Superconductivity In Hydrides Research Articles

Designing multicomponent hydrides with potential high Tc superconductivity

While hydrogen-rich materials have been demonstrated to exhibit high Tc superconductivity at high pressures, there is an ongoing search for ternary, quaternary, and more chemically complex hydrides that achieve such high critical temperatures at much lower pressures. First-principles searches are impeded by the computational complexity of solving the Eliashberg equations for large, complex crystal structures. Here, we adopt a simplified approach using electronic indicators previously established to be correlated with superconductivity in hydrides. This is used to study complex hydride structures, which are predicted to exhibit promisingly high critical temperatures for superconductivity. In particular, we propose three classes of hydrides inspired by the Fm[Formula: see text]m RH[Formula: see text] structures that exhibit strong hydrogen network connectivity, as defined through the electron localization function. The first class [RH[Formula: see text]X[Formula: see text]Y] is based on a Pm[Formula: see text]m structure showing moderately high Tc, where the Tc estimate from electronic properties is compared with direct Eliashberg calculations and found to be surprisingly accurate. The second class of structures [(RH[Formula: see text])[Formula: see text]X[Formula: see text]YZ] improves on this with promisingly high density of states with dominant hydrogen character at the Fermi energy, typically enhancing Tc. The third class [(R[Formula: see text]H[Formula: see text])(R[Formula: see text]H[Formula: see text])X[Formula: see text]YZ] improves the strong hydrogen network connectivity by introducing anisotropy in the hydrogen network through a specific doping pattern. These design principles and associated model structures provide flexibility to optimize both Tc and the structural stability of complex hydrides.

High‐Throughput Study of Ambient‐Pressure High‐Temperature Superconductivity in Ductile Few‐Hydrogen Metal‐Bonded Perovskites

AbstractSeveral multi‐hydrogen hydrides have exhibited high critical temperature (Tc) superconductivity, but the requirement for ultrahigh pressures limits their applications. Here, high‐throughput calculations are utilized to investigate the superconductivity in few‐hydrogen metal‐bonded (FHMB) perovskites (PVSKs) AHM3 characterized with perfect ambient‐pressure stability. AHM3 is classified into two groups, d and sp superconductors, and provide three indicators that accurately describe AHM3 superconductivity. i) Tc of d superconductors is positively correlated with the number of unpaired d electrons from M atoms; ii) A suitably sized octahedral interstice of H atom is essential for sp superconductors; iii) The introduction of H will further improve the superconductivity, when the M atom has a lower electronegativity than H. ZnHCr3 and ZnHAl3, perfectly meeting the requirements aforementioned, exhibit the highest Tc of 30 and 80 K among the d and sp superconductors, respectively. The results are helpful for understanding the electron–phonon coupling (EPC) mechanism in few‐hydrogen metal‐bonded perovskites and facilitate realizations of ambient‐pressure high‐Tc superconductivity in hydrides.

Prediction of ambient pressure conventional superconductivity above 80 K in hydride compounds

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. We predict that their superconducting transition temperatures are in the range of 45–80 K, or even above 100 K with appropriate electron doping of the Pt compound. These results indicate that, although very rare, high-temperature superconductivity in hydrides is achievable at room pressure.

Metal-bonded perovskite lead hydride with phonon-mediated superconductivity exceeding 46 K under ambient pressure

In the search for high-temperature superconductivity in hydrides, a plethora of multi-hydrogen superconductors have been theoretically predicted, and some have been synthesized experimentally under ultrahigh pressures of several hundred GPa. However, the impracticality of these high-pressure methods has been a persistent issue. In response, we propose a new approach to achieve high-temperature superconductivity under ambient pressure by implanting hydrogen into lead to create a stable few-hydrogen binary perovskite, Pb4H. This approach diverges from the popular design methodology of multi-hydrogen covalent high critical temperature (Tc ) superconductors under ultrahigh pressure. By solving the anisotropic Migdal–Eliashberg equations, we demonstrate that perovskite Pb4H presents a phonon-mediated superconductivity exceeding 46 K with inclusion of spin–orbit coupling, which is six times higher than that of bulk Pb (7.22 K) and comparable to that of MgB2, the highest Tc achieved experimentally at ambient pressure under the Bardeen, Cooper, and Schrieffer framework. The high Tc can be attributed to the strong electron–phonon coupling strength of 2.45, which arises from hydrogen implantation in lead that induces several high-frequency optical phonon modes with a relatively large phonon linewidth resulting from H atom vibration. The metallic-bonding in perovskite Pb4H not only improves the structural stability but also guarantees better ductility than the widely investigated multi-hydrogen, iron-based and cuprate superconductors. These results suggest that there is potential for the exploration of new high-temperature superconductors under ambient pressure and may reignite interest in their experimental synthesis in the near future.

Hysteresis loops in measurements of the magnetic moment of hydrides under high pressure: Implications for superconductivity

Measurements of magnetic moments of hydride materials under high pressure have been claimed to prove the existence of superconductivity in these materials (Drozdov et al., 2015; Minkov et al., 2021; Minkov et al., 2022; Minkov et al., 2023; Eremets et al., 2022) . However, detection of the signal from the small sample requires subtraction of a large background contribution whose details are largely unknown. Here we analyze reported measurements and point out that the resulting hysteresis loops are incompatible with the conclusion that they result from superconductivity, independent of what assumptions are made about the background signal. We argue that this also invalidates the conclusion that the magnetic moment measured after the external magnetic field is turned off is evidence for trapped magnetic flux resulting from superconducting currents, as proposed in Minkov et al. (2023). Our results imply that to date no magnetic evidence for the existence of high temperature superconductivity in hydrides under pressure exists, despite multiple claims to the contrary.

On Magnetic Field Screening and Trapping in Hydrogen-Rich High-Temperature Superconductors: Unpulling the Wool Over Readers’ Eyes

In Minkov et al. (Nat. Commun. 13:3194, 2022), Minkov et al. reported magnetization measurements on hydrides under pressure that claimed to find a diamagnetic signal below a critical temperature demonstrating the existence of superconductivity. Here, we present an analysis of raw data recently released (Minkov et al. Nat. Commun. 14:5322, 2023) by the authors of Minkov et al. (Nat. Commun. 13:3194, 2022) that shows that the measured data do not support their claim that the samples exhibit a diamagnetic response indicative of superconductivity. We also point out that Minkov et al. (Nat. Commun. 13:3194, 2022) in its original form omitted essential information that resulted in presentation of a distorted picture of reality, and that important information on transformations performed on measured data remains undisclosed. Our analysis also calls into question the conclusions of Minkov et al.’s trapped flux experiments reported in Minkov et al. (Nat. Phys. 19:1293–1300, 2023) as supporting superconductivity in these materials. This work together with earlier work implies that there is no magnetic evidence for the existence of high temperature superconductivity in hydrides under pressure.

Are hydrides under high-pressure-high-temperature superconductors?

Contrary to the current consensus, I argue that the existing evidence for high-temperature superconductivity in hydrides under high pressure is not compelling. I suggest that the focus of the field should urgently shift to establish unequivocally experimentally whether or not superconductivity in pressurized hydrides exists, instead of continuing to search for new materials that might show elusive signals of unproven superconductivity at ever higher temperatures. The implications of a negative finding for the theoretical understanding of superconductivity are discussed.

Magnetic flux trapping in hydrogen-rich high-temperature superconductors

Recent discoveries of superconductivity in various hydrides at high pressures have shown that a critical temperature of superconductivity can reach near-room-temperature values. However, experimental studies are limited by high-pressure conditions, and electrical transport measurements have been the primary technique for detecting superconductivity in hydrides. Here we implement a non-conventional protocol for the magnetic measurements of superconductors in a SQUID magnetometer and probe the trapped magnetic flux in two near-room-temperature superconductors H3S and LaH10 at high pressures. Contrary to traditional magnetic susceptibility measurements, the magnetic response from the trapped flux is almost unaffected by the background signal of the diamond anvil cell due to the absence of external magnetic fields. The behaviour of the trapped flux generated under zero-field-cooled and field-cooled conditions proves the existence of superconductivity in these materials. We reveal that the absence of a pronounced Meissner effect is associated with the very strong pinning of vortices inside the samples. This approach can also be a tool for studying multiphase samples or samples that have a low superconducting fraction at ambient pressure.

Metal borohydrides as ambient-pressure high-Tcsuperconductors

The extreme pressures required to stabilize the recently discovered superhydrides represent a major obstacle to their practical application. In this paper, we propose a novel route to attain high-temperature superconductivity in hydrides at ambient pressure, by doping commercial metal borohydrides. Using first-principles calculations based on Density Functional Theory and Migdal-Eliashberg theory, we demonstrate that in Ca(BH$_4$)$_2$ a moderate hole doping of 0.03 holes per formula unit, obtained through a partial replacement of Ca with monovalent K, is sufficient to achieve $T_c$'s as high as 110 K. The high-$T_c$ arises because of the strong electron-phonon coupling between the B-H $\sigma$ molecular orbitals and bond-stretching phonons. Using a random sampling of large supercells to estimate the local effects of doping, we show that the required doping can be achieved without significant disruption of the electronic structure and at moderate energetic cost. Given the wide commercial availability of metal borohydrides, the ideas presented here can find prompt experimental confirmation. If successful, the synthesis of high-$T_c$ doped borohydrides will represent a formidable advancement towards technological exploitation of conventional superconductors.

Enhanced superconductivity in CuH2 monolayers

Achieving superconductivity in hydrides at lower pressures is a long-standing scientific challenge. Here, we propose that reducing their effective dimensionality might be a possible route to achieve this goal. First-principles structural search calculations identify several two-dimensional transition metal hydrides (TMHs) with phonon-mediated superconductivity. Among them, the two ${\mathrm{CuH}}_{2}$ phases with $P\text{\ensuremath{-}}3m1$ and $P\text{\ensuremath{-}}6m2$ symmetries are predicted to have the highest ${T}_{\mathrm{c}}$ values up to 44.4 and 47.8 K, comparable to the well-known ${\mathrm{MgB}}_{2}$ superconductor. Their superconductivity mainly originates from the coupling of Cu $3{d}_{xz}/{d}_{yz}$ and $\mathrm{H}\text{\ensuremath{-}}1s$ electrons with in-plane H-vibrational phonons. Interestingly, both ${\mathrm{CuH}}_{2}$ structures show a unique superconducting energy gap by solving the anisotropic Eliashberg equations. Furthermore, the superconductivity of TMHs is closely related to the polarity of the TM-H bond. Our paper paves the way for finding hydride superconductors in low-dimensional materials.

Clear evidence against superconductivity in hydrides under high pressure

The Meissner effect, magnetic field expulsion, is a hallmark of superconductivity. Associated with it, superconductors exclude applied magnetic fields. Recently, Minkov et al. [Nat. Commun. 13, 3194 (2022)] presented experimental results reportedly showing “definitive evidence of the Meissner effect” in sulfur hydride and lanthanum hydride under high pressure, and Eremets et al. [J. Supercond. Nov. Magn. 35, 965 (2022)] argued that “the arguments against superconductivity (in hydrides) can be either refuted or explained.” Instead, we show here that the evidence presented in those papers does not support the case for superconductivity in these materials. Together with experimental evidence discussed in earlier papers, we argue that this strongly suggests that hydrides under pressure are not high-temperature superconductors.

Magnetic field screening in hydrogen-rich high-temperature superconductors

In the last few years, the superconducting transition temperature, Tc, of hydrogen-rich compounds has increased dramatically, and is now approaching room temperature. However, the pressures at which these materials are stable exceed one million atmospheres and limit the number of available experimental studies. Superconductivity in hydrides has been primarily explored by electrical transport measurements, whereas magnetic properties, one of the most important characteristic of a superconductor, have not been satisfactory defined. Here, we develop SQUID magnetometry under extreme high-pressure conditions and report characteristic superconducting parameters for Im-3m-H3S and Fm-3m-LaH10—the representative members of two families of high-temperature superconducting hydrides. We determine a lower critical field Hc1 of ∼0.82 T and ∼0.55 T, and a London penetration depth λL of ∼20 nm and ∼30 nm in H3S and LaH10, respectively. The small values of λL indicate a high superfluid density in both hydrides. These compounds have the values of the Ginzburg-Landau parameter κ ∼12–20 and belong to the group of “moderate” type II superconductors, rather than being hard superconductors as would be intuitively expected from their high Tcs.

Superconducting Li10Se electride under pressure.

Achieving a compound with interesting multiple coexisting states, such as electride, metallicity, and superconductivity, is of great interest in basic research and practical application. Pressure has become an effective way to realize high-temperature superconductivity in hydrides, whereas most electrides are semiconducting or insulating at high pressure. Here, we have applied swarm-intelligence structural search to identify a hitherto unknown C2/m Li10Se electride that is superconducting at high pressure. More interestingly, Li10Se is estimated to exhibit the highest Tc value of 16K at 50GPa, which is the lowest pressure among Li-based chalcogen electrides. This superconducting transition is dominated by Se-related low frequency vibration modes. The increasing electronic occupation of the Se 4d orbital and the decreasing amount of interstitial anion electrons with pressure heighten their coupling with low-frequency phonons, which is responsible for the enhancement of the Tc value. The finding of Li-based chalcogen superconducting electrides provides a reference for the realization of other superconducting electrides at lower pressures.

High-Temperature Superconductivity in Hydrides: Experimental Evidence and Details

Since the discovery of superconductivity at ~ 200 K in H3S [1], similar or higher transition temperatures, Tcs, have been reported for various hydrogen-rich compounds under ultra-high pressures [2]. Superconductivity was experimentally proved by different methods, including electrical resistance, magnetic susceptibility, optical infrared, and nuclear resonant scattering measurements. The crystal structures of superconducting phases were determined by X-ray diffraction. Numerous electrical transport measurements demonstrate the typical behavior of a conventional phonon-mediated superconductor: zero resistance below Tc, shift of Tc to lower temperatures under external magnetic fields, and pronounced isotope effect. Remarkably, the results are in good agreement with the theoretical predictions, which describe superconductivity in hydrides within the framework of the conventional BCS theory. However, despite this acknowledgement, experimental evidences for the superconducting state in these compounds have recently been treated with criticism [3–7], which apparently stems from misunderstanding and misinterpretation of complicated experiments performed under very high pressures. Here, we describe in greater detail the experiments revealing high-temperature superconductivity in hydrides under high pressures. We show that the arguments against superconductivity [3–7] can be either refuted or explained. The experiments on the high-temperature superconductivity in hydrides clearly contradict the theory of hole superconductivity [8] and eliminate it [3].

Unusual width of the superconducting transition in a hydride.

The long-sought goal of room-temperature superconductivity has reportedly recently been realized in a carbonaceous sulfur hydride compound under high pressure, as reported by Snider et al. [1]. The evidence presented in that paper is stronger than in other similar recent reports of high temperature superconductivity in hydrides under high pressure [2-7], and has been received with universal acclaim [8-10]. Here we point out that features of the experimental data shown in Ref. [1] indicate that the phenomenon observed in that material is not superconductivity. This observation calls into question earlier similar claims of high temperature conventional superconductivity in hydrides under high pressure based on similar or weaker evidence [2-7].

Synthesis and electrical transport measurement of superconducting hydrides using diamond anvil cell with boron-doped diamond electrodes

High transition-temperature superconductivity in hydrides under compression is one of the hottest topics in materials science. However, the synthesis and measurement of the superconducting properties of hydrides are quite challenging because of the required hydrogen atmosphere and high pressure. This study developed a synthesis and measurement platform for superconducting hydrides under compression using a gas loading system and diamond anvil cell with unique diamond components. The undoped diamond insulation packs hydrogen into the sample chamber, and the boron-doped diamond electrodes detect the sample properties. The synthesis and observation of superconductivity were demonstrated on PdH x using the developed system.

Nonstandard superconductivity or no superconductivity in hydrides under high pressure

Over the past six years, superconductivity at high temperatures has been reported in a variety of hydrogen-rich compounds under high pressure. That high-temperature superconductivity should exist in these materials is expected according to the conventional theory of superconductivity, as shown by detailed calculations. However here we argue that experimental observations rule out conventional superconductivity in these materials. Our results indicate that either these materials are unconventional superconductors of a novel kind, which we term `nonstandard superconductors', or alternatively that they are not superconductors. If the former, we point out that the critical current in these materials should be five orders of magnitude larger than in standard superconductors, potentially opening up the way to important technological applications. If the latter, which we believe is more likely, we suggest that the signals interpreted as superconductivity are either experimental artifacts or they signal other interesting physics but not superconductivity.

Absence of magnetic evidence for superconductivity in hydrides under high pressure

It is generally believed that magnetization measurements on sulfur hydride under high pressure performed in 2015 [1] provided “final convincing evidence of superconductivity” [2] in that material, in agreement with theoretical predictions [3,4]. Supported by this precedent, drops in resistance that were later observed in several other hydrides under high pressure [[2], 5] have been generally accepted as evidence of superconductivity without corroborating evidence from magnetic measurements. In this paper we challenge the original interpretation that the magnetic measurements on sulfur hydride performed in 2015 were evidence of superconductivity. We point out that a large paramagnetic contribution to the magnetic susceptibility was detected below Tc and argue that its temperature dependence rules out the possibility that it would be a background signal; instead the temperature dependence indicates that the paramagnetic behavior originated in the sample. We discuss possible explanations for this remarkable behavior and conclude that standard superconductors would not show such behavior. We also survey all the other published data from magnetic measurements on this class of materials and conclude that they do not provide strong evidence for superconductivity. Consequently, we call into question the generally accepted view that conventional superconductivity in hydrogen-rich materials at high temperature and pressure is a reality, and discuss the implications if it is not.

Experimental Study of Crystal Structure and Superconductivity in Hydrides under High Pressure

Experimental Study of Crystal Structure and Superconductivity in Hydrides under High Pressure

Superconductivity in La and Y hydrides: Remaining questions to experiment and theory

Recent reports of the superconductivity in hydrides of two different families (covalent lattice, as in SH3 and clathrate-type H-cages containing La and Y atoms, as in LaH10 and YH6) have revealed new families of high-Tc materials with Tc’s near room temperature values. These findings confirm earlier expectations that hydrides may have very high Tc’s due to the fact that light H atoms have very high vibrational frequencies, leading to high Tc values within the conventional Bardeen–Cooper–Schrieffer phonon mechanism of superconductivity. However, as is pointed out by Ashcroft, it is important to have the metallic hydrogen “alloyed” with the elements added to it. This concept of a metallic alloy containing a high concentration of metal-like hydrogen atoms has been instrumental in finding new high-Tc superhydrides. These new superhydride “room-temperature” superconductors are stabilized only at very high pressures above 100 GPa, making the experimental search for their superconducting properties very difficult. We will review the current experimental and theoretical results for LaH10−x and YH6−x superhydrides.