Preparation and critical currents of vapour-deposited Nb3Sn compound

Conclusion The presented results have shown that by optimizing some of the basic technological parameters of the chemical vapour-deposition process it is possible to increase substantially the critical currentI _c and the Nb₃Sn layer thickness of the produced superconductor without any other technological treatment.The critical currentI _c reported byEnstrom et al. [7] concerning the pure reaction atmosphere without doping gases, is about 13 A/mm (per 1 mm of the tape width), in our case it is 62 A/mm in transverse magnetic field of 5 T. We have shown that as far as the Nb₃Sn tape length of the order of 100 m is concerned, it seems more advantageous to work with Nb/Sn chlorides ratio for the region of 0·55 to 0·87 in contrast to the value of 0·33 reported byEnstrom et al. [7]. As we have already mentioned, further increase of theI _c andJ _c values may be achieved by means of doping gases injection into the reaction atmosphere or by additional heat treatment [11] etc.As it is also reported byEnstrom et al. [7], by doping the gaseous ambient with 0·1% CO₂, the increase ofI _c from 13 A/mm to 84 A/mm in the magnetic field of 5 T may be obtained. As the preliminary results of our investigation have shown, an increase from 32 A/mm to 106 A/mm by using the doping gases may be achieved for the tape travelling velocity of 4 m/h, the Nb/Sn chlorides ratio equal to 0·55 and the substrate heating current of the value of 4 A.We expect that by optimizing the technological parameters mentioned above together with following doping and additional heat treatment, we may obtain a substantial increase in theI _c andJ _c values.     

Soft modes and high Tc in Nb3Sn

We investigate the direct effects of phonon softening on T_c in Nb₃Sn. It is inadequate to explain the high T_c, which arises instead from an unusually large electron- phonon coupling to the two Γ'₁₂ optical phonon modes.     

Microprocessing of superconducting Nb3Sn tapes by electron-beam microetching

The unsuitable mechanical properties of Nb₃Sn (its brittleness) make the production of a conductor in the required shape and dimensions very difficult. A method has been proposed and experimentally investigated by which the existing superconducting Nb₃Sn tape could be modified to obtain intrinsically stable superconductor. The required filamentation of commercial Nb3Sn conductor is manageable by electron-beam microetching. The achieved results could serve as a starting point for the construction of a special electron-beam machine. Such device may work in connection with the Nb₃Sn producing apparatus.The authors are grateful to I.Hlásnik for the encouraging discussions, to V.ernuko for providing us with the Nb₃Sn vapour deposited tape and to S.Koina for performing the microanalysis.     

Studies of the microstructure of a superconducting Nb<Subscript>3</Subscript>Sn-Band obtained from (Cu/Nb)/Cu12Sn nanocomposite

A multilayer layer from the superconducting compound Nb₃Sn was obtained by the thermal treatment of composite (Cu/Nb)/Cu12Sn conductor consisting of Cu-12% Sn and Cu/Nb interlayers consisting in turn of nanosized copper and niobium layers. The conductor was coated with copper from the outside, which served as a stabilizer. The microstructure of the cross section of the composite band in dependence on the volume ratio of Cu/Nb and bronze interlayers and the microstructure of the Cu/Nb interlayers themselves in dependence on the annealing temperature were investigated. The optimum ratio of composite components was as follows: t _Nb N _Nb ≈ 0.288t _CuSn N _CuSn, where t and N are the thickness and number of the niobium and bronze layers, respectively. Trying to maintain the optimum design of the conductor forced us to increase the bronze volume content relative to the content of Cu/Nb interlayers, which had a negative impact on the composite microstructure. This resulted in disruptions of Cu/Nb interlayers. A technique has been developed for producing the (Cu/Nb)/Cu12Sn composite as a precursor to the band from the Nb₃Sn compound with allowance for the first experiment’s shortcomings.     

The relation betweenT c and anharmonicity in Sn-base A15 superconductors

The absolute value of the recoil-free fraction was measured accurately at three reference temperatures in the range 300 K down to 4 K, in two Sn-baseA 15 superconductors with disparate superconducting properties. The higher-T _c compound Nb₃Sn (T _c≈18K) exhibits low-temperature anharmonicity; this is in contrast to the lattice-dynamics of the low-T _c isomorph V₃Sn (T _c≈4K) in which it is observed that harmonic binding of the Sn atoms is prevalent down to low temperatures. The difference in the superconducting properties of the two compounds is shown to correlate with the considerable difference in their lattice-dynamics.     

Investigate the effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors

The effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors in BCS weak-coupling limit is investigated. An analytical specific heat jump and the numerical specific heat are shown by using anisotropic order parameters, and the electron–phonon interaction and non-electron–phonon interaction. The two models of anisotropic order parameters are used for numerical calculation that we find little effect on the numerical results. The specific heat jump of MgB₂, Lu₂Fe₃Si₅ and Nb₃Sn superconductors can fit well with both of them. By comparing the experimental data with overall range of temperature, the best fit is Nb₃Sn, MgB₂, and Lu₂Fe₃Si₅ superconductors.     

Pressure and volume dependence of thelo-to phonons in InAs

The pressure dependence of thelo-to phonons in InAs has been investigated by Raman scattering using the diamond anvil cell. Indium arsenide transforms, presumably to the rock-salt structure at 70±1 kbar. The mode Grüneisen parameters for thelo-to phonons are γ _lo =0.99±0.03, γ _to =1.2±0.03 respectively. The effective charge,e* _T , for InAs decreases slightly with pressure and this trend is in accordance with the behaviour of other III–V zinc blende structured semiconductors: The structural phase transition is discussed in the light of theoretical calculations for phase stability of III–V compounds, as well as recent high pressure x-ray diffraction studies.     

High-spin states of<Superscript>125</Superscript>Sb: Particle-core excitation coupling

Excited states of¹²⁵Sb have been studied using in-beam γ spectroscopy techniques via the¹²⁴Sn(⁷Li, α2n) reaction at a beam energy of 32 MeV. A high-spin level scheme including 21 new γ-transitions and 14 new excited states have been established. Three isomers have been identified at 1970, 2110 and 2471 keV and the ranges of their half-lives have been estimated from the delayed coincidence data. The level structure of¹²⁵Sb is discussed in terms of particle-core excitation coupling. With the help of empirical shell model calculations the three isomers are proposed to have three-quasiparticle πg_7/2⊗v(h _11/2 s _1/2)₅₋, πg_7/2⊗v(h _11/2 d _3/2)₇₋ and πg_7/2⊗v(h ₁₁^2/2)₁₀ ⁺ configurations, respectively.     

Strong electron-phonon coupling in Be<Subscript>1−x</Subscript>B<Subscript>2</Subscript>C<Subscript>2</Subscript>: ab initio studies

Several structures for off-stoichiometric beryllium diboride dicarbide Be_1−xB₂C₂ have been designed, and their properties studied from first-principles density functional methods. Among the most stable phases examined, the layered hexagonal structures are shown to exhibit various features in the electronic properties and in the lattice dynamics reminiscent of the superconducting magnesium diboride and alkaline earth-intercalated graphites. For substoichiometric composition x ∼ 1/3, the system is found metallic with a moderately strong electron-phonon coupling through a predominant contribution arising from high frequency streching modes modulating the σ-bonding of the B-C network, and a weaker contribution at medium frequency range of the phonon spectra, arising from the intercalent motion coupled to the π-bonding states. Further, anharmonicities emerging from the proximity of the Fermi level to the σ-band edge, contributes to reduce the phonon softening hence stabilizing the structure. All these effects appear to combine favourably to produce a high temperature phonon-superconductivity.     

Influence of the Grain Boundaries on the Heat Transfer in Laser Ceramics

Grain boundaries play a key role in determining several key properties of polycrystalline laser ceramics. Heat transfer measurements at low temperature constitute a good tool to probe grain boundaries. We review the results of heat transfer measurements in polycrystalline Y₃Al₅O₁₂ garnets as well as Y₂O₃ and Lu₂O₃ sesquioxide materials obtained by self-energy-driven sintering of nano-particles. The average phonon mean free path in Y₃Al₅O₁₂ was found to be significantly larger than the average grain size and to scale with temperature as T ⁻² at low temperature. Existing models describing the interaction between phonons and grain boundaries are reviewed. Correct temperature dependence of the mean free path and order of magnitude of scattering rates were found by assuming the existence of a grain boundary layer having acoustic properties different from those of the bulk. A different temperature dependence of phonon mean free path was found for the sesquioxides and was ascribed to the stronger elastic anisotropy of these materials. The thermal resistance associated to the grain boundaries of laser ceramics was found to be lower than in other dense polycrystalline ceramic materials reported in the literature.     

Lattice dynamics of Nb3Sn at 300 K

The phonon dispersion curves have been calculated from a microscopic model for Nb₃Sn at 300 K and are in good agreement with the available inelastic neutron scattering data. The calculated phonon density of states exhibits the essential features of the experimental results obtained by incoherent inelastic neutron scattering.     

Partial wave analysis of πproduction in two-photon collisions at LEP

The reaction γγ → π⁺π^-π⁰ with quasi-real photons is studied with a total integrated luminosity of 663pb^-1, collected by the L3 detector at LEP at center-of-mass energies from 183 to 209GeV. The results of an energy-dependent partial wave analysis in the mass region 1.1≤M(π⁺π^-π⁰)≤2.2GeV are presented. The reaction is dominated by a₂(1320) formation. A strong signal consistent with the first radial excitation of the isovector tensor state, a₂(1700), is present and confirms the previous L3 observation. Its two-photon partial width is found to be Br(3π) = 0.37^+0.12_-0.08keV, the relative branching ratio of ρ(770)π to f₂(1270)π is 3.4±0.4. For all observed states the product of γγ partial width and 3π branching ratios is measured.     

Isospin violations in largeP T pion inclusive processes in perturbative quantum chromodynamics

We investigate the asymmetries arising due to electromagnetic interactions in largeP _T pion inclusive processes. The hardqcd processes that contribute to such asymmetries areq+g→q+γ,q+q→g+γ etc. which are suspected to be substantial, as indicated by theqcd predictions for a significant and increasingγ/π ^o ratio at largeP _T. We calculate the expected isospin related asymmetries and propose tests that might detect them. Our estimates indicate that the effects are much smaller than may be naively expected. We also observe a remarkable scaling of asymmetries in the variableP _T/(s)^1/2.     

Leading-order 2πγ exchange NN interaction: Central potentials proportional to g<Subscript>A</Subscript><Superscript>0</Superscript> and g<Subscript>A</Subscript><Superscript>2</Superscript>

We calculate at two-loop order in chiral perturbation theory the electromagnetic corrections to the leading-order 2π exchange NN interaction proportional to g _A ⁰ and g _A ². The resulting 2πγ exchange potential contains isospin-breaking components which reach up to about -2% of the corresponding isovector 2π exchange potential. With a value of only -17keV at r = m _π ^-1 = 1.4fm the charge-independence breaking central potential obtained here is negligibly small in comparison to the one generated by the isoscalar c₃ contact vertex. Our calculation confirms that the largest long-range isospin-violating NN potentials arise from the 2πγ exchange diagrams involving the large low-energy constants c ₄ ≃ - c ₃ ≃ 3.3GeV^-1 representing the important Δ(1232) dynamics.     

Rotational bands in the doubly odd 130Cs nucleus

The band structures built on the 5^- isomeric state ( T _1/2 = 3.46 m) in the doubly odd ¹³⁰Cs nucleus have been established up to I = 24? via the ¹²⁴Sn(¹¹B, 5n)¹³⁰Cs reaction. The previously observed bands based on the πh _11/2⊗νh _11/2, πg _7/2⊗νh _11/2 and πd _5/2⊗νh _11/2 configurations and a positive-parity side band with multiple connections to the α = 0 signature partner of the yrast πh _11/2⊗νh _11/2 band have been extended to higher spins. A new band based on the πh _11/2⊗νg _7/2 configuration is observed. The yrast πh _11/2⊗νh _11/2 band exhibits anomalous signature splitting whose magnitude decreases up to spin 15 and then increases without restoring the normal signature splitting. Received: 20 February 2001 / Accepted: 9 May 2001     

The infrared spectra of tutton salts 1. A comparative study of (NH4)2 M″(SO4)2·6H2O (M″=Ni,Co or Mg)

The infrared spectra of (NH₄)₂M″(SO₄)₂.6H₂O has been analysed in the region 4000–250 cm⁻¹. The dynamics of each crystal has been discussed in terms of 234 phonon modes, including 3 acoustical ones, using the unit cell approximation. The ambiguity in the assignments of the bands in the region 900–500 cm⁻¹ has been removed by assigning the bands in this region to the libratory modes of H₂O molecules. It has been concluded that the NH ₄ ⁺ and SO ₄ ²⁻ ions have a symmetry lower thanT _dand also the complex [M″(H₂O)₆]²⁺ has a symmetry lower than O _h . The hydrogen bonding is the strongest in the Ni-salt and the weakest in the Mg-salt.     

Considerable increase in the thermomagnetic stability of multifilament superconductors internally doped by large-heat-capacity substances

It is demonstrated that the thermomagnetic stability of composite superconductors can be considerably increased by introducing into them a small quantity of a material with an extremely high specific heat at low temperatures. Measurements show that the criterion of “adiabatic” stability for a (Nb₃Sn + 7 vol. % PrB₆) wire is 70% higher than for a reference Nb₃Sn wire (at 4.2 K, the specific heat of the doped sample is seven times higher than that of the reference sample). For a (NbTi + 5vol. % Gd₂O₂S) sample, the specific heat of which at 4.2 K is nine times higher than that of a reference NbTi wire, this increase in stability is as small as 10% (because the characteristic thermal time in the transverse direction is much longer than the time of the magnetic flux jump development).     

Effect of variations in peptide parameters and charges on the unperturbed dimensions of polypeptide chains

The net charges on various atoms of poly (l-alanine), polyglycine, poly (N-methyl-l-alanine) and poly (N-methylglycine) were computed using the MOLCAO method of Del Re forσ charges and the Hückel MO method forπ charges. The characteristic ratiosC _∞ were computed for all the above polypeptide chains, with different sets of parameters for the peptide unit. The calculated values ofC _∞ are found to be very sensitive to the input peptide geometry. The calculated value of 2·2 forC _∞ of poly (N-methylglycine) obtained with set-3 parameters (derived from a crystal structure containing prolyl residue) is closer to the experimental value of 1·8±0·2 than the value of 2·7 obtained with set 1 (Pauling-Corey parameters), suggesting that the peptide parameters of N-substituted aminoacids have close similarity to set 3 rather than to set 1. The calculated values ofC _∞ of the polypeptide chains show no correlation with the number of allowed conformations, suggesting that the ratio ofC _∞/C _f need not always provide information about the flexibility or freedom of rotation of chain units. Contribution No. 57 from the Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560012, INDIA.     

The sign of interband interaction and stability of the superconducting ground state in the multiband model

Summary It is stressed that the stability of the superconducting ground state in the two-band model is guaranteed for both signs of the leading interband interactionW. Thereby the requirement for the energy minimum fixed the phase differences of two order parameters as |ϕ₁-ϕ₂|=0,2π, … ifW<0 and |ϕ₁-ϕ₂|=π 3π, … ifW>0, and this difference is reflected in the ground-state wave function.     

Nonadiabatic effects in the phonon spectra of superconductors

The nonadiabatic corrections to the self-energy part Σ_s(q, ω) of the phonon Green’s function are studied for various values of the phonon vectors q resulting from electron-phonon interactions. It is shown that the long-range electron-electron Coulomb interaction has no direct influence on these effects, aside from a possible renormalization of the corresponding constants. The electronic response functions and Σ_s(q, ω) are calculated for arbitrary vectors qand energy ω in the BCS approximation. The results obtained for q=0 agree with previously obtained results. It is shown that for large wave numbers q, vertex corrections are negligible and Σ_s(q, ω) possesses a logarithmic singularity at ω=2Δ, where Δ is the superconducting gap. It is also shown that in systems with nesting, Σ_s(Q, ω) (where Q is the nesting vector) possesses a square-root singularity at ω=2Δ, i.e., exactly of the same type as at q=0. The results are used to explain the recently published experimental data on phonon anomalies, observed in nickel borocarbides in the superconducting state, at large q. It is shown, specifically, that in these systems nesting must be taken into account in order to account for the emergence of a narrow additional line in the phonon spectral function S(q, ω)≈−π ⁻¹ Im D _s (q, ω), where D _s (q, ω) is the phonon Green’s function, at temperatures T<T _c . Zh. éksp. Teor. Fiz. 115, 1799–1817 (May 1999)