J. J. Harris

University College London, Londinium, England, United Kingdom

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Publications (244)612.84 Total impact

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    ABSTRACT: In a two-dimensional electron gas at ν=1 (where the filling factor ν=neh/eB measures how many Landau levels (LL) are filled) the ground state should be regarded as a ferromagnet since all the spin down states in the lowest LL are occupied while all the spin up states are empty. In GaAs the single particle (SP) Zeeman energy (ZE) gμBB is very small 0.3 K/T, and Coulomb interactions are very significant. This has led several authors to suggest that novel charged excitations with number of reversed spin R > 1, and with non-trivial spin order, known as charged spin-texture excitations or Skyrmions, may occur (1,2). This type of excitation is based on a spin texture that consists of a central reversed spin surrounded by rings of spin that gradually cant in a radial direction until at the edge they are aligned with the external magnetic field. The balance between the SPZE and the Coulomb energy is determined by the parameter η= gμBB /Ec which determines whether Skyrmions with R>1 (small η) or spin waves (large η) will be the lowest lying excitations. To date two transport measurements have inferred the existence of Skyrmions. Increasing η by tilting the magnetic field suggested a 7 spin excitation for η~0.01 (3). In a narrow (6.8 nm) GaAs quantum well where g is already reduced by quantum confinement energy and penetration of the wavefunction into the AlGaAs barriers, it was decreased further by hydrostatic pressure becoming zero at ~4.8kbar where the energy gap at ν=1 showed a minimum (4). This indicated a much larger Skyrmionic excitation consistent with R=33 when |η|
    Full-text · Article · Jan 2008
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    ABSTRACT: We describe measurements of electron diffraction by a magnetic-field-induced Wigner crystal formed at a GaAs heterojunction. The crystal is formed by dopant electrons in the lowest subband of the confining potential at the interface and is pinned by defects: the probe electrons are photoexcited into higher subbands. The signature of the electron crystal is the break-up of the density of states of the probe electron into Hofstadter bands which is detected directly in the luminescence spectrum. The measured value of the principal gap agrees well with theoretical estimates.
    No preview · Article · Jul 2007 · EPL (Europhysics Letters)
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    ABSTRACT: Optical measurements of substrate temperature during the growth by plasma-assisted molecular beam epitaxy of GaN on sapphire and GaAs (1 1 1)B were obtained using two commercial BandiT systems for visible and infrared wavelengths. With the visible system, an absorption edge at ∼400 nm was seen after 20–50 nm of GaN growth, and although this became clearer and more stable for the film on sapphire, strong interference oscillations precluded further measurements on the GaAs substrate. Using the infrared version, we observed a real initial increase in temperature for GaN films grown on GaAs substrates followed by a subsequent decrease to a steady-state value.
    No preview · Article · Apr 2007 · Journal of Crystal Growth
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    ABSTRACT: We report on the use of a commercially available band-edge detection system for substrate temperature monitoring of gallium arsenide substrates. The extension of the technique to the cases where strong absorption by either the substrate or substrate holder might normally preclude the use of such systems due to poor signal levels is discussed. For indium-mounted wafers, a background subtraction/removal is applied which allows unambiguous determination of the band edge across the full temperature range. An alternative method of operation of the instrument as a highly configurable pyrometer allows measurements to be made on highly conducting p-type substrates where free carrier absorption swamps the band edge.
    No preview · Article · Apr 2007 · Journal of Crystal Growth
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    ABSTRACT: The recent development of a commercial band-gap thermometry system for wide band-gap materials such as GaN (the k-Space Associates’ “BandiTTM”) has allowed a systematic study of the relationship between pyrometric or thermocouple temperature-monitoring methods and the directly determined layer temperature. The wide band gap of GaN gives a weak and sample-dependent coupling to radiant heat sources, and it is common in molecular beam epitaxy (MBE) to use a sputtered Mo coating on the rear of the sapphire substrate to improve the efficiency and consistency of heat transfer. We have investigated the role of this backing layer and the use of a PBN diffuser in an MBE chamber, but similar results are expected to be obtained from band-gap thermometry in a metal-organic vapour phase epitaxy (MOVPE) system. The wide range spectrometer used for the band-edge determination can also be employed in a pyrometric mode, at wavelengths both within the band gap of GaN and above it. The latter is insensitive to thickness oscillations, and is less affected by the presence or absence of a Mo backing layer. Results are presented comparing all these measurement techniques, and analysed to show that the Mo backing does not significantly increase the power coupling into the epitaxial layer, although it does improve the accuracy of pyrometric measurements.
    No preview · Article · Mar 2007 · Journal of Crystal Growth
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    ABSTRACT: A description is given of the occurrence of fractional quantum Hall effects, and how these are influenced by the presence of disorder. It is shown that when electrons are photoexcited into a GaAs-GaAlAs heterojunction, the fractional state existing at a Landau level occupancy of 7/5 can dominate over that at 4/3. Activation energy measurements of the resistivity show how the energy gap of the fractional states is reduced by the presence of disorder. A new method of analysis of the resistivity minima is presented which is used to give information on the correlation length of the many body ground state.
    No preview · Article · Feb 2007 · Physica Scripta
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    ABSTRACT: We present a review of magneto-optical experiments and theoretical calculations which have been used to examine the influences of dimensionality and many electron effects on electron-optic-phonon coupling in low dimensional structures of GaAs-(Ga,Al)As.
    Full-text · Article · Jan 2007 · Physica Scripta
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    ABSTRACT: Photoluminescence measurements of two-dimensional electrons in GaAs/GaAlAs heterojunctions in the integer and fractional quantum Hall regimes are reviewed. Intensity and energy anomalies observed experimentally are discussed in terms of many-body effects in the recombination process. Optical evidence for the formation of an electron solid in the extreme quantum limit is presented.
    No preview · Article · Jan 2007 · Physica Scripta
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    ABSTRACT: The interaction between 2D electrons and photoexcited holes is of fundamental importance in determining the photoluminescence spectrum in the fractional quantum Hall regime. We present new measurements on a low-density electron system which reveal a distinct Fermi edge singularity at zero and low magnetic fields, indicating dynamical response of the Fermi sea to the presence of the hole. The photoluminescence intensity from this system is very sensitive to the effects of electron correlation, and hierarchies of FQH states are clearly identified. The photoluminescence energy, however, is quite insensitive to electron ground state in the FQH regime.
    No preview · Article · Jan 2007 · Physica Scripta
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    ABSTRACT: The low-lying bulk excitations of two-dimensional electron systems subject to a strong perpendicular magnetic field have been studied using phonon absorption. Results reported earlier lead to the conclusion that the magnetoroton minimum energy in the fractional quantum Hall effect can be measured by phonon absorption. Recent time-resolved measurements have demonstrated fundamental differences in phonon absorption between fractional quantum Hall states, non-rational filling factors and filling factors close to 1/2. At non-rational filling factors the response is bolometric, whereas at filling factors corresponding to FQHE states the response also depends on the spectrum of the heater pulse. Around ν = 1/2 the response is largely independent of magnetic field with a weak maximum at ν = 1/2.
    No preview · Article · Dec 2006 · Physica Scripta
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    ABSTRACT: Shubnikov-de Haas and persistent photoconductivity measurements are used to study the mobility, free electron density (n) and the occupancy of the DX centre in heavily doped n-GaAs as a function of doping level and hydrostatic pressure. The results show that the DX centre produces a resonant donor level between the Γ - and L-conduction band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration mode measurements indicates that the DX level can be identified with SiGa. The level acts to pin the Fermi energy at electron concentrations around 1.8 × 1019 cm−3. Analysis of the results indicates that macroscopic charge separation is not responsible for persistent photoconductivity in these samples.
    No preview · Article · Nov 2006 · Physica Scripta
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    ABSTRACT: The work is devoted to interaction-induced magnetoresistance in a two-dimensional electron gas in a Si/SiGe heterostructure. Several types of magnetoresistance behavior are discussed including a magnetoresistance that changes sign with temperature. The data is analyzed using the recent theory of interaction-induced magnetoresistance for arbitrary transport regime and any type of disorder potential.
    No preview · Article · Aug 2005 · EPL (Europhysics Letters)
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    ABSTRACT: We present the study of the magnetoresitance (MR) in a two dimensional electron gas in a Si/SiGe heterostructure. We show that the sign of this MR can change with temperature depending on the sample. We analyze the observed magnetoresistance in terms of the recent theory of interaction induced MR (Phys. Rev. B, 69, 045313 (2003)).
    No preview · Article · Jun 2005
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    ABSTRACT: We report the first observation of the metal–insulator transition (MIT) in a two-dimensional electron gas (2DEG) in a Si/SiGe heterostructure at zero field. We have performed an analysis of the temperature dependence of conductivity and magnetoresistance of our sample on the metallic side of the transition using recent theories.
    No preview · Article · Apr 2004 · Physica E Low-dimensional Systems and Nanostructures
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    ABSTRACT: We report the observation of a metal-insulator transition (MIT) in a two-dimensional electron gas (2DEG) in a Si/SiGe heterostructure at zero magnetic field. On going through the MIT we observe the corresponding evolution of the magnetic field induced transition between the insulating phase and the quantum Hall (QH) liquid state in the QH regime. Similar to the previous reports for a GaAs sample, we find that the critical magnetic field needed to produce the transition becomes zero at the critical electron density corresponding to the zero field MIT. The temperature dependence of the conductivity in a metalliclike state at zero field is compared with the theory of the interaction corrections at intermediate and ballistic regimes kBTtau/h=>1. The theory yields a good fit for the linear part of the curve. However, the slope of that part of sigmaxx(T) is about two times smaller than that reported in other 2D systems with similar values of rs. At the same time, the recent theory of magnetoresistance due to electron-electron interaction in the case of arbitrary kBTtau/h, smooth disorder and classically strong fields does not seem to be quite adequate for the description of the parabolic magnetoresistance observed in our samples. We attribute these results to the fact that neither of these theories deals with the whole scattering potential in a sample but leaves either its long-range or its short-range component out of consideration.
    Full-text · Article · Aug 2003 · Physical Review B
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    ABSTRACT: We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on two silicon doped GaN epitaxial layers grown by two different metalorganic chemical vapor deposition sources on sapphire substrates. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness, for 1.62 and 3.92 μm thick films. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that in the undoped layer, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Using a model based upon a doped layer with one type of shallow donor, plus compensating acceptors, in parallel with an interface layer which shows impurity-band conduction, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors in the doped region were also found. An important result is that for the carrier concentration fitting, when the temperature dependence of the activation energy is taken into account, the fitted energies are closer to those predicted by free and bound carrier screening. Finally the Fermi level was found to be in the range ∼56–84 meV below the conduction band minimum at room temperature and moved closer to the donor levels at low temperatures.
    No preview · Article · Jun 2003 · Journal of Applied Physics
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    ABSTRACT: We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on a silicon-doped GaN epitaxial layer grown by metalorganic chemical vapor deposition on a sapphire substrate. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that close to the interface, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Over the whole temperature range, the mobility does not change appreciably for the first 1.35 μm of removed doped material, but then falls rapidly over the remaining undoped region. Using a model based upon one donor, a deep acceptor and one conducting layer, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors were also found.
    No preview · Article · Mar 2003 · Diamond and Related Materials
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    ABSTRACT: We have combined plasma etching with the Hall effect and resistivity measurements between 10 and 300 K to study the depth distribution of conduction in silicon (Si)-doped GaN epitaxial layers grown on sapphire substrates by two different metalorganic chemical vapor deposition processes. Reduction of the epitaxial layer thickness produces a linear decrease of the sheet carrier density with depth in the doped region, whilst in one sample, in the region less than ∼0.3 μm from the interface, the sheet carrier density tends to flatten out to a value of ∼3×1013 cm−2. The former is indicative of a uniform dopant distribution in the epitaxial material, and the latter reveals the existence of mobile charge near the interface. These experiments allow the properties of the doped material to be deconvoluted from those of the interface region, and the temperature dependence of these properties indicates the presence of two parallel conduction paths in the doped material: the conduction band and an impurity band. Thus a full analysis of GaN epitaxial layers is shown to require consideration of multiple parallel conduction processes, at the interface and in the bulk. © 2002 American Institute of Physics.
    Full-text · Article · Jun 2002 · Journal of Applied Physics
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    ABSTRACT: We report the fabrication and study of gated quantum wires and interferometers based on a Si/SiGe heterostructure fabricated by electron lithography and anisotropic ion etching. In the wires, negative magnetoresistance connected with weak localisation effects and in the ring, Aharonov–Bohm oscillations were investigated in the temperature range –. The phase coherence time was found to be due to electron–electron scattering with small energy transfer and magnetic impurity scattering. High magnetic field Aharonov–Bohm oscillations connected with the interference of edge states current have been observed in Si/SiGe ring for the first time.
    Full-text · Article · Mar 2002 · Physica E Low-dimensional Systems and Nanostructures
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    ABSTRACT: The rate of energy loss per electron, Pe, by a two-dimensional electron gas in an GaN/AlGaN heterostructure has been measured as a function of electron temperature, Te, in the range 0.4–35 K. A combination of zero and high magnetic field electrical transport measurements were used to determine Te as a function of the power dissipated in the device. It was found that Pe ∝ Ten, with n ≈ 5 at the lowest temperatures, Te ≪ 2 K, while for higher temperatures, Te >10 K, n → 1. The experimental results are compared with numerical calculations of the energy relaxation rate. In the range of temperatures studied, emission of piezoelectrically coupled acoustic phonons was found to be the dominant energy relaxation mechanism.
    No preview · Article · Nov 2001 · physica status solidi (b)

Publication Stats

6k Citations
612.84 Total Impact Points

Institutions

  • 1994-2008
    • University College London
      • Department of Electronic and Electrical Engineering
      Londinium, England, United Kingdom
  • 1998-2007
    • University of Oxford
      • Department of Physics
      Oxford, ENG, United Kingdom
    • University of Reading
      Reading, England, United Kingdom
    • University of New South Wales
      • School of Physics
      Kensington, New South Wales, Australia
    • University of Sussex
      • School of Mathematical and Physical Sciences
      Brighton, England, United Kingdom
    • Queen's University
      Kingston, Ontario, Canada
    • University of Essex
      Colchester, England, United Kingdom
    • Polish Academy of Sciences
      Warszawa, Masovian Voivodeship, Poland
  • 2005
    • Institut Universitaire de France
      Lutetia Parisorum, Île-de-France, France
  • 1992-2000
    • Imperial College London
      • Department of Physics
      London, ENG, United Kingdom
  • 1999
    • Arizona State University
      • Department of Electrical Engineering
      Tempe, Arizona, United States
  • 1993-1999
    • University of London
      Londinium, England, United Kingdom
  • 1987-1998
    • University of Nottingham
      • School of Physics and Astronomy
      Nottigham, England, United Kingdom
  • 1996
    • WWF United Kingdom
      Londinium, England, United Kingdom