Observation de phonons chiraux par l'effet Hall thermique dans la phase pseudogap des cuprates (Nd, Eu)-La_{2-x}Sr_xCuO4

Les cuprates sont étudiés depuis plus de trente ans, mais certaines de leurs propriétés sont encore mystérieuses. Les cuprates ont également un diagramme de phases riche qui est cible de plusieurs sondes expérimentales. Une récente technique de mesure, l'effet Hall thermique, a permis d'observer une nouvelle propriété de la phase pseudogap. En effet, le signal d'effet Hall thermique observé demeure négatif dans la phase pseudogap, en opposition à l'effet Hall électrique qui reste positif. Ce signal est également présent dans le composé parent non dopé La2CuO4 qui est un isolant de Mott, suggérant que les porteurs de chaleurs ne sont pas les électrons. Le premier chapitre de ce mémoire fait un retour sur les cuprates et l'article qui a motivé les expériences qui ont suivi. Le deuxième chapitre porte sur l'aspect théorique des mesures, particulièrement l'effet Hall thermique kappa_{xy} et kappa_{zy}, ainsi que les défis expérimentaux reliés à cette mesure. Finalement, le troisième chapitre porte sur les résultats expérimentaux importants et une discussion sur leur interprétation. La particularité des mesures d'effet Hall thermique de ce mémoire est qu'elles ont été effectuées perpendiculairement aux plans de cuivre-oxygène des cuprates, direction dans laquelle plusieurs excitations sont peu mobiles. Nos résultats démontrent que les phonons sont les principaux porteurs de chaleur et qu'ils deviennent chiraux dans la phase pseudogap. Nous pensons également que les domaines structuraux, les impuretés magnétiques et les impuretés d'oxygène ne sont pas responsables de cette chiralité. Les mesures et conclusions importantes ont été publiées en juillet 2020 dans le journal scientifique Nature Physics et apparaissent dans l'édition de novembre 2020

Effect of pressure on the pseudogap and charge density wave phases of the cuprate Nd-LSCO probed by thermopower measurements

We report thermopower measurements under hydrostatic pressure on the cuprate superconductor La1.6−xNd0.4SrxCuO4 (Nd-LSCO), at low temperature in the normal state accessed by suppressing superconduc- tivity with a magnetic field up to H = 31 T. Using an ac thermopower measurement technique suitable for high pressure and high field, we track the pressure evolution of the Seebeck coefficient S. At ambient pressure and low temperature, S/T in Nd-LSCO was recently found to suddenly increase below the pseudogap critical doping p ⋆ = 0.23, consistent with a drop in carrier density n from n = 1 + p above p⋆ to n = p below. Under a pressure of 2.0 GPa, we observe that the large S/T value just below p ⋆ is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect, which found that pressure lowers p ⋆, thereby reinforcing the interpretation that this effect is driven by the pressure-induced shift of the van Hove point. It implies that the pseudogap only exists when the Fermi surface is hole-like, which puts strong constraints on theories of the pseudogap phase. We also report thermopower measurements on Nd-LSCO and La1.8−xEu0.2SrxCuO4 in the charge density wave phase near p ∼ 1/8, which reveals a weakening of this phase under pressure.L.T. acknowledges support from the Canadian Institute for Ad- vanced Research (CIFAR) as a CIFAR Fellow and funding from the Institut Quantique, the Natural Sciences and Engi- neering Research Council of Canada (PIN:123817), the Fonds de Recherche du Québec–Nature et Technologies (FRQNT), the Canada Foundation for Innovation (CFI), and a Canada Research Chair. This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund and the Gordon and Betty Moore Foundation’s EPiQS Ini- tiative (Grant No. GBMF5306 to L.T.). The National High Magnetic Field Laboratory is supported by the National Sci- ence Foundation through Grant No. NSF/DMR-1644779 and the State of Florida. J.-S.Z. was supported by NSF MRSEC under Cooperative Agreement No. DMR-1720595.Center for Dynamics and Control of Material

Phonons become chiral in the pseudogap phase of cuprates

The nature of the pseudogap phase of cuprates remains a major puzzle. One of its new signatures is a large negative thermal Hall conductivity κxy, which appears for dopings p below the pseudogap critical doping p∗, but whose origin is as yet unknown. Because this large κxy is observed even in the undoped Mott insulator La2CuO4, it cannot come from charge carriers, these being localized at p=0. Here we show that the thermal Hall conductivity of La2CuO4 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO2 planes, i.e. κzy(T)≈κxy(T). This shows that the Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes . At p>p∗, in both La1.6−xNd0.4SrxCuO4 and La1.8−xEu0.2SrxCuO4 with p=0.24, we observe no c-axis Hall signal, i.e. κzy(T)=0, showing that phonons have zero Hall response outside the pseudogap phase. The phonon Hall response appears immediately below p∗=0.23, as confirmed by the large κzy(T) signal we find in La1.6−xNd0.4SrxCuO4 with p=0.21. The microscopic mechanism by which phonons become chiral in cuprates remains to be identified. This mechanism must be intrinsic - from a coupling of phonons to their electronic environment - rather than extrinsic, from structural defects or impurities, as these are the same on both sides of p∗. This intrinsic phonon Hall effect provides a new window on quantum materials and it may explain the thermal Hall signal observed in other topologically nontrivial insulators.Center for Dynamics and Control of Material

Genetic Interactions with Age, Sex, Body Mass Index, and Hypertension in Relation to Atrial Fibrillation: The AFGen Consortium

It is unclear whether genetic markers interact with risk factors to influence atrial fibrillation (AF) risk. We performed genome-wide interaction analyses between genetic variants and age, sex, hypertension, and body mass index in the AFGen Consortium. Study-specific results were combined using meta-analysis (88,383 individuals of European descent, including 7,292 with AF). Variants with nominal interaction associations in the discovery analysis were tested for association in four independent studies (131,441 individuals, including 5,722 with AF). In the discovery analysis, the AF risk associated with the minor rs6817105 allele (at the PITX2 locus) was greater among subjects ≤ 65 years of age than among those > 65 years (interaction p-value = 4.0 × 10-5). The interaction p-value exceeded genome-wide significance in combined discovery and replication analyses (interaction p-value = 1.7 × 10-8). We observed one genome-wide significant interaction with body mass index and several suggestive interactions with age, sex, and body mass index in the discovery analysis. However, none was replicated in the independent sample. Our findings suggest that the pathogenesis of AF may differ according to age in individuals of European descent, but we did not observe evidence of statistically significant genetic interactions with sex, body mass index, or hypertension on AF risk

Large-scale analyses of common and rare variants identify 12 new loci associated with atrial fibrillation

Atrial fibrillation affects more than 33 million people worldwide and increases the risk of stroke, heart failure, and death. Fourteen genetic loci have been associated with atrial fibrillation in European and Asian ancestry groups. To further define the genetic basis of atrial fibrillation, we performed large-scale, trans-ancestry meta-analyses of common and rare variant association studies. The genome-wide association studies (GWAS) included 17,931 individuals with atrial fibrillation and 115,142 referents; the exome-wide association studies (ExWAS) and rare variant association studies (RVAS) involved 22,346 cases and 132,086 referents. We identified 12 new genetic loci that exceeded genome-wide significance, implicating genes involved in cardiac electrical and structural remodeling. Our results nearly double the number of known genetic loci for atrial fibrillation, provide insights into the molecular basis of atrial fibrillation, and may facilitate the identification of new potential targets for drug discovery

Multi-ethnic genome-wide association study for atrial fibrillation

Atrial fibrillation (AF) affects more than 33 million individuals worldwide and has a complex heritability. We conducted the largest meta-analysis of genome-wide association studies (GWAS) for AF to date, consisting of more than half a million individuals, including 65,446 with AF. In total, we identified 97 loci significantly associated with AF, including 67 that were novel in a combined-ancestry analysis, and 3 that were novel in a European-specific analysis. We sought to identify AF-associated genes at the GWAS loci by performing RNA-sequencing and expression quantitative trait locus analyses in 101 left atrial samples, the most relevant tissue for AF. We also performed transcriptome-wide analyses that identified 57 AF-associated genes, 42 of which overlap with GWAS loci. The identified loci implicate genes enriched within cardiac developmental, electrophysiological, contractile and structural pathways. These results extend our understanding of the biological pathways underlying AF and may facilitate the development of therapeutics for AF

Genetic analyses of the electrocardiographic QT interval and its components identify additional loci and pathways

The QT interval is an electrocardiographic measure representing the sum of ventricular depolarization and repolarization, estimated by QRS duration and JT interval, respectively. QT interval abnormalities are associated with potentially fatal ventricular arrhythmia. Using genome-wide multi-ancestry analyses (>250,000 individuals) we identify 177, 156 and 121 independent loci for QT, JT and QRS, respectively, including a male-specific X-chromosome locus. Using gene-based rare-variant methods, we identify associations with Mendelian disease genes. Enrichments are observed in established pathways for QT and JT, and previously unreported genes indicated in insulin-receptor signalling and cardiac energy metabolism. In contrast for QRS, connective tissue components and processes for cell growth and extracellular matrix interactions are significantly enriched. We demonstrate polygenic risk score associations with atrial fibrillation, conduction disease and sudden cardiac death. Prioritization of druggable genes highlight potential therapeutic targets for arrhythmia. Together, these results substantially advance our understanding of the genetic architecture of ventricular depolarization and repolarization

Atrial fibrillation genetic risk differentiates cardioembolic stroke from other stroke subtypes

AbstractObjectiveWe sought to assess whether genetic risk factors for atrial fibrillation can explain cardioembolic stroke risk.MethodsWe evaluated genetic correlations between a prior genetic study of AF and AF in the presence of cardioembolic stroke using genome-wide genotypes from the Stroke Genetics Network (N = 3,190 AF cases, 3,000 cardioembolic stroke cases, and 28,026 referents). We tested whether a previously-validated AF polygenic risk score (PRS) associated with cardioembolic and other stroke subtypes after accounting for AF clinical risk factors.ResultsWe observed strong correlation between previously reported genetic risk for AF, AF in the presence of stroke, and cardioembolic stroke (Pearson’s r=0.77 and 0.76, respectively, across SNPs with p &lt; 4.4 × 10−4 in the prior AF meta-analysis). An AF PRS, adjusted for clinical AF risk factors, was associated with cardioembolic stroke (odds ratio (OR) per standard deviation (sd) = 1.40, p = 1.45×10−48), explaining ∼20% of the heritable component of cardioembolic stroke risk. The AF PRS was also associated with stroke of undetermined cause (OR per sd = 1.07, p = 0.004), but no other primary stroke subtypes (all p &gt; 0.1).ConclusionsGenetic risk for AF is associated with cardioembolic stroke, independent of clinical risk factors. Studies are warranted to determine whether AF genetic risk can serve as a biomarker for strokes caused by AF.</jats:sec

Etude comparative sur l’inclusion des groupes vulnérables dans les chaînes de valeur agricoles en Afrique de l’Ouest

Le PRESAO est un programme de recherche appliquée, de diffusion et discussion des résultats de recherche, et de renforcement des capacités dans le domaine des politiques de sécurité alimentaire en Afrique de l'Ouest. Il est mis en oeuvre conjointement par l'Université de l'État du Michigan (MSU), l'Assemblée Permanente des Chambres d'Agriculture du Mali (APCAM) et un ensemble de partenaires ouest-africains y compris des institutions universitaires, des organisations de recherche agricole, des unités gouvernementales d’analyse des politiques, des bureaux d’études ouest-africaines, et des systèmes d'information du marché. Le programme comprend divers composantes appuyées par différents partenaires au développement et mettant l'accent sur différents aspects de la politique de sécurité alimentaire et le renforcement des capacités. Le programme est mis en oeuvre de manière à exploiter les synergies entre ces différentes composantes