Strongly Correlated Electrons: NSLS-II and the Future

August 28-29, 2003

The workshop was attended by approximately 35 people, with 14 speakers. The small size was designed to facilitate discussion of, and seek input for, the scientific case for a third-generation medium-energy synchrotron such as NSLS II in the field of strongly correlated electron systems. Talks and discussions centered on the key scientific questions ("Grand Challenges") to be addressed in this field, the techniques and instrumentation advances that will be required and the role of the NSLS II in accomplishing these goals.

There was general agreement that strongly correlated electron systems will remain at the forefront of condensed matter physics in the coming decades. In particular, Millis argued that the phenomena observed in many such systems are causing a paradigm shift away from the current theoretical prescription - namely in terms of a Fermi-Liquid picture of renormalized, weakly-interacting quasi-particles, towards a more generalized description in which systems are described in terms of their correlation functions. The grand challenges in the coming years will be to measure the appropriate correlations, and to control them. Scattering techniques (inelastic scattering, photoemission, resonant scattering, coherent scattering, etc.) will all play a crucial role in addressing these challenges. Particularly interesting areas will include the investigation of new phases in nanomaterials, electronic reconstruction at surfaces and interfaces and behaviors in extreme environments. The new phases will need to be identified, requiring new materials (synthesis) and new control parameters (ultra-low temperatures, high pressures, high magnetic fields). The NSLS II will play the leading role in all these areas.

Key Points/Recommendations

1. It is clear that soft x-rays (0.25 5 keV) will be one of the major strengths of the NSLS II, which will be the world-leader in this range. The soft x-ray superconducting undulator needs to be optimized to gain the full performance of the high brightness ring.



2. Some techniques, such as inelastic x-ray scattering would benefit more from higher flux than higher brightness - the possibility of having several extended straight sections with optimized undulators should be looked into. Even if this means compromises on the spot size in those straights. Factors of two make a big difference.



3. Different classes of experiments require different properties from the beam (emittance, pulse separation, source size) and that rather than compromising, it might be better to have several modes for the ring to operate and to switch between them. For example, longer pulse lengths raise the flux, are better for photon correlation spectroscopy and may reduce beam damage.



4. The Far-IR regime is important and the number from conservative estimate of NSLS-II bending magnet is not good enough. The existing UV ring should be upgraded to keep it world-class.



5. Fast polarization switching and control of the coherence of the photon beam are important.



6. Machine stability is very important. At the ESRF MCD effects can be measured down to < 10-5 MCD and signals of 10-4 Bohr. So-called "top-off" mode will be important.



7. High magnetic fields are important. A 20 T superconducting magnet is possible. ~30 T repetitive pulse magnet (1 Hz and 107 shots) might be feasible, though very expensive to operate.



8. Low-temperatures are important in the search for new "quantal" phases.



9. ~10 nm real-space imaging is very important because of the intrinsic inhomogeneity of these systems. The sharp decrease in source size that will be achieved by NSLS II is essential to push the limits on nano-beam imaging. In general, imaging is expected to be extremely important.

Beamlines
The needs of the community would be well served with following ID lines:

a) Soft X-ray (< 5 keV) 1. Coherent scattering/imaging 2. High energy ARPES 3. Resonant inelastic x-ray scattering 4. Resonant scattering 5. High magnetic fields	b) Hard X-ray (> 5 keV) 1. Inelastic x-ray scattering 2. Resonant scattering (+ low temperatures) 3. Micro-diffraction 4. High pressure 5. High magnetic fields
c) Hard X-ray (~ 100 keV) Two wiggler beamlines (~ 6 ports) for high pressure, in-situ growth,etc.	d) Far IR It was the opinion of this workshop that this was better done on the existing UV ring - with appropriate upgrades

Participating Institutions

ANL, BNL, Carnegie Inst. of Wash, Columbia University, FSU-NHFML, Lucent Technologies, Princeton University, University of British Columbia, UCSD, University of Washington

Last Modified: January 31, 2008
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