Condensed Matter: Experiments
Scanning Probe Microscopy of nanoscale semiconductors
We are using the scanning probe microscope to investigate a variety of nanostructures, including InAs and InGaAs quantum dots and GaN nanowires. Quantum dots are structures which are so small (generally < 40 nm) that the free electrons are confined in all three dimensions. The dots are a physical realization of the standard quantum "particle-in-a-box" problem and are sometimes called artificial atoms. Nanowires confine the electrons in two dimensions.
The microscope has several scanning modes available. In addition to the AFM and STM modes which are normally part of the system, we have modified the microscope to enable ballistic electron emission microscopy (BEEM) and also conductive AFM modes.
Students who have worked on this project include Annie Erbsen (REU 2004), Stephen Poprocki (REU 2004), Austin Carter (Senior IS 2005), Kelly Patton (REU 2005), Kathy McCreary (Senior IS 2006), Danny Shai (REU Senior Researcher 2007), and Richard Sampson (REU 2008).
In this atomic force microscope image, the dots appear as bright points. These quantum dots are pyramidal islands of Indium Arsenide (InAs) approximately 5 nanometers tall. The dots are grown on a substrate of Gallium Arsenide (GaAs). The scalloped layers visible across the background are actually steps from one atomic layer to the next in the GaAs crystal.
Avalanches on a Conical Beadpile
A granular system behaves in some ways like a liquid with an ability to flow and in some ways like a solid with a stable fixed structure if undisturbed. As a result, the behavior of a granular system such as a beadpile with beads constantly added to the top is complex and fascinating. Before adding a bead, we cannot tell whether the bead will stay on the pile, fall off the pile, cause an avalanche of a few beads, or cause a catastrophically large avalanche. Avalanches are relatively infrequent -- most often the bead stays on, building the pile. Small avalanches (measured by the number of beads involved) are much more common than large avalanches, and in fact the probability P of an avalanche occurring depends on its size s as a power law according to
where the exponent τ is near 1.5 in our experimental work.
We recently began using a scaling analysis to provide a more complete description of the dynamics of the beadpile system than an analysis only of the critical exponent τ. A preprint of an article summarizing the scaling analysis and our recent results is available on the arXiv here.
Beadpile research has a long history at the College of Wooster under the guidance of Dr Jacobs; I have recently begun collaborating on this work as well. We are currently beginning an investigation on the effect on the dynamical behavior of the system of cohesion between the beads. Students who have worked on the project most recently include Alyse Marquinez (REU 2010), Ingrid Thvedt (Senior IS 2011), and Tom Gilliss (REU 2011).
Swelling of Sol-gel Derived Organosilica
Osorb ® is a sol-gel derived organosilica that instantaneously swells up to four times in volume with organic liquids. The nanoporous glass-like material is hydrophobic and does not swell in water but absorbs non-polar organic solutes from aqueous solution. When the material swells due to absorption of organic solutes, substantial mechanical force is generated. We have investigated the force exerted by placing a powdered sample in a cylinder with a freely movable piston. As solvent percolates into the cylinder from below, the exerted force is measured by a load cell. The piston is then gradually moved upward to allow the material to expand.
Osorb ® was discovered by Dr. Paul Edmiston of the College of Wooster Department of Chemistry. Our work on Osorb ® in the Physics Department began with Lily Christman (REU 2010) and continued with Amanda Logue (Senior IS 2011) and Theresa Albon (REU 2011). For more information on Osorb ®, please visit ABSMaterials.
Cavity ring-down is an optical technique which is extremely sensitive to any sources of optical loss within the measurement cavity. We have modified the cavity so that we can easily switch between an end-injected and center-injected geometries. The cavity is mainly used to study semiconductor mirrors known as DBRs.
Students who have worked on this project include Dan Utley (Senior IS 2005), Danny Tremblay (REU 2005), Ian Steward (REU 2007), Henry Timmers (Senior IS 2009), and Heather Moore (Senior IS 2010).
Whether hot water sometimes freezes faster than cold water has been greatly debated. This phenomenon is known as the Mpemba Effect. Joe Thomas began our research in this area with a Junior IS self-designed project; he found some interesting results so we continued the work for his Senior IS (2008). Ingrid Thvedt (REU 2008) continued the project that summer. Martha Roseberry (Senior IS 2009) made some significant improvements to the consistency of our set-up, adding multiple thermistors to monitor the temperature during freezing. Erin Ford (REU 2009) continued data collection with Martha's modifications. We're currently working to wrap up the project.