Explore the vast range of titles available.

Motivated by the recent observation of superconductivity with Tc ~ 80 K in pressurized La3Ni2O71, we explore the structural and electronic properties of A3Ni2O7 bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La3Ni2O7 at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations between Tc and the in-plane Ni-O-Ni bond angles for La3Ni2O7. Moreover, two structural phases with significant c+ octahedral rotations and in-plane bond disproportionations are uncovered for A = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Ni eg manifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb3Ni2O7 as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism.

We explore the optical properties of La3Ni2O7 bilayer nickelates by using density functional theory including a Coulomb repulsion term. Convincing agreement with recent experimental ambient-pressure spectra is achieved for U ~ 3 eV, which permits tracing the microscopic origin of the characteristic features. Simultaneous consistency with angle-resolved photoemission spectroscopy and x-ray diffraction suggests the notion of rather moderate electronic correlations in this novel high-Tc superconductor. Oxygen vacancies form predominantly at the inner apical sites and renormalize the optical spectrum quantitatively, while the released electrons are largely accommodated by a defect state. We show that the structural transition occurring under high pressure coincides with a significant enhancement of the Drude weight and a reduction of the out-of-plane interband contribution that acts as a fingerprint of the emerging hole pocket. We further calculate the optical spectra for various possible magnetic phases including spin-density waves and discuss the results in the context of experiment. Finally, we investigate the role of the 2–2 versus 1–3 layer stacking and compare the bilayer nickelate to La4Ni3O10, La3Ni2O6, and NdNiO2, unveiling general trends in the optical spectrum as a function of the formal Ni valence in Ruddlesden–Popper versus reduced Ruddlesden–Popper nickelates.

For a century, researchers have sought materials that superconduct — transport electricity without loss — at room temperature. Experimental data now confirm superconductivity at higher temperatures than ever before. Experiments on lanthanum hydride under extreme pressure.

The quest to identify new superconducting materials with enhanced properties is hindered by the prohibitive cost of computing electron-phonon spectral functions, severely limiting the materials space that can be explored. Here, we introduce a Bootstrapped Ensemble of Equivariant Graph Neural Networks (BEE-NET), a machine-learning model trained to predict the Eliashberg spectral function and superconducting critical temperature with a mean-absolute-error of 0.87 K relative to DFT-based Allen-Dynes calculations. Intriguingly, BEE-NET achieves a true-negative-rate of 99.4%, enabling highly efficient screening for the rare property of superconductivity. Integrated into a multi-stage, AI-accelerated discovery pipeline that incorporates elemental-substitution strategies and machine-learned interatomic potentials, our workflow reduced over 1.3 million candidate structures to 741 dynamically and thermodynamically stable compounds with DFT-confirmed T c > 5 K. We report the successful synthesis and experimental confirmation of superconductivity in two of these previously unreported compounds. This study establishes a data-driven framework that integrates machine learning, quantum calculations, and experiments to systematically accelerate superconductor discovery.

Nitrogen vacancies make for superlative sensors of material properties at high pressures We spend our entire lives at pressures near 1 atm. But most of the matter in our planet exists at far higher pressures. Experiments conducted under applied pressure are crucial to understanding condensed matter. High-pressure experiments have provided data on the matter in planetary interiors that have improved our understanding of seismic events. Most notably, applied high pressures permit the synthesis and study of new materials with extraordinary properties, such as extreme hardness. Recent experiments on hydride materials compressed to greater than 1 million atm have revealed near-room-temperature superconductivity ( 1 – 3 ), finally pushing past record critical temperatures that had stagnated since the 1990s. On pp. 1349, 1359, and 1355 of this issue, Hsieh et al. ( 4 ), Lesik et al. ( 5 ), and Yip et al. ( 6 ), respectively, report on a comprehensive set of experiments that demonstrate that quantum sensors based on so-called nitrogen vacancy (NV) centers offer powerful new tools for probing matter at extreme pressures.

Standard models for simple metals and insulators often fail for systems based on elements with unstable d - or f -electron shells, where strong electronic correlations can generate new and unexpected states of matter. Such a scenario can often be induced when a magnetic phase transition is tuned to absolute zero temperature by an external control parameter such as chemical composition, pressure or magnetic field. At the resulting quantum critical point (QCP), emergent phenomena, such as unconventional superconductivity and novel magnetic phases are frequently observed. The temperature and energy dependences of the physical properties are also found to deviate from expectations for a simple Fermi liquid. This “non-Fermi-liquid” (NFL) behavior is commonly manifested as weak power laws and logarithmic divergences in the physical properties at low temperatures and is often found in a V-shaped region near a QCP, which has become the “classic” QCP phase diagram. However, there is also a growing number of materials where the NFL behavior either occurs far away from the QCP, within an ordered phase, or may not be associated with any putative QCP. Thus, after nearly 20 years of research, it remains unknown whether NFL physics is universal, or if a multitude of unique subclasses exist. In this article, we review research that has primarily been carried out in our laboratory on systems that exhibit NFL behavior that does not conform to the “classic” QCP scenario.

Studies of the effect of high pressure on superconductivity began in 1925 with the seminal work of Sizoo and Onnes on Sn to 0.03 GPa and have continued up to the present day to pressures in the 200 - 300 GPa range. Such enormous pressures cause profound changes in all condensed matter properties, including superconductivity. In high pressure experiments metallic elements, Tc values have been elevated to temperatures as high as 20 K for Y at 115 GPa and 25 K for Ca at 160 GPa. These pressures are sufficient to turn many insulators into metals and magnetics into superconductors. The changes will be particularly dramatic when the pressure is sufficient to break up one or more atomic shells. Recent results in superconductivity to Mbar pressures wll be discussed which exemplify the progress made in this field over the past 82 years.

In this paper, I present an analysis of the electrical resistance graphs in Nature volume 586, pages 373-377 (2020), which reported the discovery of room temperature superconductivity in a carbonaceous sulfur hydride and was subsequently retracted on September 26th, 2022. I show that, over a single temperature interval, the electrical resistance data can be decomposed into at least two signals of differing digital precision, thus raising questions concerning the methods used to obtain the published data. Since the raw data-files for the electrical resistance measurements have not been made available, in order to perform this analysis, I have developed a set of python scripts to extract the data-points with high precision from the internal structure of the vector graphics image files. I describe the data extraction method. Example code and the resulting electrical resistance vs temperature data-files are made available in public repositories.

CrMnFeCoNi, also called the Cantor alloy, is a well-known high-entropy alloy whose magnetic properties have recently become a focus of attention. We present a detailed muon spin relaxation study of the influence of chemical composition and sample processing protocols on the magnetic phase transitions and spin dynamics of several different Cantor alloy samples. Specific samples studied include a pristine equiatomic sample, samples with deficient and excess Mn content, and equiatomic samples magnetized in a field of 9 T or plastically deformed in pressures up to 0.5 GPa. The results confirm the sensitive dependence of the transition temperature on composition and demonstrate that post-synthesis pressure treatments cause the transition to become significantly less homogeneous throughout the sample volume. In addition, we observe critical spin dynamics in the vicinity of the transition in all samples, reminiscent of canonical spin glasses and magnetic materials with ideal continuous phase transitions. Application of an external magnetic field suppresses the critical dynamics in the Mn-deficient sample, while the equiatomic and Mn-rich samples show more robust critical dynamics. The spin-flip thermal activation energy in the paramagnetic phase increases with Mn content, ranging from \\(3.1(3) 10^-21\\) J for 0% Mn to \\(1.2(2) 10^-20\\) J for 30% Mn content. These results shed light on critical magnetic behavior in environments of extreme chemical disorder and demonstrate the tunability of spin dynamics in the Cantor alloy via chemical composition and sample processing.

Samples of Co were grown directly in the ferromagnetic state under equilibrium conditions using a cobalt sulfide flux. Magnetic fields up to 9 T were applied during growth, and isolated Co products exhibit progressively elongated morphologies, from cubes to rectangular rods to needle-like tendrils with poorly-defined facets. The degree of elongation of the major axis was found to correlate with magnetic field direction, strength, and gradient. Two-dimensional X-ray diffraction data indicate some level of polycrystalline-like samples, and quantitative analyses (Le Bail and Rietveld) of the one-dimensional data confirm the presence of hcp and fcc phases. The magnetic responses indicate a partial alignment of the magnetic easy-axis of the hcp phase along the magnetic field present during growth.