Magnetism and transport properties of super-structure of diluted magnetic semiconductors

通过格林函数方法和相干势近似的第一性原理计算，发现了新型稀磁半导体的可能性。还开发了计算超结构输运性质的计算机程序。1)计算了II-IV-V2和I-III-VI2黄铜矿稀磁半导体的电子结构，并估算了其铁磁转变温度。结果表明，无缺陷的(Cd，Mn)GEP_2和(Zn，Mn)GEP_2不具有铁磁性。这些系统的基态是类似自旋玻璃的磁态。然而，计算表明，相同的系统，但含有不同类型的缺陷，成为铁磁性。这意味着实验观察到的铁磁性是由于晶格缺陷或Mn原子占据…更多的Ge位，而不是Cd或Mn位。还发现I-III-VI2黄铜矿，(Cu，Tm)AlS2，其中Tm为V或Cr时表现出较高的铁磁转变温度。发展了Ni/Cu/绝缘体/Ni型超结构的输运性质理论。计算了这些样品通过理想引线的隧穿电流，并研究了它与铜层厚度的关系。研究发现，当绝缘层的势垒高度足够大时，隧道磁阻随铜层厚度的增加而振荡变化。这似乎解释了实验上观察到的使用Co而不是Ni的类似结构的量子振荡。然而，计算未能再现振荡幅度的衰减。这一失败是由于忽略了在铜层中引起的散射过程，这意味着定量讨论可能需要更复杂的投资，如相干势近似。3)屏蔽KKR方法的计算机程序已经开发出来，它使我们能够计算大尺度结构的输运性质。较少

The possibility of new types of diluted magnetic semiconductors has been found through first-principies calculation using the Green's function method and the coherent potential approximation. The computer code that calculates the transport properties of super structures have been also developed. By use of this method the tunneling magneto resistance of realistic systems were calculated and compared with the experimental observations1) The electronic structure of II-IV-V2 and I-III-VI2 chalcopyrite type diluted magnetic semiconductors was calculated and the ferromagnetic transition temperature was estimated. The results show that (Cd,Mn)GeP_2 and (Zn,Mn)GeP_2 free from any defects never shows ferromagnetism. The ground state of these systems is spin-glass like magnetic state. However the calculation shows that the same system but with various types of defects becomes ferromagnetic. This implies that the experimentally observed ferromagnetism is due to lattice defects or Mn atoms occupyi … More ng Ge sites instead of Cd or Mn sites. It was also found that I-III-VI2 chalcopyrite, (Cu,TM)AlS_2, where TM is V or Cr showed rather high ferromagnetic transition temperature.) The theory of transport properties of the super structure of Ni/Cu/insulator/Ni type was developed. The tunneling current flow across these samples through ideal leads is calculated and its dependency on the thickness of the Cu layer is investigated. It was found that the tunneling magneto resistance varies oscillatory as the thickness of the Cu layer was increased when the barrier height due to the insulator layer was big enough.. This seems to explain the experimentally observed quantum oscillation of the similar structure using Co instead of Ni. The calculation, however, failed to reproduce the decay of the amplitude of the oscillation. This failure is due the neglect of scattering process caused in the Cu layer, implying more sophisticated vestment such as the coherent potential approximation might be necessary for quantitative discussion.3) The computer code for screened KKR method, which enables us to calculate the transport properties of structures of large scale, has been developed The test run has been completed and we are ready to use the code for applications. Less