crystalline regime.

The lateral structures of dried adsorbed binary mixtures of star polymers were investigated. Blends of protonated and deuterated polybutadiene stars were prepared from cyclohexane solutions and adsorbed onto silicon substrates. The number of arms and the molecular weight of the arms was varied. With grazing incidence small angle scattering techniques (GISAS) and scanning force microscopy (SFM), different dominant in-plane length scales were determined. The morphology of these structures is dominated by blob-like structures created from single stars or agglomerates of star polymers.

Phase separation of the weakly incompatible blend system polystyrene and polyparamethylstyrene confined in isolated droplets is investigated. The droplet geometry imposes a two-dimensional spatial restriction. With specular, diffuse and grazing incidence small-angle neutron scattering the surface topography as well as the chemical morphology inside the droplets is determined. Due to the differences in surface tension, a core–shell-like structure characterized by one most prominent length scale inside the droplets is installed.

The reflectometer D17 has been operational for a year and has already proved to be an excellent tool for investigating surfaces and interfaces in the realms of physics, biology and chemistry. The instrument has two modes of operation, time-of-flight and monochromatic, the latter incorporating the polarised-neutron option. Both modes are flexible in the wave-vector-transfer (q) resolution. The loosest resolution required to resolve the sample structure can be chosen (and hence the highest flux), enabling the lowest reflectivities and hence the widest q-range to be measured.

Fluid Interfaces Grazing Angles ReflectOmeter (FIGARO) is the new horizontal neutron reflectometer at the Institut Laue-Langevin, Grenoble, France. It is a versatile, high-flux time-of-flight instrument with features suitable for a range of studies in soft condensed matter, chemistry, physics and biology both at free air/liquid interfaces and buried liquid/liquid and solid/liquid interfaces. Most of the experiments exploit isotopic contrast variation to determine the structure and composition of surface layers. Multiple chopper discs allow variable wavelength resolution, with the loose-resolution options increasing the available neutron flux for fast kinetic studies of thin films and improving the data acquisition efficiency from complex samples requiring multiple contrast measurements. It is possible to approach the interface with neutrons from below or above the horizon. The instrument is equipped with a range of sample environments including free liquid adsorption troughs, a Langmuir trough, an overflowing cylinder and a range of solid/liquid sample cells. FIGARO was launched as a user instrument in April 2009 and its success has been steadily increasing since. This article includes an introduction to the scientific case, detailed sections on the instrument components and performance, and descriptions of standard sample environments developed to date as well as some selected early scientific highlights.

The vortex lattice (VL) in the high-κ superconductor YBa2Cu3O7, at 2 K and with the magnetic field parallel to the crystal c axis, undergoes a sequence of transitions between different structures as a function of applied magnetic field. However, from structural studies alone, it is not possible to determine precisely the system anisotropy that governs the transitions between different structures. To address this question, here we report new small-angle neutron scattering measurements of both the VL structure at higher temperatures and the field and temperature dependence of the VL form factor. Our measurements demonstrate how the influence of anisotropy on the VL, which in theory can be parameterized as nonlocal corrections, becomes progressively important with increasing magnetic field, and suppressed by increasing the temperature toward Tc. The data indicate that nonlocality due to different anisotropies plays an important role in determining the VL properties.

By neutron scattering, we have observed a well-ordered magnetic vortex lattice (VL) in KFe${}_{2}$As${}_{2}$ single crystals. With the field along the $c$ axis, a nearly isotropic hexagonal VL is formed, with no symmetry transitions up to high fields, indicating a small anisotropy of the superconducting state around this axis. This rules out line nodes parallel to the $c$ axis, and thus $d$-wave or anisotropic $s$-wave pairing. However, the strong temperature dependence of the signal at $T\ensuremath{\ll}{T}_{c}$ shows that extremely small gap values exist; these may arise from nodal lines perpendicular to the $c$ axis.

The microscopic magnetic field distribution in the mixed state of single-crystal ${\mathrm{Bi}}_{2.15}$${\mathrm{Sr}}_{1.85}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ has been measured using the muon spin rotation technique. A study of the line shapes as a function of temperature and magnetic field gives evidence for an abrupt change of flux-lattice structure which is attributed to the presence of a flux-lattice melting line in the B-T phase diagram. In addition, the data suggest the existence of a crossover field above which the flux structure is more 2D in nature.

We report on small angle neutron scattering observations of the flux line lattice (FLL) in a single crystal of YBa[sub 2]Cu[sub 3]O[sub 7]. To probe the mass anisotropy ratio, [ital m][sub 3]/[ital m][sub 1], measurements were made as a function of angle, [Theta], between the 8 kOe applied field and the crystallographic (001) axis for 0[degree][le][Theta][le]80[degree]. With the rotation about an [ital a]/[ital b] (or [ital y]) axis, two symmetry-related distorted hexagonal FLL domains formed. Contrary to theoretical prediction, the lattices formed are consistent with a rotation of the short basis vector, [bold a][sub 1], from the [ital x] axis by 15[degree], after the effects of anisotropy are removed. The mass ratio is 20[plus minus]2, which is slightly lower than published values. The temperature dependence of the intensity is not conventional.

Neutron diffraction has been used to measure the vortex lattice scattering for YBa2Cu3O7. A square pattern is found when the field is applied along the c axis, while a triangular pattern is found when the field is applied well away from the c axis. High‐resolution measurements for the square pattern show that the vortex lattice has long‐range orientational order but only short‐range positional order. The temperature dependence of the penetration depth is not that expected for a superconductor with a conventional s‐wave BCS‐type gap. Preliminary measurements for temperatures near the irreversibility line are consistent with the occurrence of melting of the vortex lattice or glass phase.

A small-angle neutron study of vortex lattices in YBa 2 Cu 3 O 7 showed a diffraction pattern with square symmetry when the applied field was along the c axis with the four main spots aligned with the crystallographic {110} directions. With the field 30 o from the c axis, a hexagonal lattice exists. At intermediate angles, a fraction of the flux lines follow the c axis and the two lattices coexist. The temperature dependence of the intensity deviates markedly from conventional predictions

The state of the vortex lattice extremely close to the superconducting to normal transition in an applied magnetic field is investigated in high-purity niobium. We observe that thermal fluctuations of the order parameter broaden the superconducting to normal transition into a crossover but no sign of a first-order vortex lattice melting transition is detected in measurements of the heat capacity or the small-angle neutron scattering (SANS) intensity. Direct observation of the vortices via SANS always finds a well-ordered vortex lattice. The fluctuation broadening is considered in terms of the lowest Landau-level theory of critical fluctuations and scaling is found to occur over a large ${H}_{c2}(T)$ range.

The non-centrosymmetric heavy fermion compound CePt3Si orders antiferromag-netically at TN = 2.2K, followed by a superconducting transition, where Ts = 0.45 K for high quality samples and Ts = 0.75K for lower quality samples. We have used neutron Larmor diffraction to measure the temperature dependence and distribution of the lattice constants in a single crystal of CePt3Si with Ts = 0.75K. In our study we observe an unusually wide range of lattice parameters for the a-axis and c-axis with Δa/a ≈ Δc/c ≈ 10−3. Small angle neutron scattering suggests an abundance of defects along the lattice planes. When taking into account the pressure dependence of Ts our study attributes the increased value of Ts in low quality samples to an effective negative pressure.

High-purity niobium exhibits a surprisingly rich assortment of vortex lattice (VL) structures for fields applied parallel to a fourfold symmetry axis, with all observed VL phases made up of degenerate domains that spontaneously break some crystal symmetry. Yet a single regular hexagonal VL domain is observed at all temperatures and fields parallel to a threefold symmetry axis. We report a detailed investigation of the transition between these lush and barren VL landscapes, discovering new VL structures and phase transitions at high fields. We show that the number and relative population of VL domains is intrinsically tied to the underlying crystal symmetry. We discuss how subtle anisotropies of the crystal may generate the remarkable VLs observed.

We describe the use of reverse Monte Carlo refinement to extract structural information from angle-resolved data of a Bragg peak. Starting with small-angle neutron scattering data, the positional order of an ensemble of flux lines in superconducting Nb is revealed. We discuss the uncovered correlation functions in the light of topical theories, in particular, the "Bragg glass" paradigm.

The flux line lattice in CeRu 2 has been studied by small-angle neutron scattering. The scattering potential is found to be well described in terms of a linear superposition of single flux-line profiles, while an increase in the mosaic spread of the diffraction peaks at low fields is consistent with the theory of weak collective pinning. The pinning parameter deduced from the data gives the correct onset field of a peak in the critical current observed at higher field.

A combination of XPS, reflectance infrared spectroscopy, and neutron reflectivity measurements has been used to probe the spatial and global extents to which carboxylic acid motifs in electrodeposited conducting polymer films can be functionalized by reaction with vapor phase reagents (a carbodiimide together with trifluoroethanol) with the goal of controlling hydrophobicity. Across a range of polymer deposition and reaction temperatures, neutron reflectivity showed that, surprisingly, functionalization of the polymer matrix at depths >5-10 nm into the polymer film was always significantly lower than at the exposed surface. The most efficient functionalization was found to occur when a low-density polymer matrix was prepared by elution of motifs cleaved from the polymer by base hydrolysis. Finally, when trifluoroethanol functionalization was performed, the macroscopic property of hydrophobicity was related to the surface, internal microstructure, and composition of the reacted films as elucidated by the above combination of probes.

Small-angle neutron scattering has been applied to study the vortex lattice in the intercalated graphite superconductor CaC6 (T-c=11.3 K). Scattering from the vortex lattice is in the form of a ring, most likely reflecting the absence of in-plane orientational order of the pyrolitic graphene planes. The temperature and field dependence of the scattered intensity allows the in-plane zero temperature value of the coherence length [xi=350(25) angstrom] and the London penetration depth [lambda=500(30) angstrom] to be estimated. Measurements with the applied field at 70 degrees to the c axis directly reveal the penetration depth anisotropy, gamma(lambda)=5.1(4), which, unlike MgB2, is equal to the anisotropy of the coherence length deduced from magnetization measurements. The orientation of the vortex lattice is fixed relative to the rotation axis of the crystal as predicted by anisotropic London theory.

We describe in situ neutron reflectivity (NR) and RAIRS studies of the chemical modification of films of a polypyrrole-based conducting polymer derived from the pentafluorophenyl ester of poly(pyrrole-N-propanoic acid) (PFP) electrodeposited on electrode surfaces. We explore the role of the solvent in controlling the rate of reaction with solution-based nucleophiles (amines, which react with the ester to form amides). By varying the identity of the solvent (water vs acetonitrile) and the neutron contrast (deuteration), we find that both the identity of the solvent and its population within the film are paramount in determining chemical reactivity and electroactivity. IR signatures allow monitoring of the reaction of solution-based amine-tagged species such as amino-terminated poly(propylene glycol), ferrocene ethylamine, and lysine with film-based ester functionalities: the carbonyl bands show ester/amide interconversion and some hydrolysis to acid. Time-dependent spectral analysis shows marked variations in reaction rate with (i) (co-)polymer composition (replacement of some fluorinated ester-functionalized pyrrole with unfunctionalized pyrrole), (ii) the solvent to which the polymer film is exposed, and (iii) the rate of polymer deposition. NR data provide solvent profiles as a function of distance perpendicular to the interface, the variations of which provide an explanation for film reactivity patterns. Homopolymer films are relatively hydrophobic, thus hindering reaction with species present in water solutions. Incorporating pyrrole groups raises the solvent population-dramatically for water-thereby facilitating entry and reaction of aqueous-based lysine. Changing film deposition rate yields films with different absolute levels of solvent and reactivity patterns that are dependent on the size of the reactant molecules: more rapid deposition of polymer gives films with a more open structure leading to a higher solvent content and thence increased reactivity. These results, supported by XPS and AFM data, allow assembly of composition-structure-reactivity correlations, in which the controlling feature is film solvation.

We report an extensive investigation of magnetic vortex lattice (VL) structures in single crystals of pure niobium with the magnetic field applied parallel to a fourfold symmetry axis, so as to induce frustration between the cubic crystal symmetry and hexagonal VL coordination expected in an isotropic situation. We observe new VL structures and phase transitions; all the VL phases observed (including those with an exactly square unit cell) spontaneously break some crystal symmetry. One phase even has the lowest possible symmetry of a two-dimensional Bravais lattice. This is quite unlike the situation in high-${T}_{c}$ or borocarbide superconductors, where VL structures orient along particular directions of high crystal symmetry. The causes of this behavior are discussed.

Using small-angle neutron scattering, we have measured the misalignment between an applied field of $4\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ and the flux-line lattice in $\mathrm{Mg}{\mathrm{B}}_{2}$, as the field is rotated away from the $c$ axis by an angle $\ensuremath{\theta}$. The measurements, performed at $4.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, showed the vortices canting towards the $c$ axis for all field orientations. Using a two-band/two-gap model to calculate the magnetization, we are able to fit our results yielding a penetration depth anisotropy, ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}=1.1\ifmmode\pm\else\textpm\fi{}0.1$.

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical and an experimental point of view. We have used a combination of small angle neutron scattering (SANS) and muon-spin rotation to probe the order of the vortex system on a microscopic scale and have succeeded, for the first time, to measure a well-ordered vortex lattice (VL) structure at all doping regimes of LSCO. In the optimally to overdoped regime a field-induced transition from hexagonal to square coordination is reported. The possible connections of our neutron results to photoemission data, as well as the implications for various competing theoretical models will be discussed. In the underdoped regime we observe, as a function of applied magnetic field, a transition from an ordered vortex state to a vortex glass phase that results from the presence of random pinning. Finally, recent measurements of the VL on electron doped high-temperature superconductors are presented.

In ${\mathrm{V}}_{3}\mathrm{Si}$, a first-order structural phase transition from hexagonal to square flux-line lattice occurs at approximately 1 T with $H\ensuremath{\Vert}$ to the $a$ axis. In this paper, we demonstrate the reentrant structural transition in the flux-line lattice, which reverts to hexagonal symmetry as the magnetic field approached ${H}_{c2}(T)$. This behavior is described very well by a nonlocal London theory with thermal fluctuations. The phase diagram of the flux lattice topology is mapped out for this geometry.

The interfacial width of polyolefins blends has been probed as a function of distance away from the critical point by using neutron reflectivity. For strongly immiscible polymer pairs, the width of the interface increases slowly when the degree of immiscibility is decreased and the interfacial width varies with the interaction parameter $\ensuremath{\chi}$ of the polymers. Closer to the critical point the dependence on the degree of miscibility becomes stronger and the way in which the interfacial width diverges, as criticality is approached, is related to both the chain length and $\ensuremath{\chi}$. The self-consistent field theory numerical calculations, with the additional contribution due to capillary waves, provides a good description of the width of the interface between two polymer bulk phases in particular at intermediate values of the degree of immiscibility.

Small-angle neutron diffraction measurements from the flux lattice in a single crystal of the cubic Laves' phase superconductor, CeRu2, are reported. The mixed state is described in terms of aligned rigid bundles of vortices. The bundle diameters decrease above 1 2 H c 2 (consistent with collective weak pinning theory) and become comparable with the penetration length at a field at which a ‘peak effect’ is seen in magnetisation measurements. A clear memory of field histories that pass through the ‘peak effect’ region is also found; however, some of the induced disorder can be removed by subsequently cycling the field.

We report that the flux-line lattice in the cubic superconductor Pr(Os4Sb12 is strongly distorted from an ideal hexagonal lattice at very low temperatures in a small applied field. We attribute this to the presence of gap nodes in the superconducting state on at least some Fermi-surface sheets.

The equilibrium distance between two lipid bilayers stable in bulk water and in proximity of a substrate was investigated. Samples consisted of a homogeneous lipid bilayer, floating near an identical bilayer deposited on the hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl phosphocholines, with the number of carbon atoms per chain, n, varying from 16 to 20. The average and r.m.s. positions of the floating bilayer were determined by means of neutron specular reflectivity. Samples were prepared at room temperature (i.e. with the lipids in the gel phase) and measurements performed at various temperatures so that the whole region of transition from gel to fluid phase was explored. Data have been interpreted in terms of competition between the interbilayer potential and membrane fluctuations and used to estimate the bending rigidity of the bilayer.

We present neutron scattering from the flux line lattice (FLL) in MgB2. Between 0.5 and 0.9 T the FLL undergoes a 30 degrees reorientation, and simultaneously the scattered intensity falls sharply consistent with the weaker superconducting pi band being suppressed with increasing field. We speculate that the pi and sigma bands favor different FLL orientations, and that the reorientation is driven by the suppression of the pi band. When the c axis of the crystal is rotated 45 degrees to the applied field the penetration depth anisotropy could be measured, and rises both as a function of applied field and temperature.

Small-angle neutron studies have been performed on samples of continuous inhomogeneous magnetic recording media. This has allowed the local magnetic structure to be probed at a sub-nanometre resolution, revealing some interesting information about the size and shape of the magnetic grains and their relation to the physical grains.

We report here the first direct observations of a well ordered vortex lattice in the bulk of a La(2-x)Sr(x)CuO(4+delta) single crystal (slightly overdoped, x = 0.17). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been observed in high-temperature superconductors and is indicative of the coupling of the vortex lattice to a source of anisotropy, such as those provided by a d-wave order parameter or the presence of stripes.

This chapter provides an introduction to the principles and applications of neutron reflection. In the quantum mechanical approach, the neutron can be treated as a wave with a characteristic wavelength, λ , defined by the de Broglie relation, λ = h/m n v, where h is Planck's constant, and m n and v are the neutron mass and speed, respectively. The Schrodinger equation is analogous to the wave equation for light and leads to neutrons showing characteristic optical behavior such as total reflection and refraction. A specular neutron reflection experiment measures the reflectivity as a function of the wave vector perpendicular to the reflecting surface, q . The measurement can be done by varying either the glancing angle of incidence θ at a constant wavelength or measuring the time-of-flight, hence wavelength, at constant θ . In most nuclear reactors, measurements are usually made at a fixed value of λ using long-wavelength neutrons and a θ (reflectionangle) - 2θ (detector angle) scan.

We describe how the ferromagnetic spin echo technique may be used to make a direct and microscopic measurement of the motion of flux lines in a superconductor under the influence of an applied current. Our experimental results agree well with theory and indicate that both the value and distribution of velocities of flux lines may be measured.

Thermo-responsive polymers can exhibit a demixing transition which is of the lower critical solution temperature (LCST) type. The collapse transition of polymers with such LCST behavior is of great interest for applications where a strong change of volume is desired even for small changes of an external stimulus such as temperature. Examples of applications are valves in micro-fluidics, the release of drugs in the body or sensors. A polymer which is a promising candidate in this context is poly(N-isopropylacrylamide), PNIPAM. It exhibits a LCST of about 320C that is attributed to alterations in the hydrogen-bonding interactions of the amide group. A polymer with a higher transition temperature as compared to PNIPAM is the thermosensitive acrylate poly(methoxydiethylene glycol acrylate) or PMDEGA. In thin film geometry, such thermoresponsive polymers are of particular interest for use as thermosensitive surfaces, artificial pumps and muscles, light modulation systems and optical switches. The strong volume change translates into a change of the film thickness due to the thin film geometry. We investigate the kinetics of chain collapse of thin thermosensitive films as a function of quench depth. Homopolymer films and amphiphilic block copolymer films are compared. With time resolved neutron reflectivity (NR) we follow the kinetics of the transition from a swollen to a collapsed thermoresponsive film. Within 15 seconds a full NR curve is probed, which allows to determine the evolution of film thickness and of the water content in real time. Thus, we investigate in-operando the switching behavior caused by a thermal trigger. The observed complex three-step switching of the films is discussed.

We report on detailed small-angle neutron scattering measurements with polarization analysis from the flux-line lattice in the anisotropic superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}.$ When the field was applied at an angle to the principal axes we have observed spin-flip neutron diffraction consistent with local transverse field components. From a detailed study of the angle dependence of the magnitude of the spin-flip neutron-scattered intensity we have found that the transverse-field components are larger than predicted by an anisotropic London model for reasonable mass anisotropy values. The transverse-field components are consistent with a reduced c-axis coherence length at low temperature.

We report small-angle neutron-scattering (SANS) measurements of flux line properties near ${H}_{c2}$ in an ultrapure sample of niobium with weak pinning of flux in the bulk. These confirm in detail the Abrikosov picture of the flux line lattice to within 20 mK of the upper critical field line. However, it has recently been claimed [X. S. Ling et al., Phys. Rev. Lett. 86, 712 (2001)], on the basis of SANS observations of a disordering of flux lines in niobium, that the flux lattice melts at temperatures clearly separated from the upper critical field line. This discrepancy may possibly arise from differences in sample purity and pinning.

High resolution, neutron small angle scattering experiments have been performed on the flux-line lattice within the mixed state of the type-II superconductors YNi2B2C and TmNi2B2C. In the Y compound clear evidence is found for a transition at low fields from a distorted hexagonal lattice with its diagonal along a [110] direction to a [100] direction as the field is increased. At higher fields the lattice is found to be square. These results are in

Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of the crystalline order. The resulting state of matter has been a longstanding puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into crystallites. But a different theory predicts the existence of a novel phase of matter: the so-called Bragg glass, which is a glass and yet nearly as ordered as a perfect crystal. The lattice of vortices that can contain magnetic flux in type II superconductors provide a good system to investigate these ideas. Here we show that neutron diffraction data of the vortex lattice in type II superconductors provides unambiguous evidence for a weak, power-law decay of the crystalline order characteristic of a Bragg glass. The theory also predicts accurately the electrical transport properties of superconductors; it naturally explains the observed phase transition and the dramatic jumps in the critical current associated with the melting of the Bragg glass. Moreover the model explains experiments as diverse as X-ray scattering in disordered liquid crystals and conductivity of electronic crystals., 9 pages, 4 figures

In 1957, Abrikosov1 described how quanta of magnetic flux enter the interior of a bulk type II superconductor. It was subsequently predicted that, in an isotropic superconductor, the repulsive forces between the flux lines would cause them to order in two dimensions, forming a hexagonal lattice2. Flux-line lattices with different geometry can also be found in conventional (type II) superconductors3; however, the ideal hexagonal lattice structure should always occur when the magnetic field is applied along a hexagonal crystal direction4. Here we report measurements of the orientation of the flux-line lattice in the heavy-fermion superconductor5 UPt3, for this special case. As the temperature is increased, the hexagonal lattice, which is initially aligned along the crystal symmetry directions, realigns itself with the anisotropic superconducting gap. The superconductivity in UPt3 is unusual (even compared to unconventional oxide superconductors6) because the superconducting gap has a lower rotational symmetry than the crystal structure. This special feature enables our data to demonstrate clearly the link between the microscopic symmetry of the superconductivity and the mesoscopic physics of the flux-line lattice. Moreover, our observations provide a stringent test of the theoretical description of the unconventional superconductivity in UPt3.