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Search Results: 1 - 10 of 309235 matches for " C. T. Foxon "
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Thermo-Electric Properties of Quantum Point Contacts
H. van Houten,L. W. Molenkamp,C. W. J. Beenakker,C. T. Foxon
Physics , 2005, DOI: 10.1088/0268-1242/7/3B/052
Abstract: I. Introduction II. Theoretical background (Landauer-Buttiker formalism of thermo-electricity, Quantum point contacts as ideal electron waveguides, Saddle-shaped potential) III. Experiments (Thermopower, Thermal conductance, Peltier effect) IV. Conclusions
Pulsed Magnetic Field Measurements of the Composite Fermion Effective Mass
D. R. Leadley,M. van der Burgt,R. J. Nicholas,C. T. Foxon,J. J. Harris
Physics , 1995, DOI: 10.1103/PhysRevB.53.2057
Abstract: Magnetotransport measurements of Composite Fermions (CF) are reported in 50 T pulsed magnetic fields. The CF effective mass is found to increase approximately linearly with the effective field $B^*$, in agreement with our earlier work at lower fields. For a $B^*$ of 14 T it reaches $1.6m_e$, over 20 times the band edge electron mass. Data from all fractions are unified by the single parameter $B^*$ for all the samples studied over a wide range of electron densities. The energy gap is found to increase like $\sqrt{B^*}$ at high fields.
Phonon drag thermopower and weak localization
A. Miele,R. Fletcher,E. Zaremba,Y. Feng,C. T. Foxon,J. J. Harris
Physics , 1998, DOI: 10.1103/PhysRevB.58.13181
Abstract: Previous experimental work on a two-dimensional (2D) electron gas in a Si-on-sapphire device led to the conclusion that both conductivity and phonon drag thermopower $S^g$ are affected to the same relative extent by weak localization. The present paper presents further experimental and theoretical results on these transport coefficients for two very low mobility 2D electron gases in $\delta-$doped GaAs/Ga$_x$Al$_{1-x}$As quantum wells. The experiments were carried out in the temperature range 3-7K where phonon drag dominates the thermopower and, contrary to the previous work, the changes observed in the thermopower due to weak localization were found to be an order of magnitude less than those in the conductivity. A theoretical framework for phonon drag thermopower in 2D and 3D semiconductors is presented which accounts for this insensitivity of $S^g$ to weak localization. It also provides transparent physical explanations of many previous experimental and theoretical results.
Critical Collapse of the Exchange Enhanced Spin Splitting in 2-D Systems
D. R. Leadley,R. J. Nicholas,J. J. Harris,C. T. Foxon
Physics , 1998, DOI: 10.1103/PhysRevB.58.13036
Abstract: The critical filling factor v_c where Shubnikov-de Haas oscillations become spin split is investigated for a set of GaAs-GaAlAs heterojunctions. Finite temperature magnetoresistance measurements are used to extract the value of v_c at zero temperature. The critically point is where the disorder potential has the same magnitude as the exchange energy, leading to the empirical relationship v_c = g* n t h / 2 m_0. This is valid for all the samples studied, where the density n and single particle lifetime t both vary by more than an order of magnitude and g* the exchange enhanced g-factor has a weak dependence on density. For each sample the spin gap energy shows a linear increase with magnetic field. Experiments in tilted magnetic field show the spin gap is the sum of the bare Zeeman energy and an exchange term. This explains why measurements of the enhanced g-factor from activation energy studies in perpendicular field and the coincidence method in tilted fields have previously disagreed.
Magnetism in (Ga,Mn)As Thin Films With TC Up To 173K
K. Y. Wang,R. P. Campion,K. W. Edmonds,M. Sawicki,T. Dietl,C. T. Foxon,B. L. Gallagher
Physics , 2004, DOI: 10.1063/1.1994124
Abstract: We have investigated the magnetic properties of (Ga,Mn)As thin films with Mn concentration between 1 and 9%. Ferromagnetic transition temperatures TC of up to 173K are observed. The results are compared to the predictions of the Zener mean-field theory. We find no evidence of a fundamental limit to TC.
Magneto-optical and micromagnetic simulation study the current driven domain wall motion in ferromagnetic (Ga,Mn)As
K. Y. Wang,A. C. Irvine,R. P. Campion,C. T. Foxon,J. Wunderlich,D. A. Williams,B. L. Gallagher
Physics , 2008, DOI: 10.1016/j.jmmm.2008.03.022
Abstract: We have studied current-driven domain wall motion in modified Ga_0.95Mn_0.05As Hall bar structures with perpendicular anisotropy by using spatially resolved Polar Magneto-Optical Kerr Effect Microscopy and micromagnetic simulation. Regardless of the initial magnetic configuration, the domain wall propagates in the opposite direction to the current with critical current of 1~2x10^5A/cm^2. Considering the spin transfer torque term as well as various effective magnetic field terms, the micromagnetic simulation results are consistent with the experimental results. Our simulated and experimental results suggest that the spin-torque rather than Oersted field is the reason for current driven domain wall motion in this material.
Simulating multiple quantum well solar cells
James P. Connolly,Jenny Nelson,Keith W. J. Barnham,Ian Ballard,C. Roberts,J. S. Roberts,C. T. Foxon,.
Physics , 2010, DOI: 10.1109/PVSC.2000.916130
Abstract: The quantum well solar cell (QWSC) has been proposed as a route to higher efficiency than that attainable by homojunction devices. Previous studies have established that carriers escape the quantum wells with high efficiency in forward bias and contribute to the photocurrent. Progress in resolving the efficiency limits of these cells has been dogged by the lack of a theoretical model reproducing both the enhanced carrier gen- eration and enhanced recombination due to the quantum wells. Here we present a model which calculates the incremental generation and recombination due to the QWs and is verified by modelling the experimental light and dark current-voltage characteristics of a range of III-V quantum well structures. We find that predicted dark currents are significantly greater than experiment if we use lifetimes derived from homostructure devices. Successful simulation of light and dark currents can be obtained only by introducing a parameter which represents a reduction in the quasi-Fermi level separation.
GaMnAs grown on (311) GaAs substrates: modified Mn incorporation and new magnetic anisotropies
K. Y. Wang,K. W. Edmonds,L. X. Zhao,M. Sawicki,R. P. Campion,B. L. Gallagher,C. T. Foxon
Physics , 2005, DOI: 10.1103/PhysRevB.72.115207
Abstract: We report the results of a detailed study of the structural, magnetic and magnetotransport properties of as-grown and annealed Ga0.91Mn0.09As thin films grown on (311)A and (311)B GaAs substrates. The high Curie temperature and hole density of the (311)B material are comparable to those of GaMnAs grown on (001) GaAs under the same growth conditions, while they are much lower for the (311)A material. We find evidence that Mn incorporation is more efficient for (311)B than for (001) and significantly less efficient for (311)A which is consistent with the bonding on these surfaces. This indicates that growth on (311)B may be a route to increased Curie temperatures in GaMnAs. A biaxial magnetic anisotropy is observed for the (311) material with easy axes along the [010] and [001] out-of-plane directions. An additional uniaxial in-plane anisotropy is also observed with the easy axis along for the (311)A material, and along for the (311)B material. This new observation may be of importance for the resolution of the outstanding problem of the origin of uniaxial anisotropy in (001) GaMnAs.
Achieving High Curie Temperature in (Ga,Mn)As
M Wang,R P Campion,A W Rushforth,K W Edmonds,C T Foxon,B L Gallagher
Physics , 2008, DOI: 10.1063/1.2992200
Abstract: We study the effects of growth temperature, Ga:As ratio and post-growth annealing procedure on the Curie temperature, Tc, of (Ga,Mn)As layers grown by molecular beam epitaxy. We achieve the highest Tc values for growth temperatures very close to the 2D-3D phase boundary. The increase in Tc, due to the removal of interstitial Mn by post growth annealing, is counteracted by a second process which reduces Tc and which is more effective at higher annealing temperatures. Our results show that it is necessary to optimize the growth parameters and post growth annealing procedure to obtain the highest Tc.
Anisotropic Magnetoresistance and Magnetic Anisotropy in High-quality (Ga,Mn)As Films
K. Y. Wang,K. W. Edmonds,R. P. Campion,L. X. Zhao,C. T. Foxon,B. L. Gallagher
Physics , 2005, DOI: 10.1103/PhysRevB.72.085201
Abstract: We have performed a systematic investigation of magnetotransport of a series of as-grown and annealed Ga1-xMnxAs samples with 0.011 <= x <= 0.09. We find that the anisotropic magnetoresistance (AMR) generally decreases with increasing magnetic anisotropy, with increasing Mn concentration and on low temperature annealing. We show that the uniaxial magnetic anisotropy can be clearly observed from AMR for the samples with x >= 0.02. This becomes the dominant anisotropy at elevated temperatures, and is shown to rotate by 90o on annealing. We find that the in-plane longitudinal resistivity depends not only on the relative angle between magnetization and current direction, but also on the relative angle between magnetization and the main crystalline axes. The latter term becomes much smaller after low temperature annealing. The planar Hall effect is in good agreement with the measured AMR indicating the sample is approximately in a single domain state throughout most of the magnetisation reversal, with a two-step magnetisation jump ascribed to domain wall nucleation and propagation.
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