Decoherence processes of a quantum two-level system coupled to a fermionic environment

We study decoherence processes of an S = 1/2 localized spin coupled to conduction band electrons in a metal or a semiconductor via an Ising-like interaction. We derive master equations for the density matrix of the localized spin, by tracing out all degrees of freedom in the conduction electron system based on the linked-cluster-expansion technique. It is found that the decoherence occurs more rapidly for the metallic case than for semiconducting case.Comment: to appear in J. Appl. Phy

Ab-initio Study on the Magnetic Structures in the Ordered Mn3Pt Alloy

We study the electronic states of the magnetically ordered Mn3Pt alloy within the density functional theory. Mn3Pt has been believed that one third of Mn atoms have no magnetic moment in an antiferromagnetic phase (so-called the F-phase) realized in the temperature range of 400 K < T < 475 K. We show that this experimentally suggested spin configuration is energetically so much unfavorable that it would be irrelevant to the F-phase. We discuss the possibility that the spin moments on the one third of Mn atoms are not paramagnetic but thermally fluctuating in the F-phase. The present results have an immediate connection with the recent neutron scattering study [T. Ikeda and Y. Tsunoda, J. Phys. Soc. Jpn., vol. 72, pp. 2614-2621, October. 2003.].Comment: 4 pages, 4 figure

Spin-polarized electronic structures and transport properties of Fe-Co alloys

The electrical resistivities of Fe-Co alloys owing to random alloy disorder are calculated using the Kubo-Greenwood formula. The obtained electrical esistivities agree well with experimental data quantitatively at low temperature. The spin-polarization of Fe50Co50 estimated from the conductivity (86%) has opposite sign to that from the densities of the states at the Fermi level (-73%). It is found that the conductivity is governed mainly by s-electrons, and the s-electrons in the minority spin states are less conductive due to strong scattering by the large densities of the states of d-electrons than the majority spin electrons.Comment: 3 pages, 4 figure

Local density of states around a magnetic impurity in high-Tc superconductors based on the t-J model

The local density of states (LDOS) around a magnetic impurity in high-Tc superconductors is studied using the two-dimensional t-J model with a realistic band structure. The order parameters are determined in a self-consistent way within the Gutzwiller approximation and the Bogoliubov-de Gennes theory. In sharp contrast with the nonmagnetic impurity case, the LDOS near the magnetic impurity shows two resonance peaks reflecting the presence of spin-dependent resonance states. It is also shown that these resonance states are approximately localized around the impurity. The present results have an large implication on the scanning tunneling spectroscopy observation of Bi_{2}Sr_{2}Ca(Cu_{1-x}Ni[Zn]_{x})_{2}O_{8+delta}.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let