Anomalous Spin Dephasing in (110) GaAs Quantum Wells: Anisotropy and Intersubband Effects

Abstract

A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As quantum wells grown on (110) oriented substrate. The spin lifetime of spins perpendicular to the growth direction is about one order of magnitude shorter compared to spins along (110). The spin lifetimes of both spin orientations decrease monotonically above a temperature of 80 and 120 K, respectively. The decrease is very surprising for spins along (110) direction and cannot be explained by the usual Dyakonov Perel dephasing mechanism. A novel spin dephasing mechanism is put forward that is based on scattering of electrons between different quantum well subbands.The electron spin in semiconductors has recently become a focus of intense research in the context of spinelectronics or spintronics. This new kind of electronics aims to utilize spin for devices with unprecedented properties [1,2,3]. A prime condition for the development of potential applications is the understanding of spin decoherence, i.e. the loss of spin memory, in semiconductor structures [4]. The main reason for spin decoherence at room temperature is the intrinsic spin splitting of the conduction band, which occurs in all binary semiconductors. The spin splitting, which acts as an effective magnetic field, depends on the electron's momentum and is the basis for the Dyakonov-Perel (DP) spin relaxation mechanism [5,6]. Semiconductor heterostructures are in this context of particular interest since spin splitting in conduction and valence band can be controlled via dimensionality and orientation of crystal axes [7]. Ohno et al. observed very long electron spin decoherence times at room temperature in GaAs quantum wells (QWs) grown on (110) oriented substrates that exceeded the coherence times in usual (100) grown QWs by more than one order of magnitude [8,9]. However, slow spin dephasing in (110) QWs had been demonstrated only for electron spins oriented along the crystal growth direction. The dynamics of in-plane spin was left unexplored.Starting point for the theoretical description of the spin dynamics in (110) QWs is the Dresselhaus-Hamilton for binary semiconductorswhere i = x, y, z are the principal crystal axes with i + 3 → i, Γ is the spin-orbit coefficient for the bulk semiconductor, and σ i are the Pauli spin matrices [10]. Comparing eq.(1) with the spin Hamilton for a free electron in a magnetic field (H = 1 2 i µ B σ i B i ) one easily recognizes that random scattering of electrons leads to an effective k dependent random magnetic field with components in x, y, and z direction. This random magnetic field destroys the average spin orientation of an ensemble of electrons by rotating individual spins in different directions. The DP effect increases in bulk semiconductors with temperature due to occupation of higher k-states with larger spin splittings despite a motional narrowing effect at higher temperatures (spin lifetime τ s is inversely proportional to momentum scattering time τ * p ). In (110) QWs, however, the spin splitting (effective magnetic field)

Dependence of spin dephasing on initial spin polarization in a high-mobility two-dimensional electron system

“…This leads to a drastic decrease of the spin dephasing time (SDT), shown theoretically by Wu and Kuwata-Gonokami 12 and experimentally by Döhrmann et al. 13 This is because for spins with an orientation different from [110], the Dresselhaus field again causes a precessional motion, leading to dephasing due to the DP mechanism. From the point of view of applications, the advantage of long spin dephasing time in [110]-grown QWs is thus diminished, as the manipulation of spins by an external magnetic field destroys it.…”

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells

“…Therefore, in small and medium x regimes where the DP mechanism is more important than the EY mechanism, the magnetic field induces an abrupt increase of the SRT due to the mixing of the in-plane and out-of-plane spin relaxations. 65 In large x regime, the EY mechanism is more important than the DP mechanism, and the SRT hence decreases abruptly with increasing magnetic field. Another important effect of the magnetic field is that it induces an equilibrium Mn spin polarization and thus enhances the s-d exchange scattering mechanism.…”

“…2 ] (ω L is the Larmor frequency, τ p is the momentum scattering time); 26 (ii) mixing the in-plane and out-of-plane spin relaxations, 35,65 e.g., 65 Usually, effect (i) is weak, but effect (ii) is more important. Differing from the case of non-magnetic n-type quantum wells, there are several new scenarios in the p-type Ga(Mn)As quantum wells: (i) The magnetic field can polarize the Mn spins, which alters the spin relaxation due to the s-d exchange scattering mechanism.…”

Spin polarization, dephasing, and photoinduced spin diffusion in (110)-grown two-dimensional electron systems

“…The spin dephasing in (110)-grown QWs and 2DES was studied experimentally by several groups [17][18][19][20][21][22]. Due to the suppression of the DP mechanism, other spin dephasing mechanisms can be studied, yet the measurement of the lifetimes by optical techniques remains challenging due to spin dephasing induced via optically generated holes.…”