Critical Field Bc2 Research Articles

Superconductivity in TiZrNb and TiZrNbHf bulk equimolar alloys

We have prepared and investigated superconducting equimolar medium-entropy alloys of TiZrNb and TiZrNbHf. Their basic superconducting parameters, as the values of the critical temperature Tc, upper critical magnetic field Bc2 and the superconducting energy gap Δ have been studied with the use of magnetic susceptibility and point-contact Andreev reflection (PCAR) spectroscopy measurements. Although our samples have different critical temperatures, namely Tc ∼ 8.2 K for TiZrNb and Tc ∼ 6.3 K for TiZrNbHf, their zero-temperature critical magnetic field has the same value Bc2(0) = 12 T, from the vicinity of the Clogston limit. We have observed the presence of degraded phases on the surfaces of all the investigated samples using PCAR measurements. Still, we were able to show that the bulk phase in both systems exhibits BCS weakly coupled superconductivity with 2Δ(0)/kBTc ∼ 3.5.

Vortex phase diagram and transition in (Bi1.6Pb0.4Sr2Ca2Cu3O10-δ)1-x(SnO2)x superconductors

The superconducting vortex phase diagram of (Bi1.6Pb0.4Sr2Ca2Cu3O10-δ)1-x(SnO2)x ceramics, where x = 0, 0.002, 0.004, 0.006, 0.008, and 0.010, was investigated using resistivity measurement under magnetic fields. If the value of the offset critical temperature (Tc,offset) monotonously decreased on the SnO2-added samples, then the mean-field critical temperature (Tmf) would slightly improve on the x = 0.002 sample. The excess conductivity of all samples was analyzed based on the Aslamazov–Larkin and Lawrence–Doniach models. The c-axis coherence length at 0 K (ξc(0)) and the interlayer coupling strength were optimized on the x = 0.002 sample. The activation energy (U0) calculated using the Arrhenius model was also increased, and the maximum for the x = 0.002 sample was reached. The upper critical field (Bc2) deduced using the Werthamer–Helfand–Hohenberg model was also enhanced for the x = 0.002 sample. The small bundle field (Bsb), large bundle field (Blb), irreversibility field (Birr), and Bc2 were combined for the vortex phase (B-T) diagram of the x = 0.000 and x = 0.002 samples. All pinning regimes of the x = 0.002 sample were extended, clearly revealing the improvements in the flux pinning properties in that sample.

Unconventional superconductivity in chiral molecule-TaS2 hybrid superlattices.

Chiral superconductors, a unique class of unconventional superconductors in which the complex superconducting order parameter winds clockwise or anticlockwise in the momentum space1, represent a topologically non-trivial system with intrinsic time-reversal symmetry breaking (TRSB) and direct implications for topological quantum computing2,3. Intrinsic chiral superconductors are extremely rare, with only a few arguable examples, including UTe2, UPt3 and Sr2RuO4 (refs. 4-7). It has been suggested that chiral superconductivity may exist in non-centrosymmetric superconductors8,9, although such non-centrosymmetry is uncommon in typical solid-state superconductors. Alternatively, chiral molecules with neither mirror nor inversion symmetry have been widely investigated. We suggest that an incorporation of chiral molecules into conventional superconductor lattices could introduce non-centrosymmetry and help realize chiral superconductivity10. Here we explore unconventional superconductivity in chiral molecule intercalated TaS2 hybrid superlattices. Our studies reveal an exceptionally large in-plane upper critical field Bc2,|| well beyond the Pauli paramagnetic limit, a robust π-phase shift in Little-Parks measurements and a field-free superconducting diode effect (SDE). These experimental signatures of unconventional superconductivity suggest that the intriguing interplay between crystalline atomic layers and the self-assembled chiral molecular layers may lead to exotic topological materials. Our study highlights that the hybrid superlattices could lay a versatile path to artificial quantum materials by combining a vast library of layered crystals of rich physical properties with the nearly infinite variations of molecules of designable structural motifs and functional groups11.

Phase Transition near the Filling Factor ν = 3

A phase transition accompanied by the appearance of a spike in the longitudinal resistance of a two-dimensional electron system has been studied using the electron spin resonance near the filling factor ν = 3 in the ZnO/MgZnO heterojunction. This transition occurs when the tilt angle θ of the magnetic field is increased to some critical value θc. An analysis of the spin resonance amplitude has made it possible to demonstrate the spin nature of this phenomenon. For example, the ground state of the system on both sides of the transition has a nonzero spin polarization, which changes by several times when the phase of the system is changed. Strong spin resonance is observed both at θ < θc and at θ > θc. Surprisingly, the spin resonance at the critical angle θc has been detected in only one phase, which lies in the region of magnetic fields below the critical field Bc corresponding to the spike position in the longitudinal resistance. An increase in the magnetic field to this value leads to a decrease in the resonance amplitude and an increase in the resonance width. In the field region above Bc, the spin resonance disappears completely. Such behavior of the spin resonance is most likely due to the formation of domains with different spin polarizations in the electron system.

Excess conductivity and magnetoresistance analysis for (BSF)x/(Bi, Pb)-2223 composite

This study examined the impact of adding hard ferrite Ba0.5Sr0.5Fe12O19 (BSF) nanoparticles to the Bi1.8Pb0.4Sr2Ca2Cu3.2O10+δ (Bi, Pb)-2223) superconductor phase. The investigation specifically focused on evaluating the critical current density, fluctuation-induced conductivity, and magnetoresistance of nano-(BSF)x/(Bi, Pb)-2223 composite, where 0.00 ≤ x ≤ 0.20 wt.%. The results revealed that the critical current density, Jc, increased with the addition of nano-(BSF) up to x = 0.04 wt.%, reaching a value of 441.20 A/cm2. The Aslamazov and Larkin (A–L) approach has been evaluated the fluctuation-induced conductivity. Several superconducting parameters, including coherence length ζc(0), effective layer thickness d, penetration depth λpd(0), and Fermi energy EF\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${E}_{F}$$\\end{document} showed improvement as the concentration of nano-(BSF) increased up to x = 0.04 wt.%. In addition to Ginzburg–Landau critical parameters, such as the thermodynamic critical field Bc(0), lower critical magnetic field Bc1(0), upper critical magnetic field Bc2(0), and critical current density Jc(0) demonstrated an increase up to x = 0.04 wt.%, followed by a decrease for higher concentrations. The magnetoresistance measurements were performed at various applied DC magnetic fields, with values ranging from 0.29 to 4.44 kG, and were analyzed using the thermally activated flux creep (TAFC) and Ambegaokar–Halperin (AH) models. The calculated flux pinning energy (U) increased with the addition of nano-(BSF) up to x = 0.04 wt.% and then decreased for x > 0.04 wt.%. Furthermore, the transition width (ΔT), was observed to increase as the applied magnetic field values increased. Moreover, the addition of nano-(BSF) increased the field-independent critical current density, Jc,0(0), up to x = 0.04 wt.%, after which it decreased for higher concentrations.

Excess Conductivity Analysis of an YBCO Foam Strut and Its Microstructure.

Struts of a superconducting YBa2Cu3Oy (YBCO) foam prepared by the infiltration growth method on the base of commercial polyurethane foams were extracted from the bulk, and thoroughly characterized concerning the microstructure and the magnetoresistance, measured by the four-point technique. Optical microscopy, electron microscopy, electron backscatter diffraction and atomic force microscopy observations indicate a unique microstructure of the foam struts which shows a large amount of tiny Y2BaCuO5 (Y-211) particles (with diameters between 50 and 100 nm) being enclosed in channel-like grain boundaries between the YBCO grains and a one-of-a-kind surface of the struts covered with Ba3Cu5Oy-particles. The resistance data obtained at temperatures in the range 4.2 K ≤T≤ 150 K (applied magnetic fields ranging from 0 to 7 T) were analyzed in the framework of the fluctuation-induced conductivity (FIC) approach using the models of Aslamazov-Larkin (AL) and Lawrence-Doniach (LD). The resulting FIC curves reveal the presence of five distinct fluctuation regimes, namely, the short-wave (SWF), one-dimensional (1D), two-dimensional (2D), three-dimensional (3D), and critical (CR) fluctuation domains. The analysis of the FIC data enable the coherence length in the direction of the c-axis at zero-temperature (ξc(0)), the irreversibility field (Birr), the upper critical magnetic field (Bc2), the critical current density at T= 0 K (Jc(0)) and several other parameters describing the the material's superconducting properties to be determined. The present data reveal that the minuscule Y-211 particles found along the YBCO grain boundaries alter the excess conductivity and the fluctuation behavior as compared to conventional YBCO samples, leading to a quite high value for Jc(0) for a sample with a non-optimized pinning landscape.

Multiple transition temperature enhancement in superconducting TiNbMoTaW high entropy alloy films through tailored N incorporation

We report about the influence of nitrogen incorporation on the superconducting transition temperature TC of TiNbMoTaW high entropy alloy films deposited using high power impulse magnetron sputtering. By measuring the temperature dependence of resistivity of (TiNbMoTaW)Nx nitrides, we observe a significant increase of TC, from 0.62 K for x = 0 up to 5.02 K for x = 0.74. With further increase of x, TC is decreasing and reaches 1.08 K for x = 0.97. The eightfold TC enhancement seems to be associated with the incorporation of light N atoms into the face-centered cubic lattice and with the x dependent enhancement of the electron-phonon interaction, which may be related to the high configuration entropy in high entropy alloys. Measurements in magnetic field show that the upper critical fields Bc2 of (TiNbMoTaW)Nx with x > 0.15 provide Bc2/TC > 2 T/K ratios, which are above the weak-coupling pair breaking limit. Additional heat capacity and point contact spectroscopy measurements show that the superconductivity in the ∼1 μm thick films is bulk in nature, consistent with conventional weak-coupling phonon mediated superconductivity. The proposed strategy of nitrogen incorporation into high entropy alloys may pave a pathway towards tailoring their superconducting properties, especially their TC.

The interplay between exciton- and phonon-induced superconductivity might explain the phenomena observed in LK-99

The experimental results hinting at the room temperature and ambient pressure superconductivity and magnetic levitation in LK-99 attracted an unprecedented interest. While attempts of other teams to reproduce the reported observations on similar samples failed so far, it seems worthwhile to try building a theoretical model that would explain the ensemble of the available data. One of important features that needs to be explained is an apparent contradiction between an extremely high critical temperature Tc and rather modest critical magnetic field Bc and critical current jc reported for LK-99. We show theoretically, that these data may be quantitatively reproduced assuming the interplay between exciton- and phonon-induced superconductivity, while the conventional BCS or Brinkman-Rice-Bardeen-Cooper-Schriefer (BR-BCS) mechanisms would result in a much higher Bc for the same Tc.

Unconventional superconductivity in PdxBi2Se3 whiskers

Studies of temperature dependence of magnetoresistance for PdxBi2Se3 whiskers in the temperature range 1.6-77K in magnetic field up to 10 T were carried out. Crystals were grown by chemical transport reaction method in closed bromide system. The source and crystallization zone temperatures were 1100 K and 780 K, respectively. Doping of the crystals was carried out during the growth process with palladium impurity to concentrations of (1 – 2) × 1019 cm−3. In the low-temperature region beginning at a temperature of 5 K and reaching a temperature 3.5 K, a sharp decrease in resistance was observed, which is associated with the transition to superconducting state. Based on the analysis of the temperature dependence of the resistance at fixed magnetic fields, the Curie temperature Tc1=5.3 K and Tc2=3.5 K as well as the upper critical magnetic field Bc2=1.45 T and 0.25 T were determined. The established parameters indicates in II type supercondor. This is indicated by the ratio Δ0/kBTc = 2.0, which exceeds BCS limit of 1.76 and indicates a relatively large value of the superconducting gap Δ0=0.8 meV. The determined ratio A/γ2, which establishes the relationship between the electron-electron and electron-phonon interaction, is about of 2ao, which indicates a strong fermionic interaction with phonons in the PdxBi2Se3 superconductor. The estimated value of the ratio of the Curie temperature to the effective Fermi temperature equal to 0.04 also falls within the range of 0.01 ⩽ Tc/TF ⩽ 0.1, which confirms the unconventional superconductivity in the investigated whiskers.

Non-Fermi-Liquid Behavior of Superconducting SnH4.

The chemical interaction of Sn with H2 by X-ray diffraction methods at pressures of 180-210GPa is studied. A previously unknown tetrahydride SnH4 with a cubic structure (fcc) exhibiting superconducting properties below TC = 72 K is obtained; the formation of a high molecular C2/m-SnH14 superhydride and several lower hydrides, fcc SnH2 , and C2-Sn12 H18 , is also detected. The temperature dependence of critical current density JC (T) in SnH4 yields the superconducting gap 2Δ(0)= 21.6 meV at 180GPa. SnH4 has unusual behavior in strong magnetic fields: B,T-linear dependences of magnetoresistance and the upper critical magnetic field BC2 (T) ∝ (TC - T). The latter contradicts the Wertheimer-Helfand-Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of fcc SnH4 in non-superconducting state exhibits a deviation from what is expected for phonon-mediated scattering described by the Bloch-Grüneisen model and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed.

Superconducting properties and gap structure of the topological superconductor candidate Ti3Sb

We present a study of the superconducting properties of the candidate topological superconductor Ti_(3)Sb. Electrical transport measurements show zero resistance with a T_(c,onset) of ~ 5.9 K with a transition width {\Delta}T_c~ 0.6 K. The superconducting phase boundaries as derived from magneto-transport and magnetic susceptibility measurements agree well. We estimate an upper critical field Bc2(0)~4.5 T. A Ginzburg-Landau (GL) analysis yields values of the coherence length and penetration depth of {\zeta} = 6.2 nm and {\lambda} = 340 nm, respectively, and a GL parameter ~ 55, indicating extreme type-II behavior. Furthermore, we observed a step height in the specific heat ({\Delta}C_e)/({\gamma}T_c )~1.61, a value larger than the Bardeen-Cooper-Schrieffer (BCS) value of 1.43, suggesting modest coupling. Measurements of the temperature dependence of the London penetration depth via the tunnel-diode oscillator (TDO) technique down to ~ 450 mK show a full superconducting gap, consistent with a conventional s-wave gap structure.

Impact of adding bismuth vanadate (BiVO4) nanoparticles on the electrical and mechanical properties of Cu0.5Tl0.5Ba2Ca2Cu3O10-δ ceramic

A mechanochemical route was used to prepare bismuth vanadate (BiVO4) with an average grain size of 20 nm. To obtain (BiVO4)x/Cu0.5Tl0.5Ba2Ca2Cu3O10-δ composites, appropriate amounts of BiVO4 (x) were added to Cu0.5Tl0.5Ba2Ca2Cu3O10-δ via a solid-state reaction. The superconducting critical temperature (Tc) was dropped from 119.5 to 103.5 K with x. The different conduction mechanisms were studied using the Aslamazov–Larkin and Lawrence–Doniach models. Four different fluctuation zones were observed. As the BiVO4 content rose the coherence length was increased from 19.69 to 39.30 Å. Superconducting parameters such as thermodynamic critical field Bc(0), lower critical magnetic field Bc1(0), upper critical magnetic field Bc2(0), and critical current density Jc(0) were decreased with x. Furthermore, the mechanical strength was studied through Vickers microindentation test and variety of mechanical parameters, including elastic modulus (E), yield strength (Y), fracture toughness (K), and brittleness index (B), were determined as a function of BiVO4 content.Graphical abstract

Superconductivity with large upper critical field in noncentrosymmetric Cr-bearing high-entropy alloys

A series of new Cr5+xMo35−xW12Re35Ru13C20 high-entropy alloys (HEAs) have been synthesized and characterized by x-ray diffraction, scanning electron microscopy, electrical resistivity, magnetic susceptibility and specific heat measurements. It is found that the HEAs adopt a noncentrosymmetric cubic β-Mn type structure and exhibit bulk superconductivity for 0 ≤x≤ 9. With increasing x, the cubic lattice parameter decreases from 6.7940(3) Å to 6.7516(3) Å. Meanwhile, the superconducting transition temperature Tc is suppressed from 5.49 K to 3.35 K due to the magnetic pair breaking caused by Cr moments. For all these noncentrosymmetric HEAs, the zero-temperature upper critical field Bc2(0) is comparable to the Pauli paramagnetic limit BP(0) = 1.86Tc. In particular, the Bc2(0)/BP(0) ratio reaches a maximum of ∼1.03 at x = 6, which is among the highest for β-Mn type superconductors.

Lifshitz scaling effects on the holographic model for paramagnetism-antiferromagnetism phase transition

In this work, we construct a holographic antiferromagnetism model by introducing two real antisymmetric tensor fields coupling to the background gauge field strength and interacting with each other in the four-dimensional Lifshitz black hole, and investigate the effects of Lifshitz dynamical exponent z on the paramagnetism-antiferromagnetism phase transition in detail. Using the numerical shooting method, we find that there is still a phase transition in asymptotically Lifshitz space-time in the probe limit. In the absence of an external magnetic field and in low temperature, the magnetic moments condense spontaneously in antiparallel manner with the same magnitude. But the critical temperature and the staggered magnetization decrease with increasing z, which makes paramagnetism-antiferromagnetism phase transition harder to happen. In the presence of the weak external magnetic field, the magnetic susceptibility has a peak at the critical temperature and increasing values of z makes the peak higher. In the case of the strong external magnetic field, there exists a critical magnetic field Bc in the antiferromagnetic phase. When the magnetic field reaches Bc, the system turns into the paramagnetic phase. Besides, the increasing of z results in increasing of Bc.

Investigation of transport properties, flux pinning mechanisms and fluctuations induced conductivity of SiO2 nanoparticles doped YBa2Cu3O7-d thick films on silver substrates

In this study, we examined the pinning mechanism and fluctuations induced conductivity, and we report the superconducting critical parameters variation of fine SiO2 nanoparticles added YBa2Cu3O7−d (Y123) composite thick films of ∼100 μm in thicknesses on Ag substrate. Composite films (Y123 + x SiO2)/Ag were produced by the solid-state reaction process. Here, x = 0.00 and 0.01 wt% content of SiO2 nanoparticles (12 nm in diameter) were considered. The X-ray powder diffraction (XRD) together with Rietveld refinement and the scanning electron microscopy (SEM) were used to survey the structure and the morphology of thick films. Both composite films reveal a dense granular structure and similar compositions in phases. A small quantity of fine SiO2 particles intensifies the critical current density versus magnetic field Jc(H) and gives more weight to the strong pinning component of the Jc. Some important critical physical parameters of the composite films were obtained from the fluctuations induced conductivity analyses. The electrical conductivity was deduced from the electrical resistivity measurements and explained through Aslamazov–Larkin model. A small quantity of fine SiO2 particles upgrade the superconducting features, particularly the lower and the upper critical magnetic field (Bc1 and Bc2) at temperature 0 K.

Positive magnetoconductivity and inelastic scattering time at low temperatures with magnetic field in InSb semiconductor

In this work, we investigate the temperature dependence of the perpendicular electrical conductivity in presence of magnetic field of InSb sample measurements obtained by Abboudy [16]. First we determine the value of the critical magnetic field BC for which the metal insulator transition (MIT) occurs. On the metallic side of the MIT, we are interested in the study of positive magnetoconductivity as a function of magnetic field by modeling it with complex theoretical models. The validity of these models is tested by calculating and comparing the inelastic scattering time τ ε for each model used.

Valley-filling instability and critical magnetic field for interaction-enhanced Zeeman response in doped WSe2 monolayers

Carrier-doped transition metal dichalcogenide (TMD) monolayers are of great interest in valleytronics due to the large Zeeman response (g-factors) in these spin-valley-locked materials, arising from many-body interactions. We develop an ab initio approach based on many-body perturbation theory to compute the interaction-enhanced g-factors in carrier-doped materials. We show that the g-factors of doped WSe2 monolayers are enhanced by screened-exchange interactions resulting from magnetic-field-induced changes in band occupancies. Our interaction-enhanced g-factors g* agree well with experiment. Unlike traditional valleytronic materials such as silicon, the enhancement in g-factor vanishes beyond a critical magnetic field Bc achievable in standard laboratories. We identify ranges of g* for which this change in g-factor at Bc leads to a valley-filling instability and Landau level alignment, which is important for the study of quantum phase transitions in doped TMDs. We further demonstrate how to tune the g-factors and optimize the valley-polarization for the valley Hall effect.

Creation of single chiral soliton states in monoaxial helimagnets

In monoaxial helimagnets, the Dzyaloshinskii–Moriya interaction favors inhomogeneous distributions of the magnetization with chiral modulations of solitonic character. In addition to the helical magnetic state at zero field, a chiral soliton lattice can be stabilized when a magnetic field perpendicular to the chiral axis is applied. When the magnetic field is increased, the system undergoes a phase transition to the uniform state at a critical field Bc. Above Bc, a single chiral soliton comprises the lowest level excitation over the stable uniform state, surviving as a metastable configuration. How to retain a single chiral soliton metastable state has not been addressed yet. Using micromagnetic simulations, we analyze this possibility by injecting spin polarized currents and put forward a feasible protocol to obtain a state with a single chiral soliton from the chiral soliton lattice. Our proposal could be relevant in the experimental study of metastable solitons for technological applications.

High-temperature-tolerable superconducting Nb-alloy and its application to Pb- and Cd-free superconducting joints between NbTi and Nb3Sn wires

For more than 30 years, Pb–Bi alloy and Wood's metal (50% Bi, 26.7% Pb, 13.3% Sn, and 10% Cd) have been used as representative superconducting solder intermedia to establish superconducting joints between NbTi and Nb3Sn wires in high-field nuclear magnetic resonance magnet systems. However, the use of Pb and Cd has been severely restricted by environmental regulations, such as the Restriction of Hazardous Substances Directive. Herein, a novel method of forming a superconducting joint between NbTi and Nb3Sn wires without Pb and Cd has been proposed. This approach is based on metallurgical bonding processes using a superconducting Nb-alloy intermedium, whose fine microstructure is maintained even after exposure to temperatures higher than 650 °C. Further, fine crystal defects become sources of magnetic flux pinning centers. Among transition elements close to Nb, Hf is considered the most suitable additive for realizing high-temperature-tolerable (HTT) superconducting Nb-alloy intermedia. Utilizing the HTT characteristic of Nb–Hf, a superconducting joint between Nb3Sn filaments and one end of the Nb–Hf alloy core was created by forming a superconducting Nb3Sn layer at the interface through a chemical reaction. The other end of the Nb–Hf alloy core was cold-pressed with NbTi filaments, to connect their active new surfaces to each other in order to create a superconducting joint. Ultimately, a superconducting joint between NbTi and Nb3Sn wires was realized with a high critical magnetic field (Bc2) of more than 1 T. The formation of the superconducting joint was confirmed by current decay measurements. This method of forming a superconducting joint is promising for application in environmentally friendly nuclear magnetic resonance magnet systems.Graphical abstract

Magnetic-field-controlled spin fluctuations and quantum criticality in Sr3Ru2O7

When the transition temperature of a continuous phase transition is tuned to absolute zero, new ordered phases and physical behaviour emerge in the vicinity of the resulting quantum critical point. Sr3Ru2O7 can be tuned through quantum criticality with magnetic field at low temperature. Near its critical field Bc it displays the hallmark T-linear resistivity and a T,{{{{{{mathrm{log}}}}}}},(1/T) electronic heat capacity behaviour of strange metals. However, these behaviours have not been related to any critical fluctuations. Here we use inelastic neutron scattering to reveal the presence of collective spin fluctuations whose relaxation time and strength show a nearly singular variation with magnetic field as Bc is approached. The large increase in the electronic heat capacity and entropy near Bc can be understood quantitatively in terms of the scattering of conduction electrons by these spin-fluctuations. On entering the spin-density-wave ordered phase present near Bc, the fluctuations become stronger suggesting that the order is stabilised through an “order-by-disorder” mechanism.