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Search Results: 1 - 10 of 714320 matches for " A. M. Gilbertson "
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Zero-field spin-splitting and spin lifetime in n-InSb/In1-xAlxSb asymmetric quantum well heterostructures
A. M. Gilbertson,M. Fearn,J. H. Jefferson,B. N. Murdin,P. D. Buckle,L. F. Cohen
Physics , 2008, DOI: 10.1103/PhysRevB.77.165335
Abstract: The spin-orbit (SO) coupling parameters for lowest conduction subband due to structural (SIA) and bulk (BIA) inversion asymmetry are calculated for a range of carrier densities in [001]-grown delta-doped n-type InSb/In1-xAlxSb asymmetric quantum wells using the established 8 band k.p formalism [PRB 59,8 R5312 (1999)]. We present calculations for conditions of zero bias at 10 K. It is shown that both the SIA and BIA parameters scale approximately linearly with carrier density, and exhibit a marked dependence on well width when alloy composition is adjusted to allow maximum upper barrier height for a given well width. In contrast to other material systems the BIA contribution to spin splitting is found to be of significant and comparable value to the SIA mechanism in these structures. We calculate the spin lifetime for spins oriented along [11-0] based on D'yakonov-Perel mechanism using both the theory of Averkiev et al. [J. Phys.:Condens. Matter 14 (2002)] and also the rate of precession of spins about the effective magnetic field, taking into account all three SO couplings, showing good agreement.Spin lifeime for this direction is largest in the narrow wells over the range of moderate carrier densities considered, which is attributed to the reduced magnitude of the k-cubic BIA parameter in narrow wells. The inherently large BIA induced SO coupling in these systems is shown to have considerable effect on the spin lifetime, which exhibits significant reduction in the maximum spin lifetime compared to previous studies which consider systems with relatively weak BIA induced SO coupling. The relaxation rate of spins oriented in the [001] direction is dominated by the k-linear SIA and BIA coupling parameters and at least an order of magnitude greater than in the [11-0] direction.
High field magneto-transport in high mobility gated InSb/InAlSb quantum well heterostructures
A. M. Gilbertson,W. R. Branford,M. Fearn,L. Buckle,P. D. Buckle,T. Ashley,L. F. Cohen
Physics , 2009, DOI: 10.1103/PhysRevB.79.235333
Abstract: We present high field magneto-transport data from a range of 30nm wide InSb/InAlSb quantum wells. The low temperature carrier mobility of the samples studied ranged from 18.4 to 39.5 m2V-1s-1 with carrier densities between 1.5x1015 and 3.28x1015 m-2. Room temperature mobilities are reported in excess of 6 m2V-1s-1. It is found that the Landau level broadening decreases with carrier density and beating patterns are observed in the magnetoresistance with non-zero node amplitudes in samples with the narrowest broadening despite the presence of a large g-factor. The beating is attributed to Rashba splitting phenomenon and Rashba coupling parameters are extracted from the difference in spin populations for a range of samples and gate biases. The influence of Landau level broadening and spin-dependent scattering rates on the observation of beating in the Shubnikov-de Haas oscillations is investigated by simulations of the magnetoconductance. Data with non-zero beat node amplitudes are accompanied by asymmetric peaks in the Fourier transform, which are successfully reproduced by introducing a spin-dependent broadening in the simulations. It is found that the low-energy (majority) spin up state suffers more scattering than the high-energy (minority) spin down state and that the absence of beating patterns in the majority of (lower density) samples can be attributed to the same effect when the magnitude of the level broadening is large.
Ballistic transport and boundary scattering in InSb/InxAl1-xSb mesoscopic devices
A. M. Gilbertson,M. Fearn,A. Kormányos,D. E. Read,C. J. Lambert,M. T. Emeny,T. Ashley,S. A. Solin,L. F. Cohen
Physics , 2010,
Abstract: We describe the influence of hard wall confinement and lateral dimension on the low temperature transport properties of long diffusive channels and ballistic crosses fabricated in an InSb/InxAl1-xSb heterostructure. Partially diffuse boundary scattering is found to play a crucial role in the electron dynamics of ballistic crosses and substantially enhance the negative bend resistance. Experimental observations are supported by simulations using a classical billiard ball model for which good agreement is found when diffuse boundary scattering is included.
Cyclotron-resonance-assisted photon drag effect in InSb/InAlSb quantum wells excited by terahertz radiation
S. Stachel,G. V. Budkin,U. Hagner,V. V. Bel'kov,M. M. Glazov,S. A. Tarasenko,S. K. Clowes,T. Ashley,A. M. Gilbertson,S. D. Ganichev
Physics , 2013, DOI: 10.1103/PhysRevB.89.115435
Abstract: We report on the observation of the cyclotron-resonance-assisted photon drag effect. Resonant photocurrent is detected in InSb/InAlSb quantum wells structures subjected to a static magnetic field and excited by terahertz radiation at oblique incidence. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experimental findings. We show that the resonant photocurrent originates from the transfer of photon momentum to free electrons drastically enhanced at cyclotron resonance.
Evidence for Nodal superconductivity in Sr$_{2}$ScFePO$_{3}$
K. A. Yates,I. T. M. Usman,K. Morrison,J. D. Moore,A. M. Gilbertson,A. D. Caplin,L. F. Cohen,H. Ogino,J. Shimoyama
Physics , 2009, DOI: 10.1088/0953-2048/23/2/022001
Abstract: Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature, dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7T at 2K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.
Room temperature ballistic transport in InSb quantum well nanodevices
A. M. Gilbertson,A. Kormanyos,P. D. Buckle,M. Fearn,T. Ashley,C. J. Lambert,S. A. Solin,L. F. Cohen
Physics , 2011, DOI: 10.1063/1.3668107
Abstract: We report the room temperature observation of significant ballistic electron transport in shallow etched four-terminal mesoscopic devices fabricated on an InSb/AlInSb quantum well (QW) heterostructure with a crucial partitioned growth-buffer scheme. Ballistic electron transport is evidenced by a negative bend resistance signature which is quite clearly observed at 295 K and at current densities in excess of 10$^{6}$ A/cm$^{2}$. This demonstrates unequivocally that by using effective growth and processing strategies, room temperature ballistic effects can be exploited in InSb/AlInSb QWs at practical device dimensions.
Learning to Obtain Reward, but Not Avoid Punishment, Is Affected by Presence of PTSD Symptoms in Male Veterans: Empirical Data and Computational Model
Catherine E. Myers, Ahmed A. Moustafa, Jony Sheynin, Kirsten M. VanMeenen, Mark W. Gilbertson, Scott P. Orr, Kevin D. Beck, Kevin C. H. Pang, Richard J. Servatius
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0072508
Abstract: Post-traumatic stress disorder (PTSD) symptoms include behavioral avoidance which is acquired and tends to increase with time. This avoidance may represent a general learning bias; indeed, individuals with PTSD are often faster than controls on acquiring conditioned responses based on physiologically-aversive feedback. However, it is not clear whether this learning bias extends to cognitive feedback, or to learning from both reward and punishment. Here, male veterans with self-reported current, severe PTSD symptoms (PTSS group) or with few or no PTSD symptoms (control group) completed a probabilistic classification task that included both reward-based and punishment-based trials, where feedback could take the form of reward, punishment, or an ambiguous “no-feedback” outcome that could signal either successful avoidance of punishment or failure to obtain reward. The PTSS group outperformed the control group in total points obtained; the PTSS group specifically performed better than the control group on reward-based trials, with no difference on punishment-based trials. To better understand possible mechanisms underlying observed performance, we used a reinforcement learning model of the task, and applied maximum likelihood estimation techniques to derive estimated parameters describing individual participants’ behavior. Estimations of the reinforcement value of the no-feedback outcome were significantly greater in the control group than the PTSS group, suggesting that the control group was more likely to value this outcome as positively reinforcing (i.e., signaling successful avoidance of punishment). This is consistent with the control group’s generally poorer performance on reward trials, where reward feedback was to be obtained in preference to the no-feedback outcome. Differences in the interpretation of ambiguous feedback may contribute to the facilitated reinforcement learning often observed in PTSD patients, and may in turn provide new insight into how pathological behaviors are acquired and maintained in PTSD.
Interplay of spin and orbital magnetogyrotropic photogalvanic effects in InSb/AlInSb quantum well structures
S. Stachel,P. Olbrich,C. Zoth,U. Hagner,T. Stangl,C. Karl,P. Lutz,V. V. Bel'kov,S. K. Clowes,T. Ashley,A. M. Gilbertson,S. D. Ganichev
Physics , 2011, DOI: 10.1103/PhysRevB.85.045305
Abstract: We report on the observation of linear and circular magnetogyrotropic photogalvanic effects in InSb/AlInSb quantum well structures. We show that intraband (Drude-like) absorption of terahertz radiation in the heterostructures causes a dc electric current in the presence of an in-plane magnetic field. The photocurrent behavior upon variation of the magnetic field strength, temperature and wavelength is studied. We show that at moderate magnetic fields the photocurrent exhibits a typical linear field dependence. At high magnetic fields, however, it becomes nonlinear and inverses its sign. The experimental results are analyzed in terms of the microscopic models based on asymmetric relaxation of carriers in the momentum space. We demonstrate that the observed nonlinearity of the photocurrent is caused by the large Zeeman spin splitting in InSb/AlInSb structures and an interplay of the spin-related and spin-independent roots of the magnetogyrotropic photogalvanic effect.
So Far-So Close: Remembering Clive Robbins: Invited Editorial
Simon Gilbertson
Voices: A World Forum for Music Therapy , 2012,
Abstract:
The Silent Epidemic of Road Traffic Injury: What Can Music Therapists do About It?
Simon Gilbertson
Voices: A World Forum for Music Therapy , 2008,
Abstract: Can music therapists do anything about road traffic injury and its effects? Road traffic crashes are responsible for up to 1.2 million deaths and up to 50 million injuries globally each year. One quarter of these injuries are traumatic brain injuries. In this paper, the literature related to music therapy and traumatic brain injury is reviewed. By analysing this literature, it becomes apparent that music therapists have provided for those injured almost to the exclusion of those affected by traumatic brain injury, the family, the community and the society. Using literature related to trauma, the author discusses ways in which music therapists may change the scope of music therapy in relation to caring for people affected by road traffic injury and considers the role music therapists may play in the prevention of road traffic injury in the future.
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