Exactly-solvable problems for two-dimensional excitons

Several problems in mathematical physics relating to excitons in two dimensions are considered. First, a fascinating numerical result from a theoretical treatment of screened excitons stimulates a re-evaluation of the familiar two-dimensional hydrogen atom. Formulating the latter problem in momentum space leads to a new integral relation in terms of special functions, and fresh insights into the dynamical symmetry of the system are also obtained. A discussion of an alternative potential to model screened excitons is given, and the variable phase method is used to compare bound-state energies and scattering phase shifts for this potential with those obtained using the two-dimensional analogue of the Yukawa potential. The second problem relates to excitons in a quantising magnetic field in the fractional quantum Hall regime. An exciton against the background of an incompressible quantum liquid is modelled as a few-particle neutral composite consisting of a positively-charged hole and several quasielectrons with fractional negative charge. A complete set of exciton basis functions is derived, and these functions are classified using a result from the theory of partitions. Some exact results are obtained for this complex few-particle problem.Comment: 66 pages, 9 figure

The Stability of Dispersions in Non-Aqueous Media

Results obtained in studies of coagulation behaviour of some hydrophilic (rutile) and hydrophobic (graphitised carbon black Sterling MTG) materials in solutions of the anionic surface active agent Aerosol OT (Na-di-2-ethylhexyl sulphosuccinate) in hydrocarbons (xylene) are discussed. Using particle sizes, determined by electron microscopy, and zeta-potentials, obtained from microelectrophoretic measurements, as parameters to calculate energy barriers, it was found that the Deryaguin-Landau-Verwey-Overbeek theory allows satisfactory quantitative interpretation

Precipitation of Hydrolysis Products on to Oxide Surfaces

Several complex physico-chemical phenomena occur when oxide surfaces are brought into contact with solutions of hydrolysable salts. Both the hydrolysis and the surface chemistry of the oxide are pH dependent, and by appropriate variation of pH the adsorption of solution species at the surface may be controlled to bring about such effects as charge reversal and coagulation of the oxide particles, growth of a new phase at the surface, etc., many of which have technological significance

Influence of surgery and rehabilitation conditioning on psychophysiological fitness

The purpose of this study was to assess changes in psychophysiological fitness following reconstructive knee surgery and early phase (2.5 months) physical rehabilitation. Nine patients (7 male, 2 female; mean age, 29.9 years) electing to undergo anterior cruciate ligament reconstructive surgery (central third, bone-patella tendon-bone graft) were assessed on four separate assessment occasions post-surgery. Repeated measures ANOVAs showed significant condition (injured/non-injured leg) by test occasion (2 weeks pre-surgery and 6, 8 and 10 weeks post-surgery) interactions for knee ligamentous compliance (anterior tibiofemoral displacement), peak force and electromechanical delay associated with the knee flexors of the injured and noninjured legs (F3,24 = 4.7 to 6.6; p < 0.01), together with individualized emotional profile disturbance scores that were significantly less at 10 weeks post-surgery compared to pre-surgery, 6 weeks and 8 weeks post-surgery (F3,24 = 7.6; p < 0.01). Spearman rank correlation coefficients identified significant relationships between musculoskeletal fitness and emotional profile scores at pre-surgery (r = 0.69–0.72; p < 0.05) and at 8 weeks post-surgery (r = 0.70–0.73; p < 0.05). The 6 Bi-POMS subscales and the 12 ERAIQ responses found inconsistent patterns of response and relationships across the assessment occasions. Overall, the patterning of changes and associations amongst emotional performance profile discrepancy scores in conjunction with those scores from indices of musculoskeletal fitness performance capability offered important support for the efficacy of an approach which integrates self-perceptive and objective measurements of fitness capability during rehabilitation following surgery to a synovial joint

Acute affective responses to prescribed and self-selected exercise sessions in adolescent girls: an observational study

Background Positive affective responses can lead to improved adherence to exercise. This study sought to examine the affective responses and exercise intensity of self-selected exercise in adolescent girls. Methods An observational study where twenty seven females (Age M?=?14.6?�?0.8 years) completed three 20-minute exercise sessions (2 self-selected and 1 prescribed intensity) and a graded exercise test. The intensity of the prescribed session was matched to the first self-selected session. Intensity, affective responses and ratings of perceived exertion were recorded throughout the sessions and differences examined. Repeated measures ANOVAs were conducted to examine differences. Results There were no significant differences in intensity between the prescribed and self-selected sessions, but affective responses were significantly more positive (p?<?.01) during the self-selected session. Ratings of perceived exertion were significantly lower (p?<?.01) during the self-selected session than the prescribed session. On average participants worked at 72% V?O2 peak; well within the intensity recommended by the American College of Sports Medicine. Conclusion Even though the intensity did not differ between the self-selected and prescribed sessions, there was a significant impact on affective responses, with more positive affective responses being elicited in the self-selected session. This highlights the importance of autonomy and self-paced exercise for affective responses and may have potential long-term implications for adherence

The Role of the Surface in the Behaviour of Titanium Dioxide Pigments

A review is presented on the relevance of surface properties in pigment behaviour. It is shown that the majority of titanium dioxide pigments in use have a coating of some other oxide. whose principal role is the formation of an energy barrier for the transfer of excited species from the bulk phase to the organic coating of the paint film. Results of recent studies of dispersion properties and durability of paints against photo-degradation are discussed

Precipitation of Hydrolysis Products on to Oxide Surfaces

Several complex physico-chemical phenomena occur when oxide surfaces are brought into contact with solutions of hydrolysable salts. Both the hydrolysis and the surface chemistry of the oxide are pH dependent, and by appropriate variation of pH the adsorption of solution species at the surface may be controlled to bring about such effects as charge reversal and coagulation of the oxide particles, growth of a new phase at the surface, etc., many of which have technological significance

The Stability of Dispersions in Non-Aqueous Media

Results obtained in studies of coagulation behaviour of some hydrophilic (rutile) and hydrophobic (graphitised carbon black Sterling MTG) materials in solutions of the anionic surface active agent Aerosol OT (Na-di-2-ethylhexyl sulphosuccinate) in hydrocarbons (xylene) are discussed. Using particle sizes, determined by electron microscopy, and zeta-potentials, obtained from microelectrophoretic measurements, as parameters to calculate energy barriers, it was found that the Deryaguin-Landau-Verwey-Overbeek theory allows satisfactory quantitative interpretation

Prefrontal cortex haemodynamics and affective responses during exercise: a multi-channel near infrared spectroscopy study

The dose-response effects of the intensity of exercise upon the potential regulation (through top-down processes) of affective (pleasure-displeasure) responses in the prefrontal cortex during an incremental exercise protocol have not been explored. This study examined the functional capacity of the prefrontal cortex (reflected by haemodynamics using near infrared spectroscopy) and affective responses during exercise at different intensities. Participants completed an incremental cycling exercise test to exhaustion. Changes (Δ) in oxygenation (O2Hb), deoxygenation (HHb), blood volume (tHb) and haemoglobin difference (HbDiff) were measured from bilateral dorsal and ventral prefrontal areas. Affective responses were measured every minute during exercise. Data were extracted at intensities standardised to: below ventilatory threshold, at ventilatory threshold, respiratory compensation point and the end of exercise. During exercise at intensities from ventilatory threshold to respiratory compensation point, ΔO2Hb, ΔHbDiff and ΔtHb were greater in mostly ventral than dorsal regions. From the respiratory compensation point to the end of exercise, ΔO2Hb remained stable and ΔHbDiff declined in dorsal regions. As the intensity increased above the ventilatory threshold, inverse associations between affective responses and oxygenation in (a) all regions of the left hemisphere and (b) lateral (dorsal and ventral) regions followed by the midline (ventral) region in the right hemisphere were observed. Differential activation patterns occur within the prefrontal cortex and are associated with affective responses during cycling exercise