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Byers-Yang theorem
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Byers-Yang theorem : ウィキペディア英語版
Byers-Yang theorem
In quantum mechanics, the Byers-Yang theorem states that all physical properties of a doubly connected system (an annulus) enclosing a magnetic flux \Phi through the opening are periodic in the flux with period \Phi_0=h/e (the magnetic flux quantum). The theorem was first stated and proven by Nina Byers and Chen-Ning Yang (1961), and further developed by Felix Bloch (1970).
==Proof==
An enclosed flux \Phi corresponds to a vector potential A(r) inside the annulus with a line integral \oint_C A\cdot dl=\Phi along any path C that circulates around once. One can try to eliminate this vector potential by the gauge transformation
:\psi'(\)=\exp\left(\frac\sum_j\chi(r_j)\right)\psi(\)
of the wave function \psi(\) of electrons at positions r_1,r_2,\ldots. The gauge-transformed wave function satisfies the same Schrödinger equation as the original wave function, but with a different magnetic vector potential A'(r)=A(r)+\nabla\chi(r). It is assumed that the electrons experience zero magnetic field B(r)=\nabla\times A(r)=0 at all points r inside the annulus, the field being nonzero only within the opening (where there are no electrons). It is then always possible to find a function \chi(r) such that A'(r)=0 inside the annulus, so one would conclude that the system with enclosed flux \Phi is equivalent to a system with zero enclosed flux.
However, for any arbitrary \Phi the gauge transformed wave function is no longer single-valued: The phase of \psi' changes by
:\delta\phi=(e/\hbar)\oint_C\nabla\chi(r)\cdot dl=2\pi\Phi/\Phi_0
whenever one of the coordinates r_n is moved along the ring to its starting point. The requirement of a single-valued wave function therefore restricts the gauge transformation to fluxes \Phi that are an integer multiple of \Phi_0. Systems that enclose a flux differing by a multiple of h/e are equivalent.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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