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Biomedical subjects

D A Ritchie

Publications and source records attributed to D A Ritchie.

At least 37 records · Page 2Linked to original sources
PubMed2002

Precession and motional slowing of spin evolution in a high mobility two-dimensional electron gas.

Optical spin-dynamic measurements in a high-mobility n-doped GaAs/AlGaAs quantum well show oscillatory evolution at 1.8 K consistent with a quasi-collision-free D'yakonov-Perel'-Kachorovskii regime. Above 5 K evolution becomes exponential as expected for collision-dominated spin dynamics. Momentum scattering times extracted from Hall mobility and Monte Carlo simulation of spin polarization agree at 1.8 K but diverge at higher temperatures, indicating the importance of electron-electron scattering and an intrinsic upper limit for the spin-relaxation rate.

Journal Article↗
PubMed2002

Kondo effect in a quantum antidot.

We report Kondo-like behavior in a quantum antidot (a submicron depleted region in a two-dimensional electron gas) in the quantum-Hall regime. When both spins of the lowest Landau level are present all around the antidot, the resonances between extended edge states via antidot bound states show an abnormal feature in alternate Coulomb-blockaded regions. The feature becomes suppressed when the temperature or source-drain bias is raised as for Kondo resonances in quantum dots. Although the exact mechanism is unknown, Kondo-like correlated tunneling may arise from a Skyrmion-type edge reconstruction. This observation demonstrates the generality of the Kondo phenomenon.

Journal Article↗
PubMed2002

Tunneling spectroscopy of a two-dimensionally periodic electron system.

The tunneling current between an electron gas with a periodic potential in two dimensions and a plain two-dimensional electron system (2DES) has been studied. The strength of the periodic potential, the subband energy of the plain 2DES, and an applied in-plane magnetic field were varied, mapping the Fourier transform of the periodic wave function. Periodic peaks were observed and explained by translations in the reciprocal lattice. When the potential was strongly modulated to form an array of antidots, commensurability peaks were seen in lateral transport, but, as expected, not in tunneling.

Journal Article↗
PubMed2002

Hole-hole interaction effect in the conductance of the two-dimensional hole gas in the ballistic regime.

On a high-mobility two-dimensional hole gas (2DHG) in a GaAs/GaAlAs heterostructure we study the interaction correction to the Drude conductivity in the ballistic regime, k(B)Ttau/ variant Planck's over 2pi >1. It is shown that the "metallic" behavior of the resistivity (drho/dT>0) of the low-density 2DHG is caused by the hole-hole interaction effect in this regime. We find that the temperature dependence of the conductivity and the parallel-field magnetoresistance are in agreement with this description, and determine the Fermi-liquid interaction constant Fsigma0 which controls the sign of drho/dT.

Journal Article↗
PubMed2001

Observation of charge transport by negatively charged excitons.

We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X-), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X- mobility is found to be as high as 6.5 x 10(4) cm2 V-1 s-1. The results demonstrate that X- exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.

Journal Article↗
PubMed2001

Metallic behavior in dilute two-dimensional hole systems.

We have studied the metallic behavior in low-density two-dimensional p-GaAs systems, close to the apparent metal-insulator transition. Two observations are made concerning the origins of the metallic-like behavior. Within a given sample the strength of the metallic behavior is almost independent of the asymmetry of the confining potential, and is predominantly determined by the low-temperature resistivity (i.e., by k(F)l). In all our samples we find that at low densities, close to the transition from insulating to metallic behavior, the fractional decrease in conductivity with increasing temperature scales as T/T(F).

Journal Article↗
PubMed2001

Evolution of fractal patterns during a classical-quantum transition.

We investigate how fractals evolve into nonfractal behavior as the generation process is gradually suppressed. Fractals observed in the conductance of semiconductor billiards are of particular interest because the generation process is semiclassical and can be suppressed by transitions towards either fully classical or fully quantum-mechanical conduction. Investigating a range of billiards, we identify a "universal" behavior in the changeover from fractal to nonfractal conductance, which is described by a smooth evolution rather than deterioration in the fractal scaling properties.

Journal Article↗
PubMed2001

Fermi-liquid behavior of the low-density 2D hole gas in a GaAs/AlGaAs heterostructure at large values of r(s).

We examine the validity of the Fermi-liquid description of the dilute 2D hole gas in the crossover from "metallic"-to-"insulating" behavior of rho(T). It has been established that, at r(s) as large as 29, negative magnetoresistance does exist and is well described by weak localization theory. The dephasing time, extracted from the magnetoresistance, is dominated by the T2 term due to hole-hole scattering in the clean limit. The effect of hole-hole interactions, however, is suppressed when compared with the theory derived for small r(s).

Journal Article↗
PubMed2001

Bizarre parosteal osteochondromatous proliferation with cortical invasion.

A 15-year-old male presented with an 18-month history of increasing swelling on the radial aspect of his left forearm. Radiographs and MR images showed a partly calcified mass on the lateral border of the radius with erosion of the underlying cortex. Following a CT-guided needle biopsy, the mass was excised by segmental resection of the radial shaft with replacement by a vascularised fibular graft. Histological examination of the resected specimen showed many features typical of a bizarre parosteal osteochondromatous proliferation (BPOP) but with destruction of the underlying cortical bone by cellular spindle cell tissue. Although there were no cytological signs of malignancy, we believe that this unusual disease should be regarded as a particularly aggressive form of BPOP and treated by wide excision. Clinical follow-up of our patient at 2 years shows no sign of local recurrence or metastasis.

Adolescent↗
PubMed2000

Combined molecular ecological and confocal laser scanning microscopic analysis of peat bog methanogen populations.

Confocal laser scanning microscopy, using fluorescently labelled oligonucleotide probes targeting the 16S rRNA of different physiological groups of methanogens, was used to identify which methanogenic genera were present and to describe their in situ spatial locations in samples taken at different depths from blanket peat bog cores. Total bacterial DNA was also extracted and purified from the samples and used as template for amplification of 16S rRNA and regions of methyl CoM reductase-encoding genes using the polymerase chain reaction, as well as for oligonucleotide hybridisation experiments. These techniques, used in concert, demonstrated that methanogens of several physiological groups were present in highest numbers in the mid regions of 25 cm deep peat cores. Some discrepancies were apparent in the findings of the microscopic and molecular methods, though these may be partially accounted for by the different sensitivities of the techniques employed. The combined approaches used in this study gave an insight into the diversity and distribution of methanogens in peat environments not possible using molecular ecological methods alone.

DNA, Archaeal↗
PubMed2000

A standardised neurosurgical referral letter for the inter-hospital transfer of head injured patients.

OBJECTIVES: (1) To evaluate the use of a standardised neurosurgical referral letter in terms of compliance, completeness and clinical relevance. (2) To compare the clinical information provided on the standardised neurosurgical letter with that provided by referring hospitals that used alternative documentation. DESIGN: A six month prospective audit was conducted in south west Scotland. Consultant neurosurgeons were asked to weight key clinical variables on the neurosurgical referral letter (NRL). Postal surveys of 114 referring accident and emergency (A&E) staff and 18 neurosurgical receiving staff were undertaken to determine the clinical relevance of the NRL. Case notes were examined for the presence and level of completeness of the NRL. In the absence of the NRL, a form was completed retrospectively using data from the referring hospital's letter. This enabled comparison of the NRL with routine hospital letters in terms of the availability of key clinical information. RESULTS: 139 adult patients were identified as suitable for inclusion: 99 patients were transferred from 11 hospitals with access to the NRL. The compliance rate for use of the NRLwas 82%. Forty patients were transferred from nine hospitals that did not have access to the NRL. The completion rate of key variables on the NRL was higher than when an ordinary letter was sent: 87% compared with 38%. The NRL was considered useful by 67 of 71 (94%) A&E questionnaire respondents and by 14 of 15 neurosurgeons who responded. CONCLUSIONS: The widespread acceptance of the NRL and its ability to provide essential clinical information in a concise format not available in routine hospital letters indicates that national, standardised documentation can be implemented if users are involved in both its design and implementation.

Adult↗
PubMed1999

MR imaging of synovial tumours and tumour-like lesions.

MR is the imaging method of choice for detection, staging and follow-up of synovial tumours and tumour-like conditions. The majority of lesions have non-specific features but careful analysis of lesion morphology and signal characteristics reduces the differential diagnosis and leads to a specific diagnosis in some cases.

Diagnosis, Differential↗
PubMed1998

The 'penumbra sign' on T1-weighted MR imaging in subacute osteomyelitis: frequency, cause and significance.

OBJECTIVE: We studied the frequency and cause of a feature exhibited on T1-weighted (T1-W) magnetic resonance (MR) imaging termed the 'penumbra sign' in a series of patients presenting with osteomyelitis and correlated it with the double-line sign described as a T2-weighted (T2-W) or short tau inversion recovery (STIR) feature of both the Brodie's abscess and avascular necrosis. MATERIALS AND METHODS: The clinical, radiographic, MR imaging, microbiological and histological findings in 32 patients referred to an orthopaedic oncology service, but subsequently proven to have osteomyelitis, were reviewed. The presence or absence of a rim of tissue lining an abscess cavity typified by minor signal hyperintensity relative to the main abscess contents on T1-W MR imaging (the 'penumbra sign') was identified. The sign was correlated with the radiographic and other findings. RESULTS: The penumbra sign was identified in 24 cases (75%) and appears to be a more sensitive sign than the corresponding double-line sign which was evident in only 29% of these on T2-W or fast STIR images. The lesions were unilocular in 11 cases (46%) and multilocular in 13 (54%). The thickness of the penumbra ranged from 2 to 5mm. On histological examination the tissue comprising the penumbra sign was found to be highly vascularized granulation tissue containing thick walled arterioles. CONCLUSION: The penumbra sign is characteristically seen on T1-W MR images in subacute osteomyelitis and is due to a thick layer of highly vascularized granulation tissue which may not be visible as the double-line sign on T2-W or fast STIR sequences. This characteristic, but not pathognomonic, MR finding supports the diagnosis of bone infection and helps to exclude the presence of a tumour.

Abscess↗
PubMed1997

Distribution, diversity and evolution of the bacterial mercury resistance (mer) operon.

Mercury and its compounds are distributed widely across the earth. Many of the chemical forms of mercury are toxic to all living organisms. However, bacteria have evolved mechanisms of resistance to several of these different chemical forms, and play a major role in the global cycling of mercury in the natural environment. Five mechanisms of resistance to mercury compounds have been identified, of which resistance to inorganic mercury (HgR) is the best understood, both in terms of the mechanisms of resistance to mercury and of resistance to heavy metals in general. Resistance to inorganic mercury is encoded by the genes of the mer operon, and can be located on transposons, plasmids and the bacterial chromosome. Such systems have a worldwide geographical distribution, and furthermore, are found across a wide range of both Gram-negative and Gram-positive bacteria from both natural and clinical environments. The presence of mer genes in bacteria from sediment cores suggest that mer is an ancient system. Analysis of DNA sequences from mer operons and genes has revealed genetic variation both in operon structure and between individual genes from different mer operons, whilst analysis of bacteria which are sensitive to inorganic mercury has identified a number of vestigial non-functional operons. It is hypothesised that mer, due to its ubiquity with respect to geographical location, environment and species range, is an ancient system, and that ancient bacteria carried genes conferring resistance to mercury in response to increased levels of mercury in natural environments, perhaps resulting from volcanic activity. Models for the evolution of both a basic mer operon and for the Tn21-related family of mer operons and transposons are suggested. The study of evolution in bacteria has recently become dominated by the generation of phylogenies based on 16S rRNA genes. However, it is important not to underestimate the roles of horizontal gene transfer and recombinational events in evolution. In this respect mer is a suitable system for evaluating phylogenetic methods which incorporate the effects of horizontal gene transfer. In addition, the mer operon provides a model system in the study of environmental microbiology which is useful both as an example of a genotype which is responsive to environmental pressures and as a generic tool for the development of new methodology for the analysis of bacterial communities in natural environments.

Biological Evolution↗