A5044519284- Quantum and electron transport phenomena
- Semiconductor Quantum Structures and Devices
- Physics of Superconductivity and Magnetism
- Advancements in Semiconductor Devices and Circuit Design
- Semiconductor materials and devices
- Magnetic properties of thin films
- Graphene research and applications
- Surface and Thin Film Phenomena
- Topological Materials and Phenomena
- Thermal properties of materials
- Magnetic Field Sensors Techniques
- Graphite, nuclear technology, radiation studies
- Advancements in Battery Materials
- Electronic and Structural Properties of Oxides
- Quantum optics and atomic interactions
- Chaos control and synchronization
- Theoretical and Computational Physics
- Quantum chaos and dynamical systems
- Mechanical and Optical Resonators
- Quantum-Dot Cellular Automata
- Catalysis and Oxidation Reactions
- Nonlinear Dynamics and Pattern Formation
- Atomic and Subatomic Physics Research
- Catalytic Processes in Materials Science
- Advanced Chemical Physics Studies
University of Exeter
2003-2021
National University of Singapore
2016
Phillips Exeter Academy
1992
University of Oxford
1983-1988
Measurements of activated conductivity for well-resolved Landau levels at filling factors v=1, 2, and 3 reveal a large exchange contribution to the energy gap, both v=1 (the familiar exchange-enhanced g factor) first time demonstrate similar v=2. These results provide measure large-wave-vector limits spin-wave magnetic-exciton dispersion relations. The surprisingly strong magnetic field dependences measured energies are caused by disorder present in system.
We have studied the low-temperature photoluminescence of two-dimensional electron gas in a single GaAs quantum well magnetic fields up to 50 T over four orders magnitude illumination intensity. At very highest powers, where recombination is excitonic at zero field, we find that binding energy both singlet and triplet states negatively charged exciton ${(X}^{\ensuremath{-}})$ increase monotonically with applied field above 15 T. This contradicts recent calculations for ${X}^{\ensuremath{-}},$...
While thermally cycling monolayer, bilayer, and trilayer graphene between 5 K 300 K, Raman spectroscopy has shown that cooling to induces a strain in these flakes of −0.081 ± 0.003%. This was used measure the thermal expansion coefficient (TEC), which found be (−3.2 0.2)×10−6 K−1 for monolayers, (−3.4 0.4)×10−6 bilayers, (−3.8 0.6)×10−6 trilayers at room temperature. The TEC showed similar temperature dependence across all thicknesses good agreement with theoretical predictions. study, thus,...
We have used a torque magnetometer to measure de Haas - van Alphen oscillations in the magnetization of two-dimensional electrons GaAs/AlGaAs heterostructures and multiple-quantum-well systems for temperatures ranging from 0.125 K 4.2 magnetic fields up 15 T. Our results indicate that high density states can be described by series Lorentzian-broadened Landau levels with broadening is independent field, B, level index, n. However, at low becomes indistinguishable Gaussian one proportional ....
The high magnetic field, low-temperature properties of lowdimensional electron and hole systems reveal a wealth fundamental information.Quantum oscillations the thermodynamic equilibrium magnetization yield total density states, central quantity in understanding quantum Hall effect 2D systems.The arising from non-equilibrium circulating currents reveals details, not accessible with traditional measurements, vanishingly small longitudinal resistance regime.We review how technique magnetometry...
De Haas--van Alphen (dHvA) oscillations are observed for Landau levels (LLs) with filling factors between 4 and 52, at temperatures in the range 50 mK to 1 K, experiments on high-mobility GaAs/(Al, Ga)As heterojunctions. The become sawtooth-shaped low factors, theoretical fits data, assuming two-dimensional electron gas be a non-interacting Fermi system, show shape of LLs close \ensuremath{\delta} function. small residual width (\ensuremath{\sim}0.4 meV or less) equally well either Gaussian...
We report measurements of optically induced density depletion the two-dimensional electron system formed at interface a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunction with \ensuremath{\delta}-doped layer Be acceptors in GaAs 250 \AA{} from interface. Our show that low laser power effect is controlled by recombination electrons photoexcited holes have become bound to acceptors. The point which all sample been neutralized indicated sudden...
We have developed a model of the high-current breakdown integer quantum Hall effect, as measured in contactless experiments using highly-sensitive torsion balance magnetometer. The predicts that, for empirically ``low-mobility'' samples $(\ensuremath{\mu}<75\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}\phantom{\rule{0.3em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, critical current should decrease with, and linear dependence on, temperature....
Recent studies of the electronic properties graphite have produced conflicting results regarding positions different carrier types within Brillouin zone, and possible presence Dirac fermions. In this paper we report a comprehensive study de Haas--van Alphen, Shubnikov--de Haas, Hall effects in sample highly orientated pyrolytic graphite, at temperatures range 30$ $mK to 4$ $K magnetic fields up 12$ $T. The transport measurements confirm Brillouin-zone locations assigned by Schroeder,...
We exploit the difference in thermal contraction coefficients within a device structure to create strain patterns required generate homogeneous pseudo-magnetic field suspended graphene membrane. The interplay between supported and metallic contacts creates deformations upon cooling, producing desired non-uniform This work can easily be adapted produce other configurations, providing route engineering electrical properties of two-dimensional crystals, an area with large body theoretical work,...
We have observed induced currents associated with the fractional quantum Hall effect (FQHE) in a contactless two-dimensional electron gas subjected to sweeping magnetic field. Their current-voltage characteristics exhibit pronounced nonlinearities caused by breakdown of FQHE at high currents. Our results support inter-Landau-level-scattering model and provide new local probe energy gaps $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/3$ $2/3$. They are also consistent...
Exchange enhancement of the Landau-level separation up to 30% has been observed in three high mobility GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As single heterojunctions. Analysis amplitude Shubnikov--de Haas oscillations as a function temperature and magnetic field allowed measurement this at filling factors \ensuremath{\nu}=100.
Hysteretic effects are reported in magneto-transport experiments on lateral quantum devices. The characterized by two vastly different relaxation times (minutes and days). It is shown that the observed phenomena related to long-lived eddy currents. This confirmed torsion-balance magnetometry measurements of same two-dimensional electron gas (2DEG) material. These observations show induced Hall potential at edges 2DEG reservoirs influences transport through devices, have important...
We have studied the effect of structural disorder on de Haas van Alphen (dHvA) and Shubnikov (SdH) quantum oscillations measured in single-crystal highly oriented pyrolytic graphite samples at temperatures down to 30 mK magnetic fields up 14 T. The measurements were performed different characterized by means x-ray diffraction, transmission electron microscopy atomic-force techniques. Our results reveal a correlation between amplitude surface roughness.
Torsion balance magnetometry can be used as a versatile probe of two-dimensional electron systems. We have developed highly sensitive magnetometer, utilizing capacitive proximity detection the rotor position, for use at temperatures below 10 mK. The instrument incorporates two piezo-electric linear motors to enable stator positions adjusted in situ base temperature. magnetometer responsivity is inversely proportional square rotor–stator separation and novel motor technique, accompanied by...
We have obtained a two-dimensional electron-solid phase diagram in the extreme magnetic quantum limit by studying temperature dependence of radiative recombination electrons GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunction with holes bound to \ensuremath{\delta} layer, 250 \AA{} away GaAs, Be acceptors. The low-energy shoulder luminescence line, indicating presence electron solid, is seen disappear at filling-factor-dependent critical...
The decay of quasi-persistent circulating currents in the dissipationless quantum Hall regime has been observed. induced by a time-varying magnetic field flow within two-dimensional electron system (2DES) embedded GaAs–(Al,Ga)As heterojunction. associated moment is measured using highly sensitive magnetometer. are observed to continue for many hours after sweep stopped indicating very low sheet resistivity. Two distinct current regimes observed, consisting an initial exponential lasting few...
We have investigated the time-dependent mobility and density of a modulation-doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterojunction subsequent to ionization $\mathrm{DX}$ centers using persistent photoconductivity effect. Our results are in excellent agreement with simple theory limited by independent charged scattering centers. show that these fluctuations average impurity potential which principally arise due...
We observe a novel breakdown of the quantum Hall effect in two-dimensional electron gas using contact-free method which circulating eddy currents are induced upon sweeping magnetic field, and detected via their moment. The largest around integer filling factor, where longitudinal resistivity ρxx → 0. Below about 1 K, strong "noisy" behaviour current is seen factor ν = 2. interpret this model accompanied by radial electric field consequent charge build up near sample edge. Breakdown occurs at...
We present magnetophotoluminescence (PL) investigations of a range very high-mobility low-density two-dimensional hole systems in GaAs-${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum wells, obtained using excitation both above and below the ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As band gap. At low densities, we observe two peaks at zero magnetic field an analysis temperature excitation-power dependences...