University of Minnesota
School of Physics & Astronomy


The art of 1/f noise

Jim Kakalios
Jim Kakalios
Alex Schumann

School of Physics and Astronomy Professor James Kakalios has made a career out of finding order in messy systems. From work on amorphous silicon, to helping neuroscientists map the brain, if it's noisy, Kakalios is there.

Kakalios argues that while every solid state physics class begins with or implies the phrase, "assume a perfect crystal," real life is far more complicated. Kakalios notes that if the crystalline form of solids were common in nature, then diamonds would be cheap and soot would be rare and valuable. By studying fluctuation phenomena, Kakalios seeks to gain insights into the behavior of complex, messy systems.

Two important types of noise are "white noise," where the noise amplitude is independent of frequency and 1/f noise, where the noise power decreases as the inverse of the frequency. It's the 1/f noise that interests Kakalios as it's found in such disparate systems as semiconducting resistors, to spin glasses to flood levels of the Nile and Tokyo traffic jams.

Kakalios's studies of 1/f noise on amorphous silicon, a material used in solar cells and flat panel displays, led to the discovery that in this material, the noise power itself varied with time. when the time dependent fluctuations of the noise power were investigated, he again found a 1/f spectrum - that is, in amoprhous silicon, the 1/f noise has 1/f noise! His group also observed random telegraphic switching noises (RTSN) in amorphous silicon, a phenonmenon that had previously been seen in nanofabricated, tiny samples at temperature near absolute zero. Yet they found the RTSN phenomenon in relatively large sample of amorphous silicon at 100 degrees Celsius. These studies led to a collaboration with Prof. A. David Redish in the University of Minnesota's Dept. of Neuroscience, where techniques developed to study inhomogeneous current filaments in amorphous silicon are applied to voltage fluctuations recorded in the brain.

Over the summer, Kakalios will be working in conjuction with Prof. Uwe Kortshagen in the Mechanical Engineering Department to create thin films of amorphous silicon seeded with nanodots, in order to synthesize materials with superior properties for photovoltaic devices.

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