Spin-reversed Excitations Research Articles

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range 2/5≥ ν≥1/3. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor ν≳1/3 and at ν≲2/5 identify a structure of lowest spin-split Landau levels of composite fermions (CFs) that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At ν=1/3 energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.

Evidence of Landau levels and interactions in low-lying excitations of composite fermions at 1/3

Excitation modes in the range 2/5>or=nu>or=1/3 of the fractional quantum Hall regime are observed by resonant inelastic light scattering. Spectra of spin-reversed excitations suggest a structure of lowest spin-split Landau levels of composite fermions that is similar to that of electrons. Spin-flip energies determined from spectra reveal significant composite fermion interactions. The filling factor dependence of mode energies displays an abrupt change in the middle of the range when there is partial population of a composite fermion level.

Activation energies for spin-reversed excitations in the fractional quantum Hall effect

The activation energy measured in transport experiments in the fractional Hall regime corresponds to the energy required to create a far separated particle hole pair. We calculate, for several fractional quantum Hall states, the energy gaps for the excitation in which the excited composite fermion reverses its spin quantum number. Our results indicate that such spin reversed excitations may be relevant in experimentally accessible regimes.

Low-energy spin rotons in the fractional quantum Hall effect

Motivated by the discovery of extremely low-energy collective modes in the fractional quantum Hall effect [Kang, Pinczuk et al., Phys. Rev. Lett. 84, 546 (2000)], with energies below the Zeeman energy, we study theoretically the spin-reversed excitations for fractional quantum Hall states at $\ensuremath{\nu}=2/5$ and 3/7 and find qualitatively different behavior than for $\ensuremath{\nu}=1/3.$ We find that a low-energy, charge-neutral ``spin roton,'' associated with spin-reversed excitations that involve a change in the composite-fermion Landau level index, has energy in reasonable agreement with experiment.

Spin-reversed excitations in the fractional quantum Hall effect.

We have investigated the fractional quantum Hall state at 3/5 and 4/9 fillings of the lowest Landau level. The quasiparticle-quasihole energy gaps for different spin polarizations indicate that at these filling factors, it might be possible to distinguish between the spin reversal of quasiparticles and quasiholes from activation-energy measurements. Novel features of the energy gap, such as its disappearance in a range of magnetic field, explain a similar observation in recent tilted-field experiments in the fractional-quantum-Hall-effect regime.

Spin-reversed ground state and energy gap in the fractional quantum Hall effect

The role of reversed spins in the fractional quantum Hall effect is briefly reviewed. The ground-state spin configurations for various filling fractions and the spin-reversed quasi-particles are discussed. Some new results for the spin-reversed excitations are presented and compared with recent experimental results.

Spin-reversed Quasiparticles in the Fractional Quantum Hall Effect - Finite System Calculations

The elementary charged excitations at 1/3 Landau level filling have been studied for various spin polarizations for finite electron systems in a periodic rectangular geometry. In the absence of Zeeman energy, the spin-reversed excitations, studied for three to six electron systems show significant reduction of the energy gap, compared to the fully spin-polarized quasiparticle-quasihole gap, studied for upto seven electron systems. In the presence of Zeeman energy, the lowest energy excitations involve spin-reversed quasiparticles and spin-polarized quasiholes for low magnetic fields. With increasing magnetic fields, a crossover point is reached (in the region B < 10 T), beyond which a fully spin polarized quasiparticle-quasihole state will be energetically favored. The Zeeman energy due to spin-flip explains qualitatively the linear magnetic field dependence of the activation energies observed experimentally.