1,464 research outputs found
HP-sequence design for lattice proteins - an exact enumeration study on diamond as well as square lattice
We present an exact enumeration algorithm for identifying the {\it native}
configuration - a maximally compact self avoiding walk configuration that is
also the minimum energy configuration for a given set of contact-energy
schemes; the process is implicitly sequence-dependent. In particular, we show
that the 25-step native configuration on a diamond lattice consists of two
sheet-like structures and is the same for all the contact-energy schemes,
; on a square lattice also, the
24-step native configuration is independent of the energy schemes considered.
However, the designing sequence for the diamond lattice walk depends on the
energy schemes used whereas that for the square lattice walk does not. We have
calculated the temperature-dependent specific heat for these designed sequences
and the four energy schemes using the exact density of states. These data show
that the energy scheme is preferable to the other three for both
diamond and square lattice because the associated sequences give rise to a
sharp low-temperature peak. We have also presented data for shorter (23-, 21-
and 17-step) walks on a diamond lattice to show that this algorithm helps
identify a unique minimum energy configuration by suitably taking care of the
ground-state degeneracy. Interestingly, all these shorter target configurations
also show sheet-like secondary structures.Comment: 19 pages, 7 figures (eps), 11 tables (latex files
Can coarse-graining introduce long-range correlations in a symbolic sequence?
We present an exactly solvable mean-field-like theory of correlated ternary
sequences which are actually systems with two independent parameters. Depending
on the values of these parameters, the variance on the average number of any
given symbol shows a linear or a superlinear dependence on the length of the
sequence. We have shown that the available phase space of the system is made up
a diffusive region surrounded by a superdiffusive region. Motivated by the fact
that the diffusive portion of the phase space is larger than that for the
binary, we have studied the mapping between these two. We have identified the
region of the ternary phase space, particularly the diffusive part, that gets
mapped into the superdiffusive regime of the binary. This exact mapping implies
that long-range correlation found in a lower dimensional representative
sequence may not, in general, correspond to the correlation properties of the
original system.Comment: 10 pages including 1 figur
Bulk and contact-sensitized photocarrier generation in single layer TPD devices
In this paper, we report on the photoelectronic properties of TPD studied in
sandwich geometry. In particular, we have obtained from both forward and
reverse bias measurements the "mew-tau" product for holes in TPD. "mew" is the
hole mobility and "tau" the carrier trapping time. The "mew-tau" product is a
measure of the electronic quality of the material and allows a quantitative
comparison of different samples. We have carried out numerical simulations to
understand the photocurrent in these structures. We show that in reverse bias,
the photocurrent (PC) is due to bulk. The carrier generation is governed by
field assisted exciton dissociation at electric fields greater than 10^6 V/cm.
At lower fields the generation of carriers occurs spontaneously in the bulk of
the sample. In forward bias, the photocurrent is due to exciton dissociation at
the ITO contact. We also obtain a "mew-tau" product for holes from forward bias
PC measurements which is in agreement with the value obtained from reverse bias
measurements. Based on our experiments, we demonstrate that TPD in a sandwich
structure is a good candidate for cheap large area solar blind UV detector
arrays.Comment: Submitted to J. Appl. Phy
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