1,721,058 research outputs found
Signatures of composite-fermion metals in electron bilayers at νT=1
No full textComposite-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
Metamorphosis of a quantum Hall bilayer state into a composite fermion metal
No full textComposite fermion (CF) metal states emerge in quantum Hall bilayers at total Landau level filling factor v(T) = 1 when the tunneling gap collapses by application of in-plane component of the 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. The results highlight significant differences of bilayer CF quasiparticles from those in single layers
Shining light on electrons under extreme conditions
No full textStrongly-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
Ultrafast carrier activation in resonantly excited 1.3-μm InAs/GaAs quantum dots at room temperature
No full textObservation of soft magnetorotons in bilayer quantum Hall ferromagnets
No full textInelastic 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
Spectroscopic determination of the order parameter of coupled bilayers at νT=1
No full textWe 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
Resonant Rayleigh scattering from bilayer quantum Hall phases
No full textWe observe resonant Rayleigh scattering of light from quantum Hall bilayers at Landau level filling factor 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
Seeing emergent phases in quantum Hall double layers
No full textWe review recent studies by optics methods of emergent phases in the quantum Hall (QH) regimes of double layers with finite tunneling at Landau level filling factor nu=1. In measurements of spin excitations by inelastic light scattering and of elastically scattered Rayleigh light under the application of in-plane magnetic fields, we uncovered evidence of a quantum phase transition that occurs when a many-body tunneling gap collapses. The transformation can be regarded as a transition from an incompressible highly correlated QH state to a compressible composite-fermion bilayer system. The correlated QH state is characterized by the presence of populations of bound electron-hole pairs across the tunneling gap. Quantitative determinations of the density of such excitonic pairs are obtained from inelastic light scattering spectra of spin excitations. The correlated QH state displays resonant Rayleigh scattering with unusual temperature dependence
Observation of collapse of pseudospin order in bilayer quantum Hall ferromagnets
No full textThe 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
Selective control of edge-channel trajectories by scanning gate microscopy RID B-4406-2011 RID C-6303-2008 RID C-5465-2009
No full textElectronic 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. (C) 2009 Elsevier B.V. All rights reserved
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