Magnetoquantum Oscillations at THz Frequencies in InSb

The ac magnetoconductance of bulk InSb at THz frequencies in high magnetic fields, as measured by the transmission of THz radiation, shows a field-induced transmission, which at high temperatures ($\approx 100\mathrm{K}$) is well explained with classical magnetoplasma effects (helicon waves). However, at low temperatures (4 K), the transmitted radiation intensity shows magnetoquantum oscillations that represent the Shubnikov–de Haas effect at THz frequencies. At frequencies above 0.9 THz, when the radiation period is shorter than the Drude scattering time, an anomalously high transmission is observed in the magnetic quantum limit that can be interpreted as carrier localization at high frequencies.

Figure 1

(a) Schematic experimental setup. (b),(c) Measured transmission signal of InSb as a function of the magnetic field at (b) $12.2{\mathrm{cm}}^{-1}$ and (c) $69.6{\mathrm{cm}}^{-1}$ at 100 and 4.2 K. The dotted curves correspond to theoretical calculations [using Eq. (3)]. (d) The same data as in (b),(c) but with the rapid Fabry-Pérot oscillations filtered out (solid lines; the filled areas show the original data [12]). The black solid curve at the bottom of (d) shows the longitudinal conductivity as a function of magnetic field at 4.2 K. Minima in ${\sigma }_{xx}$ (vertical dashed lines) coincide with maxima in transmission at low temperatures. Inset: Measured longitudinal (red) and Hall (black) dc resistances at 4.2 K (solid lines) and 100 K (dashed lines).

Figure 2

Normalized transmission of InSb as a function of magnetic field (solid lines) at 4.2 K for the FEL frequencies indicated. Scale bars for (a) and (b) are, respectively, 0.05% and 0.02%. The dotted lines correspond to the calculated transmission [using Eq. (4)] normalized to the peak in the experimental curve around 9 T. In the inset of (a), an enlargement of the low field data is shown, where the same coloring is used as in the (a) and (b) panels.

Figure 3

Measured (open symbols) and calculated (filled symbols) transmission amplitudes at 9 T (red circles, $I_1$) and 19 T (blue triangles, $I_2$) at 4.2 K. The red and blue solid lines are drawn to guide the eye. The inset shows how the average amplitudes ($I_1$, $I_2$; dashed curves) are determined from the experimental data (solid curves).