1,504 research outputs found
Vernier spectrometer using counter-propagating soliton microcombs
Acquisition of laser frequency with high resolution under continuous and
abrupt tuning conditions is important for sensing, spectroscopy and
communications. Here, a single microresonator provides rapid and broad-band
measurement of frequencies across the optical C-band with a relative frequency
precision comparable to conventional dual frequency comb systems. Dual-locked
counter-propagating solitons having slightly different repetition rates are
used to implement a Vernier spectrometer. Laser tuning rates as high as 10
THz/s, broadly step-tuned lasers, multi-line laser spectra and also molecular
absorption lines are characterized using the device. Besides providing a
considerable technical simplification through the dual-locked solitons and
enhanced capability for measurement of arbitrarily tuned sources, this work
reveals possibilities for chip-scale spectrometers that greatly exceed the
performance of table-top grating and interferometer-based devices
May the Force be with You: Unified Force-Centric Pre-Training for 3D Molecular Conformations
Recent works have shown the promise of learning pre-trained models for 3D
molecular representation. However, existing pre-training models focus
predominantly on equilibrium data and largely overlook off-equilibrium
conformations. It is challenging to extend these methods to off-equilibrium
data because their training objective relies on assumptions of conformations
being the local energy minima. We address this gap by proposing a force-centric
pretraining model for 3D molecular conformations covering both equilibrium and
off-equilibrium data. For off-equilibrium data, our model learns directly from
their atomic forces. For equilibrium data, we introduce zero-force
regularization and forced-based denoising techniques to approximate
near-equilibrium forces. We obtain a unified pre-trained model for 3D molecular
representation with over 15 million diverse conformations. Experiments show
that, with our pre-training objective, we increase forces accuracy by around 3
times compared to the un-pre-trained Equivariant Transformer model. By
incorporating regularizations on equilibrium data, we solved the problem of
unstable MD simulations in vanilla Equivariant Transformers, achieving
state-of-the-art simulation performance with 2.45 times faster inference time
than NequIP. As a powerful molecular encoder, our pre-trained model achieves
on-par performance with state-of-the-art property prediction tasks
PolyGET: Accelerating Polymer Simulations by Accurate and Generalizable Forcefield with Equivariant Transformer
Polymer simulation with both accuracy and efficiency is a challenging task.
Machine learning (ML) forcefields have been developed to achieve both the
accuracy of ab initio methods and the efficiency of empirical force fields.
However, existing ML force fields are usually limited to single-molecule
settings, and their simulations are not robust enough. In this paper, we
present PolyGET, a new framework for Polymer Forcefields with Generalizable
Equivariant Transformers. PolyGET is designed to capture complex quantum
interactions between atoms and generalize across various polymer families,
using a deep learning model called Equivariant Transformers. We propose a new
training paradigm that focuses exclusively on optimizing forces, which is
different from existing methods that jointly optimize forces and energy. This
simple force-centric objective function avoids competing objectives between
energy and forces, thereby allowing for learning a unified forcefield ML model
over different polymer families. We evaluated PolyGET on a large-scale dataset
of 24 distinct polymer types and demonstrated state-of-the-art performance in
force accuracy and robust MD simulations. Furthermore, PolyGET can simulate
large polymers with high fidelity to the reference ab initio DFT method while
being able to generalize to unseen polymers
First-principles calculation of topological invariants Z2 within the FP-LAPW formalism
In this paper, we report the implementation of first-principles calculations
of topological invariants Z2 within the full-potential linearized augmented
plane-wave (FP-LAPW) formalism. In systems with both time-reversal and spatial
inversion symmetry (centrosymmetric), one can use the parity analysis of Bloch
functions at time-reversal invariant momenta to determine the Z2 invariants. In
systems without spatial inversion symmetry (noncentrosymmetric), however, a
more complex and systematic method in terms of the Berry gauge potential and
the Berry curvature is required to identify the band topology. We show in
detail how both methods are implemented in FP-LAPW formalism and applied to
several classes of materials including centrosymmetric compounds Bi2Se3 and
Sb2Se3 and noncentrosymmetric compounds LuPtBi, AuTlS2 and CdSnAs2. Our work
provides an accurate and effective implementation of first-principles
calculations to speed up the search of new topological insulators
Moving a step closer towards environmental sustainability in Asian countries: focusing on real income, urbanization, transport infrastructure, and research and development
Environmental pollution has become the matter of concern all
over the world with the increase in urbanization, transport, industrialization
and several other factors. The researcher has therefore
designed this study to investigate the impact of urbanization,
research and development R&D expenditure, infrastructure development
and real income on the emission of carbon dioxide in
Asian countries. The data collection process involved six Asian
countries from 1997 and ending 2019. The panel data estimation
and analysis tools and techniques were applied on the collected
data and the results were obtained. The results of regression estimation
suggest that as per MG estimator, all the variables have
significant and positive impact on CO2 emission but infrastructure
development has insignificant impact. In case of FMOLS, again all
the variables have significant and positive impact on CO2 emission
but infrastructure development has insignificant impact.
However, in case of DOLS, all the variables have shown significant
impact on CO2 emission. In the last, DK estimator indicates that
urbanization, real income and population density have significant
and positive impact on CO2 emission but R&D expenditure and
infrastructure development has insignificant impact. In this way,
the impacts of all independent and control variables on CO2 emission
were estimated
Synthesis and Characterization of the Inclusion Complex of Dicationic Ionic Liquid and β-Cyclodextrin
The supramolecular structure of the inclusion complex of β-cyclodextrin (β-CD) with 1,1′,2,2′-tetramethyl-3,3′-(p-phenylenedimethylene) diimidazolium dibromide (TetraPhimBr), a dicationic ionic liquid, has been investigated. The inclusion complex with 1:1 molar ratio was prepared by a kneading method. Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD) analysis, 1H NMR and thermogravimetric analysis (TGA) confirmed the formation of the inclusion complex. The results showed that the host-guest system is a fine crystalline powder. The decomposition temperature of the inclusion complex is lower than that of its parent molecules, TetraPhimBr and β-CD individually
A novel transflectance near infrared spectroscopy technique for monitoring hot melt extrusion
yesA transflectance near infra red (NIR) spectroscopy approach has been used to simultaneously measure drug and plasticiser content of polymer melts with varying opacity during hot melt extrusion. A high temperature reflectance NIR probe was mounted in the extruder die directly opposed to a highly reflective surface. Carbamazepine (CBZ) was used as a model drug, with polyvinyl pyrollidone-vinyl acetate co-polymer (PVP-VA) as a matrix and polyethylene glycol (PEG) as a plasticiser. The opacity of the molten extrudate varied from transparent at low CBZ loading to opaque at high CBZ loading. Particulate amorphous API and voids formed around these particles were found to cause the opacity. The extrusion process was monitored in real time using transflectance NIR; calibration and validation runs were performed using a wide range of drug and plasticiser loadings. Once calibrated, the technique was used to simultaneously track drug and plasticiser content during applied step changes in feedstock material. Rheological and thermal characterisations were used to help understand the morphology of extruded material. The study has shown that it is possible to use a single NIR spectroscopy technique to monitor opaque and transparent melts during HME, and to simultaneously monitor two distinct components within a formulation
Transport in three-dimensional topological insulators: theory and experiment
This article reviews recent theoretical and experimental work on transport
due to the surface states of three-dimensional topological insulators. The
theoretical focus is on longitudinal transport in the presence of an electric
field, including Boltzmann transport, quantum corrections and weak
localization, as well as longitudinal and Hall transport in the presence of
both electric and magnetic fields and/or magnetizations. Special attention is
paid to transport at finite doping, to the -Berry phase, which leads to
the absence of backscattering, Klein tunneling and half-quantized Hall
response. Signatures of surface states in ordinary transport and
magnetotransport are clearly identified. The review also covers transport
experiments of the past years, reviewing the initial obscuring of surface
transport by bulk transport, and the way transport due to the surface states
has increasingly been identified experimentally. Current and likely future
experimental challenges are given prominence and the current status of the
field is assessed.Comment: Review article to appear in Physica
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