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A picture of Dr. Stephen Arnason           
Stephen Arnason

Associate Professor 

Department of Physics
University of Massachusetts at Boston
100 Morrissey Blvd
Boston, MA 02125

Office: S–3–83
Email: here
Phone: 617 287 6068
Fax: 617 287 6053

Research Interests
The interface between the disciplines of materials science, condensed matter physics and electrical engineering
Current Research Topics

My research examines the electronic properties of new materials. I am particularly interested in the ways in which structure and the quantum mechanical corrections to the semi-classical theory of conduction modify the behavior of electrons. My work spans three disciplines; Physics, Materials Science, and Electrical Engineering. I grow thin films of materials with interesting electronic properties, fabricate devices that allow me to control aspects of the electronic conduction in these films, and use the devices to experiment on the transport of the electrons through the material.

Ongoing Research:

Quench Condensed growth of Silver
When a thin film is grown onto a cryogenically cooled substrate the atoms that arrive on the substrate surface are not free to diffuse around on the surface. Under these circumstances we say that the surface mobility is quenched. This means that the atoms are not able to find their equilibrium positions in the growing Silver film. This non-equilibrium growth process results in a thin film which is in a meta-stable disordered phase. This phase is not only disordered but it is an insulating phase of Silver a material which is normally thought of as a good conductor. If we take such a quench condensed film that is a few layers of Silver atoms thick and allow it to warm up to room temperature slowly, it will go through a metamorphosis into an ultra-thin conducting film of silver. This ultra-thin film is also meta-stable but is stabilized a quantum size effect. My interest is in understanding the details of the kinetic pathway that leads to these ultra-thin Silver films.

Percolating Silver and Weak Localization.
If you grow a Silver thin film on a non-wetting substrate at room temperature the film will grow via the nucleation of isolated grains. As the grains expand, via the diffusion of the atoms arriving on the substrate surface, the eventually grow into one another. The coalescing grains form larger and larger clusters until there is a single cluster that spans the entire substrate. The point at which a single cluster spans the substrate is known as the percolation threshold. This so called infinite cluster has a quasi-self similar fractal geometry over a range of length scales from the grain size up to the dimension of the substrate. The electrons that move on this cluster are confined to the fractal geometry. Classically their motion is diffusive, but because of the resulting geometric constraints that diffusion becomes anomalous, with a diffusion constant that now depends on length scale. My interest is in how the constraints of the fractal cluster modify the Quantum corrections to conduction known as weak localization. Weak localization is a manifestation of the self interference of an electron. Thinking of the electron as a wave, the scattering of that wave off of the impurities leads to an enhancement of the probability that the electron stays where it is rather than propagating away. This is called coherent backscatter. Since the electron is more likely to stay in one place than we would predict classically this increases the resistance of the Silver film. The Quantum coherence necessary for the interference breaks down as the temperature of the sample increases; so, the sample has an extra, increasing resistance as the temperature goes down. My interests are in the details of how this excess resistance changes on the fractal cluster where the ways in which it can self interfere are limited by the geometry.

Noise in an Electron Glass.
Amorphous Indium Oxide is one of the few materials that are known to undergo the Superconductor to Insulator transition. As the number of electrons in the sample increases the material becomes a superconductor. At low carrier densities it is an insulator. On the insulating side of the Superconductor to Insulator transition Indium Oxide manifests what is know as electron glass behavior. One of the consequences of this glassiness is that if electrons in the glassy phase are pushed out of equilibrium they decay back towards their equilibrium configuration via an extremely constrained set of transitions. This leads to a non-ergodic relaxation. The constraints on the relaxation processes come about as a consequence of the correlations that result from the strong coulomb interactions between the electrons in the material. They cannot find their equilibrium positions because to do so would require them to move past other electrons which repel them. These same charge correlations lead to an enhancement of the conductance noise is this materials: the resistance changes a lot as a function of time. I am developing protocols for performing conductance noise spectroscopy in Amorphous Indium Oxide Field Effect Transistors that have been pushed away from their equilibrium and are relaxing back as a way to understand the dynamics of the relaxation in the glassy phase.

Physics 181: Physics Laboratory I  
Physics 182: Physics Laboratory II

Selected Publications 
Magnetocapacitance: probe of spin-dependent potentials
K.T McCarthy, A.F. Hebard and S.B. Arnason
Phys. Rev. Lett. 90,117201 (2003)  

Growth of the dilute magnetic semiconductor GaMnN by molecular-beam epitaxy
M.E. Overberg, G.T. Thaler, C.R. Abernathy, N.A. Theodoropoulou, K.T. McCarthy, S.B. Arnason et. al.
J. Electronic Materials 32, 298 (2003)  

Magnetic properties of P-type GaMnP grown by molecular-beam epitaxy
M.E. Overberg, B.P. Gila, C.R. Abernathy, S.J. Pearton, N.A. Theodoropoulou, K.T. McCarthy, S.B. Arnason, A.F. Hebard, Appl. Physics Lett. 79, 3128 (2001)  

Carbon nanotube-modified cantilevers for improved spatial resolution in electrostatic force microscopy
S.B. Arnason, A.G. Rinzler, Q. Hudspeth and A.F. Hebard
Appl. Physics Lett. 75, 2842 (1999)  

Bad metals made with good-metal components
S.B. Arnason, S.P. Herschfield and A.F. Hebard
Phys. Rev. Lett. 81, 3936 (1998)