Reading list

Can hole doping give LiBC the boron-carbon sigma-band coupling that makes MgB2 superconduct, and push it past 39 K at ambient pressure?

1. 001
   Prediction of High Tc Superconductivity in Hole-doped LiBC

   arXiv:cond-mat/0111592 · 2001 · H. Rosner, A. Kitaigorodsky, W. E. Pickett

   The layered lithium borocarbide LiBC, isovalent with and structurally similar to the superconductor MgB2, is an insulator due to the modulation within the hexagonal layers (BC vs. B2). We show that hole-doping of LiBC results in Fermi surfaces of B-C p sigma character that couple very strongly to B-C bond stretching modes, precisely the features that lead to superconductivity at Tc = 40 K in MgB2. Comparison of Li{0.5}BC with MgB2 indicates the former to be a prime candidate for electron-phonon coupled superconductivity at substantially higher temperature than in MgB2.

2. 002
   Electronic and optical properties of LiBC

   arXiv:cond-mat/0207299 · 2002 · A. V. Pronin, K. Pucher, P. Lunkenheimer et al.

   LiBC, a semiconducting ternary borocarbide constituted of the lightest elements only, has been synthesized and characterized by x-ray powder diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing an infrared microscope the phonon spectrum has been investigated in single crystals. The in-plane B-C stretching mode has been detected at 150 meV, noticeably higher than in AlB2, a non-superconducting isostructural analog of MgB2. It is this stretching mode, which reveals a strong electron-phonon coupling in MgB2, driving it into a superconducting state below 40 K, and is believed to mediate predicted high-temperature superconductivity in hole-doped LiBC [H. Rosner, A. Kitai

3. 003
   Synthesis and search for superconductivity in LiBC

   arXiv:cond-mat/0207448 · 2002 · A. Bharathi, S. Jemima Balaselvi, M. Premila et al.

   Following the recent theoretical prediction of superconductivity in hole doped LiBC by Rosner et al [1], we have attempted to synthesise Li deficient LixBC (x = 1, 0.8, 0.6 and 0.4) and look for superconductivity in this system. Our synthesis procedure, following the recipe for MgB2, involves reaction of elemental components in a Ta crucible at 900o C under 50 bar of argon pressure. X-ray diffraction measurements indicate the formation of P63/mmc structure up to x=0.6. However, no diamagnetic signal or zero resistance, corresponding to the superconducting transition, was observed in the temperature range of 300 to 4 K. This is possibly related to the presence of disorder in the B-C stacking;

4. 004
   Vibrational Modes in LiBC: Theory Compared with Experiment

   arXiv:cond-mat/0209256 · 2002 · J. M. An, H. Rosner, S. Y. Savrasov et al.

   The search for other superconductors in the MgB2 class currently is focussed on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with the potential to be a better superconductor than MgB2. Here we present the calculated phonon spectrum of the parent semiconductor LiBC. The calculated Raman-active modes are in excellent agreement with a recent observation, and comparison of calculated IR-active modes with a recent report provides a prediction of the LO–TO splitting for these four modes, which is small for the B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes at 540 cm^{-1} and 620 cm^{-1}.

Which Mg2XH6-type hydrides keep hydrogen-derived coupling strong enough for Tc above 77 K when made at ambient pressure?

1. 005
   Searching Materials Space for Hydride Superconductors at Ambient Pressure

   arXiv:2403.13496 · 2024 · Tiago F. T. Cerqueira, Yue-Wen Fang, Ion Errea et al.

   We employed a machine-learning assisted approach to search for superconducting hydrides under ambient pressure within an extensive dataset comprising over 150 000 compounds. Our investigation yielded around 50 systems with transition temperatures surpassing 20 K, and some even reaching above 70 K. These compounds have very different crystal structures, with different dimensionality, chemical composition, stoichiometry, and arrangement of the hydrogens. Interestingly, most of these systems displayed slight thermodynamic instability, implying that their synthesis would require conditions beyond ambient equilibrium. Moreover, we found a consistent chemical composition in the majority of these s

2. 006
   Feasible route to high-temperature ambient-pressure hydride superconductivity

   arXiv:2310.07562 · 2023 · Kapildeb Dolui, Lewis J. Conway, Christoph Heil et al.

   A key challenge in materials discovery is to find high-temperature superconductors. Hydrogen and hydride materials have long been considered promising materials displaying conventional phonon-mediated superconductivity. However, the high pressures required to stabilize these materials have restricted their application. Here, we present results from high-throughput computation, considering a wide range of high-symmetry ternary hydrides from across the periodic table at ambient pressure. This large composition space is then reduced by considering thermodynamic, dynamic, and magnetic stability, before direct estimations of the superconducting critical temperature. This approach has revealed a m

3. 007
   Superconductivity in atom-intercalated quaternary hydrides under ambient pressure

   arXiv:2508.10912 · 2025 · Bo-Wen Yao, Zhenfeng Ouyang, Xiao-Qi Han et al.

   Hydrogen-rich materials are the most promising candidates for high-temperature conventional superconductors under ambient pressure. Multinary hydrides have abundant structural configurations and are more promising to find high-temperature superconductors at ambient pressure, but searching for multinary materials in complex phase space is a great challenge. In this work, we used our developed AI search engine (InvDesFlow) to perform extensive investigations regarding ambient stable superconducting hydrides. Several quaternary hydrides with high superconducting temperature~(Tc) are predicted. In particular, the superconducting Tc of K2GaCuH6 and K2LiCuH6 are calculated to be

4. 008
   Search for thermodynamically stable ambient-pressure superconducting hydrides in GNoME database

   arXiv:2508.19781 · 2025 · Antonio Sanna, Tiago F. T. Cerqueira, Ekin Dogus Cubuk et al.

   Hydrides are considered to be one of the most promising families of compounds for achieving high temperature superconductivity. However, there are very few experimental reports of ambient-pressure hydride superconductivity, and the superconducting critical temperatures (Tc) are typically less than 10 K. At the same time several hydrides have been predicted to exhibit superconductivity around 100 K at ambient pressure but in thermodynamically unfavorable phases. In this work we aim at assessing the superconducting properties of thermodynamically stable hydride superconductors at room pressure by investigating the GNoME material database, which has been recently released and includes

How far can boron doping raise Tc in diamond before the lattice gives way, and does the coupling scale as theory predicts?

1. 009
   Superconducting boron doped nanocrystalline diamond on boron nitride ceramics

   arXiv:1905.02979 · 2019 · Soumen Mandal, Henry A. Bland, Jerome A. Cuenca et al.

   In this work we have demonstrated the growth of nanocrystalline diamond on boron nitride ceramic. We measured the zeta potential of the ceramics to select the diamond seeds. Diamond was then grown on the seeded ceramics using a microwave chemical vapour deposition system. A clear difference was found between the samples which were seeded with nanodiamond and the ones not seeded before growth. Raman spectroscopy confirmed the excellent quality of the diamond film. Dielectric measurements showed an increase in the dielectric constant of the material after diamond growth. The diamond was also doped with boron to make it superconducting. The film had a transition temperature close to 3.4K. Simil

2. 010
   Optical properties of boron-doped diamond

   arXiv:cond-mat/0508540 · 2005 · Dan Wu, Y. C. Ma, Z. L. Wang et al.

   We report optical reflectivity study on pure and boron-doped diamond films grown by a hot-filament chemical vapor deposition method. The study reveals the formation of an impurity band close to the top of the valence band upon boron-doping. A schematic picture for the evolution of the electronic structure with boron doping was drawn based on the experimental observation. The study also reveals that the boron doping induces local lattice distortion, which brings an infrared-forbidden phonon mode at 1330 cm-1 activated in doped sample. The antiresonance characteristic of the mode in conductivity spectrum evidences the very strong coupling between electrons and this phonon mode.

3. 011
   Electron-Phonon Coupling in Boron-Doped Diamond Superconductor

   arXiv:cond-mat/0406446 · 2004 · H. J. Xiang, Zhenyu Li, Jinlong Yang et al.

   The electronic structure, lattice dynamics, and electron-phonon coupling of the boron-doped diamond are investigated using the density functional supercell method. Our results indicate the boron-doped diamond is a phonon mediated superconductor, con rming previous theoretical conclusions deduced from the calculations employing the virtual crystal approximation. We show that the optical phonon modes involving B vibrations play an important role in the electron-phonon coupling. Di erent from previous theoretical results, our calculated electron-phonon coupling constant is 0.39 and the estimated superconducting transition temperature Tc is 4.4 K for the boron doped diamond with 2.78% boron cont

4. 012
   Advantage on Superconductivity of Heavily Boron-Doped (111) Diamond Films

   arXiv:cond-mat/0503303 · 2005 · Hitoshi Umezawa, Tomohiro Takenouchi, Yoshihiko Takano et al.

   The superconductivity transition temperatures Tc(onset) of 11.4 K and Tc(offset) of 7.4 K, which are the highest in diamond at present, are realized on homoepitaxially grown (111) diamond films with a high boron doping concentration of 8.4E21 cm-3 (4.7 atomic percent). Tc values of (111) diamond films are more than twice as high as those of (100) films at the equivalent boron concentration. The Tc of boron-doped (111) diamond increases as the boron content increases up to the maximum incorporated concentration and is agrees with the value estimated using McMillan's equation. The advantageous Tc for (111) diamond films is due to the higher carrier concentration which exceeds its boron concent

Can strain or a different intercalant lift graphite intercalation compounds above the 11.5 K of CaC6?

1. 013
   Na-catalyzed rapid synthesis and characterization of intercalated graphite CaC6

   arXiv:2309.01942 · 2023 · Akira Iyo, Hiroshi Fujihisa, Yoshito Gotoh et al.

   In this study, we conducted experiments on CaC6 for elucidating the Na-catalyzed formation mechanism and achieving rapid mass synthesis of graphite intercalation compounds (GICs). Rapidly synthesized CaC6 was characterized by analysis of its crystal structure and physical properties. We found that the formation of the reaction intermediate Na-GIC (NaCx, x = 64) requires a larger amount of Na than is intercalated between the graphite interlayers. The requirement for excess Na may provide insights into the mechanism of Na-catalyzed GIC formation. A Na-to-C molar mixing ratio of 1.5-2.0:6 was suitable for the efficient formation of CaC6 under heat treatment at 250°C for 2 h, and the catalytic N

2. 014
   Far-infrared signature of the superconducting gap in intercalated graphite CaC6

   arXiv:0804.2606 · 2008 · U. Nagel, D. Huvonen, E. Joon et al.

   Terahertz reflectance spectra of the Ca-intercalated graphite CaC6 reveal a superconducting gap below 11K. The gap signature lacks a sharp onset to full reflectivity at 2Delta, but rather shows a distribution of gap values consistent with an anisotropic gap. The experimental data were successfully fitted to the gap distribution obtained from density functional calculations by Sanna et al. (Phys.Rev. B75, 020511, 2007). The temperature dependence of the superconducting gap is characteristic for a BCS type superconductor.

3. 015
   Superconductivity in Li3Ca2C6 intercalated graphite

   arXiv:cond-mat/0512114 · 2005 · Nicolas Emery, Claire Herold, Jean-Francois Mareche et al.

   In this letter, we report the discovery of superconductivity in Li3Ca2C6. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li3Ca2C6 is a ternary GIC, for which the intercalated sheets are very thick and poly-layered (five lithium layers and two calcium ones). It contains a great amount of metal (five meta

4. 016
   Muon spin rotation study of the intercalated graphite superconductor CaC6 at low temperatures

   arXiv:1101.4386 · 2011 · F. Murányi, M. Bendele, R. Khasanov et al.

   Muon spin rotation (muSR) experiments were performed on the intercalated graphite CaC6 in the normal and superconducting state down to 20 mK. In addition, AC magnetization measurements were carried out resulting in an anisotropic upper critical field Hc2, from which the coherence lengths xi_ab(0)=36.3(1.5) nm and xi_c(0)=4.3(7) nm were estimated. The anisotropy parameter gamma_H= H_c2_ab/H_c2_c increases monotonically with decreasing temperature. A single isotropic s-wave description of superconductivity cannot account for this behaviour. From magnetic field dependent muSR experiments the absolute value of the in-plane magnetic penetretion depth lambda_ab=78(3) nm was determined. The tempera

Do boron-carbon clathrate frameworks such as SrB3C3 keep useful superconductivity once recovered to ambient pressure?

1. 017
   High-Tc Superconductivity in doped boron-carbon clathrates

   arXiv:2110.05333 · 2021 · Simone Di Cataldo, Shadi Qulaghasi, Giovanni B. Bachelet et al.

   We report a high-throughput ab-initio study of the thermodynamic and superconducting proper- ties of the recently synthesized XB3C3 clathrates. These compounds, in which boron and carbon form a sponge-like network of interconnected cages each enclosing a central X atom, are attractive candidates to achieve high-Tc conventional superconductivity at ambient pressure, due to the simultaneous presence of a stiff B-C covalent network and a tunable charge reservoir, provided by the guest atom. Ternary compounds like CaB3C3, SrB3C3 and BaB3C3 are predicted to exhibit Tc 50 K at moderate or ambient pressures, which may further increase up to 77 K if the original comp

2. 018
   Guest metal-driven quantum anharmonic effects on stability and two-gap superconductivity in carbon-boron clathrates

   arXiv:2512.05497 · 2025 · Xianghui Meng, Yanqing Shen, Xin Yang et al.

   Traditionally, strong quantum anharmonic effects have been considered a characteristic of hydrogen-rich compounds. Here we propose that these effects also play a decisive role in boron-carbon clathrates. The stability and superconducting transition temperature (Tc) of carbon-boron clathrates XYB6C6, whose metal atoms have an average oxidation state of +1.5, have long remained under debate. At this oxidation state, some combinations (e.g., RbSrB6C6) are dynamically stable, whereas others (e.g., RbPbB6C6) are not. Using the stochastic self-consistent harmonic approximation combined with machine learning, we find that the anharmonicity originates primarily from guest metal atoms. For comparison

3. 019
   Superconductivity near 70 K in boron-carbon clathrates MB2C8 (M = Na, K, Rb, Cs) at ambient pressure

   arXiv:2405.13752 · 2024 · Bin Li, Yulan Cheng, Cong Zhu et al.

   Inspired by the first boron-carbon (B-C) clathrate SrB3C3 and the ternary borohydride KB2H8 [Miao et al., Phys. Rev. B 104 L100504 (2021)], we have performed first-principles density functional theory calculations of the electronic and phonon band structures for B-C compounds MB2C8 (M = Na, K, Rb, Cs). Our calculations reveal that these materials are dynamically stable and can potentially exhibit superconductivity at ambient pressure. However, only the K, Rb, and Cs compounds exhibit thermodynamic stability below 50 GPa, while NaB2C8 remains thermodynamically unstable at all pressures considered. Based on the Allen and Dynes modified McMillan equation, we predict the

4. 020
   Hydride Units Filled B–C Clathrate: A New Pathway for High-Temperature Superconductivity at Ambient Pressure

   arXiv:2311.01656 · 2023 · Ying Sun, Li Zhu

   The pursuit of room-temperature superconductors has recently advanced with the discovery of high-temperature superconductivity in compressed hydrides, although sustaining the superconductivity of hydrides at ambient pressure remains challenging. In parallel, sp3-bonded frameworks comprising lightweight elements (\textit{e.g.}, boron and carbon) have emerged as another avenue for developing ambient-pressure superconductors. However, despite their stability at low pressures, the critical temperature (Tc) values observed in these materials have not yet reached the impressive benchmarks set by hydride-based superconductors. Here we propose a novel design strategy for achieving high-

Which combinations of coupling λ and phonon scale ω_log clear 77 K under Allen-Dynes for μ* between 0.10 and 0.13, and which known families sit closest?

1. 021
   The Maximum Tc of Conventional Superconductors at Ambient Pressure

   arXiv:2502.18281 · 2025 · Kun Gao, Tiago F. T. Cerqueira, Antonio Sanna et al.

   The theoretical maximum critical temperature (Tc) for conventional superconductors at ambient pressure remains a fundamental question in condensed matter physics. Through analysis of electron-phonon calculations for over 20,000 metals, we critically examine this question. We find that while hydride metals can exhibit maximum phonon frequencies of more than 5000 K, the crucial logarithmic average frequency ωlog rarely exceeds 1800 K. Our data reveals an inherent trade-off between ωlog and the electron-phonon coupling constant λ, suggesting that the optimal Eliashberg function that maximizes Tc is unphysical. Based on our calculations, we identify Li2AgH6 and

2. 022
   Prediction of Ambient Pressure Conventional Superconductivity above 80K in Thermodynamically Stable Hydride Compounds

   arXiv:2310.06804 · 2023 · Antonio Sanna, Tiago F. T. Cerqueira, Yue-Wen Fang et al.

   The primary challenge in the field of high-temperature superconductivity in hydrides is to achieve a superconducting state at ambient pressure rather than the extreme pressures that have been required in experiments so far. Here, we propose a family of compounds, of composition Mg2XH6 with X=Rh, Ir, Pd, or Pt, that achieves this goal. These materials were identified by scrutinizing more than a million compounds using a machine-learning accelerated high-throughput workflow. They are thermodynamically stable, indicating that they are serious candidates for experimental synthesis. We predict that their superconducting transition temperatures are in the range of 45-80K, or even above 100

3. 023
   Fundamental limits on the electron-phonon coupling and superconducting Tc

   arXiv:2407.12922 · 2024 · Dmitrii V. Semenok, Boris L. Altshuler, Emil A. Yuzbashyan

   Fundamental upper bounds on the electron-phonon interaction strength and superconducting transition temperature Tc in metals are established based on the intrinsic instability of the equilibrium between electrons and the crystal lattice under strong interaction. This instability explains why observed electron-phonon coupling constants are limited to λ4. The theory also accounts for the mechanism of metastable superconductivity with enhanced Tc, which emerges near the instability threshold. Based on theoretical analysis and comparison with experimental data, room-temperature phonon-mediated superconductivity is found to be feasible exclusively in hydrogen compounds.

4. 024
   Upper limit for superconducting transition temperature in electron-phonon superconductors: very strong coupling

   arXiv:2506.19326 · 2025 · M. V. Sadovskii

   We present a brief review of some recent work on the problem of highest achievable temperature of superconducting transition Tc in electron-phonon systems. The discovery of record-breaking values of Tc in quite a number of hydrides under high pressure was an impressive demonstration of capabilities of electron-phonon mechanism of Cooper pairing. This lead to an increased interest on possible limitations of Eliashberg-McMillan theory as the main theory of superconductivity in a system of electrons and phonons. We shall consider some basic conclusions following from this theory and present some remarks on the limit of very strong electron-phonon coupling. We shall discuss possible limita