Evolution of the second lowest extended state as a function of the effective magnetic field in the fractional quantum hall regime

It has been shown that, at a Landau level filling factor v=1/2, a two-dimensional electron system can be mathematically transformed into a composite fermion system interacting with a Chern-Simons gauge field. At v=1/2, the average of this Chern-Simons gauge field cancels the external magnetic field B-ext so that the effective magnetic field B-eff acting on the composite fermions is zero. Away from v=1/2, the composite fermions experience a net effective magnetic field B-eff. We present the first study of the evolution of the second lowest extended state in a vanishing effective magnetic field in the fractional quantum Hall regime. Our result shows that the evolution of the second lowest extended state has a good linear dependence on the effective magnetic field Beff within the composite fermion picture

Anticrossing of spin-split subbands in quasi-one-dimensional wires

In quantum Hall systems, both anticrossings and magnetic phase transitions can occur when opposite-spin Landau levels coincide. Our results indicate that both processes are also possible in quasi-1D quantum wires in an in-plane B field, B-parallel to. Crossings of opposite-spin 1D subbands resemble magnetic phase transitions at zero dc source-drain bias, but display anticrossings at high dc bias. Our data also imply that the well-known 0.7 structure may evolve into a spin-hybridized state in finite dc bias

Acoustic transport of electrons in parallel quantum wires

Over the last few years we have developed a new method to control single-electrons by isolating and moving them through a submicron width channel formed in a GaAs/AlGaAs heterostructure using a surface acoustic wave. The acoustic wave acts to push electrons through the depleted submicron channel in packets each containing an integer number of electrons. Our primary motivation for studying this system has been to develop a new standard of dc current for metrological purposes, but our recent focus has widened to investigate the possibility of single-photon emission. Here we show new experimental results which demonstrate acoustoelectric current flow in adjacent 1D wires. These results have relevance both to the use of the system in a single-photon emission scheme, as well as in the creation of a proposed acoustoelectric quantum computer

Possible evidence of a spontaneous spin polarization in mesoscopic two-dimensional electron systems

We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero B, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at B=0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems

Zero-bias anomaly and kondo-assisted quasiballistic 2D transport

Nonequilibrium transport measurements in mesoscopic quasiballistic 2D electron systems show an enhancement in the differential conductance around the Fermi energy. At very low temperatures, such a zero-bias anomaly splits, leading to a suppression of linear transport at low energies. We also observed a scaling of the nonequilibrium characteristics at low energies which resembles electron scattering by two-state systems, addressed in the framework of two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an intrinsic phenomenon in unconfined 2D systems in the low electron-density regime

Characterisation of spin-incoherent transport in one dimension

Spin-incoherent transport in quantum wires, whereby exchange coupling between neighbouring electrons is overcome by thermal energy, leading to the suppression of spin modes of transport expected in a Luttinger liquid, has been observed in the form of a conductance plateau at e(2)/h in the absence of a magnetic field. We present here further characterisation of this spin-incoherent plateau in a source-drain bias, which causes it to evolves to 0.85 x 2e(2)/h. Laterally shifting the channel and illuminating the sample allows us to verify its origin

Kondo effect from a tunable bound state within a quantum wire

We investigate the conductance of quantum wires with a variable open quantum dot geometry, displaying an exceptionally strong Kondo effect and most of the 0.7 structure characteristics. Our results indicate that the 0.7 structure is not a manifestation of the singlet Kondo effect. However, specific similarities between our devices and many of the clean quantum wires reported in the literature suggest a weakly bound state is often present in real quantum wires

Low-temperature collapse of electron localization in two dimensions

We report direct experimental evidence that the insulating phase of a disordered, yet strongly interacting two-dimensional electron system becomes unstable at low temperatures. As the temperature decreases, a transition from insulating to metal-like transport behavior is observed, which persists even when the resistivity of the system greatly exceeds the quantum of resistivity h/e(2). The results have been achieved by measuring transport on a mesoscopic length scale while systematically varying the strength of disorder

Origin of the oscillator strength of the triplet state of a trion in a magnetic field

The dynamics of the spin-triplet trion state, under high magnetic field in a GaAs/AlGaAs quantum well, are studied using time resolved spectroscopy. The oscillator strength of the triplet transition is shown to rise with increasing electron density, in good agreement with a theoretical model where the trion interacts with excess electrons in the quantum well. This analysis suggests that the spin-triplet trion state, which is expected to be an optically "dark" state, is experimentally observable due to the interactions with the excess electrons, demonstrating that X- cannot be regarded as an isolated three particle complex