7,116 research outputs found
Quantum field theory in de Sitter and quasi-de Sitter spacetimes: Revisited
It is possible to associate temperatures with the non-extremal horizons of a
large class of spherically symmetric spacetimes using periodicity in the
Euclidean sector and this procedure works for the de Sitter spacetime as well.
But, unlike e.g., the black hole spacetimes, the de Sitter spacetime also
allows a description in Friedmann coordinates. This raises the question of
whether the thermality of the de Sitter horizon can be obtained, working
entirely in the Friedmann coordinates, without reference to the static
coordinates or using the symmetries of de Sitter spacetime. We discuss several
aspects of this issue for de Sitter and approximately de Sitter spacetimes, in
the Friedmann coordinates (with a time-dependent background and the associated
ambiguities in defining the vacuum states). The different choices for the
vacuum states, behaviour of the mode functions and the detector response are
studied in both (1+1) and (1+3) dimensions. We compare and contrast the
differences brought about by the different choices. In the last part of the
paper, we also describe a general procedure for studying quantum field theory
in spacetimes which are approximately de Sitter and, as an example, derive the
corrections to thermal spectrum due to the presence of pressure-free matter.Comment: 26 page
Comparative study of evolution of residual stress state by local mechanical tensioning and laser processing of ferritic and austenitic structural steel welds.
Complex thermal stresses generated in welded structures are undesirable but inevitable in fusion welding. The presence of residual stresses can be detrimental to the integrity of a welded joint. In this research, redistribution of residual stress magnitude and profile was studied and compared in two multi-pass welded structural alloys (API X100 and 304L stainless steel) after cold rolling and laser processing. The residual stress field was studied by neutron diffraction using the SALSA strain scanner at their reactor neutron source at ILL, Grenoble. In addition to a complex distribution of residual stress state, multi-pass welds also forms dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. Dendritic grain structure is weaker and segregation of alloying elements may result in formation of corrosion microcells as well as reduction in overall corrosion prevention due to depletion of alloying elements in certain areas. The modification of as-welded residual stress state was done by cold rolling which was followed by laser processing to create a recrystallized microstructure to minimise segregation. The main objective of this study is to understand the suitability of this novel manufacturing technique to create a stress free weldment with recrystallised grain structure. Hardness evolution in the welded structures was scanned following welding, post weld cold rolling and cold rolling followed by laser processing. Hardness distribution in both the structural alloys showed a significant evidence of plastic deformation near the cap pass of the weld metal. Residual stress redistribution was observed up to 4 mm from the capping pass for ferritic steel, while in austenitic steel weld, post weld cold rolling was effective in modifying the residual stress redistribution throughout the entire thickness. Laser processing in both cases reinstated the as-welded residual stress distribution and resulted in softening of the strained area
Electrically modulated photoluminescence in ferroelectric liquid crystal
Electrical modulation and switching of photoluminescence (PL) have been
demonstrated in pure deformed helix ferroelectric liquid crystal (DHFLC)
material. The PL intensity increases and peak position shifts towards lower
wavelength above a threshold voltage which continues up to a saturation
voltage. This is attributed to the helix unwinding phenomenon in the DHFLC on
the application of an electric field. Moreover, the PL intensity could be
switched between high intensity (field-on) and low intensity (field-off)
positions. These studies would add a new dimension to ferroelectric liquid
crystal's application in the area of optical devices.Comment: 4 figure
Experiments with flowing gases in an open photoacoustic cell
A simple gas-microphone photoacoustic cell is described in which there is no sizable loss of signal on opening the cell to the atmosphere or even under conditions of gas flow. Results obtained under different rates of flow of gases and chopping frequencies are reported. Except for carbon black, the photoacoustic signal is found to be independent of flow-rate for all the solid substances studied
Structure, bonding and magnetism in cobalt clusters
The structural, electronic and magnetic properties of Co clusters
(20) have been investigated using density functional theory within the
pseudopotential plane wave method. An unusual hexagonal growth pattern has been
observed in the intermediate size range, 20. The cobalt atoms are
ferromagnetically ordered and the calculated magnetic moments are found to be
higher than that of corresponding hcp bulk value, which are in good agreement
with the recent Stern-Gerlach experiments. The average coordination number is
found to dominate over the average bond length to determine the effective
hybridization and consequently the cluster magnetic moment.Comment: 12 pages and 9 figure
Time lower bounds for nonadaptive turnstile streaming algorithms
We say a turnstile streaming algorithm is "non-adaptive" if, during updates,
the memory cells written and read depend only on the index being updated and
random coins tossed at the beginning of the stream (and not on the memory
contents of the algorithm). Memory cells read during queries may be decided
upon adaptively. All known turnstile streaming algorithms in the literature are
non-adaptive.
We prove the first non-trivial update time lower bounds for both randomized
and deterministic turnstile streaming algorithms, which hold when the
algorithms are non-adaptive. While there has been abundant success in proving
space lower bounds, there have been no non-trivial update time lower bounds in
the turnstile model. Our lower bounds hold against classically studied problems
such as heavy hitters, point query, entropy estimation, and moment estimation.
In some cases of deterministic algorithms, our lower bounds nearly match known
upper bounds
Shape-invariant quantum Hamiltonian with position-dependent effective mass through second order supersymmetry
Second order supersymmetric approach is taken to the system describing motion
of a quantum particle in a potential endowed with position-dependent effective
mass. It is shown that the intertwining relations between second order partner
Hamiltonians may be exploited to obtain a simple shape-invariant condition.
Indeed a novel relation between potential and mass functions is derived, which
leads to a class of exactly solvable model. As an illustration of our
procedure, two examples are given for which one obtains whole spectra
algebraically. Both shape-invariant potentials exhibit harmonic-oscillator-like
or singular-oscillator-like spectra depending on the values of the
shape-invariant parameter.Comment: 16 pages, 5 figs; Present e-mail of AG: [email protected]
Emergence of a non-scaling degree distribution in bipartite networks: a numerical and analytical study
We study the growth of bipartite networks in which the number of nodes in one
of the partitions is kept fixed while the other partition is allowed to grow.
We study random and preferential attachment as well as combination of both. We
derive the exact analytical expression for the degree-distribution of all these
different types of attachments while assuming that edges are incorporated
sequentially, i.e., a single edge is added to the growing network in a time
step. We also provide an approximate expression for the case when more than one
edge are added in a time step. We show that depending on the relative weight
between random and preferential attachment, the degree-distribution of this
type of network falls into one of four possible regimes which range from a
binomial distribution for pure random attachment to an u-shaped distribution
for dominant preferential attachment
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