774 research outputs found
Crystallisation study of the Cu<sub>2</sub>ZnSnS<sub>4</sub> chalcogenide material for solar applications
Second generation thin-film chalcogenide materials, in particular CuInGa(S,Se)2 (CIGS) and CdTe, have been among the most promising and quickly became commercial candidates for large-scale PV manufacturing. These materials offer stable and efficient (above 10%) photovoltaic modules fabricated by scalable thin-film technologies and cell efficiencies above 20 % (CIGS). Indium-free kesterite-related materials such as Cu2ZnSnS4 have attracted significant research interest due to their similar properties to CIGS. In these materials, indium is replaced with earth-abundant zinc and tin metals. The quaternary semiconductor Cu2ZnSnS4(CZTS) is a relatively new photovoltaic material and is expected to be interesting for environmentally amenable solar cells, as its constituents are nontoxic and abundant in the Earth's crust. The CZTS thin films show p-type conductivity, a band gap of 1.44–1.51 eV that is ideal to achieve the highest solar-cell conversion efficiency, and relatively high optical absorption in the visible light range
Fine structure of excitons in CuO
Three experimental observations on 1s-excitons in CuO are not consistent
with the picture of the exciton as a simple hydrogenic bound state: the
energies of the 1s-excitons deviate from the Rydberg formula, the total exciton
mass exceeds the sum of the electron and hole effective masses, and the
triplet-state excitons lie above the singlet. Incorporating the band structure
of the material, we calculate the corrections to this simple picture arising
from the fact that the exciton Bohr radius is comparable to the lattice
constant. By means of a self-consistent variational calculation of the total
exciton mass as well as the ground-state energy of the singlet and the
triplet-state excitons, we find excellent agreement with experiment.Comment: Revised abstract; 10 pages, revtex, 3 figures available from G.
Kavoulakis, Physics Department, University of Illinois, Urban
Development of wirelessly-powered, extracranial brain activator (ECBA) in a large animal model for the future non-invasive human neuromodulation
As transcranial electrical stimulation (tES) is an emerging and promising technique for neuromodulation, we developed a novel device; wirelessly-powered, extracranial brain activator (ECBA), which is mounted subcutaneously, and its neuromodulation effect was investigated. The oscillatory changes in electrocorticography (EcoG) were analyzed from two types of stimulation. Two weeks prior to the recording experiment, we underwent surgery for implantation of subdural strips and ECBA module over centroparietal regions of anesthetized beagles. Low-frequency stimulation (LFS) and subsequent high-frequency stimulation (HFS) protocols (600 pulses respectively) were applied. Then, the power changes before and after each stimulation in five different bands were compared. A significantly larger voltage difference with subcutaneous than transcutaneous stimulation measured at EcoG channels indicated a substantial current attenuation between the skin and skull. Compared with the baseline, all subjects showed consistently decreased delta power and increased gamma power after HFS. LFS also induced a similar, but opposite, pattern of power change in four beagles. The results from this study indicate that LFS and HFS with our novel ECBA can consistently and effectively modulate neural activity of the cortex, inducing neural inhibition and facilitation functions, respectively. Future studies are necessary to further ensuring a consistent efficacy and long-term safety.11Ysciescopu
Self-intersection local time of planar Brownian motion based on a strong approximation by random walks
The main purpose of this work is to define planar self-intersection local
time by an alternative approach which is based on an almost sure pathwise
approximation of planar Brownian motion by simple, symmetric random walks. As a
result, Brownian self-intersection local time is obtained as an almost sure
limit of local averages of simple random walk self-intersection local times. An
important tool is a discrete version of the Tanaka--Rosen--Yor formula; the
continuous version of the formula is obtained as an almost sure limit of the
discrete version. The author hopes that this approach to self-intersection
local time is more transparent and elementary than other existing ones.Comment: 36 pages. A new part on renormalized self-intersection local time has
been added and several inaccuracies have been corrected. To appear in Journal
of Theoretical Probabilit
Finite Size and Current Effects on IV Characteristics of Josephson Junction Arrays
The effects of finite size and of finite current on the current-voltage
characteristics of Josephson junction arrays is studied both theoretically and
by numerical simulations. The cross-over from non-linear to linear behavior at
low temperature is shown to be a finite size effect and the non-linear behavior
at higher temperature, , is shown to be a finite current effect.
These are argued to result from competition between the three length scales
characterizing the system. The importance of boundary effects is discussed and
it is shown that these may dominate the behavior in small arrays.Comment: 5 pages, figures included, to appear in PR
The Role of Nonequilibrium Dynamical Screening in Carrier Thermalization
We investigate the role played by nonequilibrium dynamical screening in the
thermalization of carriers in a simplified two-component two-band model of a
semiconductor. The main feature of our approach is the theoretically sound
treatment of collisions. We abandon Fermi's Golden rule in favor of a
nonequilibrium field theoretic formalism as the former is applicable only in
the long-time regime. We also introduce the concept of nonequilibrium dynamical
screening. The dephasing of excitonic quantum beats as a result of
carrier-carrier scattering is brought out. At low densities it is found that
the dephasing times due to carrier-carrier scattering is in picoseconds and not
femtoseconds, in agreement with experiments. The polarization dephasing rates
are computed as a function of the excited carrier density and it is found that
the dephasing rate for carrier-carrier scattering is proportional to the
carrier density at ultralow densities. The scaling relation is sublinear at
higher densities, which enables a comparison with experiment.Comment: Revised version with additional refs. 12 pages, figs. available upon
request; Submitted to Phys. Rev.
Dynamic instabilities induced by asymmetric influence: Prisoners' dilemma game on small-world networks
A two-dimensional small-world type network, subject to spatial prisoners'
dilemma dynamics and containing an influential node defined as a special node
with a finite density of directed random links to the other nodes in the
network, is numerically investigated. It is shown that the degree of
cooperation does not remain at a steady state level but displays a punctuated
equilibrium type behavior manifested by the existence of sudden breakdowns of
cooperation. The breakdown of cooperation is linked to an imitation of a
successful selfish strategy of the influential node. It is also found that
while the breakdown of cooperation occurs suddenly, the recovery of it requires
longer time. This recovery time may, depending on the degree of steady state
cooperation, either increase or decrease with an increasing number of long
range connections.Comment: 5 pages, 6 figure
Cognitive Ability and Cardiovascular Control in Intellectually and Developmentally Disabled People
Earlier, we examined positive effects of hyperoxic air on the cognitive ability of intellectually
and developmentally disabled people (IDDP). In this study, correlation between cognitive
performance in the visual matching task and heart rate (HR) was investigated under normal
air conditions. Eighteen men (mean age 28.7 ± 5.0 year) and 22 women (mean age 35.5 ± 6.9
year) with an assessed disability level of 2.3 ± 0.6 participated. The experiment consisted of
three phases, a total of 7 min, including the rest (3 min), control (2 min), and visual matching
task 2 min phases. The HR in visual matching task phase increased, compared to those in the
rest and control phases. The cognitive ability in the visual matching task correlated with the
HR values; the response time showed a negative correlation with HR, while the accuracy rate
showed a positive correlation. Thus, adaptive changes in cardiovascular regulation probably
related to cognitive efforts and emotional excitation should be considered a noticeable factor
influencing brain supply with oxygen in IDDP (similarly to healthy people). The result of this
study agrees with the earlier obtained indications that hyperoxic air can positively affect the
cognitive performance in IDDP.Раніше ми вивчали позитивні впливи повітря, збагаченого
киснем, на когнітивні здатності людей з недостатнім інтелектуальним розвитком (НІР). У нашій роботі ми досліджували кореляцію між показниками когнітивної активності в
тесті візуальної відповідності та частотою серцевих скорочень (ЧСС) у таких суб’єктів в умовах дихання нормальним
повітрям. У тестах брали участь 18 чоловіків (середній вік
28.7 ± 5.0 року) та 22 жінки (35.5 ± 6.9 року) з оцінкою рівня
інтелектуального розвитку 2.3 ± 0.6. Експеримент (загальна
тривалість 7 хв) складався з трьох фаз: стан розслаблення
(3 хв), контроль (2 хв) та тест візуальної відповідності
(2 хв). Величини ЧСС у межах цього тесту порівняно з величинами в умовах розслаблення й контролю зростали. Показники когнітивних здібностей корелювали з величинами
ЧСС; кореляція часу відповіді була негативною, а точності відповіді – позитивною. Отже, адаптивні зміни регуляції
серцево-судинної системи, мабуть, пов’язані з когнітивними
зусиллями та емоційним збудженням, є істотним фактором,
який впливає на постачання мозку киснем у людей із НІР
(як і у здорових людей). Результати нашої роботи узгоджуються з отриманими раніше свідоцтвами про те, що дихання
повітрям, збагаченим киснем, позитивно впливає на когнітивні можливості людей із НІР
Wavelets and graph -algebras
Here we give an overview on the connection between wavelet theory and
representation theory for graph -algebras, including the higher-rank
graph -algebras of A. Kumjian and D. Pask. Many authors have studied
different aspects of this connection over the last 20 years, and we begin this
paper with a survey of the known results. We then discuss several new ways to
generalize these results and obtain wavelets associated to representations of
higher-rank graphs. In \cite{FGKP}, we introduced the "cubical wavelets"
associated to a higher-rank graph. Here, we generalize this construction to
build wavelets of arbitrary shapes. We also present a different but related
construction of wavelets associated to a higher-rank graph, which we anticipate
will have applications to traffic analysis on networks. Finally, we generalize
the spectral graph wavelets of \cite{hammond} to higher-rank graphs, giving a
third family of wavelets associated to higher-rank graphs
Proximity induced metal/insulator transition in superlattices
The far-infrared dielectric response of superlattices (SL) composed of
superconducting YBaCuO (YBCO) and ferromagnetic La%
CaMnO (LCMO) has been investigated by ellipsometry. A drastic
decrease of the free carrier response is observed which involves an unusually
large length scale of d20 nm in YBCO and d10
nm in LCMO. A corresponding suppression of metallicity is not observed in SLs
where LCMO is replaced by the paramagnetic metal LaNiO. Our data suggest
that either a long range charge transfer from the YBCO to the LCMO layers or
alternatively a strong coupling of the charge carriers to the different and
competitive kind of magnetic correlations in the LCMO and YBCO layers are at
the heart of the observed metal/insulator transition. The low free carrier
response observed in the far-infrared dielectric response of the magnetic
superconductor RuSrGdCuO is possibly related to this effect
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