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J. G. S. Lok

Publications and source records attributed to J. G. S. Lok.

8 recordsLinked to original sources
arXiv2007

Exciton condensate at a total filling factor of 1 in Corbino 2D electron bilayers

Magneto-transport and drag measurements on a quasi-Corbino 2D electron bilayer at the systems total filling factor 1 (v_tot=1) reveal a drag voltage that is equal in magnitude to the drive voltage as soon as the two layers begin to form the expected v_tot=1 exciton condensate. The identity of both voltages remains present even at elevated temperatures of 0.25 K. The conductance in the current carrying layer vanishes only in the limit of strong coupling between the two layers and at T->0 K which suggests the presence of an excitonic circular current.

cond-mat.mes-hall↗
arXiv2005

Electron spin resonance on a 2-dimensional electron gas in a single AlAs quantum well

Direct electron spin resonance (ESR) on a high mobility two dimensional electron gas in a single AlAs quantum well reveals an electronic $g$-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 Gauss. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin orbit-interaction and a modulation of the electron wavevector caused by the microwave electric field. This contrasts markedly to conventional ESR that detects through the microwave magnetic field.

cond-mat.mes-hall↗
arXiv2004

Activated Transport in the individual Layers that form the $ν_T$=1 Exciton Condensate

We observe the total filling factor $ν_{T}$=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunnelling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing $d/\ell_B$ below 1.65. In the imbalanced system we find activated transport in each of the layers separately, yet the activation energies show a striking asymmetry around the balance point. This implies that the gap to charge-excitations in the {\em individual} layers is substantially different for positive and negative imbalance.

cond-mat.mes-hall↗
arXiv2003

Coulomb Drag as a Probe of the Nature of Compressible States in a Magnetic Field

Magneto-drag reveals the nature of compressible states and the underlying interplay of disorder and interactions. At ν=3/2 a clear T^{4/3} dependence is observed, which signifies the metallic nature of the N=0 Landau level. In contrast, drag in higher Landau levels reveals an additional contribution, which anomalously grows with decreasing T before turning to zero following a thermal activation law. The anomalous drag is discussed in terms of electron-hole asymmetry arising from disorder and localization, and the crossover to normal drag at high fields as due to screening of disorder.

cond-mat.mes-hall↗
arXiv2001

Closely spaced and separately contacted 2d electron and 2d hole gases by in situ focussed ion implantation

Separately contacted double layers of a 2d electron - 2d hole gases have been prepared in GaAs separated by thin AlGaAs barriers with thicknesses down to 15 nm. The molecular-beam-epitaxial growth was interrupted just before the barrier in order to use in situ focussed-ion-beam implantation to pattern contacts which extend underneath the barrier. The two charge gases form upon biasing the p- and n-type contacts underneath and above the barrier in forward direction and show independent transistor-like behavior.

cond-mat.mes-hall↗
arXiv2000

Spin effects in the magneto-drag between double quantum wells

We report on the selectivity to spin in a drag measurement. This selectivity to spin causes deep minima in the magneto-drag at odd fillingfactors for matched electron densities at magnetic fields and temperatures at which the bare spin energy is only one tenth of the temperature. For mismatched densities the selectivity causes a novel 1/B-periodic oscillation, such that negative minima in the drag are observed whenever the majority spins at the Fermi energies of the two-dimensional electron gasses (2DEGs) are anti-parallel, and positive maxima whenever the majority spins at the Fermi energies are parallel.

cond-mat.mes-hall↗
arXiv1998

Hysteretic behavior and evidence for domain formation in a double-layer quantum Hall system at total filling factor 2

We report anomalous behavior in a double-layer two dimensional hole gas (2DHG) at even integer filling factors which includes hysteresis in the longitudinal and Hall resistances and a very weak temperature dependence of the resistance minima. All anomalies disappear and the conventional quantum Hall effect behavior recovers when a thin metal film is placed on top of the 2DHG. The behavior is attributed to presence of the theoretically predicted magnetic ordering at even integer filling factors which causes the formation of macroscopic spin-charge domains.

cond-mat.mes-hall↗