Strong Quantum Coherence between Fermi Liquid Mahan Excitons

Abstract

In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.

Biexcitons in monolayer transition metal dichalcogenides tuned by magnetic fields

“…Coherent nonlinear spectroscopy can separate these different contributions. Two-dimensional Fourier-transform spectroscopy based on the four-wave mixing (FWM) signal is an excellent example where the two and four particle correlations generate different features in the two-dimensional frequency spectra 19 , 34 – 36 . Alternatively, time-integrated FWM spectroscopy, combined with microscopic theoretical calculations can already provide important insights.…”

“…The magnetic field further tunes these interactions leading to an increase of the dephasing time. Time-dependent density-functional theory (DFT) calculations, in their density-matrix representation can microscopically track the different contributions to the FWM signal and provide very interesting insights into the effect of the external magnetic fields on the quasiparticle ordering 34 , 35 . These calculations reveal that in magnetic fields, the experimentally observed dephasing process is dominated by biexcitons for both positive and negative delays.…”

Coulomb-interaction induced coupling of Landau levels in intrinsic and modulation-doped quantum wells

“…Such peculiar elongated line shapes along the ω τ observed for the Landau levels have in the past been observed in the absence of magnetic fields and have been attributed to interactions with the continuum [47,51,52]. However, in the present high quality sample the discrete nature of the Landau levels and the quantum confinement of the quantum well in the magnetic field direction should not lead to such continuum state interactions.…”

“…Two quantum well samples were used in the present studies, one undoped (intrinsic) with 12 nm well thickness and a modulation-doped 18 nm GaAs/AlGaAs quantum well with in-well carrier concentration of ∼ 4 × 10 11 cm −2 . The samples were studies using different optical techniques and the carrier concentration has been unambiguously determined [47,63].…”

“…Recently, the two-dimensional Fourier transform (2DFT) spectroscopy was developed, which can provide details of the may-body interactions that cannot be obtained using other methods. The correlated nature of the frequency axes can reveal underlying physics in the form of two-dimensional line shapes and additional peaks in the 2DFT frequency spectra [42][43][44][45][46][47][48].…”