Coupled two-layer plasmon modes induced in a single quantum well by in-plane magnetic fields

We present a complete investigation of the collective excitations in quantum wells subject to in-plane magnetic fields $\mathbf{B},$ based on the random phase approximation and self-consistently calculated ground states in the Hartree approximation. We find that, for a symmetric quantum well of width w, high in-plane $\mathbf{B}$ fields ${(l}_B\ll w)$ can, besides the usual optical intrasubband plasmons (OP’s), give rise to an additional branch of undamped acoustic plasmons (AP’s), which does not exist in a single well without $\mathbf{B}.$ In low $\mathbf{B}$ fields ${(l}_B\gg w),$ the magnetic-field broken symmetry of electron wave functions causes an anticrossing feature of the plasmon dispersions even in the symmetric well. In intermediate $\mathbf{B}$ fields ${(l}_B\lesssim w),$ an unusual anisotropy effect occurs: the number of the plasmon branches depends on the angle between the magnetic field and the in-plane plasmon wave vector $\mathbf{q}.$ For $\mathbf{B}\Vert \mathbf{q},$ there exist two branches of plasmons (OP’s and AP’s), whereas only one (OP) exists for $\mathbf{B}\perp \mathbf{q}.$