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1. Road to Room-Temperature Superconductivity: Tc above 260 K in Lanthanum Superhydride under Pressure

   Russell J. Hemley, Muhtar Ahart (January 2019, Proceedings of the International Symposium Superconductivity and Pressure: A Fruitful Relationship on the Road to Room Temperature Superconductivity. May, 21-22 - 2018. Madrid - Spain)

   The use of high pressure to realize superconductivity in the vicinity of room temperature has a long history, much of it focused on achieving this in hydrogen-rich materials. This paper provides a brief overview of the work presented at this May 2018 conference, together with background on motivation and techniques, the theoretical predictions of superconductivity in lanthanum hydride, and the subsequent experimental confirmation. Theoretical calculations using density-functional based structure-search methods combined with BCS-type models predicted a new class of dense, hydrogen-rich materials – superhydrides (MHx, with x > 6 and M selected rare earth elements) – with superconducting critical temperatures (Tc) in the vicinity of room-temperature at and above 200 GPa pressures. The existence of a series of these phases in the La-H system was subsequently confirmed experimentally, and techniques were developed for their syntheses and characterization, including measurements of structural and transport properties, at megabar pressures. Four-probe electrical transport measurements of a cubic phase identified as LaH10 display signatures of superconductivity at temperatures above 260 K near 200 GPa. The results are supported by pseudo-four probe conductivity measurements, critical current determinations, low-temperature x-ray diffraction, and magnetic susceptibility measurements. The measured high Tc is in excellent agreement with the original calculations. The experiments also reveal additional superconducting phases with Tc between 150 K and above 260 K. This effort highlights the novel physics in hydrogen-rich materials at high densities, the success of ‘materials by design’ in the discovery and creation of new materials, and the possibility of new classes of superconductors Tc‘s at and above room temperature. 

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2. Unlocking the Origin of High‐Temperature Superconductivity in Molecular Hydrides at Moderate Pressures

   https://doi.org/10.1002/adfm.202415910  

   Zhao, Wendi; Ellis, Austin; Duan, Defang; Wang, Hongwei; Jiang, Qiwen; Du, Mingyang; Cui, Tian; Miao, Maosheng (November 2024, Advanced Functional Materials)

   Abstract The current pressing challenge in the field of superconducting hydride research is to lower the stable pressure of such materials for practical applications. Molecular hydrides are usually stable under moderate pressure, but the underlying metallization mechanism remains elusive. Here, the important role of chemical interactions in governing the structures and properties of molecular hydrides is demonstrated. A new mechanism is proposed for obtaining high‐temperature and even room‐temperature superconductivity in molecular hydrides and report that the ternary hydride NaKH₁₂hostsT_cvalues up to 245 K at moderate pressure of 60 GPa. Both the excellent stability and superconductivity of NaKH₁₂originate from the fact that the localized electrons in the interstitial region of the metal lattice occupying the crystal orbitals well matched with the hydrogen lattice and forming chemical templates to assist the assembly of H₂units. These localized electrons weaken the H─H covalent bonds and improve the charge connectivity between the H₂units, ensuring the strong coupling between electrons and hydrogen‐dominated optical phonons. The theory provides a key perspective for understanding the superconductivity of molecular hydrides with various structural motifs, opening the door to obtaining high‐temperature superconductors from molecular hydrides at moderate pressures. 

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3. On the Remarkable Superconductivity of FeSe and Its Close Cousins

   https://doi.org/10.3390/sym12091402  

   Kreisel, Andreas; Hirschfeld, Peter J.; Andersen, Brian M. (September 2020, Symmetry)

   null (Ed.)

   Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity. 

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4. The 2021 room-temperature superconductivity roadmap

   https://doi.org/10.1088/1361-648X/ac2864  

   Boeri, Lilia; Hennig, Richard; Hirschfeld, Peter; Profeta, Gianni; Sanna, Antonio; Zurek, Eva; Pickett, Warren E; Amsler, Maximilian; Dias, Ranga; Eremets, Mikhail I; et al (March 2022, Journal of Physics: Condensed Matter)

   - (Ed.)

   Abstract Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms. In memoriam, to Neil Ashcroft, who inspired us all. 

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5. Strain-stabilized superconductivity

   https://doi.org/10.1038/s41467-020-20252-7  

   Ruf, J. P.; Paik, H.; Schreiber, N. J.; Nair, H. P.; Miao, L.; Kawasaki, J. K.; Nelson, J. N.; Faeth, B. D.; Lee, Y.; Goodge, B. H.; et al (December 2021, Nature Communications)

   null (Ed.)

   Abstract Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO 2 thin films on (110)-oriented TiO 2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals. 

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