Showing papers by "Andre K. Geim published in 1993"

Mesoscopic fluctuations in high magnetic fields: Change in behavior due to boundary diffusion.

Abstract: We have studied universal conductance fluctuations in the magnetoresistance of n+-type GaAs submicrometer wires, which represent an orthodox mesoscopic system but also allow us to reach the high-magnetic-field regime, omega(c)tau > 1, where omega(c) is the cyclotron frequency and tau the electron scattering time. The Lee-Stone correlation field B(c) increases by more than an order of magnitude as the magnetic field increases from 0 to 18 T, but the amplitude of the fluctuations remains unchanged. This implies that the universal scaling of conductance fluctuations is not valid in high magnetic fields, in strong disagreement with theoretical predictions. We show that this behavior is not specific to the nonlocal geometry of measurements, where the breakdown has been reported earlier, but that it also occurs in the local magnetoresistance and rectification fluctuations. The violation of universal scaling is attributed to the appearance of a second phase-breaking length, in the regime omega(c)tau > 1, due to extended electron diffusion near the sample boundaries.

Quantum-resolved investigations of flux dynamics: Collective and single vortex effects.

Abstract: The dynamics of flux vortices in Pb films has been investigated with single fluxon resolution. With increasing magnetic field, there is a crossover from a region where single vortex effects dominate to one where vortex motion becomes correlated. We have measured the flux bundle correlation radius directly and tracked single vortices or correlated bundles as they moved under the action of an applied dc current or in response to a change in temperature. The signatures of flux flow and flux creep have been identified, enabling us to determine the pinning force at individual pinning sites.

Microscopic investigation of the flux dynamics of type-II superconducting films.

Abstract: Type-II superconducting Pb and Pb-In alloy films have been deposited on top of the two-dimensional electron gas (2DEG) in a GaAs/Al1-xGaxAs heterostructure in a Hall bar geometry with 10 mum characteristic dimensions. The superconductor was sufficiently close to the 2DEG for the inhomogeneous field of the flux vortices to be projected almost intact through the electronic system below. Measurements of the Hall voltage and magnetoconductivity at the 2DEG in an applied magnetic field show abrupt correlated changes that reflect the motion of very small bundles of flux (typically 4-10 flux quanta). Consequently we have been able to study vortex-bundle nucleation, flux pinning, and vortex-antivortex annihilation on this microscopic scale. In the low-field region where the magnetoconductivity is expected to be linear in an applied field, a simple model has been used to calculate the density of vortices and antivortices present at a given applied field.

Nonlocal magnetoresistance of diffusive wires in high magnetic fields

Abstract: We have used a nonlocal configuration of measurement probes to study the magnetoresistance of diffusive n+-GaAs wires at high magnetic fields where ωcτ>1. At low temperatures we observe universal conductance fluctuations (UCF). For ωcτ>1 there is a large increase in the Lee-Stone correlation field but the amplitude is unchanged in strong disagreement with the theoretical prediction. We conclude that the UCF in this regime cannot be scaled in terms of a single parameter, the phase coherence length. At higher temperatures, where the UCF are quenched, we observe qualitatively new magnetoresistance oscillations which have the same periodicity as Shubnikov-de Haas oscillations but differ from them in many important respects. In particular, the new oscillations disappear at low temperatures. This indicates the importance of dissipation in the measurement of nonlocal resistance. There is excellent agreement between the temperature dependence of the new oscillations and a simple model.

The observation of Hall plateaux in diffusive quasi-two-dimensional wires of submicron size

Abstract: Plateaux in the Hall effect have been observed in heavily doped n+-GaAs wires with a mobility as low as 015 m2/V s and widths of 200 nm and 800 nm The observed plateaux are strongly dependent on sample width and electron temperature The usual vanishing of the longitudinal resistance is not seen Pronounced plateaux are only observed in the 200 nm sample An explanation is suggested in terms of conduction along edge states In this picture the quantized Hall effect is observed if the mean free path of an electron traveling in an edge state, or the backscattering mean free path, well exceeds the Hall probe separation

Mesoscopic fluctuations in high magnetic fields — are they still universal?

Abstract: Universal conductance fluctuations (UCF) arise in mesoscopic systems as a direct consequence of the quantum interference of electron waves. The universal feature of the fluctuations is that, at finite temperature, their behaviour can be expressed in terms of one parameter, the phase breaking length of electrons Lϕ. Though the mesoscopic fluctuations have also been observed in high magnetic fields, ωcτ > 1, where ωc is the cyclotron frequency and τ the electron scattering time, it was widely believed that the magnetic field had to suppress the fluctuations. However, experiments on an orthodox mesoscopic system have found out no signs of the suppression. The fluctuation amplitude remains unchanged in high magnetic fields while the period of fluctuations changes dramatically. This indicates the breakdown of universal scaling at high magnetic fields.