Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2

We report synthesis and superconductivity at 3.7 K in PrO_0.5F_0.5BiS₂. The newly discovered material belongs to the layered sulfide based REO_0.5F_0.5BiS₂ compounds having a ZrCuSiAs-type structure. The bulk polycrystalline compound is synthesized by the vacuum encapsulation technique at 780 ^°C in a single step. Detailed structural analysis has shown that the as synthesized PrO_0.5F_0.5BiS₂ is crystallized in a tetragonal P4/nmm space group with lattice parameters a=4.015(5) Å, c=13.362(4) Å. Bulk superconductivity is observed in PrO_0.5F_0.5BiS₂ below 4 K from magnetic and transport measurements. Electrical transport measurements showed superconducting transition temperature (T _c ) onset at 3.7 K and T _c (ρ=0) at 3.1 K. The hump at T _c related to the superconducting transition is not observed in the heat capacity measurement and rather a Schottky-type anomaly is observed at below ～6 K. The compound is slightly semiconducting in a normal state. Isothermal magnetization (MH) exhibited typical type II behavior with a lower critical field (H _c1) of around 8 Oe.     

Atomic Motion in Low-Coverage Helium Films Adsorbed in FSM Nanochannels

⁴He and ³He films adsorbed in nanoporous silicates have shown similar heat capacities until the quantum-fluid layer appears at coverages over the first-layer completion n ₁. To obtain information on dynamics of adatoms at low coverages below the quantum fluid region, we have done pulsed-NMR experiment at 3.3 MHz for ³He films adsorbed in straight 2.4 nm nanochannels of FSM silicates. The spin-lattice and spin-spin relaxation times T ₁ and T ₂ observed at 0.54–7 K were well described by the two-dimensional version of the Bloembergen-Purcell-Pound model. At coverages 0.4–1.7n ₁, minima of T ₁, indicating the spin correlation time τ _c of 4.8×10^?8 sec, were observed at temperatures between 6 and 3 K. With decreasing temperature, changes in T ₁ and T ₂ become small below about 1.5 K, suggesting crossover from thermally-activated motion to quantum tunneling. In contrast to large variations below n ₁, both relaxation times above n ₁ are almost independent of coverage, which is likely to indicate that τ _c is determined by interlayer exchange of adatoms. Below n ₁, onsets for localization of adatoms were suggested by a decrease in T ₂.     

Magnetic Phase Transitions in Cobalt Chromite Nanoparticles

Cobalt chromite (CoCr₂O₄), an insulating normal spinel compound, is a potential multiferroic material. We report that pure, nanosize CoCr₂O₄ particles are synthesized through a conventional coprecipiation technique by controlling the pH of the precipitation. Both single-crystal and polycrystalline samples develop long-range ferrimagnetic order below the Curie temperature, T _c (97 K), and a sharp phase transition at T _s??31 K, attributed to the onset of long-range spiral magnetic order. However, we observed a transition from paramagnetic to superparamagnetic phase at T _c. Further lowering the temperature below T _c (97 K), the superparamagnetic phase transforms to ferrimagnetic phase at blocking temperature, T _b, which is found to be between 50 and 60 K. This intermediate superparamagnetic phase in between paramagnetic and long-range ferrimagnetic phases is attributed to a nanosize effect.     

Multiple Superconducting Gaps and Anisotropic Spin Fluctuations in Hole-Doped and Electron-Doped Iron-Pnictides: NMR Studies

We present extensive ⁷⁵As-NMR data on the electron-doped pnictides PrFeAsO_0.89F_0.11 (T _c=45 K), LaFeAsO_0.92F_0.08 (T _c=23 K), and the hole-doped Ba_0.72K_0.28Fe₂As₂ (T _c=31.5 K) single crystal. We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/T ₁ decreases below T _c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T _c and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps.     

Superconductivity of Bi Confined in an Opal Host

Superconductivity is observed in a composite of rhombohedral crystalline bismuth nanoparticles imbedded in an insulating porous opal host via electrical transport and AC magnetic susceptibility. The onset of superconductivity in this system occurs in two steps, with upper transition temperature T _c,U =4.1 K and lower transition temperature of T _c,L =0.7 K, which we attribute to the granular nature of the composite. The transition at T _c,U is observed to split into two transitions with the application of a magnetic field, and these have upper critical fields extrapolated to T=0 K of H _c2,1(0)=0.7 T and H _c2,2(0)=1.0 T, corresponding to coherence lengths of ξ ₁(0)=21 nm and ξ ₂(0)=18 nm, respectively. We suggest that because of the lack of bulk-like states in the Bi nanoparticles due to confinement effects, superconductivity originates from surface states arising from Rashba spin-orbit scattering at the interface.     

3He Vapor Pressure near Its Critical Point

Between 0.65 K and 3.2 K, the temperature dependence of the vapor pressure P of ³He is defined by the International Temperature Scale of 1990 (ITS-90). However, the ITS-90 vapor pressure equation was not designed to be consistent with the scaling law required for the second temperature derivative of the vapor pressure in the vicinity of the liquid-vapor critical point. In this paper, two scaling-type equations are used to describe the ³He vapor pressure in the region near the critical point. The first scaling equation contains two unknown coefficients which are obtained by taking as reference the temperature $\bar{T}$ at which the product (T _c ?T)P presents a maximum ( $\bar{T}=2.56736$  K). The second scaling equation contains three unknown coefficients which are obtained by using as references $\bar{T}$ and T _up=3.2 K, the upper value of the ITS-90 interval. In both equations we take for the critical temperature and pressure the values T _c =3.31554 K and P _c =114?632.7 Pa. The proposed equations, specially the second one, are satisfactorily compared with experimental data for P and dP/dT within the temperature range (T _c ?T)/T _c ≤0.065 and with semiempirical data for d ² P/dT ² within the temperature range 0.0001≤(T _c ?T)/T _c ≤0.03.     

Fabrication of BSCCO Films on SrTiO3 Substrates Using Ultrasonic Spray Pyrolysis (USP) Technique

In this study, the BiSrCaCuO (Bi-2212) films on SrTiO₃ substrates were fabricated using an ultrasonic spray pyrolysis technique (USP). Structural, electrical, magnetic, and critical current density, J _c, properties of the films fabricated were investigated under different heat treatment conditions. XRD analysis showed that the films mainly consisted of the Bi-2212 phase, but the Bi-2223 phase was also detected. T _c values of the films were found between 81 K and 88 K, depending on the heat treatment conditions. J _c values of the films were calculated using the Beans’ equation. Highest J _c value was found to be 2.93×10⁵ A?cm^?2 at 5 K and 0 T for Film C. The results obviously showed that USP method is a very effective technique for fabrication of the HT _c films having high J _c values as well as its simplicity, low cost, and easily coating.     

Upper Critical Field Measurements up to 60 T in Arsenic-Deficient LaO0.9F0.1FeAs1−δ : Pauli Limiting Behavior at High Fields vs. Improved Superconductivity at Low Fields

We report resistivity and upper critical field B _c2(T) data for As-deficient LaO_0.9F_0.1FeAs_1?δ in a wide temperature and field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T _c =29 K and a significantly enlarged slope dB _c2/dT=?5.4 T/K near T _c which contrasts with a flattening of B _c2(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behavior (PLB) with B _c2(0)≈63 T. We compare our results with B _c2(T)-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B _c2(T) for applied fields H ∥ ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity.     

Proton-induced reduction ofR s,Jc, andT c in YBa2Cu3O7−δ thin films

We have explored the effect of 2-MeVH⁺ irradiation on the superconducting transport properties of thin films of YBa₂Cu₃O_7?δ [T _c, J_c(B=0; 77 K, 4.2 K), andR _s(36 GHz;T)]. The inductively measured critical temperatureT _c changed slowly and uniformly (～2 K per 10¹⁶/cm²) for fluences less than ～3×10¹⁶/cm². Beginning at ～3–4×10¹⁶/cm², the superconducting transition broadened and dropped more quickly with fluence. The critical current density measured at 77 and 4.2 K changed roughly linearly with fluence. The microwaveT _c (as defined by the sharp transition inR _s as a function of temperature) resembled the low-frequency inductiveT _c measurement at low fluences but was depressed more strongly for large fluences. The residual surface resistance (～6–10 mΩ) was not affected for fluences up to 5×10¹⁶/cm². We have interpreted the sudden and reproducible reduction in the microwaveT _c transition as a sensitive indicator of disruption in the copper-oxygen chain sublattice and compared the proton-induced change to that observed in oxygen gettering studies of bulk materials.     

The Effect of Eu Substitution onto Ca Site in Bi(Pb)-2223 Superconductor Via Co-precipitation Method

The samples with nominal composition of Bi₁₆Pb_0.4Sr₂Ca_2?x Eu _x Cu₃O _y where x=0.000, 0.025, 0.050, 0.100, and 0.200 prepared by co-precipitation method (COP) have been investigated. They were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), electrical and resistivity measurement using the four-probe method. The temperature dependency on electrical resistance showed the superconducting behavior for all the samples. The critical current density (J _c ) and superconductivity transition temperature (T _c?zero) of Eu substituted were found to be lower than those of the pure sample. T _c?zero varied between 100 and 73 K towards Eu concentration with the highest T _c?zero in the Eu substitution was found at 98 K for x=0.025 and decreased gradually for further substitution of Eu corresponding to a small change in the carrier concentration. J _c decreased with increasing Eu substitution, and it was measured to be at 5.7512 A/cm² in the Eu free sample and at 2.1223 A/cm² for the x=0.025 sample at 77 K. XRD analyses showed the decrease of the volume fraction of Bi-2223/Bi-2212 (%) which were estimated from 78.13/21.87 for x=0.000 to 23.18/76.82 for x=0.200. The crystallographic structure was found to change slightly from tetragonal to orthorhombic in Eu substituted samples. The lattice parameter c of the Eu samples decreased due to the incorporation of Eu³⁺ (0.95 Å) with smaller ionic size at the Ca²⁺ (0.99 Å) site. From the SEM investigation, the grain connectivity became weak and the porosity increased with the increment of Eu concentration, resulting in the decrease of J _c .     

Cu, Pu and Fe High T c Superconductors: All the Same Mechanism

The more than 20 years old Cu high-T _c superconductors exhibit as undoped parent materials antiferromagnetism. Upon doping the long-range antiferromagnetism disappears and only short-range antiferromagnetic clusters remain which show a spin pseudo-gap. There are no good ideas why long-range antiferromagnetism disappears upon the appearance of superconductivity because antiferromagnetism and superconductivity are compatible. A breakthrough has come about with the discovery of a plutonium (Pu) containing alloy PuCoGa₅ with a T _c of 18.5 K. In principle not very exciting, but in the field of actinides T _c??s are not more than 3 K because of the high mass and corresponding low phonon energies. The compound is a high-T _c material in the field of actinides. But also this Pu-containing compound is a short-range antiferromagnet with a spin pseudo-gap. A pattern starts to develop! As well in the Cu as in the Pu compounds, some magnetic ions Cu²⁺ and Pu³⁺ are replaced upon doping with nonmagnetic Cu³⁺ or spontaneously with nonmagnetic Pu²⁺ ions, thus a mixed valence configuration appears with nonmagnetic states (spin holes) in antiferromagnetic clusters. The newly discovered Fe pnictide superconductors, however, have only one valence, Fe²⁺ above and below T _N, the Néel temperature of 150 K, as well above and below T _c, as judged by the isomer shift of the Mössbauer effect. However, doping with fluorine, replacing oxygen, not only introduces electrons, but changes locally the crystal field acting on the iron ions. Divalent iron 3d⁶ has a high-spin configuration $\mathrm{t}_{2}^{4}\mathrm{e}^{2}$ in a magnetic ??₅ configuration and a nonmagnetic low-spin configuration $\mathrm{t}_{2}^{6}$ in a ??₁ state. So with the same valence we can have a magnetic and a nonmagnetic configuration, triggered by variation of the local crystal field induced by doping, causing again spin holes. We show that these spin holes in antiferromagnetic clusters have an attractive interaction and combine to make nonmagnetic bipolarons, which can condense and lead to superconductivity.     

Investigation of the properties of superconducting niobium nitride films

Niobium nitride films with a value for T_c of up to 17.3 K have been prepared by reactive magnetron and diode sputtering in Ar and N₂ gas mixture. Alteration of T_c and N₂ ion implantation into NbN films was studied. It is shown that small doses of implanted ions cause an abrupt decrease of T_c to 12.8 K. Annealing at 900°C restores the high T_c By the tunnel effect, the magnitude of the energy gap is 3.05 Me V at 1.56 K for NbN with T_c = 17.1 K and $\text{22/kT}{}_{\mathrm{c}}\text{= 4.14}$. The existence of an undersurface layer has been discovered, whose width is of the order of the coherence length, with a lower T_c, of 12.0–12.8 K. The superconductivity of this layer above T_c is due to the proximity effect. NbN tunnel junctions are made with a density of the Josephson current i=(1.7-2.87) · 10³ A cm^?2.     

Recent advances in iron-based superconductors toward applications

Iron with a large magnetic moment was widely believed to be harmful to the emergence of superconductivity because of the competition between the static ordering of electron spins and the dynamic formation of electron pairs (Cooper pairs). Thus, the discovery of a high critical temperature (T_c) iron-based superconductor (IBSC) in 2008 was accepted with surprise in the condensed matter community and rekindled extensive study globally. IBSCs have since grown to become a new class of high-T_c superconductors next to the high-T_c cuprates discovered in 1986. The rapid research progress in the science and technology of IBSCs over the past decade has resulted in the accumulation of a vast amount of knowledge on IBSC materials, mechanisms, properties, and applications with the publication of more than several tens of thousands of papers. This article reviews recent progress in the technical applications (bulk magnets, thin films, and wires) of IBSCs in addition to their fundamental material characteristics. Highlights of their applications include high-field bulk magnets workable at 15–25 K, thin films with high critical current density (J_c) > 1 MA/cm² at ～10 T and 4 K, and an average J_c of 1.3 × 10⁴ A/cm² at 10 T and 4 K achieved for a 100-m-class-length wire. These achievements are based on the intrinsically advantageous properties of IBSCs such as the higher crystallographic symmetry of the superconducting phase, higher critical magnetic field, and larger critical grain boundary angle to maintain high J_c. These properties also make IBSCs promising for applications using high magnetic fields.     

Relationship Between Annealing Time and Magnetic Properties in Bi-2212 Textured Composites

In this study, we report the physical and magnetic properties of Bi₂Sr₂CaCu₂O _x textured materials prepared by a LFZ melting technique and annealed for different times (60, 72, 96 and 120 h). SEM images of the annealed samples for 96 and 120 hours indicate very good alignment with the longitudinal rod axis. In all cases, X-ray diffraction patterns show that the Bi-2212 phase is the major one. The magnetization measurements have been carried out as a function of the magnetic field up to 9 kOe. J _c values of the samples were calculated by using the Bean model. The results indicate that the different annealing time has no significant effects on the T _c values but, significant change on the critical current values of samples, J _c, has been obtained for sample annealed at 96 hours. We also found that the maximum critical density of J _c is 5.5×10⁵ A/cm² at 10 K for the 96 hours annealed sample.     

Terahertz Conductivity of Overdoped Y1−x Ca x B2Cu3O7−δ

In the current work we have investigated the terahertz response of Ca overdoped YBaCuO thin films using both time and frequency domain spectroscopy methods. For both methods a basic data analysis was performed using the two fluid and a variable dielectric function (VDF) models. The imaginary part of the conductivity was proportional to 1/ω, known from the delta-function response. The real part of conductivity showed a well known frequency and temperature dependence, where it increases below T _c and obtains maxima at about 50 K. However, a sharp decrease of the real part of the conductivity was observed at about 10 cm^?1. This decrease occurs below T _c and becomes dominant as temperature decreases. It was observed on the 5% and 10% Ca doped samples but it was more dominant on the 10% case. Moreover, this sharp decrease in σ ₂(ω)at 10 cm^?1 was not observed in optimally doped YBCO samples. We would like to stress at this point that these values are much smaller than those obtained by Microwave and Tunneling measurements, arguing for the existence of a complex order parameter in the overdoped regime with an imaginary component of about 1.8 meV.     

Evolution of Superconducting Properties of LiFeAs Single Crystals Doped with Magnetic or Nonmagnetic Impurities

The effect of magnetic Co²⁺ and nonmagnetic Ga³⁺ impurities on the crystal structure and superconducting properties of LiFeAs single crystals has been investigated. A large T _c decrease of about 4.8 K/at% is observed in Ga-doped LiFeAs. This rate is higher than that of the material doped with magnetic Co impurities (～3.7 K/at%). The greater T _c suppression in the Ga case is likely due to the pair breaking associated with the significant changes in the crystal structure of the doped material. The increase of the critical current densities in intermediate magnetic fields (H⊥?ab) indicates that a very small amount of Ga (0.5 at%) acts as an effective pinning site for flux pinning enhancement in the material. The analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning model for the Co-doped material, while the magnetic relaxation measurements combined with the peak position of the critical current density in the B–T phase diagram of Ga-doped LiFeAs suggest an elastic–plastic transition of the vortex lattice at higher temperatures and fields.     

3D like vortex behavior and the thermally activated flux flow mechanism in (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting films

Approximately 1 μm thick high quality epitaxial c-axis oriented (Hg_0.8Re_0.2)Ba₂Ca₂Cu₃O_x superconducting films have been prepared on MgO (100) substrates using spraying technique and post-Hg-Vapor annealing. The effect of the heating temperature–time combinations and the filling factor of Hg (ff_Hg) on the physical, electrical and magnetic properties of the thick films have been investigated. The XRD investigations showed that the a–b plane of HgRe-1223 phase align parallel to the substrate surface. The best T _c and T _zero were found to be 130.6 and 127.2 K, respectively. The superconducting transition of the films has been measured under applied magnetic field up to 6 T. The results obtained suggested that dissipative resistivity can be explained by thermally activated flux motion below critical temperature under applied magnetic field. The temperature and field dependences of the activation energy in the thermally activated flux flow region have also been investigated. The calculated values of m and α values were found to be 1.42–1.49 and 0.498–0.518 respectively and suggesting a 3D like behavior and the thermally activated flux flow mechanism for all films fabricated. Magnetic properties of the films up to 6 T have also been investigated. The calculated value of critical current density, J _c, was found to be 4.7 × 10⁶ A/cm² at 10 K for the optimally treated films.     

Observance of Improved Magneto-resistance and Magnetic Entropy Change in La0.7(Ca0.2Sr0.1)MnO3:Pd Composite

Magneto-resistive and magneto-caloric properties of polycrystalline La_0.7(Ca_0.2Sr_0.1)MnO₃ (LCSMO) and La_0.7(Ca_0.2Sr_0.1)MnO₃:10 %PdO (LCSMO:Pd10) composites sintered at 1400 ^°C have been investigated. Rietveld refinement of their X-ray diffraction (XRD) patterns confirms the single-phase crystalline structure with orthorhombic Pbnm space group, showing no significant change in their lattice parameters with Pd addition. Disappearance of the low temperature resistivity hump in the Pd composite has been attributed to the suppression of the grain boundary effect and the conducting channels due to the presence of metallic Pd. While the Curie temperature T _c remained nearly unchanged, the peak value of the temperature coefficient of resistance (TCR) increased in the composite. $\mathit{TCR}_{\operatorname{MAX}}$ increased to 6.4 % (at 308 K) in the composite from 2 % (at 305 K) for the pristine LCSMO. Magneto-resistance (MR) and magnetic entropy change (ΔS _M ) also increased markedly in the composite material. This could be ascribed to the observed sharpness of both the magnetic and resistive transitions resulting from better grain connectivity. Maximum MR of 12.9 % (1 T) and 19.6 % (2 T) has been observed close to its T _c (309 K) in the pristine LCSMO. These values increased to 24.1 % (1 T) and 33.9 % (2 T) with the addition of PdO. The maximum values of |ΔS _M | are found to be 4.4, 8.2 and 11.7 J?kg^?1?K^?1 at field values of 1, 2 and 3 T, respectively, for LCSMO:Pd10 composite, which are far better than those reported for LCSMO:Ag10 composite.     

Electrical and Magnetic Behaviour of PrFeAsO_{\mathbf{0.8}}F_{\mathbf{0.2}} Superconductor

The superconducting and ground state samples of PrFeAsO_0.8F_0.2 and PrFeAsO have been synthesised via the easy and versatile single step solid state reaction route. X-ray and Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic-like step in resistivity below 150 K followed by another step at 12 K. The former is associated with the spin density wave (SDW)-like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of the superconducting PrFeAsO_0.8F_0.2 sample. Detailed high field (up to 12 Tesla) electrical and magnetization measurements are carried out for the superconducting PrFeAsO_0.8F_0.2 sample. The PrFeAsO_0.8F_0.2 exhibited superconducting onset ( $T_{c}^{\mathrm{onset}}$ ) at around 47 K with T _c (ρ=0) at 38 K. Though the $T_{c}^{\mathrm{onset}}$ remains nearly invariant, the T _c (ρ=0) is decreased with applied field, and the same is around 23 K under an applied field of 12 Tesla. The upper critical field (H _c2) is estimated from the Ginzburg–Landau equation (GL) fitting, which is found to be ～182 Tesla. Critical current density (J _c ), being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 10³ A/cm². Summarily, the superconductivity characterization of the single step synthesised PrFeAsO_0.8F_0.2 superconductor is presented.     

Ag5Pb2O6/CuO Composite, an Approach to Ambient Temperature Superconductivity

Byström?CEvers compound Ag₅Pb₂O₆ is highly conducting oxide, and it was prepared from the components Ag₂O and PbO₂ fired in a 300 bar O₂ atmosphere and 623 K. Compound was milled, and particles were dispersed in a powdered CuO matrix. For low concentrations of metallic particles (<38 w/w percents), the composite is semiconductor or insulator, while for concentrations >48 w/w percents material is metallic down to liquid helium temperatures. For intermediate concentrations, the composite exhibits properties characteristic for superconductors with transition temperatures 178 K<T _c<356 K, dependent on the size of dissolved metallic particles and their mutual distance. Magnetic data support a presumption of novel superconductivity. Samples were prepared with SC transition temperature T _c=138 K, and in the temperature range 145?C800 K electric resistance obeys the temperature dependence ln?R??(T ₀/T)^1/2, and this indicates a possible 1-dimensional conduction mechanism.