Ambient-pressure Superconductor Research Articles

Hydride units filled boron–carbon clathrate: a pathway for high-temperature superconductivity at ambient pressure

Recent advances in the search for room-temperature superconductors have focused on high-temperature superconductivity in compressed hydrides, though sustaining this at ambient pressure remains challenging. Concurrently, sp3-bonded frameworks comprising lightweight elements offer another avenue for ambient-pressure superconductors. However, their critical temperatures (Tc) still fall short of those in hydrides. Here we propose a design strategy for achieving high-temperature superconductivity at ambient pressure by integrating hydride units into B–C clathrate structures. This approach exploits the beneficial properties of hydrogen, the lightest element, to enhance superconductivity beyond that of the parent compounds. Our computational predictions indicate that doping SrB3C3 with ammonium (NH4) produces SrNH4B6C6, with an estimated Tc of 85 K at ambient pressure—over twice that of its precursor (31 K). Further substitutions yield a family of MNH4B6C6 superconductors, with PbNH4B6C6 predicted to reach a Tc of 115 K. These findings offer a promising route to high-Tc superconductors at ambient pressure.

Room-Temperature Superconductivity in 1D

We review the theoretical model underpinning the recently reported room-temperature, ambient-pressure superconductivity along line defects on the surface of highly oriented pyrolytic graphite. The main ingredients for this 1D room-temperature superconductivity are pairing by effective strain gauge fields, the formation of an effective Josephson junction array in its Bose metal state on the surface and the suppression of phase slips by dimensional embedding in an extremely well-conducting 3D bulk structure.

Electron–Phonon Coupling in Copper-Substituted Lead Phosphate Apatite

Recent reports of room-temperature, ambient pressure superconductivity in copper-substituted lead phosphate apatite, commonly referred to as LK99, have prompted numerous theoretical and experimental studies into its properties. As the electron–phonon interaction is a common mechanism for superconductivity, the electron–phonon coupling strength is an important quantity to compute for LK99. In this work, we compare the electron–phonon coupling strength among the proposed compositions of LK99. The results of our study are in alignment with the conclusion that LK99 is a candidate for low-temperature, not room-temperature, superconductivity if electron–phonon interaction is to serve as the mechanism.

Integrating Superconductivity in Cu Replace Lead Apatite by Nuclear Magnetic Moment Theory of RBL

Recently, scientists proclaimed superconductivity under ambient conditions of room temperature and 1 atmosphere pressure in Cu partial substituted lead apatite: Pb(10-x)Cux(PO4)6. This paper highlights the application of RBL’s stable isotope of positive and negative nuclear magnetic moments (NMMs) theory for explaining the heavy isotopic enrichment of this materials as {207Pb{10-x}63Cux(‘31PιO4)6}, where ι = 17 or 18 and the resulting superconductivity and novel room temperature atmospheric pressure superconductivity of this heavy isotopic enriched substance. On the basis of such analysis by RBL theory, the synthesis of high temperature, regular pressure superconductivity can be explained and this recent experimental observation of ambient pressure and temperature superconductivity in 207Pb{10-x}63Cux(‘31PιO4)6, where ι = 17 or 18, proves RBL’s NMMs theory of high temperature superconductivity. After 1 month on July 22, 2023 of the archiving of such data for superconductivity Pb(10-x)Cux(PO4)6 by Lee and coworkers, a huge experimental effort by hundreds of researchers around the world has attempted to replicate this room temperature, ambient pressure superconductivity in Pb(10-x)Cux(PO4)6 without success. The resulting inability to replicate the ambient superconductivity and note of some fractional component in the mixture causing the ambient superconductivity is explained by RBL’s theory as the fractional component may be clumped isotopes as in {207Pb{10-x}63Cux(‘31PιO4)6}, where ι = 17 or 18, and RBL here notes the vapor deposition process alleged by Lee and coworkers for producing the superconductivity was previously predicted by RBL to cause isotopic fractionation during solid to liquid and liquid to gas and gas to solid physical changes.

Evolution of Superconducting-Transition Temperature with Superfluid Density and Conductivity in Pressurized Cuprate Superconductors

What factors fundamentally determine the value of superconducting transition temperature T c in high temperature superconductors has been the subject of intense debate. Following the establishment of an empirical law known as Homes’ law, there is a growing consensus in the community that the T c value of the cuprate superconductors is closely linked to the superfluid density (ρ s) of its ground state and the conductivity (σ) of its normal state. However, all the data supporting this empirical law (ρ s = AσT c) have been obtained from the ambient-pressure superconductors. In this study, we present the first high-pressure results about the connection of the quantities of ρ s and σ with T c, through the studies on the Bi1.74Pb0.38Sr1.88CuO6+δ and Bi2Sr2CaCu2O8+δ , in which the value of their high-pressure resistivity (ρ = 1/σ) is achieved by adopting our newly established method, while the quantity of ρ s is extracted using Homes’ law. We highlight that the T c values are strongly linked to the joint response factors of magnetic field and electric field, i.e., ρ s and σ, respectively, implying that the physics determining T c is governed by the intrinsic electromagnetic fields of the system.

Prediction of ambient pressure superconductivity in cubic ternary hydrides with MH6 octahedra

Exploring high-temperature superconducting (high-Tc) material at ambient pressure holds immense significance for physics, chemistry, and materials science. In this study, we perform a high-throughput screening of strong electron-phonon interactions in X2MH6 compounds (X = Li, Na, Mg, Al, K, Ca, Ga, Rb, Sr, and In; M are 3d, 4d, and 5d transition metals). These compounds have a cubic structure featuring an MH6 octahedron motif. Our screening calculations suggest that 26 compounds exhibit dynamic stability and strong electron-phonon coupling. Among them, Mg2RhH6, Mg2IrH6, Al2MnH6, and Li2CuH6 show promising energetic stability and Tc of more than 50 K at ambient pressure. This study underscores promising high-Tc compounds at ambient pressure with distinctive MH6 motifs.

Realizing high-Tc ambient-pressure superconductivity in hole-doped hydride Mg(BH4)2

Hydrogen-based superconductors provide a promising route to the long-sought goal of room-temperature superconductivity (SC), however, ultrahigh pressure required to maintain their structural stability hinders the experimental investigation and potential applications. Here, we propose a novel route to design high-Tc hydride superconductors at ambient pressure by hole doping into previously reported hydrogen storage material Mg(BH4)2. Based on the first-principles calculations, we found that with a moderate hole doping of 0.1 holes per formula unit, the insulator Mg(BH4)2 transforms into the metal accompanied by the emergency of large B–H σ electronic states at the Fermi level (EF), and thus strong electron-phonon coupling SC is obtained with Tc up to ∼ 100 K, and with further increase of hole-doped concentration, the Tc can be promoted to 140 K. Additionally, the topological analysis demonstrates that the hole-doped Mg(BH4)2 has symmetry-protected Dirac nodal lines around the EF, giving rise to the delicate surface states. These findings indicate that Mg(BH4)2 not only provides an ideal candidate to attain high-Tc SC by hole doping at ambient pressure, but also to explore more exotic physics for the coexistence of high-Tc SC and topology.

Ab initio investigations on the electronic properties and stability of Cu-substituted lead apatite (LK-99) family with different doping concentrations (x = 0, 1, 2)

The pursuit of room-temperature ambient-pressure superconductivity in novel materials has sparked interest, with recent reports suggesting such properties in Cu-substituted lead apatite, known as LK-99. However, these claims lack comprehensive experimental and theoretical support. In this study, we address this gap by conducting ab initio calculations to explore the impact of varying doping concentrations (x = 0, 1, 2) on the stability and electronic properties of five compounds in the LK-99 family. Our investigations confirm the isolated flat bands that intersect the Fermi level in LK-99 (Pb9Cu(PO4)6O:Cu<Pb(1)>). In contrast, the other four parent compounds exhibit insulating behavior with wide band gaps. X-ray diffraction spectra based on the DFT simulations at 0 K confirm the presence of Cu substitution on Pb(1) sites in the originally synthesized LK-99 sample, while an extra peak suggests potential alternative like Pb8Cu2(PO4)6 phases due to compositional variations in the original LK-99 samples. Furthermore, the LK-99 structure undergoes substantial lattice constriction, resulting in a significant 5.5% reduction in volume and 6.8% in area of two mutually inverted triangles formed by Pb(2) atoms. Meanwhile, energy calculations reveal a marginal energy preference for substituting Cu on Pb(2) sites over Pb(1) sites, with a difference of approximately 0.010 eV per atom (roughly 0.9645 kJ/mol). Intriguingly, at pressures exceeding 73 GPa, stability shifts towards LK-99 containing Cu substitutions on Pb(1) sites. Despite exhibiting higher electronic conductivity than parent compounds, Pb9Cu(PO4)6O:Cu<Pb(1)> falls short of the conductivity levels observed in metals or advanced oxide conductors with the simulation based on the Boltzmann transport theory.

The interplay between exciton- and phonon-induced superconductivity might explain the phenomena observed in LK-99

The experimental results hinting at the room temperature and ambient pressure superconductivity and magnetic levitation in LK-99 attracted an unprecedented interest. While attempts of other teams to reproduce the reported observations on similar samples failed so far, it seems worthwhile to try building a theoretical model that would explain the ensemble of the available data. One of important features that needs to be explained is an apparent contradiction between an extremely high critical temperature Tc and rather modest critical magnetic field Bc and critical current jc reported for LK-99. We show theoretically, that these data may be quantitatively reproduced assuming the interplay between exciton- and phonon-induced superconductivity, while the conventional BCS or Brinkman-Rice-Bardeen-Cooper-Schriefer (BR-BCS) mechanisms would result in a much higher Bc for the same Tc.

Ferromagnetic and insulating behavior in both half magnetic levitation and non-levitation LK-99 like samples

Finding materials exhibiting superconductivity at room temperature has long been one of the ultimate goals in physics and material science. Recently, room-temperature superconducting properties have been claimed in a copper substituted lead phosphate apatite (Pb10-xCux(PO4)6O, or called LK-99) (Lee et al. in J. Korean Cryst. Growth Cryst. Technol. 33:61, 2023; Lee et al. in The first room-temperature ambient-pressure superconductor, 2023; Lee et al. in Superconductor Pb10-xCux(PO4)6O showing levitation at room temperature and atmospheric pressure and mechanism, 2023). Using a similar approach, we have prepared LK-99 like samples and confirmed the half-levitation behaviors in some small specimens under the influence of a magnet at room temperature. To examine the magnetic properties of our samples, we have performed systematic magnetization measurements on the as-grown LK-99 like samples, including the half-levitated and non-levitated samples. The magnetization measurements show the coexistence of soft-ferromagnetic and diamagnetic signals in both half-levitated and non-levitated samples. The electrical transport measurements on the as-grown LK-99 like samples including both half-levitated and non-levitated samples show an insulating behavior characterized by the increasing resistivity with the decreasing temperature.

Few-Hydrogen High-Tc Superconductivity in (Be4)2H Nanosuperlattice with Promising Ductility under Ambient Pressure.

The multi-hydrogen lanthanum hydride LaH10 is well recognized as having the highest critical temperature (Tc) of 250-260 K under unrealistically ultrahigh pressures of about 170-200 GPa. Here, we propose a novel idea for designing a new ambient-pressure high-Tc superconductor by inserting a hexagonal H-monolayer into two close-packed Be monolayers to form a new and stable few-hydrogen metal-bonded layered beryllium hydride (Be4)2H nanosuperlattice, with better ductility than multi-hydrogen, cuprate, and iron-based superconductors, completely contrary to the conventional design strategy for multi-hydrogen covalent high-Tc superconductors with poor ductility at several hundred GPa. We find that (Be4)2H is a phonon-mediated Eliashberg superconductor with a large electron-phonon coupling constant of 1.41 and a high Tc of 84-72 K with Coulomb repulsion pseudopotential μ* = 0.07-0.13. Importantly, (Be4)2H is the only new high-Tc superconductor and fills the gap in the absence of ambient-pressure superconductors around the liquid-nitrogen temperature with good ductility, which is highly beneficial for practical applications.

First-principles study on the electronic structure of Pb10−xCux(PO4)6O (x = 0, 1)

Recently, Lee et al. claimed the experimental discovery of room-temperature ambient-pressure superconductivity in a Cu-doped lead-apatite (LK-99) (arXiv:2307.12008, arXiv:2307.12037). Remarkably, the claimed superconductivity can persist up to 400 K at ambient pressure. Despite the experimental implication, the electronic structure of LK-99 has not yet been studied. Here, we investigate the electronic structures of LK-99 and its parent compound using first-principles calculations, aiming to elucidate the doping effects of Cu. Our results reveal that the parent compound Pb10(PO4)6O is an insulator, while Cu doping induces an insulator-metal transition and thus volume contraction. The band structures of LK-99 around the Fermi level are featured by a half-filled flat band and a fully-occupied flat band. These two very flat bands arise from both the 2p orbitals of 1/4-occupied O atoms and the hybridization of the 3d orbitals of Cu with the 2p orbitals of its nearest-neighboring O atoms. Interestingly, we observe four van Hove singularities on these two flat bands. Furthermore, we show that the flat band structures can be tuned by including electronic correlation effects or by doping different elements. We find that among the considered doping elements (Ni, Cu, Zn, Ag, and Au), both Ni and Zn doping result in the gap opening, whereas Au exhibits doping effects more similar to Cu than Ag. Our work establishes a foundation for future studies to investigate the role of unique electronic structures of LK-99 in its claimed superconducting properties.

A new polymorph of graphene monoxide: an all-sp3 bonded metal and ambient pressure superconductor

In this brief report, we propose the ab initio prediction of a new polymer of carbon monoxide, previously overlooked in extensive searches for its phases.

Prediction of ambient-pressure superconductivity in ternary hydride PdCuHx

We present an ab initio study of the ternary hydride PdCuHx, a parent compound of the superconducting PdH, at different hydrogen content (x=1,2). We investigate its structural, electronic, dynamical, and superconducting properties, demonstrating that, at low hydrogen content, the system is not a superconductor above 1 K; however, the highly hydrogenated structure is a strongly coupled superconductor. We give a solid rationale for the unusual increase of the superconducting critical temperature in hydrogenated palladium when alloyed with noble metals (Cu, Ag, and Au), as observed in Stritzker’s experiments in 1972 [B. Stritzker, Z. Phys. 268, 261–264 (1974)] but never investigated with modern experimental and theoretical techniques. We highlight the important role played by H-derived phonon modes at intermediate frequencies, dynamically stabilized by anharmonic effects, as they strongly couple with states at the Fermi level. We hope that the present results will stimulate additional experimental investigations of structural, electronic, and superconducting properties of hydrogenated palladium–noble metal alloys. Indeed, if confirmed, these compounds could be considered a novel class of superconducting hydrides, showing different coupling mechanisms, which can be exploited to engineer new ambient-pressure superconductors.

Charge doping to flat AgF2 monolayers in a chemical capacitor setup

Flat monolayers of silver(II) fluoride, which could be obtained by epitaxial deposition on an appropriate substrate, have been recently predicted to exhibit very strong antiferro-magnetic superexchange and to have large potential for ambient pressure superconductivity if doped to an optimal level. It was shown that AgF2 could become a magnetic glue-based superconductor with a critical superconducting temperature approaching 200 K at optimum doping. In the current work we calculate the optimum doping to correspond to 14% of holes per formula unit, i.e. quite similar to that for oxocuprates(II). Furthermore, using DFT calculations we show that flat [AgF2] single layers can indeed be doped to a controlled extent using a recently proposed "chemical capacitor" setup. Hole doping associated with the formation of Ag(III) proves to be difficult to achieve in the setup explored in this work as it falls at the verge of charge stability of fluoride anions and does not affect the d(x2 - y2) manifold. However, in the case of electron doping, manipulation of different factors - such as the number of dopant layers and the thickness of the separator - permits fine tuning of the doping level (and concomitantly TC) all the way from the underdoped to overdoped regime (in a similar manner to chemical doping for the Nd2CuO4 analogue).

Pressure-induced boron clathrates with ambient-pressure superconductivity

A pressure-induced boron clathrate LaB8 structure exhibits potential superconductivity of 20 K at ambient pressure. This current result provides a new route to find high-Tc cage superconductors at low-pressures.

The chromium pnictide materials: A tunable platform for exploring new exciting phenomena

Among intermetallic compounds between 3d transition metals and pnictogens, compounds containing chromium and arsenide ions are of special interest due to their rich variety of electronic, magnetic and superconducting properties. The monopnictide CrAs compound, although known since over fifty years ago, was observed to exhibit pressure-induced superconductivity in the vicinity of an antiferromagnetic quantum critical point, making it the first superconductor among Cr-based compounds. After this discovery, seeking for ambient pressure superconductivity in Cr-based compounds, bulk superconductivity was discovered in the pnictide K2Cr3As3. These discoveries boost the search and the investigation of the several interesting phenomena exhibited by these materials that actually need to be properly addressed. Here, we try to give a comprehensive overview on these materials showing that they may provide a tunable platform for emphasising the role played by the quantum criticality, the low dimensionality, the magnetic fluctuations and the topology.

2D Molecular Superconductor to Insulator Transition in the β''-(BEDT-TTF)2[(H2O)(NH4)2M(C2O4)3]·18-crown-6 Series (M = Rh, Cr, Ru, Ir).

The series of salts β''-(BEDT-TTF)2[(H2O)(NH4)2M(C2O4)3]·18-crown-6 show ambient-pressure superconductivity when M = Cr, Rh. Evidence indicates that the previously reported Cr and Rh salts show a bulk Berezinski-Kosterlitz-Thouless superconducting transition. The isostructural ruthenium and iridium salts are reported here. The Ir salt represents the first radical-cation salt to contain a 5d tris(oxalato)metalate anion. The Ru and Ir salts do not show superconductivity but instead undergo a broad chemically induced metal to insulator transition at 155 K for ruthenium and at 100 K for iridium. The c axes of the Ru and Ir salts are much shorter than those of the Rh and Cr salts. Thus, the more stable metallic state of the Cr and Rh salts is associated with the more strongly 2D electronic systems. The different low-temperature behavior of the Ru and Ir salts, which exhibit a smaller interlayer spacing, could originate from a structural change in the anionic layer which thus can be easily transmitted to the donor layers and generate a localized state. However, another possibility is that it originates from Berezinski-Kosterlitz-Thouless effects.

Emergence of intrinsic superconductivity below 1.178 K in the topologically non-trivial semimetal state of CaSn3

Topological materials which are also superconducting are of great current interest, since they may exhibit a non-trivial topologically-mediated superconducting phase. Although there have been many reports of pressure-tuned or chemical-doping-induced superconductivity in a variety of topological materials, there have been few examples of intrinsic, ambient pressure superconductivity in a topological system having a stoichiometric composition. Here, we report that the pure intermetallic CaSn3 not only exhibits topological fermion properties, but also has a superconducting phase at ~1.178 K under ambient pressure. The topological fermion properties, including the nearly zero quasi-particle mass and the non-trivial Berry phase accumulated in cyclotron motions, were revealed from the de Haas–van Alphen (dHvA) quantum oscillation studies of this material. Although CaSn3 was previously reported to be superconducting with Tc = 4.2 K, our studies show that the Tc = 4.2 K superconductivity is extrinsic and caused by Sn on the degraded surface, whereas its intrinsic bulk superconducting transition occurs at 1.178 K. These findings make CaSn3 a promising candidate for exploring new exotic states arising from the interplay between non-trivial band topology and superconductivity, e.g. topological superconductivity (TSC).

High-pressure phase diagram of NdFeAsO0.9F0.1 : Disappearance of superconductivity on the verge of ferromagnetism from Nd moments

We investigated transport and magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$ single crystal under hydrostatic pressures up to 50\,GPa. The ambient pressure superconductivity at $T_{c} \sim$ 45.4\,K is fully suppressed at $P_{c} \sim$ 21 GPa. Upon further increase of the pressure, the ferromagnetism associated with the order of rare-earth subsystem is induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M$($H$) loops and the strong increase in the field cooled data, $M$($T$), toward low temperatures. We also show that the temperature evolution of the electrical resistivity as a function of pressure is consistent with a crossover from a Fermi-liquid to non-Fermi-liquid to Fermi-liquid. These results give access to the high-pressure side of the superconducting phase diagram in 1111 type of materials.