Derek W. Lawther

BSc,MSc,PhD
Associate Professor
Phone: 
(902) 566-0338
Department: 
Physics
Office: 
School of Sustainable Design Engineering Building, 318

Dr. Derek Lawther, an experimental physicist, uses positron annihilation to identify defects in materials such as metals. He probes for defects at the atomic level and works to understand better how to manipulate these defects to produce enhanced physical properties such as increased strength or conductivity.

  • Doppler Broadening and Positron Lifetime measurements on disordered metallic alloys (e.g. quasicrystals and nanocrystals). This work includes supervision of undergraduate (UPEI) and graduate (Dalhousie University) student researchers.Postdoctoral Research, 1993-1996
  • Positron Beam Laboratory, The University of Western Ontario: (i) operation and maintenance of the variable-energy positron beam apparatus; (ii) study of defects in silicon-based films and structures, particularly their influence on dopant electrical-deactivation, segregation and  diffusion; (iii) development of defect-spectroscopy techniques using a positron beam. Graduate and Undergraduate Research, 1986-1993
  • Nuclear Physics Laboratory, Dalhousie University: study of physical properties of quasicrystalline and amorphous alloys.

Research techniques

  • Positron annihilation spectroscopy
  • Monte Carlo simulation
  • Mossbauer spectroscopy
  • Differential thermal analysis
  • Resistivity
  • X-ray diffraction

Theses

  • D.W. Lawther, "Positron annihilation study of Al-Cu-Fe FCI quasicrystals", Ph.D. thesis, Dalhousie University (1993).
  • D.W. Lawther, "Transition metal site distributions of the icosahedral phase in as-quenched aluminum-transition metal alloys", M.S. thesis, Dalhousie University (1988).

Chapters in Books

  • R.A. Dunlap, D.W. Lawther and V. Srinivas, "Analysis of x-ray powder diffraction patterns of rapidly quenched Al-based alloys on the basis of quasicrystalline and cubic models", in Quasicrystals, Networks, and Molecules of Fivefold Symmetry, ed. I. Hargittai, VCH Publishers, New York (1990), 83-96.

Review Articles

  • D.W. Lawther and P.J. Simpson, "Positron-beam techniques for materials characterization", Defect and Diffusion Forum 138-139(1996), 1-18.
  • R.A. Dunlap and D.W. Lawther, "Applications of Møssbauer effect spectroscopy to the study of quasicrystalline materials", Material Science and Engineering R: Reports 10(1993), 141-185.

Refereed Journal Publications

  • L.S. Adam, M.E. Law, S. Szpala, P.J. Simpson, D.W. Lawther, O. Dokumaci, and S. Hegde, "Experimental identification of nitrigen-vacancy complexes in nitrogen implanted silicon", Appl. Phys. Lett. 79 (2001), 623-625.
  • U. Myler, P.J. Simpson, D.W. Lawther, and P.M. Rousseau "On the Electrical Deactivation of Arsenic in Silicon", J. Vacuum Sci. Technology 15 (1997), 757-759.
  • U. Myler, R.D. Goldberg, A.P. Knights, D.W. Lawther, and P.J. Simpson, "Chemical information in positron annihilation spectra", Appl. Phys. Lett. 69 (1996), 3333-3335.
  • D.W. Lawther, U. Myler, P.J. Simpson, P. Rousseau, P.B. Griffin and J.D. Plummer, "Vacancy generation resulting from electrical deactivation of arsenic", Appl. Phys. Lett. 67 (1995), 3575-3577.
  • D. Landheer, G.C. Aers, G.I. Sproule, D.W. Lawther, P.J. Simpson, W. Lennard and S.Y. Tong, "Positron-beam study of annealed silicon nitride films", J. Appl. Phys. 79 (1996), 2458-2462.
  • R.A. Dunlap, J. Kyriakidis, Z. Wang and D.W. Lawther "A Doppler broadening positron annihilation technique for the study of defects in bulk samples'', Nuclear Instrum. Methods B 108 (1996), 339-342.
  • D.W. Lawther and R.A. Dunlap, "In situ Cu-64 Doppler-broadening positron-annihilation methods for elevated temperature study of defect formation in metals'', Can. J. Phys. 73 (1995), 11-17.
  • R.A. Dunlap, Z. Wang and D.W. Lawther, "Møssbauer effect study of one-dimensional and two-dimensional quasicrystalline alloys in the Al-Pd-Fe system", Solid State Communications 95 (1995), 45-47.
  • Z.J. Yang, M. Yewondwossen, D.W. Lawther, S.P. Ritcey, D.J.W. Geldart and R.A. Dunlap, "Thermal expansion of Bi(2.2)Sr(1.8)CaCu(2)O(x) superconductor single crystals", J. Superconductivity 8 (1995), 233-239.
  • D.W. Lawther and R.A. Dunlap, "Positron annihilation study of equilibrium defects in Al-Cu-Fe FCI quasicrystals", Phys. Rev. B 49 (1994), 3183-3189.
  • Z. Wang, R.A. Dunlap and D.W. Lawther, "Structural and Møssbauer effect studies of quasicrystalline Al-Pd-Cr-Fe alloys", J. Phys.: Condens. Matter 5 (1993), 663-668.
  • D.W. Lawther, G. Beydaghyan and R.A. Dunlap, "A positron annihilation study of crystalline, quasicrystalline and amorphous Al-Cu-T (T=Fe,V) alloys", J. Phys.: Condens. Matter 2 (1990), 6239-6244.
  • D.W. Lawther, D.J. Lloyd and R.A. Dunlap, "Transition metal site distributions in binary aluminum-transition metal quasicrystals: Al-V and Al-Cr", Mater. Sci. and Eng. A 123 (1990), 33-39.
  • D.W. Lawther, R.A. Dunlap and V. Srinivas, "On the question of stability and disorder in icosahedral aluminum-transition metal alloys", Can. J. Phys. 67 (1989), 463-467.
  • D.W. Lawther, R.A. Dunlap, D.J. Lloyd and M.E. McHenry, "Structure and stability of rapidly quenched Al(86)Cr(14-x)Fe(x) alloys", J. Mater. Sci. 24 (1989), 3076-3080.
  • R.A. Dunlap, D.W. Lawther, P. Hargraves and P. Sinclair, "Correlated hyperfine interactions in amorphous Cr(72-x)Fe(x)C(17)Si(8)Al(3) alloys", J. Phys. F: Metal Phys. 18 (1988), 2479-2486.
  • R.A. Dunlap, D.W. Lawther and D.J. Lloyd, "Fe-57 Møssbauer-effect study of preferential site occupancy in quasicrystalline Al(86)Cr(14-x)Fe(x) alloys", Phys. Rev. B 38 (1988), 3649-3652.
  • R.A. Dunlap, D.W. Lawther and R.H. March, "A positron annihilation study of crystalline, quasicrystalline and amorphous Al-Mn-Si alloys", J. Phys. F: Metal Phys. 17 (1987), L39-L43.

Conference Proceedings

  • D.W. Lawther and R.A. Dunlap, "Positron annihilation in quasicrystals", In Proceedings of the Tenth International Conference on Positron Annihilation, Mater. Sci. Forum 175-178 (1995), 431-434.
  • D.W. Lawther, R. Khatri, P.J. Schultz, P.J. Simpson, I. Calder and L. Weaver, "Variable-energy positron-beam study of arsenic diffusion in poly-silicon", In Proceedings of the Sixth Workshop on Slow Positron Beam Technology, Appl. Surface Sci. 85 (1995), 265-270.
  • V. Srinivas, A.K. Nigam, G. Chandra, D.W. Lawther, M. Yewondwossen and R.A. Dunlap, "Electrical transport in amorphous Fe-Mn-Zr alloys", In Proceedings of the Sixth Joint Magnetism and Magnetic Materials- Intermag Conference , J. Appl. Phys. 76 (1994), 6501-6503.
  • D.W. Lawther and R.A. Dunlap, "Møssbauer effect studies of local Fe environments in crystalline and quasicrystalline Al-Cu-Fe alloys", In Proceedings of the Fourth International Conference on Quasicrystals, J. Non-Cryst. Solids 153-154 (1993), 45-48.
  • D.W. Lawther and R.A. Dunlap, "Møssbauer effect study of the stable Al(75)Pd(15)Fe(10) decagonal quasicrystal", In Proceedings of the Fourth International Conference on Quasicrystals, J. Non-Cryst. Solids 153-154 (1993), 49-52.
  • D.W. Lawther and R.A. Dunlap, "Positron annihilation study of Al(63)Cu(25)Fe(12) as a function of thermal treatment", In Proceedings of the Fourth International Conference on Quasicrystals, J. Non-Cryst. Solids 153-154 (1993), 611-614.
  • R.A. Dunlap, M.H. Yewondwossen and D.W. Lawther, " Møssbauer effect studies of amorphous Al-Y-Ni-Fe alloys", In Proceedings of the Eighth International Conference on Liquid and Amorphous Metals, J. Non-Cryst. Solids 156-158} (1993), 192-195.
  • R.A. Dunlap, D.W. Lawther, M.E. McHenry, V. Srinivas and G. Beydaghyan, "Physical properties of amorphous Al-Gd-transition metal alloys", In Proceedings of the Eighth International Conference on Liquid and Amorphous Metals, J. Non-Cryst. Solids 156-158 (1993), 344-346.
  • D.W. Lawther and R.A. Dunlap, "Observation of a structural phase transition in Al-Cu-Fe quasicrystals using positron annihilation techniques", In Proceedings of the Third International Conference on Amorphous Metallic Materials, Key Eng. Mater. 81-83 (1993), 95-104.
  • M.H. Yewondwossen, R.A. Dunlap, D.W. Lawther and D.J. Lloyd, "Structural, thermal and electrical properties of amorphous Al-Ce-Fe- transition metal alloys", In Proceedings of the Third International Conference on Amorphous Metallic Materials, Key Eng. Mater. 81-83 (1993), 351-356.
  • R.A. Dunlap, D.W. Lawther, V. Srinivas, G. Beydaghyan and M.E. McHenry, "Møssbauer effect studies of amorphous Al-Fe-rare earth alloys", In Proceedings of the Third International Conference on Amorphous Metallic Materials, Key Eng. Mater. 81-83 (1993), 413-418.
  • R.A. Dunlap, D.W. Lawther and V. Srinivas, "Møssbauer studies of quasicrystals", In Proceedings of The NATO Advanced Research Workshop: Physics and Chemistry of Finite Systems: From Clusters to Crystals , Vol I, eds. P. Jena, S.N. Khanna and B.K. Rao, Kluwer Academic, Dordrecht (1992), 527-536.
  • D.W. Lawther, R.A. Dunlap, V. Srinivas, D.J. Lloyd and S. Jha, "Fe-57 Møssbauer-effect studies of quadrupole splitting distributions in icosahedral Al-TM-Fe quasicrystals", In Proceedings of the Eighth International Conference on Hyperfine Interactions, Hyper. Int. 60 (1990) 785-790.

Presentations

  • "Positron annihilation investigations of related amorphous, quasicrystalline and crystalline phases", Third International Conference on Amorphous Metallic Materials, Bratislava, Czecho-Slovakia, September 7-11, 1992.
  • Poster Presentation: "Positron Annihilation Study of Age Hardening Phenomenon in Aluminium Alloys", 2002 Congress of the Canadian Association of Physicsts, Quebec City, Canada, June 2-5, 2002
  • Conference Summary: 1999 Workshop on Positron Studies of Semiconductor Defects, Hamilton, Canada, July 21-23, 1999.
  • Poster Presentation: "Materials Research at the DALUPEI Positron Facility", 1998 Congress of the Canadian Association of Physicsts, Waterloo, Canada, June 14-17, 1998.
  • Oral Presentation: "Positron annihilation in quasicrystals", Tenth International Conference on Positron Annihilation, Beijing, China, May 23-28, 1994.
  • Oral Presentation: "Variable-energy positron-beam study of arsenic diffusion in poly-silicon", Sixth Workshop on Slow Positron Beam Technology, Makuhari, Japan, May 18-22, 1994.
  • Poster Presentations: "Møssbauer effect studies of amorphous Al-Fe-rare earth alloys" and "Structural, thermal and electrical properties of amorphous Al-Ce-Fe-transition metal alloys", Third International Conference on Amorphous Metallic Materials, Bratislava, Czecho-Slovakia, September 7-11, 1992.
  • Poster Presentations: "Møssbauer effect studies of amorphous Al-Y-Ni-Fe alloys" and "Physical properties of amorphous Al-Gd-transition metal alloys", Eighth International Conference on Liquid and Amorphous Metals, Vienna, Austria, August 31-September 6, 1992.
  • Poster Presentations: "Møssbauer effect studies of local Fe environments in crystalline and quasicrystalline Al-Cu-Fe alloys", "Møssbauer effect study of the stable Al(75)Pd(15)Fe(10) decagonal quasicrystal" and "Positron annihilation study of Al(63)Cu(25)Fe(12) as a function of thermal treatment", Fourth International Conference on Quasicrystals, St. Louis, U.S.A., May 31-June 5, 1992.
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