Michael Burns - Associate Director for Science
Over the years I've participated in a number of, to me, fascinating projects, most of them at the Rowland Institute. I have found no particular common thread other than simple curiosity coupled with an opportunity to indulge that curiosity. Some of the more interesting ones are described below. (For a complete list of publications.)
Superheating water and nanobubble nucleation
Currently I’m working with two of Jene Golovchenko’s wonderful grad students (Edlyn Levine and Gaku Nagashima) on a fun project of making a billion 10 nanometer bubbles a second by applying a voltage across a solid-state nanopore. The extreme focused current from an applied voltage across the tiny pore causes Joule heating which raises the temperature locally very close to the thermodynamic limit of superheat of water - almost 330 degrees Centigrade. This is far above the normal boiling point of water so within a few nanoseconds a bubble nucleates, and starts to grow. But as soon as it gets large enough to completely block the pore, the current stops and the temperature drops and the bubble goes away. Which means the current can start to flow again, heating the water, forming the next bubble, and so on - a very nice example of a relaxation oscillator. Its so fast (we’re pushing to get a gigahertz repetition rate) because the scale is so small and everything - even thermal conduction - happens very fast. Understanding the details of the process is difficult and we’re working on that now.
Atomic level metal cluster structure analysis
One of the Rowland Senior Fellows, Joel Parks, has built an instrument which is a real tour-de-force and combines an ion trap to select and hold metal clusters with tens of silver atoms along with an electron beam apparatus that allows him to get electron diffraction measurements of the structures of those trapped metal clusters. A silver atom is very different from a clump of silver and the interesting question is what the transition from one to many is like in metals. Because of the exquisite control the trap affords, Joel can select silver clusters of any specified size - say 43 atoms - and measure its structure. Then he traps 44 atom clusters, measures them and so on. From such a sequence he can watch the transition from the atomic properties to the bulk properties. Just as in atomic nuclei or elements there are “magic numbers” of constituents that are particularly noteworthy, and understanding how and why these occur is important. My role in this work was in data analysis and in particular trying to quantify and characterize the local symmetries of the clusters. For infinite assemblies of atoms such symmetry measures are well known but for these clusters there is a combination of amorphous and symmetric character that is more difficult to characterize.
Active control of DNA in a nanopore
Amit Meller, a Rowland Senior Fellow at the time, was continuing his work on using biological nano pores to measure single molecule DNA translocations. When a voltage is applied across a single pore, an ionic current flow occurs which can be measured. When a stringy DNA molecule starts to translocate across the pore, it blocks the current by some amount during the time that it is transiting the pore. This provides a signal that can be related to the length of the strand, and if you could measure the current accurately and fast enough, you might even be able to tell which nucleotide base was in the pore at a given moment. This would be a really cool way to sequence DNA, and in fact a related technology is just now coming to market. But this particular work was long before that and arose because one day while we were talking about it we realized that since we were measuring the current in real time and also had complete control over the applied voltage, we could close a feedback loop and modify the applied voltage depending on the current that we measured. There are lots of things that could be done with this capability, but we chose to investigate what the translocations were like with zero applied voltage by measuring escape times when no voltage was applied. We found there were two distinct escape times - fast and slow - which reflected different binding interactions between the DNA and the pore.
Electromigration in single crystal copper
In a metal conducting a current the electrons, in the process of colliding with the atoms in the lattice, can actually knock the atoms out of their lattice positions. Since the electrons are all flowing in one direction, there can be a net movement of atoms in the wire. At high enough current densities this can lead to a measurable macroscopic movement of the metal along the wire (one mode of failure in modern integrated circuits). We are studying some of the effects of surface electromigration in thin single crystal copper wires in an effort to better understand the electromigration process itself. Copper is a particularly interesting metal in this regards technologically since the most recent semiconductor processes have moved from using aluminum interconnects to using copper. Understanding (and controlling) electromigration is necessary for successful use of copper in this new role.
Single crystal metal whiskers
To observe the binding of cold atoms around a wire (described below), we needed a long, thin wire. One potential method for producing such a wire is to use an old technique of growing metallic single crystal metal whiskers. Although it is surprisingly easy to grow forests of these whiskers in a rather uncontrolled fashion, it is a much more difficult problem to grow them on demand, at a certain location, with a particular crystal orientation. Although the wires needed for the atom-orbits are now being produced by other methods, the technique of metal whisker growth is still being pursued. One reason is that single crystal iron whiskers produced by this technique look like they can be used as the tip in an STM (scanning-tunneling microscope), perhaps as a source and/or detector of spin polarized electrons. This would be of great interest in the emerging field of spintronics. Additionally they are the source of the wires in the electromigration experiments described above.
Atoms orbiting a wire
This project is an exploration of the binding of neutral atoms in stable orbits around a wire charged by a time-varying sinusoidal voltage. Both classical and quantum-mechanical theories for this system have been studied and a unified approach to the Kapitza picture of effective potentials associated with high-frequency fields produced. It appears that cavities and waveguides for neutral atomic matter waves may be constructed using these principles. [Phys. Rev. A, 45, 6468, (1992)]
One may also bind a magnetic atom to a current in a wire through the interaction between the atomic dipole moment and the wire's magnetic field. The theoretical description is based on an extension of the concept of supersymmetry to multi-component wave functions. An analytic solution for spin 1/2 particles can be obtained directly in coordinate space. Experimentally, the system should be realizable for 25 micro-Kelvin sodium atoms around a wire with a diameter of 0.5 microns and a current of 400 micro-amps. [Phys. Rev. Lett. 74,3138 (1995)]
Spin 1 particles present more of a problem theoretically. We have found bound states via numerical and approximate analytic results, and have calculated the decay due to various effects, which bodes well for experimental realizations. Theoretically the supersymmetry method that was so successful in the spin 1/2 state is only approximate for the spin 1 state. The eigenvalues are not degenerate in the angular quantum number, but they almost are. This presents an interesting, unanswered, question: what is the small physical parameter responsible for the small breaking of the symmetry, which we observe in the numerically computed energy eigenvalues? [Phys. Rev. A, 53, 1653, (1996)]
Properly fashioned electromagnetic fields coupled to microscopic dielectric objects can be used to create arrays of extended crystalline and noncrystalline structures. Organization can be achieved in two ways: In the first, dielectric matter moves in direct response to the externally applied standing wave optical fields. In the second, the external optical fields induce interactions between dielectric objects that can also result in the creation of complex structures. In either case, these new ordered structures, whose existence depends on the presence of both light and polarizable matter, are referred to as optical matter. [Science, 249, 749 (1990)]
The interactions in the second group are formed by the significant forces between dielectric objects induced by intense optical fields. These forces are very long range (the forces decay only as 1/r) and oscillate in sign at the optical wavelength. We performed an experiment that demonstrates the simplest case by observing a series of bound states between two 1.43 micron diameter plastic spheres in water. The application to more complex cases (with more spheres) is not currently understood: naive integration of the 1/r forces leads to infinities, which in practice must somehow saturate. It is not known, for example, what the ground-state equilibrium configuration is and hence we do not yet even know what optical matter formed in this fashion would look like. [Phys. Rev. Lett., 63, 1233 (1989)]
Human vision has the remarkable property that, over a wide range, changes in the wavelength composition of the source light illuminating a scene result in very little change in the color of any of the objects. Computations for the color perception of an object depend on knowing more than the amount of light from a point on that object (as in Dr. Land's retinex theory, for example); hence long-range interactions of neural signals is necessary. It was not clear whether these long-range interactions take place right in the retina or further along the pathway in the cortex. We tested the role of the cortex in a human subject in whom the nerve fibers connecting cortical areas subserving two separate parts of the visual field had been severed, and found that the cortex is necessary for long-range color computations. [Nature, 303, 616 (1983)]