

There are a hundred billion trillion electrons in a typical
solid, and every one of them interacts with every other.
Luckily, understanding a solid's electronic structure
doesn't require precise quantumstate solutions for the
whole kit and kaboodle.
Instead of coping with 10tothe23rd entangled electrons
(not to mention oppositely charged atomic nuclei), solidstate
physicists base their calculations on quasielectrons,
entities with the same spin and charge as electrons but
with fictitious masses and finite lifetimes, properties
that allow realistic approximations of their local interactions
as they move through a crystal. Quasiparticle theory,
which dates from the 1950s, is unsurpassed at modeling
a host of materials.
Among other things, quasiparticle theory modifies the
straightforward energyversusmomentum relation that applies
to free particles by taking into account interactions
of electrons with other nearby electrons, with the ion
cores of atoms, and with phonons, which are quantized
vibrations of the crystal lattice. A key parameter is
the Fermi surface, giving the momentum values of all electrons
at the Fermi energy, which separates occupied electronic
levels from unoccupied ones.
"ARPES is a good way to look at these features with
extreme sensitivity and very high resolution," says
YiDe Chuang, "especially using the improved HERS
endstation and the extremely bright monochromatic beam
available at the Advanced Light Source's new undulator
beamline 10."
When a sample is struck by the beam's energetic photons
it emits photoelectrons, which are collected by HERS,
the High Resolution Energy Spectrometer. The Fermi surface,
energy versus momentum relations, and other features can
be reconstructed by measuring the precise angles and energies
of the photoelectrons; from ARPES data gathered by HERS,
the energies and lifetimes of the material's quasielectrons
can be derived.
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