No abstract

No abstract

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 crystalline order. The resulting state of matter has been a long-standing 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 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 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 transitions 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 the conductivity of electronic crystals.

No abstract

No abstract

The skyrmion lattice state (SkL), a crystal built of mesoscopic spin vortices, gains its stability via thermal fluctuations in all bulk skyrmion host materials known to date. Therefore, its existence is limited to a narrow temperature region below the paramagnetic state. This stability range can drastically increase in systems with restricted geometries, such as thin films, interfaces and nanowires. Thermal quenching can also promote the SkL as a metastable state over extended temperature ranges. Here, we demonstrate more generally that a proper choice of material parameters alone guarantees the thermodynamic stability of the SkL over the full temperature range below the paramagnetic state down to zero kelvin. We found that GaV4Se8, a polar magnet with easy-plane anisotropy, hosts a robust Néel-type SkL even in its ground state. Our supporting theory confirms that polar magnets with weak uniaxial anisotropy are ideal candidates to realize SkLs with wide stability ranges.

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. [S0031-9007(98)07842-9] PACS numbers: 68.15. + e, 61.12.Ha, The process by which an initially uniform film of a liquid on a nonwetting substrate breaks up into droplets is known as dewetting. Because of the importance of thin films and coatings in technology much experimental and theoretical work has been devoted to the study of the dynamics of dewetting. Much of this work has involved polymer films [1][2][3][4][5][6][7], both because of their technological importance and because their high viscosity means that experiments can be carried out on convenient time scales.The usual scenario for dewetting, whether for liquids on solids or liquids on liquids, involves the nucleation of holes. While this scenario for dewetting has been extensively studied, there is another possibility-that the initial stages of dewetting consist of the unstable growth of a capillary wave driven by dispersion forces across the film. This process, known as spinodal dewetting, has been analyzed theoretically and identified in physical systems such as metal films and polymer films on the basis of the characteristics of the final dewetted structure [1,8]. While there has been a suggestion, following analysis of the spatial correlations in dewetted structures of polymer thin films, that in some cases morphologies ascribed to spinodal dewetting in fact result from nucleation and growth of holes [9], very recent atomic force microscopy (AFM) measurements provide good evidence that, at least for ultrathin, very low molecular weight polystyrene on silicon, spinodal dewetting is indeed the mechanism in this case of liquid/solid dewetting [7].The dewetting of a polymer melt substrate by a polymer thin film is an example of liquid/liquid dewetting; this is a more complicated situation than the more commonly studied liquid/solid case, because both the polymer/polymer interface and the free film surface are deformable. Liquid/ liquid dewetting also presents an experimental challenge, since the interface of interest is buried, in contrast to the dewetting of a solid by a liquid film, where it is the liquid surface that is unstable.In this Letter, we characterize the earliest stages of dewetting of polystyrene (PS) by a thin poly(methyl methacrylate) (PMMA) film, and we show clear evidence that an instability of the buried, polymer/polymer, interface is a precursor to the dewetting process. The instability was monitored by the diffraction, at grazing incidence, of thermal neutrons from the growing capillary waves at the polymer/polymer interface. The characteristic wavelength of the fastest unstable mode and the way the diffu...

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.