Understanding Type I & II Superconductors and La2CuO4 in High TC Copper Oxides - Prof. R. , Assignments of Materials science

Download Understanding Type I & II Superconductors and La2CuO4 in High TC Copper Oxides - Prof. R. and more Assignments Materials science in PDF only on Docsity! Materials 218/UCSB: Class XVII: High TC copper oxide superconductors (continued): Ram Seshadri (seshadri@mrl.ucsb.edu) • Type I and type II superconductors: superconducting superconducting H H HC HC1 HC2 Type IIType I vortex state 4πM 4πM In type I superconductors (pure metals), the superconductivity is destroyed by small magnetic fields; above some critical magnetic field HC , the material is no longer superconducting. Typical critical fields are of a 1000 Oe. In type II superconductors, above the first critical field HC1, the material is no longer a pure su- perconductor. It is in what is called the vortex state, comprising regions that are superconducting, interspersed with “flux tubes” that are normal (not superconducting). The magnetic field can penetrate the superconducting sample through the flux tubes, which often form an ordered lattice. flux tube H HC1 HC1 H HC2 1 Examples of type II superconductor are most superconducting alloys and intermetallics (such as the so-called A15 compounds) and the high TC copper oxide superconductors. The record for the second highest critical field HC2 is held by the Chevrel phase PbMo6S8 of 54 T at 4 K. Such materials are good candidates for forming superconducting solenoids. • La2−xAxCuO4 (A = Sr, Ba etc.): The compound La2CuO4 has the K2NiF4 structure, which is the structure of the n = 1 Ruddlesden- Popper phase (SrTiO3)1(SrO). A sketch as well as the “real” structure are shown below: La “LaCuO3” The large spheres are the La3+ ions. The formula suggests that in La2CuO4, Cu is in the +2 oxidation state. Cu2+ has 9 d electrons. Since it is octahedral, we would expect the crystal field diagram and the DOS to look as follows: Jahn-Teller distorted elongated O Cu dz2 dxy dx2 y2 EF dxz
dyz 2