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arXiv:1202.2496 · Semiconductor Science and Technology

Electron spin rephasing in n-type (0 0 1) GaAs quantum wells

Ka Shen, M. W. Wu

Published May 11, 2012• 0 citations
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Abstract

We investigate the electron spin dephasing in the presence of spin phase recovering by a serried $π$-pulse sequence in $n$-type (001) GaAs quantum wells in a wide range of temperature and density regimes. Our numerical calculation is based on the kinetic spin Bloch equation approach with all the relevant scatterings explicitly included. We find that the rephasing pulse sequence with a long inter-pulse spacing only has a marginal influence on the spin lifetime in both the strong and weak scattering limits. We show that the spin lifetime can be significantly increased by reducing the inter-pulse spacing. More interestingly, we show that the temperature and density dependences of the spin lifetime in the case of short inter-pulse spacing coincide with those of the momentum scattering time in the low temperature regime, where nonmonotonic behaviors can appear. The origin of this feature is that the scattering under the quick rephasing manipulation mainly performs as the source of the dephasing channel instead of the key to suppress the inhomogeneous broadening. The contributions of the relevant scattering mechanisms are also discussed.

DOI
10.1088/0268-1242/27/6/065015

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We investigate the electron spin dephasing in the presence of spin phase recovering by a serried πππ-pulse sequence in nnn-type (001) GaAs quantum wells in a wide range of temperature and density regimes. Our numerical calculation is based on the kinetic spin Bloch equation approach with all the relevant scatterings explicitly included. We find that the rephasing pulse sequence with a long inter-pulse spacing only has a marginal influence on the spin lifetime in both the strong and weak scattering limits. We show that the spin lifetime can be significantly increased by reducing the inter-pulse spacing. More interestingly, we show that the temperature and density dependences of the spin lifetime in the case of short inter-pulse spacing coincide with those of the momentum scattering time in the low temperature regime, where nonmonotonic behaviors can appear. The origin of this feature is that the scattering under the quick rephasing manipulation mainly performs as the source of the dephasing channel instead of the key to suppress the inhomogeneous broadening. The contributions of the relevant scattering mechanisms are also discussed.
Based on fully microscopic kinetic spin Bloch equation approach, we show the non-Markovian effect in spin dephasing of heavy holes in ppp-type GaAs quantum wells. The non-Markovian effect manifests itself in spin dephasing when the mean spin precession time is shorter than the momentum relaxation time. The spin dephasing becomes slower when the non-Markovian effect is important. Moreover, quantum spin beats due to the memory effect of the non-Markovian hole--longitudinal-optical phonon scattering are predicted.
We investigate resonant time-resolved four-wave mixing on GaAs quantum wells at T=300KT=300\mathrm{K}T=300K and at elevated carrier densities using excitation with 11 fs optical pulses. The experiments are compared with non-Markovian quantum-kinetic calculations including carrier-carrier and carrier-LO-phonon scattering. A self-consistently determined two-time-dependent screened random-phase approximation Coulomb potential is used for the carrier-carrier scattering. In experiment and theory we find an unusual real-time response, which is far from an ideal photon echo. Surprisingly, we also find that the dependence of the decay time \ensuremath{\tau} of the four-wave-mixing signal versus the optically excited nonequilibrium electron-hole pair density neh{n}_{\mathrm{eh}}neh​ is quantitatively similar to the corresponding behavior of bulk GaAs. Thus both data sets can be fitted by the same form ${\ensuremath{\tau}}^{\ensuremath{-}1}={\ensuremath{\tau}}_{0}^{\ensuremath{-}1}{+cn}_{\mathrm{eh}}^{1/3}.$ This is not consistent with previous experimental work in a more limited range of carrier densities, which determined different behavior for quasi-two-dimensional (2D) and 3D, i.e., exponents of 0.3 and $0.55\ifmmode\pm\else\textpm\fi{}0.04,$ respectively.
Carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a diluted hole gas is studied by picosecond pump-probe Kerr rotation. For resonant optical excitation of the positively charged exciton the spin precession shows two types of oscillations: Electron spin beats decaying with the charged exciton radiative lifetime of 50 ps, and long-lived hole spin beats with dephasing times up to 650 ps, which decrease with increasing temperature, underlining the importance of hole localization. The mechanism of hole spin coherence generation is discussed.
We report time-resolved photoluminescence (TRPL) measurements of the radiative lifetime (τr) at 1.8K of resonantly excited heavy-hole excitons in a 27nm GaAs single quantum well as a function of exciton density. By the use of AlAs interface smoothing layers [1], the sample exhibits very little inhomogeneous broadening (≈0.15meV). The exciton centre of mass motion is therefore not significantly restricted by localisation in well width fluctuation islands. However, measurements of the exciton homogeneous linewidth (Γ h ) using time-resolved degenerate four wave mixing in the self-diffraction configuration (Fig.1) [2], additionally show that the sample imperfection scattering contribution to the Γ h of these delocalised excitons is also small (0.2meV). Thus, as indicated by the dependence of Γ h on exciton density (N x ), even at moderate exciton densities, exciton-exciton scattering is the dominant dephasing interaction within the photocreated exciton ensemble.
We resonantly excite the n=2 excitons at 8 K in 13 nm and 17.5 nm GaAs quantum wells using 0.5 ps pulses from a Ti-Sapphire laser and perform four wave mixing (FWM) measurements. The exciton dephasing time is deduced by fitting the FWM spectra to a numerical solution of the optical Bloch equations for the excitonic resonance. The finite pulse width and presence of small inhomogeneous broadening are taken into account. The homogeneous linewidth thus obtained at different low excitation intensities has a linear dependence on the intensity. The zero intensity intercept is essentially governed by the n=2 exciton lifetime due to intersubband relaxation, the pure transverse dephasing contribution being relatively less important. We deduce the exciton lifetime to be 0.9 ps and 2.6 ps for the 13 nm and 17.5 nm QWs, respectively.
Systematic investigation of the spin dephasing times in GaAs/(Al,Ga)As quantum wells and its depending on parameters like temperature, magnetic fields and doping will be shown in Chapter 3.In Chapter 4 are presented results of investigation of the nuclear spin systems in quantum well heterostructures.The chapter 5 presents the results of studying the carrier spin dynamics in a n-doped (In,Ga)As/GaAs quantum wells by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 mil-liKelvin.In Chapter 6 is devoted to experimental and theoretically studying of optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization.For that purpose the coherent electron spin dynamics in a n-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state.on the eigenstates |s z ; ↑ and |s z ; ↓ yields the states themselves independent of time.Along the field axis (longitudinal direction) these quantum-mechanical spin states are thus stationary.Any loss of coherence is induced by coupling of those states with the environment.Applying the time-evolution operator on the spin states perpendicular