114 research outputs found
Asymptotic charges in 3d gravity with torsion
We discuss some new developments in three-dimensional gravity with torsion,
based on Riemann-Cartan geometry. Using the canonical approach, we study the
structure of asymptotic symmetry, clarify its fundamental role in defining the
gravitational conserved charges, and explore the influence of the asymptotic
structure on the black hole entropy.Comment: 6 pages, LATEX file (+jpconf.cls,jpconf11.clo), Talk presented at
Constrained Dynamics and Quantum Gravity 05, Cala Gonone (Sardinia, Italy),
September 12-16, 200
Black hole entropy in 3D gravity with torsion
The role of torsion in quantum three-dimensional gravity is investigated by
studying the partition function of the Euclidean theory in Riemann-Cartan
spacetime. The entropy of the black hole with torsion is found to differ from
the standard Bekenstein-Hawking result, but its form is in complete agreement
with the first law of black hole thermodynamics.Comment: 17 pages, RevTeX, minor revision
Hamiltonian formulation of tetrad gravity: three dimensional case
The Hamiltonian formulation of the tetrad gravity in any dimension higher
than two, using its first order form when tetrads and spin connections are
treated as independent variables, is discussed and the complete solution of the
three dimensional case is given. For the first time, applying the methods of
constrained dynamics, the Hamiltonian and constraints are explicitly derived
and the algebra of the Poisson brackets among all constraints is calculated.
The algebra of the Poisson brackets among first class secondary constraints
locally coincides with Lie algebra of the ISO(2,1) Poincare group. All the
first class constraints of this formulation, according to the Dirac conjecture
and using the Castellani procedure, allow us to unambiguously derive the
generator of gauge transformations and find the gauge transformations of the
tetrads and spin connections which turn out to be the same found by Witten
without recourse to the Hamiltonian methods [\textit{Nucl. Phys. B 311 (1988)
46}]. The gauge symmetry of the tetrad gravity generated by Lie algebra of
constraints is compared with another invariance, diffeomorphism. Some
conclusions about the Hamiltonian formulation in higher dimensions are briefly
discussed; in particular, that diffeomorphism invariance is \textit{not
derivable} as a \textit{gauge symmetry} from the Hamiltonian formulation of
tetrad gravity in any dimension when tetrads and spin connections are used as
independent variables.Comment: 31 pages, minor corrections, references are added, to appear in
Gravitation & Cosmolog
Influence of Carbon Nanotubes on Polyamide Properties
In this study, the addition of carbon nanotubes (MWCNT) and modified carbon nanotubes (MWCNT-COOH) in the range of 0.5 wt. % to 5 wt. % in polyamide (PA) obtained as a residue upon 3D printing, was investigated. PA and nanocomposite samples were prepared by melt mixing. PA/MWCNT and PA/MWCNT-COOH nanocomposites were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), THB thermal conductivity determination method, electrochemical impedance spectroscopy (EIS), and tensile test. Results of DSC analysis showed that both types of carbon nanotubes (MWCNTs) acted as nucleation centres of PA matrix, but had no effect on the order of the crystalline structure. Due to the polar nature of the surface and better dispersion, MWCNT-COOH filler accelerated PA crystallization more significantly compared
to MWCNT. Due to the presence of nanofiller, the PA chains had limited motion space, which interfered with the crystallization process of the matrix. The thermal stability of the PA matrix increased with the addition of both MWCNT and MWCNT-COOH fillers. Higher thermal conductivity was achieved with the addition of MWCNT-COOH filler compared to the addition of MWCNT. The results of the tensile test showed that with the addition of both types of MWCNT fillers in the PA matrix, the modulus of elasticity and yield stress had reduced, but the yield strain increased. Results of the EIS showed that MWCNT nanofiller had not changed the electrical conductivity regardless of modification
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A RISC-V Vector Processor With Simultaneous-Switching Switched-Capacitor DC-DC Converters in 28 nm FDSOI
This work demonstrates a RISC-V vector microprocessor implemented in 28 nm FDSOI with fully integrated simultaneous-switching switched-capacitor DC-DC (SC DC-DC) converters and adaptive clocking that generates four on-chip voltages between 0.45 and 1 V using only 1.0 V core and 1.8 V IO voltage inputs. The converters achieve high efficiency at the system level by switching simultaneously to avoid charge-sharing losses and by using an adaptive clock to maximize performance for the resulting voltage ripple. Details about the implementation of the DC-DC switches, DC-DC controller, and adaptive clock are provided, and the sources of conversion loss are analyzed based on measured results. This system pushes the capabilities of dynamic voltage scaling by enabling fast transitions (20 ns), simple packaging (no off-chip passives), low area overhead (16%), high conversion efficiency (80%-86%), and high energy efficiency (26.2 DP GFLOPS/W) for mobile devices
Tur\'an numbers for -free graphs: topological obstructions and algebraic constructions
We show that every hypersurface in contains a large grid,
i.e., the set of the form , with . We use this to
deduce that the known constructions of extremal -free and
-free graphs cannot be generalized to a similar construction of
-free graphs for any . We also give new constructions of
extremal -free graphs for large .Comment: Fixed a small mistake in the application of Proposition
Translational invariance of the Einstein-Cartan action in any dimension
We demonstrate that from the first order formulation of the Einstein-Cartan
action it is possible to derive the basic differential identity that leads to
translational invariance of the action in the tangent space. The
transformations of fields is written explicitly for both the first and second
order formulations and the group properties of transformations are studied.
This, combined with the preliminary results from the Hamiltonian formulation
(arXiv:0907.1553 [gr-qc]), allows us to conclude that without any modification,
the Einstein-Cartan action in any dimension higher than two possesses not only
rotational invariance but also a form of \textit{translational invariance in
the tangent space}. We argue that \textit{not} only a complete Hamiltonian
analysis can unambiguously give an answer to the question of what a gauge
symmetry is, but also the pure Lagrangian methods allow us to find the same
gauge symmetry from the \textit{basic} differential identities.Comment: 25 pages, new Section on group properties of transformations is
added, references are added. This version will appear in General Relativity
and Gravitatio
Optical far infrared properties of FeS2
In this paper are presented the far-infrared reflectivity spectra for a cleaved FeS 2 sample measured at roam temperature and at 77 K
Optical far infrared properties of PtSb2
Far infrared reflection spectra measured at room temperature were used to investigate vibrational properties of PtSb2 single crystals. The experimental results were analyzed using a dielectric function taking into account the existence of plasmonionised impurity-phonon interactions. Together with strong coupling three infrared active lattice modes at about 143, 187 and 202 cm-1 were observed. These results were discussed with respect to calculated literature vibrational frequencies. Electrical properties of single crystal PtSb2 were also measured at room temperature
Dynamics of BPS Dyons: Effective Field Theory Approach
Based on a detailed analysis of nonlinear field equations of the SU(2)
Yang-Mills-Higgs system, we obtain the effective field theory describing
low-energy interaction of BPS dyons and massless particles (i.e., photons and
Higgs particles). Our effective theory manifests electromagnetic duality and
spontaneously broken scale symmetry, and reproduces the multimonopole moduli
space dynamics of Manton in a suitable limit. Also given is a generalization of
our approach to the case of BPS dyons in a gauge theory with an arbitrary gauge
group that is maximally broken.Comment: 48 pages, uses revte
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