Happy Birthyday, Bardeen

Today is the birthday physicist John Bardeen, born in Madison, Wisconsin in 1908. Before he came along, the only way to transmit and amplify electric impulses was through a vacuum tube, a large glass device, which was bulky, fragile, expensive, and used a lot of energy. Plus it took awhile for the filaments to heat up, so if you turned on a radio powered by a vacuum tube, you had to wait awhile for it to get up and running.

After he got a master's in electric engineering, taking five years to do that plus his BS, he went to work for Gulf Research Laboratories, a division of Gulf Oil Company. He worked on methods to do gravitational and magnetic surveys that would give hints about what might be underground, but the work bored him. After a few years, he applied and was accepted to the graduate school of mathematics at Princeton University.

There he worked with Eugene Wigner, a major figure for us physicists. He was the first to apply methods of a branch of mathematics called group theory, essentially the study of symmetries, to quantum mechanics. This work was foundational to later developments in quantum field theory. Bardeen, however, worked on a problem is solid state physics, which was to be the focus of much of his professional life.

After getting his PhD in mathematical physics in 1936, he was a fellow at Harvard for three years, working for two future Nobel Prize winners. After that, he spent a few years at the University of Minnesota until war broke out. During the war, he worked at the Naval Ordnance Laboratory, working on magnetic mines and torpedoes as well as countermeasures for them.

In October of 1945, he went to Bell Labs and joined a solid state group headed by William Shockley. They were looking for an alternative to vacuum tubes for amplifiers. Those of you who are my age will remember the glowing tubes inside TVs and radios. They didn't work until the filaments heated up so there was always a time delay between switching it on and getting to listen to something, and they had fundamental limits on their ability to amplify signals.

The group's efforts failed mysteriously time after time until Bardeen suggested that perhaps electrons operated differently on the surface of a metal than they did in the interior. The group changed its focus to investigate surface states. Bardeen and his friend Walter Brattain created the first transistor.

The critical experiment, carried out on December 16, 1947, consisted of a block of germanium, a semiconductor, with two very closely spaced gold contacts held against it by a spring. Brattain attached a small strip of gold foil over the point of a plastic triangle — a configuration which is essentially a point-contact diode. He then carefully sliced through the gold at the tip of the triangle. This produced two electrically isolated gold contacts very close to each other.

The piece of germanium used had a surface layer with an excess of electrons. When an electric signal traveled in through the gold foil, it removed electrons, creating what are called holes (points which lack electrons). This created a thin layer which had a scarcity of electrons, limiting its ability to conduct a signal.

A small positive current applied to one of the two contacts had an influence on the current which flowed between the other contact and the base upon which the block of germanium was mounted. In fact, a small change in the first contact current caused a greater change in the second contact current, thus it was an amplifier.

Shockley publicly took the lion's share of the credit for the invention of transistor; this led to a deterioration of Bardeen's relationship with Shockley. Bell Labs management, however, consistently presented all three inventors as a team. Shockley eventually infuriated and alienated Bardeen and Brattain, and he essentially blocked the two from further work on the transistor. Bardeen began pursuing a theory for superconductivity and left Bell Labs in 1951. Brattain refused to work with Shockley further and was assigned to another group.

Shockley, Bardeen and Brattain were jointly awarded the Nobel Prize in Physics in 1956 for the discovery of the "transconductance resistor" or transistor. Bardeen found out about the Nobel by hearing it on the radio while he was making eggs in the kitchen, and he was so excited that he dropped the frying pan on the floor.

Bardeen left Bell Labs to join the University of Illinois at Urbana-Champagne, with a joint appointment in Electrical Engineering and Physics. There he pursued his interest in superconductivity.

Superconductivity appears in some metals at very low temperatures, hundereds of degrees below zero, when electrical resistance suddenly disappears. If maintaineed at that temperature, just a few degrees above absolute zero, a current can run forever. If you've ever had an MRI, you've been inside a superconducting electromagnet.

It had been discovered by Heike Kammerlingh Onnes in1911, winning him a Nobel Prize, but no one really understood how it worked or why it even existed. Together with his Illinois colleague Leon Cooper and graduate student John Schrieffer. They developed the BCS theory, in which electrons pair up by interacting via vibrations in the crystal lattice of the metal and move collectively even if they are separated by great distances.

An electron moving through a conductor will attract nearby positive charges in the lattice. This deformation of the lattice causes another electron, with opposite spin, to move into the region of higher positive charge density. The two electrons then become correlated. Because there are a lot of such electron pairs in a superconductor, these pairs overlap very strongly and form a highly collective condensate. In this "condensed" state, the breaking of one pair will change the energy of the entire condensate - not just a single electron, or a single pair. Thus, the energy required to break any single pair is related to the energy required to break all of the pairs. Because the pairing increases this energy barrier, kicks from vibrating atoms in the conductor (which are small at sufficiently low temperatures) are not enough to affect the condensate as a whole, or any individual "member pair" within the condensate. Thus the electrons stay paired together and resist all kicks, and the electron flow as a whole (the current through the superconductor) will not experience resistance. Thus, the collective behavior of the condensate is a crucial ingredient necessary for superconductivity.

This was a major breakthrough in understanding superconductivity. It was published in 1957, the year after Bardeen won the Nobel Prize. In 1972, Bardeen, Cooper and Schrieffer were awarded the Nobel Prize in Physics for this work, making John Bardeen the only person ever to win the prize twice.

Bardeen died of heart disease in 1991, and the University of Illinoise named the Engineering Quadrangle in his honor, the Bardeen Triangle. A year before his death, the Sony Corporation, which basically wouldn't exist without the transistor, endowed a #3 million John Bardeen Chair at Illinois in Electrical and Computer Engineering and Physics. The current John Bardeen Professor is one of his former PhD students, Nick Holnyak.