Oxygen Isotope Effect in High Temperature Superconductors

Conventional superconductivity is described by its interactions of electrons through a vibrational mode called phonons. For high temperature superconductors (HTS), the mechanism which explains its superconductivity is still unknown. Various measurement techniques have been established to study the mechanism behind HTS including looking at the effect of altering the mass of oxygen in the copper-oxide planes of cuprate crystal lattices from 16 g/mol to 18 g/mol, also known as the isotope effect. While the results of previous experiments are still inconclusive, I aim to investigate this effect using magnetometry, Raman spectroscopy, and laser angle resolved photoemission spectroscopy (ARPES) in order to find evidence for a particular pairing mechanism

Electronic collective modes in two dilute conductors with momentum-resolved electron energy-loss spectroscopy

Much of modern condensed matter research tries to understand strongly correlated electron systems. These materials exhibit a variety of interesting quantum phenomena that stem from strong electron interactions and the emergent quasiparticles that define the system. Understanding the collective mode dynamics of these particles is key to understanding the macroscopic behavior of these materials. One quantity that contains fundamental information about the about these boson collective modes is the charge susceptibility χ(q,ω) which contains information about the propagation of density fluctuations that are mediated by bosonic excitations. Until recently though, it was not possible to measure χ(q, ω) at low energy (< 100 meV) and with the momentum resolution and accuracy needed to see interesting phenomena. With the development of momentum-resolved electron energy-loss spectroscopy (M-EELS) in the Abbamonte Group at UIUC, χ(q, ω) can now be probed at low energy scales of interest. Here we use M-EELS to study the collective modes of several strongly correlated materials with interesting low energy physics. Bose condensed phases of excitons have the potential to realize macroscopic quantum phenomena at unprecedented high temperatures [1,2]. When excitons Bose condense in a real material, however, some rearrangement of the charge density inevitably results, leading to a distortion of the crystal lattice [3–6]. This raises the question of whether there can ever be a distinction between a Bose condensate of excitons and a conventional, structural phase transition that breaks the same symmetry. Here, we use inelastic electron scattering (M-EELS) to study copper-intercalated TiSe2, in which exciton condensation can be directly observed as a soft electronic mode at the exciton condensation temperature, T_XC [7]. While the lattice distortion in CuxTiSe2 persists to x > 0.10 [8], we find the exciton condensate is fully suppressed by x = 0.014, which is short of the semimetal-metal transition we identify at x = 0.025. Our observations indicate that the excitonic and lattice instabilities split as x is increased, showing that structural and excitonic subsystems can exhibit separate transitions and may be distinct subsystems that break different symmetries. In addition we found that although exciton condensation is suppressed, exciton fluctuations remain for most of the doping phase diagram. These fluctuations have been theorized to aid in superconducting pairing in this system [9,10]. SrTi(1−x)NbxO3, is an electron doped ionic semiconductor that exhibits aborted ferroelectricity due to quantum fluctuations [11], dilute (unconventional) superconductivity that survives even when the plasmon energy (ω_p) is lower than the Fermi energy (E_F ) [12], and charge transport properties that suggests bad metal behavior [13]. In general, the polar nature of ionic semiconductors leads to coupling of the longitudinal optical (LO) phonons to collective charge modes, such as plasmons, due to long range polarization fields. Understanding the dynamics of these collective modes, which have been implicated in superconducting pairing [14] [15] [16], can provide insight into the nature of these unusual properties. Here, we use inelastic electron scattering (M-EELS) with a fracturing surface preparation technique to study the doping, temperature, and momentum dependence of the charge collective modes in SrTi(1−x)NbxO3. We measure propagating and diffusive acoustic phonons, Fuchs-Kliewer optical phonons and overtones that get suppressed with doping due to metallic screening, a 93 meV phonon mode that becomes highly damped and asymmetric at x=0.002, and a very broad plasmon that blue-shifts with decreasing temperature. We find that the plasmon in SrTi(1−x)NbxO3 remains dispersion-less at all temperatures and dopings even with other propagating collective modes (acoustic phonon), contrary to RPA predictions and dispersions in other polar doped semiconductors [17]. The width of the plasmon deviates from RPA predictions by an order of magnitude. In addition, the energy of the plasmon measured in M-EELS is less than that of other techniques like infrared spectroscopy for samples of comparable carrier densities

JCuda vectorized and parallelized computation strategy for solving integral equations in electromagnetism on a standard personal computer

International audienceThe paper presents a computation strategy for solving integral equations in electromagnetism. Nowadays, powerful programmable Graphic Processing Units (GPU) can be found in any standard computer. The paper investigates the benefits of the use of GPUs in addition to the CPU one in order to improve computation speed by using integral methods. Java language and the JCuda library, not often used in speed calculation by the computing community, has been used here. A 100 time speed-up is reported in matrix assembly between an optimized traditional CPU computation and a CPU+GPU one. FMM… Index Terms-Fast Multipole Method on GPU, JCuda computing, Pure Java ..

Calcul hautes performances pour les formulations intégrales en électromagnétisme basses fréquences. Intégration, compression matricielle par ondelettes et résolution sur architecture GPGPU

Les méthodes intégrales sont des méthodes particulièrement bien adaptées à la modélisation des systèmes électromagnétiques car contrairement aux méthodes par éléments finis elles ne nécessitent pas le maillage des matériaux inactifs tel que l'air. Ces modèles sont donc légers en terme du nombre de degrés de liberté. Cependant ceux sont des méthodes à interactions totales qui génèrent des matrices de systèmes d'équations pleines. Ces matrices sont longues à calculer en temps processeur et coûteuses à stocker dans la mémoire vive de l'ordinateur. Nous réduisons dans ces travaux les temps de calcul grâce au parallélisme, c'est-à-dire l'utilisation de plusieurs processeurs, notamment sur cartes graphiques (GPGPU). Nous réduisons également le coût du stockage mémoire via de la compression matricielle par ondelettes (il s'agit d'un algorithme proche de la compression d'images). C'est une compression par pertes, nous avons ainsi développé un critère pour contrôler l'erreur introduite par la compression. Les méthodes développées sont appliquées sur une formulation électrostatique de calcul de capacités, mais elles sont à priori également applicables à d'autres formulations.Integral equation methods are widely used in electromagnetism modeling because, in opposition to finite element methods, they do not require the meshing of non-active materials like air. Therefore they lead to formulations with small degrees of freedom. However, they also lead to fully dense systems of equations. Computation times are expensive and the storage of the matrix is very expensive. This work presents different parallel computation strategies in order to speed up the computation time, in particular the use of graphical processing units (GPGPU) is focused. The next point is to reduce the memory requirements thanks to wavelets compression (it is an algorithm similar to image compression). The compression technique introduces errors, therefore a control criterion is proposed. The methodology is applied to an electrostatic formulation but it is general and it could also be used with others integral formulations.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Safety aspects and procedural characteristics of ambulatory diagnostic cerebral catheter angiography

Purpose: Diagnostic cerebral catheter angiography is used to assess a variety of neurovascular pathologies especially in patients before and after endovascular neurointerventional treatment. In many centers diagnostic cerebral angiographies are performed with the patient staying for one night in the hospital because there are not yet sufficient data on the safety of ambulatory cerebral angiography. At the same time hospitals face a growing demand to perform ambulatory medical procedures. Methods: A total of 426 ambulatory diagnostic cerebral angiographies were retrospectively analyzed. Technical details of the angiographies were analyzed to identify procedural risk factors. Results: Out of 426 patients 14 (3.3%) had some form of complication, 3 developed minor transient neurological symptoms, 1 patient developed Quincke's edema probably as an adverse reaction to contrast agent, 1 patient had an asymptomatic carotid dissection and 1 had a fall of unknown etiology. Of the 14 complications 8 were puncture site complications with 1 re-bleeding, 1 dissection, and 6 minor complications, 421 punctures were femoral, 3 radial and 2 brachial. Out of 333 patients with magnetic resonance imaging (MRI) after angiography 21 showed focal diffusion-weighted imaging (DWI) lesions but none of these lesions were symptomatic. The rate of DWI lesions was significantly higher in selectively angiography territories than in other territories. The use of a Simmons 2 catheter significantly increased the rate of DWI lesions (p = 0.047), whereas 3D rotational angiography did not (p = 0.55). The rate of DWI lesions per selectively accessed vessel was 4.6% with a higher rate in the anterior than in the posterior circulation. Conclusion: Diagnostic cerebral catheter angiography can be safely performed in an ambulatory setting