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Register a new userDouble-exciton component of the cyclotron spin-flip mode in a quantum Hall ferromagnet
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We report on the calculation of the cyclotron spin-flip excitation (CSFE) in a spin-polarized quantum Hall system at unit filling. This mode has a double-exciton component which contributes to the CSFE correlation energy but can not be found by means of a mean field approach. The result is compared with available experimental data.
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Experimental and theoretical studies of the coherent spin dynamics of two-dimensional GaAs/AlGaAs electron gas were performed. The system in the quantum Hall ferromagnet state exhibits a spin relaxation mechanism that is determined by many-particle Coulomb interactions. In addition to the spin exciton with changes in the spin quantum numbers of $delta S!=!delta S_z !=!-1$, the quantum Hall ferromagnet supports a Goldstone spin exciton that changes the spin quantum numbers to $delta S!=!0$ and $delta S_z!=!-1$, which corresponds to a coherent spin rotation of the entire electron system to a certain angle. The Goldstone spin exciton decays through a specific relaxation mechanism that is unlike any other collective spin state.
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 Schroedinger 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 stochastizationa mechanism (determined by spatial fluctuations of the Lande g-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.
Spin relaxation in quantum Hall ferromagnet regimes is studied. As the initial non-equilibrium state, a coherent deviation of the spin system from the ${vec B}$ direction is considered and the breakdown of this Goldstone-mode state due to hyperfine coupling to nuclei is analyzed. The relaxation occurring non-exponentially with time is studied in terms of annihilation processes in the Goldstone condensate formed by zero spin excitons. The relaxation rate is calculated analytically even if the initial deviation is not small. This relaxation channel competes with the relaxation mechanisms due to spin-orbit coupling, and at strong magnetic fields it becomes dominating.
Spin-flip excitations in a quantum Hall electron system at fixed filling factor nu=2 are modelled and studied under conditions of a strong Coulomb interaction when the `Landau level mixing is a dominant factor determining the excitation energy. The `one-exciton approach used for the purely electronic excitations in question allows us to describe the Stoner transition from the unpolarized/paramgnet state to the polarized/ferromagnet one. The theoretical results are compared with the available experimental data.
The cyclotron spin-flip modes of spin unpolarized integer quantum Hall states ($ u =2,4$) have been studied with inelastic light scattering. The energy of these modes is significantly smaller compared to the bare cyclotron gap. Second order exchange corrections are held responsible for a negative energy contribution and render these modes the lowest energy excitations of unpolarized integer quantum Hall states.
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