Daniel C. Ralph

Research Group Web Pages

Selected Publications

Nanostructures and Quantum Transport

Studies of nm-scale devices. Magnetism and Superconductivity. Development of new nanofabrication techniques. Ph.D., Cornell University, 1993.

Quantized energy levels in metal nanoparticles

When conduction electrons are confined within a metal particle smaller than about 10 nm in diameter, it is not appropriate to use the usual assumption that the electronic energy levels inside the metal form a continuum of states. The energy spacing between the "electrons-in-a-box" quantum levels can be greater than the thermal energy, kBT, at low temperature, so that the energy levels must be considered discrete. We have developed a technique by which the spectrum of levels inside a single metal particle can be measured for the first time. This is useful because it provides a powerful, fundamentally new probe into the physics of the interacting electrons inside a metal. The effects of a wide variety of different forces which determine the electronic properties can now be examined directly at the level of individual eigenstates, as in atomic physics. Nanoparticles also provide a well-controlled model system for understanding the dynamics of electron flow in any nanometer-scale system with discrete energy levels. We have recently completed studies of magnetic interactions in cobalt nanoparticles; spin-orbit coupling in copper, silver, and gold; and the effects of non-equilibrium excitations that arise under conditions of current flow. Experiments under way include studies of the anisotropic response of energy levels to the direction of an applied magnetic field, studies of spin-dependent electron tunneling through individual states, and the physics of magnetic reversal in nanometer-scale ferromagnets.

Manipulating nanomagnets with spin-polarized currents

In ferromagnetic devices such as magnetic memory elements, the direction of the magnetic moments is ordinarily controlled by the application of magnetic fields. We are investigating a second, alternative approach, whereby a current of spin-polarized electrons scatters from a ferromagnetic element, and in the process the current applies a torque directly to the magnetic moment by means of the magnetic exchange interaction. We have demonstrated that this torque can be used to produce magnetic switching -- reversibly switching two magnetic layers between parallel and antiparallel configurations depending on the direction of the current. In an applied magnetic field, the same effect can also be used to generate high-frequency magnetic precession under the application of a DC electrical current. We are currently investigating the microscopic mechanism which gives rise to the torque, and trying to understand the resulting dynamics of the magnets for use in magnetic memory technologies and high-speed signal processing. (This work is being done in collaboration with the groups of Bob Buhrman and Piet Brouwer.)

Molecular Electronics

The ultimate limit of small-scale electronics would be the ability to control electron flow through individual molecules, assembled into structures capable of performing useful tasks. While this field is very far from any useful applications, there are many interesting puzzles associated with the processes that affect electron motion in molecules. We have developed techniques to make electrical connections to single molecules and to assemblies of molecules in order to conduct systematic studies of electron transport. We have succeeded in forming single-electron transistor devices in which electron flow is controlled through one well-defined atom in a single molecule. We are also incorporating nanocrystals of organic molecules such as pentacene into single-electron transistors, so that we can perform spectroscopic studies of the active electronic states and the processes by which electrons traverse interfaces between metals and organic crystals. (Collaborations with the groups of Paul McEuen, Hector Abruna, George Malliaras, Jim Sethna, and Paul Houston).

Advanced scanning-probe microscopies

Scanning-probe microscopy is an extremely useful technique for imaging sample topology (STM or AFM), forces (AFM), magnetic structure (MFM), electric fields (EFM), and a variety of other quantities. To extend this technique even further, we are developing a general process for fabricating structures and electronic devices on the 10-nm size scale directly on the apex of scanning tips. These devices will serve as scannable sensors and probes to provide new types of imaging capabilities. The idea is to form the structures by depositing metal directly onto the tip through a nm-scale stencil structure. Our plans include the deposition of well-controlled magnetic particles for improved magnetic imaging, a photon antenna for improved near-field optical microscopy, single-electron transistors for electrical imaging, and electrically-separate metal electrodes on one tip for gated STM studies.

Other new nanofabrication techniques

Research in our group is driven by the development of new techniques for fabricating small devices with interesting physical properties. Therefore a considerable fraction of our effort is devoted to experimenting with novel ways to make and characterize nanostructures. The presence of the Cornell Nanofabrication Facility on campus is an invaluable resource. We are actively pursuing projects based on advanced electron-beam lithography, chemical self-assembly techniques, mechanically-tunable break junctions, modification using scanning probes, electrodeposition, and the use of electromigration to sculpt wires on the nanoscale.

I encourage graduate students and undergraduates who wish to find out more about doing research in the group to come talk with me.

Last modified: January 17, 2007

Douglas E. Milton, Sr., dem8@cornell.edu