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Roberto Colella

Emeritus Professor of Physics

colella@purdue.edu
Office: Physics 170
Telephone: (765) 494-3029
Fax: (765) 494-0706
Vitae

Italian Doctoral Degree (Laurea), Physics 1958 , University of Milan

Research Interests

Current

Diffraction Physics and Solid State.
  • Solution of the Phase Problem in Crystallography by Multiple Bragg Scattering
  • Resonant Scattering
  • Isotope Effect

Previous Area(s)

Charge density studies; phonons; x-ray and neutron interferometry; quasicrystals. The work on neutron interferometry and gravity, in collaboration with Overhauser, Werner, Eagen and Staudenmann, has received ample coverage in the U.S. and international press. See, for example: Physics Today (August 1975, p. 17); La Recherche (France) 7, n. 63, p. 66 and 60 (1976), and other journals. Popular descriptions of these experiments have been given in: American Scientist, 68, 70 (1980) and Scientific American (May 1980). They are mentioned in standard textbooks on Quantum Mechanics and Optics, such as: "Quantique, Rudiments" by J.M. Lévy-Leblond and F. Balibar (Inter Editions, Paris 1984), "The Investigation of the Physical World," by G. Toraldo di Francia (Cambridge University Press, 1981), and: "Modern Quantum Mechanics," by J.J. Sakurai (Benjamin/Cummings, Menlo Park, CA, 1985).
Cited in the 1977 Yearbook of the MacGraw Hill Encyclopedia of Science and Technology, under the entry: "Quantum Theory, Non Relativistic," (by R. Colella).

Research Activities

My general field of interest is in the area of diffraction physics, mostly by x-rays. The present description covers the last ten years of research, and the work in progress.

The Phase Problem in Diffraction

Over the last few years a research project has been pursued in my laboratory for perfecting a method suitable for phase determination of x-ray reflections by making use of multiple Bragg scattering. It has been shown, in a sequence of progressive stages, that the method can be successfully applied to perfect and mosaic crystals, and, more recently, to organic crystals. Use of synchrotron x-rays (Brookhaven, Stanford) has been a key ingredient for success of this work.

This work has attracted some attention. One conclusive short paper was published in Nature ( 329 , 232 (1987)) with an editorial comment (p. 201 on the same issue). An article in reference to this work has also appeared in Scientific American ("The Science and the Citizen", January 1988, p. 10). The feasibility of applying this method for phasing Bragg reflections in quasicrystals has been investigated. At the present time the method is being applied to resonant scattering.

Quasi-Crystals

New high quality single quasi-crystals are now available, in the family of the Al Cu Fe and Al Pd Mn alloys, and can be used for quantitative studies of quasi-crystallography. A project was initiated, aimed at a quantitative understanding of the diffraction spots. Experiments using synchrotron x-rays, at the National Synchrotron Light Source of Brookhaven National Laboratory, have shown evidence of multiple diffraction effects which may help in determining atomic locations within unit cells through phase information. The present results are in support of a non-centrosymmetric atomic arrangement.

Work has been pursued in the following areas:
  • Debye-Waller factors in Al Pd Mn, temperature dependence, non-Debye behavior at low temperature
  • diffuse scattering studies near Bragg reflections, anisotropy and phason fluctuations.

Resonant Scattering in Germanium

Some x-ray reflections that are forbidden by the symmetry of the lattice become observable when the x-ray energy is tuned to one of the resonances of the germanium atom. The space-group of the crystal is then modified by the x-ray beam in resonant conditions.

Work is in progress to study this effect along the following directions:

   a) apply multiple beam diffraction to determine the phases of the scattered beams
   b) to characterize the energy dependence of the cross section for resonant scattering
   c) to study the polarization and the temperature dependence of the scattered beam
   d) to solve phase problems in manganates

Isotope Effect in Germanium

The lattice constant of Ge is slightly dependent on the nuclear mass. This phenomenon is due to the effect of nuclear mass on the zero point energy of the crystal. We are in the process of measuring the lattice constants, at various temperatures, of several highly enriched isotopes. The variations in lattice constants are quite small, and non-conventional techniques are being used. The experiments are all performed at synchrotron installations. This research is done in collaboration with Prof. S. Rodriguez for the theoretical part.

Graduate Students

  • D. H. Bilderback, "X-Ray Determination of the Valence Charged Density of Grey Tin, Indium Antimonide, and Gallium Arsenide". Ph.D. degree, August 1975. Cornell University, High Energy Synchrotron Source.
  • S. D. LeRoux, "X-Ray Studies of Acoustoelectrically Amplified Phonons". Ph.D. degree, December 1975. Atomic Energy Commission in South Africa.
  • D. R. Yoder, "Valence Electron Density in Silicon and InSb Under High Pressure". Ph.D. degree, January 1982. Michigan Technological University, Physics Department, Houghton, MI.
  • L. D. Chapman, "X-Ray Diffraction Studies of Acoustoelectrically Amplified Phonons". Ph.D. degree, June 1981, National Synchrotron Light Source - Brookhaven National Laboratory.
  • D.D.S. Kaufman, "X-ray Measurements of Lattice Parameter Changes in Electron Irradiated Silicon". Ph.D. degree, August 1984. (In absentia).
  • S. M. Hsieh, "X-Ray Diffraction Studies of Layered Crystal Structures: Graphite and 1T-TaS2". Ph.D. degree, December 1985. Texas Instruments, Dallas, TX.
  • M. C. Schmidt, "X-Ray Scattering from Forbidden Reflections in Vanadium Silicide". Ph.D. Degree. December 1986. Amoco Research Center. Naperville, Illinois.
  • Q. Shen, "Solution of the Phase Problem Using Multi-Beam X-Ray Diffraction". Ph.D. Degree. August 1987. Cornell University, Cornell High Energy Synchrotron Source Ithaca, New York.
  • J. R. Buschert, "Time Resolved X-Ray Diffraction Studies of Laser Annealing and Photostriction in Silicon". Ph. D. degree. December 1989. Goshen College, Physics Department, Goshen Indiana.
  • Q. Zhao, Transferred to Electrical Engineering, Purdue University, 1990.
  • Hyunsook Lee, "Phase Determination by Multiple Bragg Scattering of X-Rays in an Icosahedral Quasicrystal". Ph.D. degree, May 1993. Postdoctoral Research Associate, University of California at San Diego.
  • Rachel Eisenhower, "Multiple Bragg Scattering in Decagonal Quasicrystals". Ph.D. degree, May 1996. Postdoctoral Research Associate at the National Synchrotron Light Source, Brookhaven National Laboratory.
  • Yi Zhang, "X-Ray Diffraction Studies of Al-Pd-Mn Quasicrystal. Structural and Thermal Properties". Ph.D. degree, December 1997. Staff Scientist at Qualcomm Inc., San Diego, California.
  • John Sutter, "Applications of Special X-ray Diffraction Cases in Silicon Crystals". Ph.D. Degree, May 2000. Performed "in absentia" at the Advanced Photon Source of Argonne National Laboratory. Postdoctoral Research Associate at HASYLAB (the German Synchrotron Radiation Source), Hamburg, Germany.

Awards and Honors

  • On November 15, 2003, several former students and postdocs organized a "Day of Recognition" in honor of Prof. Colella, to recognize the role he played in their lives.
  • Recipient of the McCoy Award (April 2002), the most prestigious research award bestowed by Purdue University, for "significant contributions in the natural sciences".
  • Member (2001) of the Editorial Board for the Encyclopedia of Condensed Matter Physics, to be published by Elsevier.
  • Member of the Organizing Committee of the "International School on X-ray and Neutron Dynamical Diffraction: "Theory and Applications". (Erice, Italy, April 1996).
  • JSPS Fellow (2/1/95-4/30/95); JSPS=Japanese Society for the Promotion of Science,1995
  • Fellow of the American Physical Society
  • Co-Editor, Acta Crystallographica, 1980-90
  • Chairman and Founder of the first Gordon Conference on "X-Ray Physics", New London, N.H., August 7-11, 1989.
  • Honorable Mention, Gravity Research Foundation, 1980
  • Several citations in college general physics texts

Professional Experience

  • Emeritus Professor of Physics, (Retired), July 1, 2008
  • Professor of Physics, Purdue University, 1977-present
  • Associate Professor of Physics, Purdue University, 1975-77
  • Assistant Professor of Physics, Purdue University, 1971-75
  • Postdoctoral Research Associate, Catholic University of America, 1970-71
  • Postdoctoral Research Associate, Cornell University, 1967-70
  • Staff Research Scientist, Euratom Nuclear Research Center (Ispra, Italy), 1961-67
  • Comitato Nazionale per l'Energia Nucleare, Rome, Italy, 1960-61

Past Research Associates:

  • Spencer Chang, September 1, 1974 - June 30, 1976.
  • Frank Sinclair, September 1, 1976 - June 30, 1978.
  • Roger Moret, January 1, 1978 - June 30, 1980.
  • Vladislaw Minor, October 23, 1985 - July 31, 1988.
  • Keiichi Hirano, from the Photon Factory (Tsukuba), Sept. 1, 1999 Aug. 31, 2000.

Selected Publications

  1. "Observation of Gravitationally Induced Quantum Interference," Phys. Rev. Lett. 34, 1472-1474 (1975). (w/A. Overhauser and S. Werner.)
  2. "Effect of Acoustoelectric Phonons on Anomalous Transmission of X-Rays," Phys. Rev. Lett. 37, 1056, (1976). (w/S. D. LeRoux and R. Bray.)
  3. "Virtual Bragg Scattering: A Practical Solution to the Phase Problem in Diffraction," Phys. Rev. Lett. 46, 1578 (1981). (w/L. D. Chapman and D. R. Yoder.)
  4. "Experimental Evidence of Phase Excitations in Solids by X-Ray Diffraction," Phys. Rev. Lett. 52, 652 (1984). (w/L. D. Chapman.)
  5. "Solution of Phase Problem for Crystallography at a Wavelength of 3.5 Å," Nature, 329, 232 (1987). (w/Q. Shen). (See Editorial Comment on p. 201. ibid.)
  6. "Dynamical Diffraction and X-ray Standing Waves from Atomic Planes Normal to a 2-fold Symmetry Axis of the Quasicrystal A1-Pd-Mn" (w/T. Jach, Y. Zhang, M. de Boissieu, M. Boudard, A.I. Goldman, T.A. Lograsso, D.W. Delane and S.W. Kycia). Phys. Rev. Letters 82 , 2904-07 (1999).
  7. "Resonant Scattering in Germanium''. (T.L. Lee, R. Felici, K. Hirano, B. Cowie, J. Zegenhagen, and R. Colella). Phys. Rev. B 64 , 201316(R) (2001) (4 pages). Rapid Communication.
  8. "X-ray Diffuse Scattering in the Icosahedral Quasicrystal A1-Pd-Mn". (Y. Zhang, S.N. Ehrlich, R. Colella, M. Kopecky, and M. Widom). Phys. Rev B 66 , 104202, 1-7, (2002).
  9. "The Effect of Isotopic Composition on the Lattice Parameter of Germanium Measured by X-ray Backscattering". (M.Y. Hu, H. Sinn, A. Alatas, W. Sturhahn, E.E. Alp, H.C. Wille, Yu.V. Shvyd'ko, J.P. Sutter, J. Bandaru, E.E. Haller, V.I. Ozhogin, S. Rodriguez, R. Colella, E. Kartheuser, M.A. Villeret. Phys. Rev. 67 , 113306; 1-4 (2003).
  10. "Determination of the Mechanism for Resonant Scattering in LaMnO3". (Q. Shen, I.S. Elfimov, P. Fanwick, Y. Tokura, T. Kimura, K. Finkelstein, R. Colella, and G. A. Sawatzky). Phys. Rev. Letters 96, 246405 (2006) 1-4.
  11. "Resonant Scattering and Multiple Bragg X-ray diffraction in LaMnO3; A Classical View." (R. Colella & Q. Shen). Acta Cryst. A62, 459---462 (featured in the Newsletter of the International Union of Crystallography, Vol. 14, n. 4, p. 4) (2006). (PDF)