1,790 research outputs found
Thermodynamics and the intrinsic stability of lead halide perovskites CH3NH3PbX3
The role of thermodynamics in assessing the intrinsic instability of the CH3NH3PbX3 perovskites (X = Cl,Br,I) is outlined on the basis of the available experimental information. Possible decomposition/degradation pathways driven by the inherent instability of the material are considered. The decomposition to precursors CH3NH3X(s) and PbX2(s) is first analysed, pointing out the importance of both the enthalpic and the entropic factor, the latter playing a stabilizing role making the stability higher than often asserted. For CH3NH3PbI3 the disagreement between the available calorimetric results makes the stability prediction uncertain. Subsequently, the gas-releasing decomposition paths are discussed, with emphasis on the discrepant results presently available, probably reflecting the predominance of thermodynamic or kinetic control. The competition between the formation of NH3(g)+CH3X(g), CH3NH2(g)+HX(g) or CH3NH3X(g) is analysed, in comparison with the thermal decomposition of methylammonium halides. In view of the scarce and inconclusive thermodynamic studies to-date available, the need for further experimental data is emphasized
Metric projective geometry, BGG detour complexes and partially massless gauge theories
A projective geometry is an equivalence class of torsion free connections
sharing the same unparametrised geodesics; this is a basic structure for
understanding physical systems. Metric projective geometry is concerned with
the interaction of projective and pseudo-Riemannian geometry. We show that the
BGG machinery of projective geometry combines with structures known as
Yang-Mills detour complexes to produce a general tool for generating invariant
pseudo-Riemannian gauge theories. This produces (detour) complexes of
differential operators corresponding to gauge invariances and dynamics. We
show, as an application, that curved versions of these sequences give geometric
characterizations of the obstructions to propagation of higher spins in
Einstein spaces. Further, we show that projective BGG detour complexes generate
both gauge invariances and gauge invariant constraint systems for partially
massless models: the input for this machinery is a projectively invariant gauge
operator corresponding to the first operator of a certain BGG sequence. We also
connect this technology to the log-radial reduction method and extend the
latter to Einstein backgrounds.Comment: 30 pages, LaTe
Poincare-Einstein Holography for Forms via Conformal Geometry in the Bulk
We study higher form Proca equations on Einstein manifolds with boundary data
along conformal infinity. We solve these Laplace-type boundary problems
formally, and to all orders, by constructing an operator which projects
arbitrary forms to solutions. We also develop a product formula for solving
these asymptotic problems in general. The central tools of our approach are (i)
the conformal geometry of differential forms and the associated exterior
tractor calculus, and (ii) a generalised notion of scale which encodes the
connection between the underlying geometry and its boundary. The latter also
controls the breaking of conformal invariance in a very strict way by coupling
conformally invariant equations to the scale tractor associated with the
generalised scale. From this, we obtain a map from existing solutions to new
ones that exchanges Dirichlet and Neumann boundary conditions. Together, the
scale tractor and exterior structure extend the solution generating algebra of
[31] to a conformally invariant, Poincare--Einstein calculus on (tractor)
differential forms. This calculus leads to explicit holographic formulae for
all the higher order conformal operators on weighted differential forms,
differential complexes, and Q-operators of [9]. This complements the results of
Aubry and Guillarmou [3] where associated conformal harmonic spaces parametrise
smooth solutions.Comment: 85 pages, LaTeX, typos corrected, references added, to appear in
Memoirs of the AM
Quantum Gravity and Causal Structures: Second Quantization of Conformal Dirac Algebras
It is postulated that quantum gravity is a sum over causal structures coupled
to matter via scale evolution. Quantized causal structures can be described by
studying simple matrix models where matrices are replaced by an algebra of
quantum mechanical observables. In particular, previous studies constructed
quantum gravity models by quantizing the moduli of Laplace, weight and
defining-function operators on Fefferman-Graham ambient spaces. The algebra of
these operators underlies conformal geometries. We extend those results to
include fermions by taking an osp(1|2) "Dirac square root" of these algebras.
The theory is a simple, Grassmann, two-matrix model. Its quantum action is a
Chern-Simons theory whose differential is a first-quantized, quantum mechanical
BRST operator. The theory is a basic ingredient for building fundamental
theories of physical observables.Comment: 4 pages, LaTe
Rediscovering the scientific and didactic value of minor herbarium collections: the seeds and fruits collection by Gustavo Bonaventura
Seeds and fruits collections are very important from a systematic point of view and represent useful references in several disciplines and research fields. The Herbarium of Sapienza University of Rome (RO) hosts a Spermoteque/Carpoteque, which was organized by Gustavo Bonaventura (1902-1976). The purpose of this paper is to describe the heritage of Bonaventura's collection. It consists of 42 wooden boxes, globally hosting 3411 glass tubes containing seeds, fruits, and other materials. The collection was first of all catalogued; then, analysis were conducted regarding taxonomic composition, temporal and geographic coverage, institutions of provenience, collectors, content, and preservation status. The specimens refer to 2740 taxa, belonging to 890 genera and 135 families. Many genera of agricultural interest are present, each one with different cultivars. The collection spans across 130 years (1843-1975) and hosts specimens coming from all over the world. Materials were provided by several herbaria, botanical gardens and agrarian institutes, and by 50 collectors. The Bonaventura's collection is still a useful reference collection, testifying biodiversity over times and thus being useful for diachronic studies; moreover, it documents the interests of collectors and the past network activity between institutions
Charge dynamics in molecular junctions: Nonequilibrium Green's Function approach made fast
Real-time Green's function simulations of molecular junctions (open quantum
systems) are typically performed by solving the Kadanoff-Baym equations (KBE).
The KBE, however, impose a serious limitation on the maximum propagation time
due to the large memory storage needed. In this work we propose a simplified
Green's function approach based on the Generalized Kadanoff-Baym Ansatz (GKBA)
to overcome the KBE limitation on time, significantly speed up the
calculations, and yet stay close to the KBE results. This is achieved through a
twofold advance: first we show how to make the GKBA work in open systems and
then construct a suitable quasi-particle propagator that includes correlation
effects in a diagrammatic fashion. We also provide evidence that our GKBA
scheme, although already in good agreement with the KBE approach, can be
further improved without increasing the computational cost.Comment: 13 pages, 13 figure
Quantum Darboux theorem
The problem of computing quantum mechanical propagators can be recast as a computation of a Wilson line operator for parallel transport by a flat connection acting on a vector bundle of wave functions. In this picture, the base manifold is an odd-dimensional symplectic geometry, or quite generically a contact manifold that can be viewed as a "phase-spacetime,"while the fibers are Hilbert spaces. This approach enjoys a "quantum Darboux theorem"that parallels the Darboux theorem on contact manifolds which turns local classical dynamics into straight lines. We detail how the quantum Darboux theorem works for anharmonic quantum potentials. In particular, we develop a novel diagrammatic approach for computing the asymptotics of a gauge transformation that locally makes complicated quantum dynamics trivial
The symplectic origin of conformal and Minkowski superspaces
Supermanifolds provide a very natural ground to understand and handle
supersymmetry from a geometric point of view; supersymmetry in and
dimensions is also deeply related to the normed division algebras.
In this paper we want to show the link between the conformal group and
certain types of symplectic transformations over division algebras. Inspired by
this observation we then propose a new\,realization of the real form of the 4
dimensional conformal and Minkowski superspaces we obtain, respectively, as a
Lagrangian supermanifold over the twistor superspace and a
big cell inside it.
The beauty of this approach is that it naturally generalizes to the 6
dimensional case (and possibly also to the 10 dimensional one) thus providing
an elegant and uniform characterization of the conformal superspaces.Comment: 15 pages, references added, minor change
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