Room-temperature Superconductivity Research Articles

Strategies for Improving the Superconductivity of Hydrides Under High Pressure.

The successful prediction and confirmation of unprecedentedly high-temperature superconductivity in compressed hydrogen-rich hydrides signify a remarkable advancement in the continuous quest for attaining room-temperature superconductivity. The recent studies have established a broad scope for developing binary and ternary hydrides and illustrated correlation between specific hydrogen motifs and high-Tcs under high pressures. The analysis of the microscopic mechanism of superconductivity in hydrides suggests that the high electronic density of states (DOS) at the Fermi level (EF), the large phonon energy scale of the vibration modes and the resulting enhanced electron-phonon coupling are crucial contributors towards the high-Tc phonon-mediated superconductor. The aim of our efforts is to tackle forthcoming challenges associated with elevating the Tc and reducing the stabilization pressures of hydrogen-based superconductors, and offer insights for the future discoveries of room-temperature superconductors. Our present Review offers an overview and analysis of the latest advancements in predicting and experimentally synthesizing various crystal structures, while also exploring strategies to enhance the superconductivity and reducing their stabilization pressures of hydrogen-rich hydrides.

Superconductivity of thulium substituted clathrate hexahydrides at moderate pressure

Due to the BCS theory, hydrogen, the lightest element, would be the prospect of room-temperature superconductor after metallization, but because of the difficulty of the hydrogen metallization, the theory about hydrogen pre-compression was proposed that the hydrogen-rich compounds could be a great option for the high Tc superconductors. The superior properties of TmH6, YbH6 and LuH6 indicated the magnificent potential of heavy rare earth elements for low-pressure stability. Here, we designed XTmH12 (X = Y, Yb, Lu, and La) to obtain higher Tc while maintaining low pressure stability. Most prominently, YbTmH12 can stabilize at a pressure of 60 GPa. Compared with binary TmH6 hydride, its Tc was increased to 48 K. The results provide an effective method for the rational design of moderate pressure stabilized hydride superconductors.

Superconductivity above 105 K in Nonclathrate Ternary Lanthanum Borohydride below Megabar Pressure.

Hydrides are promising candidates for achieving room-temperature superconductivity, but a formidable challenge remains in reducing the stabilization pressure below a megabar. In this study, we successfully synthesized a ternary lanthanum borohydride by introducing the nonmetallic element B into the La-H system, forming robust B-H covalent bonds that lower the pressure required to stabilize the superconducting phase. Electrical transport measurements confirm the presence of superconductivity with a critical temperature (Tc) of up to 106 K at 90 GPa, as evidenced by zero resistance and Tc shift under an external magnetic field. X-ray diffraction and transport measurements identify the superconducting compound as LaB2H8, a nonclathrate hydride, whose crystal structure remains stable at pressures as low as ∼ half megabar (59 GPa). Stabilizing superconductive stoichiometric LaB2H8 in a submegabar pressure regime marks a substantial advancement in the quest for high-Tc superconductivity in polynary hydrides, bringing us closer to the ambient pressure conditions.

Superior Superconducting Properties Realized in Quaternary La-Y-Ce Hydrides at Moderate Pressures.

The recent revolution in the superconductivity field stems from hydride superconductors. Multicomponent hydrides provide a crucial platform for tracking high-temperature superconductors. Besides high superconducting transition temperature (Tc), achieving both giant upper critical magnetic field [μ0Hc2(0)] and high critical current density [Jc(0)] is also key to the latent potential of the application for hydride superconductors. In this work, we have successfully synthesized quaternary La-Y-Ce hydrides with excellent properties under moderate pressure by using the concept of "entropy engineering." The obtained temperature dependence of the resistance provides evidence for the superconductivity of Fm3m-(La,Y,Ce)H10, with the maximum Tc ∼ 190 K (at 112 GPa). Notably, Fm3m-(La,Y,Ce)H10 boasts exceptional properties: μ0Hc2(0) reaching 292 T and Jc(0) surpassing 4.61 × 107 A/cm2. Compared with the binary LaH10/YH10, we find that the Fm3m structure in (La,Y,Ce)H10 can be stable at relatively low pressures (112 GPa). These results indicate that multicomponent hydrides can significantly enhance the superconducting properties and regulate stabilizing pressure through the application of "entropy engineering." This work stimulates the experimental exploration of multihydride superconductors and also provides a reference for the search of room-temperature superconductors in more diversified hydride materials in the future.

Unusual metallic state in superconducting A15-type La4H23

Abstract Hydride superconductors continue to fascinate the communities of condensed matter physics and material sciences because they host the promising room-temperature superconductivity. The current research has been concentrated on the new hydride superconductors with the enhancement of the superconducting transition temperature (Tc). The multiple extreme conditions (high pressure/temperature and magnetic field) will introduce new insights on the hydride superconductors. The study of transport properties under very high magnetic fields facilitates the understanding of the superconductivity in the conventional hydride superconductors. In the present work, we report experimental evidence of an unusual metal state in a newly synthesized cubic A15-type La4H23 that exhibits superconductivity with a Tc reaching 105 K at 118 GPa. A large negative magnetoresistance is observed for strong pulsed magnetic fields in the non-superconducting state below 40 K. Moreover, we construct the full magnetic phase diagram of La4H23 up to 68 T at high pressure. The present work reveals anomalous electronic structural properties of A15-La4H23 under high magnetic fields, therefore holding great importance for advancing the understanding of quantum transport behaviors in hydride superconductors.

Hydrogen bond conversion and near room temperature superconductivity in oxygen hydrides

Hydrogen bond conversion and near room temperature superconductivity in oxygen hydrides

Berry curvature and quantum metric in copper-substituted lead phosphate apatite

The recent discovery of copper-substituted lead phosphate apatite, also known as LK-99, has caught much attention owing to certain experimental evidence of room-temperature superconductivity, although this claim is currently under intensive debate. Be it superconducting or not, we show that the normal state of this material has peculiar quantum geometrical properties that may be related to the magnetism and the mechanism for flat band superconductivity. Based on a recently proposed spinless two-band tight-binding model for the Pb–Cu hexagonal lattice subset of the crystalline structure, which qualitatively captures the two flat bands in the band structure, we elaborate the highly anisotropic Berry curvature and quantum metric in the regions of Brillouin zone where one flat band is above and the other below the Fermi surface. In these regions, the Berry curvature has a pattern in the planar momentum that remains unchanged along the out-of-plane momentum. Moreover, the net orbital magnetization contributed from the Berry curvature is zero, signifying that the magnetism in this material should come from other sources. The quantum metric has a similar momentum dependence, and its two planar components are found to be roughly the same but the out-of-plane component vanishes, hinting that the superfluid stiffness of the flat band superconductivity, shall it occur, may be quite anisotropic.

High temperature superconductivity of quaternary hydrides XM3Be4H32 (X, M = Ca, Sr, Ba, Y, La, Ac, Th) under moderate pressure

The compressed hydrogen-rich compounds have received extensive attention as promising candidates for room temperature superconductivity, however, the high pressure required to stabilize such materials hinders their wide practical application. In order to search for potential superconducting hydrides that are stable at low pressures, we have investigated the crystal structures and properties of quaternary hydrides, XM3Be4H32 (X, M = Ca, Sr, Ba, Y, La, Ac, Th) via first-principles calculations. We identified nine dynamically stable quaternary hydrides at 20 GPa. Strikingly, their superconducting transition temperatures (Tc) are much higher than that of liquid nitrogen, especially CaTh3Be4H32 (124 K at 5 GPa), ThLa3Be4H32(134 K at 10 GPa), LaAc3Be4H32 (135 K at 20 GPa) and AcLa3Be4H32 (153 K at 20 GPa) exhibit outstanding superconductivity at mild pressures. Metal atoms acting as pre-compressors donate abundant electrons to hydrogen, weakening the H–H covalent bond and thus facilitating the metallization of the hydrogen sublattice. At the same time, the appropriate combination of metal elements with different ionic radius and electronegativity can effectively tune the electronic structure near the Fermi level and improve the superconductivity. Molecular dynamics simulations reveal that these quaternary hydrides remain thermodynamically stable in the experimental acceptable pressure range (above 50 GPa). These findings fully reveal the great promise of hosting high-temperature superconductivity of quaternary hydrides at moderate pressures and will further promote related exploration.

Thermodynamic properties and enhancement of diamagnetism in nitrogen doped lutetium hydride synthesized at high pressure

Nitrogen doped lutetium hydride has drawn global attention in the pursuit of room-temperature superconductivity near ambient pressure and temperature. However, variable synthesis techniques and uncertainty surrounding nitrogen concentration have contributed to extensive debate within the scientific community about this material and its properties. We used a solid-state approach to synthesize nitrogen doped lutetium hydride at high pressure and temperature (HPT) and analyzed the residual starting materials to determine its nitrogen content. High temperature oxide melt solution calorimetry determined the formation enthalpy of LuH1.96N0.02 (LHN) from LuH2 and LuN to be -28.4 ± 11.4 kJ/mol. Magnetic measurements indicated diamagnetism which increased with nitrogen content. Ambient pressure conductivity measurements observed metallic behavior from 5 to 350 K, and the constant and parabolic magnetoresistance changed with increasing temperature. High pressure conductivity measurements revealed that LHN does not exhibit superconductivity up to 26.6 GPa. We compressed LHN in a diamond anvil cell to 13.7 GPa and measured the Raman signal at each step, with no evidence of any phase transition. Despite the absence of superconductivity, a color change from blue to purple to red was observed with increasing pressure. Thus, our findings confirm the thermodynamic stability of LHN, do not support superconductivity, and provide insights into the origins of its diamagnetism.

Synthesis of cubic lutetium hydride at HPHT conditions and its implications

Binary rare earth hydrides are ideal hydrogen rich materials for the search of room temperature superconductivity. Even though there are plethora of such theoretical predictions, only a few binary hydrides show high TC experimentally. The recent progress in the search for new ternary hydrides have led to the controversial discovery of room temperature superconductivity in the nitrogen doped lutetium hydride (LuH3-δNε) (NLH). This has stimulated the interest in the synthesis of lutetium hydrides and their detailed understanding. Phase pure LuH2 has been synthesized using pure Lutetium (Lu) metal and paraffin as hydrogen source at high pressure and high temperature conditions. The sample inside diamond anvil cell (DAC) has been characterised with optical micrographs using high resolution optical microscope. The recovered sample is characterised by employing synchrotron based angle dispersive x-ray diffraction (ADXRD) measurements. The lattice parameter of as synthesized LuH2 is in excellent agreement with the earlier reported one. The comparison of high pressure evolution of the unit cell volumes of synthesized LuH2, cubic LuH3 and NLH imparts broader understanding and opens the way for future research in lutetium based hydride systems. Data analysis at high pressures reveals that the parent structural framework of NLH is LuH2 rather than LuH3. Here, we have proposed a novel method for determination of hydrogen stoichiometry in ternary rare earth hydride systems. In this method, hydrogen to metal ratio (H:M) and metal-hydrogen (M − H) distance as a function of hydrogen-solubilities systematics for lanthanide dihydrides solid solution are utilized to constrain the concentration of hydrogen in these compounds.

Replication and study of anomalies in LK-99—the alleged ambient-pressure, room-temperature superconductor

We have studied LK-99 [Pb10−x Cu x (PO4)6O], alleged by Lee et al to exhibit superconductivity above room temperature and at ambient pressure, and have reproduced all anomalies in electric and magnetic measurements that they reported as evidence for the claim of LK-99 being an ambient-pressure, room-temperature superconductor. We found that these anomalies are associated with the structural transition of the Cu2S impurity in their sample and not with superconductivity.

The current status and future development of high-temperature conventional superconductivity

The robust evidence and reproducibility of high-temperature superconductivity in hydrogen-rich materials under challenging experimental conditions of megabar pressures is presented.

Superconductivity and electron self-energy in tungsten-sulfur-hydride monolayer

Hydrogen-rich structures have recently gained attention as a candidate for room-temperature superconductors. Hydrogen has high phonon frequencies and can be an ideal component for superconductors if it also exhibits strong electron–phonon coupling. In bulk materials, this has been achieved only under very high pressure. Two-dimensional hydrogen-decorated materials can also be expected to become superconductors. Recently, it was shown that a Janus MoSH monolayer can be synthesized (Lu et al 2017 Nat. Nanotechnol. 12 744–9), and a theoretical investigation of this MoSH monolayer claimed that T c = 28.58 K at atmospheric pressure (Liu et al 2022 Phys. Rev. B 105 245420). In this work, we propose that tungsten sulfur hydride (WSH) is also a superconducting Janus monolayer. The Tc is carefully calculated with very high resolution via the Eliashberg spectral function and the electron self-energy. We find that WSH is a conventional BCS superconductor with T c = 12.2 K at ambient pressure. For practical applications, sensitive dependence on substrate is inferred. We also reported the electron self-energy of WSH, which can be compared directly with future measurements from angle-resolved photoelectron spectroscopy.

The characteristics of Cu-doped lead apatite (LK-99) synthesized with the removal of Cu2S using ammonia solution: A diamagnetic semiconductor

This study evaluated the superconducting properties of Cu-doped lead apatite “LK-99” and synthesized LK-99 samples using the process proposed by a team from Korea, who first described the process. Additionally, whether the results of this Korean team were related to Cu2S was also examined using an ammonia solution (NH3·H2O) to remove Cu2S. Through x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDX) analyses, a distinct Cu2S phase was identified in the LK-99 samples. This phase was effectively eliminated through a subsequent treatment using an ammonia solution. The appearance of blue Cu+2 ions in the solution and the elimination of the Cu2S peak in XRD supported the conclusion. The magnetic and electrical properties of LK-99 with and without Cu2S suggest that the superconductor-like behavior in LK-99 predominantly arises from a transition in resistivity due to the influence of Cu2S. As such, LK-99 is better classified as a diamagnetic semiconductor than a room-temperature superconductor. The room-temperature superconductors still require further research.

The 1/4 occupied O atoms induced ultra-flatband and the one-dimensional channels in the Pb10−xCux(PO4)6O4 (x = 0, 0.5) crystal

The search for room-temperature superconductors has been a long-standing goal in condensed matter physics. In this study, we investigate the electronic and geometric properties of lead apatite with and without Cu doping within the framework of the density functional theory. Based on our calculations, we found that without Cu doping, the lead apatite PPOO shows an insulator character with flatbands that straddle the Fermi level. Once we introduce O1 vacancies, the flatbands disappear. Furthermore, we analyze the effects of Cu doping on the crystal structure and the electronic band structure of the material PCPOO4. Our calculations reveal the presence of one-dimensional channels induced by fully occupied O1 atoms that are only 1/4 occupied in the literature, which may play a crucial role in the realization of room-temperature superconductivity. Based on our findings, we propose a possible solution to improve the quality of superconductivity by annealing the material in an oxygen atmosphere. These results contribute to a better understanding of the unusual properties of Cu-doped lead apatite PCPOO4 and will pave the way for further exploration of its potential as a room-temperature superconductor.

Superconducting ternary hydrides in Ca-U-H under high pressure

The research on hydrogen-rich ternary compounds attract tremendous attention for it paves new route to room-temperature superconductivity at lower pressures. Here, we study the crystal structures, electronic structures, and superconducting properties of the ternary Ca-U-H system, combining crystal structure predictions with ab-initio calculations under high pressure. We found four dynamically stable structures with hydrogen clathrate cages: CaUH12-Cmmm, CaUH12-Fd-3m, Ca2UH18-P-3m1, and CaU3H32-Pm-3m. Among them, the Ca2UH18-P-3m1 and CaU3H32-Pm-3m are likely to be synthesized below 1 megabar. The f electrons in U atoms make dominant contribution to the electronic density of states around the Fermi energy. The electron-phonon interaction calculations reveal that phonon softening in the mid-frequency region can enhance the electron–phonon coupling significantly. The T c value of Ca2UH18-P-3m1 is estimated to be 57.5–65.8 K at 100 GPa. Our studies demonstrate that introducing actinides into alkaline-earth metal hydrides provides possibility in designing novel superconducting ternary hydrides.

On the dynamical stability of copper-doped lead apatite

The recent claim of room temperature superconductivity in a copper-doped lead apatite compound, called LK-99, has sparked remarkable interest and controversy. Subsequent experiments have largely failed to reproduce the claimed superconductivity, while theoretical works have identified multiple key features including strong electronic correlation, structural instabilities, and dopability constraints. A puzzling claim of several recent theoretical studies is that both parent and copper-doped lead apatite structures are dynamically unstable at the harmonic level, questioning decades of experimental reports of the parent compound structures and the recently proposed copper-doped structures. In this work, we demonstrate that both parent and copper-doped lead apatite structures are dynamically stable at room temperature. Anharmonic phonon–phonon interactions play a key role in stabilizing some copper-doped phases, while most phases are largely stable even at the harmonic level. We also show that dynamical stability depends on both volume and correlation strength, suggesting controllable ways of exploring the copper-doped lead apatite structural phase diagram. Our results fully reconcile the theoretical description of the structures of both parent and copper-doped lead apatite with the experiment.

Pressure-induced hydrogen-dominant high-temperature superconductors

The century-old pursuit of room temperature superconductivity has finally been reached in highly compressed hydrogen-dominant compounds. Future efforts will be focused on understanding the high-pressure hydrogen physics and ambient-pressure applications.

Ternary superhydrides for high-temperature superconductivity at low pressures

Focusing on the ternary hydrides, the new hope of Room-Temperature Superconductivity, this perspective delves into the research background, highlights current challenges, and illuminates promising avenues for future studies.

Exceptional metal-semiconductor-metal transition of lead apatites via oxygen defect tuning.

Lead apatites, distinguished and compelling bulk materials with the stoichiometric arrangement as Pb10(POx)6Oy, are renowned for their structural complexity. Recently, the discovery of possible room-temperature superconductivity under ambient pressure in copper-substituted lead apatites has engendered considerable interest within both the physics community and beyond. Nevertheless, exploration of pristine Pb10(POx)6Oy parent structures has hitherto remained elusive. In this study, we employ density functional theory (DFT) calculations to investigate the effects of oxygen defects on the electronic structures of Pb10(POx)6Oy and Pb9Cu(POx)6Oy. We scrutinize two distinct categories of defects: oxygen atoms enmeshed within POx groups (Ox) and solitary oxygen atoms (Oy). Our investigation uncovers a profound influence of these defects on the band structure. Specifically, the introduction of Oy defects prompts a remarkable transition in Pb10(PO4)6Oy from a metal to semiconductor to metal state, accompanied by pivotal shifts in the principal electronic contributors from p orbitals of Oy to those of Pb atoms. Furthermore, the introduction of Ox defects in Pb10(POx)6O1 engenders metamorphosis in the band structure, transmuting it from a semiconductor to a metallic state. Significantly, our findings pinpoint the suitable range of x in the Pb10(POx)6O1 configuration as lying between 2 and 4. Additionally, our study also demonstrates that the oxygen defects (Ox/Oy) do not affect the metallic properties of copper-substituted lead apatites. This study elucidates the significant role of oxygen defects in modulating the electronic properties of apatite materials, offering insights into potential interdisciplinary applications. This establishes a crucial link between material composition and electronic behavior, revealing key mechanisms for engineering functionality in lead apatites and other advanced materials.