Erik J. Nelson, Joseph C. Woicik, M. Zahid Hasan, Zhi-Xun Shen, David Heskett,
and Lonny E. Berman

 In the vicinity of an x-ray Bragg reflection, the incident
and reflected x-rays coherently superpose to form an x-ray standing
wave, with a periodicity equal to the diffracting plane spacing.  The
phase relationship between the two x-rays can be tuned by changing
either the photon energy or the angle of the incident x-ray.  This
tuning results in a shift of the maxima of the x-ray standing wave
field intensity relative to the diffracting planes.  Unlike
conventional x-ray diffraction, where diffracted intensities are
measured and the phase relationship between the incident and
scattered waves and hence the phase of the crystal structure factor
are lost, the interferometric x-ray standing wave (XSW) technique
directly determines the amplitude and phase of the structure factor
of absorbing atoms.  By examining the atomic absorption as the
standing wave is swept though the unit cell, site-specific structural
data are measured.  By using both valence band and core-level
photoelectron yields to monitor the absorption, both the
site-specific electronic structure of the valence band and the
element-specific geometric structure of the crystal may be determined.

 To demonstrate the applicability of this technique to
materials of technological interest, we present our results on the
perovskite material La1/2Sr3/2MnO4.  The perovskite structure is
common to colossal magnetoresistive (manganite) and high-Tc
superconducting (cuprate) materials.  These effects originate in the
Mn-O (or Cu-O) planes in the layered planar tetragonal structure of
the perovskites, and "charge ordering" and "orbital ordering" of
valence electrons in the Mn-O plane have been observed.

 The position-tunable electric field is the basis of a unique
technique to directly probe the photoemission partial densities of
states of crystalline materials.  For La1/2Sr3/2MnO4, we find that Mn
valence-state emission is enhanced at the top and bottom parts of the
valence band, while the middle part is higher in La, Sr, and O
valence-state density.  These results are consistent with theoretical
partial densities of states for the related perovskite compound
La0.7Sr0.3MnO3.

 In addition to retaining phase information, the x-ray
standing waves technique using core-level photoemission yields also
has an advantage over conventional x-ray diffraction in that it is
element-specific.  The atomic position distribution of each of the
four elements in La1/2Sr3/2MnO4 was separated, and each was found to
be consistent with the model structure verified by x-ray diffraction.
The identical lineshapes of the La and Sr core-level yields directly
verify that La substitutes in the Sr sites.