Amorphization induced by pressure: results for zeolites and general implications

We report an {\sl ab initio} study of pressure-induced amorphization (PIA) in zeolites, which are model systems for this phenomenon. We confirm the occurrence of low-density amorphous phases like the one reported by Greaves {\sl et al.} [Science {\bf 308}, 1299 (2005)], which preserves the crystalline topology and might constitute a new type of glass. The role of the zeolite composition regarding PIA is explained. Our results support the correctness of existing models for the basic PIA mechanim, but suggest that energetic, rather than kinetic, factors determine the irreversibility of the transition.Comment: 4 pages with 3 figures embedded. More information at http://www.icmab.es/dmmis/leem/jorg

Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6

Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi$_2$O$_6$ in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below $\sim$100 K. This lattice modulation is characterized by a resolution limited wave vector {\bf q}$_{IC}$=(0,$\sim$0.13,0) and its 2$^{nd}$ and 3$^{rd}$ harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.Comment: 4 pages, 4 figure

Roughening of close-packed singular surfaces

An upper bound to the roughening temperature of a close-packed singular surface, fcc Al (111), is obtained via free energy calculations based on thermodynamic integration using the embedded-atom interaction model. Roughening of Al (111) is predicted to occur at around 890 K, well below bulk melting (933 K), and it should therefore be observable, save for possible kinetic hindering.Comment: RevTeX 4 pages, embedded figure

First-principles study of the ferroelastic phase transition in CaCl_2

First-principles density-functional calculations within the local-density approximation and the pseudopotential approach are used to study and characterize the ferroelastic phase transition in calcium chloride (CaCl_2). In accord with experiment, the energy map of CaCl_2 has the typical features of a pseudoproper ferroelastic with an optical instability as ultimate origin of the phase transition. This unstable optic mode is close to a pure rigid unit mode of the framework of chlorine atoms and has a negative Gruneisen parameter. The ab-initio ground state agrees fairly well with the experimental low temperature structure extrapolated at 0K. The calculated energy map around the ground state is interpreted as an extrapolated Landau free-energy and is successfully used to explain some of the observed thermal properties. Higher-order anharmonic couplings between the strain and the unstable optic mode, proposed in previous literature as important terms to explain the soft-phonon temperature behavior, are shown to be irrelevant for this purpose. The LAPW method is shown to reproduce the plane-wave results in CaCl_2 within the precision of the calculations, and is used to analyze the relative stability of different phases in CaCl_2 and the chemically similar compound SrCl_2.Comment: 9 pages, 6 figures, uses RevTeX

Are Vicinal Metal Surfaces Stable?

Quantum Matter and Optic

Is bisexuality invisible? A review of sexualities scholarship 1970–2015

This article provides a review of sexualities scholarship within the social sciences between 1970 and 2015. It takes an innovative approach by focusing on the way in which bisexuality is addressed in this body of literature. The article reveals the marginalisation, under-representation and invisibility of bisexuality within and across the social sciences in relation to both bisexual experience and identity. Reasons for this varied across the different eras, including the heterosexist nature of the literature, the impact of gay and lesbian-focused identity politics, and queer deconstructionism. In addition, patterns of bisexual erasure and invisibility were uneven, with some scholarship taking inclusive approaches or criticising prejudice against bisexuality. The initial findings of the review were enriched by critical commentary from key relevant sociologists and political scientists. The article concludes that future sexualities scholarship could be enhanced by greater consideration of bisexuality