BCS Theory of Superconductivity turns 50

December 2007 marks the fiftieth anniversary of one of the major scientific theories of the twentieth century, the Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity, formulated by the late ECE and physics professor John Bardeen along with postdoc Leon Cooper and graduate student J. Robert Schrieffer.

“Theory of Superconductivity” by Bardeen, Cooper, and Schrieffer appeared in the December 1, 1957, issue of Physical Review. The classic paper explained the mysterious loss of electrical resistance in certain metals at extremely low temperatures—a puzzle that for several decades had stumped such physics luminaries as Einstein, Bohr, and Feynman. The BCS theory quickly won acceptance among specialists and still stands today as a pillar of the field of condensed matter physics. For the theory, the 1972 Nobel Prize in Physics went to Bardeen, Cooper, and Schrieffer.

The University of Illinois celebrated the birthday with a sold-out conference entitled “BCS@50,” held October 10–13 at the Beckman Institute for Advanced Science and Technology. Among the many Nobel laureates in attendance were Cooper and Schrieffer. The American Physical Society commemorated the BCS theory by designating the old Physics Building (now the Materials Science and Engineering Building) as a site of historical significance to physics. Only 12 sites nationwide have been so honored by APS.

Bardeen had been fascinated by superconductivity his entire professional life, but wartime research during WW II and subsequent work on the transistor while at Bell Labs (for which he earned his first Nobel Prize in 1956) kept him from giving sustained attention to the problem. Thanks to the enlightened leadership of Physics Department head Wheeler Loomis and College of Engineering dean William Everitt, Bardeen, a Wisconsin native, came home to the Midwest in 1951 as a professor of ECE and physics at Illinois, where he remained until his death in 1991 at age 82.

Early on, Bardeen established a semiconductor lab in ECE’s old Electrical Engineering Research Laboratory, where his first graduate student was Nick Holonyak, Jr, the future LED inventor and decorated ECE faculty member. But Bardeen gradually relinquished oversight of the semiconductor lab in order to devote more time and energy to exploring superconductivity with physics colleagues such as experimentalist Charles Slichter and theorist David Pines. Schrieffer enrolled as a graduate student under Bardeen in 1953 and Cooper came on board as a postdoc in 1955, completing the team that would crack one of physics’ toughest riddles.

Bardeen set the direction for the group by framing the problem around the existence of an energy gap between the normal and the superconducting states of the metal. The gap seemed to arise from the interaction of electrons and phonons, the quantized acoustic vibrations of the crystal lattice, but no one could identify the precise mechanism. It was Cooper in early 1956 who accounted for the energy gap with his notion of “Cooper pairs,” electron pairs created when the attractive force caused by the electron-phonon interaction dominated the normal repelling force, or Coulomb force. Nearly a year later, Schrieffer convinced Cooper’s skeptics (including Bardeen) by calculating a wave function for the Cooper pairs that satisfied the requirements of quantum theory. The team spent the next several months deeply immersed in checking, fleshing out, and refining the BCS theory, before submitting their paper in July 1957.

Fifty years later, BCS is the model for the patient, collaborative, and experimentally based way of doing physics that has come to be called the “Urbana style.” Aspects of the theory have been adopted to solve fundamental problems of particle physicists. The technological impact of the theory has been felt in fields ranging from medicine to astronomy to transportation.