The Discovery of Superconductivity

H. K. Onnes, Commun. Phys. Lab.12,120, (1911) H. K. Onnes, Commun. Phys. Lab.12,120, (1911)（专名或术语）

H. Kamerlingh Onnes, after having successfully liquified helium in 1908, investigated the low temperature resistivity of mercury in 1911. The mercury could be made very pure by distillation, and this was important because the resistivity at low temperatures tends to be dominated by impurity effects. He found that the resistivity suddenly dropped to zero at 4.2K, a phase transition to a zero resistance state. This phenomenon was called superconductivity, and the temperature at which it occurred is called its critical temperature. Compare to others 与其他对比 Magnetic levitation 磁悬浮 Is the resistance zero, or just small? 电阻是零，还是只是很小？ H. Kamerlingh Onnes 在1908年成功液化氦之后，于1911年研究了汞在低温下的电阻率。汞可以通过蒸馏使其非常纯净，这一点很重要，因为低温下的电阻率往往主要由杂质效应决定。他发现汞的电阻率在4.2K时突然降为零，这标志着一种零电阻状态的相变。这种现象被称为超导性，发生这种现象的温度称为临界温度。

Lead as Superconductor

Evidence for zero resistance

Lead is a Type I superconductor with a critical temperature of 7.2 K. Although such superconductors can conduct currents with zero resistance, their usefulness is limited because of low critical magnetic fields. Above a certain current, the magnetic field created by the current drives the material into a normal resistive state.

If a current is generated in a superconducting lead ring, it will persist because of the zero resistivity. Currents have been maintained in lead rings for several years to test the zero resistance condition. An induced current in an ordinary metal ring would decay rapidly from the dissipation of ordinary resistance, but superconducting rings had exhibited a decay constant of over a billion years!

An exactly zero resistance implies a quantum effect - an energy gap. If the charge carriers do not interact with their environment to reduce their energy even a little bit, it must be because they can't - they are forbidden to by conservation of energy. This implies that there are no available quantum states within reach of the energy they have. The evidence for an energy gap was one of the steps which led to the BCS theory of superconductivity.