Superradiant Decay of Cyclotron Resonance of Two-Dimensional Electron Gases

We report on the observation of collective radiative decay, or superradiance, of cyclotron resonance (CR) in high-mobility two-dimensional electron gases in GaAs quantum wells using time-domain terahertz magnetospectroscopy. The decay rate of coherent CR oscillations increases linearly with the electron density in a wide range, which is a hallmark of superradiant damping. Our fully quantum mechanical theory provides a universal formula for the decay rate, which reproduces our experimental data without any adjustable parameter. These results firmly establish the many-body nature of CR decoherence in this system, despite the fact that the CR frequency is immune to electron-electron interactions due to Kohn’s theorem.

Figure 1

(a) Polarization-resolved THz magnetotransmission setup in the Faraday geometry. (b) Coherent cyclotron resonance oscillations in the time domain. Each blue dot represents the tip of the THz electric field at a given time. The red traces are the projections of the waveforms onto the $E_x\text{−}t$ and $E_x\text{−}{E}_y$ planes. The bottom trace is the difference between the top (0 T) and middle (2.5 T) traces.

Figure 2

(a) Magnetic field dependence of CR oscillations, showing peaks (blue) and valleys (red). (b) The frequency-domain version of (a). Black dashed line: linear fit with a cyclotron mass of $0.069m_e$. (c) Magnetic field dependence of ${\tau }_{\mathrm{CR}}$ at 3 K. (d) Temperature dependence of ${\tau }_{\mathrm{CR}}$ at 2.5 T and ${\tau }_{\mathrm{dc}}$. All the data are for sample 1.

Figure 3

(a) Low-density sample (sample 2) exhibiting the longest ${\tau }_{\mathrm{CR}}$ of $\sim 40\mathrm{ps}$. (b) CR oscillations in sample 1 with different densities by controlling the illumination time. (c) Decay rate as a function of density. Blue solid circle: sample 2. Red solid circles: sample 1. The blue dashed line represents Eq. (11) without any adjustable parameter (with $n_{\text{GaAs}}=3.6$ and $m^{*}=0.069m_e$). (d) Incident ($E_{\mathit{i}}$), reflected ($E_r$), and transmitted ($E_t$) THz pulses at the 2DEG. (e) The decay of the THz-pulse-excited energy in the 2DEG. The red dashed line is an exponential fit. About 80% of the energy relaxes through CR superradiance.

Figure 4

(a) The measured values of ${\mathrm{\Gamma }}_{\mathrm{CR}}-{\mathrm{\Gamma }}_{\mathrm{dc}}$ versus ${\mathrm{\Gamma }}_{\mathrm{SR}}$ given by Eq. (11) for four representative data points in the present study and values from Refs. [34, 35]. The solid line has a slope of 1. (b) ${\mathrm{\Gamma }}_{\text{scatt}}\equiv {\mathrm{\Gamma }}_{\mathrm{CR}}-{\mathrm{\Gamma }}_{\mathrm{SR}}$ as a function of $\sqrt{B}$. Red solid line: linear fit. Blue dashed line: prediction based on short-range scattering [14].