Thermal conductivity and phonon hydrodynamics in transition metal dichalcogenides from first-principles

We carry out a systematic study of the thermal conductivity of four single-layer transition metal dichalcogenides, MX$_2$ (M = Mo, W; X = S, Se) from first-principles by solving the Boltzmann Transport Equation (BTE). We compare three different theoretical frameworks to solve the BTE beyond the Relaxation Time Approximation (RTA), using the same set of interatomic force constants computed within density functional theory (DFT), finding that the RTA severely underpredicts the thermal conductivity of MS$_2$ materials. Calculations of the different phonon scattering relaxation times of the main collision mechanisms and their corresponding mean free paths (MFP) allow evaluating the expected hydrodynamic behaviour in the heat transport of such monolayers. These calculations indicate that despite of their low thermal conductivity, the present TMDs can exhibit large hydrodynamic effects, being comparable to those of graphene, especially for WSe$_2$ at high temperatures.Comment: 16 pages, 9 figure

Class-Weighted Convolutional Features for Visual Instance Search

Image retrieval in realistic scenarios targets large dynamic datasets of unlabeled images. In these cases, training or fine-tuning a model every time new images are added to the database is neither efficient nor scalable. Convolutional neural networks trained for image classification over large datasets have been proven effective feature extractors for image retrieval. The most successful approaches are based on encoding the activations of convolutional layers, as they convey the image spatial information. In this paper, we go beyond this spatial information and propose a local-aware encoding of convolutional features based on semantic information predicted in the target image. To this end, we obtain the most discriminative regions of an image using Class Activation Maps (CAMs). CAMs are based on the knowledge contained in the network and therefore, our approach, has the additional advantage of not requiring external information. In addition, we use CAMs to generate object proposals during an unsupervised re-ranking stage after a first fast search. Our experiments on two public available datasets for instance retrieval, Oxford5k and Paris6k, demonstrate the competitiveness of our approach outperforming the current state-of-the-art when using off-the-shelf models trained on ImageNet. The source code and model used in this paper are publicly available at http://imatge-upc.github.io/retrieval-2017-cam/.Comment: To appear in the British Machine Vision Conference (BMVC), September 201

Design of a distributed data acquisition system for the ITER’s neutral beam

The International Thermonuclear Experimental Reactor (ITER) is a groundbreaking interna- tional collaboration aimed at developing fusion energy as a clean, safe, and virtually limitless source of power that brings together scientists, engineers, and experts from 35 countries to con- struct and operate the world’s largest experimental fusion reactor. Through the fusion of hy- drogen isotopes, ITER seeks to replicate the process that powers the sun and stars, harnessing the immense energy released to generate electricity. With its ambitious goals and cutting-edge technology, ITER represents a significant milestone in the pursuit of sustainable and abundant energy for the future. As part of the ITER project, the development of several systems of plasma heating is needed to achieve fusion conditions in order to reach plasma ignition. One of such heating systems is the Heating Neutral Beam (HNB), which is designed to inject a energetic beam of neutral atoms into the plasma and heat the fusion plasma by coulomb collisions of such with the plasma. This system requires of several components such as power supplies, cryopumps and cooling components working together in order to achieve a controlled and safe operation of the HNB. It also needs to work coordinated with the experimental control with high availability. The neutral beam control system is, therefore, responsible for the correct and safe operation of the two HNB units installed at ITER. The project presents an overview of the instrumentation and control system currently being developed for the Neutral Beam units and presents the development and design of a remote distributed data acquisition system prototype for the Neutral Beam instrumentation and control system. The performance of the prototype will be measured and evaluated to determine if such solution is fit for ITER requirements and can therefore be implemented into the Neutral Beam control system and other control systems within the reactor components. This project was developed under the Traineeship program by the European Joint Undertaking for ITER and the Development of Fusion Energy, Fusion For Energy (F4E). This report presents the work the author performed during such contract and under the guidance of the program’s supervisor

Coexistence of Pairing Tendencies and Ferromagnetism in a Doped Two-Orbital Hubbard Model on Two-Leg Ladders

Using the Density Matrix Renormalization Group and two-leg ladders, we investigate an electronic two-orbital Hubbard model including plaquette diagonal hopping amplitudes. Our goal is to search for regimes where charges added to the undoped state form pairs, presumably a precursor of a superconducting state.For the electronic density $\rho=2$, i.e. the undoped limit, our investigations show a robust $(\pi,0)$ antiferromagnetic ground state, as in previous investigations. Doping away from $\rho=2$ and for large values of the Hund coupling $J$, a ferromagnetic region is found to be stable. Moreover, when the interorbital on-site Hubbard repulsion is smaller than the Hund coupling, i.e. for $U'<J$ in the standard notation of multiorbital Hubbard models, our results indicate the coexistence of pairing tendencies and ferromagnetism close to $\rho=2$. These results are compatible with previous investigations using one dimensional systems. Although further research is needed to clarify if the range of couplings used here is of relevance for real materials, such as superconducting heavy fermions or pnictides, our theoretical results address a possible mechanism for pairing that may be active in the presence of short-range ferromagnetic fluctuations.Comment: 8 pages, 4 Fig

Implementation of the SU(2) Hamiltonian Symmetry for the DMRG Algorithm

In the Density Matrix Renormalization Group (DMRG) algorithm, Hamiltonian symmetries play an important role. Using symmetries, the matrix representation of the Hamiltonian can be blocked. Diagonalizing each matrix block is more efficient than diagonalizing the original matrix. This paper explains how the the DMRG++ code has been extended to handle the non-local SU(2) symmetry in a model independent way. Improvements in CPU times compared to runs with only local symmetries are discussed for the one-orbital Hubbard model, and for a two-orbital Hubbard model for iron-based superconductors. The computational bottleneck of the algorithm and the use of shared memory parallelization are also addressed.Comment: elsarticle, 1 figur

Subjetividades on line: Entre la vigilancia y la autorreferencia global

A partir de lecturas antropológicas, comunicacionales y sociológicas, el presente ensayo buscar reflexionar como el uso y apropiación de las tecnologías de la información han desembocado en nuevas formas de subjetividades, vigilancia, control  y seguridad social. Conceptos como panóptico, sinóptico, seguridad e identidad se han vuelto recurrentes en los debates sociales sobre las tecnologías, especialmente cuando asistimos a un mundo en que lo digital ha cambiado la manera de reconocernos, representarnos y estar en el mundo

Implementation of the K-Means Algorithm on Heterogeneous Devices: A Use Case Based on an Industrial Dataset

This paper presents and analyzes a heterogeneous implementation of an industrial use case based on K-means that targets symmetric multiprocessing (SMP), GPUs and FPGAs. We present how the application can be optimized from an algorithmic point of view and how this optimization performs on two heterogeneous platforms. The presented implementation relies on the OmpSs programming model, which introduces a simplified pragma-based syntax for the communication between the main processor and the accelerators. Performance improvement can be achieved by the programmer explicitly specifying the data memory accesses or copies. As expected, the newer SMP+GPU system studied is more powerful than the older SMP+FPGA system. However the latter is enough to fulfill the requirements of our use case and we show that uses less energy when considering only the active power of the execution.This work is partially supported by the European Union H2020 project AXIOM (grant agreement n. 645496), HiPEAC (grant agreement n. 687698), and Mont-Blanc (grant agreements n. 288777, 610402 and 671697), the Spanish Government Programa Severo Ochoa (SEV-2015-0493), the Spanish Ministry of Science and Technology (TIN2015- 65316-P) and the Departament d’Innovació, Universitats i Empresa de la Generalitat de Catalunya, under project MPEXPAR: Models de Programaci´o i Entorns d’Execució Paral·lels (2014-SGR-1051).Peer ReviewedPostprint (author's final draft

Ungewöhnliche Reaktionen und neuartige Glycosylierungen mit Benzotriazol- und Nitrobenzolderivaten

1. Glycosyl sulfonates have been somewhat neglected in recent years, despite of their considerable potential in stereoselective glycosylation. For this reason, the generation of glycosyl donors with nonafluorobutanesulfonyl derivatives has been studied. Using the commercially available nonafluorobutane sulfonyl fluoride, it was found that either trehalose or the glycosylated product could be formed depending on the order in which the reagents were added. The use of 1H-benzotriazole for the transfer of the nonafluorobutane sulfonyl group to glucose and the generation of the glycosyl donor was studied. By studying how to cleave the nonafluorobutane sulfonyl group from thebenzotriazole moiety with phenol, the heterocyclic ring was opened between positions N1 and N2, resulting in a new methodology for the formation of azobenzenes. This reaction was exploited in phthalocyanine chemistry in case of benzotriazole-substituted phthalocyanines. The sulphonated benzotriazole ring could also be opened with phosphor ylides, giving new phosphoranylidenes. When mesyl and tosyl benzotriazoles were used, these groups were transferred to phenol giving the corresponding esters. Triflyl benzotriazole gave first the azo compound and then formed ester. When sugars were used as nucleophiles, 1H and 2H-benzotriazole substituted sugars were obtained, in the case of tosyl and tryflyl and nonaflyl 1H-benzotriazoles. 1-hydroxybenzotriazole was also used for this purpose. In theses cases substraction of the sulphonate group occurred, giving a sulphonate substituted 1H-1,2,3-benzotriazoles that could not further be used for glycosilation. The mechanisms of all new reactions were investigated. 2. Carbohydrates attached to porphyrin-like compounds have been previously considered for the treatment of cancer with visible light (photodynamic therapy). Since phthalocyanines are effective substances in photodynamic therapy, as well, the attachment of one carbohydrate to the phthalocyanine ring at the anomeric carbon was considered. The tested glycosilation reactions were not successful when 3,4-dicyanophenol was the substrate. But the substitution of the nitro group in 4-Nitrophthalonitrile was successful. Mainly alpha-linked phenyl glucosides were obtained in good yields. More compounds were synthesised with this method and several phthalocyanines were formed. This reaction was also studied with dinitro or nitrocyano phenyls that need particular conditions for the formation of phenyl glycoside.1. Glycosylsulfonate haben, trotz ihres Potentials für die stereoselektive Synthese, in den letzten Jahren wenig Beachtung gefunden. Aus diesem Grunde wurde die Synthese von Glycosyldonoren mit Nonafluorbutansulfonyl-Derivaten untersucht. Dabei wurde gefunden, dass man unter Verwendung des handelsüblichen Nonafluorbutansulfonsäurefluorids entweder Trehalose oder aber das glykosylierte Produkt erhalten konnte, abhängig von der Reihenfolge, in der die Reagenzien zugegeben wurden. Die Verwendung von 1H-Benzotriazol als Überträger der Nonafluorbutansulfonyl- Gruppe auf Glucose wurde untersucht. Während der Untersuchungen zur Abspaltung der Nonafluorbutansulfonyl-Gruppe vom Benzotriazolrest mittels Phenol, wurde der Heterozyklus zwischen den Positionen N1 und N2 geöffnet, woraus sich eine neue Synthesemethode für Azobenzole ergab. Diese Methode wurde in der Phthalocyaninchemie speziell für die Synthese von Benzotriazol-substituierten Phthalocyaninen angewandt. Der Nonafluorbutansulfonyl-substituierte Benzotriazolring konnte ebenfalls mit Phosphoryliden geöffnet werden, wobei neue Phosphoranylide entstanden. Verwendete man Mesyl- oder Tosyl-Gruppen, wurden diese Gruppen auf Phenol übertragen und bildeten die korrespondierenden Ester. Triflylbenzotriazole bildeten zuerst die Azokomponente und anschließend die Ester. Wenn Zuckerderivate als Nukleophile eingesetzt wurden, erhielt man im Fall von Tosyl-, Triflyl- und Nonaflylbenzotriazolen die 1H- und 2H-Benzotriazol-substituierten Zuckerderivate. Zum gleichen Zweck wurde 1-Hydroxybenzotriazol eingesetzt. Dabei fand eine Abspaltung des Sulfonates und das entstandene Sufonat-substituierte 1H-Benzotriazol konnte nicht weiter zur Glykosylierung verwendet werden. Die Mechanismen aller neuen Reaktionen wurden untersucht. 2. An porphyrinähnliche Substanzen gekoppelte Kohlenhydrate wurden schon früher für die Krebstherapie mit sichtbarem Licht (Photodynamiktherapie) in Betracht gezogen. Da Phthalocyanine ebenfalls als effektive Wirkstoffe in der Photodynamiktherapie eingesetzt werden, wurde geplant, ein Kohlenhydrat mit dem anomeren Zentrum an einen Phthalocyaninring anzubinden. Direkte Glykosylierungsreaktionen waren nicht erfolgreich, wenn 3,4-Dicyanophenol als Gykosylakzeptor verwendet wurde. Allerdings verlief die Substitution der Nitrogruppe in 4-Nitrophthalonitril erfolgreich. Als Hauptprodukt wurden alpha-substituierte Glycoside in sehr guter Ausbeute erhalten. Mehrere ähnliche Substanzen wurden mit dieser Methode synthetisiert und einige Phthalocyanine gebildet. Diese Reaktion wurde auch mit 1,2-Dinitrobenzol und 4-Nitrobenzoesäurenitril untersucht, die spezielle Bedingungen für die Bildung von Phenylglykosiden benötigen

Long period variable stars: galactic populations and infrared luminosity calibrations

In this paper HIPPARCOS astrometric and kinematic data are used to calibrate both infrared lumi-nosities and kinematical parameters of Long Period Variable stars (LPVs). Individual absolute K and IRAS 12and 25 luminosities of 800 LPVs are determined and made available in electronic form?. The estimated meankinematics is analyzed in terms of galactic populations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limit of the initial mass is given for starsof each population. A di erence of 1.3 mag in K for the upper limit of the Asymptotic Giant Branch is foundbetween the disk and old disk galactic populations, con rming its dependence on the mass in the main sequence.LPVs with a thin envelope are distinguished using the estimated mean IRAS luminosities. The level of attraction(in the classi cation sense) of each group for the usual classifying parameters of LPVs (variability and spectraltypes) is examine

Retention-pH profiles of acids and bases in hydrophilic interaction liquid chromatography

The high proportion of acetonitrile used in many HILIC mobile phases significantly changes the acid-base properties of pH buffers and analytes foreseen from available data in water. In this paper, the recommended stability pH range for chromatographic columns is examined with various acetonitrile water mixtures, resulting in a significant broadening in the operational pH window with the content of organic solvent. Additionally, the challenge of buffer selection in HILIC is also addressed. Commonly used ammonium acetate shrinks its pH buffering range in acetonitrile-rich mobile phases due to variations in the dissociation constants of the buffer constituents (acetic acid and ammonium). Thus, other organic acids such as formic acid, TFA, and succinimide have been studied as buffers in order to fully cover the pH range of use of the column. Also the retention-pH profiles of several acids and bases have been studied in 80% and 90% acetonitrile using the proposed buffers and their behavior compared to that obtained with buffers prepared from oxalic acid, pyrrolidine, and triethylamine. The latter two show additional interactions in 80% acetonitrile that distort the expected retention-pH profiles of acid analytes, but not the ones of bases. In 90% acetonitrile the profiles are affected by significant additional solute-buffer interactions that might be caused by ion pairing, homo- and heteroassociation in this low ion solvating medium