16 research outputs found
A theoretical study of the structural phases of Group 5B - 6B metals and their transport properties
In order to predict the stable and metastable phases of the bcc metals in the
block of the Periodic Table defined by groups 5B to 6B and periods 4 to 6, as
well as the structure dependence of their transport properties, we have
performed full potential computations of the total energies per unit cell as a
function of the c/a ratio at constant experimental volume. In all cases, a
metastable body centered tetragonal (bct) phase was predicted from the
calculations. The total energy differences between the calculated stable and
metastable phases ranged from 0.09 eV/cell (vanadium) to 0.39 eV/cell
(tungsten). The trends in resistivity as a function of structure and atomic
number are discussed in terms of a model of electron transport in metals.
Theoretical calculations of the electrical resistivity and other transport
properties show that bct phases derived from group 5B elements are more
conductive than the corresponding bcc phases, while bct phases formed from
group 6B elements are less conductive than the corresponding bcc phases.
Special attention is paid to the phases of tantalum where we show that the
frequently observed beta phase is not a simple tetragonal distortion of bcc
tantalum
Magneto-optical properties of (Ga,Mn)As: an ab--initio determination
The magneto-optical properties of (Ga,Mn)As have been determined within
density functional theory using the highly precise full-potential linear
augmented plane wave (FLAPW) method. A detailed investigation of the electronic
and magnetic properties in connection to the magneto-optic effects is reported.
The spectral features of the optical tensor in the 0-10 eV energy range are
analyzed in terms of the band structure and density of states and the essential
role of the dipole matrix elements is highlighted by means of Brillouin zone
dissection. Using an explicit representation of the Kerr angle in terms of real
and imaginary parts of the tensor components, a careful analysis of the Kerr
spectra is also presented. The results of our study can be summarized as
follows: i) different types of interband transitions do contribute in shaping
the conductivity tensor; ii) the dipole matrix elements are important in
obtaining the correct optical spectra; iii) different regions in the
irreducible Brillouin zone contribute to the conductivity very differently; iv)
a minimum in the Re spectra can give rise to a large Kerr
rotation angle in the same energy region; and v) materials engineering via the
magneto-optical Kerr effect is possible provided that the electronic structure
of the material can be tuned in such a way as to \emph{enhance} the depth of
the minima of Re .Comment: 33 pages, 7 figures, accepted for publication in Phys. Rev.
A review of the optical properties of alloys and intermetallics for plasmonics
Alternative materials are required to enhance the efficacy of plasmonic
devices. We discuss the optical properties of a number of alloys, doped metals,
intermetallics, silicides, metallic glasses and high pressure materials. We
conclude that due to the probability of low frequency interband transitions,
materials with partially occupied d-states perform poorly as plasmonic
materials, ruling out many alloys, intermetallics and silicides as viable. The
increased probability of electron-electron and electron-phonon scattering rules
out many doped and glassy metals.Comment: 26 pages, 10 figures, 3 table
Preliminary mapping of void fractions and sound speeds in gassy marine sediments from subbottom profiles
Bubbles of gas (usually methane) in marine sediments affect the load-bearing properties of the seabed and act as a natural reservoir of “greenhouse” gas. This paper describes a simple method which can be applied to historical and future subbottom profiles to infer bubble void fractions and map the vertical and horizontal distributions of gassy sediments, and the associated sound speed perturbations, even with single-frequency insonification. It operates by identifying horizontal features in the geology and interpreting any perceived change of depth in these as a bubble-mediated change in sound speed