Stephen Fahy -

I grew up in Cork and took my undergraduate degrees in Mathematical Science and Physics and masters degree in Mathematics at University College Cork. I attended the University of California at Berkeley for my PhD in Physics, where I specialized in condensed matter theory under the supervision of Steven Louie. I was a member of technical staff in the physical science division at Bell Laboratories, 1988-1990. I joined the faculty at the University of Michigan in 1990 and returned to UCC in 1993. 

My research interests are in the theory of electronic properties of solids, particularly semiconductors and semimetals. 

Semiconductor Transport: A major part of my effort in recent years has been given to the formulation of transport in semiconductors in a form to which modern first-principles electronic structure methods can be applied. The systems under study have included conventional semiconductor alloys (SiGe) and the extreme dilute nitride alloys GaAsN and GaInNAs. In collaboration with the group of E. P. O'Reilly, I have investigated the physical origins of poor mobility in the dilute nitrides and have related this to strong band-shifts arising from the presence of nitrogen clusters in these materials. In the conventional alloys, we have calculated for the first time the mobility of a semiconductor alloy, SiGe, from first principles. This represents a major new development in the quantitative understanding of carrier transport in semiconductors.
Vibrational Spectroscopy: The use of vibration mode frequencies as a probe of extreme physical environments has been a recurrent theme in my work, starting with early investigations of the Raman shift in diamond as a gauge for ultra-high pressure in diamond anvil systems. Subsequent work in collaboration with the experimental group of Roy Clarke on the infra-red absorption of cubic boron nitride thin films demonstrated conclusively that these films exhibit islanding in the initial stages of growth and ruled out extreme stress as the instigating factor in the growth of the cubic phase, thereby resolving a long-standing controversy in the field. Seminal work with R. Merlin on the extreme anharmonic excitation of coherent phonons in ferroelectric materials has prompted on-going experimental attempts by several groups to reverse domains on a picosecond time-scale by the application of intense, ultra-short optical pulses. 

My most recent work in this area has investigated impulsive optical excitation of coherent phonons in semiconductors and simulated the resulting pump-probe reflectivity oscillations. Band structure calculations on tellurium and bismuth have demonstrated how their strong electron-phonon coupling is directly related to the Peierls distortion that stabilises the low-pressure phase of these materials. Results of these simulations have been used to understand the diffusion of carriers and their coupling to phonons in optically excited electron-hole plasmas, demonstrating both softening of the phonons at high levels of electronic excitation, slow electron-hole recombination and a substantial reduction in the carrier diffusion rate in the dense plasma. My work on ultrafast coherent dynamics has involved a particularly fruitful interaction with the experimental groups of David Reis, Roberto Merlin and Philip Bucksbaum at the FOCUS centre of the University of Michigan and Stanford University. This has culminated in recent x-ray probe measurements at the Stanford Linear Accelerator, Sub-Picosecond Pulse Source (SPPS), of the energy surface of photo-excited bismuth, which have verified our earlier calculations.
Recent calculations of nitrogen-related localized vibrational modes in dilute nitride semiconductors have provided signature spectra of various defects that are important in determining the microscopic structure and electronic transport properties of these materials. This work has led to a joint experimental-theoretical collaboration with the groups of Rachel Goldman and Cagliyan Kurdak at the University of Michigan. We are currently working on the design and growth of dilute nitride heterostructures in which we plan to use low-temperature transport as a spectroscopic probe of localized state scattering.
Quantum Monte Carlo: In the late 1980s and throughout the 1990s, my work was largely focused on the development of quantum Monte Carlo me

Honours and Awards

• [2005] - Fellow of the American Physical Society
• [1985] - Victor Lenzen Scholarship, University of California
• [1982] - Regents Fellowship, University of California
• [1981] - Travelling Studentship in Mathematical Science, National University of Ireland
• [1976] - College Scholarship, University College Cork

Professional Associations

• Fellow

Patents

• [1995] 5,477,519 - Data storage using pulsed optical domain reversal

Committees

• [1995] European TMR Network Coordinator,
• [2001] Committee for Physics of Royal Irish Academy ,
• [1998] Irish Atomistic Simulation Conference , 1998, 2001, 2004, 2007
• [2005] Scientific Council, Irish Centre for High-end Computing (Chair, 2005-2008),
• [2006] Academic Council Research Committee, University College Cork (Chair, 2006-2009),
• [2007] Research Committee, Tyndall National Institute (Chair),

Outreach Activities

• Irish Primary Schools Outreach Programme - Bridging the Gap (2003-2005)

   Tyndall Sound Show - demonstration lecture for primary school students

   Sound Show, presented at the British Association Meeting, Dublin, 2005 

• SFI UREKA Programme for undergraduate research experience (2004-2007)
• SFI STARS Programme for Secondary School Teachers (2004-2005)

Employment

• [1992] Department of Physics / Tyndall Institute - Associate Professor / Academic Member
• [1990] University of Michigan, USA - Assistant Professor of Physics and Applied Physics
• [1988] AT&T Bell Laboratories, Murray Hill, USA - Member of Technical Staff
• [1987] University of California, Berkeley - Postdoctoral fellow

Education

• [1987] University of California, Berkeley - PhD
• [1981] University College Cork - MSc
• [1979] University College Cork - BSc

Consultancy

• [1997] Enterprise Ireland/Forbairt - Basic Science Grants Scheme 
• [2007] Science Foundation Ireland - Research Frontiers Programme 2007
• US National Science Foundation - : Reviewer for Division of Materials Research

Journal Activities

• Phys Rev B - Referee
• Physical Review Letters - Referee
• Journal of Physics A - Referee
• Applied Physics Letters - Referee

Teaching Interests

My teaching interests are in general physics, condensed matter physics, theoretical & computational physics. Currently I am teaching:

Introductory Physics (PY1052 and PY1053) for physical and mathematical science majors

Condensed Matter Physics (PY3105 and PY4104)

Computational Physics (PY2105 and PY4109) 

INSPIRE graduate course on Numerical Methods