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Hopfield dielectric : ウィキペディア英語版
Hopfield dielectric
Hopfield dielectric – in quantum mechanics a model of dielectric consisting of quantum harmonic oscillators interacting with the
modes of the quantum electromagnetic field. The collective interaction of the charge polarization modes with the vacuum excitations, photons
leads to the perturbation of both the linear dispersion relation of photons and constant dispersion of charge waves by the avoided crossing between the two dispersion lines of polaritons.
Similarly to the acoustic and the optical phonons and far from the resonance one branch is photon-like while the other charge wave-like.
Mathematically the Hopfield dielectric for the one mode of excitation is equivalent to the Trojan wave packet in the harmonic
approximation. The Hopfield model of the dielectric predicts the existence of eternal trapped frozen photons similar to
the Hawking radiation inside the matter with
the density proportional to the strength of the matter-field coupling.
==Theory==

The Hamiltonian of the quantized Lorentz dielectric consisting of N harmonic oscillators interacting with the
quantum electromagnetic field can be written in the dipole approximation as:
:H=\sum\limits_^N(a_A+^+)\cdot E(r_A)+\sum_\lambda\sum_k
a_^+a_\hbar c k
Assuming oscillators to be on some kind of the regular solid lattice and applying the polaritonic
Fourier transform
:B_k^+=\sum\limits_^N\exp(ikr_A)a_A^+,
:B_k=\sum\limits_^N\exp(-ikr_A)a_A
and defining projections of oscillator charge waves onto the electromagnetic field
polarization directions
:B_^+=e_(k)\cdot B_k^+
:B_=e_(k)\cdot B_k,
after dropping the longitudinal contributions not interacting with the electromagnetic field one may obtain the Hopfield Hamiltonian
:H=\sum_\sum_k(B_^+B_+)\hbar \omega +\hbar cka_^+a_
+\sqrt
a_ +B_^+a_-B_^+a_^+
-B_a_^+ )
Because the interaction is not
mixing polarizations this can be transformed to the normal form with the eigen-frequencies of two polaritonic branches:
:H=\sum_\sum_k \left(\Omega_(k)C_^+C_+\Omega_(k)C_^+C_\right )+const
with the eigenvalue equation
:(tight-binding charge wave dispersion).
One may notice that unlike in the vacuum of the electromagnetic field without matter the expectation
value of the average photon number is non zero in the ground state of the polaritonic Hamiltonian
C_|\mathbf 0>=0 similarly to the Hawking radiation in the neighbourhood of the black hole because of the Unruh-Davies effect. One may readily notice that the lower eigenfrequency \Omega_ becomes imaginary when the coupling constant becomes critical
at g>1 which suggests
that Hopfield dielectric will undergo the superradiant phase transition.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Hopfield dielectric」の詳細全文を読む



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