Tuning terahertz transitions in a double-gated quantum ring

We theoretically investigate the optical functionality of a semiconducting quantum ring manipulated by two electrostatic lateral gates used to induce a double quantum well along the ring. The well parameters and corresponding inter-level spacings, which lie in the THz range, are highly sensitive to the gate voltages. Our analysis shows that selection rules for inter-level dipole transitions, caused by linearly polarized excitations, depend on the polarization angle with respect to the gates. In striking difference from the conventional symmetric double well potential, the ring geometry permits polarization-dependent transitions between the ground and second excited states, allowing the use of this structure in a three-level lasing scheme.Comment: 7 pages, 6 figure

UV Spectroscopy of AB Doradus with the Hubble Space Telescope. Impulsive flares and bimodal profiles of the CIV 1549 line in a young star

We observed AB Doradus, a young and active late type star (K0 - K2 IV-V, P= 0.514 d) with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with the time and spectral resolutions of 27 s and 15 km, respectively. The wavelength band (1531 - 1565 A) included the strong CIV doublet (1548.202 and 1550.774, formed in the transition region at 100 000 K). The mean quiescent CIV flux state was close to the saturated value and 100 times the solar one. The line profile (after removing the rotational and instrumental profiles) is bimodal consisting of two Gaussians, narrow (FWHM = 70 km/s) and broad (FWHM =330km/s). This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that TR transient events (random multiple velocities), with a large surface coverage, give rise to the broadening of the narrow component,while true microflaring is responsible for the broad one. The transition region was observed to flare frequently on different time scales and magnitudes. The largest impulsive flare seen in the CIV 1549 emission reached in less than one minute the peak differential emission measure (10**51.2 cm-3) and returned exponentially in 5 minutes to the 7 times lower quiescent level.The 3 min average line profile of the flare was blue-shifted (-190 km/s) and broadened (FWHM = 800 km/s). This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam, accelerated (by reconnection) at the apex of a coronal loop.Comment: to be published in AJ (April 98), 3 tables and 7 figures as separate PS-files, print Table 2 as a landscap

Pattern formation by lateral inhibition with feedback: a mathematical model of Delta-Notch intercellular signalling

In many developing tissues, adjacent cells diverge in character so as to create a fine-grained pattern of cells in contrasting states of differentiation. It has been proposed that such patterns can be generated through lateral inhibition—a type cells–cell interaction whereby a cell that adopts a particular fate inhibits its immediate neighbours from doing likewise. Lateral inhibition is well documented in flies, worms and vertebrates. In all of these organisms, the transmembrane proteins Notch and Delta (or their homologues) have been identified as mediators of the interaction—Notch as receptor, Delta as its ligand on adjacent cells. However, it is not clear under precisely what conditions the Delta-Notch mechanism of lateral inhibition can generate the observed types of pattern, or indeed whether this mechanism is capable of generating such patterns by itself. Here we construct and analyse a simple and general mathematical model of such contact-mediated lateral inhibition. In accordance with experimental data, the model postulates that receipt of inhibition (i.e. activation of Notch) diminishes the ability to deliver inhibition (i.e. to produce active Delta). This gives rise to a feedback loop that can amplify differences between adjacent cells. We investigate the pattern-forming potential and temporal behavior of this model both analytically and through numerical simulation. Inhomogeneities are self-amplifying and develop without need of any other machinery, provided the feedback is sufficiently strong. For a wide range of initial and boundary conditions, the model generates fine-grained patterns similar to those observed in living systems

High aspect ratio silicon dioxide-coated single-walled carbon nanotube scanning probe nanoelectrodes

We have fabricated high aspect ratio, hydrophilic nanoelectrodes from individual single-walled carbon nanotubes (SWNTs) mounted on conductive atomic force microscope (AFM) tips for use as electrochemical probes. Individual SWNTs with an average diameter of 5 nm and up to 1.5 μm in length were passivated with nanometer-thick SiO_2 films, deposited conformally in an inductively coupled plasma reactor. The electrically insulating SiO_2 films improved the nanotube rigidity and stabilized the nanotube−AFM tip contact to enable use in aqueous environments. The nanotube tip was successfully exposed by subjecting the probe to nanosecond electrical pulse etching but only after electron beam irradiation in a transmission electron microscope (TEM). Probe functionality was verified by electrodepositing gold nanoparticles from aqueous solution only at the exposed tip

Bistable molecular conductors with a field-switchable dipole group

A class of bistable "stator-rotor" molecules is proposed, where a stationary bridge (stator) connects the two electrodes and facilitates electron transport between them. The rotor part, which has a large dipole moment, is attached to an atom of the stator via a single sigma bond. Hydrogen bonds formed between the rotor and stator make the symmetric orientation of the dipole unstable. The rotor has two potential minima with equal energy for rotation about the sigma bond. The dipole orientation, which determines the conduction state of the molecule, can be switched by an external electric field that changes the relative energy of the two potential minima. Both orientation of the rotor correspond to asymmetric current-voltage characteristics that are the reverse of each other, so they are distinguishable electrically. Such bistable stator-rotor molecules could potentially be used as parts of molecular electronic devices.Comment: 8 pages, 7 figure

Investigating the beneficial experiences of online peer support for those affected by alopecia: An interpretative phenomenological analysis using online interviews

BACKGROUND: Those affected by alopecia are at risk of experiencing a range of psychosocial consequences. Alopecia UK provide online peer support via facilitated Facebook pages. Online peer support has been found to provide a variety of benefits in other long term conditions, such as providing information and emotional support. OBJECTIVES: This study sought to gain an experiential account as to how online support provides benefit to people living with alopecia. METHODS: A purposive sample of 12 participants was recruited directly from Alopecia UK's Facebook group. A total of 11 were diagnosed with alopecia and 1 participant was a family member of someone with alopecia. The qualitative approach used to guide data collection and analysis was Interpretative Phenomenological Analysis. Participants took part in online synchronous interviews, which drew on review of participant-selected screenshots of online activity. RESULTS: Four themes were identified, including 'Gradual Healing', 'Image Concern', 'Belonging', and 'New Identity and Self-Acceptance'. The participants made use of the group through expressing emotion, finding practical advice, and also as a place to connect and share experiences. Participants also reported that the online support facilitated the return of social confidence. CONCLUSIONS: This study provides a detailed understanding as to how online peer support groups can be beneficial. Online groups have the potential to create a feeling of belonging, that can develop a sense of being accepted and understood and as such can be important in maintaining psychological well-being. The findings indicate that online groups have the potential to be used as a platform for assisting people in developing both effective coping styles and feeling understood, and such platforms warrant further investment by healthcare professionals

Performance measurement and management in portuguese law enforcement

Performance measurement of police services is complicated by ambiguous and complex goal- and objectives-setting, and by the difficulties of measuring outputs. This article looks at the organizational and management changes being made in Portuguese police forces. The authors fill a gap in the literature on performance measurement in Portugal by taking a national approach to the study of how law enforcement agencies are introducing new management accounting changes. The article therefore widens the debate on performance measurement and performance improvements in law enforcement

The resilience of indigenous knowledge in small-scale African agriculture: key drivers

The successful use of indigenous knowledge (IK) in development practice in rural Africa over the last couple of decades has proved to be elusive and disappointing. Using empirical field data from northern Malawi, this study suggests that the two key drivers for farmers in this area are household food security and the maintenance of soil fertility. Indigenous ways of knowing underpin the agricultural system which has been developed, rather than the adoption of more modern, ‘scientific’ ways, to deliver against these drivers. Such IKs, however, are deeply embedded in the economic, social and cultural environments in which they operate