Physics@CU-Boulder—Spring 2006: Bonus Content

A New Light Source for Photoemission Spectroscopy

Understanding the electronic structure and phase transitions of high temperature superconducting materials and unusual magnetic materials is the goal of Dan Dessau’s research group. For the last decade, the most powerful and direct method for probing the electronic structure of solids has been angle resolved photoemission spectroscopy (ARPES). This technique hails from Hertz’s 1887 experiment and Einstein’s Nobel-winning explanation in 1905, though as Dessau explains, “the technique has advanced a bit since then.”

The technique fires photons into a material and analyzes the energy and momentum of the electrons that are knocked out. With knowledge of the energy and momentum of the incident photons, the energy and momentum of the electrons in the solid can be studied in detail. Typically the photons for these experiments are obtained at synchrotron radiation facilities – large 100 million-dollar-plus facilities that users can perhaps get a few weeks of beam time at per year. Dessau is a seasoned synchrotron user, but when he came to CU-Boulder he realized that there may be a better way.

The goal was to replace the photons from the synchrotron with those from a laser. An ultraviolet laser which produces photons with energy of about 6 eV or more (wavelength of about 200 nm) is needed, as these can overcome the work function of the sample, which is typically about 4.5 eV. This gives electrons of maximum energy 1.5 eV, which is much lower than what is typically used at the synchrotrons (20-50 eV). Electrons with this low a kinetic energy had not been previously been used successfully for ARPES because the construction of the electron spectrometer to work at these low energies is much more difficult.

Physics graduate student Jake Koralek and Dessau began the project a few years ago. For the laser development portion of the project, they began collaborating with physics and JILA colleagues Steve Cundiff, Margaret Murnane, Henry Kapteyn, and Jun Ye. “Originally Jake and I knew almost nothing about lasers. They lent equipment to us, helped steer us in the right direction, and gave critical advice. Only in a place like this could we have made this happen,” said Dessau, referring to the collegial atmosphere as well as to the world-class optics capabilities in physics/JILA. Dessau says he now has become somewhat of a laser expert in his own right, and is working with the university’s technology transfer office to patent some of his latest laser innovations.

The laser-based ARPES system built by Koralek, Dessau, and other grad students, especially Fraser Douglas and Nick Plumb, not only replaces the synchrotron for many of their studies, but “for many problems it does it much better.” Compared to the synchrotron, Dessau says they can now get far superior momentum resolution, energy resolution, and count rates. They also win in other ways, including less sensitivity to the surfaces of the samples, which has been the scourge of all prior studies, and they are working to make use of the femtosecond nature of their lasers to directly access the electron dynamics. Their system is currently unique, though many groups around the world are working to duplicate it. “What is exciting is that we can now directly obtain the most intimate quantum mechanical details of the electrons in the superconductor.” These details are rich and complex, and have attracted the attention of many of the leading condensed matter physicists around the world who are hoping to explain the exotic physics of high temperature superconductivity.

Publication about this work:
J. D. Koralek, J.F. Douglas, N.C. Plumb, Z. Sun, A. Fedorov, M. Murnane, H. Kapteyn, S. Cundiff, Y. Aiura, K. Oka, H. Eisaki, D.S. Dessau “Laser ARPES, the sudden approximation, and quasiparticle-like peaks in Bi2Sr2CaCu2O8+d” Phys. Rev. Letters (to appear).

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