D. K. Maude

Approaching the dirac point in high-mobility multilayer epitaxial graphene.

Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed 250,000 cm2/(V x s). In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.

Plasma wave detection of terahertz radiation by silicon field effects transistors: Responsivity and noise equivalent power

Si metal oxide semiconductor field effect transistors (MOSFETs) with the gate lengths of 120–300nm have been studied as room temperature plasma wave detectors of 0.7THz electromagnetic radiation. In agreement with the plasma wave detection theory, the response was found to depend on the gate length and the gate bias. The obtained values of responsivity (⩽200V∕W) and noise equivalent power (⩾10−10W∕Hz0.5) demonstrate the potential of Si MOSFETs as sensitive detectors of terahertz radiation.

Plasma wave detection of sub-terahertz and terahertz radiation by silicon field-effect transistors

We report on experiments on photoresponse to sub-THz (120GHz) radiation of Si field-effect transistors (FETs) with nanometer and submicron gate lengths at 300K. The observed photoresponse is in agreement with predictions of the Dyakonov–Shur plasma wave detection theory. This is experimental evidence of the plasma wave detection by silicon FETs. The plasma wave parameters deduced from the experiments allow us to predict the nonresonant and resonant detection in THz range by nanometer size silicon devices—operating at room temperature.

Quantum oscillations and quantum Hall effect in epitaxial graphene

Johannes Jobst, Daniel Waldmann, Florian Speck, Roland Hirner, Duncan K. Maude, Thomas Seyller, and Heiko B. Weber ∗ Lehrstuhl für Angewandte Physik, Universität Erlangen-Nürnberg, 91056 Erlangen, Germany Lehrstuhl für Technische Physik, Universität Erlangen-Nürnberg, 91056 Erlangen, Germany Laboratoire des Champs Magnétiques Intenses, 25 Avenue des Martyrs, 38042 Grenoble,France (Dated: August 14, 2009)

High Electron Mobility in AlGaN/GaN Heterostructures Grown on Bulk GaN Substrates

Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities.

Optical manipulation of the exciton charge state in single-layer tungsten disulfide

Raman scattering and photoluminescence (PL) emission are used to investigate a single layer of tungsten disulfide (WS

Graphite from the Viewpoint of Landau Level Spectroscopy: An Effective Graphene Bilayer and Monolayer

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High mobility two-dimensional electron gas in AlGaN∕GaN heterostructures grown on bulk GaN by plasma assisted molecular beam epitaxy

) obtained by exfoliating n-type bulk crystals. Direct gap emission with both neutral and charged exciton recombination is observed in the low temperature PL spectra. The ratio between the trion and exciton emission can be tuned simply by varying the excitation power. Moreover, the intensity of the trion emission can be independently tuned using additional sub band gap laser excitation.

Photoluminescence spectroscopy of self-assembled InAs quantum dots in strong magnetic field and under high pressure

We describe an infrared transmission study of a thin layer of bulk graphite in magnetic fields up to B=34 T. Two series of absorption lines whose energy scales as sqrt[B] and B are present in the spectra and identified as contributions of massless holes at the H point and massive electrons in the vicinity of the K point, respectively. We find that the optical response of the K point electrons corresponds, over a wide range of energy and magnetic field, to a graphene bilayer with an effective interlayer coupling 2gamma_{1}, twice the value for a real graphene bilayer, which reflects the crystal ordering of bulk graphite along the c axis. The K point electrons thus behave as massive Dirac fermions with a mass enhanced twice in comparison to a true graphene bilayer.

Dirac fermions at the H point of graphite: magnetotransmission studies.

The results on growth and magnetotransport characterization of AlGaN∕GaN heterostructures obtained by plasma assisted molecular beam epitaxy on dislocation-free (below 100cm−2) GaN high pressure synthesized bulk substrates are presented. The record mobilities of the two dimensional electron gas (2DEG) exceeding 100000cm2∕Vs at liquid helium temperature and 2500cm2∕Vs at room temperature are reported. An analysis of the high field conductivity tensor components allowed us to discuss the main electron scattering mechanisms and to confirm unambiguously the 2DEG room temperature mobility values.