4,736 research outputs found
Non-existence of the Luttinger-Ward functional and misleading convergence of skeleton diagrammatic series for Hubbard-like models
The Luttinger-Ward functional , which expresses the
thermodynamic grand potential in terms of the interacting single-particle
Green's function , is found to be ill-defined for fermionic models
with the Hubbard on-site interaction. In particular, we show that the
self-energy is not a single-valued functional of : in addition to
the physical solution for , there exists at least
one qualitatively distinct unphysical branch. This result is demonstrated for
several models: the Hubbard atom, the Anderson impurity model, and the full
two-dimensional Hubbard model. Despite this pathology, the skeleton Feynman
diagrammatic series for in terms of is found to
converge at least for moderately low temperatures. However, at strong
interactions, its convergence is to the unphysical branch. This reveals a new
scenario of breaking down of diagrammatic expansions. In contrast, the bare
series in terms of the non-interacting Green's function
converges to the correct physical branch of in all cases
currently accessible by diagrammatic Monte Carlo. Besides their conceptual
importance, these observations have important implications for techniques based
on the explicit summation of diagrammatic series.Comment: 5 pages, 5 figure
Orbital Polarization in Strained LaNiO: Structural Distortions and Correlation Effects
Transition-metal heterostructures offer the fascinating possibility of
controlling orbital degrees of freedom via strain. Here, we investigate
theoretically the degree of orbital polarization that can be induced by
epitaxial strain in LaNiO films. Using combined electronic structure and
dynamical mean-field theory methods we take into account both structural
distortions and electron correlations and discuss their relative influence. We
confirm that Hund's rule coupling tends to decrease the polarization and point
out that this applies to both the and local
configurations of the Ni ions. Our calculations are in good agreement with
recent experiments, which revealed sizable orbital polarization under tensile
strain. We discuss why full orbital polarization is hard to achieve in this
specific system and emphasize the general limitations that must be overcome to
achieve this goal.Comment: 13 pages, 13 figure
Theoretical prediction and spectroscopic fingerprints of an orbital transition in CeCu2Si2
We show that the heavy-fermion compound CeCu2Si2 undergoes a transition
between two regimes dominated by different crystal-field states. At low
pressure P and low temperature T the Ce 4f electron resides in the atomic
crystal-field ground state, while at high P or T the electron occupancy and
spectral weight is transferred to an excited crystal-field level that
hybridizes more strongly with itinerant states. These findings result from
first-principles dynamical-mean-field-theory calculations. We predict
experimental signatures of this orbital transition in X-ray spectroscopy. The
corresponding fluctuations may be responsible for the second high-pressure
superconducting dome observed in this and similar materials.Comment: 5 pages, 4 figures + 5 supplementary page
Quench front progression in a superheated porous medium: experimental analysis and model development
In case of severe accident in a nuclear reactor, the fuel rods may be highly damaged and oxidized and finally collapse to form a debris bed. Removal of decay heat from a debris bed is a challenging issue because of the difficulty for water to flow inside. Currently, IRSN has started experimental program PEARL with two experimental facilities PRELUDE and PEARL, to investigate the reflood process at high temperature, for various particle sizes. On the basis of PRELUDE experimental results, the thermal hydraulic features of the quench front have been analysed and the intensity of heat transfers was estimated. From a selection of experimental results, a reflooding model was improved and validated. The model is implemented in the code ICARE-CATHARE developed by IRSN which is used for severe accident reactor analysis
Microfluidic synthesis and assembly of reactive polymer beads to form new structured polymer materials
Monodisperse and size-controlled polymer particles were produced without surfactant or wash-coat from O/W monomer emulsions and ‘‘on the fly’’ polymerization under UV irradiation in a very simple needle/tubing system. The effect of the viscosity of the continuous phase on the size of final particles was investigated. The capillary number ratio was found to be relevant to predict the size of the droplets. A relation between dimensionless numbers predicts particle diameter as a function of the needle inner diameter and both velocity and viscosity ratios of continuous and dispersed phases. A functional comonomer was incorporated in the monomer phase so as to obtain polymer microparticles bearing reactive groups on their surface. Polymer beads necklaces were thus formed by linking polymer particles together
Фразеологическая единица как объект изучения
Проблема выделения и определения языковых единиц – одна из наиболее трудных в лингвистике. Лингвист идет «по следам» живых носителей языка, пытаясь установить, как сами говорящие выделяют для себя какие-то единицы из того речевого потока, который только и является реальным существованием и проявлением языковой системы. Обучаясь речи естественным образом (т.е. в процессе общения, а не специального обучения), мы стремимся расчленить речевой поток, выделить из него и запомнить какие-то минимальные компоненты (разных уровней) и модели, с помощью которых можно потом строить свою собственную речь. Тем самым отбрасывается индивидуальное речевое, и усваиваются определенные стабильные элементы, обладающие относительной самостоятельностью формы и содержания
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