Crystal structure of solid Oxygen at high pressure and low temperature

Results of X-ray diffraction experiments on solid oxygen at low temperature and at pressures up to 10 GPa are presented.A careful sample preparation and annealing around 240 K allowed to obtain very good diffraction patterns in the orthorhombic delta-phase. This phase is stable at low temperature, in contrast to some recent data [Y. Akahama et al., Phys. Rev. B64, 054105 (2001)], and transforms with decreasing pressure into a monoclinic phase, which is identified as the low pressure alpha-phase. The discontinuous change of the lattice parameters, and the observed metastability of the alpha-phase increasing pressure suggest that the transition is of the first order.Comment: 4 pages with three figure

Temperature dependence of surface reconstructions of Au on Pd(110)

Surface reconstructions of Au film on Pd(110) substrate are studied using a local Einstein approximation to quasiharmonic theory with the Sutton-Chen interatomic potential. Temperature dependent surface free energies for different coverages and surface structures are calculated. Experimentally observed transformations from $(1\times1)$ to $(1 \times 2)$ and $(1 \times 3)$ structures can be explained in the framework of this model. Also conditions for Stranski-Krastanov growth mode are found to comply with experiments. The domain of validity of the model neglecting mixing entropy is analyzed.Comment: 7 pages, REVTeX two-column format, 3 postscript figures available on request from [email protected] To appear in Phys. Rev. Letter

First principles study of strain/electronic interplay in ZnO; Stress and temperature dependence of the piezoelectric constants

We present a first-principles study of the relationship between stress, temperature and electronic properties in piezoelectric ZnO. Our method is a plane wave pseudopotential implementation of density functional theory and density functional linear response within the local density approximation. We observe marked changes in the piezoelectric and dielectric constants when the material is distorted. This stress dependence is the result of strong, bond length dependent, hybridization between the O $2p$ and Zn $3d$ electrons. Our results indicate that fine tuning of the piezoelectric properties for specific device applications can be achieved by control of the ZnO lattice constant, for example by epitaxial growth on an appropriate substrate.Comment: accepted for publication in Phys. Rev.

Daylight photodynamic therapy:patient willingness to undertake home treatment

In the UK, almost one in four individuals over 60 are affected by actinic keratoses (AK), and this is a cause of significant morbidity in an ageing population, with risk of progression to squamous cell carcinoma. Daylight photodynamic therapy (dPDT) is an effective and simple treatment for field change AK, with similar efficacy to conventional PDT. Commonly, skin surface preparation is performed in a Dermatology clinic prior to dPDT. However, a recent German study by Karrer and colleagues investigated dPDT as a patient‐applied home‐delivered treatment for face and scalp AK and reported that patients who undertook this self‐administered treatment had high levels of efficacy, tolerance and patient satisfaction.<br/

Hydrogen atom in a spherical well: linear approximation

We discuss the boundary effects on a quantum system by examining the problem of a hydrogen atom in a spherical well. By using an approximation method which is linear in energy we calculate the boundary corrections to the ground-state energy and wave function. We obtain the asymptotic dependence of the ground-state energy on the radius of the well.Comment: Revised version to appear in European Journal of Physic

The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics

A longstanding limitation of first-principles calculations of substitutional alloy phase diagrams is the difficulty to account for lattice vibrations. A survey of the theoretical and experimental literature seeking to quantify the impact of lattice vibrations on phase stability indicates that this effect can be substantial. Typical vibrational entropy differences between phases are of the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of configurational entropy differences in binary alloys (at most 0.693 k_B/atom). This paper describes the basic formalism underlying ab initio phase diagram calculations, along with the generalization required to account for lattice vibrations. We overview the various techniques allowing the theoretical calculation and the experimental determination of phonon dispersion curves and related thermodynamic quantities, such as vibrational entropy or free energy. A clear picture of the origin of vibrational entropy differences between phases in an alloy system is presented that goes beyond the traditional bond counting and volume change arguments. Vibrational entropy change can be attributed to the changes in chemical bond stiffness associated with the changes in bond length that take place during a phase transformation. This so-called ``bond stiffness vs. bond length'' interpretation both summarizes the key phenomenon driving vibrational entropy changes and provides a practical tool to model them.Comment: Submitted to Reviews of Modern Physics 44 pages, 6 figure

Is there an optimal irradiation dose for photodynamic therapy: 37 Jcm^-2 or 75 Jcm^-2?

Topical photodynamic therapy (PDT) is widely used for the treatment of Bowen’s disease (BD) and superficial basal cell carcinoma (BCC). The process involves oxygen‐dependent light activation of the tissue‐localised photosensitiser, protoporphyrin IX, three hours after pro‐drug application to the lesion. Current approved and licensed regimens involve application of either methylaminolevulinate (MAL, Metvix, Galderma, Switzerland) or 5‐aminolaevulinic acid in nanocolloid emulsion (ALA, Ameluz gel, Biofrontera, Germany) and subsequent light activation using narrow spectrum (630‐635 nm) red light (typically light emitting diodes (LEDs)), at an unweighted radiant exposure (“dose”) of 37 Jcm‐2 (1‐3)