Electron spin relaxation as tracer of excitons in a two-dimensional electron-hole plasma inside a (110)-GaAs quantum well

Abstract

We exploit the influence of the Coulomb interaction between electrons and holes on the electron spin relaxation in a (110)-GaAs quantum well to unveil excitonic signatures within the many particle electron-hole system. The temperature dependent time-and polarization-resolved photoluminescence measurements span five decades of carrier density, comprise the transition from localized excitons over quasi free excitons to an electron-hole plasma, and reveal strong excitonic signatures even at relatively high densities and temperatures. 78.55.Cr, 78.67.De Shortly after the big bang, the hot gas of negatively charged electrons and positively charged protons cooled to temperatures below the Rydberg energy and formed a new state of matter, called hydrogen atoms. 13.7 billion years later, researchers shoot short laser pulses on direct semiconductors at liquid helium temperature creating a hot gas of negatively charged electrons in the conduction and positively charged holes in the valence band. The hot carrier gas cools with time by emission of optical and acoustical phonons and forms excitons, i.e., hydrogen like quasi-particles. These quasi-particles have in bulk GaAs a binding energy of 5 meV and strongly influence the photoluminescence (PL) spectrum. Consequently, the PL emission at the exciton transition energy has been used as an indicator for the existence of excitons[1] -as the emission and absorption lines of hydrogen are used in astronomy. However, S. Koch and coworkers demonstrated within the framework of many body semiconductor Bloch equations that distinct excitonic like emission lines from an electron-hole plasma do not proof the existence of excitons but can be explained equally by a Coulomb correlated electron-hole plasma. [2] Manyfold interband pump-probe absorption[3], reflection [4], and four wave mixing experiments[5], high resolution time-resolved PL[6], and experimentally very demanding quasi-particle THz spectroscopy [7] have been carried out to understand the many-body and quantum-optical character and the diligent interaction of excitons in an electron-hole plasma but besides fifty years of intense research the exciton quest remains a fascinating and active field. The challenge in the interpretation of most interband experiments concerning the incoherent exciton population results from the fact that the interaction of classical light and matter is induced by optical polarization and not directly by incoherent population. The challenge in the interpretation of timeresolved PL experiments results generally from the finite excitation density and that thereby the PL does not result from a two but a many particle interaction which strongly depends on the frequency-dependent strength of light-matter interaction and only to some extend on exciton population. Especially THz experiments support that excitonic like PL lines exist without the population of K=0 excitons. [8] In this letter, we present an entirely different experimental approach to study the existence of excitons in a two-dimensional electr...

The rise of spin noise spectroscopy in semiconductors: From acoustic to GHz frequencies

“…However, any continuous or modulated injection of carriers due to absorption can alter the spin dynamic significantly. Especially in semiconductors optical pumping induces spin dephasing mechanisms, which often overshadow the inherent spin dynamic . A widely employed alternative to investigate the spin dynamic without the obstacles of optical pumping is optical Faraday rotation with probe photon energies in the transparent regime.…”

“…Henceforth, (110)‐GaAs quantum wells attracted great attention in the realm of semiconductor spintronics since here the electron spin relaxation times are extremely long even at room temperature and mostly limited by intersubband electron scattering induced spin relaxation (ISR) . However, at low temperatures the impact of ISR is weak and without the DP mechanism spin relaxation only takes place via the Bir–Aronov–Pikus mechanism resulting from electron–hole exchange interaction of free carriers or excitonic electron–hole spin interaction , which relevance increases with excitation densities . SNS avoids optical excitation and is therefore perfectly suited to determine the experimentally unaltered contingently long intrinsic spin‐relaxation time τ z .…”

Anisotropic homogeneous linewidth of the heavy-hole exciton in (110)-oriented GaAs quantum wells

“…11 A recent coherent control study on (110)-oriented GaAs QWs revealed novel photocurrents that directly depend on exciton polarization dephasing. 12 The contribution of excitons to spin currents 13 and the electron spin relaxation time 14 were also inferred. However, a clear understanding of excitonic effects in these QWs is lacking.…”