Quasi-2D Systems: Superconductors and Insulators

This experimental condensed matter physics project focuses on quasi-two-dimensional, nanostructured films near their superconductor to nonsuperconductor quantum phase transitions. Nanostructured proximity effect arrays, consisting of nanoscale superconducting grains embedded in a metal film and nanoscale wire arrays of superconducting films will be tuned through their superconductor to non-superconductor transitions and probed with electron magnetotransport and tunneling measurements at dilution refrigerator temperatures. These systems are attractive because their properties are predicted to include unconventional metallic states and a pseudogap in the tunneling density of states similar to those observed in other quasi-two dimensional strongly correlated electronic systems such as the underdoped high Tc superconductors. Moreover, the physics behind the quantum phase transitions in the nano-arrays, while similar to the well-known superconductor to insulator transitions is expected to sharply contrast with them. Because of the nanoscale dimensions, the phase and amplitude degrees of freedom in the superconducting order parameter, rather than just the phase, will vary strongly through these transitions.

These experiments have the potential to reveal new classes of quantum phase transitions, and to provide physical insight into the origin of pseudogaps, and unconventional metallic phases in two-dimensional correlated electron systems. This project will train new PhD physics level scientists by immersing two graduate students at Brown in important current problems in condensed matter physics. These students will develop expertise in nanotechnology, scanning probe microscopies and low temperature techniques that will make them potentially attractive to industry and prepare them to be future leaders in science.

Magnetic Manipulation of Biological Systems

We investigate the swimming response of microorganisms to intense magnetic fields and magnetic forces. We have developed a method based on magneto-Archimedes principle that employs magnetic forces to simulate variable gravity environments for the study of gravi-sensitivity of cells. Paramecium picture The May 2006 issue of Scientific American has a short paragraph about our research.

My experiments are performed on single cell protozoan Paramecium caudatum. A rather large (200 micrometer long) and commonly used ciliate, which possesses gravi-sensing abilities. We carried out most of our experiments at the National High Magnetic Field Laboratory.

All our experiments are performed in solenoid magnet systems that produce non-homogenous magnetic fields. The field is at its maximum at the center of the magnet whereas the magnetic force, since it is proportional to the field-field gradient, has two maxima off the center as shown with the blue curve. When a diamagnetic object is placed in non-homogenous magnetic field, the induced magnetic moments are such that the object is repelled from the maximum field region (away from the center). Therefor the objects placed above the center of the magnet feel a decreased gravitation pull and vice versa. The total force profile is shown in red, the arrows show the relative magnitude of the force. Since biological matters are very weakly diamagnetic, we need strong forces of order of few thousand T2m-1 to manipulate them.