512 research outputs found
Systematic treatment of displacements, strains and electric fields in density-functional perturbation theory
The methods of density-functional perturbation theory may be used to
calculate various physical response properties of insulating crystals including
elastic, dielectric, Born charge, and piezoelectric tensors. These and other
important tensors may be defined as second derivatives of the total energy with
respect to atomic-displacement, electric-field, or strain perturbations, or as
mixed derivatives with respect to two of these perturbations. The resulting
tensor quantities tend to be coupled in complex ways in polar crystals, giving
rise to a variety of variant definitions. For example, it is generally
necessary to distinguish between elastic tensors defined under different
electrostatic boundary conditions, and between dielectric tensors defined under
different elastic boundary conditions. Here, we describe an approach for
computing all of these various response tensors in a unified and systematic
fashion. Applications are presented for two materials, wurtzite ZnO and
rhombohedral BaTiO3, at zero temperature.Comment: 14 pages. Uses REVTEX macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/xfw_sys/index.htm
A Low Temperature Nonlinear Optical Rotational Anisotropy Spectrometer for the Determination of Crystallographic and Electronic Symmetries
Nonlinear optical generation from a crystalline material can reveal the
symmetries of both its lattice structure and underlying ordered electronic
phases and can therefore be exploited as a complementary technique to
diffraction based scattering probes. Although this technique has been
successfully used to study the lattice and magnetic structures of systems such
as semiconductor surfaces, multiferroic crystals, magnetic thin films and
multilayers, challenging technical requirements have prevented its application
to the plethora of complex electronic phases found in strongly correlated
electron systems. These requirements include an ability to probe small bulk
single crystals at the micron length scale, a need for sensitivity to the
entire nonlinear optical susceptibility tensor, oblique light incidence
reflection geometry and incident light frequency tunability among others. These
measurements are further complicated by the need for extreme sample
environments such as ultra low temperatures, high magnetic fields or high
pressures. In this review we present a novel experimental construction using a
rotating light scattering plane that meets all the aforementioned requirements.
We demonstrate the efficacy of our scheme by making symmetry measurements on a
micron scale facet of a small bulk single crystal of SrIrO using
optical second and third harmonic generation.Comment: 8 pages, 5 figure
The Myth of Cyberwar: Bringing War in Cyberspace Back Down to Earth
Cyberwar has been described as a revolution in military affairs, a transformation of technology and doctrine capable of overturning the prevailing world order. This characterization of the threat from cyberwar, however, reflects a common tendency to conflate means and ends; studying what could happen in cyberspace (or anywhere else) makes little sense without considering how conflict over the internet is going to realize objectives commonly addressed by terrestrial warfare. To supplant established modes of conflict, cyberwar must be capable of furthering the political ends to which force or threats of force are commonly applied, something that in major respects cyberwar fails to do. As such, conflict over the internet is much more likely to serve as an adjunct to, rather than a substitute for, existing modes of terrestrial force. Indeed, rather than threatening existing political hierarchies, cyberwar is much more likely to simply augment the advantages of status quo powers. </jats:p
Phase vortices from a Young's three-pinhole interferometer
An analysis is presented of the phase vortices generated in the far field, by
an arbitrary arrangement of three monochromatic point sources of complex
spherical waves. In contrast with the case of three interfering plane waves, in
which an infinitely-extended vortex lattice is generated, the spherical sources
generate a finite number of phase vortices. Analytical expressions for the
vortex core locations are developed and shown to have a convenient
representation in a discrete parameter space. Our analysis may be mapped onto
the case of a coherently-illuminated Young's interferometer, in which the
screen is punctured by three rather than two pinholes.Comment: 10 pages, 8 figures, REVTeX4, Submitted to Phys. Rev.
Negative compressibility in platinum sulfide using density-functional theory
Copyright © 2010 The American Physical SocietyThe structural and dynamic properties of the mineral Cooperite (PtS) are investigated using density-functional theory. The results show that a competition with the less symmetric but more compact PdS structure leads to a phase transition when the pressure is increased. However, before the phase transition, PtS displays a rare anomalous elastic behavior by expanding along its long axis under hydrostatic pressure. We report the elastic constants of PtS and interpret this negative linear compressibility in the context of a displacive phase transition. We also show that the real structure of PtS is less symmetric than originally determined by experiment
Nothing Is True? The Credibility of News and Conflicting Narratives during “Information War” in Ukraine
In international politics, the strategic narratives of different governments compete for public attention and support. The Russian government’s narrative has prompted western concern due to fears that it exerts a destabilizing effect on societies in Eastern Europe and elsewhere. However, the behavior and thought processes of news consumers targeted by contradictory strategic narratives are rarely subjected to analysis. This paper examines how Ukrainian news consumers decide where to get their news and what to believe in a media environment where “propaganda” and “disinformation” are regarded as major threats to national security. Evidence comes from thirty audio-diaries and in-depth interviews conducted in 2016 among adult residents of Odesa Region. Through qualitative analysis of the diary and interview transcripts, the paper reveals how participants judged the credibility of news and narratives based on their priorities (what they considered important), not just “facts” (what they believed had happened). The attribution of importance to different foreign policy issues was associated, in turn, with varying personal experiences, memories, and individual cross-border relationships
Spontaneous polarization and piezoelectric constants of III-V nitrides
The spontaneous polarization, dynamical Born charges, and piezoelectric
constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using
the Berry phase approach to polarization in solids. The piezoelectric constants
are found to be up 10 times larger than in conventional III-V's and II-VI's,
and comparable to those of ZnO. Further properties at variance with those of
conventional III-V compounds are the sign of the piezoelectric constants
(positive as in II-VI's) and the very large spontaneous polarization.Comment: RevTeX 4 pages, improved upon revie
JunD, not c-Jun, is the AP-1 transcription factor required for Ras-induced lung cancer.
The AP-1 transcription factor c-Jun is required for Ras-driven tumorigenesis in many tissues and is considered as a classical proto-oncogene. To determine the requirement for c-Jun in a mouse model of K-RasG12D-induced lung adenocarcinoma, we inducibly deleted c-Jun in the adult lung. Surprisingly, we found that inactivation of c-Jun, or mutation of its JNK phosphorylation sites, actually increased lung tumor burden. Mechanistically, we found that protein levels of the Jun family member JunD were increased in the absence of c-Jun. In c-Jun-deficient cells, JunD phosphorylation was increased, and expression of a dominant-active JNKK2-JNK1 transgene further increased lung tumor formation. Strikingly, deletion of JunD completely abolished Ras-driven lung tumorigenesis. This work identifies JunD, not c-Jun, as the crucial substrate of JNK signaling and oncogene required for Ras-induced lung cancer
Swords into ploughshares? China's Soft Power strategy in Southeast Asia and its challenges1
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