National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

Effect of disorder on Raman scattering of single-layer Mo S 2

Abstract: We determine the effect of defects induced by ion bombardment on the Raman spectrum of single-layer molybdenum disulfide. The evolution of both the linewidths and frequency shifts of the first-order Raman bands with the density of defects is explained with a phonon confinement model, using density functional theory to calculate the phonon dispersion curves. We identify several defect-induced Raman scattering peaks arising from zone-edge phonon modes. Among these, the most prominent is the $\mathrm{LA}(M)$ peak at $\ensuremath{\sim}227\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and its intensity, relative to the one of first-order Raman bands, is found to be proportional to the density of defects. These results provide a practical route to quantify defects in single-layer $\mathrm{Mo}{\mathrm{S}}_{2}$ using Raman spectroscopy and highlight an analogy between the $\mathrm{LA}(M)$ peak in $\mathrm{Mo}{\mathrm{S}}_{2}$ and the $D$ peak in graphene.

The field and radiation impedance of mechanical radiators on the free surface of a semi-infinite isotropic solid

Abstract: The problem of wave-motion in a semi-infinite solid has been studied by several workers in connexion with seismological disturbances. In a classic paper (1904), Lamb investigated the propagation of vibrations over the surface of a semi-infinite isotropic solid, due to the application of force at a point or along a line in the free surface. A related problem is studied by Lapwood (I 949) who considers the case of an infinitely long dilating line-source situated parallel to and just below the free surface. Both authors obtain solutions in the form of definite integrals which they evaluate asymptotically to obtain the various components of the field at points on or near the surface at large distances from the source. In a more recent paper by Margery Newlands (I 952) this work has been extended to include the case of a source situated in a thin surface layer and propagating into a semi-infinite solid. The static problem of the distortion of a semi-infinite solid by stresses on the free surface was investigated by Boussinesq in a series of papers (1878-83) and by Love (1929). In the present paper Lamb's method has been adapted to furnish definite-integral representations of the field at an arbitrary point in a semi-infinite isotropic solid, due to prescribed periodic stresses on the free surface, the particular stress distributions considered here being such as would be produced by certain types of electro-mechanical transducer. Asymptotic formulae are obtained for the field at infinity, and the results for the various displacement components are presented in the form of polar diagrams. To obtain the radiation impedance integral expressions are derived for the mean displacement over the part of the surface which lies beneath the source; these expressions have been evaluated by numerical quadrature in a number of cases and a table shows the corresponding values of the radiation impedance.

Molecular orbital theory of nuclear spin coupling constants

Abstract: Molecular orbital theory in the LCAO form is used in a study of the indirect coupling between nuclear spins through the electronic environment. By retaining only the largest one-centre integrals, approximate formulae involving the LCAO coefficients are derived for the three contributions due to (1) the electron orbital effect, (2) the dipolar interaction between nuclear and electron spins and (3) the Fermi contact effect. The theory is then applied in detail to the coupling between directly bonded atoms, when the contact term is usually dominant. Approximate calculations indicate that the reduced coupling constant K AB (defined as (2πtħγAγB)J AB, where γ A and γB are the nuclear magnetogyric ratios and J AB is the usual constant in cycles/second) is negative if one of the atoms is fluorine. Broad agreement is obtained with the available experimental data for atoms up to fluorine connected by single bonds and a tentative pattern for signs and magnitudes is suggested.