Magnetoplasmons in high electron mobility CdTe/CdMgTe quantum wells

Terahertz magnetospectroscopy experiments on high quality CdTe/CdMgTe quantum wells were carried out at low temperatures and high magnetic fields. Samples of two different geometries were considered: a large-area grid-gated sample and a split-gate quantum point contact (QPC). The spectra show features originating from a cyclotron resonance transition and magnetoplasmon excitations. Depending on the sample geometry, plasmons characterized by an effective dielectric function of a mixed gated/ungated (in grid-gated samples) or ungated type (in the QPC) were excited. In each case, the resulting plasmon dispersion relation was determined and we show that it can be precisely described within a theory based on a local approximation of a high-frequency magnetoconductivity tensor of a two-dimensional electron gas by taking into account a polaron effect and plasmon-longitudinal optical (LO) phonon interaction.

Figure 1

A layout of wafers used for the fabrication of (a) the grid-gated samples and (c) a quantum point contact. (b) shows a sketch of a metallic grid pattern, lithographically defined on the wafer surface. A scanning electron microscope picture of the quantum point contact device is presented in (d), showing a split gate (G) and a conduction channel (C).

Figure 2

(a) Transmission dependence on the magnetic field for samples GR [no grid, curve (a)], G1 [$\Lambda =2$ $\mu \mathrm{m}$, curves (b) and (c)], and G2 $\Lambda =3$ $\mu \mathrm{m}$, curves (d) and (e)] at laser frequencies of 2.52 and 3.11 THz. Triangles mark the peaks of the magnetoplasma resonances. Lorentzians resulting from the deconvolution procedure on curves (c) and (d) are represented by dotted lines. Squares and circles in (b): Experimental plasmon dispersion for $\Lambda =2$ and 3 $\mu \mathrm{m}$, respectively, at 2.52 THz. Up (down) triangles in (c): Experimental plasmon dispersion for $\Lambda =2$ $\mu \mathrm{m}$ ($\lambda =3$ $\mu \mathrm{m}$) at 3.11 THz. Solid (dotted) lines in (b) and (c): Theoretical plasmon dispersion [Eq. (3)] with (without) a plasmon-LO phonon interaction.

Figure 3

(a) Photocurrent signal recorded on the QPC at different $V_g$. Spectra are shifted vertically for clarity. Vertical lines are guides to the eye to mark the positions of the maxima in the spectra. The cyclotron resonance position is denoted by CR. Deconvolution to Lorentzians is shown (dotted curves) for a spectrum measured at $V_g=0$ V. (b) Experimental frequencies of the plasmon modes (open circles) fitted with a theoretical dispersion relation including a plasmon-LO phonon interaction (solid curve) and neglecting it (dotted curve).