Competing hyperfine and spin-orbit couplings: Spin relaxation in a quantum Hall ferromagnet

Abstract

We measure the spin-resolved transport of dipolar excitons in a biased GaAs double quantum well structure. From these measurements we extract both spin lifetime and mobility of the excitons. We find that below a temperature of 4.8K, there is a sharp increase in the spin lifetime of the excitons, together with a sharp reduction in their mobility. At T = 1.5K, where the excitons have the lowest mobility, we observe an anomalous non-monotonous dependence of the exciton spin lifetime on the mobility. Below a critical power the spin lifetime increases with increasing mobility and density, while above the critical power the opposite trend is observed. We interpret this transition as an evidence of the interplay between two different spin dephasing mechanisms: at low mobility the dephasing is dominated by the hyperfine interaction with the lattice nuclei spins, while at higher mobility the spin-orbit interaction dominates, and a Dyakonov-Perel spin relaxation takes over. The excitation power and temperature regime where the hyperfine interaction induced spin dephasing is observed correlates with the regime where a dark dipolar quantum liquid was reported recently on a similar sample.

Slow spin relaxation in a quantum Hall ferromagnet state

“…In addition, it is remarkable that all theoretically calculated times are much longer than the times observed experimentally. In the present Letter, we report on spin relaxation measurements with relaxation times reaching 150 ns, which strongly agree with theoretical estimates for the "disorder relaxation" mechanism [9] (i.e., in which the energy dissipation occurring simultaneously with the spin relaxation is determined by a smooth random potential). Meanwhile, the observed exponential time dependence for spin relaxation requires a special explanation and therefore a more comprehensive theoretical analysis.…”

“…In particular, we first consider the importance of spin density spatial fluctuations stimulated by the disorder.The 2DES under quantum Hall conditions represents a quantum object where Coulomb correlations radically modify the energy spectrum. To date, the most elaborated spin-relaxation theory is the one describing the quantum Hall ferromagnet at the electron filling factor ν = 1 [8,9]. This state is definitely a spin dielectric where a deviation of the spin system from equilibrium is treated as the appearance of spin excitons comprising effective holes in the lowest spin sublevel of the zero Landau level and electrons promoted to the next spin sublevel of the same Landau level.…”

“…This state is definitely a spin dielectric where a deviation of the spin system from equilibrium is treated as the appearance of spin excitons comprising effective holes in the lowest spin sublevel of the zero Landau level and electrons promoted to the next spin sublevel of the same Landau level. The spin exciton with a nonzero momentum leads to a reduction of the total spin and the spin projection along the magnetic field of the 2DES by one; in turn, a zero momentum spin exciton rotates the total spin of the 2DES in space [8,9]. Relaxation in both cases is considered to represent an annihilation of spin excitons due to the SO interaction.…”

Goldstone mode stochastization in a quantum Hall ferromagnet

“…[9] the relaxation was supposed to occur via the mechanism of spinorbit coupling affected by the smooth random potential that always takes place in 2D systems. (In fact, a similar relaxation channel is realized for nonzero momentum spin excitons where S and S z are equally reduced and where the relaxation was studied not only theoretically [12] but also experimentally [13]. )…”

“…The most comprehensive concept of spin relaxation was developed for the quantum Hall ferromagnet (QHF), ν = 2n + 1 [9][10][11][12][13], in which the n − 1 low Landau levels at T → 0 are fully occupied and the nth level is filled by spin-up electrons aligned along B. The QHF is in fact a high symmetry system for investigating the influence of many-particle Coulomb interactions on the spin excitation spectrum [3][4][5][6][7].…”