pdf, 9 MiB - Infoscience - EPFL

Chapter 1
Introduction
1.1 History of high temperature superconductivity
In 1986 superconductivity was found in a range of temperature which was well
above all previous signature: Bednorz and Müller found a transition temperature
of about 30K [1] in the layered copper-oxide material La 2−x Ba x CuO 4 ,forwhich
they won the Nobel Prize in Physics. The psychological barrier of the liquid
Nitrogen condensation temperature was crossed shortly after with YBaCu 3 O 7
and the hope was growing that one might find an increasing number of every-day
life applications.
Since then, copper-oxide superconductors, which are structural derivatives of
the class of perovskites, have been at the center of a tremendous scientific activity.
A number of high-temperature superconducting compounds has been reported.
Starting out at T c =30K for La 2−x Ba x CuO 4 in 1986 the transition temperatures
have climbed to T c = 156K in members of the HgBa 2 CuO 4+x family which are
among the recently discovered compounds. Moreover, new materials found more
recently like the mercury-type copper-oxides still increased accessible transition
temperatures without any obvious upper bound, and a critical temperature as
high as 164K was reported [2], though it was obtained for a compound under
pressure. In the meantime, progress in the preparation of high-quality single
crystals has allowed to remove many of the uncertainties in the interpretation of
experimental data obtained from polycrystals. In Fig. 1.1 we show the lattice
structure of an YBCO perovskite structure. Despite the apparent complexity of
the structures of the different cuprates compounds, they all have two dimensional
CuO 2 planes. In essence all the high-temperature superconductors (HTCS) consist
of two-dimensional CuO 2 planes which are sandwiched between intervening
atomic layers. These layers are composed mostly out of alkaline-earths, rareearths,
oxygen and halogenides. Depending on the number of CuO 2 planes per
unit cell the materials have a single , a double-, or a triple-plane form as in
15