254 research outputs found
Carrier multiplication yields in CdSe and CdTe nanocrystals by transient photoluminescence
Engineering semiconductors to enhance carrier multiplication (CM) could lead
to increased photovoltaic cell performance and a significant widening of the
materials range suitable for future solar technologies. Semiconductor
nanocrystals (NCs) have been proposed as a favourable structure for CM
enhancement, and recent measurements by transient absorption have shown
evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report
here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis
of biexciton and exciton signatures in transient photoluminescence decays.
Although the technique is particularly sensitive due to enhanced biexciton
radiative rates relative to the exciton, kradBX > 2 kradX, we find no evidence
for CM in CdSe and CdTe NCs up to photon energies E > 3 Eg, well above
previously reported relative energy thresholds.Comment: 9 pages, 6 figure
The Changing Fractions of Type Ia Supernova NUV-Optical Subclasses with Redshift
UV and optical photometry of Type Ia supernovae (SNe Ia) at low redshift have
revealed the existence of two distinct color groups, NUV-red and NUV-blue
events. The color curves differ primarily by an offset, with the NUV-blue u-
color curves bluer than the NUV-red curves by 0.4 mag. For a sample of 23 low-z
SNe~Ia observed with Swift, the NUV-red group dominates by a ratio of 2:1. We
compare rest-frame UV/optical spectrophotometry of intermediate and high-z SNe
Ia with UVOT photometry and HST spectrophotometry of low-z SNe Ia, finding that
the same two color groups exist at higher-z, but with the NUV-blue events as
the dominant group. Within each red/blue group, we do not detect any offset in
color for different redshifts, providing insight into how SN~Ia UV emission
evolves with redshift. Through spectral comparisons of SNe~Ia with similar peak
widths and phase, we explore the wavelength range that produces the UV/OPT
color differences. We show that the ejecta velocity of NUV-red SNe is larger
than that of NUV-blue objects by roughly 12% on average. This velocity
difference can explain some of the UV/optical color difference, but differences
in the strengths of spectral features seen in meanspectra require additional
explanation. Because of the different b-v colors for these groups, NUV-red SNe
will have their extinction underestimated using common techniques. This, in
turn, leads to under-estimation of the optical luminosity of the NUV-blue
SNe~Ia, in particular, for the high-redshift cosmological sample. Not
accounting for this effect should thus produce a distance bias that increases
with redshift and could significantly bias measurements of cosmological
parameters.Comment: submitted to Ap
Where there’s ‘willingness’ there’s a way: barriers and facilitators to maternal, newborn and child health data sharing by the private health sector in Uttar Pradesh, India
In India and Uttar Pradesh (UP), the private health sector plays an important role in health care services, including institutional deliveries, but there is limited information on the availability of maternal, newborn and child health (MNCH) data that private facilities maintain and share with the public health information system. Sharing data could help the public sector plan their resources more efficiently.
Aim of the study: To explore current practices of MNCH data availability and sharing/reporting by private health facilities and the barriers and facilitators to data sharing
Real-Time Reinforcement Learning for Vision-Based Robotics Utilizing Local and Remote Computers
Real-time learning is crucial for robotic agents adapting to ever-changing,
non-stationary environments. A common setup for a robotic agent is to have two
different computers simultaneously: a resource-limited local computer tethered
to the robot and a powerful remote computer connected wirelessly. Given such a
setup, it is unclear to what extent the performance of a learning system can be
affected by resource limitations and how to efficiently use the wirelessly
connected powerful computer to compensate for any performance loss. In this
paper, we implement a real-time learning system called the Remote-Local
Distributed (ReLoD) system to distribute computations of two deep reinforcement
learning (RL) algorithms, Soft Actor-Critic (SAC) and Proximal Policy
Optimization (PPO), between a local and a remote computer. The performance of
the system is evaluated on two vision-based control tasks developed using a
robotic arm and a mobile robot. Our results show that SAC's performance
degrades heavily on a resource-limited local computer. Strikingly, when all
computations of the learning system are deployed on a remote workstation, SAC
fails to compensate for the performance loss, indicating that, without careful
consideration, using a powerful remote computer may not result in performance
improvement. However, a carefully chosen distribution of computations of SAC
consistently and substantially improves its performance on both tasks. On the
other hand, the performance of PPO remains largely unaffected by the
distribution of computations. In addition, when all computations happen solely
on a powerful tethered computer, the performance of our system remains on par
with an existing system that is well-tuned for using a single machine. ReLoD is
the only publicly available system for real-time RL that applies to multiple
robots for vision-based tasks.Comment: Appears in Proceedings of the 2023 International Conference on
Robotics and Automation (ICRA). Source code at
https://github.com/rlai-lab/relod and companion video at
https://youtu.be/7iZKryi1xS
Correcting discount-factor mismatch in on-policy policy gradient methods
The policy gradient theorem gives a convenient form of the policy gradient in
terms of three factors: an action value, a gradient of the action likelihood,
and a state distribution involving discounting called the \emph{discounted
stationary distribution}. But commonly used on-policy methods based on the
policy gradient theorem ignores the discount factor in the state distribution,
which is technically incorrect and may even cause degenerate learning behavior
in some environments. An existing solution corrects this discrepancy by using
as a factor in the gradient estimate. However, this solution is not
widely adopted and does not work well in tasks where the later states are
similar to earlier states. We introduce a novel distribution correction to
account for the discounted stationary distribution that can be plugged into
many existing gradient estimators. Our correction circumvents the performance
degradation associated with the correction with a lower variance.
Importantly, compared to the uncorrected estimators, our algorithm provides
improved state emphasis to evade suboptimal policies in certain environments
and consistently matches or exceeds the original performance on several OpenAI
gym and DeepMind suite benchmarks
Carrier multiplication yields in PbS and PbSe nanocrystals measured by transient photoluminescence
We report here an assessment of carrier multiplication (CM) yields in PbSe
and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton
dynamics in transient photoluminescence decays. Interest in CM, the generation
of more than one electron and hole in a semiconductor after absorption of one
photon, has renewed in recent years because of reports suggesting greatly
increased efficiencies in nanocrystalline materials compared to the bulk form,
in which CM was otherwise too weak to be of consequence in photovoltaic energy
conversion devices. In our PbSe and PbS NC samples, however, we estimate using
transient photoluminescence that at most 0.25 additional e-h pairs are
generated per photon even at energies hv > 5Eg, instead of the much higher
values reported in the literature. We argue by comparing NC CM estimates and
reported bulk values on an absolute energy basis, which we justify as
appropriate on physical grounds, that the data reported thus far are
inconclusive with respect to the importance of nanoscale-specific phenomena in
the CM process.Comment: 10 pages, 7 figure
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