Suppression of superconductivity in YNi2B2C at the atomic disordering

The behavior of the electrical resistivity ρ(T), the superconducting transition temperature T _c , and the upper critical field H _c2(T) of a polycrystalline sample of YNi₂B₂C irradiated by thermal neutrons with the subsequent high-temperature isochronous annealing in the temperature interval T _ann = 100–1000°C has been studied. It has been found that the irradiation of YNi₂B₂C with a fluence of 10¹⁹cm^?2 leads to the suppression of the superconductivity. The final disordered state is reversible; i.e., the initial ρ(T), T _c , and H _c2(T) values are almost completely recovered upon annealing at up to T _ann = 1000°C. The quadratic dependence ρ(T) = ρ₀ + a ₂ T ² is observed for the sample in the superconducting state (T _c = 5.5?14.5 K). The coefficient a ₂ (proportional to the square of the electron mass m*) hardly changes. The form of the dependence of T _c on ρ₀ can be interpreted as the suppression of the two superconducting gaps, Δ₁ and Δ₂ (Δ₁ ～ 2Δ₂). The degradation rate of Δ₁ is about three times higher than that of Δ₂. The dependences dH _c2/dT on ρ₀ and T _c may be described by the relations for a superconductor in the intermediate limit (the coherence length ζ₀ is on the order of the electron mean free path l _tr) under the assumption of a nearly constant electron density of states on the Fermi level N(E _F). The observed behavior of T _c obviously does not agree with the widespread opinion about the purely electron-phonon mechanism of superconductivity in the compounds of this type supposing the anomalous type of superconducting pairing.     

Effects of low temperature irradiation with heavy ions on superconductor niobium

To study the effects of heavy ion irradiation at low temperature on type II superconductor Nb, the transition temperatureT _c , the normal state residual resistivityρ _B , the transition widthΔT _ph using oxygen ions of 25 MeV and subsequent thermal annealing were measured. The samples were held at temperatures <20 K during irradiation in a cryostat for in situ measurements. The maximum oxygen fluence was about 2·10¹⁵ cm^?2 corresponding a relatively high defect concentration. The heavy ion irradiation experiments are described. The critical temperatureT _c decreases with increasing residual resistivityρ _B . In agreement with the theory and experiments, the gap anisotropy parameter is 〈a ²〉=0.008, subsequent annealing shows a hysteresis ofT _c versusρ _B . The resistivity saturation value Δρ_BS = 2.55 μΩ cm was obtained and different recovery stages were found. Significant broadening of transition width during irradiation was observed.T _c andΔT _ph anneal to 60% in the temperature interval of (60–90) K. Oxygen induced effects as a simulation method of high neutron damage are compared with irradiation measurements using neutrons and deuterons.     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Normal state resistivity and Tç studies of superconducting Ti1−xSbx system

The superconducting transition temperature (T_c) and the temperature dependence of the normal state resistivity of the Ti_1?xSb_x system between T_c and 300 K have been studied. The T_c values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T² dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.     

Superconductivity of neutron irradiated Mo3Os

The effect of high energy neutron irradiation (E>1 MeV) on the superconducting transition temperature, T_c, of the A-15 compound Mo₃Os is reported. T_c was found to decrease with increasing neutron dose, but at a rate considerably less than observed in other A-15 compounds composed of non-transition metals. The results lend support to the idea that the effect of ordering on T_c is smaller for A-15 compounds composed only of transition metals than those composed of transition and non-transition metals.     

Pulsed laser deposition conditions and superconductivity of FeSe thin films

We report the effects of growth conditions on the superconducting properties of FeSe films epitaxially grown on LaAlO₃ substrates by pulsed laser deposition (PLD). Customary materials characterization techniques [X-ray diffraction (XRD), in-situ X-ray photoelectron spectroscopy (XPS), in-situ ultra-violet photoelectron spectroscopy (UPS), and scanning electron microscopy (SEM)] revealed the films had a c-axis oriented tetragonal structure with lattice constants dependent on the growth temperature (varied from 100 to 600°C). The standard four-point probe method was used to measure the resistivity and superconducting transitions. Films grown at 400–550°C showed a clear superconducting onset but no zero resistance down to 2 K. The highest superconducting onset temperature (T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}}) of 8 K was observed in films grown at 500°C and the onset temperature was clearly correlated to the ratio of the lattice constants (c/a). As the thickness of the FeSe films increased from 27 nm to 480 nm, T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}} also increased as the strain in the system was relaxed.     

Effect of electron irradiation on the structure and properties of the MgB2 superconductor

This paper reports on the results of an investigation into the effect of irradiation of the Bardeen-Cooper-Schriefer superconductor MgB₂ by electrons with a mean energy ē ～ 10 MeV at low doses (0 ≤ Φt ≤ ～5 × 10¹⁶ cm^?2) on the lattice parameters, the intensity and width of diffraction lines, the superconducting transition temperature T _c , and the temperature dependence of the resistivity ρ(T) in the normal state. The results of structural investigations have revealed regularities in the defect formation in the magnesium and boron sublattices of the MgB₂ compound as a function of the electron fluence. At the initial stage, irradiation leads to the formation of vacancies, originally in the magnesium sublattice and then in the boron sublattice. For fluences Φt ≥ ～1 × 10¹⁶ cm^?2, vacancies are formed in both sublattices. The evolution of the electrical and physical properties [T _c , ρ_{273 K}, residual resistivity ratio RRR = ρ_{273 K}/ρ_{50 K}, parameters of the dependence ρ(T)] under electron irradiation is in agreement with the regularities revealed in the formation of radiation-induced defects in the crystal lattice of the MgB₂ compound.     

Fast neutron irradiation of YBa2Cu3O7

The influence of fast neutron irradiation on the superconducting transition temperature,T _c , the transition width, upper critical field,B _c2 , and critical current density,j _c , has been investigated in YBa₂Cu₃O₇ up to a fluence of 10¹⁹ cm^–2 (E>1 MeV). TheT _c degradation with fluence is slightly less than in PbMo₆S₈, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainj _c and the superconducting volume fraction.     

Chaleur spécifique et transformation martensitique dans le système Nb1−xSnx

Experimental data on the superconducting temperature, and low temperature specific heat, together with X-ray and neutron investigations are reported for two sets of Nb_xSn_1?x, samples with the A15 type structure. The first one, intended for a study of the effect of stoichiometry, consists of cast alloys with lattice constants ranging from 5.273 to 5.289 Å, γ's from 4.9 to $\text{9}\text{mJ}\text{K}{}^2$ g-at and T_c's from 6 to 17.9 K. The second one consists of nearly stoichiometric sintered alloys with lattice constants ranging from 5.288 to 5.291 Å, γ's from 5.6 to 13.5 $\text{mJ}\text{K}{}^2$ g-at and T_c's close to 18 K. The martensitic transformation ratio has been determined by neutron diffraction in the latter set and the values 32, 77 and 95% were obtained. The analysis of the specific heat jumps at T_c shows that the superconducting T_c of the tetragonal phase is 17.9 K and both T_c and γ are reduced in the cubic phase.Phonon information inferred from specific heat data are indicative of a structure with a typical energy of 9 meV in the density of states. The agreement with spectra obtained by inelastic neutron scattering is good.     

Irradiation effects on the YBa2Cu3O7-δ superconducting compound

The effects of neutron irradiation on the magnetic and superconducting properties of the YBa₂Cu₃O_7- (Y123) superconducting compound have been investigated. No significant change of the superconducting transition temperature (T _c) was found. The effect of irradiation on the magnetization critical current density (J _c) was studied. Bean's critical state model was used to estimate J _c. Around 40% increase in J _c was found. The temperature dependence of J _c was also studied.     

Low-flux neutron irradiation of ceramic YBa2 Cu3O7−δ

Summary Pellets of sintered YBa₂Cu₃O_7−δ with three different oxygen contents have been irradiated with fast neutron beams of energies 6.5, 3.3 and 4.4 MeV at fluences of 7.7·10⁴, 1.3·10⁵ and 1.4·10⁹ n/cm², respectively. The radiation damage has been investigated by comparing the critical temperature (T _c ^mid ), the zero resistivity and the onset temperature before and after neutron irradiation. The critical current has been measured for a few samples in the same experimental conditions. In all transport measurements two different responses to the neutron radiation are observed and discussed.     

Unusual behavior of nuclear relaxation in CeCu2Si2 “possible evidence for triplet superconductivity”

Nuclear relaxation of ⁶³Cu in the superconducting state of the Kondo-lattice system CeCu₂Si₂ has been studied with the use of the ⁶³Cu nuclear quadrupole resonance technique under zero field and down to 65mK. The nuclear spin-lattice relaxation rate (1/T₁) decreases drastically just below T_c=0.67 K down to 0.5T_c without the apparent enchanced behavior and then is found to be almost temperature independent below 0.3T_c. These results suggest that the superconductivity in CeCu₂Si₂ is not in the usual BCS regime. The analysis based upon the existing triplet pairing model with an anisotropic energy gap describes well the behavior from T_c down to 0.5T_c, while the temperature independence below 0.3T_c remains unexplained.     

Effect of structural water on the critical characteristics of highly textured YBa2Cu3O6.9

The effect of low-temperature treatment (200°C) in a humid argon atmosphere and subsequent annealing (930°C) on the critical parameters of a highly textured YBa₂Cu₃O_6.9 has been studied. During annealing at T = 200°C, the absorbed water is incorporated into the structure of the compound, which is accompanied by the deterioration of its superconducting properties. However, after the recovery annealing at T = 930°C and subsequent oxidation, the superconducting characteristics (j _c , B _1c , and F _p ) are improved. This is explained by the formation of 124-type planar defects, which are effective pinning centers, especially in high fields applied perpendicular to the c axis (⊥ c). The optimum conditions of double annealing substantially increasing the critical current density (j _c ≥ 10⁴ A/cm²) in an external magnetic field up to 10 T and also the first critical fields have been found. In fields up to ～3 T, the critical current density j _c is isotropic despite the conservation of high texture in the samples.     

Sputtered magnesium diboride thin films: Growth conditions and surface morphology

Magnesium diboride (MgB₂) thin films were deposited on C-plane sapphire substrates by sputtering pure B and Mg targets at different substrate temperatures, and were followed by in situ annealing. A systematic study about the effects of the various growth and annealing parameters on the physical properties of MgB₂ thin films showed that the substrate temperature is the most critical factor that determines the superconducting transition temperature (T_c), while annealing plays a minor role. There was no superconducting transition in the thin films grown at room temperature without post-annealing. The highest T_c of the samples grown at room temperature after the optimized annealing was 22 K. As the temperature of the substrate (T_s) increased, T_c rose. However, the maximum T_s was limited due to the low magnesium sticking coefficient and thus the T_c value was limited as well. The highest T_c, 29 K, was obtained for the sample deposited at 180 °C, annealed at 620 °C, and was subsequently annealed a second time at 800 °C. Three-dimensional (3D) AFM images clearly demonstrated that the thin films with no transition, or very low T_c, did not have the well-developed MgB₂ grains while the films with higher T_c displayed the well-developed grains and smooth surface. Although the T_c of sputtered MgB₂ films in the current work is lower than that for the bulk and ex situ annealed thin films, this work presents an important step towards the fabrication of MgB₂ heterostructures using rather simple physical vapor deposition method such as sputtering.     

MgB2 coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical–chemical vapor deposition

MgB₂ coated conductors (CCs), which can avoid the low packing density problem of powder-in-tube (PIT) processed wires, can be a realistic solution for practical engineering applications. Here we report on the superior superconducting properties of MgB₂ CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to 600 °C by using hybrid physical–chemical vapor deposition (HPCVD) technique. The superconducting transition temperatures (T_c) are in the range of 38.5–39.4 K, comparable to bulk samples and high quality thin films. Clear (101) and (002) reflection peaks of MgB₂ are observed in the X-ray diffraction patterns without any indication of chemical reaction between MgB₂ and Hastelloy tapes. From scanning electron microscopy, it was found that connection between MgB₂ grains and voids strongly depend on the growth temperature. A systematic increase in the flux pinning force density and thereby the critical current density with decreasing growth temperature was observed for the MgB₂ CCs. The critical current density (J_c) of J_c(5 K, 0 T) ～10⁷ A/cm² and J_c(5 K, 2.5 T) ～10⁵ A/cm² has been obtained for the sample fabricated at a low growth temperature of 520 °C. The enhanced J_c (H) behavior can be understood on the basis of the variation in the microstructure of MgB₂ CCs with growth temperature.     

Heat capacity measurements on irradiated and non-irradiated Mo3Ge and Mo5Ge3

We have measured the heat capacity of irradiated and non-irradiated Mo₃Ge and Mo₅Ge₃ in the temperature range ≈ 1.6 to 10 K. An irradiation of 2.2 × 10¹⁹ neutrons cm^-2 results in an increase in the superconducting transition temperature T_c from < 1.6 K in the non-irradiated state to ～ 4 K after irradiation for Mo₃Ge and a corresponding change from < 1.6 K to ～ 3 K for Mo₅Ge₃. Our analysis shows that this change in T_c is not accompanied by a change in the electronic density of states (within experimental error) but rather a decrease in the Debye temperature from 392 to 322 K for Mo₃Ge and 377 to 320 K for Mo₅Ge₃.     

Effects of neutron irradiation on superconducting properties of orthorhombic Y–Ba–Cu oxides

Abstract

We have studied the effects of fast neutron (E>0.1 MeV) irradiation at reactor (～ 360 K) and low (～ 20 K) temperatures on the superconducting properties of polycrystalline orthorhombic YBa₂Cu₃O_7?y . Measurements were made on the superconducting critical temperature T_c , critical current J_c , Meissner effect and magnetic field dependence of J_c . The T_c drops by an irradiation at reactor temperature and J_c increases with increasing fluence. On the other hand with the irradiation at low temperature, T_c rises and J_c increases. Results of observation of Meissner effect and the magnetic field dependence of J_c are consistent with the behavior of T_c and J_c .     

Effect of external pressure on Tc of as‐grown and thermally treated superconducting Rbx Fe2–ySe2 single crystals

We report on the effect of external pressure on the superconducting transition temperature (T_c) of as‐grown and thermally treated single crystals of superconducting iron chalcogenide Rb_0.85Fe_1.9Se₂. The superconducting transition temperature of 27.1 K at ambient pressure for the as‐grown sample was found to increase up to 33.2 K for the sample annealed for 3 h at 215 °C in vacuum. An increase of T_c up to 28.2 K was observed for the as‐grown sample at a pressure of 0.83 GPa. For all the studied crystals, annealed in the temperature range between 215 °C and 290 °C, the external pressure seems to decrease the superconducting transition temperature and a negative pressure coefficient of T_c was observed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Composition dependences of Tc,Jc, physical property and crystal structure of Bi1.8Pb0.3Sr2.0Ca0.9Y0.1Cu2.0−xMxOy (M = Zr,Zn) superconducting oxide

In this study, we focused on bulks and thin films of Bi_1.8Pb_0.3Sr_2.0Ca_0.9Y_0.1Cu_2.0O_y, and replaced Cu by Zr⁴⁺ and Zn²⁺. From the electrical conductivity measurements on the bulk samples, it was found that T_c became higher by a partial substitution of Zr⁴⁺, and exhibited a maximum value in the sample with the Zr⁴⁺ content of 0.05. On the other hand, T_c was decreased by the Zn²⁺ substitution. These results suggested that it was possible to control a carrier concentration by the Zr- and Zn-substitutions. Indeed, an average valence of the Cu, determined by a chelate titration, was changed by these substitutions. Similar tendency of the superconducting properties was also observed in the case of the film samples. In order to investigate crystal structures in detail, we measured neutron diffraction patterns, and then performed the Rietveld analysis. As a result, it was clarified that the (Cu, Zr)–O2 layers became flat by the Zr-substitution. Such a structure change may be one of the reasons why the substitution improved the superconducting properties.     

The superconducting transition temperature of bulk samples of the niobium-germanium A15-phase after implantation of Ge,Si, O and Ar ions

Bulk material of Nb₃ (Ge_0.8Nb_0.2) with A15 structure and a superconducting transition temperature T_c of 6.5 K has been implanted with Ge, Si, Ar and O ions and subsequently annealed at high temperatures. After annealing between 700 and 750°C the Ge implanted samples showed a strong increase in T_c up to 16.2 K. With Si ions only a T_c of 13 K was obtained, with Ar and O ions T_c remained below 9 K. From X-ray measurements carried out on high T_c Ge implanted samples it could be concluded that the implanted surface layer grows up to a high degree epitaxially on the single crystallites of the bulk material. The lattice constant $\text{a}$₀ of the implanted film was reduced by 0.02 Å with respect to the bulk material. This reduction in $\text{a}$₀ is stronger than expected from the transition temperature of the implanted surface layer.