Statics and dynamics of domain patterns in hexagonal-orthorhombic ferroelastics

We study the statics and the dynamics of domain patterns in proper hexagonal-orthorhombic ferroelastics; these patterns are of particular interest because they provide a rare physical realization of disclinations in crystals. Both our static and dynamical theories are based entirely on classical, nonlinear elasticity theory; we use the minimal theory consistent with stability, symmetry and ability to explain qualitatively the observed patterns. After scaling, the only parameters of the static theory are a temperature variable and a stiffness variable. For moderate to large stiffness, our static results show nested stars, unnested stars, fans and other nodes, triangular and trapezoidal regions of trapped hexagonal phase, etc observed in electron microscopy of Ta4N and Mg-Cd alloys, and also in lead orthovanadate (which is trigonal-monoclinic); we even find imperfections in some nodes, like those observed. For small stiffness, we find patterns like those observed in the mineral Mg-cordierite. Our dynamical studies of growth and relaxation show the formation of these static patterns, and also transitory structures such as 12-armed bursts, streamers and striations which are also seen experimentally. The major aspects of the growth-relaxation process are quite unlike those in systems with conventional order parameters, for it is inherently nonlocal; for example, the changes from one snapshot to the next are not predictable by inspection.Comment: 9 pages, 3 figures (1 b&w, 2 colour); animations may be viewed at http://huron.physics.utoronto.ca/~curnoe/sim.htm

Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2

Weyl semimetal is a new quantum state of matter [1-12] hosting the condensed matter physics counterpart of relativisticWeyl fermion [13] originally introduced in high energy physics. The Weyl semimetal realized in the TaAs class features multiple Fermi arcs arising from topological surface states [10, 11, 14-16] and exhibits novel quantum phenomena, e.g., chiral anomaly induced negative mag-netoresistance [17-19] and possibly emergent supersymmetry [20]. Recently it was proposed theoretically that a new type (type-II) of Weyl fermion [21], which does not have counterpart in high energy physics due to the breaking of Lorentz invariance, can emerge as topologically-protected touching between electron and hole pockets. Here, we report direct spectroscopic evidence of topological Fermi arcs in the predicted type-II Weyl semimetal MoTe2 [22-24]. The topological surface states are confirmed by directly observing the surface states using bulk-and surface-sensitive angle-resolved photoemission spectroscopy (ARPES), and the quasi-particle interference (QPI) pattern between the two putative Fermi arcs in scanning tunneling microscopy (STM). Our work establishes MoTe2 as the first experimental realization of type-II Weyl semimetal, and opens up new opportunities for probing novel phenomena such as exotic magneto-transport [21] in type-II Weyl semimetals.Comment: submitted on 01/29/2016. Nature Physics, in press. Spectroscopic evidence of the Fermi arcs from two complementary surface sensitive probes - ARPES and STS. A comparison of the calculated band structure for T_d and 1T' phase to identify the topological Fermi arcs in the T_d phase is also included in the supplementary informatio

Fractional deuteration applied to biomolecular solid-state NMR spectroscopy

Solid-state Nuclear Magnetic Resonance can provide detailed insight into structural and dynamical aspects of complex biomolecules. With increasing molecular size, advanced approaches for spectral simplification and the detection of medium to long-range contacts become of critical relevance. We have analyzed the protonation pattern of a membrane-embedded ion channel that was obtained from bacterial expression using protonated precursors and D2O medium. We find an overall reduction of 50% in protein protonation. High levels of deuteration at Hα and Hβ positions reduce spectral congestion in (1H,13C,15N) correlation experiments and generate a transfer profile in longitudinal mixing schemes that can be tuned to specific resonance frequencies. At the same time, residual protons are predominantly found at amino-acid side-chain positions enhancing the prospects for obtaining side-chain resonance assignments and for detecting medium to long-range contacts. Fractional deuteration thus provides a powerful means to aid the structural analysis of complex biomolecules by solid-state NMR

Structural and vibrational study of pseudocubic CdIn2Se4 under compression

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jp5077565We report a comprehensive experimental and theoretical study of the structural and vibrational properties of a-CdIn2Se4 under compression. Angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy evidence that this ordered-vacancy compound with pseudocubic structure undergoes a phase transition (7 GPa) toward a disordered rocksalt structure as observed in many other ordered-vacancy compounds. The equation of state and the pressure dependence of the Raman-active modes of this semiconductor have been determined and compared both to ab initio total energy and lattice dynamics calculations and to related compounds. Interestingly, on decreasing pressure, at similar to 2 GPa, CdIn2Se4 transforms into a spinel structure which, according to calculations, is energetically competitive with the initial pseudocubic phase. The phase behavior of this compound under compression and the structural and compressibility trends in AB(2)Se(4) selenides are discussed.This study was supported by the Spanish government MEC under Grant Nos: MAT2013-46649-C4-3-P, MAT2013-46649-C4-2-P, MAT2010-21270-C04-03/04, and CTQ2009-14596-C02-01, by MALTA Consolider Ingenio 2010 Project (CSD2007-00045) and by Generalitat Valenciana (GVA-ACOMP-2013-1012). A.M. and P.R-H. acknowledge computing time provided by Red Espanola de Supercomputacion (RES) and MALTA-Cluster, and also to S. Munoz-Rodriguez for providing a data-parsing application. J.A.S. acknowledges Juan de la Cierva fellowship program for financial support.Santamaría Pérez, D.; Gomis, O.; Pereira, ALJ.; Vilaplana Cerda, RI.; Popescu, C.; Sans Tresserras, JÁ.; Manjón Herrera, FJ.... (2014). Structural and vibrational study of pseudocubic CdIn2Se4 under compression. Journal of Physical Chemistry C. 118(46):26987-26999. https://doi.org/10.1021/jp5077565S26987269991184

Dispersion of electro-optic coefficients in sillenite crystals.

The photorefractive crystals of the sillenite family (Bi12SiO20 or BSO, Bi12GeO20 or BGO, and Bi12TiO20 or BTO) that belong to the cubic 23 point group are optically active, and exhibit the piezoelectric, elasto-optic, electro-optic and electrogyration effect. In this paper we measure the dispersion of the electro-optic coefficient for all the crystals of the sillenite family in the visible spectral range (500-800 nm). For this we measure by ellipsometry the polarization properties of a beam transmitted in the transverse configuration under the action of an externally applied field. The experimental data are fitted with an analytical expression of the beam polarization to find the electro-optic coefficient. The results show a normal dispersion of the electro-optic coefficient in all three sillenite crystals, similarly to other electro-optic crystals

The orientation of interfaces between a prototype phase and its ferroelastic derivatives : theoretical and experimental study

A method is proposed to determine the strainfree interfaces between a paraelastic prototype phase and its ferroelastic derivatives. The predictions of the theory are tested by means of electron microscope observations on a number of examples. Reasonable agreement is found between the predictions and the observations.Une méthode est proposée pour déterminer les interfaces non déformées entre une phase prototype para-élastique et les variantes d'orientation ferro-élastiques qui en résultent. Les prédictions de cette théorie sont comparées à quelques exemples d'observations obtenus par microscopie électronique. L'accord entre les prédictions et les observations est raisonnable