Low Temperature Research at CCNY
The Low Temperature Research Group at the City College of New York (CCNY) Physics Department is dedicated to the study of condensed matter properties at low temperature. We are currently interested in two areas:
(I) Molecular Nanomagnets
(II) Novel Behavior of Two Dimensional Electron Systems .
(I) Molecular Nanomagnets
Computing power/speed and the density of memory elements for storing and manipulating information has been steadily increasing, while the size of the component memory " bits " have been decreasing very rapidly. Considerable effort is currently being devoted to methods for storing information at molecular length scales. In order to continue our steep trajectory to better, smaller and faster computers, we must learn to understand and manipulate physics and chemistry at the molecular level. Moreover, quantum computation, a new and entirely different computing paradigm based on quantum phenomena, is being widely explored, both mathematically and experimentally. Rather than having two possible " classical " values, 0 or 1, the quantum mechanical elements of quantum computers, called " qubits " , represent a far broader set of possibilities, enabling much greater computing power.
A number of potential candidates for high-density information storage and quantum computation are under investigation, among them molecular nanomagnets, sometimes referred to as " single molecule magnets "
We are studying Mn 12 -Acetate, a prototypical member of this class of materials. Mn 12 -Acetate is an organic molecular crystal containing a very large number of regularly spaced, magnetically identical spin-10 clusters, a size that is borderline between the quantum and classical regimes. The first demonstration of resonant tunneling of a mesoscopic spin was done in our laboratory in 1996. This finding led to an enormous increase in interest and activity in the study of molecular nanomagnets, sometimes referred to as "single molecule magnets".
Current research includes spectroscopic measurements at low temperatures at the synchrotron light source at Brookhaven, and measurements at CCNY of the local magnetization at low temperatures on a length scale of microns using very small micro-Hall bars. Recent studies of magnetic avalanches in Mn12-acetate have yielded particularly exciting results. Local time-resolved measurements of the fast reversal of the magnetization of single crystals have shown that magnetic avalanches spread as a narrow interface that propagates through the crystal at a constant velocity roughly two orders of magnitude smaller than the speed of sound. This phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance
For a schematic animated view of "magnetic burning" click here (courtesy Kevin M. Mertes)
(II) Novel Phenomena in Dilute Two dimensional Electron Systems.
One of the most interesting current questions in condensed matter physics is whether the unusual behavior observed in dilute, strongly-interacting two dimensional systems of electrons (or holes) signals the presence of a metallic phase and a metal-insulator transition.
According to well established theory, two-dimensional systems of weakly interacting electrons (or holes) are expected to be insulating in zero magnetic field B=0 in the zero-temperature limit. Experiments performed in the early 1980's provided confirmation of these expectations for relatively high electron densities. The availability within the last decade of samples of unusually high mobilities have allowed access to much lower electron (hole) densities, where electron interactions are quite strong. Unexpected metallic behavior has been observed in this low-density regime: for electron densities above some critical (low) density the resistivity decreases with decreasing temperature down to the lowest accessible temperatures while exhibiting insulating behavior at lower densities. This suggests there exists a true metallic phase in 2D.
The response of these systems to external magnetic field is also unusual and dramatic. A (density-dependent) external magnetic field of the order of a few Tesla applied parallel to the plane of the electrons causes the resistivity to increase sharply to a much larger, constant value. The parallel magnetic field suppresses or "quenches" the metallic temperature dependence.
These findings have fueled a lively debate. The issue is whether these novel effects represent fundamentally new physics, or whether they can be explained by an extension of physics that is already understood, or perhaps a combination of these. Experiments on these and similar low-density two-dimensional electron systems has opened an entirely new area of research where the strong interaction between electrons provides the dominant energy scale.
Detailed transport measurements are continuing in our laboratory to gain a better understanding of these materials. We are investigating the dynamical response using microwave radiation, as well as the thermopower and thermal conductivity.