Impure spin states

Also in the vapour pressure thermometer, the presence of impurities, isotopes or different spin states must be evidenced and minimized. 

We first note that an isolated atom with an odd number of electrons will necessarily have a magnetic moment. In this book we discuss mainly moments on impurity centres (donors) in semiconductors, which carry one electron, and also the d-shells of transitional-metal ions in compounds, which often carry several In the latter case coupling by Hund s rule will line up all the spins parallel to one another, unless prevented from doing so by crystal-field splitting. Hund s-rule coupling arises because, if a pair of electrons in different orbital states have an antisymmetrical orbital wave function, this wave function vanishes where r12=0 and so the positive contribution to the energy from the term e2/r12 is less than for the symmetrical state. The antisymmetrical orbital state implies a symmetrical spin state, and thus parallel spins and a spin triplet. The two-electron orbital functions of electrons in states with one-electron wave functions a(x) and b(x) are, to first order,... 

At the top of systems proposed for processors of quantum computers, there are systems in which electronic and nuclear spins of various defects and impurities in diamond are used as stationary qubits [1,2]. Single NV-centers having electronic spin S=1 in the ground electronic state are the most promising [3]. To improve optical read-out of such spin-states, various three-dimensional nanostructures in diamond such as micro resonators, waveguides, photon-crystal structures, etc. [1,4,5] are being developed. Besides, the methods of NV-center... 

The high-spin states of the iodine isotopes allow definition of many of the parameters of an iodine impurity atom in a host lattice, and some interesting results are beginning to emerge. 

In matrix-impurity systems in which the matrix is a superconductor and Tg is sufficiently low compared to the critical temperature of the pure host, the temperature dependent scattering of conduction electrons by impurity spins may even lead to the striking phenomenon of re-entrant superconductivity (where alloys within a certain impurity concentration range exhibit a transition to the superconducting state at a critical temperature Tg, which is followed by a return to the normal state at a second lower critical temperature as well as pronounced deviations of the specific heat jump from the BCS law of corresponding states. 

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