Spin-split Cyclotron Resonance Research Articles

Effects of nonequilibrium electron distribution and electron-electron interaction observed in spin-split cyclotron resonance of InAs/AlSb single quantum wells at high magnetic fields

We have observed two remarkable phenomena in spin-split cyclotron resonance of InAs/AlSb single quantum wells under short-pulse high magnetic fields produced by the single-turn coil technique. The first phenomenon is a hysteretic phenomenon, which was observed with a rapid sweep of the magnetic field in increasing and decreasing directions (\ensuremath{\sim}100 T in a few microseconds). The sweep-rate dependence of the cyclotron resonance spectra revealed that the hysteretic phenomenon is attributable to the nonequilibrium electron distribution in the lowest Landau levels $(N=0)$ with plus and minus spins induced by short-pulse high magnetic fields. At low temperatures of around 15 K, a disagreement in intensity between the two different spins is observed in such a way that the total intensity is conserved. The hysteresis is attributed to a slow spin-flip relaxation between the two spin states of the $N=0$ Landau level with a time constant of 1 \ensuremath{\mu}s comparable with the field pulse duration. At high temperatures of around 100 K, a different type of hysteresis is found, where the absorption intensity of the minus spin is smaller in the down sweep than in the up sweep. The sweep-rate dependence shows that the up sweep does not change significantly, but the minus spin decreases with increasing the sweep rate in the down sweep. This is attributed to a slow electron transfer from donor levels in AlSb to a Landau level in InAs. The second phenomenon is the electron-electron interaction effect on the spin-split cyclotron resonance, which displays an intriguing dependence on the filling factor \ensuremath{\nu} and the temperature at around 45 T, where $0.3<~\ensuremath{\nu}<~1.2.$ With increasing temperature, the absorption intensity of the down spin increases, exceeding the amount expected from the Boltzmann distribution, while that of the up spin shows the opposite behavior. For samples with different carrier concentrations, the spin splitting at $\ensuremath{\nu}=0.7$ is smaller than that at $\ensuremath{\nu}=0.4$ by 1 T at around 50 K. These features can be explained qualitatively by a recent theory by Asano and Ando that calculates the effect of the electron-electron interaction on cyclotron resonance spectra by numerical diagonalization for a finite-size system with a finite number of electrons.

Cyclotron resonance of InAs/AlSb single quantum well in high magnetic fields and spin relaxation process

Spin-split cyclotron resonance (CR) of InAs/AlSb single quantum well exhibits a new type hysteresis in strong magnetic fields at low temperature. Pulsed high magnetic fields up to 150T with a duration of 6μs were generated by the single turn coil technique. The hysteresis was observed in the up and down sweep process of the fields with Bmax≳ 60T. This suggests that a non-equilibrium spin-state in the lowest Landau level is induced due to a slow spin reversal rate 1/T1≈107s−1. Two mechanisms for the spin reversal process are proposed: (i) Direct LO–phonon process and (ii) Raman process associated with LO–phonon emission and acoustic phonon absorption. These processes can well explain the characteristic feature of the hysteresis.

Blocking of the Polaron Effect and Spin-Split Cyclotron Resonance in a Two-Dimensional Electron Gas

Cyclotron resonance (CR) of high density GaAs quantum wells exhibits well-resolved spin splitting above the LO-phonon frequency. The spin-up and spin-down CR frequencies are reversed relative to the order expected from simple band nonparabolicity. We demonstrate that this is a consequence of the blocking of the polaron interaction which is a sensitive function of the filling of the Landau levels.

Breakdown of single mode approximation in quantum hall cyclotron resonance

Spin-split cyclotron resonance (CR) in a two dimensional electron system in a strong magnetic field is calculated by the exact diagonalization method and compared with results for a generalized single-mode approximation. The agreement is excellent at low filling factors ν≲1/4 and their dependence on electron–electron interactions show a mode repulsion behavior. At high filling factors ν≳6/5, the single-mode approximation breaks down and the lineshape shows a highly complicated behavior.

Cyclotron resonance and subband-Landau level coupling in 2D electron and hole gases

We discuss the far-infrared absorption in quantum wells in the presence of a quantizing magnetic field. Our theory is applicable both to electrons and holes. Band structure effects are fully taken into account and our approach allows us to consider different sample geometries and potential profiles. As an example we discuss the spin-split cyclotron resonance in the non-parabolic 2D electron gas in InAs AlSb quantum wells. Our parameter-free calculations are in excellent agreement with recent experiments. The complex valence band structure results in anticrossings between Landau levels of different hole subbands.

Effects of Electron-Electron Interactions on Spin-Split Cyclotron Resonance

The effects of electron-electron interactions on two-component cyclotron resonance are studied numerically in a two-dimensional system. The resonance can be considered roughly as consisting of two peaks if a broadening introduced phenomenologically is sufficiently large. At low temperatures the two peaks repel each other and their intensities are modified strongly, while they merge into a single peak at high temperatures. The results qualitatively explain recent experiments on spin-split cyclotron resonance in GaAs/AlGaAs heterostructures.

Landau-split and spin-split cyclotron resonance of two-dimensional electron systems in HgTe quantum wells

Cyclotron resonances of high-mobility electrons in inverted semiconducting type HgTe quantum wells were investigated at liquid helium temperatures by far-infrared Fourier transform spectroscopy. Due to the energy dependence of the electron effective mass and the high electron mobilities in excess of 70 000 cm2 V−1 s−1, the spin-splitting and the Landau-splitting of the cyclotron resonance could be resolved. At the conduction subband edge an effective mass m∗0 ≈ 0.016 me and a Landég-factor|g0| ≈ 20 was obtained. The experiment is in good agreement with predictions based on the single-particle approximation.

Interface and layer thickness dependence of the effective mass in [formula omitted] superlattices studied by high field cyclotron resonance

The effective mass has been studied in a series of semiconducting and semimetallic InAs GaSb superlattices as a function of superlattice period and band gap. The mass is found to be determined almost entirely by the superlattice period, and is relatively insensitive to both the ratio of the InAs to GaSb thickness, and the structure of the interface layer, which was grown as both a monolayer of InSb and GaAs. Using pulsed magnetic fields of up to 180 T it was possible to observe spin-split cyclotron resonance at room temperature for all samples studied. On cooling the spin-splitting was found to be temperature dependent. This is attributed to the importance of electron-electron interactions which couple the two spin transitions as observed recently in high purity GaAs GaAlAs heterojunctions at low temperatures.

Spin-split cyclotron resonance and spatial distribution of interacting electrons.

Spin-split cyclotron resonance and spatial distribution of interacting electrons.

Theory of spin-split cyclotron resonance in the extreme quantum limit.

We present an interpretation of recent cyclotron resonance experiments on the two-dimensional electron gas in GaAs/AlGaAs heterostructures. We show that the observed dependence of the resonance spectrum on Landau level occupancy and temperature arises from the interplay of three factors: spin-splitting of the cyclotron frequency; thermal population of the two spin states; and coupling of the resonances for each spin orientation by Coulomb interactions. In addition, we derive an $f$-sum rule which allows spin polarisation to be determined directly from resonance spectra.

Spin-and Landau-splitting of the cyclotron resonance in a nonparabolic two-dimensional electron system

We study cyclotron resonance (CR) in a two-dimensional electron system on InAs/AlSb-quantum wells observing discrete transitions between adjacent sets of spin-split Landau states. While in three-dimensional electron systems such splittings are well established, no spin-split CR has been observed in a two-dimensional electron system, to date. Theoretical attempts link this fact to the reduced dimensionality and strong electron-electron coupling. Our results, however, can be interpreted quantitatively in a straightforward single-particle model offering new clues to this major puzzle.

Spin Split Cyclotron Resonance in a 2-D Electron System at Very High Magnetic Fields

Cyclotron resonance of the 2-Dimensional Electron System in GaAs/Ga 0.66 Al 0.34 As heterojunctions has been studied in very high (∼ 90 T) magnetic fields. The resonance is found to split into two components due to the energy dependence of the electron g-factor. The exact magnitude of the splitting is slightly smaller than observed in bulk GaAs, but is found to be carrier concentration dependent. The intensity of the two different transitions is also found to be carrier concentration dependent, despite the fact that the electron g-factor of the Lowest Landau state is close to zero. This behaviour is interpreted as due to collective electron effects similar to results observed at lower densities and lower temperatures in lower magnetic fields.