Module 1 : Introduction
Lecture 1 : Historical review and a survey of properties of superconductors.
 
This was the first ever observation of superconductivity (called supraconductivity by Onnes at that time). He was deciding between gold and mercury for his first measurement and it is fortunate that he chose mercury since gold is not superconducting. Onnes realized the commercial potential of his discovery of superconductivity and began examining other metals. Tin and lead were the next elements to be found superconducting by him. Since the Leiden lab. had a near monopoly in the production of liquid helium, Onnes and his co-workers were the leaders in low-temperature physics for many years. Subsequent to the original discovery of Onnes of zero resistance in Hg at 4.2 K, many new superconductors and allied phenomena were discovered. In one of his experiments Onnes started a current in a loop of lead wire cooled to 4 K. Even after a year the current was still flowing without any noticeable change. This was called a persistent current by Onnes. Kammerlingh Onnes was awarded the Nobel prize in 1913. The citation read that the prize was for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium, though not really for the discovery of superconductivity! Finally Onnes was the person who also found that superconductivity could be destroyed by an applied magnetic field called the critical field. The empirical relation for the temperature variation of the critical field was found to be $H_{c}(T)=H_{c}(0)[1-(\frac{T}{T_{c}})^{2}]$. Here, $T_{c}$(the critical temperature) is the temperature below which the substance is superconducting when no magnetic field is applied and $H_{c}(T)$ is the magnetic field necessary to decrease the critical temperature to $T$.