The Studies On D~0 Half-metallic Magnetism And Electric Characteristics Of Josephson Junction

Half-metallic (HM) magnets are seen as the most promising candidates of spin-injector materials due to the complete (100%) spin-polarization of conduction electrons around the Fermi level, and thus HM magnets are the key materials of the realization of spintronic devices. HM magnets are named HM ferromagnets and HM antiferromagnets (precisely called HM fully compensated ferrimagnets) when the total magnetic moment is non zero and zero, respectively. HM antiferromagnets are more meaningful in practice spintronic application due to their zero magnetization and thus little effect by external fields.Because of Josephson junetion is one kind of typical weak link superconductors, it has high non-linear characteristies, sush as chaotic property, the nonlinearⅠ-Ⅴrelationship, and so on. Josephson junctions have wide application fields, it has been used in many values such as military, medicine, communication technology, geology exploration as well as computer science.In this thesis, using the pseudopotential plane-wave method based on density functional theory, we perform calculational studies on theⅣ-Ⅰ-Ⅱcompounds with half-Heusler structure and the ordered Cr1-xCaxSb alloy. Furthermore, the effects of fluctuations on the voltage-current characteristics of Josephson junction are studied by using the Langevin theory based on Brownian motion.The study on the half-HeuslerⅣ-Ⅰ-Ⅱcompounds (IV=C, Si, Ge, Sn; I=Li, Na, K; II=Mg, Ca) indicates that most of them are exclent d0 HM ferromagnets, and the magnetic moment mainly originates from the p electrons of IV elements. This is the first prediction of d0 HM ferromagnets in half-Heusler alloys. We also find the d0 HM ferromagnetism is preserved when the lattice constant is compressed and expanded. The strong Coulomb interaction does not destroy the d0 HM ferromagnetism of CKCa, but the d0 HM ferromagnetism of GeKCa and SnKCa loses when the Coulomb interaction U is larger than 2eV.The ordered alloys of Cr1-xCaxSb with x=0,0.125,0.25,0.5,0.75 and 1 all exhibit half-metallicity. However, for x=0.75, i.e., CrCa3Sb4, besides the half-metalicity, the total magnetic moment is zeroμB, and thus CrCa3Sb4 is a HM antiferromagnet. The Cr d and Sb p electrons carried the main magnetic moment, and there are fully compensated ferromagnetic interactions between Cr and Sb, Ca. We also find that the HM antiferromagnetism of CrCa3Sb4 is robust against the lattice compression and expansion and the choice of different electronic exchange and correlation functional.The study of the effects of fluctuations on the Josephson junction shows that the effects of the white noise are opposite to the O-U noise in the Linear responses of driving current of Josephson junction. In the case of small capacitance, the effects of the O-U noise on the current-voltage characteristics of Josephson are discussed. When the noise is dealed with colored noise, we find that the average normalization voltage of the Josephson junction decreases because of the correlation time of noises. The effects of external fluctuations play an important role in many realistic systems. And thus the effects of external noises on dc current-voltage characteristic of the Josephson junction are studied. Considering the effects of fluctuations in Josephson junction, the curve of voltage-current characteristics is rounded. Zero- voltage current increases with increasing multiplicative noise intensity, but rounding curve of voltage-current characteristics in Josephson junction will vanish when the thermal fluctuations parameter is less than a certainty numerical value.