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Tham khảo tài liệu 'applications of high tc superconductivity part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Preparation of Existing and Novel Superconductors using a Spatial Composition Spread Approach 229 magnitude in going from cubic Celn3 to its tetragonal analogues CeMIn5 M Rh Ir or Co as anticipated by the magnetic interaction model Monthoux Lonzarich 2002 . Thus in the search for higher temperature superconductors one should explore the border of antiferromagnetism in a quasi two-dimensional tetragonal system with high characteristic spin fluctuation frequencies. The conditions favourable for magnetic pairing include i strong quasi two-dimensional antiferromagnetic correlations large J for spin singlet pairing and for large amplitude oscillations of the spin-spin interaction gives small correlation length which is inversely proportional to Tc ii a single band of relatively high characteristic energy scale and iii a crystal structure that enables the repulsive regions of the pairing potential to be optimally neutralized. Favourable Tc s can be achieved in layered d-electron systems of moderate electron densities n and bandwidths t and can be controlled by chemical doping or hydrostatic pressure Monthoux et al 2007 . One system which satisfies most of these requirements is the perovskite-type single layer compounds of composition A2MX4 and double layer compounds of composition A3M2X7 where A1 K Rb Cs M2 Mg Mn Fe Co Cu Cd and X F Cl or Br see Geick 2001 for a review . In these perovskite-type layer structures the dominant magnetic interaction is the nearest-neighbour Heisenberg exchange within the layers which causes their 2D character. These compounds have a metal ion M surrounded by 6 halides X in an octahedral arrangement. The magnetic properties depend on the intralayer superexchange interaction J mediated by the halide X between two M ions. Theory predicts an exponential dependence of J on the nearest neighbour distance aMXM and experiments find a power law dependence J ann J ann 0 ann ann 0 12for small ann. A classic perovskite layer compound is La2CuO4 X O2- M
