Heisenberg ferromagnet ferromagnet Heisenberg model : nearest-neighbor ferromagnetic coupling conserved quantity mean field T c periodic boundary conditions
Heisenberg ferromagnet ferromagnet Heisenberg equation of motion Tyablikov decoupling
Heisenberg ferromagnet ferromagnet Heisenberg decoupled equation of motion Fourier transformation:
Heisenberg ferromagnet ferromagnet Heisenberg spectrum of excitations dynamical structure factor
Heisenberg ferromagnet ferromagnet instability condition Heisenberg instability condition mean field onsite equal-spin correlation function instability fluctuation-dissipation theorem d= 1,2 d=3 Hohenberg fluctuation renormiert determine m(T) Mermin-Wagner
spin wave in in ferromagnet ferromagnet - - magnon magnon spin wave Heisenberg ferromagnet ground state: excited state:
spin wave in in ferromagnet ferromagnet - - magnon magnon spin wave excited state: "spin hopping" nearest neighbors of i Fourier transformation "moving particle"
spin wave in in ferromagnet ferromagnet - - magnon magnon spin wave flip up magnon flip down probe measurement
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