The orientational order parameters of a homologous series of 4-cyanophenyl 4-alkylbenzoates have been determined at different temperatures from ¹³C-NMR, x-ray diffraction, optical birefringence, high resolution density and diamagnetic susceptibility anisotropy measurements. To determine the temperature dependence of the orientational order parameter from the ¹³C chemical shift values, the two-dimensional polarization inversion spin exchange at the magic angle (PISEMA) method was also used for the measurement of ¹³C– ¹H dipolar couplings at different sites in the oriented phase. The nematic order parameters determined from each of these methods have been compared. Apart from a slight shift in their values their trends with temperature are very similar. The differences among the results obtained by these five different methods have been discussed. From the high resolution density data, the values of the critical exponents near the T

As a generic model system for phase separation in polymer solutions, a coarse-grained model for hexadecane/carbon dioxide mixtures has been studied in two-dimensional geometry. Both the phase diagram in equilibrium (obtained from a finite size scaling analysis of Monte Carlo data) and the kinetics of state changes caused by pressure jumps (studied by large scale molecular dynamics simulations) are presented. The results are compared to previous work where the same model was studied in three-dimensional geometry and under confinement in slit geometry. For deep quenches the characteristic length scale ℓ( t) of the formed domains grows with time t according to a power law close to . Since in this problem both the polymer density ρ _p and the solvent density ρ _s matter, the time evolution of the density distribution P

We show that a metallic surface state is formed on Tl/Ge(111)-(1 × 1). The surface state forms electron pockets around of the surface Brillouin zone. A first-principles calculation reveals that the electron pockets are composed of a single branch of a spin-split surface-state band. The spin quantization axis is along the surface normal and inverts according to the time-reversal symmetry. Since this spin-split branch is the unique metallic band on this surface, the surface conductivity should be governed by this spin-split branch, suggesting a possible spin-polarized electric current.

X-ray fluorescence holography (XFH) is a method of atomic resolution holography which utilizes fluorescing atoms as a wave source or a monitor of the interference field within a crystal sample. It provides three-dimensional atomic images around a specified element and has a range of up to a few nm in real space. Because of this feature, XFH is expected to be used for medium-range local structural analysis, which cannot be performed by x-ray diffraction or x-ray absorption fine structure analysis. In this article, we explain the theory of XFH including solutions to the twin-image problem, an advanced measuring system, and data processing for the reconstruction of atomic images. Then, we briefly introduce our recent applications of this technique to the analysis of local lattice distortions in mixed crystals and nanometer-size clusters appearing in the low-temperature phase of a shape-memory alloy.

To develop a model system containing regularly spaced misfit dislocations for studies of the radiation resistance of nanoscale defects, epitaxial thin films of Cr, Mo, and Cr _x Mo _{1− x} alloys were deposited on MgO(001) by molecular beam epitaxy. Film compositions were chosen to vary the lattice mismatch with MgO. The film structure was investigated by x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and scanning transmission electron microscopy (STEM). Epitaxial films with reasonably high crystalline quality and abrupt interfaces were achieved at a relatively low deposition temperature, as confirmed by STEM. However, it was found by XRD and RBS in the channeling geometry that increasing the Mo content of the CrMo alloy films degraded the crystalline quality, despite the improved lattice match with MgO. XRD rocking curve data indicated that regions of different crystalline order may be present within the films with higher Mo con...

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2–Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2–Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.

The structure and electronic properties of different terminations of the α-Fe ₂O ₃(0001) surface were studied with spin-polarized density functional theory (DFT) and the DFT +  U method accounting for the effect of strong on-site Coulomb correlations. The results for lattice relaxation, electronic and magnetic properties are presented and discussed. Though the DFT and DFT +  U methods provide qualitatively very similar surface geometries they differ very much in the prediction of the electronic and magnetic properties, and the surface energetics of the clean α-Fe ₂O ₃(0001). The most stable single iron terminated (0001) surface and the oxygen-rich termination were chosen to study Au and Pd atom adsorption. The results show that both Au and Pd bind strongly to hematite surfaces and induce large changes in their geometry. The DFT +  U bonding is weaker by 0.3–0.6 eV than DFT on the iron terminated surface and about 2 eV stro...

The microscopic mechanism for H and He trapping by vacancy defects and bubble formation in a Be host lattice is investigated using first-principles calculations. A single He atom prefers to occupy a vacancy centre while H does not. He can segregate towards the vacancy from the interstitial site much more easily than H. Both H and He exhibit lower diffusion barriers from a remote interstitial to a vacancy with regard to their diffusion barriers inside a perfect Be solid. Up to five H or 12 He atoms can be accommodated into the monovacancy space, and the Be–He interaction is much weaker than Be–H. The physical origin for aggregation of multiple H or He atoms in a vacancy is further discussed. The strong tendency of H and He trapping at vacancies provides an explanation for why H and He bubbles were experimentally observed at vacancy defects in materials. We therefore argue that vacancies provide a primary nucleation site for bubbles of H and He gases inside Be materials.

Higher manganese silicide nanowires have been grown on the Si(001)-2 × 1 surface by the pre-growth of Bi nanolines. Scanning tunnelling microscope (STM) observations show that the nanowire has a linear surface reconstruction with a periodicity of 0.56 nm, and we propose a reconstruction on their surface to reduce the density of dangling bonds, which forms linear structures matching the dimensions from STM. Scanning tunnelling spectroscopy (STS) data agree with previous calculation results and reveal that the nanowires are degenerate semiconductors, with potential application for spintronics.

We study the Pb growth on both √3 × √3-In and 4 × 1-In reconstructed Si(111) surfaces at room and low temperature (160 K). The study takes place with complementary techniques, to investigate the role of the substrate reconstruction and temperature in determining the growth mode of Pb. Specifically, we focus on the correlation between the growth morphology and the electronic structure of the Pb films. The information is obtained by using Auger electron spectroscopy, low energy electron diffraction, soft x-ray photoelectron spectroscopy, scanning tunneling microscopy and spot profile analysis–low energy electron diffraction. The results show that, at low temperature and coverage ≤12 ML on the Si(111)√3 × √3–In surface, Pb does not alter the initial semiconducting character of the substrate and three-dimensional Pb islands with poor crystallinity are grown on a wetting layer. On the other hand, for the same coverage range, Pb growth on the Si(111)4 × 1–In surface results in metallic...