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Electronic Mach–Zehnder interferometers in the quantum Hall (QH) regime are currently discussed for the realization of quantum information schemes. A recently proposed device architecture employs interference between two co-propagating edge channels. Here we demonstrate the precise control of individual edge-channel trajectories in quantum point contact devices in the QH regime. The biased tip of an atomic force microscope is used as a moveable local gate to pilot individual edge channels. Our results are discussed in light of the implementation of multi-edge interferometers.

We report inelastic light scattering measurements of spin excitations on coupled electron bilayers with relatively large tunneling gaps at total filling factor nu(T) = 1. We show that the pseudospin polarization order parameter, where the pseudospin labels the occupation of symmetric and antisymmetric levels, can be determined from the energy of long wavelength spin excitations. Our experiments indicate that the order parameter in the quantum Hall ground state collapses at the incompressible-compressible phase transition. The latter is driven by decreasing the tunneling gap through the application of an in-plane magnetic field. (c) 2006 Elsevier B.V. All rights reserved.

Strongly-correlated two-dimensional (2D) electron fluids occur in artificial semiconductor heterostructures of high perfection that are subjected to quantizing magnetic fields. Under conditions of high magnetic fields, low temperatures in the millikelvin range and low electron density, the 2D quantum fluids display dispersive low-energy collective excitations that represent time- and space-dependent oscillations in the charge and/or the orientations of spin. These collective modes manifest fundamental interactions that are responsible for electron correlation. Studies of low-lying collective excitation modes play pivotal roles in the low-temperature phases of the electron liquids and offer venues of studying energetics, coherence, magnetization, instabilities and quantum phase transitions of the 2D system. In 1978 a proposal by Burstein, Pinczuk and Buchner suggested that resonant inelastic light scattering methods would have the sensitivity required to Study elementary excitations of 2D electron systems in semiconductors. We review here recent light scattering results obtained from 2D electron fluids in semiconductor quantum structures under extreme conditions of low temperature and large magnetic field. In these experiments, resonant inelastic light scattering methods probe fundamental behaviors due to interactions with a sensitivity that will keep light scattering studies at the frontiers of research of quantum fluids in low dimensional electron systems. (C) 2005 Elsevier Ltd. All rights reserved.

Composite-fermion metals occur in the quantum Hall bilayers at total Landau level filling fraction v(T) = 1 as the tunneling gap Delta(SAS) collapses by application of in-plane magnetic fields. Experimental evidence is obtained from the observation of a spin continuum below the Zeeman energy by resonant inelastic light scattering. These low-lying spin modes are assigned to quasi-particle excitations, where spin and composite-fermion Landau level index change simultaneously. (C) 2007 Elsevier B.V. All rights reserved.

Composite fermion metal states emerge in quantum Hall bilayers at total Landau level filling factor $\nu_T$=1 when the tunneling gap collapses by application of in-plane components of the external magnetic field. Evidence of this transformation is found in the continua of spin excitations observed by inelastic light scattering below the spin-wave mode at the Zeeman energy. The low-lying spin modes are interpreted as quasiparticle excitations with simultaneous changes in spin orientation and composite fermion Landau level index., Comment: 4 pages 4 figures

We observe resonant Rayleigh scattering of light from quantum Hall bilayers at Landau level filling factor $\ensuremath{\nu}=1$. The effect arises below 1 Kelvin when electrons are in the incompressible quantum Hall phase with strong interlayer correlations. Marked changes in the Rayleigh scattering signal in response to application of an in-plane magnetic field indicate that the unexpected temperature dependence is linked to formation of a nonuniform electron fluid close to the phase transition towards the compressible state. These results demonstrate a new realm of study in which resonant Rayleigh scattering methods probe quantum phases of electrons in semiconductor heterostructures.

The Hartree-Fock paradigm of bilayer quantum Hall states with finite tunneling at filling factor $\nu$=1 has full pseudospin ferromagnetic order with all the electrons in the lowest symmetric Landau level. Inelastic light scattering measurements of low energy spin excitations reveal major departures from the paradigm at relatively large tunneling gaps. The results indicate the emergence of a novel correlated quantum Hall state at $\nu$=1 characterized by reduced pseudospin order. Marked anomalies occur in spin excitations when pseudospin polarization collapses by application of in-plane magnetic fields., Comment: ReVTeX4, 4 pages, 3 EPS figures

The recent report on the observation of soft magnetorotons in the dispersion of charge-density excitations across the tunneling gap in coupled bilayers at total Landau level filling factor nu(T) = 1 is reviewed. The inelastic light scattering experiments take advantage of the breakdown of wave vector conservation that occurs under resonant excitation. The results offer evidence that in the quantum Hall state there is a roton that softens and sharpens markedly when the phase boundary for transitions to highly correlated compressible states is approached. These findings are interpreted with Hartree-Fock evaluations of the dynamic structure factor. The model includes the effect of disorder in the breakdown of wave vector conservation and resonance enhancement profiles within a phenomenological approach. These results link the softening of magnetorotons to enhanced excitonic Coulomb interactions in the ferromagnetic bilayers.

Strongly-correlated two-dimensional electrons in coupled semiconductor bilayers display remarkable broken symmetry many-body states under accessible and controllable experimental conditions. In the cases of continuous quantum phase transitions soft collective modes drive the transformations that link distinct ground states of the electron double layers. In this paper we consider results showing that resonant inelastic light scattering methods detect soft collective modes of the double layers and probe their evolution with temperature and magnetic field. The light scattering experiments offer venues of research of fundamental interactions and continuous quantum phase transitions in low-dimensional electron liquids., Comment: 10 pages, 7 figures

Inelastic light scattering measurements of low-lying collective excitations of electron double layers in the quantum Hall state at total filling nu_T=1 reveal a deep magnetoroton in the dispersion of charge-density excitations across the tunneling gap. The roton softens and sharpens markedly when the phase boundary for transitions to highly correlated compressible states is approached. The findings are interpreted with Hartree-Fock evaluations that link soft magnetorotons to enhanced excitonic Coulomb interactions and to quantum phase transitions in the ferromagnetic bilayers., Comment: ReVTeX4, 4 pages, 4 EPS figures