Réseaux de tenseurs et solutionneurs d’impureté pour la théorie du champ moyen dynamique

Cette thèse porte sur l'obtention de solutions au problème d'impureté de la théorie du champ moyen dynamique sur amas. Le problème d'impureté consiste en des sites en interaction submergées dans un bain d'électrons sans interaction. Ce bain est déterminé par la condition d'autocohérence du champ moyen dynamique. Dans le contexte du champ moyen dynamique, la solution prend la forme d'une fonction de Green, une observable dynamique. La fonction de Green est riche en information: elle encode la densité d'état à une particule, l'énergie potentielle ainsi que les valeurs moyennes de toute observable à un corps. Plusieurs classes de solutionneurs existent pour ce problème d'impureté; on se concentre sur ceux usant du formalisme hamiltonien. Dans ce formalisme, la qualité des résultats est limitée par la représentation hamiltonienne du bain. Dans le formalisme hamiltonien, le bain du champ moyen dynamique a une grande liberté de jauge: il existe de nombreuses façons de le représenter. Cette liberté crée certaines difficultés: bien que toutes les représentations soient formellement équivalentes, elles ne sont pas toutes également faciles à traiter numériquement. Il faut donc choisir judicieusement. Historiquement, on obtenait une paramétrisation pratique en imposant des contraintes heuristiques au bain. On montre qu'en utilisant les symétries du réseau on peut créer une paramétrisation complète qui se prête au traitement numérique. Avec cette paramétrisation et un solutionneur par diagonalisation exacte, on a revisité le problème de la compétition entre l'antiferromagnétisme et la supraconductivité dans le modèle de Hubbard pour les cuprates. Cette étude montre que la coexistence homogène est possible avec un dopage en électrons, mais impossible avec un dopage aux trous, contrairement aux résultats déjà publiés. La taille du bain est aussi limitée avec un solutionneur hamiltonien: il doit être discret et fini. Avec un solutionneur par diagonalisation exacte, la difficulté du problème croît exponentiellement avec le nombre d'orbitales. On est limité à moins de vingt orbitales au total avec cette méthode. En utilisant les méthodes sur réseaux de tenseurs, la difficulté numérique des problèmes hamiltoniens croît plutôt de façon polynomiale. Avec les réseaux de tenseurs, calculer la fonction de Green demeure ardu. On pallie cette difficulté en extrapolant la représentation en fractions continues de la fonction de Green. Contrairement aux méthodes par diagonalisation exacte, cette méthode donne une densité d'état qui peut être continue. On démontre le potentiel de cette méthode en l'applicant au modèle de Hubbard sur le réseau de Bethe

Direct solution of multiple excitations in a matrix product state with block Lanczos

Matrix product state methods are known to be efficient for computing ground states of local, gapped Hamiltonians, particularly in one dimension. We introduce the multi-targeted density matrix renormalization group method that acts on a bundled matrix product state, holding many excitations. The use of a block or banded Lanczos algorithm allows for the simultaneous, variational optimization of the bundle of excitations. The method is demonstrated on a Heisenberg model and other cases of interest. A large of number of excitations can be obtained at a small bond dimension with highly reliable local observables throughout the chain.Comment: 18 page

Effet des impuretés sur la supraconductivité et l'antiferromagnétisme dans le modèle de Hubbard

Nous présentons dans ce document une étude théorique de l’effet d’une impureté répétée sur le modèle de Hubbard à une bande appliqué au YBCO, un cuprate supraconducteur. Pour résoudre approximativement ce problème, on a fait appel à la théorie du champ moyen dynamique sur amas à température nulle. Il s’agit de la première application de cette méthode à l’étude des impuretés dans les cuprates

Pyqcm: An open-source Python library for quantum cluster methods

Pyqcm is a Python/C++ library that implements a few quantum cluster methods with an exact diagonalization impurity solver. Quantum cluster methods are used in the study of strongly correlated electrons to provide an approximate solution to Hubbard-like models. The methods covered by this library are Cluster Perturbation Theory (CPT), the Variational Cluster Approach (VCA) and Cellular (or Cluster) Dynamical Mean Field Theory (CDMFT). The impurity solver (the technique used to compute the cluster's interacting Green function) is exact diagonalization from sparse matrices, using the Lanczos algorithm and variants thereof. The core library is written in C++ for performance, but the interface is in Python, for ease of use and inter-operability with the numerical Python ecosystem. The library is distributed under the GPL license.Comment: Submission to SciPost. arXiv admin note: substantial text overlap with arXiv:0806.269

Science Hackathons for Cyberphysical System Security Research: Putting CPS testbed platforms to good use

A challenge is to develop cyber-physical system scenarios that reflect the diversity and complexity of real-life cyber-physical systems in the research questions that they address. Time-bounded collaborative events, such as hackathons, jams and sprints, are increasingly used as a means of bringing groups of individuals together, in order to explore challenges and develop solutions. This paper describes our experiences, using a science hackathon to bring individual researchers together, in order to develop a common use-case implemented on a shared CPS testbed platform that embodies the diversity in their own security research questions. A qualitative study of the event was conducted, in order to evaluate the success of the process, with a view to improving future similar events

Performance of photosensors in a high-rate environment for gas Cherenkov detectors

The solenoidal large intensity device (SoLID) at Jefferson Lab will push the boundaries of luminosity for a large-acceptance detector, which necessitates the use of a light-gas threshold Cherenkov counter for online event selection. Due to the high luminosity, the single-photon background rate in this counter can exceed 160 kHz/cm$^2$ at the photosensors. Therefore, it is essential to validate the high-rate limits of the planned photosensors and readout electronics in order to mitigate the risk of failure. We report on the design and an early set of studies carried out using a small telescopic Cherenkov device in a high-rate environment up to 60 kHz/cm$^2$, in Hall C at Jefferson Lab. Commercially available multi-anode photomultipliers (MaPMT) and low-cost large-area picosecond photodetectors (LAPPD) were tested using the JLab FADC250 modules for readout. The test beam results show that the MaPMT array and the internal stripline LAPPD can detect and identify single-electron and pair-production events in high-rate environments. Due to its higher quantum efficiency, the MaPMT array provided a better separation between the single-electron and the pair-production events compared to the internal stripline LAPPD. A GEANT4 simulation confirms the experimental performance of our telescopic device.Comment: 16 pages, 11 figure

Low-volume intraplate volcanism in the Early/Middle Jurassic Pacific basin documented by accreted sequences in Costa Rica

Countless seamounts occur on Earth that can provide important constraints on intraplate volcanism and plate tectonics in the oceans, yet their nature and origin remain poorly known due to difficulties in investigating the deep ocean. We present here new lithostratigraphic, age and geochemical data from Lower/Middle Jurassic and Lower Cretaceous sequences in the Santa Rosa accretionary complex, Costa Rica, which offer a valuable opportunity to study a small-sized seamount from a subducted plate segment of the Pacific basin. The seamount is characterized by very unusual lithostratigraphic sequences with sills of potassic alkaline basalt emplaced within thick beds of radiolarite, basaltic breccia and hyaloclastite. An integration of new geochemical, biochronological and geochronological data with lithostratigraphic observations suggests that the seamount formed ~175 Ma ago on thick oceanic crust away from subduction zones and mid-ocean ridges. This seamount travelled ~65 Ma in the Pacific before accretion. It resembles lithologically and compositionally “petit-spot” volcanoes found off Japan, which form in response to plate flexure near subduction zones. Also, the composition of the sills and lava flows in the accreted seamount closely resembles that of potassic alkaline basalts produced by lithosphere cracking along the Line Islands chain. We hypothesize based on these observations, petrological constraints and formation of the accreted seamount coeval with the early stages of development of the Pacific plate that the seamount formed by extraction of small volumes of melt from the base of the lithosphere in response to propagating fractures at the scale of the Pacific basin

Epigenetic alterations differ in phenotypically distinct human neuroblastoma cell lines

<p>Abstract</p> <p>Background</p> <p>Epigenetic aberrations and a CpG island methylator phenotype have been shown to be associated with poor outcomes in children with neuroblastoma (NB). Seven cancer related genes (<it>THBS-1, CASP8, HIN-1, TIG-1, BLU, SPARC</it>, and <it>HIC-1</it>) that have been shown to have epigenetic changes in adult cancers and play important roles in the regulation of angiogenesis, tumor growth, and apoptosis were analyzed to investigate the role epigenetic alterations play in determining NB phenotype.</p> <p>Methods</p> <p>Two NB cell lines (tumorigenic LA1-55n and non-tumorigenic LA1-5s) that differ in their ability to form colonies in soft agar and tumors in nude mice were used. Quantitative RNA expression analyses were performed on seven genes in LA1-5s, LA1-55n and 5-Aza-dC treated LA1-55n NB cell lines. The methylation status around <it>THBS-1, HIN-1, TIG-1 </it>and <it>CASP8 </it>promoters was examined using methylation specific PCR. Chromatin immunoprecipitation assay was used to examine histone modifications along the <it>THBS-1 </it>promoter. Luciferase assay was used to determine <it>THBS-1 </it>promoter activity. Cell proliferation assay was used to examine the effect of 5-Aza-dC on NB cell growth. The soft agar assay was used to determine the tumorigenicity.</p> <p>Results</p> <p>Promoter methylation values for <it>THBS-1</it>, <it>HIN-1</it>, <it>TIG-1</it>, and <it>CASP8 </it>were higher in LA1-55n cells compared to LA1-5s cells. Consistent with the promoter methylation status, lower levels of gene expression were detected in the LA1-55n cells. Histone marks associated with repressive chromatin states (H3K9Me3, H3K27Me3, and H3K4Me3) were identified in the <it>THBS-1 </it>promoter region in the LA1-55n cells, but not the LA1-5s cells. In contrast, the three histone codes associated with an active chromatin state (acetyl H3, acetyl H4, and H3K4Me3) were present in the <it>THBS-1 </it>promoter region in LA1-5s cells, but not the LA1-55n cells, suggesting that an accessible chromatin structure is important for <it>THBS-1 </it>expression. We also show that 5-Aza-dC treatment of LA1-55n cells alters the DNA methylation status and the histone code in the <it>THBS-1 </it>promoter modifies cell morphology, and inhibits their ability to form colonies in soft agar.</p> <p>Conclusion</p> <p>Our results suggest that epigenetic aberrations contribute to NB phenotype, and that tumorigenic properties can be inhibited by reversing the epigenetic changes with 5-Aza-dC.</p