High Critical Temperature Research Articles

Comparison of Magnetic Field Detectors Based On SQUID And/Or Josephson Junction Arrays With HTSc

We compare the potential of SQUID arrays and Josephson junction arrays as magnetic field detectors in an open-loop configuration, accounting for such limitations as technological parameter fluctuations, current density distribution and self-field effects originating from the bias current. Because of the simplicity of its single layer process, the ion-damaged barrier technology is used to fabricate the Josephson junctions (JJ) and to make the evaluations by extrapolation to larger area detectors. From preliminary experimental results on series arrays of Josephson junctions, we expect a less detrimental effect of self-flux and parameter dispersion, which is particularly important with high critical temperature technologies.

Thermodynamic comparison of modified Rankine cycle configurations for LNG cold energy recovery under different working conditions

In this study, we present the evaluation of two modified single-stage Rankine cycles (RC) and two two-stage RCs to provide a guidance for configuration selection under different working conditions. Moreover, a regenerative-reheat RC, which is the combination of the modified single-stage RCs, is proposed to utilize the cold energy of liquefied natural gas (LNG) and the thermal energy of low grade heat. The key parameters of these five configurations using eight working fluids are optimized with exergy efficiency as the optimization objective. The results show that the modified single-stage RCs are usually better than the two-stage RCs since they have simpler configurations with similar or higher efficiencies. The regenerative-reheat RC performs even better under most working conditions and it achieves the maximum efficiency of 24.57% by using ethane as the working fluid under the heat source temperature of 180 °C and the pipeline pressure of 3 MPa. The addition of direct expansion cycle can improve the exergy efficiencies of most fluids and configurations under lower pipeline pressures. However, the improvements decrease greatly for fluids with low normal boiling point temperatures (NBPTs) and especially under higher pipeline pressures. As to the fluid selection, high critical temperature and low NBPT are both important for reheat RC and parallel two-stage RC. NBPT weights more than critical temperature for regenerative RC and regenerative-reheat RC. Besides, wet working fluids are more suitable for reheat RC while dry and isentropic fluids are more suitable for regenerative RC and regenerative-reheat RC. The optimal regenerative ratio and reheat pressure ratio vary with different working fluids and working conditions. Lower reheat pressure ratio usually leads to higher regenerative ratio.

Macromolecular regulators have matching effects on the phase equilibrium and interfacial tension of biomolecular condensates.

The interfacial tension of phase-separated biomolecular condensates affects their fusion and multiphase organization, and yet how this important property depends on the composition and interactions of the constituent macromolecules is poorly understood. Here we use molecular dynamics simulations to determine the interfacial tension and phase equilibrium of model condensate-forming systems. The model systems consist of binary mixtures of Lennard-Jones particles or chains of such particles. We refer to the two components as drivers and regulators; the former has stronger self-interactions and hence a higher critical temperature (Tc ) for phase separation. In previous work, we have shown that, depending on the relative strengths of driver-regulator and driver-driver interactions, regulators can either promote or suppress phase separation (i.e., increase or decrease Tc ). Here we find that the effects of regulators on Tc quantitatively match the effects on interfacial tension (γ). This important finding means that, when a condensate-forming system experiences a change in macromolecular composition or a change in intermolecular interactions (e.g., by mutation or posttranslational modification, or by variation in solvent conditions such as temperature, pH, or salt), the resulting change in Tc can be used to predict the change in γ and vice versa. We also report initial results showing that disparity in intermolecular interactions drives multiphase coexistence. These findings provide much needed guidance for understanding how biomolecular condensates mediate cellular functions.

Prediction of Phonon-Mediated Superconductivity with High Critical Temperature in the Two-Dimensional Topological Semimetal W2N3.

Two-dimensional superconductors attract great interest both for their fundamental physics and for their potential applications, especially in the rapidly growing field of quantum computing. Despite intense theoretical and experimental efforts, materials with a reasonably high transition temperature are still rare. Even more rare are those that combine superconductivity with a nontrivial band topology that could potentially give rise to exotic states of matter. Here, we predict a remarkably high superconducting critical temperature of 21 K in the easily exfoliable, topologically nontrivial 2D semimetal W2N3. By studying its electronic and superconducting properties as a function of doping and strain, we also find large changes in the electron-phonon interactions that make this material a unique platform to study different coupling regimes and test the limits of current theories of superconductivity. Last, we discuss the possibility of tuning the material to achieve coexistence of superconductivity and topologically nontrivial edge states.

Charge and isospin fluctuations in a non-ideal pion gas with dynamically fixed particle number

We study the behavior of the non-ideal pion gas with the dynamically fixed number of particles, formed on an intermediate stage in ultra-relativistic heavy-ion collisions. The pion spectrum is calculated within the self-consistent Hartree approximation. General expressions are derived for cross-covariances of the number of various particle species in the pion gas of an arbitrary isospin composition. The behavior of the cross-variances is analyzed for the temperature approaching from above the maximal critical temperature of the Bose–Einstein condensation for the pion species $$a=\pm ,0$$ , i.e. for $$T>\max T_\mathrm{cr}^a$$ . It is shown that in the case of the system with equal averaged numbers of isospin species, the variance of the charge, $$Q=N_{+}-N_{-}$$ , diverges at $$T\rightarrow T_\mathrm{cr}=T_\mathrm{cr}^a$$ , whereas variances of the total particle number, $$N=N_{+} + N_{-} + N_{0}$$ , and of a relative abundance of charged and neutral pions, $$G=(N_{+}+N_{-})/2 - N_{0}$$ , remain finite in the critical point. Then, fluctuations are studied in the pion gas with small isospin imbalance $$0<|G|\ll N$$ and $$0<|Q|\ll N$$ and shifts of the effective masses, chemical potentials, and values of critical temperatures are calculated for various pion species, and the highest critical temperature, $$\text{ max }T_\mathrm{cr}^{{a}}$$ is found, above which the pion system exists in the non-condensed phase. Various pion cross variances are calculated for $$T>\text{ max }T_\mathrm{cr}^{{a}}$$ , which prove to be strongly dependent on the isospin composition of the system, whereas the variances of N and G are found to be independent on the isospin imbalance up to the term linear in G/N and Q/N.

One-dimensional edge contact to encapsulated MoS2 with a superconductor

Establishing ohmic contact to van der Waals semiconductors such as MoS2 is crucial to unlocking their full potential in next-generation electronic devices. Encapsulation of few layer MoS2 with hBN preserves the material’s electronic properties but makes electrical contacts more challenging. Progress toward high quality edge contact to encapsulated MoS2 has been recently reported. Here, we evaluate a contact methodology using sputtered MoRe, a type II superconductor with a relatively high critical field and temperature commonly used to induce superconductivity in graphene. We find that the contact transparency is poor and that the devices do not support a measurable supercurrent down to 3 K, which has ramifications for future fabrication recipes.

Cobalt substitution effects on critical current, inter-grain full penetration field, and critical temperature study for Tl2Ba2Ca2Cu3-xCoxOy bulk superconductor, where x is from 0.005 to 0.050

Thallium-based cuprates are known to exhibit high critical temperatures and currents. Usually, partial substitution of copper by transition elements leads to the suppression of superconducting properties. This contribution reports on the studies of Tl2Ba2Ca2Cu3-xCoxOy. All specimens were found to be superconductors with critical temperatures of 102.4 K, 112.9 K, 112.2 K, 113.5 K, 112.8 K, and 94.8 K for x = 0.005, 0.010, 0.020, 0.030, 0.040, and 0.050, respectively. The samples were characterised by scanning electron microscopy, X-ray diffraction, AC susceptibility, resistance, and vibrating sample magnetometry.

Thermal management of single and multiple PCMs based heat sinks for electronics cooling

The present study focuses on passive thermal management of electronic devices using heat sinks embedded with single or multiple phase change materials (PCMs). RT58, RT44 and n-Eicosane are used as PCMs, while aluminium fins are used as thermal conductivity enhancer (TCE). Single PCM cases are investigated independently while multiple PCMs pairs are studied through their filling in alternate enclosures separately. Three constant heat fluxes of 1.5, 2.5 and 3.5 kW/m2 are applied to the heat sinks to investigate PCMs’ thermal performance. The numerical model is validated against available experimental results where an acceptable 2.9% difference in the transient temperature is achieved. High values of Nusselt number were discovered at the beginning of melting and the end of solidification stages. The RT44 shows the lowest peak temperature and longest melting duration among single PCM cases, due to its high latent heat of fusion. When n-Eicosane and RT44 are paired in a heat sink, the best overall thermal performance is achieved where its operational time is increased by 3.3–12% comparing to single n-Eicosane or RT44. The results show that the multiple PCMs pair of n-Eicosane/RT44 is feasible for high critical temperature devices due to its longer operating time and also lowest average transient temperature.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO3

PbMO3 (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO3 remain unclear. Herein, we reveal that PbFeO3 crystallizes into an unusual 2ap × 6ap × 2ap orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb2+0.5Pb4+0.5FeO3 lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb2+ and Pb4+) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe3+ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Current Status in Building a Compact and Mobile HTS MRI Instrument

Magnetic Resonance Imaging (MRI) is an indispensable tool in monitoring lipid-rich tissues and the musculoskeletal system. It however requires maintaining a huge homogeneous (1 to 7 T) field. That is where superconductors come into play since they are the only feasible way to create such fields. The state-of-the-art technology utilizes low critical temperature superconductors (LTS) which require cooling down to liquid helium temperatures (4.2K). The new generation superconductors, aka the high critical temperature superconductors (HTS), can operate up to relatively elevated temperatures (technically above 100K's). In addition to that, they can withstand larger magnetic field strengths which can theoretically go up to the order of hundred Teslas in magnitude. It is evident that LTS wires of current MRI magnet technology is at their limits and new pursuits such as reaching higher fields and reducing operational/maintenance costs are in the favor of HTS tapes. There is, in fact, another advantage beyond minimization of the cryogenic costs, namely the compactness of the overall system. Utilizing certain auxiliary techniques such as flux pumping and conductive cooling of the tapes, MRI magnets can be reduced in size substantially. That, in turn, enables constructing much smaller thus mobile MRI scanners which can be brought to patients' doorsteps. Here we present the current status of our project aiming to build an HTS MRI machine dedicated to use as a mobile head scanner.

Quantum and Classical Proton Diffusion in Superconducting Clathrate Hydrides.

The discovery of near room temperature superconductivity in clathrate hydrides has ignited the search for both higher temperature superconductors and deeper understanding of the underlying physical phenomena. In a conventional electron-phonon mediated picture for the superconductivity for these materials, the high critical temperatures predicted and observed can be ascribed to the low mass of the protons, but this also poses nontrivial questions associated with how the proton dynamics affect the superconductivity. Using clathrate superhydride Li_{2}MgH_{16} as an example, we show through abinitio path integral simulations that proton diffusion in this system is remarkably high, with a diffusion coefficient, for example, reaching 6×10^{-6} cm^{2}/s at 300K and 250GPa. The diffusion is achieved primarily through proton transfer among interstitial voids within the otherwise rigid Li_{2}Mg sublattice at these conditions. The findings indicate the coexistence of proton quantum diffusion together with hydrogen-induced superconductivity, with implications for other very-high-temperature superconducting hydrides.

Two-Dimensional Cr2X3S3 (X = Br, I) Janus Semiconductor with Intrinsic Room-Temperature Magnetism.

The exploration of two-dimensional (2D) semiconductors with intrinsic room-temperature magnetism for use in nanoscale spintronic devices is of particular interest. Recently, the ferromagnetic CrX3 monolayer (X = Br, I) has received growing attention, but low critical temperature hinders its practical applications in spintronics. Here, using first-principles calculations, we report 2D Cr2X3S3 (X = Br, I) Janus semiconductors with room-temperature magnetism by replacing one layer of halogon atoms with sulfur atoms in CrX3 monolayer. Our results demonstrate that Cr2Br3S3 and Cr2I3S3 Janus crystals are ferrimagnetic semiconductors, that maintain their magnetic order, with a direct bandgap of 1.19 and 0.61 eV and high critical temperature of 387 and 447 K, respectively. The residual unpaired p electrons on the S anions lead to a strong direct-exchange interaction between the Cr and S atoms. Moreover, their room-temperature magnetism is robust under biaxial strain, while the bandgap can be remarkably modulated with strain. The novel magnetic properties in 2D Cr2X3S3 Janus magnetic semiconductors give them promising applications in spintronics.

Phonon-mode specific contributions to room-temperature superconductivity in atomic hydrogen at high pressures

We investigate the role of specific phonon mode symmetries for the room temperature superconductivity in atomic hydrogen under large pressure. Using anisotropic Migdal-Eliashberg theory with ab initio input from density functional theory, we show that the $E_u$ phonon modes are the dominant driving force for obtaining such high critical temperatures. When going from 400 to 600 GPa, we find an increased transition temperature, however, the total electron-phonon coupling strength is counterintuitively reduced. Our analysis reveals that this is due to an enhanced contribution to the coupling strength by the $E_u$ phonon mode.

New Application Method of Static-Field Magnetization to Magnetically Levitated Mover System With High T c Superconductors

Application method of static-field magnetization to magnetically levitated mover system with high critical temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) superconductors is discussed. The system is composed of a levitated mover, magnetic rails, a linear induction motor (LIM) and some power supplies. In the paper, static-field magnetization is applied to superconductors in the levitated system from bottom superconductor surfaces. Then, magnetizations in superconductors are tried to control using static-field on superconductor surfaces. That is, magnetic flux density distributions are successfully forced to change to initial flux density distributions.

The superconductivity of N–Si–H compounds at high pressure

Recently, the study of superconductive hydrides with high critical temperature (Tc) has attracted considerable attention in high-pressure community due to the finding of 240 K in YH9 at ~200 GPa and 250–260 K in LaH10 at 170–190 GPa. In this work, we performed structure searches on the exploration of the crystal structure and the superconductivity in ternary N–Si–H system at high pressure via a particle swarm optimization structure prediction methodology in combination with the first-principles electronic structure framework. As a result, our first-principles calculations show that a metastable phase NSiH11 is a potential superconductor Tc of 98–110 K at 300 GPa. This high superconductivity is associated with the H–H interaction among H2 units.

Emergence of intrinsic superconductivity in monolayer W2N3

Two-dimensional (2D) materials possessing intrinsic superconductivity with high transition temperature and unconventional pairing are highly desired, but their realizations are few and far between. Recently, ${\mathrm{W}}_{2}{\mathrm{N}}_{3}$ nanosheets down to three layers were successfully prepared [Yu et al., Adv. Mater. 30, 1805655 (2018)]. By performing solid ab initio calculations based on the anisotropic Migdal-Eliashberg theory, we predict that monolayer ${\mathrm{W}}_{2}{\mathrm{N}}_{3}$ is an unexplored intrinsic (without the assistance of external gating, strain, or a special substrate) 2D superconductor with large electron-phonon coupling (EPC) and high critical temperature ${T}_{c}=38\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ accompanied with a single and broad superconducting gap \ensuremath{\sim}7.5 meV. The ratio between the gap and the critical temperature is much larger than the value derived from Bardeen-Cooper-Schrieffer (BCS) theory, further confirming the strong coupling feature of monolayer ${\mathrm{W}}_{2}{\mathrm{N}}_{3}$. The extremely strong EPC originates from the large deformation potential of low-frequency acoustic phonons rather than Fermi-surface nesting. Due to the partially filled $d$ orbitals, electron-electron correlation leads to remarkable enhancement of EPC based on frozen phonon analysis. Here, ${T}_{c}$ can be further enhanced via hydrogen passivation based on the McMillian-Allen-Dynes formula. In addition, the symmetry-restricted spin-orbit coupling (SOC) brings forth exotic type-I Ising pairing whose in-plane upper critical field is far beyond the Pauli paramagnetic limit. Our predictions provide a fascinating and highly feasible platform for realizing high-temperature 2D superconductivity and studying the interplay between electron-phonon coupling, electron-electron correlation, and SOCs.

Anisotropic superconductivity in the spin-vortex antiferromagnetic superconductor CaK(Fe0.95Ni0.05)4As4

High critical temperature superconductivity often occurs in systems where an antiferromagnetic order is brought near $T=0K$ by slightly modifying pressure or doping. CaKFe$_4$As$_4$ is a superconducting, stoichiometric iron pnictide compound showing optimal superconducting critical temperature with $T_c$ as large as $38$ K. Doping with Ni induces a decrease in $T_c$ and the onset of spin-vortex antiferromagnetic order, which consists of spins pointing inwards to or outwards from alternating As sites on the diagonals of the in-plane square Fe lattice. Here we study the band structure of CaK(Fe$_{0.95}$Ni$_{0.05}$)$_4$As$_4$ (T$_c$ = 10 K, T$_N$ = 50 K) using quasiparticle interference with a Scanning Tunneling Microscope (STM) and show that the spin-vortex order induces a Fermi surface reconstruction and a fourfold superconducting gap anisotropy.

Predicting doped Fe-based superconductor critical temperature from structural and topological parameters using machine learning

Abstract Recently, Fe-based superconductors have shown promising properties of high critical temperature and high upper critical fields, which are prerequisites for applications in high-field magnets. Critical temperature, T c, is an important characteristic correlated with crystallographic and electronic structures. By doping with foreign ions in the crystal structure, T c can be modified, which however requires significant manpower and resources for materials synthesis and characterizations. In this study, we develop the Gaussian process regression model to predict T c of doped Fe-based superconductors based on structural and topological parameters, including the lattice constants, volume, and bonding parameter topological index H 31. The model is stable and accurate, contributing to fast T c estimations.

Route to phonon-mediated high-temperature unconventional superconductivity

We investigate the appearance of superconductivity in a model with Peierls electron-phonon coupling for very low carrier concentrations. Superconductivity with very high critical temperatures is found to occur at all carrier concentrations if the electron-phonon coupling is not too weak, with a gap of $s+{s}^{*}$ symmetry that closes on the Fermi surface for appropriate parameters. The possibility of finding the elusive $p$-type superconductivity is also revealed, although this turns out not to be stable within the approximations we use.

Superfluid weight and Berezinskii-Kosterlitz-Thouless transition temperature of strained graphene

We obtain the superfluid weight and Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for highly unconventional superconducting states with the coexistence of chiral d-wave superconductivity, charge density waves and pair density waves in the strained graphene. Our results show that the strain-induced flat bands can promote the superconducting transition temperature approximately 50% compared to that of the original doped graphene, which suggests that the flat-band superconductivity is a potential route to get superconductivity with higher critical temperatures. In particular, we obtain the superfluid weight for the pure superconducting pair-density-wave states from which the deduced superconducting transition temperature is shown to be much lower than the gap-opening temperature of the pair density wave, which is helpful to understand the phenomenon of the pseudogap state in high-Tc cuprate superconductors. Finally, we show that the BKT transition temperature versus doping for strained graphene exhibits a dome-like shape and it depends linearly on the spin-spin interaction strength.