High Superconducting Transition Temperature Research Articles

First-principles study of two-dimensional transition metal carbide Mn+1CnO2 (M=Nb,Ta).

In the present work, the three stable MXenes Mn+1CnO2 (M=Nb,Ta) are explored based on first-principles calculations. These materials are important derivatives of 2D materials and exhibit distinctive properties, holding vast potential in nanodevices. All these Mn+1CnO2 (M=Nb,Ta) materials exhibit outstanding superconducting performance, with corresponding superconducting transition temperatures of 23.00K, 25.00K, and 29.00K. Analysis reveals that the high superconducting transition temperatures of MXenes Mn+1CnO2 (M=Nb,Ta) are closely associated with the high value of the logarithmic average of phonon frequencies, !log, and the strong electron-phonon coupling (EPC), attributed to the crucial contribution of low-frequency phonons. Additionally, we applied strain treatments of 2% and 4% to Mn+1CnO2 (M=Nb,Ta), resulting in varying changes in superconducting transition temperatures under different strains.&#xD.

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.

Arrays of nano-high-transition temperature superconductor quantum interference devices

We report the fabrication and testing of arrays of nanoscale superconducting quantum interference devices (SQUIDs) directly written into a thin film of the high-transition temperature superconductor YBa2Cu-3O7−δ with a focused helium ion beam. We compare three array configurations with 400 nm by 400 nm nanoSQUIDs connected in series and parallel and a two-dimensional (2D) combination of both. Our electrical transport measurements show that series arrays of three nanoSQUIDs exhibit modulation voltages greater than 1 mV and that combining the devices in parallel greatly enhances the slope of the voltage–magnetic field characteristic. A 2D array with 3 SQUIDS in series and 7 in parallel exhibited a transfer function of 5.51 mV/mT.

Magnetic suppression for a possible Fe-poor organic–inorganic hybrid superconductor Fe14Se16(tepa)0.8 (tepa = tetraethylenepentamine) with a superconducting transition at ∼42 K

Composition/structure-dependent superconductivity for FeSe-based superconductors attracted great attention not only due to their high superconducting transition temperatures (TC), but also for understanding the origin of iron-based superconductivity. Here, we report a new Fe-poor organic-inorganic hybrid material Fe14Se16(tepa)0.8 with a paramagnetic-diamagnetic transition at ∼42 K grown by a high-temperature organic-solution-phase method with soluble iron/selenium sources in a tepa solution, alternative to previous intercalation strategies. The Fe14Se16(tepa)0.8 phase is in a tetragonal layered hybrid structure with a nanoplate shape. Composition analyses reveal a Fe-poor characteristic of the hybrid in contrast to previous FeSe-intercalated superconductor, and selected area electron diffraction pattern is featured by Fe3Se4 superstructures with a √2 × √2 of Fe vacancy order. Ab initio density functional calculations show that minus Fe3Se4 ions are stable in the hybrid and ∼0.25e−/Fe0.75Se is obviously larger than the reported values of approximately 0.2e−/FeSe in other FeSe-intercalated superconductors. Typical hysteresis loops and temperature dependence of dc/ac susceptibilities of the Fe14Se16(tepa)0.8 measured below ∼42 K suggest a presence of the Meissner effect in this material. Effects of synthesis conditions on structures and magnetic properties of the hybrids show a magnetic evolution from a long-range ferrimagnetic (FIM) order of Fe14Se16(tepa) to a coexistence of FIM and superconducting (SC) orders of Fe14Se16(tepa)0.9 and an SC order of Fe14Se16(tepa)0.8. X-ray absorption spectrum (XAS) confirms the presence of ferric/ferrous irons. Mössbauer studies reveal that the high-TC superconductivity originates from a suppression of the FIM order through tuning the spin states of irons from high-spin Fe3+ (S = 5/2) and Fe2+ (S = 2) in the Fe14Se16(tepa) to low-spin Fe3+ (S = 1/2) and Fe2+ (S = 0) in the Fe14Se16(tepa)0.8. Although no zero resistance is detected even at a temperature of 2 K, the resistivity at 2 K decreases by more than 1600 times compared to that in a normal state calculated by a variable range hopping (VRH) model, suggesting that the high-TC superconductivity of Fe14Se16(tepa)0.8 is possible.

Superconducting ternary hydrides: progress and challenges

ABSTRACT Since the discovery of the high-temperature superconductors H3S and LaH10 under high pressure, compressed hydrides have received extensive attention as promising candidates for room-temperature superconductors. As a result of current high-pressure theoretical and experimental studies, it is now known that almost all the binary hydrides with a high superconducting transition temperature (Tc) require extremely high pressure to remain stable, hindering any practical application. In order to further lower the stable pressure and improve superconductivity, researchers have started exploring ternary hydrides and had many achievements in recent years. Here, we discuss recent progress in ternary hydrides, aiming to deepen the understanding of the key factors regulating the structural stability and superconductivity of ternary hydrides, such as structural motifs, bonding features, electronic structures, electron–phonon coupling, etc. Furthermore, the current issues and challenges of superconducting ternary hydrides are presented, together with the prospects and opportunities for future research.

2D High-Temperature Superconductor Integration in Contact Printed Circuit Boards.

Inherent properties of superconducting Bi2Sr2CaCu2O8+x films, such as the high superconducting transition temperature Tc, efficient Josephson coupling between neighboring CuO layers, and fast quasiparticle relaxation dynamics, make them a promising platform for advances in quantum computing and communication technologies. However, preserving two-dimensional superconductivity during device fabrication is an outstanding experimental challenge because of the fast degradation of the superconducting properties of two-dimensional flakes when they are exposed to moisture, organic solvents, and heat. Herein, to realize superconducting devices utilizing two-dimensional (2D) superconducting films, we develop a novel fabrication technique relying on the cryogenic dry transfer of printable circuits embedded into a silicon nitride membrane. This approach separates the circuit fabrication stage requiring chemically reactive substances and ionizing physical processes from the creation of the thin superconducting structures. Apart from providing electrical contacts in a single transfer step, the membrane encapsulates the surface of the crystal, shielding it from the environment. The fabricated atomically thin Bi2Sr2CaCu2O8+x-based devices show a high superconducting transition temperature of Tc ≃ 91 K close to that of the bulk crystal and demonstrate stable superconducting properties.

Anisotropy and isotope effect in superconducting solid hydrogen

Elucidating the phase diagram of solid hydrogen is a key objective in condensed matter physics. Several decades ago, it was proposed that at low temperatures and high pressures, solid hydrogen would be a metal with a high superconducting transition temperature. This transition to a metallic state can happen through the closing of the energy gap in the molecular solid or through a transition to an atomic solid. Recent experiments have managed to reach pressures in the range of 400–500 GPa, providing valuable insights. There is strong evidence suggesting that metallization via either of these mechanisms occurs within this pressure range. Computational and experimental studies have identified multiple promising crystal phases, but the limited accuracy of calculations and the limited capabilities of experiments prevent us from determining unequivocally the observed phase or phases. Therefore, it is crucial to investigate the superconducting properties of all the candidate phases. Recently, we reported the superconducting properties of the C2/c-24, Cmca-12, Cmca-4 and I41/amd-2 phases, including anharmonic effects. Here, we report the effects of anisotropy on superconducting properties using Eliashberg theory. Then, we investigate the superconducting properties of deuterium and estimate the size of the isotope effect for each phase. We find that the isotope effect on superconductivity is diminished by anharmonicity in the C2/c-24 and Cmca-12 phases and enlarged in the Cmca-4 and I41/amd-2 phases. Our anharmonic calculations of the C2/c-24 phase of deuterium agree closely with the most recent experiment by Loubeyre et al (2022 Phys. Rev. Lett. 29 035501), indicating that the C2/c-24 phase remains the leading candidate in this pressure range, and has a strong anharmonic character. These characteristics can serve to distinguish among crystal phases in experiment. Furthermore, expanding our understanding of superconductivity in pure hydrogen holds significance in the study of high-Tc hydrides.

Superconductivity in two-dimensional MB4 (M = V, Nb, and Ta)

The study of two-dimensional (2D) superconductors is one of the most prominent areas in recent years and has remained a long-standing scientific challenge. Since the introduction of the new member of 2D material family consisting of transition-metal elements with Borides (MBenes), the study of various properties of 2D metal borides has attracted widespread interest. In this work, we systematically investigate the phonon-mediated superconductivity in 2D MB4 (M = V, Nb, and Ta), by using the first-principles calculations. Starting from the dynamical stabilities of these structures, we perform a detailed analysis of the electron–phonon coupling and superconductivity of AB-stacked MBenes by solving the anisotropic Eliashberg equation. NbB4 has the largest electron–phonon coupling and the highest superconducting transition temperature T c of 35.4 K. Our study broadens the 2D superconducting boride family, which is of great significance for the study of 2D superconductivity.

Synthesis of possible room temperature superconductor LK-99: Pb9Cu(PO4)6O

The quest for room temperature superconductors has been teasing scientists and physicists, since its inception in 1911 itself. Several assertions have already been made about room temperature superconductivity, but have never been verified or reproduced across the labs. The cuprates were the earliest high transition temperature (T c) superconductors, and it seems that copper has done the magic once again. In July 2023, a Korean group synthesized a lead apatite based compound LK-99, showing a T c of above 400 K (Lee et al 2023 arXiv: 2307.12008; 2023 arXiv: 2307.12037; Lee et al 2023 J. Korean Cryst. Growth Cryst. 33 61). The signatures of superconductivity in the compound are very promising, in terms of resistivity (ρ = 0) and diamagnetism at T c. Although, the heat capacity (C p) did not show the obvious transition at T c. Inspired by the interesting claims of the above room temperature superconductivity in LK-99, in this article, we report the synthesis of polycrystalline samples of LK-99, by following the same heat treatment as reported in Lee et al (2023 arXiv: 2307.12008; 2023 arXiv: 2307.12037) using a two-step precursor method. The phase is confirmed through x-ray diffraction measurements, performed after each heat treatment. The room temperature diamagnetism is not evidenced by the levitation of a permanent magnet over the sample or vice versa. The isothermal magnetization measurement at 280 K shows that as synthesized sample of LK-99 is paramagnetic. Further measurements for the confirmation of bulk superconductivity in variously synthesized samples are underway. Our results on the present LK-99 sample, synthesized at 925 °C, as of now do not confirm the appearance of bulk superconductivity at room temperature. Further studies with different heat treatments are underway.

Niobium Nitride Preparation for Superconducting Single-Photon Detectors.

Niobium nitride (NbN) is widely used in the production of superconducting nanowire single-photon detectors (SNSPDs) due to its high superconducting transition temperature and suitable energy gap. The processing parameters used for the preparation of NbN films and the subsequent processing of nanowires have a significant effect on the performance of the SNSPD. In this review, we will present various thin film growth methods, including magnetron sputtering, atomic layer deposition (ALD), and chemical vapor deposition (CVD). The relationships between the superconducting performance of each thin film and the corresponding deposition process will be discussed. Subsequently, NbN nanowire fabrication methods and microstructures based on thin film etching will be summarized, and their impact on the qualities of the finished SNSPDs will be systematically analyzed. Finally, we will provide an outlook for the future development of preparation for SNSPD.

Extremely Narrow Superconducting Band with Crystal Spin 32ℏ in LaH10

We show that the high-temperature superconductor LaH10 possesses an unusual superconducting band in its band structure. We examine this band by group-theoretical methods and report evidence that the special features of the band support the characteristics of the superconducting state in LaH10 at megabar pressures, i.e., the strong electron–phonon coupling and the high superconducting transition temperature.

Three-Dimensional Reconstruction of Nb3Sn Films by Focused Ion Beam Cross Sectional Microscopy

Niobium has been the material of choice for SRF cavities for several decades due to its formability and superconducting properties. The accelerating gradient of niobium cavities is, however, rapidly approaching a theoretical limit. To achieve higher accelerating gradients a new material is needed that can sustain high fields. Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn is a promising competitor with a higher superconducting transition temperature and a higher critical field than pure niobium. However, Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn is very brittle and cannot be formed readily into a cavity. The main method for creating Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cavities is to form a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn film into a niobium surface using a tin vapor-diffusion method. This technique creates a microcrystalline Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn thin film on the inner surface of the cavity. Tin depleted regions are known to form in the film during this process. Previous studies have analyzed these regions using transmission electron microscopy on cross-sectional lamellae prepared by focused ion beam/scanning electron microscope (FIB/SEM). This method does not provide any three-dimensional (3-D) information about the distribution of tin-deficient regions. In this study we employ a focused ion beam tomographic technique to analyze the 3-D structure of the film. Electron dispersive X-ray spectroscopy is used to image the tin concentration of the film in 3-D. Tin-deficient regions are discovered close to the surface of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn film.

Elemental Substitution at Tl Site of Tl1-xXx(Ba, Sr)CaCu2O7 Superconductor with X = Cr, Bi, Pb, Se, and Te.

The effects of elemental substitutions at the Tl site of a Tl1-xXx(Ba, Sr)CaCu2O7 superconductor with X = Cr, Bi, Pb, Se, and Te were investigated. This study aimed to determine the elements that enhance and suppress the superconducting transition temperature of the Tl1-xXx(Ba, Sr)CaCu2O7 (Tl-1212) phase. The selected elements belong to the groups of transition metal, post-transition metal, non-metal, and metalloid. The relationship between the transition temperature and ionic radius of the elements was also discussed. The samples were prepared by the solid-state reaction method. The XRD patterns showed a single Tl-1212 phase was formed in the non- and Cr-substituted (x = 0.15) samples. The Cr-substituted samples (x = 0.4) showed a plate-like structure with smaller voids. The highest superconducting transition temperatures (Tc onset, Tcχ', and Tp) were also achieved by the Cr-substituted samples for x = 0.4 compositions. However, the substitution of Te suppressed the superconductivity of the Tl-1212 phase. Jc inter (Tp) for all samples was calculated to be in the range of 12-17 A/cm2. This work shows that substitution elements with a smaller ionic radius tend to be more favorable in improving the superconducting properties of the Tl-1212 phase.

Light-induced giant Rashba spin-orbit coupling at superconducting KTaO3 (110) heterointerfaces.

The two-dimensional electron system (2DES) at KTaO3 surface or heterointerface with 5d orbitals hosts extraordinary physical properties including the stronger Rashba spin-orbit coupling (RSOC), higher superconducting transition temperature, and potential of topological superconductivity. Herein, we report a huge enhancement of RSOC under light illumination achieved at a superconducting amorphous-Hf0.5 Zr0.5 O2 /KTaO3 (110) heterointerfaces. The superconducting transition is observed with Tc = 0.62 K and temperature dependent upper critical field reveals the interaction between spin-orbit scattering and superconductivity. A strong RSOC with Bso =1.9 T is revealed by weak antilocalization in normal state, which undergoes sevenfold enhancement under light illumination. Furthermore, RSOC strength develops a dome-shaped dependence of carrier density with the maximum of Bso =12.6 T achieved near the Lifshitz transition point nc ∼ 4.1×1013 cm-2 . The highly tunable giant RSOC at KTaO3 (110)-based superconducting interfaces show great potential for spintronics. This article is protected by copyright. All rights reserved.

Superconductivity Observed in Tantalum Polyhydride at High Pressure

We report experimental discovery of tantalum polyhydride superconductor. It was synthesized under high-pressure and high-temperature conditions using diamond anvil cell combined with in situ high-pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature T c was found to be ∼ 30 K at 197 GPa in the sample that was synthesized at the same pressure with ∼ 2000 K heating. The transitions are shifted to low temperature upon applying magnetic fields that support the superconductivity nature. The upper critical field at zero temperature μ 0 H c2(0) of the superconducting phase is estimated to be ∼ 20 T that corresponds to Ginzburg–Landau coherent length ∼ 40 Å. Our results suggest that the superconductivity may arise from phase of TaH3. It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transition metals.

УРАВНЕНИЕ ЭЛИАШБЕРГА, ЗАПИСАННОЕ НА МНИМОЙ ОСИ ДЛЯ ИЗОТРОПНОЙ СРЕДЫ: АЛГОРИТМ РЕШЕНИЯ И ПРИМЕРЫ РАСЧЕТОВ

At present, the task of searching for compounds with a high superconducting transition temperature ТС is a very relevant scientific direction. Usually, the calculation of ТС is performed by numerically solving the system of Eliashberg equations. This system is a system of integral equations, which, by discretizing the integrals included in it, is reduced to a system of transcendental non-linear equations, suitable for a numerical solution. In this paper, we present a set of programs for solving this system written in various forms on the imaginary axis for an isotropic medium. In addition, formulas for the Jacobian of this system are presented for the first time, the use of which in numerical calculations increases the speed and stability of calculations. As an example of the developed methods applications, the results of calculations of ТС and thermodynamic properties for the metallic hydrogen I41/amd phase under a pressure of 500 GPa are given. Also, the results of calculations of ТС for some metallic hydrides superconducting at high pressures, are given.

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 times ;10^{5} A/cm2.

Prediction of π-electrons mediated high-temperature superconductivity in monolayer LiC12

Prediction and synthesis of two-dimensional high transition temperature (T C) superconductors is an area of extensive research. Based on calculations of the electronic structures and lattice dynamics, we predict that graphene-like layered monolayer LiC12 is a π-electrons mediated Bardeen–Cooper–Schrieffer-type superconductor. Monolayer LiC12 is theoretically stable and expected to be synthesized experimentally. From the band structures and the phonon dispersion spectrum, it is found that the saddle point of π-bonding bands induces large density of states at the Fermi energy level. There is strongly coupled between the vibration mode in the in-plane direction of the lithium atoms and the π-electrons of carbon atoms, which induces the high-T C superconductivity in LiC12. The T C can reach to 41 K under an applied 10% biaxial tensile strain based on the anisotropic Eliashberg equation. Our results show that monolayer LiC12 is a good candidate as π-electrons mediated electron-phonon coupling high-T C superconductor.

Superconductivity in an Orbital‐Reoriented SnAs Square Lattice: A Case Study of Li0.6Sn2As2 and NaSnAs

AbstractSearching for functional square lattices in layered superconductor systems offers an explicit clue to modify the electron behavior and find exotic properties. The trigonal SnAs3 structural units in SnAs‐based systems are relatively conformable to distortion, which provides the possibility to achieve structurally topological transformation and higher superconducting transition temperatures. In the present work, the functional As square lattice was realized and activated in Li0.6Sn2As2 and NaSnAs through a topotactic structural transformation of trigonal SnAs3 to square SnAs4 under pressure, resulting in a record‐high Tc among all synthesized SnAs‐based compounds. Meanwhile, the conductive channel transfers from the out‐of‐plane pz orbital to the in‐plane px+py orbitals, facilitating electron hopping within the square 2D lattice and boosting the superconductivity. The reorientation of p‐orbital following a directed local structure transformation provides an effective strategy to modify layered superconducting systems.

Superconductivity in an Orbital-Reoriented SnAs Square Lattice: A Case Study of Li0.6 Sn2 As2 and NaSnAs.

Searching for functional square lattices in layered superconductor systems offers an explicit clue to modify the electron behavior and find exotic properties. The trigonal SnAs3 structural units in SnAs-based systems are relatively conformable to distortion, which provides the possibility to achieve structurally topological transformation and higher superconducting transition temperatures. In the present work, the functional As square lattice was realized and activated in Li0.6 Sn2 As2 and NaSnAs through a topotactic structural transformation of trigonal SnAs3 to square SnAs4 under pressure, resulting in a record-high Tc among all synthesized SnAs-based compounds. Meanwhile, the conductive channel transfers from the out-of-plane pz orbital to the in-plane px +py orbitals, facilitating electron hopping within the square 2D lattice and boosting the superconductivity. The reorientation of p-orbital following a directed local structure transformation provides an effective strategy to modify layered superconducting systems.