arXiv subjects

R. Airey

Publications and source records attributed to R. Airey.

8 recordsLinked to original sources

arXiv2016

A microscopic model for the magnetic field driven breakdown of the dissipationless state in the integer and fractional quantum Hall effect

Intra Landau level thermal activation, from localized states in the tail, to delocalized states above the mobility edge in the same Landau level, explains the $B_c(T)$ (half width of the dissipationless state) phase diagram for a number of different quantum Hall samples with widely ranging carrier density, mobility and disorder. Good agreement is achieved over $2-3$ orders of magnitude in temperature and magnetic field for a wide range of filling factors. The Landau level width is found to be independent of magnetic field. The mobility edge moves, in the case of changing Landau level overlap to maintain a sample dependent critical density of states at that energy. An analysis of filling factor $ν=2/3$ shows that the composite Fermion Landau levels have exactly the same width as their electron counterparts. An important ingredient of the model is the Lorentzian broadening with long tails which provide localized states deep in the gap which are essential in order to reproduce the robust high temperature $B_c(T)$ phase observed in experiment.

cond-mat.mes-hall↗

arXiv2013

Dispersive line shape in the vicinity of the ν = 1 quantum Hall state: Coexistence of Knight shifted and unshifted resistively detected NMR responses

The frequency splitting between the dip and the peak of the resistively detected nuclear magnetic resonance (RDNMR) dispersive line shape (DLS) has been measured in the quantum Hall effect regime as a function of filling factor, carrier density and nuclear isotope. The splitting increases as the filling factor tends to ν = 1 and is proportional to the hyperfine coupling, similar to the usual Knight shift versus ν-dependence. The peak frequency shifts linearly with magnetic field throughout the studied filling factor range and matches the unshifted substrate signal, detected by classical NMR. Thus, the evolution of the splitting is entirely due to the changing Knight shift of the dip feature. The nuclear spin relaxation time, T1, is extremely long (hours) at precisely the peak frequency. These results are consistent with the local formation of a ν = 2 phase due to the existence of spin singlet D$^-$ complexes.

cond-mat.mes-hall↗

arXiv2013

Classical percolation fingerprints in the high-temperature regime of the integer quantum Hall effect

We have performed magnetotransport experiments in the high-temperature regime (up to 50 K) of the integer quantum Hall effect for two-dimensional electron gases in semiconducting heterostructures. While the magnetic field dependence of the classical Hall law presents no anomaly at high temperatures, we find a breakdown of the Drude-Lorentz law for the longitudinal conductance beyond a crossover magnetic field B_c ~ 1 T, which turns out to be correlated with the onset of the integer quantum Hall effect at low temperatures. We show that the high magnetic field regime at B > B_c can be understood in terms of classical percolative transport in a smooth disordered potential. From the temperature dependence of the peak longitudinal conductance, we extract scaling exponents which are in good agreement with the theoretically expected values. We also prove that inelastic scattering on phonons is responsible for dissipation in a wide temperature range going from 1 to 50 K at high magnetic fields.

cond-mat.mes-hall↗

arXiv2007

Influence of the single-particle Zeeman energy on the quantum Hall ferromagnet at high filling factors

In a recent paper [B. A. Piot et al., Phys. Rev. B 72, 245325 (2005)], we have shown that the lifting of the electron spin degeneracy in the integer quantum Hall effect at high filling factors should be interpreted as a magnetic-field-induced Stoner transition. In this work, we extend the analysis to investigate the influence of the single-particle Zeeman energy on the quantum Hall ferromagnet at high filling factors. The single-particle Zeeman energy is tuned through the application of an additional in-plane magnetic field. Both the evolution of the spin polarization of the system and the critical magnetic field for spin splitting are well described as a function of the tilt angle of the sample in the magnetic field.

cond-mat.mes-hall↗

arXiv2007

Anisotropic magnetic field dependence of many-body enhanced electron tunnelling through a quantum dot

We investigate the effect of an applied magnetic field on resonant tunneling of electrons through the bound states of self-assembled InAs quantum dots (QDs) embedded within an (AlGa)As tunnel barrier. At low temperatures (no more than 2 K), a magnetic field B applied either parallel or perpendicular to the direction of current flow causes a significant enhancement of the tunnel current. For the latter field configuration, we observe a strong angular anisotropy of the enhanced current when B is rotated in the plane of the quantum dot layer. We attribute this behavior to the effect of the lowered symmetry of the QD eigenfunctions on the electron-electron interaction.

cond-mat.mes-hall↗

arXiv2005

The quantum Hall ferromagnet at high filling factors: A magnetic field induced Stoner transition

Spin splitting in the integer quantum Hall effect is investigated for a series of Al$_{x}$Ga$_{1-x}$As/GaAs heterojunctions and quantum wells. Magnetoresistance measurements are performed at mK temperature to characterize the electronic density of states and estimate the strength of many body interactions. A simple model with no free parameters correctly predicts the magnetic field required to observe spin splitting confirming that the appearance of spin splitting is a result of a competition between the disorder induced energy cost of flipping spins and the exchange energy gain associated with the polarized state. In this model, the single particle Zeeman energy plays no role, so that the appearance of this quantum Hall ferromagnet in the highest occupied Landau level can also be thought of as a magnetic field induced Stoner transition.

cond-mat.mes-hall↗

arXiv2003

High temperature gate control of quantum well spin memory

Time-resolved optical measurements in (110)-oriented GaAs/AlGaAs quantum wells show a ten-fold increase of the spin-relaxation rate as a function of applied electric field from 20 to 80 kV cm-1 at 170 K and indicate a similar variation at 300 K, in agreement with calculations based on the Rashba effect. Spin relaxation is almost field-independent below 20 kV cm-1 reflecting quantum well interface asymmetry. The results indicate the achievability of voltage-gateable spin-memory time longer than 3 ns simultaneously with high electron mobility.

cond-mat↗

arXiv2001

Ferromagnetic GaMnAs/GaAs superlattices - MBE growth and magnetic properties

We have studied the magnetic properties of (GaMnAs)m/(GaAs)n superlattices with magnetic GaMnAs layers of thickness between 8 and 16 molecular layers (ML) (23-45 Å), and with nonmagnetic GaAs spacers from 4 ML to 10 ML (11-28 Å). While previous reports state that GaMnAs layers thinner than 50 Åare paramagnetic in the whole Mn composition range achievable using MBE growth (up to 8% Mn), we have found that short period superlattices exhibit a paramagnetic-to-ferromagnetic phase transition with a transition temperature which depends on both the thickness of the magnetic GaMnAs layer and the nonmagnetic GaAs spacer. The neutron scattering experiments have shown that the magnetic layers in superlattices are ferromagnetically coupled for both thin (below 50 Å) and thick (above 50 Å) GaMnAs layers.

cond-mat.mtrl-sci↗