Critical Field Bc2 Research Articles

Superconducting interstitial MoReRuCx medium-entropy alloys with a hexagonal structure

We report the results of structural and physical studies on the MoReRuCx medium-entropy alloys (MEAs) with x varying from 0 to 0.20. In the whole x range, a single hexagonal phase is formed. The variation of lattice parameters indicates that carbon atoms are located at the interstitial site for x ≤ 0.10 while occupy both the interstitial and substitutional sites at higher x values, which is corroborated by the x-ray photonelectron spectroscopy (XPS) results. Moreover, the resistivity, magnetic and thermodynamic measurements evidence that these MEAs are bulk fully gapped superconductors. In particular, the transition temperature Tc displays a dome-like dependence on carbon content with a maximum of 9.62 K at x = 0.05, whereas a maximal zero-temperature upper critical field Bc2(0) of 10.5 T is achieved at x = 0.10. For all MEAs, the specific heat data reveal the existence of an inhomogeneous superconducting state, which is discussed in relation to the chemical inhomogeneity and strong atomic disorder. In addition, we show that the key factors that control superconductivity are essentially the same for both MoReRuCx and ReCx, despite their very different Tc values. Our work not only provides the first series of interstitial MEAs with a hexagonal structure, but also demonstrates a feasible way to tune the superconductivity in multicomponent alloys.

Extrinsic and Intrinsic Anomalous Metallic States in Transition Metal Dichalcogenide Ising Superconductors.

The metallic ground state in two-dimensional (2D) superconductors has attracted much attention but is still under intense scrutiny. Especially, the measurements in the ultralow temperature region are challenging for 2D superconductors due to the sensitivity to external perturbations. In this work, the resistance saturation induced by external noise, named as the "extrinsic anomalous metallic state", is observed in 2D transition metal dichalcogenide (TMD) superconductor 4Ha-TaSe2 nanodevices. However, with further decreasing temperature, credible evidence of the intrinsic anomalous metallic state is obtained by adequately filtering external radiation. Our work indicates that, at ultralow temperatures, the anomalous metallic state can be experimentally revealed as the quantum ground state in 2D crystalline TMD superconductors. Besides, Ising superconductivity revealed by ultrahigh in-plane critical field (Bc2∥) going beyond the Pauli paramagnetic limit (Bp) is detected in 4Ha-TaSe2, from the one-unit-cell device to the bulk situation, which might be due to the weak coupling between the TaSe2 submonolayers.

Superconductivity at 253 K in lanthanum–yttrium ternary hydrides

Here we report the high-pressure synthesis of a series of lanthanum–yttrium ternary hydrides obtained at pressures of 170–196GPa via the laser heating of P63/mmc La–Y alloys with ammonia borane. As a result, we discovered several novel compounds: cubic hexahydride (La,Y)H6 and decahydrides (La,Y)H10 with a maximum critical temperature TC ∼ 253 K and an extrapolated upper critical magnetic field BC2(0) of up to 135 T at 183GPa. The current–voltage measurements show that the critical current density JC in (La,Y)H10 is 12–27.7 kA/mm2 at 4.2 K, which is comparable with that of commercial superconducting wires such as NbTi and Nb3Sn. (La,Y)H6 and (La,Y)H10 are among the first examples of ternary high-TC superconducting hydrides. Our experiments show that part of metal atoms in the structures of recently discovered Im3¯m-YH6 and Fm3¯m-LaH10 can be replaced with lanthanum (∼70%) and yttrium (∼25%), respectively, with the formation of unique ternary superhydrides containing metal-encapsulated cages La@H24 and Y@H32, which are specific for Im3¯m-LaH6 and Fm3¯m-YH10. This work demonstrates that hydrides, unstable in pure form such as LaH6 and YH10, may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the desired structure.

Investigation of Pinning Force Density F<sub>p</sub> and Critical Current Density J<sub>c</sub> of Nd<sub>0.33</sub>Eu<sub>0.33</sub>Gd<sub>0.33</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>y</sub>(NEG-123) Superconductors with Addition of 211 Phase Particles and Analysis with a Theoretical Evaluation Based on Flux Creep-Flow Model

The pinning force density Fp and the critical current density Jc were investigated in Nd0.33Eu0.33Gd0.33Ba2Cu3O(NEG-123) superconductors with addition of NEG-211 and EG-211 secondary phase particles of the volume fractions up to 10 mol% by analyzing experimental data of DC magnetization and compared with theoretical calculation based on flux creep-flow model taking the pinning parameter into account. The pinning parameters such that the number of flux lines in the flux bundle (g2), the most probable value of pinning strength (Am), distribution width (σ2), upper critical field (Bc2) were determined so that a good fit was obtained between theoretical and experimental results. The Chittagong Univ. J. Sci. 42(1): 24-38, 2020

Anisotropic lattice expansion and enhancement of superconductivity induced by interstitial carbon doping in Rhenium

We report the effect of carbon doping on the structural and superconducting properties of elemental Re studied in a series of polycrystalline ReCx samples with x up to 0.10. Structural analyses indicate that the C atoms occupy the octahedral interstitial sites in the hexagonal close-packed Re lattice. This leads to an expansion of unit cell mainly along the a-axis and a shift of the binding energies of Re towards higher values. Furthermore, these ReCx samples are found to be weakly coupled, fully gapped, type-II superconductors. With increasing C content x, both Tc and zero temperature upper critical field Bc2(0) increase monotonically. At x = 0.10, Tc and Bc2(0) reach 2.65 K and 1.18 T, respectively, which are more than 50% and 60 times larger than the corresponding values of pure Re. Combined with theoretical calculations, we show that the enhancement in Tc induced by C doping is due to the increase in both the density of states at the Fermi level and electron-phonon coupling. Our results suggest that the introduction of interstitial atoms provides a viable way to tune the superconductivity in transition metal elements.

Synthesis of Superconducting InxSn1−xTe (0.04 < x < 0.1) Large Single Crystal by Liquid Transport Method

In this work, a new crystal growth technique called the liquid transport method was introduced to synthesize single crystals of a topological superconductor candidate, InxSn1−xTe (IST). Crystals with the size of several millimeters were successfully synthesized, and were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy as well as electronic transport measurements. Lattice parameters decreased monotonously with the increase of indium content while hole density varied in reverse. Superconductivity with the critical temperature (Tc) around 1.6 K were observed, and the hole densities were estimated to be in the order of 1020 cm−3. The upper critical fields (Bc2) were estimated to be 0.68 T and 0.71 T for In0.04Sn0.96Te and In0.06Sn0.94Te, respectively. The results indicated that the quality of our crystals is comparable to that grown by the chemical vapor transport method, but with a relatively larger size. Our work provides a new method to grow large single crystals of IST and could help to solve the remaining open questions in a system that needs large crystals, such as a superconducting pairing mechanism, unconventional superconductivity, and so on.

On the mechanisms governing the critical current reduction in Nb3Sn Rutherford cables under transverse stress

Within the framework of the HiLumi-LHC project, CERN is currently manufacturing 11 T dipole and quadrupole accelerator magnets using state-of-the-art Nb3Sn Rutherford cables. Even higher magnetic fields are considered by the Hadron Future Circular Collider (FCC-hh) design study, which plans to develop 16 T Nb3Sn bending dipoles. In such high-field magnets, the design pre-stress can reach considerable values (150–200 MPa) and, since Nb3Sn is a brittle compound, this can constitute a technological difficult challenge. Due to the significant impact that a transverse load can have on the performances of a Nb3Sn magnet, CERN has launched a campaign of critical current measurements of reacted and impregnated Nb3Sn cables subjected to transverse pressure up to about 210 MPa. In this paper, results obtained on 18-strand 10-mm-wide cable sample based on a 1-mm-diameter powder-in-tube (PIT) wire are presented. The tests were carried out on a 2-m-long sample by using the FReSCa test station, at T = 4.3 K and background magnetic fields up to 9.6 T. For applied pressures below ≈ 130 MPa, only reversible reductions of the critical current, Ic, are observed. At higher pressures, a permanent Ic reduction occurs; such irreversible behaviour is due to the residual stresses generated by the plastic deformations of the copper stabilizer. This type of current reduction, whether reversible or not, is fully governed by the strain-induced variations of the upper critical field, Bc2. At higher pressures, estimated between 180 and 210 MPa, it is indeed plausible to believe that cracking of filaments occurs, with detrimental consequences for the Nb3Sn cable’s electrical performances. The complete set of critical current data here presented, collected at different pressures and as a function of the applied magnetic field, allows for the first time to investigate the evolution of superconducting parameters such as the upper critical field Bc2 in the irreversibility region, where both the effects of Cu matrix plasticization and/or cracking of filaments may occur. The experimental approach and data interpretation have a general value and can be applied to any typology of Rutherford cable.

Superconductivity in Cubic A15-type V–Nb–Mo–Ir–Pt High-Entropy Alloys

We report the crystal structure and superconducting properties of new V5+2xNb35−xMo35−xIr10Pt15 high-entropy alloys (HEAs) for x in the range of 0 ≤x≤ 10. These HEAs are found to crystallize in a cubic A15-type structure and have a weakly coupled, fully gapped superconducting state. A maximum Tc of 5.18 K and zero-temperature upper critical field Bc2(0) of 6.4 T are observed at x = 0, and both quantities decrease monotonically with the increase of V content x. In addition, Tc shows an increase with increasing valence electron concentration from 6.4 to 6.5, which is compared with other A15-type HEA and binary superconductors.

Recent Advances in 2D Superconductors.

The emergence of superconductivity in 2D materials has attracted much attention and there has been rapid development in recent years because of their fruitful physical properties, such as high transition temperature (Tc ), continuous phase transition, and enhanced parallel critical magnetic field (Bc ). Tremendous efforts have been devoted to exploring different physical parameters to figure out the mechanisms behind the unexpected superconductivity phenomena, including adjusting the thickness of samples, fabricating various heterostructures, tuning the carrier density by electric field and chemical doping, and so on. Here, different types of 2D superconductivity with their unique characteristics are introduced, including the conventional Bardeen-Cooper-Schrieffer superconductivity in ultrathin films, high-Tc superconductivity in Fe-based and Cu-based 2D superconductors, unconventional superconductivity in newly discovered twist-angle bilayer graphene, superconductivity with enhanced Bc , and topological superconductivity. A perspective toward this field is then proposed based on academic knowledge from the recently reported literature. The aim is to provide researchers with a clear and comprehensive understanding about the newly developed 2D superconductivity and promote the development of this field much further.

Superior engineering critical current density obtained via hot isostatic pressing of MgB2 wires manufactured using nano-amorphous isotopic boron

The effects of heat treatment temperature, time and isostatic pressure on the grain connectivity and superconducting properties of Mg11B2 wires prepared using nano-amorphous isotopic boron powder (11B) have been investigated. The article presents detailed studies of the Mg11B2 material structure of the wires using scanning electron microscope (SEM). Heat treatment at low and high temperature for different annealing time and isostatic pressure does not create an internal macro-defect that disrupts connectivity in the Mg11B2 superconducting wires. Our study shows that dense longitudinal superconducting layers are formed at high annealing temperature of 800 °C for 60 min. Formation of dense longitudinal superconducting layers under high isostatic pressure (HIP) allow improved grain connectivity and high density of pinning centers and as a result, high engineering critical current density, Jec, at 4.2 K has been achieved. Also, high upper critical field (Bc2) and high irreversible magnetic field (Birr) were obtained using HIP process while superconducting transition temperature (Tc) was not significantly affected. Better grain connectivity obtained using high annealing temperature and long annealing time under low isotopic pressure leads to Jec enhancement at high magnetic fields at 20 K. Our results show that the annealing temperature of 800 °C for 60 min under isostatic pressure of 1 GPa yields the required Jec of 25 A/mm2 and 50 A/mm2 for ITER applications in magnetic fields of 8 T and 7 T, respectively. The promising results obtained in this work could pave the way for designing low activation superconducting Mg11B2 wires to be considered for next generation fusion magnets.

Anomalous High-Temperature Superconductivity in YH6.

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors, which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here, the synthesis of one of the best-known high-TC superconductors-yttrium hexahydride -YH6 is reported, which displays a superconducting transition at ≈224K at 166GPa. The extrapolated upper critical magnetic field Bc2 (0)of YH6 is surprisingly high: 116-158T, which is 2-2.5 times larger than the calculated value. A pronounced shift of TC in yttrium deuteride YD6 with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements show that the critical current IC and its density JC may exceed 1.75A and 3500A mm-2 at 4K, respectively, which is higher than that of the commercial superconductors, such as NbTi and YBCO. The results of superconducting density functional theory (SCDFT) and anharmonic calculations, together with anomalously high critical magnetic field, suggest notable departures of the superconducting properties from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories, and presence of an additional mechanism of superconductivity.

Tailoring superconducting phases observed in hyperdoped Si:Ga for cryogenic circuit applications

Hyperdoping with gallium (Ga) has been established as a route to observe superconductivity in silicon (Si). The relatively large critical temperatures (Tc) and magnetic fields (Bc) make this phase attractive for cryogenic circuit applications, particularly for scalable hybrid superconductor–semiconductor platforms. However, the robustness of Si:Ga superconductivity at millikelvin temperatures is yet to be evaluated. Here, we report the presence of a re-entrant resistive transition below Tc for Si:Ga whose magnitude strongly depends on the distribution of the Ga clusters that precipitate in the implanted Si after annealing. By monitoring the re-entrant resistance over a wide parameter space of implantation energies and fluences, we determine conditions that significantly improve the coherent coupling of Ga clusters, therefore eliminating the re-entrant transition at temperatures as low as 20 mK.

Critical fields and features of electromagnetic transport of Bi2Se3 whiskers at low temperatures

The temperature dependences of resistance of n-type conductivity Bi2Se3 whiskers with doping concentration (1–2)⋅1019 cm−3 were studied in the temperature range 1.6–300 K. The sharp drop of the resistance was detected that is a result of a partial transition to the superconductive state at the critical temperature Tc = 5.3 K and probably due to the inclusion of β-PdBi2 phase in the studied samples. The transverse magnetoresistance of Bi2Se3 whiskers with various concentration of Pd doping impurity corresponding to the metal side of metal-insulator transition was studied in a magnetic field of 0–10 T. Superconductivity suppression effect by the magnetic field was found that permits to create the basic parameters such as the upper critical field Bc2 = 1.5 T, the coherence length of the superconductor ξ (0) = 15 nm and the superconducting gap Δ ≈ 0.8 meV. Studies of the n-type conductivity Bi2Se3 whiskers allow to create on their basis magnetic field sensors with a sensitivity of about 3.5% / T, capable in the temperature range of 4.2–77 K. The investigated crystals can also be used in the magnetic switch-control sensors magnetic switches due to the transition to the superconducting state at the critical temperature Tc, depending on a magnetic field induction.

High-field thermal transport properties of the Kitaev quantum magnet α−RuCl3 : Evidence for low-energy excitations beyond the critical field

We investigate the phononic in-plane longitudinal low-temperature thermal conductivity kappa_ab of the Kitaev quantum magnet alpha-RuCl3 for large in-plane magnetic fields up to 33 T. Our data reveal for fields larger than the critical field Bc ~ 8 T, at which the magnetic order is suppressed, a dramatic increase of kappa_ab at all temperatures investigated. The analysis of our data shows that the phonons are not only strongly scattered by a magnetic mode at relatively large energy which scales roughly linearly with the magnetic field, but also by a small-energy mode which emerges near Bc with a square-root-like field dependence. While the former is in striking agreement with recent spin wave theory (SWT) results of the magnetic excitation spectrum at the Gamma point, the energy of the latter is too small to be compatible with the SWT-expected magnon gap at the M point, despite the matching field dependence. Therefore, an alternative scenario based on phonon scattering off the thermal excitation of random-singlet states is proposed.

Excess Conductivity Analysis of Polycrystalline FeSe Samples with the Addition of Ag.

Bulk FeSe superconductors of the iron-based (IBS) “11” family containing various additions of silver were thoroughly investigated concerning the microstructure using optical microscopy and electron microscopy (TEM and SEM). The measurements of electrical resistivity were performed through the four-point technique in the temperature interval 2–150 K. The Aslamazov–Larkin model was employed to analyze the fluctuation-induced conductivity (FIC) in all acquired measurements. In all studied products, we found that the FIC curves consist of five different regimes of fluctuation, viz. critical region (CR), three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and shortwave fluctuation (SWF) regimes. The critical current density (), the lower and upper critical magnetic fields ( and ), the coherence length along the c-axis at zero-temperature ((0)), and further parameters were assessed with regards to the silver amount within the products. The analyses discloses a diminution in the resistivity and a great reduction in (0) with Ag addition. The optimal silver doping amount is achieved for 7 wt.%, which yields the best superconducting transition and the greatest value.

Phase Formation and Conductivity Fluctuation Investigation in Nanoparticle SnO2-Added Y3Ba5Cu8O18 ± δ Polycrystalline Superconductor

The influence of 0, 0.20, 0.40, 0.50, and 0.60 wt % nano-sized tin oxide (SnO2) particles on electrical conductivity fluctuation in normal and superconducting state of the Y3Ba5Cu8O18 ± δ (denoted as Y‑358) polycrystalline samples is studied. Phase formation and microstructures have been systematically examined. By increasing the content of SnO2 in YBCO matrix, X-ray diffraction technique showed slight variation in lattice parameters and overall reduction in the orthorhombicity. Scanning electron microscopy observations and the crystallite size calculation also revealed that the grain size and the average crystallite size decreased compared to the SnO2-free sample. Aslamazov–Larkin and Lawrence–Doniach prototypes were used to analyze conductivity fluctuations based on the electrical resistivity ρ(T) measurements. Superconducting transition temperatures TC and TLD have been reported. The influence of SnO2 addition on the superconducting properties indicates that with the addition of SnO2 nanoparticles into Y-358 compound, some parameters values such as zero-resistance critical temperature TC zero, coherence distance alongside the c axis at 0 K ξc(0), and super-layer length d decrease in total, while anisotropy γ, critical magnetic fields Bc1(0), Bc2(0), and critical current density Jc(0) increase in SnO2-added Y-358 specimens compared to the pure one. The reasons corresponding to these scenarios are discussed in details.

Superconductivity and equation of state of lanthanum at megabar pressures

Lanthanum (La) is the first member of the rare-earth series of elements that has recently raised considerable interest because of its unique high-Tc superhydride LaH10. Although several studies have found superconductivity and phase transitions in metallic La, there was a lack of experimental evidence for the equation of state (EoS) and superconductivity above one megabar pressure. Here, we extend the pressure range up to 140 GPa to study EoS and superconductivity of lanthanum via electrical transport and X-ray diffraction measurements. The experimental XRD patterns point to a phase transition sequences R3m-Fm3m-Fmmm above 78 GPa. All the experimental pressure-volume data were fitted by the 3rd order Birch-Murnaghan equation: V0 = 35.2 (4) A^3, B0 = 27 (1) GPa and B0' = 4. Superconducting critical temperature Tc(onset) of lanthanum is 9.6 K at 78 GPa, which decreases to 2.2 K at 140 GPa. The upper critical magnetic field Bc2(0) was found to be 0.32-0.43 T at 140 GPa. Ab initio calculations give predicted Tc(A-D)=2.2 K (mu*=0.195), dTc/dP = 0.11-0.13 K/GPa and Hc=0.4 T at 140 GPa.

Improvement of superconducting properties by chemical pressure effect in Eu-doped La2-xEuxO2Bi3Ag0.6Sn0.4S6

We have investigated the substitution effect of Eu on the superconductivity in La2-xEuxO2Bi3Ag0.6Sn0.4S6. Recently, we reported an observation of superconductivity at 0.5 K in a layered oxychalcogenide La2O2Bi3AgS6. The Sn doping at the Ag site was found to raise the superconducting transition temperature, Tc, to 2.5 K in La2O2Bi3Ag0.6Sn0.4S6. To further improve the superconducting properties, we have partially substituted Eu for the La site to increase the chemical pressure in La2-xEuxO2Bi3Ag0.6Sn0.4S6 (x = 0.1–0.6). La2-xEuxO2Bi3Ag0.6Sn0.4S6 is crystallized in a tetragonal phase with the space group P4/nmm. With the increase in Eu concentration, x, the lattice constant a decreased, while the lattice constant c was marginally shortened, which suggests that the chemical pressure induced by the Eu doping is uniaxial along the a-axis. Tc increased with increasing x up to x = 0.4, and the suppression of superconductivity was observed for higher Eu concentrations of x = 0.5 and 0.6. From the magnetic susceptibility and resistivity measurements, a superconducting transition has been confirmed for x = 0.1–0.6 with Tc = 2.5–4.0 K, respectively. The estimated upper critical field (Bc2) was 3.5 T for x = 0.4, which also has the highest Tc. We observed the jump in the temperature dependence of heat capacity near Tc = 4.0 K for La1.6Eu0.4O2Bi3Ag0.6Sn0.4S6, which confirms the emergence of bulk superconductivity in La1.6Eu0.4O2Bi3Ag0.6Sn0.4S6.

Magnetic‐Field‐Controlled Quantum Critical Points in the Triangular Antiferromagnetic Cs2CuCl4‐xBrx Mixed System

AbstractThe triangular antiferromagnetic Cs2CuCl4‐xBrx mixed system is studied by neutron single‐crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field Bc is found for a Br concentration x = 0.8 at Bc = 8.10(1) T and for x = 1.1 at Bc = 7.73(1) T and from regime IV (3.2 < x < 4) for x = 3.3 at Bc = 0.99(3) T. For magnetic fields larger than the respective Bc, magnetic superlattice reflections of these compounds are not found. The incommensurate magnetic wave vector q = (0, 0.470, 0) appears below the ordering temperature TN = 0.51(1) K for Cs2CuCl3.2Br0.8, and q = (0, 0.418, 0) below TN = 1.00(6) K for Cs2CuCl0.3Br3.7. Neutron diffraction experiments at around 60 mK for x = 3.7 in a magnetic field show the critical magnetic field at Bc = 7.94(16) T and the formation of the second magnetic phase at around 8.5 T depending on the temperature. Inelastic neutron scattering experiments for the compound from regime III (2 < x ≤ 3.2) with x = 2.2 show dynamical correlations at a temperature around 50 mK giving evidence for a spin liquid phase.

Superconductivity in hexagonal Nb-Mo-Ru-Rh-Pd high-entropy alloys

We report the superconducting properties of new hexagonal Nb10+2xMo35−xRu35−xRh10Pd10 high-entropy alloys (HEAs) (0 ≤ x ≤ 5). With increasing x, the superconducting transition temperature Tc shows a maximum of 6.19 K at x = 2.5, while the zero-temperature upper critical field Bc2(0) increases monotonically, reaching 8.3 T at x = 5. For all x values, the specific heat jump deviates from the Bardeen-Cooper-Schreiffer behavior. In addition, we show that Tc of these HEAs is not determined mainly by the density of states at the Fermi level and would be enhanced by lowering the valence electron concentration.