31 research outputs found
Hyperscaling violation, quasinormal modes and shear diffusion
We study quasinormal modes of shear gravitational perturbations for
hyperscaling violating Lifshitz theories, with Lifshitz and hyperscaling
violating exponents and . The lowest quasinormal mode frequency
yields a shear diffusion constant which is in agreement with that obtained in
previous work by other methods. In particular for theories with where is the boundary spatial dimension, the shear
diffusion constant exhibits power-law scaling with temperature, while for
, it exhibits logarithmic scaling. We then calculate certain
2-point functions of the dual energy-momentum tensor holographically for , identifying the diffusive poles with the quasinormal modes
above. This reveals universal behaviour for the
viscosity-to-entropy-density ratio for all .Comment: v2: Latex, 21pgs, more details of analysis, review of shear diffusion
from membrane paradigm, references added, matches version to be publishe
Structured semiconductor fibers for mid=infrared transmission
Thesis (M.S.)--Boston UniversityThe mid-Infrared (mid-IR: 3-12 µm wavelength range) spectral range represents a part of the electromagnetic spectrum that impacts practically every aspect of human society, and is used for biomedical surgery, chemical sensing to technologies that enhance defense capabilities. Semiconductors like silicon and germanium can effectively transmit light in the mid infrared region of the electromagnetic spectrum. In addition to remote transmission ofmid-Infrared light, by tailoring the index of refraction of a semiconductor fiber waveguide, one can manipulate the nonlinear properties of light pulses, which leads to temporally and spectrally shaping them, thus using the passive optical fiber itself as a medium to create new color sources, or detection systems. To create low loss waveguides and to be able to manipulate light, there is the necessity to fabricate refractive index structures. Current semiconductor waveguides that have refractive index structures are short in length (~30cm), while long semiconductor waveguides do not possess refractive index structures.
Optical fiber is fabricated by drawing preforms in a fiber draw tower. The most versatile technique for preform fabrication with refractive index structures is the Modified Chemical V apor Deposition (NICVD) process. In this research, the MCVD and the draw tower in the BU Photonics Center were used to deposit silicon and germanium inside glass tubes which has not been done before. Precursor gases like silane and germane were flown through Duran, Vycor and Fused silica tubes in a tube fm·nce to deposit silicon and germanium in the inner surface of the tubes. The as-deposited glass tubes were collapsed in the flame on the MCVD lathe to form solid preforms. The preforms were characterized using X-Ray Diffraction, optical and electron microscopy to understand the MCVD reaction kinetics. The characterization data was used to model the deposition behaviour with temperature and gas flow rates. Fluid dynamics software was used to model the tube collapse parameters. Several of these preforms were subsequently drawn to fiber in the fiber draw tower. This was the first time a refractive index structure was created in a semiconductor preform
Hyperscaling violation and the shear diffusion constant
We consider holographic theories in bulk -dimensions with Lifshitz and
hyperscaling violating exponents at finite temperature. By studying
shear gravitational modes in the near-horizon region given certain
self-consistent approximations, we obtain the corresponding shear diffusion
constant on an appropriately defined stretched horizon, adapting the analysis
of Kovtun, Son and Starinets. For generic exponents with , we
find that the diffusion constant has power law scaling with the temperature,
motivating us to guess a universal relation for the viscosity bound. When the
exponents satisfy , we find logarithmic behaviour. This relation
is equivalent to where is the effective
boundary spatial dimension (and the actual spatial dimension). It is
satisfied by the exponents in hyperscaling violating theories arising from null
reductions of highly boosted black branes, and we comment on the corresponding
analysis in that context.Comment: Latex, 17pgs, v3: clarifications added on z<2+d_{eff} and standard
quantization, to be publishe
Higher Spin Gravity in and Folds on Fermi Surface
In this paper, we introduce new sets of boundary conditions for higher spin
gravity in where the boundary dynamics of spin two and other higher
spin fields are governed by the interacting collective field theory Hamiltonian
of Avan and Jevicki. We show that the time evolution of spin two and higher
spin fields can be captured by the classical dynamics of folded fermi surfaces
in the similar spirit of Lin, Lunin and Maldacena. We also construct infinite
sequences of conserved charges showing the integrable structure of higher spin
gravity under the boundary conditions we considered. Further, we observe that
there are two possible sequences of conserved charges depending on whether the
underlying boundary fermions are non-relativistic or relativistic.Comment: 26 pages, 2 figure
Flow of shear response functions in hyperscaling violating Lifshitz theories
We study the flow equations of the shear response functions for hyperscaling
violating Lifshitz (hvLif) theories, with Lifshitz and hyperscaling violating
exponents and . Adapting the membrane paradigm approach of
analysing response functions as developed by Iqbal and Liu, we focus
specifically on the shear gravitational modes which now are coupled to the
perturbations of the background gauge field. Restricting to the zero momenta
sector, we make further simplistic assumptions regarding the hydrodynamic
expansion of the perturbations. Analysing the flow equations shows that the
shear viscosity at leading order saturates the Kovtun-Son-Starinets (KSS) bound
of . When , ( being the number of spatial
dimension in the dual field theory) the first-order correction to shear
viscosity exhibits logarithmic scaling, signalling the emergence of a scale in
the UV regime for this class of hvLif theories. We further show that the
response function associated to the gauge field perturbations diverge near the
boundary when . This provides a holographic understanding of
the origin of such a constraint and further vindicates results obtained in
previous works that were obtained through near horizon and quasinormal mode
analysis.Comment: Includes new subsection on Markovianity index and breakdown of
hydrodynamic expansion; Matches with published version; 19 + 3 page
N = 3 Poincare Supergravity in Four Dimensions
In this paper, we use the superconformal approach to derive the action for N
= 3 Poincare supergravity in four space-time dimensions. We first study the
coupling of N = 3 vector multiplets to conformal supergravity. Thereafter we
combine it with the pure N = 3 conformal supergravity action and use a minimum
of three vector multiplets as compensators to arrive at Poincare supergravity
with higher derivative corrections. We give a general prescription on how to
eliminate the auxiliary fields in an iterative manner and obtain the
supergravity action order by order in derivatives. We also show that the
truncation of the action at fourth order in derivatives is a consistent
truncation.Comment: 31 pages,minor change
Scale-invariant inflation
We examine a scalar-tensor model of gravity that is globally scale-invariant.
When adapted to a spatially flat Robertson-Walker metric, the equations of
motion describe a dynamical system that flows from an unstable de Sitter space
to a stable one. We show that during this transition inflation can occur.
Moreover, at the final fixed point, a mass scale naturally emerges that can be
identified with the Planck mass. We compute the inflationary spectral indices
and the tensor perturbation and we compare them with observations. We also
study the possibility that primordial magnetic fields are generated during
inflation.Comment: Proceedings of the Workshop "Avenues of Quantum Field Theory in
Curved Spacetime" (AQFTCS 2022), University of Genova, Ital
Atomic Scale Measurement of Polar Entropy
Entropy is a fundamental thermodynamic quantity that is a measure of the
accessible microstates available to a system, with the stability of a system
determined by the magnitude of the total entropy of the system. This is valid
across truly mind boggling length scales - from nanoparticles to galaxies.
However, quantitative measurements of entropy change using calorimetry are
predominantly macroscopic, with direct atomic scale measurements being
exceedingly rare. Here for the first time, we experimentally quantify the polar
configurational entropy (in meV/K) using sub-\r{a}ngstr\"{o}m resolution
aberration corrected scanning transmission electron microscopy. This is
performed in a single crystal of the prototypical ferroelectric
through the quantification of the niobium and oxygen atom
column deviations from their paraelectric positions. Significant excursions of
the niobium - oxygen polar displacement away from its symmetry constrained
direction is seen in single domain regions which increases in the proximity of
domain walls. Combined with first principles theory plus mean field effective
Hamiltonian methods, we demonstrate the variability in the polar order
parameter, which is stabilized by an increase in the magnitude of the
configurational entropy. This study presents a powerful tool to quantify
entropy from atomic displacements and demonstrates its dominant role in local
symmetry breaking at finite temperatures in classic, nominally Ising
ferroelectrics.Comment: 23 pages, 21 figures (5 main, 16 supplemental