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Berry connection and curvature
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Berry connection and curvature : ウィキペディア英語版
Berry connection and curvature
In physics, Berry connection and Berry curvature are related concepts, which can be viewed, respectively, as a local gauge potential and gauge field associated with the Berry phase. These concepts were introduced by Michael Berry in a paper published in 1984〔

emphasizing how geometric phases provide a powerful unifying concept in several branches of classical and quantum physics. Such phases have come to be known as Berry phases.
==Berry phase and cyclic adiabatic evolution==

In quantum mechanics, the Berry phase arises in a cyclic adiabatic evolution. The quantum adiabatic theorem applies to a system whose Hamiltonian H(\mathbf R) depends on a (vector) parameter \mathbf R that varies with time t. If the n'th eigenvalue \varepsilon_n(\mathbf R) remains non-degenerate everywhere along the path and the variation with time ''t'' is sufficiently slow, then a system initially in the eigenstate
\, |n(\mathbf R(0))\rangle will remain in an instantaneous eigenstate \, |n(\mathbf R(t))\rangle of the Hamiltonian \, H(\mathbf R(t)), up to a phase, throughout the process. Regarding the phase, the state at time ''t'' can be written as〔

:
|\Psi_n(t)\rangle =e^\,
e^\,
| n(\mathbf R(t))\rangle,

where the second exponential term is the "dynamic phase factor." The first exponential term is the geometric term, with \gamma_n being the Berry phase. By plugging into the time-dependent Schrödinger equation, it can be shown that
:
\gamma_n(t)=i\int_0^t dt'\,\langle n(\mathbf R(t'))||n(\mathbf R(t'))\rangle=i\int_^ d\mathbf R\,\langle n(\mathbf R)|\nabla_|n(\mathbf R)\rangle,

indicating that the Berry phase only depends on the path in the parameter space, not on the rate at which the path is traversed.
In the case of a cyclic evolution around a closed path \mathcal C such that \mathbf R(T)=\mathbf R(0), the closed-path Berry phase is
:
\gamma_n=i\oint_ d\mathbf R\,\langle n(\mathbf R)|\nabla_|n(\mathbf R)\rangle.

An example of physical system where an electron moves along a closed path is cyclotron motion (details are given in the page of Berry phase). Berry phase must be considered to obtain the correct quantization condition.

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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