Introduction

Abstract

One of the most exciting and fascinating fields in condensed matter physics is high-temperature and unconventional superconductivity, for example in hole- and electron-doped cuprates, in $\mbox{Sr}_2\mbox{RuO}_4$, in organic superconductors, in $\mbox{MgB}_2$, and in $C_{60}$ compounds. In cuprates, the highest transition temperature (without application of pressure) $T_c \simeq 134$ K has been measured in $\mbox{HgBa}_2\mbox{Ca}_2\mbox{Cu}_3\mbox{O}_{8+\delta}$, followed by -- to name just a few -- $\mbox{Bi}_2\mbox{Sr}_2\mbox{CaCu}_2\mbox{O}_{8+\delta}$ ($\delta=0.15\leftrightarrow T_c \simeq 95$ K), $\mbox{YBa}_2\mbox{Cu}_3\mbox{O}_{6+x}$ ($x=0.93\leftrightarrow T_c \simeq 93$ K), Nd$_{2-x}$Ce$_{x}$CuO$_4$ ($x=0.15\leftrightarrow T_c \simeq 24$ K), and $\mbox{La}_{2-x}\mbox{Sr}_x\mbox{CuO}_4$, where, for an optimum doping concentration $x=0.15$, a maximum value of $T_c \simeq 39$ K occurs. Since 77 K is the boiling temperature of nitrogen, it is now possible that new technologies, based for example on SQUIDs (superconducting quantum interference devices) or Josephson integrated circuits [1], might be developed. However, at present, the critical current densities are still not high enough for most technology applications. A recent overview an account of the possible prospects can be found in [2] and references therein. ...