THE ELECTRONIC ALADDIN NEWSLETTER NO. 8
September / October 1996
1) First Results from the Plane Grating Monochromator (PGM)
The first of the new generation of undulator beam lines at the SRC is
starting to move into action. Finally, the moment came for so many at SRC
who worked hard during the last few years. Undulator radiation was sent
through the new PGM beamline. To get a first impression about the spectral
distribution of the undulator radiation, the monochromator was scanned over
the full energy range of the beamline while recording the flux after the
exit slit with a gold diode. Even in the very first measurement of the
undulator spectrum, its high performance was evident from the abundance and
relative strength of the higher
order harmonics. Photoionization measurements were taken around the Ne
2s-to-np transitions to test the monochromator. For details see PGM News.
2) Magnetron Tube Fixed
The refurbished magnetron tube is up and running, producing again the
familiar 200 mA beams. SRC would like to thank the users for their patience
during the trying time of working with backups.
3) Neil Lane at the SRC
The head of the National Science Foundation, Neil Lane, visited the SRC on
Sept. 6 and gave an interview in front of the new PGM beam line that was
broadcast in the evening news on Channel 3. In his visit, he stressed the
importance of science education.
4) Workshop on MCD and Magnetic Nanostructures
The program for the Nov. 16 workshop at the SRC is close to final.
Information on the program and on housing can be found on the SRC web page.
Contact Bill O'Brien or Franz Himpsel for detailed questions.
5) Pseudogap in Underdoped HiTc Superconductors
H. Ding et al. (Argonne National Laboratory and other Institutions around
the globe) explored a second energy gap that develops in underdoped
high-temperature superconductors below a second critical temperature T* that
is higher than the superconducting transition temperature Tc. Using
angle-resolved photoemission for measuring momentum-dependence and symmetry
of the pseudogap it is found that both gaps exhibit similar, d-wave
character. T* and Tc coincide at optimum doping and diverge when the doping
level is decreased. These observations suggest that the two gaps are
related. Possibly, the pseudogap is related to local pairing that persists
above the critical temperature where long-range pairing vanishes. Such local
ordering, in turn, might be related to local antiferromagnetic ordering
these materials, a key feature of magnetic mechanisms for high-temperature
superconductivity. See H. Ding et al., Nature 382, 51 (1996) and a write-up
in Physics Today, June 1996, p. 17.
6) Line Shape in Photoemission Experiments
Tom Miller et al. (University of Illinois at Urbana-Champaign) address a
long-standing puzzle in photoelectron spectroscopy from solids. A three-step
model is commonly employed to interpret photoemission spectra to obtain
electronic structure information. For silver, which is chosen as prototype
metal, this model predicts a single, symmetric peak from direct transitions
in the bulk s,p-band. However, this is not seen experimentally. The observed
line shape consists of a markedly-asymmetric transition, plus a plateau
extending all the way to the top of the band. By accurately modeling the
spectra it is found that the asymmetric main line is the result of quantum
interference between bulk and surface photoemission channels - an effect
similar to the Fano resonances seen in atomic and molecular spectra. The
surface channel is driven by the change in the electromagnetic field upon
entering the sample, and its magnitude can be related to the optical
properties of the sample. The underlying plateau is also due to the surface,
via the influence of the surface boundary condition on the photoemission
matrix element. Surface-mediated transitions are expected to occur generally
and will be of interest for interpreting the unusual line shapes observed in
high-resolution photoemission from high-temperature superconductors and
other correlated electron systems. See T. Miller et al., Phys. Rev. Lett.
77, 1167 (1996).