Critical Temperature Tc Research Articles

The Measurement and Analysis of the Hysteresis Loss in a Multifilament MgB2 Superconducting Wire

The article presents the results obtained from measuring the magnetization of and hysteresis loss in a multifilament MgB2 superconducting wire (produced by ASG Superconductors) for magnetic field applied in two directions: in parallel and perpendicular to the conductor axis. The measurement temperatures were varied in the range of 5–45 K. The MgB2 superconducting fraction magnetization curves were obtained by subtracting the magnetization of the wire ferromagnetic matrix measured at temperature T = 45 K, which is above the superconductor critical temperature Tc , from the magnetization value of the entire specimen. From these magnetization curves, the dependences of the hysteresis energy loss on the external magnetic field strength Qh(Н) were obtained. It is shown that, with the field oriented perpendicular to the MgB2 wire axis and at the external field characteristic values, the hysteresis loss is twice as large. A method for calculating the hysteresis loss in a superconducting wire and methods for reducing it are proposed. The obtained results can be used to optimize NMR and MRI facilities, superconducting motors and generators made using MgB2 superconducting wire produced by ASG Superconductors.

ZrNb(CO) RF Superconducting Thin Film with High Critical Temperature in the Theoretical Limit

AbstractSuperconducting radio‐frequency (SRF) resonators are critical components for particle accelerator applications, such as free‐electron lasers, and for emerging technologies in quantum computing. Developing advanced materials and their deposition processes to produce RF superconductors that yield nΩ surface resistances is a key metric for the wider adoption of SRF technology. Here, ZrNb(CO) RF superconducting films with high critical temperatures (Tc) achieved for the first time under ambient pressure are reported. The attainment of a Tc near the theoretical limit for this material without applied pressure is promising for its use in practical applications. A range of Tc, likely arising from Zr doping variation, may allow a tunable superconducting coherence length that lowers the sensitivity to material defects when an ultra‐low surface resistance is required. The ZrNb(CO) films are synthesized using a low‐temperature (100 – 200 °C) electrochemical recipe combined with thermal annealing. The phase transformation as a function of annealing temperature and time is optimized by the evaporated Zr‐Nb diffusion couples. Through phase control, one avoids hexagonal Zr phases that are equilibrium‐stable but degrade Tc. X‐ray and electron diffraction combined with photoelectron spectroscopy reveal a system containing cubic β‐ZrNb mixed with rocksalt NbC and low‐dielectric‐loss ZrO2. Proof‐of‐concept RF performance of ZrNb(CO) on an SRF sample test system is demonstrated. BCS resistance trends lower than reference Nb, while quench fields occur at approximately 35 mT. The results demonstrate the potential of ZrNb(CO) thin films for particle accelerators and other SRF applications.

Phase purity and surface morphology of high-Jc superconducting Bi2Sr2Ca1Cu2O8+δ thin films

Bi2Sr2Ca1Cu2O8+δ (Bi-2212) thin films with thicknesses less than 50 nm (<20 unit cells) are grown by pulsed-laser deposition (PLD) onto (001) LaAlO3 (LAO) single crystal substrates. Phase-pure and smooth c-axis oriented Bi-2212 films with optimal oxygen doping, critical temperature Tc0 up to 86 K, and critical current density Jc(60 K) above 1 MA/cm2 are obtained for samples that are annealed in situ at temperatures below 700 °C. At higher temperature Bi-2212 films on LAO substrates partially decompose to non-superconducting impurity phases, while films on MgO and SrTiO3 substrates are stable. The broadening ΔTc of the metal-to-superconductor resistive phase transition in magnetic fields is much larger for thin films of Bi-2212 as compared to YBa2Cu3O7. The magnetic field-induced suppression of Tc0 is stronger for Bi-2212 films containing impurity phases as compared to the phase-pure Bi-2212 films. The degradation of LAO substrate crystals after several steps of deposition and chemical removal of the Bi-2212 layer is investigated. New, commercially prepared substrates provide Bi-2212 films with smallest surface roughness (3 nm) and strong out-of-plane texture. However, thin films of almost the same quality are obtained on re-used LAO substrates that are mechanically polished after the chemical etching.

Pressure dependence of superconducting properties of layered BaNi2P2

The discovery of unconventional superconductivity in a broad class of iron-based materials invoked extensive research on the corresponding compounds of other transition metals. For instance, BaNi2P2 exhibits a superconducting transition with a critical temperature Tc = 2.5 K, and is isostructural to the 122 families of the iron arsenides, suggesting that this Ni-based compound may be another example of unconventional superconductivity. Here we report a detailed analysis of the high-pressure behavior of the angle- and magnetic field-dependent electrical resistivity. The experimentally observed decrease of Tc under the applied pressure matches well with the theoretical prediction based on Migdal-Eliashberg theory, providing evidence in favor of a conventional nature of superconductivity in the nickel analogs of the iron superconductors.

The pairing symmetry in quasi-one-dimensional superconductor Rb[formula omitted]Mo[formula omitted]As[formula omitted

Quasi-one-dimensional electron systems display intrinsic instability towards long-range ordered phases at sufficiently low temperatures. The superconducting orders are of particular interest as they can possess either singlet or triplet pairing symmetry and frequently compete with magnetism. Here we report on muon spin rotation and relaxation (μSR) study of Rb2Mo3As3 characterised by one of the highest critical temperatures Tc=10.4 K among quasi-one-dimensional superconductors. The transverse-field μSR signal shows enhanced damping below Tc due to the formation of vortex lattice. Comparison of vortex lattice broadening against single gap s−, p− and d−wave models shows the best agreement for the s−wave scenario but with the anomalously small superconducting gap, Δ0, to Tc ratio of 2Δ0/kBTc=2.74(1). The alternative nodal p−wave or d−wave scenarios with marginally worse goodness of fit would yield more realistic 2Δ0/kBTc=3.50(2) and 2Δ0/kBTc=4.08(1), respectively, and thus they cannot be ruled out when accounting for the superconducting state in Rb2Mo3As3.

Crosslink-Induced Conformation Change of Intrinsically Disordered Proteins Have a Nontrivial Effect on Phase Separation Dynamics and Thermodynamics.

Biomolecule condensates formed via liquid-liquid phase separation (LLPS) play crucial roles within various cellular processes. Despite numerous theoretical and experimental discoveries, the general principle by which the protein conformation affects the propensity for LLPS remains poorly understood. Here, we systematically address this issue using a general coarse-grained model of intrinsically disordered proteins (IDPs) with different degrees of intrachain crosslinks. We find that an increased conformation collapse due to higher intrachain crosslink ratio f enhances the thermodynamic stability of protein phase separation and found the critical temperature Tc has a good scaling law with the proteins' average radius of gyration Rg. Such correlation is robust regardless of interaction types and sequence patterns. Strikingly, the growth dynamics of the LLPS process, contrary to the thermodynamic observation, is generally more favored at proteins with extended conformation. Faster condensate growing speed is again observed for higher-f collapsed IDPs, resulting altogether in a nonmonotonic dynamics as a function of f. A phenomenological understanding of the phase behavior is provided by a mean-field model with an effective Flory interaction parameter χ, which is found to have a good scaling law with conformation expansion. Our study shed lights on the general mechanism for understanding and modulation of phase separation with different conformation profiles and may provide new evidence in reconciling the contradictions in thermodynamic- and dynamic-controlled experimental LLPS observations.

Molecular dynamics simulations of liquid ethane up to 298.15 K

Equilibrium molecular dynamics (EMD) simulations are used to model density ρ L, dynamic viscosity η L, and self-diffusivity D self of liquid ethane at pressures close to saturation conditions and temperatures T between (184–298) K, close to the critical temperature T c. To correct for a T-dependent drift of the simulated results from experimental data, a T-dependent force field modification from our previous studies is extended to a wider T-range. The modification is validated within this work by comparing predicted ρ L, η L, and D self to experimental reference data. With the modification applied, EMD simulations are able to predict ρ L, η L, and D self of liquid ethane at T = 298 K with relative deviations of 6.3%, 13%, and 28% from literature references, respectively. Simulations of binary mixtures of liquid ethane with different dissolved gases were performed to predict the Fick diffusivity D 11. At a mole fraction of dissolved nitrogen x N2 = 0.03, a deviation of more than a factor of three is found between simulation results and experimental results determined using the dynamic light scattering method. This shows that EMD simulations are not able to predict the critical slowing down of D 11 approaching T c, despite excellent agreement between simulations and reference data for neat ethane.

Vertex dominated superconductivity in intercalated FeSe

Bulk FeSe becomes superconducting below 9 K, but the critical temperature (Tc) is enhanced almost universally by a factor of ~4–5 when it is intercalated with alkali elements. How intercalation modifies the structure is known from in-situ X-ray and neutron scattering techniques, but why Tc changes so dramatically is not known. Here we show that there is one-to-one correspondence between the enhancement in magnetic instabilities at certain q vectors and superconducting pairing vertex, even while the nuclear spin relaxation rate 1/(T1T) may not reflect this enhancement. Intercalation modifies electronic screening both in the plane and also between layers. We disentangle quantitatively how superconducting pairing vertex gains from each such changes in electronic screening. Intercalated FeSe provides an archetypal example of superconductivity where information derived from the single-particle electronic structure appears to be insufficient to account for the origins of superconductivity, even when they are computed including correlation effects. We show that the five-fold enhancement in Tc on intercalation is not sensitive to the exact position of the dxy at Γ point, as long as it stays close to EF. Finally, we show that intercalation also significantly softens the collective charge excitations, suggesting the electron-phonon interaction could play some role in intercalated FeSe.

Prediction of Superconductivity in Clathrate Er Hydrides under High Pressure

In this paper, we perform unbiased structure searches combined with first-principles calculations to predict the stable structures and possible superconductivity of ErHn (n = 4~6) under pressures of 50~300 GPa. Two novel compounds, ErH4 and ErH6, are identified as thermodynamically and dynamically stable above 50 GPa; ErH4 and ErH6 can stabilize in clathrate structures with the I4/mmm and Im3¯m space groups, respectively. An analysis of the electronic density of states (DOS) suggests the metallic nature of the two phases. Then, the superconducting critical temperature (Tc) is estimated using the Allen–Dynes modified McMillan equation; the results are 130.9~181.2 K for Im3¯m-ErH6 at 100~300 GPa, and 74.4~79.8 K for I4/mmm-ErH4 at 150~300 GPa. The resultant high Tc superconductivity in this system can be traced back to the combination of high density of states at the Fermi level and strong electron–phonon interactions.

Pressure-induced superconductivity in topological heterostructure (PbSe)5(Bi2Se3)6

Recently, the natural heterostructure of (PbSe)5-(Bi2Se3)6 has been theoretically predicted and experimentally confirmed as a topological insulator. In this work, we induce superconductivity in (PbSe)5(Bi2Se3)6 by implementing high pressures. As the pressure increases up to 10 GPa, superconductivity with a critical temperature (Tc) ~ 4.6 K suddenly appears, which is followed by an abrupt decrease. Remarkably, upon further compression above 30 GPa, a new superconducting state arises, where pressure raises the Tc to an unsaturated 6.0 K within the limit of our research. Combining X-ray diffraction and Raman spectroscopy, we suggest that the emergence of the two distinct superconducting states occurs concurrently with the pressure-induced structural transition in this topological heterostructure (PbSe)5(Bi2Se3)6.

Enhanced Superconductivity and Rashba Effect in a Buckled Plumbene-Au Kagome Superstructure.

Plumbene, with a structure similar to graphene, is expected to possess a strong spin-orbit coupling and thus enhances its superconducting critical temperature (Tc ). In this work, a buckled plumbene-Au Kagome superstructure grown by depositing Au on Pb(111) is investigated. The superconducting gap monitored by temperature-dependent scanning tunneling microscopy/spectroscopy shows that the buckled plumbene-Au Kagome superstructure not only has an enhanced Tc with respect to that of a monolayer Pb but also possesses a higher value than what owned by a bulk Pb substrate. By combining angle-resolved photoemission spectroscopy with density functional theory, the monolayer Au-intercalated low-buckled plumbene sandwiched between the top Au Kagome layer and the bottom Pb(111) substrate is confirmed and the electron-phonon coupling-enhanced superconductivity is revealed. This work demonstrates that a buckled plumbene-Au Kagome superstructure can enhance superconducting Tc and Rashba effect, effectively triggering the novel properties of a plumbene.

Conventional s-wave superconductivity in the non-centrosymmetric compounds Re3W, Re3W0.5Nb0.5, and Re3W0.5Ta0.5

We carried out an investigation of the structural and superconducting properties of Re3W1−xMx alloys, which can crystallize in both a centrosymmetric (CS) hexagonal, and a non-centrosymmetric (NCS) cubic α-Mn phase. The superconducting transition temperatures (Tc) of the CS and NCS phases in pure Re3W are ∼ 9.1 and ∼ 7.7 K, respectively. While boules fast cooled from the melt have a mixture of CS phases, the NCS phase can be stabilized upon annealing at very high temperatures (∼ 1700 °C), or upon the partial substitution of the transition metals M = Ta, or Nb for W. X-ray diffraction and wavelength dispersive X-ray spectroscopy (WDS) data suggest that the optimal composition for the formation of the α-Mn phase directly from the melt in Re3W1−xMx is x ≈ 0.5. A detailed study of the superconducting properties of the Re3W1−xMx solid solutions was carried out by means of measurement of magnetization, ac magnetic susceptibility, electrical transport, and low temperature specific heat. As the M content is increased, the superconducting critical temperature Tc progressively decreases, though the overall superconducting properties remain similar to those of the parent NCS Re3W compound. The partial substitution at the W site does not seem to affect the pairing mechanism. Given the exponentially vanishing character of the electronic specific heat Ce(T) at low temperatures, the superconducting behavior of the NCS alloys is consistent with a nodeless, s-wave superconducting gap symmetry in all NCS samples. These findings add the pseudo-binary Re-M-W systems to the small roll of NCS materials where the pairing symmetry of the superconducting state can be explored.

Patterning of Superconducting Two‐Dimensional Electron Gases based on AlOx/KTaO3 (111) Interfaces

AbstractThe versatility of properties displayed by two‐dimensional electron gases (2DEGs) at oxide interfaces has fostered intense research in hope of achieving exotic electromagnetic effects in confined systems. Of particular interest is the recently discovered superconducting state appearing in (111)‐oriented KTaO3 interfaces, with a critical temperature Tc≈2 K, almost 10 times higher than that of SrTiO3‐based 2DEGs. Just as in SrTiO3‐based 2DEGs, fabricating devices in this new system is a technical challenge due to the fragility of the 2DEG and the propensity of bulk KTaO3 to become conducting outside the devices upon adventitious oxygen vacancy doping. Here, three different techniques are presented for patterning Hall bars in AlOx/KTaO3 (111) heterostructures. The devices show superconducting transitions ranging from 1.3 to 1.78 K, with limited degradation from the unpatterned thin film, and enable an efficient tuning of the carrier density by electric field effect. The array of techniques allows for the definition of channels with a large range of dimensions for the design of various kinds of devices to explore the properties of this system down to the nanoscale.

Superconducting and transport properties of biphenylene network monolayer with alkaline metal adatoms

To explore novel two-dimensional superconductors, the recently prepared biphenylene network (BPN) monolayer with alkaline metal adatoms was investigated theoretically. Our study suggests that the superconducting critical temperature (Tc) of BPN monolayer could be increased from 0.03 to 11.22 K by adsorption of Li atoms, which induces increased density of states at the Fermi Level, softened vibrations and electron pockets at the Fermi surface. The diffusion barrier of Li/Na adatoms is very low (<0.25 eV), and it tends to decrease for high coverage. Average conductance was proposed to quantitatively evaluate the intrinsic conductivity and electronic transport anisotropy of two-dimensional materials. The adsorption of alkaline metal atoms enhances the transport anisotropy more than twice, and the direction with better conductance derives from the connected delocalized multi-center π bonds. Our study suggests that adsorption of metal atoms is an effective way to functionalize BPN monolayers, which endows them potential applications in future superconducting and electronic devices. It also implies that carbon monolayer allotropes like BPN monolayer may provide good platforms for both fundamental research and device applications, which stimulates more explorations in various fields.

Evidence for an emergent anomalous metallic state in compressed titanium

The anomalous metallic state (AMS) emerging from a quantum superconductor-to-metal transition is a subject of great current interest since this exotic quantum state exhibits unconventional transport properties that challenge the core physics principles of Fermi liquid theory. As the AMS concept is historically derived from disordered two-dimensional (2D) systems, related studies have predominately concentrated on 2D materials. The AMS behaviors in three-dimensional (3D) systems have been rarely reported to date, which raises intriguing questions on the fundamental nature of pertinent physics. Here, we report experimental evidence for a 3D AMS in highly compressed titanium metal that exhibits superconductivity with a critical temperature (Tc) reaching near-record 25.1K among elemental superconductors, offering a favorable material template for exploring 3D AMS. At sufficiently strong magnetic fields, unusual transport behaviors set in over a wide pressure range, showcasing AMS hallmarks of a low-temperature saturation resistance below the Drude value and giant positive magnetoresistance. These findings reveal a 3D AMS in simple elemental systems and, more importantly, provide a fresh platform for probing the decades-long enigmatic underlying physics.

Superconducting Stiffness and Coherence Length of FeSe0.5Te0.5 Measured in a Zero-Applied Field

Superconducting stiffness ρs and coherence length ξ are usually determined by measuring the penetration depth λ of a magnetic field and the upper critical field Hc2 of a superconductor (SC), respectively. However, in magnetic SC, which is iron-based, this could lead to erroneous results, since the internal field could be very different from the applied one. To overcome this problem in Fe1+ySexTe1−x with x∼0.5 and y∼0 (FST), we measured both quantities with the Stiffnessometer technique. In this technique, one applies a rotor-free vector potential A to a superconducting ring and measures the current density j via the ring’s magnetic moment m. ρs and ξ are determined from London’s equation, j=−ρsA, and its range of validity. This method is particularly accurate at temperatures close to the critical temperature Tc. We find weaker ρs and longer ξ than existing literature reports, and critical exponents which agree better with expectations based on the Ginzburg–Landau theory.

Atomic, electronic, and superconducting properties of Zr[formula omitted]Ir compound

We have investigated the structural, electronic, mechanical, phononic, and superconducting properties of the Zr2Ir compound with a body-centered tetragonal crystal structure using first-principles calculations. Our analysis reveals that the Zr2Ir compound shows mechanical and dynamically stable by using with and without spin–orbit coupling (SOC) effect. After calculating some properties such as elastic constants, Bulk modulus, Young’s modulus, Poisson ratio, Debye temperature, and sound velocity, we found that Zr2Ir is ductile. When the elastic constants C11 and C33 are compared, it is determined that the situation changes in the opposite direction under the effect of SOC, that is, more compressibility along the x-axis turns into the z-axis. Here, the electronic band structure and intensity of the states calculated for the compound show a metallic character. The superconducting critical temperature (Tc) and electron–phonon coupling constant (λ) were found to be 7.50 K and 0.93 without SOC and 7.62 K and 0.96 with SOC, respectively. We determined that although the Zr2Ir compound has a strong electron–phonon coupling regime, the inclusion of SOC slightly reduces its critical temperature and electron–phonon coupling constant.

High-temperature superconductivity up to 223K in the Al stabilized metastable hexagonal lanthanum superhydride.

As compressed hydrides constantly refresh the records of superconducting critical temperatures (Tc) in the vicinity of room temperature, this further reinforces the confidence to find more high-temperature superconducting hydrides. In this process, metastable phases of superhydrides offer enough possibilities to access superior superconducting properties. Here we report a metastable hexagonal lanthanum superhydride (P63/mmc-LaH10) stabilized at 146GPa by introducing an appropriate proportion of Al, which exhibits high-temperature superconductivity with Tc ∼ 178K, and this value is enhanced to a maximum Tc ∼ 223K at 164GPa. A huge upper critical magnetic field value Hc2(0) reaches 223 T at 146GPa. The small volume expansion of P63/mmc-(La, Al) H10 compared with the binary LaH10 indicates the possible interstitial sites of Al atoms filling into the La-H lattice, instead of forming conventional ternary alloy-based superhydrides. This work provides a new strategy for metastable high-temperature superconductors through the multiple-element system.

First‐Principles Estimation of Low‐Pressure Superconductivity in KC2H8 Ternary Hydride

Very recently, much research progress has been made on the pressure‐induced superconducting state in ternary hydrides. Herein, an ab initio study of the KC2H8 ternary system is presented at a pressure in the range of 0–100 GPa. Its structural, electronic, dynamical, and superconducting properties are investigated, demonstrating that, at low pressure, KC2H8 is dynamically stable in a clathrate structure with space group Fmm and exhibits superconducting behavior with critical temperatures Tc in the range of 4–94 K. The important role played by C‐ and K‐derived phonon modes at low frequencies and H‐derived phonon modes at high frequencies is highlighted. Moreover, through the analysis of the hydrogen isotope effect, the conventional phonon‐mediated pairing mechanism in this system is proved.

CRITICAL CURRENT DENSITIES IN MAGNETIC SUPERCONDUCTORS Dy1–xErxRh3,8Ru0,2B4 (x=0,2;0,4)

Rhodium borides of the compositions Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4 were synthesized on the basis of the compound DyRh3,8Ru0,2B4 by partial substitution of dysprosium (Dy) with erbium (Er) with a LuRu4B4 type structure. The dependences of χ-1(T), M(B) and Bc2(T) were studied for these compounds. It is established that the compounds Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4 are magnetic superconductors with a critical temperature Tc ≈ 5.1 and 5.8 K, respectively; at T ≈ 3 K the magnetic subsystem of these compounds goes into the antiferromagnetic state. Based on measurements of the magnetic moment from the field (M(B)) for samples of compounds Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4, using the Bean model, dependences on the field of the critical current density jc(B) and the reduced pinning force fp(h) are obtained. The value of jc in the studied samples did not exceed 400 A/cm2 at T ≈ 2 K. It is established that in the case of magnetic superconductors with antiferromagnetic ordering (Dy0,8Er0,2Rh3,8Ru0,2B4 and Dy0,6Er0,4Rh3,8Ru0,2B4) at h 0.4 there is a significant deviation from the law of similarity.