Superconductivity Physics 355

Introduction The topic of superconductivity brings together many of the topics we’ve covered: phonons structure magnetism metals electrons Band Structure Resistivity temperature

Discovery Onnes and van der Waals He became interested in investigating the electrical properties of pure metals in this newly available region of low temperatures. The question was: will the resistance increase, or decrease or remain constant when cooling the samples? He decided to work with mercury which can be repeatedly distilled at room temperature in order to obtain a pure sample. H. Kamerlingh Onnes was the first to liquefy Helium.

Discovery Some scientists, such as William Kelvin, believed that electrons flowing through a conductor would come to a complete halt as the temperature approached absolute zero. Other scientists, including Onnes, felt that a cold wire's resistance would dissipate. This suggested that there would be a steady decrease in electrical resistance, allowing for better conduction of electricity. At some very low temperature point, scientists felt that there would be a leveling off as the resistance reached some ill-defined minimum value allowing the current to flow with little or no resistance. Onnes passed a current through a very pure mercury wire and measured its resistance as he steadily lowered the temperature. Much to his surprise there was no leveling off of resistance, let alone the stopping of electrons as suggested by Kelvin. At 4.2 K the resistance suddenly vanished. Current was flowing through the mercury wire and nothing was stopping it, the resistance was zero.

Onnes wrote of his discovery: Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state. Discovery

Exploration

Element T C (K) Al1.2 In3.4 La4.8 Pb7.2 Nb9.3 Sn3.7 V5.4 Transitions of some Pure elements… But is the resistance really zero?

Exploration I I Use Biot-Savart Law to find the current: If you then make measurements of the field each day over several weeks, you would find that the current remains approximately the same value.

Exploration With zero resistance, we might expect to see no decrease in the current as time passes, but due to phenomena that we’ll see later, called flux creep and flux flow, there is a decay that may be observed. In an experiment, you might start the current and then monitor the field over several weeks. What you observe is that the current goes as where R is the effective resistance, L is the self-inductance of the ring, and t is the elapsed time. In one such experiment with L = 5 nH, the effective resistance was found to be about 10  15 . Using this value for the resistance, the time to reach one- half of the initial current would be more than years. This indicates that for most practical purposes, the current is permanently stored in the ring superconductor.

Exploration