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What Can Einstein's Electrons Tell Us About Superconductivity?

Kyle Shen
Applied Physics
Stanford University
November 2002

Trying to predict the motions of a pedestrian jostling through rush-hour Manhattan might seem impossible, but it pales to the problem of understanding how trillions of sub-atomic electrons move and bump into each other in a metal. Sometimes these collisions between electrons can even give rise to unexpected and extraordinary phenomena such as superconductivity - where an electrical current can flow with absolutely zero resistance. Because these electron interactions are remarkably complex, it is necessary to perform experiments to help us develop a theoretical grasp of the relevant physics. Additionally, this understanding will help in developing applications involving superconductivity which could revolutionize electronics, power transmission, medical technologies, and even levitation. To help ascertain the driving mechanism behind superconductivity, I use the "photoelectric effect", first explained by Einstein a century ago, to study how electrons move and interact in a superconductor.

If you wanted to learn about some object, you might perform simple experiments such as shaking it or taking it apart. This is impossible on an atomic scale, so physicists must resort to other tricks. My technique involves shining intense beams of ultraviolet light on materials and measuring the electrons which are kicked off - photoemitted - otherwise known as Einstein's photoelectric effect. The particular materials I study are called "high-temperature superconductors", and are particularly interesting for two reasons. First, the relatively high temperatures at which they superconduct make them the most practical materials for applications. Secondly, their underlying physics remains very mysterious. In fact, a Nobel Prize is practically guaranteed to whoever can explain the mechanism behind high-temperature superconductivity.

Superconductivity was first discovered almost a century ago in common metals, such as lead and tin, when they were cooled down to temperatures barely above "absolute zero" (-460 F), almost as cold as outer space. Such extremely low temperatures made potential applications impractical. Then in 1987, the high-temperature superconductors were accidentally discovered; this class of materials became superconducting at temperatures nearly ten times higher than the earlier materials, and led to a Nobel Prize awarded the same year. This breakthrough touched off a flurry of activity and research to try to understand what makes these high-temperature superconductors so special. Despite the fact that the microscopic principles governing the interactions between individual electrons, known as quantum mechanics, have been known for decades, physicists are often astonished and baffled by phenomena such as high-temperature superconductivity. Why is this? Take for example the game of chess, whose rules can be explained in a few minutes but whose mastery can take decades. This mastery is only achieved upon developing a sophisticated understanding of the game's subtleties which are not evident from the rules themselves, but emerge from the collective interactions and motions of the pieces on the board. The rules of quantum mechanics are far more complex than those of chess, and instead of 32 pieces, we deal with trillions of atoms and electrons. The wonderful and bizarre principles of quantum mechanics, which give rise to beautiful phenomena such as superconductivity, unfortunately make calculations incredibly difficult. Even the largest supercomputers can barely make a dent on simple problems involving just a few electrons, to say nothing of billions or trillions of electrons. When attacking the problem with pen, paper, and Pentiums fails, physicists need to rely on experiments for enlightenment.

In the photoelectric effect, an electron inside some material can absorb a bundle of light and even jump out of the surface. As it flies off the material, we can measure the electron's speed and direction using a sophisticated detector. From this information, we can deduce their previous motions inside the material, before they were photoemitted and even reconstruct how they were originally scattering and bumping into each other. Measuring these interactions is crucial, as there is something special about these collisions that gives rise to superconductivity. To perform our experiments, we use a football field-sized particle accelerator, called a synchrotron (located inside SLAC) which emits an extraordinarily powerful, laser-like beam of x-rays and ultraviolet light, which we use to knock the electrons out of the superconductor.

Ultimately, thousands of researchers worldwide are making experimental and theoretical contributions to solving the mystery of high-temperature superconductivity. Our research will comprises key piece of the puzzle, and when we finally unlock the secret to high-temperature superconductivity, we will not only have solved one of the greatest problems in modern physics, but will hopefully have developed new and exciting technologies along the way.