198 research outputs found
Electron-electron and electron-hole pairing in graphene structures
The superconducting pairing of electrons in doped graphene due to in-plane
and out-of-plane phonons is considered. It is shown that the structure of the
order parameter in the valley space substantially affects conditions of the
pairing. Electron-hole pairing in graphene bilayer in the strong coupling
regime is also considered. Taking into account retardation of the screened
Coulomb pairing potential shows a significant competition between the
electron-hole direct attraction and their repulsion due to virtual plasmons and
single-particle excitations.Comment: 13 pages with 4 figures; accepted for publication in Phil. Trans.
Roy. Soc.
Coupled system of electrons and exciton-polaritons: Screening, dynamical effects, and superconductivity
Bose-Fermi systems such as mixtures of electrons with excitons or
exciton-polaritons are extensively discussed as candidates to host a variety of
intriguing phenomena, including polaron formation, drag effects, supersolidity,
and superconductivity. In this work, assuming the strong-coupling regime
between the semiconductor excitons and cavity photons, we develop the many-body
theory approach addressing the interplay of different types of interaction
among various species in such a mixture, wherein we take into account dynamical
density responses of both the Bose-condensed exciton-polaritons and the
two-dimensional electron gas inside an optical microcavity. As was anticipated
previously, at high enough polariton densities the lower hybrid mode of the
system's excitation spectrum acquires a roton minimum, making the system prone
to superconducting pairing in the vicinity of the roton instability. We analyze
the possibility of polariton-BEC-mediated superconductivity in the electron gas
taking into account full momentum and frequency dependence of the gap, as well
as in the Eliashberg approach where the momentum dependence is neglected, and
in the Bardeen-Cooper-Schrieffer approach that discards the frequency
dependence and dynamical effects. Considering the interaction screening in
Thomas-Fermi and in random-phase approximations, we estimate the critical
temperatures of superconductivity to be not larger than 0.1 K in the vicinity
of instability. As possible realizations of the coupled polariton-electron
system, semiconductor quantum wells and two-dimensional transition metal
dichalcogenides are considered.Comment: 19 pages, 8 figure
Cooper pairing of electrons and holes in graphene bilayer: Correlation effects
Cooper pairing of spatially separated electrons and holes in graphene bilayer
is studied beyond the mean-field approximation. Suppression of the screening at
large distances, caused by appearance of the gap, is considered
self-consistently. A mutual positive feedback between appearance of the gap and
enlargement of the interaction leads to a sharp transition to correlated state
with greatly increased gap above some critical value of the coupling strength.
At coupling strength below the critical, this correlation effect increases the
gap approximately by a factor of two. The maximal coupling strength achievable
in experiments is close to the critical value. This indicated importance of
correlation effects in closely-spaced graphene bilayers at weak substrate
dielectric screening. Another effect beyond mean-field approximation considered
is an influence of vertex corrections on the pairing, which is shown to be very
weak.Comment: 6 pages, 5 figures; some references were adde
A Review of and Perspectives on Global Change Modeling for Northern Eurasia
Northern Eurasia is made up of a complex and diverse set of physical, ecological, climatic and human systems, which provide important ecosystem services including the storage of substantial stocks of carbon in its terrestrial ecosystems. At the same time, the region has experienced dramatic climate change, natural disturbances and changes in land management practices over the past century. For these reasons, Northern Eurasia is both a critical region to understand and a complex system with substantial challenges for the modeling community. This review is designed to highlight the state of past and ongoing efforts of the research community to understand and model these environmental, socioeconomic, and climatic changes. We further aim to provide perspectives on the future direction of global change modeling to improve our understanding of the role of Northern Eurasia in the coupled human-Earth system. Major modeling efforts have shown that environmental and socioeconomic impacts in Northern Eurasia can have major implications for the biodiversity, ecosystems services, environmental sustainability, and carbon cycle of the region, and beyond. These impacts have the potential to feedback onto and alter the global Earth system. We find that past and ongoing studies have largely focused on specific components of Earth system dynamics and have not systematically examined their feedbacks to the global Earth system and to society. We identify the crucial role of Earth system models in advancing our understanding of feedbacks within the region and with the global system. We further argue for the need for Integrated Assessment Models (IAMs), a suite of models that couple human activity models to Earth system models, which are key to address many emerging issues that require a representation of the coupled human-Earth system.We acknowledge the funding from the US National Aeronautics and Space Administration (NASA) Land-Cover and Land-Use Change (LCLUC) Program, which provided support for Erwan Monier, David Kicklighter, Andrei Sokolov, Qianlai Zhuang and Sergey Paltsev under grant NNX14AD91G and Irina Sokolik under grant NNX14AD88G. Support for Pavel Groisman was provided by Grant 14.B25.31.0026 of the Ministry of Education and Science of the Russian Federation and by Project “Arctic Climate Change and its Impact on Environment, Infrastructures, and Resource Availability” sponsored by ANR (France), RFBR (Russia), and NSF (USA) in response to Belmont Forum Collaborative Research Action on Arctic Observing and Research for Sustainability. The Joint Program on the Science and Policy of Global Change is funded by a number of federal agencies and a consortium of 40 industrial and foundation sponsor (for the complete list see http://globalchange.mit.edu/sponsors)
Transcription Factor Competition Allows Embryonic Stem Cells to Distinguish Authentic Signals from Noise
Stem cells occupy variable environments where they must distinguish stochastic fluctuations from developmental cues. Here, we use optogenetics to investigate how the pluripotency network in embryonic stem cells (ESCs) achieves a robust response to differentiation cues but not to gene expression fluctuations. We engineered mouse ESCs to allow quantitative control over the endogenous mechanism of neural differentiation through a light-inducible Brn2 transgene and monitored differentiation status through a genome-integrated Nanog-GFP reporter. By exposing cells to pulses of Brn2, we find that the pluripotency network rejects Brn2 inputs that are below specific magnitude or duration thresholds, but allows rapid differentiation when both thresholds are satisfied. The filtering properties of the network arise through its positive feedback architecture and the intrinsic half-life of Nanog, which determines the duration threshold in the network. Together our results suggest that the dynamic properties of positive feedback networks might determine how inputs are classified as signal or noise by stem cells
3ARM: A Fast, Accurate Radiative Transfer Model for use in Climate Models
A new radiative transfer model combining the efforts of three groups of researchers is discussed. The model accurately computes radiative transfer in a inhomogeneous absorbing, scattering and emitting atmospheres. As an illustration of the model, results are shown for the effects of dust on the thermal radiation
The Construction of Quantum Field Operators: Something of Interest
We draw attention to some tune problems in constructions of the quantum-field
operators for spins 1/2 and 1. They are related to the existence of
negative-energy and acausal solutions of relativistic wave equations.
Particular attention is paid to the chiral theories, and to the method of the
Lorentz boosts.Comment: 31 pages, no figures. The invited talk at the VIII International
Workshop "Applied Category Theory. Graph-Operad-Logic", San Blas, Nayarit,
Mexico, January 9-16, 2010, and at the 6th International Conference on the
Dark Side of the Universe (DSU2010), Leon, Gto, Mexico, June 1-6, 201
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