ABSTRACT
The superconducting state is one of the most fascinating states of condensed matter. Scientific research within this domain has constantly evolved over the last century. During this period very important discoveries were made and some of these were awarded the Nobel Prize. In 1911, three years after the first sucessful attempt at the liquefaction of helium. H.K. Onnes1 discovered that the electrical resistance of mercury suddenly droped to zero for a certain critical temperature TC below 4.2 kelvins. He called this new phenomenon “superconductivity”. A year later, he observed that the current density values greater than a threshold value, called critical current density JC, restored the resistance of mercury2 and that this phenomenon also occured from a well-defined value of the applied field called a critical field HC3. H.K. Onnes also discovered the superconductivity of tin and lead with critical temperatures of 3.7K and 7.2K, respectively4. In the years that followed, other pure superconducting elements were discovered at very low transition temperatures. The record for the critical temperatures of the pure elements is held by niobium with a transition temperature TC of 9.2K. In 1933, W. Meissner and R. Ochsenfeld5 showed that a superconducting material expels the magnetic field when it is submitted to a weaker magnetic field and cooled to temperatures below its critical temperature. This remarkable phenomenon was called the Meissner effect. In 1935, F. and H. London6 proposed a macroscopic two-fluid model, normal fluid constituted of unpaired electrons and fluid formed by superconducting electrons (an idea originally introduced by C. Gorter and H.B.G. Casimir in 19347). This model explained the Meissner effect and predicted the existence of a characteristic length of the superconducting state: the penetration depth called “penetration depth of London”.
