ABSTRACT
We address damping of a Goldstone spin-rotation mode emerging in a quantum Hall ferromagnet due to laser pulse excitation. Recent experimental data show that the attenuation mechanism, dephasing of the observed Kerr precession, is apparently related not only to spatial fluctuations of the electron Landé factor in the quantum well, but to a hyperfine interaction with nuclei, because local magnetization of GaAs nuclei should also experience spatial fluctuations. The motion of the macroscopic spin-rotation state is studied microscopically by solving a non-stationary Schrödinger equation. Comparison with the previously studied channel of transverse spin relaxation (attenuation of Kerr oscillations) shows that relaxation via nuclei involves a longer quadratic stage of time-dependance of the transverse spin, and, accordingly, an elongated transition to a linear stage, so that a linear time-dependance may not be revealed.
ABSTRACT
A spin-rotation mode emerging in a quantum Hall ferromagnet due to laser pulse excitation is studied. This state, macroscopically representing a rotation of the entire electron spin-system to a certain angle, is not microscopically equivalent to a coherent turn of all spins as a single-whole and is presented in the form of a combination of eigen quantum states corresponding to all possible S z spin numbers. The motion of the macroscopic quantum state is studied microscopically by solving a non-stationary Schrödinger equation and by means of a kinetic approach where damping of the spin-rotation mode is related to an elementary process, namely, transformation of a 'Goldstone spin exciton' to a 'spin-wave exciton'. The system exhibits a spin stochastization mechanism (determined by spatial fluctuations of the Landé factor) ensuring damping, transverse spin relaxation, but irrelevant to decay of spin-wave excitons and thus not involving longitudinal relaxation, i.e. recovery of the S z number to its equilibrium value.
ABSTRACT
Coherent condensate states of particles obeying either Bose or Fermi statistics are in the focus of interest in modern physics. Here we report on condensation of collective excitations with Bose statistics, cyclotron magnetoexcitons, in a high-mobility two-dimensional electron system in a magnetic field. At low temperatures, the dense non-equilibrium ensemble of long-lived triplet magnetoexcitons exhibits both a drastic reduction in the viscosity and a steep enhancement in the response to the external electromagnetic field. The observed effects are related to formation of a super-absorbing state interacting coherently with the electromagnetic field. Simultaneously, the electrons below the Fermi level form a super-emitting state. The effects are explicable from the viewpoint of a coherent condensate phase in a non-equilibrium system of two-dimensional fermions with a fully quantized energy spectrum. The condensation occurs in the space of vectors of magnetic translations, a property providing a completely new landscape for future physical investigations.
ABSTRACT
An experimental technique for the indirect manipulation and detection of electron spins entangled in two-dimensional magnetoexcitons has been developed. The kinetics of the spin relaxation has been investigated. Photoexcited spin-magnetoexcitons were found to exhibit extremely slow relaxation in specific quantum Hall systems, fabricated in high mobility GaAs/AlGaAs structures; namely, the relaxation time reaches values over one hundred microseconds. A qualitative explanation of this spin-relaxation kinetics is presented. Its temperature and magnetic field dependencies are discussed within the available theoretical framework.
ABSTRACT
Cyclotron spin-flip excitation in a ν=2 quantum Hall system, being separated from the ground state by a slightly smaller gap than the cyclotron energy and from upper magnetoplasma excitation by the Coulomb gap [S. Dickmann and I. V. Kukushkin, Phys. Rev. B 71, 241310(R) (2005); L. V. Kulik, I. V. Kukushkin, S. Dickmann, V. E. Kirpichev, A. B. Van'kov, A. L. Parakhonsky, J. H. Smet, K. von Klitzing, and W. Wegscheider, Phys. Rev. B 72, 073304 (2005)] cannot relax in a purely electronic way except only with the emission of a shortwave acoustic phonon (k~3×10(7)/cm). As a result, relaxation in a modern wide-thickness quantum well occurs very slowly. We calculate the characteristic relaxation time to be ~1 s. Extremely slow relaxation should allow the production of a considerable density of zero-momenta cyclotron spin-flip excitations in a very small phase volume, thus forming a highly coherent ensemble-the Bose-Einstein condensate. The condensate state can be controlled by short optical pulses (~1 µs), switching it on and off.
ABSTRACT
Inelastic light scattering spectroscopy discloses a novel type of cyclotron spin-flip excitation in a quantum Hall system around the nu = 1/3 filling. The excitation energy follows qualitatively the degree of electron spin polarization, reaching a maximum value at nu = 1/3. This characterizes the new excitation as a nu = 1/3 ferromagnet eigenmode. The mode energy exceeds drastically the theoretical prediction obtained within the renowned single-mode approximation. We develop a new theoretical approach where the basis set is extended by adding a double-exciton component representing the cyclotron magnetoplasmon and spin wave coupled together. This double-mode approximation, inferred to be responsible for substantially reducing the gap between theoretical and experimental results, shows that the cyclotron spin-flip excitation is effectively a four-particle state.
ABSTRACT
We report an inelastic light scattering study of the cyclotron spin-flip mode in the two-dimensional electron system at filling nu=1. The energy of this mode can serve as a probe of the many-body exchange interaction on short length scales. Its magnetic field dependence is compared with predictions based on Hartree-Fock theory. They agree well when including the nonzero width of the electron system. From the measured energies, the exchange enhanced g factor is extracted. It diverges at small fields and differs largely from g factors obtained via transport activation studies.
ABSTRACT
We report on a study of the spin relaxation of a strongly correlated two-dimensional electron gas in the nu=2kappa+1 quantum Hall regime. As the initial state we consider a coherent deviation of the spin system from the B direction and investigate a breakdown of this Goldstone-mode (GM) state due to the spin-orbit coupling and smooth disorder. The relaxation is considered in terms of annihilation processes in the system of spin waves. The problem is solved at an arbitrary value of the deviation. We predict that the GM relaxation occurs nonexponentially with time.
