Intersubband Spin-density Excitations Research Articles

Intersubband spin-density excitations in quantum wells with Rashba spin splitting

In inversion-asymmetric semiconductors, spin-orbit coupling induces a $\mathbf{k}$-dependent spin splitting of valence and conduction bands, which is a well-known cause for spin decoherence in bulk and heterostructures. Manipulating nonequilibrium spin coherence in device applications thus requires understanding how valence and conduction band spin splitting affects carrier spin dynamics. This paper studies the relevance of this decoherence mechanism for collective intersubband spin-density excitations (SDE's) in quantum wells. A density-functional formalism for the linear spin-density matrix response is presented that describes SDE's in the conduction band of quantum wells with subbands that may be nonparabolic and spin split due to bulk or structural inversion asymmetry (Rashba effect). As an example, we consider a 40 nm ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum well, including Rashba spin splitting of the conduction subbands. We find a coupling and wave-vector-dependent splitting of the longitudinal and transverse SDE's. However, decoherence of the SDE's is not determined by subband spin splitting, due to collective effects arising from dynamical exchange and correlation.

Canted antiferromagnetic and spin-singlet quantum Hall states in double-layer systems

We present details of earlier studies [Zheng et al., Phys. Rev. Lett. 78, 310 (1997) and Das Sarma et al., 79, 917 (1997)] and additional results on double-layer quantum Hall systems at a total filling $\ensuremath{\nu}=2{\ensuremath{\nu}}_{1}$, where a single layer at filling ${\ensuremath{\nu}}_{1}$ forms a ferromagnetic, fully spin-polarized, gapped incompressible quantum Hall state. For the case ${\ensuremath{\nu}}_{1}=1$, a detailed Hartree-Fock analysis is carried out on a realistic, microscopic Hamiltonian. Apart from the state continuously connected to the ground state of two well-separated layers, we find two double-layer quantum Hall phases: one with a finite interlayer antiferromagnetic spin ordering in the plane orthogonal to the applied field (the ``canted'' state), and the other a spin singlet. The quantum transitions between the various quantum Hall states are continuous, and are signaled by the softening of collective intersubband spin-density excitations. For the case of general ${\ensuremath{\nu}}_{1}$, closely related results are obtained by a semi-phenomenological continuum quantum field theory description of the low-lying spin excitations using a nonlinear $\ensuremath{\sigma}$ model. Because of its broken symmetry, the canted phase supports a linearly dispersing Goldstone mode and has a finite-temperature Kosterlitz-Thouless transition. We present results on the form of the phase diagram, the magnitude of the canted order parameter, the collective excitation dispersions, the specific heat, the form of the dynamic light-scattering spectrum at finite temperature, and the Kosterlitz-Thouless critical temperature. Our findings are consistent with recent experimental results.

Collective intersubband spin-density excitations in a quantum wire in a magnetic field

We study an interacting electron gas parabolically confined to a quantum wire in a perpendicular magnetic field. We consider the case of more than one subband occupied in the ground state. The intersubband spin-density excitations are calculated within the Hartree-Fock random-phase approximation. Similarly to the case in the absence of a magnetic field, vertex corrections to the electron spin polarizability are important, leading to collective spin-density excitations red-shifted with respect to the Hartree-Fock single-particle energies. In a finite magnetic field, the triplet of spin-density excitations splits. The splitting is at its maximum if the chemical potential is in between the bottom of a spin-up subband and the bottom of a spin-down subband, when the g-factor is greatly enhanced due to the exchange interaction.

Spin-Excitation-Instability-Induced Quantum Phase Transitions in Double-Layer Quantum Hall Systems

We study intersubband spin density collective modes in double-layer quantum Hall systems at $\nu=2$ within the time-dependent Hartree-Fock approximation. We find that these intersubband spin density excitations may soften under experimentally accessible conditions, signaling a phase transition to a new quantum Hall state with interlayer inplane antiferromagnetic spin correlations. We show that this novel canted antiferromagnetic phase is energetically stable and that the phase transition is continuous.

Raman-scattering spectra of elementary electronic excitations in coupled double-quantum-well structures.

Using the time-dependent local-density approximation (TDLDA) within a self-consistent-linear response theory, we calculate the elementary excitation energies and the associated inelastic light-scattering spectra of a strongly coupled two-component plasma in a double-quantum-well system with electron occupation of symmetric and antisymmetric subbands. We find, consistent with the results of a recent experimental Raman-scattering study, that the intersubband spin-density excitations tend to merge with the single-particle excitations (i.e., the excitonic shift decreases monotonically) as the Fermi energy increases beyond the symmetric-antisymmetric energy gap ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{SAS}}$. However, our TDLDA calculation does not show the abrupt suppression of the excitonic shift seen experimentally at a finite value of the subband occupancy parameter \ensuremath{\eta}==${\mathrm{\ensuremath{\Delta}}}_{\mathrm{SAS}}$/${\mathit{E}}_{\mathit{F}}$.

Inelastic light scattering by electrons in GaAs quantum wires: Spin-density, charge-density and single-particle excitations

In inelastic light scattering experiments we observe for the first time clearly resolved one dimensional (1D) intersubband spin-density excitations. Together with new structure at energies close to the 2D intersubband transitions, these observations display the formation of 1D subbands. The depolarization shift ( W dep) and the excitonic shift ( W xc) can be deduced approximately from our experiments. These shifts are of special interest because they are related to the direct and exchange-correlation terms of the electron-electron interaction. We find ratios of the shifts ( W xc/ W dep) of up to 55%.