Non-existence of the Luttinger-Ward functional and misleading convergence of skeleton diagrammatic series for Hubbard-like models

The Luttinger-Ward functional $\Phi[\mathbf{G}]$, which expresses the thermodynamic grand potential in terms of the interacting single-particle Green's function $\mathbf{G}$, is found to be ill-defined for fermionic models with the Hubbard on-site interaction. In particular, we show that the self-energy $\mathbf{\Sigma}[\mathbf{G}] \propto \delta\Phi[\mathbf{G}]/\delta \mathbf{G}$ is not a single-valued functional of $\mathbf{G}$: in addition to the physical solution for $\mathbf{\Sigma}[\mathbf{G}]$, 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 $\mathbf{\Sigma}$ in terms of $\mathbf{G}$ 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 $\mathbf{G}_0$ converges to the correct physical branch of $\mathbf{\Sigma}$ 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 LaNiO3_{3}: 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$_3$ 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 $d^8\underline{L}$ and $d^7$ 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

Development path and capital structure of belgian biotechnology firms

This study investigates the relationship between the evolution of real options values and associated financing policies for Belgian companies in the sector of bio-industries. Each firm's situation regarding the relevant types of real options is stylistically represented through a scenario tree. The consumption of a time-to-build or a growth option is respectively considered as a success or a failure in company development. Empirically, several variables enable us to locate each company along the tree at any time. The study of transitions leads us to discover that failures tend to trigger higher leverage, unlike in the trade-off theory. Yet, the increases in debt maturity, in lease and in convertible financing confirm our predictions. Overall, we emphasize evidence of undercapitalization and of proper, yet insufficient, use of hybrid financing by biotech companies.