At temperatures that approach absolute zero and in strong magnetic fields, electrons start to behave very strangely. As carriers of electric current, they appear to possess only a fraction of their normal charge. They can be made to travel as waves in quantum wires, and can be bound into new, artificial atoms called quantum dots. They can even enter superfluid states where they seem to move without friction or resistance.
Two new tenured faculty at Columbia University, Aron Pinczuk and Horst Stormer, are exploring these and other behaviors as they research the fundamental properties of semiconductors. The pair have conducted their research at Bell Labs, the research and development arm of Lucent Technologies Corp., and will remain affiliated with the laboratory.
Professor Pinczuk is known as a leading experimentalist of inelastic light scattering in semiconductors and Professor Stormer as a co-discoverer of the fractional quantum Hall effect. Each accepted a joint appointment in the Department of Physics and the Department of Applied Physics effective January 1, and will be known as 'professor of applied physics and physics.'
Both work in the field of condensed matter physics, the study of condensed phases of matter such as solids and liquids. The field has grown into the largest specialty within physics, with tremendous intellectual and technological importance. The two new faculty members add important new strength to Columbia's program in condensed matter physics and material science and will begin teaching advanced physics topics in the fall 1998 semester at both the graduate and undergraduate levels.
It is the unique properties of semiconductors, the materials from which transistors are made, that lie at the heart of the computer revolution. Millions of interconnected transistors, each switching on and off hundreds of millions of times per second, provide the semiconductor "brain" controlling desktop personal computers as well as the fastest supercomputers. At present, semiconductors are used to switch electric currents, but a new generation of optoelectronic materials is being developed that can switch light instead of electricity, offering even higher levels of speed and miniaturization.
The two physicists are investigating the fundamental properties of modern semiconductor structures, research that may eventually help create improved electronic devices such as computer chips or optoelectronic devices such as solid-state lasers. Their research focuses on structures at the scale of nanometers, or billionths of a meter, that are only a few hundred atoms across.
By creating and studying systems of this size, these physicists are able to develop new methods to unravel the mysteries of the quantum theory of electrons, atoms and light. Structures this small are of immense technological importance because they will be capable of faster switching speeds and can be used to construct higher-density computer memories than is now possible.
"We are extremely pleased to have, in Aron Pinczuk and Horst Stormer, two brilliant new lights to add to the physics and applied physics departments at Columbia," said Zvi Galil, dean of the Fu Foundation School of Engineering and Applied Science at Columbia. "Our research and teaching both rise a notch with their arrival, as do our opportunities to collaborate in research projects with Lucent Technologies."
William F. Brinkman, vice president physical sciences research at Lucent's Bell Labs, said, "Bell Labs has a long tradition of collaborating with universities on research and development. We hope the joint appointment will help us establish a strong collaborative program with Columbia faculty and students on the physics of semiconductors and related sciences."
Professors Pinczuk and Stormer study systems in which the current-carrying electrons do not move in all three spatial dimensions throughout the material but are confined to an extremely thin two-dimensional layer or one-dimensional line only a few hundred atoms thick. Electrons can be so confined by building up the semiconductor material in layers in such a way that one layer is different from the rest. This special layer provides a small attraction to the electrons as they move through the material. As the temperature is lowered, moving electrons have less energy and tend to occupy only quantum states located very near the special layer, finally abandoning all other quantum states at temperatures near absolute zero.
Electrons forced to move in such confined structures display new, unexpected behavior, especially if a very intense magnetic field is applied. For example, such confined electrons display a new sort of electrical resistance that is precisely quantized into steps. This strange behavior implies that the fundamental current carriers are no longer individual electrons but instead electrons that as a group carry precisely a third to a fifth of the normal electronic charge. Professor Stormer and colleagues Daniel Tsui at Princeton and Arthur Gossard of the University of California, Santa Barbara, were the first to observe this effect, called the fractional quantum Hall effect.
"Understanding these bizarre properties has been a major success of condensed matter theory during the past decade," Professor Stormer said. "The fractional quantum Hall effect is very important for our understanding of many-particle physics, introducing as it does new ideas that may well have an important future effect on other areas of science.
"While these new phenomena may not directly affect device technology, the structures we use in these experiments are essentially the same as those being used in high-performance current amplifiers."
For example, the same two-dimensional system in which Professor Stormer has unraveled this counterintuitive behavior of electrons is the central component of a metal-oxide semiconductor field-effect transistor, or MOSFET, in which an input signal can control electrical current through the same two- dimensional electron system, called a channel; the device is central to the semiconductor industry. And the identical materials that are being used in Professor Stormer's studies are also used to make high electron mobility transistors, or HEMTs. Such transistors are used as highly-sensitive gatekeepers in many of the new high-frequency, 2-gigahertz cellular phones.
More recently, both condensed matter physicists have turned to the investigation of yet lower-dimensional systems, such as quantum wires embedded in semiconductor materials, where electrons are free to move only back and forth along a line. In such wires, electrons reveal their wave-like nature and travel like light waves down a glass fiber. Physicists believe that by adding mirrors to the ends of the wires, the quantum states of the electrons can be excited, creating a new, very narrow-frequency type of laser. They are also studying quantum dots, fully confined, zero-dimensional systems that act like artificial atoms.
Professor Stormer observed the fractional quantum Hall effect by electrical transport -- simply measuring the flow of electrical current -- while Professor Pinczuk used a different method, inelastic light scattering experiments.
Light scattering involves shining a light source, often a laser, on the medium to be studied and then examining the light that scatters from it. Some light reflects from the medium as from a mirror, but a small amount scatters in many directions, much as the beams from a car's headlights scatter in the fog. Details of the structure of a material can be learned by looking at the directions of the scattering and whether there are any changes in the wavelength -- or color -- of the light. When scientists examine elastic light scattering, they are looking at light that scatters at the same wavelength as the reflected laser beam. When they examine inelastic light scattering, they are looking at light that has its wavelength shifted as a result of scattering.
"One can say that the energy of the photons, or quantized units of light, here produced by a laser, that are reflected from the medium either gain or lose energy from the medium," Professor Pinczuk said. The magnitude of this change in photon energy -- or, equivalently, the shift in the wavelength of the reflected light -- shows scientists critical details of the structure of the material, here semiconductors.
Professor Pinczuk developed light scattering methods to study low-dimensional electron systems in low temperatures and intense magnetic fields. Such experiments have uncovered new behaviors that emerge when electrons condense into exotic liquids with unexpected properties, similar to those of superfluids, such as supercooled helium, which flow with no apparent friction, and superconductors, which conduct electricity with little or no electrical resistance. His research has demonstrated that optical methods can contribute to our understanding of these remarkable states of matter.
Both Professors Stormer and Pinczuk have won the American Physical Society's prestigious Oliver E. Buckley Prize in Condensed Matter Physics, Stormer in 1984 and Pinczuk in 1994.
Professor Pinczuk, who is from Buenos Aires, earned a Ph.D. in physics from the University of Pennsylvania in 1969. After several research and teaching positions in Buenos Aires, he took visiting scientist positions at the Max-Planck-Institut in Stuttgart and then at IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y., before joining the technical staff at AT&T Bell Labs. He is a fellow of the American Physical Society and is the recipient of an honorary doctorate from the Universidad Autonoma of Madrid.
Professor Stormer, a native of Frankfurt, earned a Ph.D. in physics from the University of Stuttgart in 1977, having conducted his thesis research at the Max-Planck-Institut's High Field Magnet Laboratory in Grenoble, France. He joined AT&T Bell Laboratories in Murray Hill, N.J., in 1978 and was appointed director of the Physical Research Laboratory there in 1992. He is a fellow of the American Physical Society and of the American Academy of Arts and Sciences. He will be awarded the Franklin Institute Medal in Physics with Dr. Tsui and Robert Laughlin of Stanford on April 30 in Philadelphia.
Lucent Technologies, headquartered in Murray Hill, N.J., designs, builds and delivers a wide range of public and private networks, communications systems and software, data networking systems, business telephone systems and microelectronics components. Bell Labs is the research and development arm of the company. For more information on Lucent Technologies, visit the company's web site at http://www.lucent.com.
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