Upper critical fields of cubic and tetragonal single crystal and polycrystalline Nb3Sn in DC fields to 30 tesla

Measurements of the upper critical field, H_c2(T), in single crystal Nb₃Sn were extended to 30 tesla (300 kG) with dc fields produced by a Hybrid magnet. Observations of H_c2(T) were made for materials which remain in the cubic (c) phase and those which show a martensitic transformation at the tetragonal (t) phase. H_c2(T) measurements of Nb₃Sn for a pure crystal for which the de Haasvan Alphen (DHVA) effect was observed and for polycrystalline (t) phase and (c) phase materials are also reported. Measured values of H_c2(4.2 K) and calculated values of H_c2(0) are: 1) along the [100] direction for our earlier Nb₃Sn, H_c2(4.2 K) = 26 T for the (c) phase and 21.5 T for the (t) phase; H_c2(0) = 29T for the (c) phase and 24 T for the (t) phase; 2) along the [100] direction for the DHVA material H_c2(4.2 K) = 18 T and H_c2(0) = 20 T; 3) for polycrystalline Nb₃Sn (t) phase material H_c2(4.2 K) = 23 T and H_c2(0) = 25 T and for (c) phase material H_c2(4.2 K) = 26 T and H_c2(0) = 29 T. The values of (dH_c2/dT)_T=Tc vary from 2.4T/K for the highest H_c2(T) material to 1.6T/K for the DHVA material. The anisotropy for various Nb₃Sn single crystal materials is small and independent of temperature from T_c to 0.1 T_c. δT_c between the (c) and (t) phase is <0.3 K. Within experimental error excellent fits of H_c2(T) with theory are obtained assuming a dirty or clean Type II superconductor with no Pauli paramagnetic limiting. Experimental details and strong-coupling effects are discussed. When strong-coupling is included, the effects of any paramagnetic limiting would be small and not detectable within our present experimental error. Brief comments also are made concerning H_c2 of V₃Si.     

The electronic structure of hypothetical Nb3Si and V3In

Self-consistent LMTO (Linear Muffin Tin Orbital) band calculations have been performed for Nb₃Si and V₃In. Calculated pressures show agreement with the results from existing Nb₃X and V₃X compounds, when the lattice parameters are 5.125 and 5.025 Å (± 0.025 Å) respectively. At these lattice dimensions, the superconducting transition temperatures are predicted to be approximately 18 K for Nb₃Si and 31 K for V₃In.     

Upper critical fields of Nb3Ge thin film superconductors

The upper critical field H_c2(T) of the highest T_c(～23K) Nb₃Ge superconducting films has been found to be ≈370kG at 4.2K. Measurements on lower T_c films show very broad transitions reflecting nonuniformity. The H_c2(T) characteristics are consistent with other Nb₃X type II superconductors.     

Experimentelle Bestimmung der Supraleitungsparameter einiger Verbindungen vom β-Wolframtyp aus Magnetisierungsmessungen

A complete analysis of magnetization curves is given for bulk sintered samples of Nb₃Sn with low hysteresis and for finely powdered sintered and melted samples of Nb₃Sn, Nb₃Al, V₃Si and V₃Ga. The Ginzburg-Landau parameterκ and the penetration depthλ of weak magnetic fields is calculated from the temperature dependence of the thermodynamic and upper critical fieldsH _c andH _c2. The slope of theH _c ?T curve is about 360 Oe/°K and approximately the same for all measured samples. There is an extremely high increase of the lower critical fieldH _c1 if the grain size approachesλ. The occurence of field expulsion and partially reversible magnetization curves of sintered Nb₃Sn samples with high sintering temperatures and high pore volumes is explained by means of connections between single grains which have lower transition temperatures.     

The sputtering of high Tc A15 Nb3Si and V3Ge

We report some initial results on the preparation of A15 Nb₃Si and V₃Ge using a getter sputtering technique. Under sufficiently clean conditions we observe an increase in the superconducting transition temperature. DC onsets in excess of 14 and 11 K have been observed for Nb₃Si and V₃Ge respectively. In each case a positive identification of the A15 phase has been made.     

Das Eindringen eines schwachen Magnetfeldes in Supraleiter aus Nb3Sn,Nb3Al und V3Si

The temperature variation of the initial suszeptibility has been measured for bulk sintered and diffusion samples as well as for finely ground sintered and melted samples of superconducting Nb₃Sn, Nb₃Al, and V₃Si. Penetration effects must be accounted for the width of the inductive transition curve if the interior of the sample is not completely shielded by current loops. Absolute values of the penetration depths after extrapolating to zero temperature were estimated to (0,18±0,04) μ for Nb₃Sn, (0,19±0,04) μ for Nb₃Al, and (0,27±0,05) μ for V₃Si. The transition temperature obtained by suszeptibility measurements is lowered with decreasing grain size.     

Influence of irradiation induced atomic disorder on Hc2 of Nb3Sn

A significant monotonous decrease of H_c2(0) almost proportional to T_c has been observed after heavy ion irradiation of the A15 superconductor Nb₃Sn at low temperature < 30 K and isochronal annealing up to 300 K has been measured. The effects are discussed in connection with the Goodman—Gorkov-relation.     

X-ray photoemission spectra of valence electrons in V3X and Nb3X compounds

The X-ray photoemission spectra (XPS) of the A15 type compounds V₃Au, Nb₃Os, Nb₃Ir, Nb₃Pt and Nb₃Au have been studied. The inner level binding energies of the different components and the valence electron distribution were measured. The Nb4d and the X5d energy bands of the Nb₃X compounds appear to be more and more separate with increasing atomic number of the X component. The comparison between the results from X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) of corresponding V₃X and Nb₃X compounds points out the similarity of their electronic structures.     

Upper critical fields of Nb∼3Au,Nb3Au0.7Pt0.3 and Nb3Pt

Upper critical field measurements of the A15 type compounds Nb_～3Au,Nb₃Au_0.7Pt_0.3 and Nb₃Pt having T_c values of 10.7, 13.0 and 8.7K, respectively, show H_c2(0) values of 235, 295 and 125 kG, respectively. The data for the compounds containing Au fit closely calculations for a dirty type II superconductor with no paramagnetic limiting, but data for Nb₃Pt are slightly above these calculations.     

XPS valence band spectra of Nb3Sn,Nb and Sn

XPS valence band spectra of Nb₃Sn, Nb and Sn have been obtained under high resolution. The 4d band structure in the Nb₃Sn spectrum is similar to that seen in Nb metal.     

Die Übergangstemperatur verschiedener Supraleiter zweiter Art unter hydrostatischem Druck

An apparatus for production of hydrostatic pressure is described using a mixture of n-pentane and iso-pentane, which transmits pressure uniformly still at low temperatures. The superconductive transitions of the A 15(β- W type)-compounds Nb₃Sn, Nb₃Al, V₃Si and the B 1(NaCl type)-compound NbN have been measured under pressure up to 18 · 10³ kp/cm². The transition temperatures of the compounds V₃Si and NbN are increasing with pressure. Further measurements have been done on some selected ternary compounds, e.g. Nb₃(Al_1?x Ge _x ), to prove some predictions for the effect of pressure on superconductive transition.     

Magnetic phase diagram of Ni(NO3)2 6NH3

The magnetic phase diagram of Ni(NO₃)₂6NH₃ was determined from the field and temperature dependence of the magnetic susceptibility. The zero temperature exchange and anisotropy fields were determined to be H_E(0) ≈ 26 kOe and H_A(0) ≈ 0.7 kOe respectively.     

Nb/Nb3Sn复合超导材料的高压电子显微镜观察

用1000kV高压电子显微镜观察了单芯和多芯Nb/Nb₃Sn复合材料的显微组织,看到了由Nb₃Sn/Nb₃Sn晶粒重叠而成的叠栅图和Nb基体/Nb₃Sn晶粒重叠而成的叠栅图。 关键词：     

Low temperature heat-capacity measurements of V3Ga and Nb3Al

By low temperature heat capacity measurements using the a.c. technique, we show indirectly that martensitic transformations do occur in V₃Ga and Nb₃Al.     

Ultrasonic attenuation of surface acoustic waves in a thin film of superconducting Nb3Sn

The attenuation of 660 MHz surface acoustic waves propagating in a thin film of Nb₃Sn 5000 Å thick has been measured as a function of temperature from 4.2 K to 16 K. The A 15 Nb₃Sn, electron-beam codeposited on YZ lithium niobate and annealed at 700°C, was studied using 5.1 μm wavelength interdigital electrodes. The film revealed a transition temperature of 14.2 ± 0.1 K and using the BCS theory, an energy gap 2Δ(0) = 3.5 k_BT_c.     

Nb3Sn复合超导体晶粒生长机制

本文研究了Nb/固态Cu-Sn和Nb/液态Cu-Sn界面上Nb₃Sn晶粒的生长。实验表明:固态-固态界面上生长的晶粒尺寸虽小(约0.1μm),但Nb₃Sn晶粒的长大仍符合通常的固态晶粒长大规律;固态-液态界面上生长的Nb₃Sn分成两层,靠近Nb的内层晶粒细小,排列致密,外层晶粒粗大,分布零散,后者是前者经过溶解/沉积过程引起的,晶体形貌大多数呈菱形十二面体,部分呈正交平行六面体,说明Nb₃Sn的{110},{100}面     

Theoretical prediction of the structural,elastic, electronic and thermodynamic properties of V3M (M = Si,Ge and Sn) compounds

Density functional theory (DFT), is used in our calculations to study the V₃M (M = Si, Ge and Sn) compounds, we are found that V₃Sn compound is mechanically unstable because of a negative C₄₄ = −19.41 GPa. For each of these compounds considered, the lowest energy structure is found to have the lowest N(E_f) value. Also there is a strong interaction between V and V, the interaction between M (M = Si, Ge, Sn) and V (M and M) is negative, not including Si [Sn]. In phonon density of states PDOS, the element contributions varies from lighter (high frequency) to heaviest (low frequency).     

Stress effects on the anisotropic thermal expansions of “martensitic” A15 compounds

Precision capacitance dilatometry provides a sensitive measure of the thermal strain developed in a sample undergoing a structural distortion with its varying temperature. The A15 structure compounds, V₃Si and Nb₃Sn, are well known to undergo distortion from their cubic structures at room temperature to tetragonal structures (c/a > 1 for V₃Si and c/a < 1 for Nb₃Sn) at low temperatures. In the past, highly anomalous thermal expansion behaviour recorded for these materials has been attributed to a strongly anharmonic lattice potential manifesting itself in unusually high, and strongly temperature-dependent, Grüneisen parameters. Further studies on polycrystalline material revealed this anomalous expansion to be highly anisotropic at temperatures for which, according to conventional diffraction data, the materials are cubic. This behaviour was linked to control of sample morphology by a residual stress field resulting from sample preparation.

More recent experiments, in which the transformation morphology has been controlled by the application of external stresses to single crystal V₃Si and polycrystalline samples of Nb₃Sn and Nb₃(Sn_1-x Sb _x ), have confirmed the occurrence of significant anisotropy in the thermal strain in the cubic phase, well above the structural transformation.

We link this departure from cubic symmetry with the well-known soft-mode character of these materials and the associated “central peak” scattering which is also observed well above the transformation temperature. We are led to propose that the “central peak” is the precursor to a Bragg reflection for the transformation structure. This coincidence between “central peak” scattering and the reciprocal lattice for the transformed phase in Ti-Ni has been termed a “ghost lattice”.     

Electrical characteristics of polycrystalline Si layers embedded into high-k Al2O3 gate layers

The electrical characteristics of polycrystalline Si (poly Si) layers embedded into high-k Al₂O₃ (alumina) gate layers are investigated in this work. The capacitance versus voltage (C-V) curves obtained from the metal-alumina-polysilicon-alumina-silicon (MASAS) capacitors exhibit significant threshold voltage shifts, and the width of their hysteresis window is dependent on the range of the voltage sweep. The counterclockwise hysteresis observed in the C-V curves indicates that electrons originating from the p-type Si substrate in the inversion condition are trapped in the floating gate layer consisting of the poly Si layer present between the top and bottom Al₂O₃ layers in the MASAS capacitor. Also, current versus voltage (I-V) measurements are performed to examine the electrical characteristics of the fabricated capacitors. The I-V measurements reveal that our MASAS capacitors show a very low leakage current density, compared to the previously reported results.     

Single-calcination synthesis of pyrochlore-free Pb(Ni1/3Nb2/3)O3-PbTiO3 using a coating method

Pyrochlore-free 0.64Pb(Ni_1/3Nb_2/3)O₃-0.36PbTiO₃ (0.64PNN-0.36PT) powder has been successfully synthesized by only one calcination step using a coating method. The formation of pyrochlore phase is prevented by coating NiCO₃·2Ni(OH)₂·2H₂O on Nb₂O₅ particles. NiCO₃·2Ni(OH)₂·2H₂O-coated Nb₂O₅ powder is prepared by heterogeneous precipitation method. The coating structure is confirmed by transmission electron microscope (TEM) with energy dispersive spectroscope (EDS). Single calcination treatment of the coating powder mixed with appropriate amounts of Pb₃O₄ and TiO₂ powders at 900 °C for 2 h produces the pure-perovskite 0.64PNN-0.36PT powder. The elimination of the pyrochlore phase can be explained in terms of the separating of Pb₃O₄ and Nb₂O₅ by the NiCO₃·2Ni(OH)₂·2H₂O coating layer.