high-Tc Superconductors Research Articles

Effects of pressure and doping on Ruddlesden-Popper phases Lan+1NinO3n+1

Recently, the discovery of superconductivity with a critical temperature Tc up to 80 K in Ruddlesden–Popper phases Lan+1NinO3n+1 (n = 2) under pressure has garnered considerable attention. Up to now, the superconductivity was only observed in La3Ni2O7 single crystal grown with the optical-image floating zone furnace under oxygen pressure. It remains to be understood the effect of chemical doping on superconducting La3Ni2O7 as well as other Ruddlesden–Popper phases. Here, we systematically investigate the effect of external pressure and chemical doping on polycrystalline Ruddlesden–Popper phases. Our results demonstrate that the application of pressure and doping effectively tunes the transport properties of Ruddlesden–Popper phases. We find pressure-induced superconductivity up to 86 K in La3Ni2O7 polycrystalline sample, while no signatures of superconductivity are observed in La2NiO4 and La4Ni3O10 polycrystalline samples under high pressure up to 50 GPa. Our study sheds light on the exploration of high-Tc superconductivity in nickelates.

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 insights into the structural, elastic, bonding, and thermophysical properties of UHx (x = 1, 2, 3, 5, 6, 7, 8) under pressure: Possible relevance to high-Tc superconductivity

Binary uranium hydrides, UHx (x = 1, 2, 3, 5, 6, 7, 8), with different crystal symmetries are potentially interesting compounds for high-Tc superconductivity and as hydrogen storage systems. In this work we have explored the structural, elastic, mechanical, bonding, and thermophysical properties of these systems under uniform pressure via density functional theory based computations. Most of the results disclosed in this work are novel. From the calculations of the cohesive energy and enthalpy of formation, we have found that the titled compounds are thermodynamically stable. The computed elastic constants at different pressures ensure elastic stability. All the binary hydrides are mechanically anisotropic. Pressure induced brittle-ductile transition takes place under high pressure. The compounds are machinable with the cubic α-UH3-Pm-3n showing very high value of the machinability index. All the compounds are fairly hard with cubic UH8 showing superhard character. The Debye temperatures and acoustic velocities of these compounds are high; the highest value is found for the cubic UH8. This is highly relevant to the predicted high-Tc superconductivity in cubic UH8 under hydrostatic pressure. The melting temperature, Grüneisen parameter, minimal phonon thermal conductivity, and the thermal expansion coefficient of these compounds have also been studied at different pressures. All these parameters show excellent correspondence with the estimated Debye temperature, elastic parameters and bonding characteristics.

The ternary phase diagram of nitrogen doped lutetium hydrides can not explain its claimed high Tc superconductivity

This paper presents the results of an extensive structural search of ternary solids containing lutetium, nitrogen and hydrogen. Based on thousands of thermodynamically stable structures the convex hull of the formation enthalpies is constructed. To obtain the correct energetic ordering, the highly accurate RSCAN DFT functional is used in high quality all-electron calculations, eliminating possible pseudopotential errors. In this way, a novel lutetium hydride structure (HLu2) is found that is on the convex hull. An electron phonon analysis however shows that it is not a candidate structure for near ambient superconductivity. Besides this structure, which appears to have been missed in previous searches, possibly due to different DFT methodologies, our results agree closely with the results of previously published structure search efforts. This shows, that the field of crystal structure prediction has matured to a state where independent methodologies produce consistent and reproducible results, underlining the trustworthiness of modern crystal structure predictions. Hence it is quite unlikely that a structure, that would give rise within standard BCS theory to the superconducting properties, claimed to have been observed by Dasenbrock-Gammon et al (2023 Nature 615 244), exists. This solidifies the evidence that structures with high Tc conventional superconductivity, that could give rise to the experimental claims, do not exist in this material.

Non-equilibrium dynamics of spin-lattice coupling

Quantifying the dynamics of normal modes and how they interact with other excitations is of central importance in condensed matter. Spin-lattice coupling is relevant to several sub-fields of condensed matter physics; examples include spintronics, high-Tc superconductivity, and topological materials. However, experimental approaches that can directly measure it are rare and incomplete. Here we use time-resolved X-ray diffraction to directly access the ultrafast motion of atoms and spins following the coherent excitation of an electromagnon in a multiferroic hexaferrite. One striking outcome is the different phase shifts relative to the driving field of the two different components. This phase shift provides insight into the excitation process of such a coupled mode. This direct observation of combined lattice and magnetization dynamics paves the way to access the mode-selective spin-lattice coupling strength, which remains a missing fundamental parameter for ultrafast control of magnetism and is relevant to a wide variety of materials.

A Quenched Disorder in the Quantum-Critical Superconductor CeCoIn5.

Emergent inhomogeneous electronic phases in metallic quantum systems are crucial for understanding high-Tc superconductivity and other novel quantum states. In particular, spin droplets introduced by nonmagnetic dopants in quantum-critical superconductors (QCSs) can lead to a novel magnetic state in superconducting phases. However, the role of disorders caused by nonmagnetic dopants in quantum-critical regimes and their precise relation with superconductivity remain unclear. Here, the systematic evolution of a strong correlation between superconductive intertwined electronic phases and antiferromagnetism in Cd-doped CeCoIn5 is presented by measuring current-voltage characteristics under an external pressure. In the low-pressure coexisting regime where antiferromagnetic (AFM) and superconducting (SC) orders coexist, the critical current (Ic ) is gradually suppressed by the increasing magnetic field, as in conventional type-II superconductors. At pressures higher than the critical pressure where the AFM order disappears, Ic remarkably shows a sudden spike near the irreversible magnetic field. In addition, at high pressures far from the critical pressure point, the peak effect is not suppressed, but remains robust over the whole superconducting region. These results indicate that magnetic islands are protected around dopant sites despite being suppressed by the increasingly correlated effects under pressure, providing a new perspective on the role of quenched disorders in QCSs.

Transport Properties of Strongly Correlated Fermi Systems

Physicists are actively debating the nature of the quantum critical phase transition that determines the low-temperature properties of metals with heavy fermions. Important experimental observations of their transport properties incisively probe the nature of the quantum critical phase transition. In our short review, we consider the transport properties of strongly correlated Fermi systems like heavy fermion metals and high—Tc superconductors. Their transport properties are defined by strong inter-particle interactions, forming flat bands in these compounds. These properties do not coincide with those of conventional metals. Indeed, in contrast to the behavior of the transport properties of conventional metals, the strongly correlated compounds exhibit linear temperature resistivity ρ(T)∝T. We analyze the magnetoresistance and show that under the application of the magnetic field, it becomes negative. It is shown that near a quantum phase transition, when the density of the electronic states diverges, semiclassical physics remains applicable to describe the resistivity ρ of strongly correlated metals due to the presence of a transverse zero-sound collective mode, representing the phonon mode in solids. We demonstrate that when T exceeds the extremely low Debye temperature TD, the resistivity ρ(T) changes linearly with T since the mechanism of formation of the T-dependence ρ(T) is a similar electron-phonon mechanism, which predominates at high temperatures in ordinary metals. Thus, in the region of T-linear resistance, electron-phonon scattering leads to a lifetime of τ quasiparticles practically independent of the material, which is expressed as the ratio of the Planck constant ℏ to the Boltzmann constant kB, Tτ∼ℏ/kB. We explain that due to the non-Fermi-liquid behavior, the real part of the frequency-dependent optical conductivity σoptR(ω) exhibits a scaling behavior and demonstrates the unusual power law behavior σoptR(ω)∝ω−1, rather than the well-known one shown by conventional metals, σoptR(ω)∝ω−2. All our theoretical considerations are illustrated and compared with the corresponding experimental facts. Our results are in a good agreement with experimental observations.

Rotational symmetry breaking in superconducting nickelate Nd0.8Sr0.2NiO2 films

The infinite-layer nickelates, isostructural to the high-Tc cuprate superconductors, have emerged as a promising platform to host unconventional superconductivity and stimulated growing interest in the condensed matter community. Despite considerable attention, the superconducting pairing symmetry of the nickelate superconductors, the fundamental characteristic of a superconducting state, is still under debate. Moreover, the strong electronic correlation in the nickelates may give rise to a rich phase diagram, where the underlying interplay between the superconductivity and other emerging quantum states with broken symmetry is awaiting exploration. Here, we study the angular dependence of the transport properties of the infinite-layer nickelate Nd0.8Sr0.2NiO2 superconducting films with Corbino-disk configuration. The azimuthal angular dependence of the magnetoresistance (R(φ)) manifests the rotational symmetry breaking from isotropy to four-fold (C4) anisotropy with increasing magnetic field, revealing a symmetry-breaking phase transition. Approaching the low-temperature and large-magnetic-field regime, an additional two-fold (C2) symmetric component in the R(φ) curves and an anomalous upturn of the temperature-dependent critical field are observed simultaneously, suggesting the emergence of an exotic electronic phase. Our work uncovers the evolution of the quantum states with different rotational symmetries in nickelate superconductors and provides deep insight into their global phase diagram.

A simple study for an optimized operation of the ITER Cryodistribution cold rotating machines

The ITER superconducting (SC) magnet system is composed of 4 distinct units: the Central Solenoid (CS) coils, the Toroidal Field (TF) coils, the coil encasing and supporting Structures (ST), and the Poloidal Field and Correction Coils (PF&CC or simply PF). Including another cold component unit, which is the Cryopump (CP) system, each unit is assigned its own Auxiliary Cold Box (ACB) with cold rotating machines (CRMs) to ensure efficient operation from a cryogenic perspective. The CRMs, which are the cold circulator (CCr) and cold compressor (CCp), are respectively in charge of generating the high mass flow rate (2.0–2.7 kg/s) and low temperature (3.7–4.2 K) of supercritical helium (SHe) that is supplied to each magnet unit and CP system. To maintain the proper cryogenic conditions of the magnets and CPs during fusion experiments, the ITER Cryogenic System can produce an average cooling power of 75 kW at 4.5 K, including the need for the 50 K gaseous helium (GHe) cooled high-Tc SC current leads. Depending on the operation mode of the ITER Tokamak, up to 28 kW of that power can be consumed by the CRMs due to compression of helium.In this study, we will explore possibilities for optimized operation of the CRMs by adjusting the mass flow rate and temperature of the SHe to minimize the heat of compression while maintaining the thermal stability of the ITER SC magnets and CP. By doing so, the CRM-heat-of-compression at 4.5 K can be reduced by more than 3 kW, providing additional “cryogenic” margins for higher-heat-load plasma experiments.

Effective Bi-Layer Model Hamiltonian and Density-Matrix Renormalization Group Study for the High-T c Superconductivity in La3Ni2O7 under High Pressure

High-T c superconductivity with possible T c ≈ 80 K has been reported in the single crystal of La3Ni2O7 under high pressure. Based on the electronic structure given by the density functional theory calculations, we propose an effective bi-layer model Hamiltonian including both 3d z 2 and 3d x 2–y 2 orbital electrons of the nickel cations. The main feature of the model is that the 3d z 2 electrons form inter-layer σ-bonding and anti-bonding bands via the apical oxygen anions between the two layers, while the 3d x 2–y 2 electrons hybridize with the 3d z 2 electrons within each NiO2 plane. The chemical potential difference of these two orbital electrons ensures that the 3d z 2 orbitals are close to half-filling and the 3d x 2–y 2 orbitals are near quarter-filling. The strong on-site Hubbard repulsion of the 3d z 2 orbital electrons gives rise to an effective inter-layer antiferromagnetic spin super-exchange J. Applying pressure can self dope holes on the 3d z 2 orbitals with the same amount of electrons doped on the 3d x 2–y 2 orbitals. By performing numerical density-matrix renormalization group calculations on a minimum setup and focusing on the limit of large J and small doping of 3d z 2 orbitals, we find the superconducting instability on both the 3d z 2 and 3d x 2–y 2 orbitals by calculating the equal-time spin singlet pair–pair correlation function. Our numerical results may provide useful insights in the high-T c superconductivity in single crystal La3Ni2O7 under high pressure.

Zeeman splitting in doped antiferromagnetic semiconductors

Zeeman splitting of conduction electrons in doped antiferromagnetic materials in the presence of a strong spin–orbit interaction is considered. Using symmetry arguments the Hamiltonian and the spectrum of conduction electrons near some time-reversal invariant momenta were constructed. It was shown, that in the vicinity of some time-reversal invariant momenta the external magnetic field perpendicular to the Neel axis does not lead to the Zeeman splitting. The Kramers degeneracy is preserved in that case. The absence of the Zeeman splitting in antiferromagnets leads to the absence of spin zeros in quantum oscillation experiments. Recent experimental confirmation of the predicted effects in organic antiferromagnet and High-Tc Superconductors is discussed.

Superconducting joints for the 1.3 GHz persistent NMR magnet under JST-Mirai Program

Superconducting joint technologies connecting high-T c superconducting (HTS) tapes, such as REBCO-coated conductors and Bi2223 silver-sheathed tapes, have been developed for the fabrication of superconducting magnets generating 30.5 T with persistent current operation at 4.2 K under the JST-Mirai Program since 2017. These are indirect joints using a superconducting intermediate layer to join highly crystallized HTS layers of tapes. Thus far, high I c values of above 100 A at 77 K in the self-field have been achieved by both joints. These performances are sufficient for installation to the 30.5 T magnet, while efforts to further improve the reproducibility of high I c for joints connecting long-length HTS tapes have been continuously made. The current status of HTS joint technologies is summarized in this paper partly from the view point of material science.

Enhanced cuprate superconductivity from elongated to compressed coordination

In 1986 Professor Müller and Bednorz discovered the historic superconductor of LaBaCuO (La214) that opens the prelude of unprecedented era for high Tc superconductivity research worldwide. The Jahn Teller effects combining with strong interlayer Coulomb interactions lead to the elongated copper oxygen coordination in La214, giving rise to the orbital order scenario that the Cu3dx2-y2 locates near Fermi surface while the 3dz2 is far below without contributions to the conductivity, hence single band model is favorite. We recently discovered a new type of cuprate superconductor Ba2CuO4-y (Ba214) synthesized at extreme high oxygen pressure that shares a similar crystal structure to the classical La214 but with notably a very rare compressive type octahedron. The compressed octahedron coordination results from the longer in plane CuO bond length than that for apical oxygen bond. The compressed octahedron will lead to a reversed orbital order, i.e. the 3dz2 will be lifted above 3dx2-y2,which is generally considered unfavorable to superconductivity based on the current theories of high Tc cuprates. Ba214 however exhibits bulk superconducting transition with Tc above 70 K at ambient pressure, a surprisingly 80 % enhancement from that for isostructural La214. Here we briefly introduce the unique structure features as well research progress for Ba214, indicating the paradigm shifts roles of 3dz2 orbital in (I) enhancing cuprate superconductivity, (II) favoring multiband scenario; (III) enabling the s wave pairing mechanism.

Molecular insights into the role of tetrafluoromethane in mitigating the boiling crisis suffered by liquid nitrogen in superconducting apparatus

Nitrogen/tetrafluoromethane (N2/CF4) has emerged as an effective refrigerant mixture, which plays a significant role in various high-Tc superconducting (HTS) power devices and energy systems. Several studies have documented the superior performance of such zeotropic binary mixtures in heat transfer and energy conservation compared to pure liquids. However, boiling heat transfer characteristics of liquid mixtures on the molecular scale are not fully understood, especially the mechanisms associated with the role of the additive in regulating thermal properties. Here, we performed the molecular dynamics simulations for the pure N2 and the mixtures containing 20 and 40 mol% of CF4 to probe into the boiling heat transfer process on an ideal copper substrate. Analyses suggest that the boiling process of the N2/CF4 mixture shares similar features with the pure N2, but the binary refrigerant manifests advantages at higher substrate temperatures. Specifically, the additive CF4 delays the onset of film boiling, and the operational temperature range could be enlarged by almost 40% compared to pure N2 in terms of the 40 mol% mixture. Moreover, the mixture with CF4 additive maintains a small interfacial resistance even if the substrate temperature exceeds the critical value of film boiling for pure N2, highlighting the potential to use such mixtures for devices that may suffer from high heat flux levels. Finally, analyses regarding the mixture composition and the solid-liquid interactions confirm the essential role of the additive CF4 in mitigating the mismatch of the vibrational density of states at a high substrate temperature, which may reveal the mechanisms concerning the CF4 improving the heat transfer performance of the original N2. These findings provide a better understanding of the advantage of N2/CF4 at high substrate temperatures, and they lay a foundation for designing the cooling systems of HTS apparatuses.

Investigating the Cuprates as a platform for high-order Van Hove singularities and flat-band physics

Beyond the two-dimensional saddle-point Van Hove singularities (VHSs) with logarithmic divergences in the density of states, recent studies have identified higher-order VHSs with faster-than-logarithmic divergences that can amplify electron correlation effects. Here we show that the cuprate high-Tc superconductors harbor high-order VHSs in their electronic spectra and unveil a new correlation that the cuprates with high-order VHSs display higher Tc’s. Our analysis indicates that the normal and higher-order VHSs can provide a straightforward new marker for identifying propensity of a material toward the occurrence of correlated phases such as the excitonic insulators and supermetals. Our study suggests cuprates and related high-Tc superconductors as materials for exploring the interplay between high-order VHSs, superconducting transition temperatures, and electron correlation effects.

An exponential-potential description for the guest–host interactions in rattler-containing cage materials

Anomalous low-energy excitations (ALEs) associated with the vibrations of atoms accommodated in a space with large freedom of motion are frequently observed in cage materials. Such ALEs have been of great interest for their important roles in large electron–phonon coupling targeting high-TC superconductivity as well as phonon–phonon scattering giving rise to low thermal conductivity to achieve high thermoelectric performance. In our previous work, we showed that van der Waals-like repulsive interactions between the atoms forming the host cage and the atoms accommodated inside the cage (rattler) play a crucial role in giving small force constants and consequently nearly dispersionless low-energy phonons. In the present work, we show that the exponential law derived from the revised Morse potential model that we used previously could provide a universal trend for the ALEs of three typical cage materials: clathrates, skutterudites, and pyrochlores. This not only confirms the effectiveness of the potential model but also provides a clear description for different types of rattler vibrational modes based on the deduced parameters from the potential model. In addition to the free space in which a rattler can move and the mass of the rattler, which we considered in our previous work, structural anisotropy, electronegativity of the component cage atoms, and the coordination number of the rattler are shown to be important factors for description of an ALE.

High-temperature superconductivity and strange metallicity: Simple observations with (possibly) profound implications

Bednorz and Müller's discovery of superconductivity in the cuprate perovskite La2-xBaxCuO4 gave the modern condensed matter community arguably two of its greatest gifts – high-temperature superconductivity and strange metallicity; certainly two of its most profound and challenging problem. While it was the multi-fold increase in Tc that rightly catapulted cuprates to the summit of global media attention and scientific curiosity, it is the nature of their normal (non-superconducting) state that has sustained interest over the intervening years. This in turn has inspired condensed-matter experimentalists and theoreticians to revolutionise their respective fields. Yet despite such boundary-pushing endeavors, the origin of both phenomena remains elusive. In this brief note, I offer a personal reflection on the link between strange metallicity and high-Tc superconductivity, highlighting a number of simple observations that may hold profound implications for their ultimate resolution.

Spin resonances in heavy-fermion superconductors

Historically heavy-fermion superconductors have been the first discovered unconventional superconductors and can to some extent be regarded as model systems for high-Tc superconductors. A hallmark of the unconventional superconductivity in high-Tc cuprates has been the observation of a spin resonance in the magnetic excitation spectrum probed by inelastic neutron scattering. We review the situation in heavy-fermion superconductors, the possible existence of a spin resonance in the superconducting state and its implications.

Energy-scale considerations of unconventional superconductors—implications to condensation and pairing

Discovery of high-Tc cuprate superconductors (HTSC) in 1986 by Bednorz and Muller, followed by synthesis of A3C60, iron-pnictides/chalcogenides and other exotic superconducting (SC) systems, introduced unconventional superconductors (UCSC) having their mechanisms of condensation and/or pairing distinctly different from those of simpler metals which can be explained by BCS theory. This article will show how one can demonstrate their new mechanisms by examining correlations among key energy-scale parameters, including the transition temperature Tc, the superfluid density ns/m*, the effective Fermi energy εF, the excitation energy of the magnetic resonance mode (MRM), the onset temperatures of Nernst effect and light-induced transient superconductivity, and the spin fluctuation energy scale ℏωsf, and by resorting to analogy / comparisons with superfluid 4He as a representative system undergoing Bose Einstein Condensation (BEC). We will propose a paring mechanism in HTSC based on resonance of spin (ℏωsf) and charge (εF) energy scales, and apply that concept for explaining unusual behaviors in the overdoped region. We will also discuss modifications of a simple BEC-BCS crossover picture to account for actual situations with additional effects of competing order and phase separation.

In Pursuit of the Exceptional: Research Directions for Machine Learning in Chemical and Materials Science.

Exceptional molecules and materials with one or more extraordinary properties are both technologically valuable and fundamentally interesting, because they often involve new physical phenomena or new compositions that defy expectations. Historically, exceptionality has been achieved through serendipity, but recently, machine learning (ML) and automated experimentation have been widely proposed to accelerate target identification and synthesis planning. In this Perspective, we argue that the data-driven methods commonly used today are well-suited for optimization but not for the realization of new exceptional materials or molecules. Finding such outliers should be possible using ML, but only by shifting away from using traditional ML approaches that tweak the composition, crystal structure, or reaction pathway. We highlight case studies of high-Tc oxide superconductors and superhard materials to demonstrate the challenges of ML-guided discovery and discuss the limitations of automation for this task. We then provide six recommendations for the development of ML methods capable of exceptional materials discovery: (i) Avoid the tyranny of the middle and focus on extrema; (ii) When data are limited, qualitative predictions that provide direction are more valuable than interpolative accuracy; (iii) Sample what can be made and how to make it and defer optimization; (iv) Create room (and look) for the unexpected while pursuing your goal; (v) Try to fill-in-the-blanks of input and output space; (vi) Do not confuse human understanding with model interpretability. We conclude with a description of how these recommendations can be integrated into automated discovery workflows, which should enable the discovery of exceptional molecules and materials.