729 research outputs found
Probing the Fermi surface by positron annihilation and Compton scattering
Positron annihilation and Compton scattering are important probes of the Fermi surface. Relying on conservation
of energy and momentum, being bulk sensitive and not limited by short electronic mean-free-paths, they can
provide unique information in circumstances when other methods fail. Using a variety of examples, their contribution
to knowledge about the electronic structure of a wide range of materials is demonstrated
Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}
Materials that exhibit colossal magnetoresistance (CMR) are currently the
focus of an intense research effort, driven by the technological applications
that their sensitivity lends them to. Using the angular correlation of photons
from electron-positron annihilation, we present a first glimpse of the Fermi
surface of a material that exhibits CMR, supported by ``virtual crystal''
electronic structure calculations. The Fermi surface is shown to be
sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure
Experimental determination of the state-dependent enhancement of the electron-positron momentum density in solids
The state-dependence of the enhancement of the electron-positron momentum
density is investigated for some transition and simple metals (Cr, V, Ag and
Al). Quantitative comparison with linearized muffin-tin orbital calculations of
the corresponding quantity in the first Brillouin zone is shown to yield a
measurement of the enhancement of the s, p and d states, independent of any
parameterizations in terms of the electron density local to the positron. An
empirical correction that can be applied to a first-principles state-dependent
model is proposed that reproduces the measured state-dependence very well,
yielding a general, predictive model for the enhancement of the momentum
distribution of positron annihilation measurements, including those of angular
correlation and coincidence Doppler broadening techniques
Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys
The first direct experimental evidence for the Fermi surface (FS) driving the
helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported.
The presence of a FS sheet capable of nesting is revealed, and the nesting
vector associated with the sheet is found to be in excellent agreement with the
periodicity of the helical ordering.Comment: 4 pages, 4 figure
Fermi surface of an important nano-sized metastable phase: AlLi
Nanoscale particles embedded in a metallic matrix are of considerable
interest as a route towards identifying and tailoring material properties. We
present a detailed investigation of the electronic structure, and in particular
the Fermi surface, of a nanoscale phase ( AlLi) that has so far been
inaccessible with conventional techniques, despite playing a key role in
determining the favorable material properties of the alloy (Al\nobreakdash-9
at. %\nobreakdash-Li). The ordered precipitates only form within the
stabilizing Al matrix and do not exist in the bulk; here, we take advantage of
the strong positron affinity of Li to directly probe the Fermi surface of
AlLi. Through comparison with band structure calculations, we demonstrate
that the positron uniquely probes these precipitates, and present a 'tuned'
Fermi surface for this elusive phase
The electronic structure of {\em R}NiC intermetallic compounds
First-principles calculations of the electronic structure of members of the
NiC series are presented, and their Fermi surfaces investigated for
nesting propensities which might be linked to the charge-density waves
exhibited by certain members of the series ( = Sm, Gd and Nd). Calculations
of the generalized susceptibility, , show strong
peaks at the same -vector in both the real and imaginary parts for
these compounds. Moreover, this peak occurs at a wavevector which is very close
to that experimentally observed in SmNiC. In contrast, for LaNiC (which
is a superconductor below 2.7K) as well as for ferromagnetic SmNiC, there
is no such sharp peak. This could explain the absence of a charge-density wave
transition in the former, and the destruction of the charge-density wave that
has been observed to accompany the onset of ferromagnetic order in the latter.Comment: 5 pages, 7 figures. Accepted for publication in Phys. Rev.
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