High Critical Temperature Research Articles

Direct evidence of high Tc superconductivity in glossy carbon films

Recent achievements and predictions in the field of intrinsic superconductivity in column IV-based covalent semiconductors and the nature of coupling, which determines the prospects for higher transition temperatures as well as the influence of disorder, are debated issues that can determine the future of the field. With this context, here we report the experimental findings of intrinsic superconductivity in glossy carbon (GC) films. These films are one of the forms of carbon materials obtained by pyrolysis of Pongamia glabra oil under high pressure and temperature. A zero-resistance state for films was observed at a critical Zero Resistance temperature of 134 K. Such relatively high superconducting critical temperature of films puts it among the superconductors with the strongest pairing strength between electrons and therefore an ideal material for investigations of high critical temperature superconductors.

Ca2FeOsO6/Sr2FeOsO6 superlattice: Multiferroicity above room temperature with giant hybrid-improper ferroelectric polarization and high photovoltaic efficiency

Using first-principles calculations, the structure and electronic properties of the room-temperature ferrimagnetic ${\mathrm{Ca}}_{2}{\mathrm{FeOsO}}_{6}/{\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ superlattice are investigated. We show that the superlattice hosts hybrid-improper ferroelectricity despite the fact that bulk ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ realizes an ${a}^{0}{a}^{0}{c}^{\ensuremath{-}}$ tilting pattern of the O octahedra. The magnitude is comparable to that of conventional ferroelectric materials and is found to increase under both compressive and tensile strain. In contrast to competing superlattices, a ferrimagnetic critical temperature above room temperature is realized. An indirect-to-direct band-gap transition is observed between $+$1% and $+$2% strain, coming along with localization of the valence and conduction states on different transition-metal sublattices, which enables efficient electron-hole separation upon photoexcitation. The potential gradient due to the ferroelectric polarization supports the electron-hole separation and a spectroscopic limited maximum efficiency of 27% confirms excellent potential in solar cell applications. The tunable room-temperature ferroelectricity, high critical temperature of the ferrimagnetic ordering with high magnetization, and favorable photoabsorption properties of the ${\mathrm{Ca}}_{2}{\mathrm{FeOsO}}_{6}/{\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ superlattice open up a broad range of technological applications.

Topological superfluid of s -wave-interacting fermions by engineered orbital hybridization in an optical lattice

Recent advanced experimental implementations of optical lattices with highly tunable geometry open up new regimes for exploring quantum many-body states of matter that had not been accessible previously. Here we report that a topological fermionic superfluid with higher Chern number emerges spontaneously from $s$-wave spin-singlet pairing in an orbital optical lattice when its geometry is tuned to explicitly break reflection symmetry. Qualitatively distinct from the conventional scheme that relies on higher partial-wave pairing, the crucial ingredient of our model is topology originating from mixing higher Wannier orbitals. It leads to unexpected changes in the topological band structure at the single-particle level, i.e., the bands are transformed from possessing two flux-$\ensuremath{\pi}$ Dirac points into a single quadratic touching point with flux $2\ensuremath{\pi}$. Based on such engineered single-particle bands, spin-singlet pairing of ultracold fermions arising from standard $s$-wave attractive interaction is found to induce higher Chern number (Chern number of 2) and topologically protected chiral edge modes, all occurring at a higher critical temperatures in relative scales, potentially circumventing one of the major obstacle for its realization in ultracold gases.

Evaluation of CO2-doped blends in single-stage with IHX and parallel compression refrigeration architectures

CO2 is the standard for medium to large-sized commercial applications, as it combines security and low environmental impact. However, it requires the use of advanced and complex cycles. Recently, CO2-doping (the addition of a small quantity of another fluid) has attracted scientific attention, as when CO2 is mixed with fluids with higher critical temperatures, the optimum operation moves to subcritical, providing COP increments in relation to pure-CO2 operation. This work, from a theoretical perspective, evaluates CO2-doping with the fluids R-152a, R-1234yf, R-1234ze(E) and R-1233zd(E) considering the two most used CO2 cycles: the base cycle with an internal heat exchanger (IHX) and the cycle with parallel compression (PC), fractionation taking place. The work analyses the COP improvements for an evaporating level of -10 °C and from 10 to 40 °C of environment temperature. Predicted maximum COP increments reach up to 5.8% for the IHX cycle and 10.0% for the PC cycle.

AlN/Si 기판 상에 성장된 바나듐 이산화물 박막 기반 전자 소자에서 광열 효과로 제어되는 고반복률 전류 스위칭

In this study, an electronic device based on a vanadium dioxide (VO₂) heterostructure thin film grown on an aluminum nitride(AlN)/silicon(Si) substrate was fabricated to realize high-repetition-rate current switching controlled by the photothermal effect. In the conventional VO₂/sapphire(Al₂O₃) device, the current switching controlled by the photothermal effect takes a long time for the cooling process to return the device to the off-state. This problem made high-repetition-rate switching impossible. On the other hand, it was assumed that the VO₂/AlN/Si device would be suitable for high-repetition-rate switching because it has a higher critical temperature and superior thermal conductivity than the VO₂/Al₂O₃ device. And, we demonstrated the photothermally-controlled current switching with a repetition rate of up to 60㎐ while maintaining a high on-state current of 50㎃ using an optimized experimental setup and anewly fabricated device. The switching response time was also improved to 1ms level.

Fabrication of Meter‐Long Class Fe(Se,Te)‐Coated Conductors with High Superconducting Performance

Previous research results have demonstrated that Fe(Se,Te) films with high critical current density Jc can be realized on flexible metal substrates and are promising for high field applications at 4.2 K. For practical applications, the length of the coated conductor is a key factor. Regarding the fabrication of long Fe(Se,Te)‐coated conductor tapes, the biggest difficulty is how to achieve a breakthrough from static coating to continuous dynamic coating process. Herein, it is demonstrated that Fe(Se,Te)‐coated conductor with high superconducting performance can be continuously fabricated. 1 m‐long Fe(Se,Te)‐coated conductors have been successfully fabricated by multiturn pulsed laser deposition on CeO2‐buffered ion beam‐assisted deposition MgO tape. The Fe(Se,Te)‐coated conductors have high critical temperature of 17.5 K and Jc over 2 MA cm−2 at 4.2 K and self‐field. The end‐to‐end critical current of 1 m‐long Fe(Se,Te)‐coated conductor is as high as 108 A cm−1‐width at 4.2 K and self‐field, and the corresponding Jc is 1.54 MA cm−2, which is above the threshold for industrial applications. The upper critical field at 0 K is deduced to be about 53 T. To the best of knowledge, this is the first demonstration of continuous fabrication of long Fe(Se,Te)‐coated conductors with high performance.

EFFECT OF SUPERHEATING DEGREE AND PRESSURE ON EFFICIENCY OF AN ORES (ORGANIC RANKINE ENERGY STORAGE) SYSTEM

The increased share of intermittent renewable energy sources has led to the development of new energy storage solutions to mitigate the effects of the variability of energy supply. CAES (Compressed Air Energy Storage) and LAES (Liquid Air Energy Storage) are two of the main solutions for medium to large-scale systems for long-term energy storage. However, both are limited; the first requires the availability of suitable geological formations and the second requires considerable investment because of the required liquefaction process. This work aims to evaluate the thermodynamic performance of an energy storage system, called Organic Rankine Energy Storage (ORES), with a focus on the effects of pressure and superheating degree at the expander inlet on the round-trip efficiency (i.e. ratio of generated energy during energy discharge over consumed energy during energy storage) of the system. The system was evaluated for six organic fluids which were selected based on commercial maturity, environmental impacts, and safety conditions, namely R-134a, R-152a, R-142b, R-236ea, R-365mfc, and R-141b. The efficiency of the system was obtained for each of those fluids using a steady-state model approximation of the operation of the system and for pressures at the expander inlet varying from 675 kPa up to 4,300 kPa (or 95% of the critical pressure, if lower) and for superheating degrees from 0 up to 40 K. The evaluation of the system for six organic fluids as working fluid resulted in round-trip efficiencies around 70 % (comparable to both CAES and LAES when subject to similar methodologies) with higher sensitivity to pressure than to superheating degree. For all fluids, an increase of 5 K in the superheating degree resulted in an absolute decrease of 2-5% in the round-trip efficiency. Effects of pressure were more diverse, R-152b, R-134a and R-142b showed an average reduction of 10% in efficiency for each reduction of 500 kPa in pressure (in the high efficiency operation region, while R-365mfc and R-141b were much less affected, around 5% decrease in efficiency for each reduction of 500 kPa. The fluids that had the highest efficiencies and that also presented a high efficiency for a wider range of pressures were the R141b and R365mfc, which are also the fluids with the highest critical temperatures.

Prediction of superconductivity in sandwich XB4 (X = Li, Be, Zn and Ga) films.

Borophenes and 2D boron allotropes are metallic and exhibit a BCS superconducting state, unlike graphene. In-plane stretching vibrational modes in bulk MgB2 boron layers induce phonon-mediated superconductivity. However, the effect of stretching vibrational phonon modes on transition temperature (Tc) still requires further investigations. Here, we use first-principles calculations combined with conventional BCS theory to explore the superconducting properties in a series of dynamically stable boron-based sandwich films that have not been realized experimentally. The sandwich films of XB4 (where X = Li, Be, Zn, Ga) are predicted to exhibit good phonon-mediated superconductivity with high Tc values of 25.1 K, 28.7 K, 38.7 K, and 36.2 K, respectively. The origin of the superconducting states is mainly caused by the high metallicity and strong electron-phonon coupling (EPC), which can be attributed to the presence of intercalated atoms within the borophene layers. It is further demonstrated in the XB4 compounds (where X = Li, Be, Zn, Ga) that the pronounced EPC is not solely attributable to the in-plane vibrations of B atoms, but it is also influenced significantly by the out-of-plane vibrations of B atoms. Sandwich (Li,Be,Zn,Ga)B4 films may be a great choice for nanoscale superconductors as the electron-phonon coupling parameter becomes greater than unity, thereby providing a powerful approach for investigating these systems with high critical temperatures.

Charge disproportionation-induced multiferroics and electric field control of magnetism in a 2D MXene - Mo2NCl2.

Two-dimensional (2D) magnetoelectric multiferroic materials with the coexistence of magnetization and ferroelectric polarization hold potential for application for the development of next-generation nano-memory devices. However, intrinsic 2D multiferroics with a high critical temperature and strong magnetoelectric coupling are still rare to date. Here, we propose a novel mechanism of 2D monolayer multiferroicity. Based on density functional theory (DFT), we predicted that in a Mo2NCl2 monolayer, the non-equilibrium charge disproportionation of Mo ions will induce an out-of-plane electric polarization, making this material a 2D monolayer multiferroic material. More importantly, the magnetic critical temperature is calculated to be ∼168 K, which is larger than those of the recently reported 2D multiferroic and ferromagnetic systems. Our findings also provide a promising platform to control the magnetic properties and electric behavior in 2D multiferroics using an external electric field.

Enhancement for phonon-mediated superconductivity up to 37 K in few-hydrogen metal-bonded layered magnesium hydride under atmospheric pressure.

The discovery of superconductivity in layered MgB2 has renewed interest in the search for high-temperature conventional superconductors, leading to the synthesis of numerous hydrogen-dominated materials with high critical temperatures (Tc) under high pressures. However, achieving a high-Tc superconductor under ambient pressure remains a challenging goal. In this study, we propose a novel approach to realize a high-temperature superconductor under ambient pressure by introducing a hexagonal H monolayer into the hexagonal close-packed magnesium lattice, resulting in a new and stable few-hydrogen metal-bonded layered magnesium hydride (Mg4)2H1. This compound exhibits superior ductility compared to multi-hydrogen, cuprate, and iron-based superconductors due to its metallic bonding. Our unconventional strategy diverges from the conventional design principles used in hydrogen-dominated covalent high-temperature superconductors. Using anisotropic Migdal-Eliashberg equations, we demonstrate that the stable (Mg4)2H1 compound is a typical phonon-mediated superconductor, characterized by strong electron-phonon coupling and an excellent Tc of 37 K under ambient conditions, comparable to that of MgB2. Our findings not only present a new pathway for exploring high-temperature superconductors but also provide valuable insights for future experimental synthesis endeavors.

Coexistence of superconductivity and charge density wave instability in A15-Nb3Sn.

A15-type compound Nb3Sn has attracted much attention due to its relatively high critical temperature and critical field of superconductivity, making it a leading material for superconducting applications. In this study, we investigate the structural instability and superconductivity of Nb3Sn under hydrostatic pressure using first-principles calculations. We determine the electronic properties, phonon dispersion, electron-phonon coupling and the superconducting gap for Nb3Sn at pressures ranging from ambient to 9 GPa. Our results show that a significant electron density is present near the Fermi level due to the van Hove singularity, indicating the strong electron-phonon coupling. The phonon dispersion of Nb3Sn exhibits Kohn anomalies at three different wave vectors at a lower temperature. Moreover, above a pressure of 6 GPa, the charge density wave (CDW) instability disappeared, suggesting that pressure inhibits the CDW phase. The superconducting temperature is predicted to be TC = 18.62 K under ambient conditions, which is well consistent with the experimental results. We find that both the CDW and superconducting orders respond to pressure, with their transition temperatures decreasing as the pressure increases below 6 GPa. Above 6 GPa, the superconducting transition temperature increases slowly with pressure. Our results suggest that the instability in Nb3Sn is driven by the softening of the phonon modes due to the CDW caused by strong electron-phonon coupling. Therefore, the CDW phase and superconducting phase of Nb3Sn coexist at low pressure.

An antiferromagnetic semiconducting FeCN2 monolayer with a large magnetic anisotropy and strong magnetic coupling.

Two-dimensional antiferromagnetic (AFM) materials with an intrinsic semiconductivity, a high critical temperature, and a sizable magnetic anisotropy energy (MAE) have attracted extensive attention because they show promise for high-performance spintronic nanodevices. Here, we have identified a new FeCN2 monolayer with a unique zigzag Fe chain through first-principles swarm structural search calculations. It is an AFM semiconductor with a direct band gap of 2.04 eV, a Néel temperature (TN) of 176 K, and a large in-plane MAE of 0.50 meV per Fe atom. More interestingly, the intrinsic antiferromagnetism, contributed by the strong magnetic coupling of neighbouring Fe ions, can be maintained under the external biaxial strains. A large cohesive energy and high dynamical stability favor synthesis and application. Therefore, these fascinating properties of the FeCN2 monolayer make it a promising nanoscale spintronic material.

Superconductivity Experiments Confirm an Important Mechanism for Increasing the Maximum Critical Temperature in Cuprates

Some cuprate superconductors exhibit high critical temperatures at atmospheric pressure. To understand the underlying processes, researchers recently carried out high-pressure nuclear magnetic resonance experiments on the cuprate YBa2Cu3O6+y to determine the charge densities at the Cu and O sites of its CuO2 plane with high precision. Their results revealed that the more generous the in-plane Cu atoms are in transferring their hole content to the O atoms, the higher the critical temperatures that can be achieved.

High-temperature ferromagnetism and strong π-conjugation feature in two-dimensional manganese tetranitride

Two-dimensional (2D) magnetic materials have attracted tremendous research interest because of the promising application in the next-generation microelectronic devices. Here, by the first-principles calculations, we propose a two-dimensional ferromagnetic material with high Curie temperature, manganese tetranitride MnN4 monolayer, which is a square-planar lattice made up of only one layer of atoms. The structure is demonstrated to be stable by the phonon spectra and the molecular dynamic simulations, and the stability is ascribed to the π–d conjugation between π orbital of N=N bond and d orbital of Mn. More interestingly, the MnN4 monolayer displays robust 2D ferromagnetism, which originates from the strong exchange couplings between Mn atoms due to the π–d conjugation. The high critical temperature of 247 K is determined by solving the Heisenberg model using the Monte Carlo method.

Critical Current Density in d-Wave Hubbard Superconductors.

In this work, the Generalized Hubbard Model on a square lattice is applied to evaluate the electrical current density of high critical temperature d-wave superconductors with a set of Hamiltonian parameters allowing them to reach critical temperatures close to 100 K. The appropriate set of Hamiltonian parameters permits us to apply our model to real materials, finding a good quantitative fit with important macroscopic superconducting properties such as the critical superconducting temperature (Tc) and the critical current density (Jc). We propose that much as in a dispersive medium, in which the velocity of electrons can be estimated by the gradient of the dispersion relation ∇ε(k), the electron velocity is proportional to ∇E(k) in the superconducting state (where E(k)=(ε(k)-μ)2+Δ2(k) is the dispersion relation of the quasiparticles, and k is the electron wave vector). This considers the change of ε(k) with respect to the chemical potential (μ) and the formation of pairs that gives rise to an excitation energy gap Δ(k) in the electron density of states across the Fermi level. When ε(k)=μ at the Fermi surface (FS), only the term for the energy gap remains, whose magnitude reflects the strength of the pairing interaction. Under these conditions, we have found that the d-wave symmetry of the pairing interaction leads to a maximum critical current density in the vicinity of the antinodal k-space direction (π,0) of approximately 1.407236×108 A/cm2, with a much greater current density along the nodal direction (π2,π2) of 2.214702×109 A/cm2. These results allow for the establishment of a maximum limit for the critical current density that could be attained by a d-wave superconductor.

Warm Mix Asphalt Binder Utilizing Water Foaming and Fluxing Using Bio-Derived Agent.

The present paper investigates the effects of simultaneous mechanical foaming using water and fluxing with a bio-derived agent on the properties of three distinct asphalt binders: 50/70 paving-grade bitumen, 45/80-55 polymer-modified bitumen, and 45/80-80 highly modified asphalt binder. The testing involved classical tests for assessing binder consistency (penetration at 25 °C, ring and ball softening point, Fraass breaking point, and dynamic viscosity) as well as performance tests (high and low Superpave critical temperatures and multiple stress creep recovery). The tests included assessment directly after asphalt binder foaming and were repeated after a 14-day period. It was shown that bitumen foaming had only short-term effects on the asphalt binders, which did not persist in the repeated tests after 14 days. The fluxing agent that was utilized caused significant changes in the consistency of all asphalt binders. The changes in the performance characteristics of the 50/70 and 45/80-55 binders were severe and amounted to a significant decrease in high-temperature performance of these binders. On the other hand, an improvement in all performance characteristics in the case of the 45/80-80 asphalt binder was observed as a result of the applied processes, particularly when measured 14 days after foaming. This study shows that the simultaneous use of foaming and the fluxing additive decreased the dynamic viscosity of the 45/80-80 binder, while improving its properties relating the pavement performance.

Synthesis of infinite-layer nickelates and influence of the capping-layer on magnetotransport

The recent discovery of a zero-resistance state in nickel-based compounds has generated a re-excitement about the long-standing problem in condensed matter of high-critical temperature superconductivity, in light of the similarities between infinite-layer nickelates and cuprates. However, despite some formal valence and crystal symmetry analogies, the electronic properties of infinite-layer nickelates are remarkably original accounting, among other properties, of a unique Nd 5d-Ni 3d hybridization. This designates infinite-layer nickelates as a new class of oxide superconductors which should be considered on their own. Here we report about Nd1-x Sr x NiO2 (x = 0, 0.05 and 0.2) thin films synthesized with and without a SrTiO3 (STO) capping-layer, showing very smooth and step-terraced surface morphologies. Angle-dependent anisotropic magnetoresistance (AMR) measurements performed with a magnetic field rotating in-plane or out-of-plane with respect to the sample surface, rendered important information about the magnetic properties of undoped STO-capped and uncapped samples. The results point at a key role of the capping-layer in controlling the magnitude and anisotropy of the AMR. We discuss this control in terms of a combined effect between the Nd 5d-Ni 3d hybridization and an intra-atomic Nd 4f-Nd 5d exchange coupling. Further studies foresee the influence of the capping layer on infinite-layer nickelates with no magnetic rare-earth.

Predicted High-Temperature Superconductivity in Rare Earth Hydride ErH2 at Moderate Pressure

Hydrides offer an opportunity to study high critical temperature (high-T c) superconductivity at experimentally achievable pressures. However, the pressure needed remains extremely high. Using density functional theory calculations, herein we demonstrate that a new rare earth hydride ErH2 could be superconducting with T c ∼ 80 K at 14.5 GPa, the lowest reported value for compressed hydrides to date. Intriguingly, due to Kondo destruction, superconductivity was prone to exist at 15 GPa. We also reveal an energy gap at 20 GPa on the background of normal metallic states. At 20 GPa, this compressed system could act as a host of superconductor judged from a sharp jump of spontaneous magnetic susceptibility with an evanescent spin density of state at Fermi level. Finally, electron pairing glue for ErH2 at these three typical pressures was attributed to the antiferromagnetic spin fluctuation.

Acrylamidevinyl acetate copolymer as a new UCST copolymer: Phase transition and aggregation behavior; an experimental and theoretical study

A high sensitive upper critical solution temperature (UCST) copolymer was synthesized through free radical copolymerization with acrylamide (AAm) and vinyl acetate (VA). The UCST behavior is due to a combination of hydrogen bonds and electrostatic potentials between copolymer chains. Turbidimetry, dynamic light scattering, and rheology studies were used to determine the phase transition and aggregation behavior. It is indicated that copolymer with 80% AAm has a phase transition temperature between 38 and 38.2 °C, which is suitable for releasing fever medicine. Also, the temperature gradient for the complete dissolution of the copolymer is 2 °C. The molecular dynamics simulations at a molecular level confirmed the experimental results and indicated that aggregation of copolymer chains below UCST temperature had increased binding energy between polymer chains. The low cytotoxicity and its desirable UCST behavior make the copolymer promising for biomedical applications in the future.

High-critical temperature iron-based superconductor epitaxial films for expanding the potential applications

Abstract This is a viewpoint of the letter by Dongyi Qin et al (2022 Supercond. Sci. Technol. 35, 09LT01).