Microscopic annealing process and its impact on superconductivity in T'-structure electron-doped copper oxides

High-transition-temperature superconductivity arises in copper oxides when holes or electrons are doped into the CuO2 planes of their insulating parent compounds. While hole-doping quickly induces metallic behavior and superconductivity in many cuprates, electron-doping alone is insufficient in materials such as R2CuO4 (R is Nd, Pr, La, Ce, etc.), where it is necessary to anneal an as-grown sample in a low-oxygen environment to remove a tiny amount of oxygen in order to induce superconductivity. Here we show that the microscopic process of oxygen reduction repairs Cu deficiencies in the as-grown materials and creates oxygen vacancies in the stoichiometric CuO2 planes, effectively reducing disorder and providing itinerant carriers for superconductivity. The resolution of this long-standing materials issue suggests that the fundamental mechanism for superconductivity is the same for electron- and hole-doped copper oxides.Comment: 23 pages, 3 figures, accepted for publication in Nature Material

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Chemical tuning of the colossal magnetoresistance of ferromagnetic perovskites

The colossal magnetoresistance (CMR) of a series of ferromagnetic $\rm L_{2/3}Ca_{1/3}MnO_3$ perovskites is explored for a wide range of lanthanides (L) having different ionic radii. It is shown that the CMR can be tuned through the appropriate size of the lanthanide. The negative magnetoresistance has been modified in this way by two orders of magnitude, reaching $\Delta R/R$ values higher than $3\cdot 10^5$%. We will show that the temperature (TM) where magnetoresistance is maximum correlates with its intensity ($\Delta R/R$) and we will argue that both effects are simply controlled by the bending of the Mn—O—Mn bond. Above TM, the enhancement of resistivity when decreasing the temperature can be described by a Variable Hopping model in which the relevant localization length l is found to be reduced when increasing the lattice distortion; inversely l enhances when applying a magnetic field, thus explaining the observation of a negative magnetoresistance. The giant modification of the electrical resistance goes in parallel with an unusually high magnetic polarizability. We argue that the coupling of itinerant charge carriers (holes) and their reduced mobility when they polarize the localized moments, thus forming magnetic polarons, lie at the heart of the CMR

The electron-doped cuprates: superconducting properties and pressure effects

We report some of the results that we have obtained from the study of the electromagnetic properties of the electron-doped L2-xCexCuO4 high temperature superconductors. We will show evidence of the strong anisotropic character of the superconducting properties of these materials, displaying 2D behavior. The competing effects of lattice contraction and charge transfer on the dependence of the critical temperature under pressure are analyzed