Nb3Ge Films Research Articles

Influence of pinning states on dynamic phasetransitions in a driven vortex lattice

We have investigated the effect of an applied current on thevortex dynamics in superconducting amorphous Nb3Ge andcrystalline Pb78Bi22 thin films. We have observed amajor difference between the scenarios for the vortex dynamicphase transition at high currents for the samples studied. Inthe amorphous Nb3Ge films the transition of the flux linelattice from a pinning state to a moving coherent state occurswithout an intermediate plastically deformed phase. In thePb78Bi22 films the initial motion exhibits thecharacteristics of plastic (or incoherent) flow, which is endedby a sharp transition into the ordered state. We compare our datawith the recent theories of ordering and phase transition in themoving vortex lattice.

Strain effect on the critical temperatures of superconducting Nb 3Ge tapes

The resistively measured critical temperature, T c, was found to increase by 0.4–0.6 K due to an applied strain of about 0.5% for chemical-vapour-deposited Nb 3Ge tapes that had different Nb 3Ge layer thicknesses and different Nb Ge composition ratios. The T cs have also been measured using a magnetization method for these tapes and the Nb 3Ge films made by removing the 0.1-mm thick Hastelloy substrates from the Nb 3Ge tapes. The triaxial strain analysis that includes measurements of lattice spacing showed that both the hydrostatic and the non-hydrostatic strain components may considerably contribute to the T c strain effect in the present Nb 3Ge tapes.

Negative magnetoresistance in amorphous 3D superconducting films at high (up to 30 T) magnetic fields

The superconducting transition in amorphous Mo3Si and Nb3Ge films (as tracked by the resistivity-vs-magnetic field measurements) is followed by a resistivitydecrease at further increase of the fieldH up to 30 T. The high-field resistivity increases on cooling and flattens out at low temperatures. This behavior is attributed to the destructive influence of both magnetic field and temperature on the interference effects. A possible relation between the negative magnetoresistance and the superconductivity is discussed.

The non-metallic conductivity of Nb 3Ge films sputtered on silicon substrates

In an attempt to study the transport properties of Nb 3Ge thin films on silicon substrates, a series of resistance versus temperature curves for various samples are measured. We use the X-ray diffraction and transmission electron microscopy to characterize the film quality. To realize the electron scattering mechanism in Nb 3Ge films, some theories are quoted by which we can explain the anomalous resistivity phenomenon. In this theoretical approach, we find that the metallic behavior of the samples can be explained by Bloch-Grüneisen's law while the non-metallic conductivity can be regarded as due to weak localization. Meanwhile, grain boundary scattering is found inappropriate to address the contribution in these films. The hopping conductivity of Nb 3Ge films deposited on MgO substrates is also studied.

Superconductivity at 23 K in yttrium palladium boride carbide

COPPER oxide compounds have dominated superconductivity research since 1986 because of their very high transition temperatures (Tcs). In contrast, no new families of high-Tc intermetallic compounds have been discovered since the A15-type Nb3X compounds were first reported in 19531. The intermetallies with highest Jcs have all been based on niobium, with the highest Tcs being 20.7 K for bulk Nb3Ga and 23.2 K for sputtered films of Nb3Ge (refs 2, 3). Here we report the observation of superconductivity at 23 K in a multiple-phase bulk sample of a quaternary intermetallic, yttrium palladium boride carbide. This is higher than any Tc reported previously for a bulk intermetallic compound. Although the materials are not yet single-phase, the superconducting volume fraction is large. We propose that this compound may represent the first of a new family of superconducting intermetallics with relatively high Tcs.

Electron Microscopy of Planar Defects in A15 Nb3Ge

Planar defects in coevaporated Nb3Ge films with the A15 structure are investigated by means of high resolution electron microscopy. The defects are usually on {100} and {110}. The high resolution electron micrographs of these defects allow one to deduce the main translation vector; i.e., (1/2) · 〈III〉. The atomic configurations of these defects are investigated by comparing the observed images and the theoretical Contrast calculations. There is a good fit between experimental and calculated images for the assumed defect models.

Flux-creep and viscous flux-line motion in thin films of A- Nb 3Ge

The resistance transition and IV curves of thin amorphous Nb3Ge films (Tc ≈ 3K) were measured in magnetic fields perpendicular to the film surface. The vortex lattice (VL) in this geometry consists of straight flux-lines (two dimensional VL). In a narrow regime just below the transition line, Bc2(T), we find clear evidence for thermally assisted flux-flow followed by melting of the vortex lattice.

Peak effects in two- and three-dimensional collective pinning

Flux pinning in amorphous Nb 3Ge and Mo 3Si films with thicknesses ranging from 60 nm to 20 μm has been studied as a function of temperature, magnetic induction and sample thickness. The theory of two-dimensional collective pinning agrees well with the critical current data. Close to the upper critical field, B c2, a peak in the volume pinning force, F p, is observed. It is demonstrated that this enhancement in F p is caused by the structural transition of the flux line lattice from an elastically deformed lattice below to a plastically distorted lattice in the peak. For thin samples (in our case typically d < 3 μm) the deformation of the flux line lattice remains two-dimensional, i.e., L c ⪢ d, whereas for thicker samples ( d > 5 μm) the transition is accompanied by a dimensional cross-over to three-dimensional deformation of the flux line lattice in the peak. The cross-over occurs when the longitudinal correlation length, L c, is approximately equal to d 2 . The cross-over is a first order phase transition and is characterized by a jump in the pinning force by up to a factor 10 and non-linear V I characteristics in the peak regime.

Possible high Tc superconductivity and large fluctuation effect in Nb 3Ge films

The electrical resistivity ϱ( T) of RF sputter-deposited Nb 3Ge films has been investigated in detail in the neighborhood of the transition temperature. After cooling to below room temperature, many samples exhibited a semiconductive type of increase in the resistivity before undergoing the transition to superconductivity. These films have very large resistivity in the normal state (ϱ n=10 3-10 5 μΩ cm) and show fairly high onset temperatures. The maximum value of the onset temperature T c-on reaches 32 K. On the other hand, the films with rather small resistivity exhibited a metallic behavior in the normal state. However, the fluctuation effect on the electrical conductivity starts from a higher temperature than the usual onset temperature where we can see a clear drop of the resistivity. The films were examined by EPMA and SEM to reveal the correlation between the superconductive properties and the microstructure.

Superconducting properties of Nb3Ge samples prepared by d.c. cathode sputtering technique

Thin Nb3Ge films were prepared using the d.c. sputtering technique. The dependence of critical temperatureTc on various conditions during the sputtering process was studied. For increasingTc in this system it is necessary to reach the optimum oxygen content in the layers. The critical current densityJc of the samples was found to depend strictly on the microstructure and defects formed in the layers during the sputtering process.

SURFACE STUDY OF SUPERCONDUCTING Nb-Ge FILM BY AUGER ELECTRON SPECTROSCOPY AND ELECTRON SPECTROSCOPY FOR CHEMICAL ANALYSIS

AES and ESCA has been applied to the analysis of the superconducting Nb-Ge film which were deposited onto sapphire substrate by a low-energy sputtering technique. The results of surface analysis and depth profiles showed that there are impurities such as C, O and Al etc. contained in the Nb-Ge film, and O being especially effective impurity which determines Tc. The segregation of Ge at surface has not been found. The concentration retio of Nb/Ge in the film decreased from 3.1 to 2.26 from the surface to the substrate interface. XPS depth profiles showed that the photoelectron spectrum of Nb, Ge, C and O exhibited apparant chemical shifts and variations of the peak shape, i.e. the properties and the chemical states of these elements are complicated and varing along the depth profiles. It may be estimated that Nb probably stabilize the growth of A15 Nb3Ge phase during sputtering deposition.

Critical current densities of Nb 3Ge tapes prepared by chemical vapour deposition on resistively heated tape substrates

Superconducting Nb 3Ge tapes were prepared continuously by the CVD process, changing the deposition temperature and the composition. Nb 3Ge films were deposited on 2.5 mm wide and 0.1 mm thick Hastelloy tapes, resistively heated by electric currents. The substrate tape temperatures were measured during continuous depositions. The maximum value of J c 2 × 10 4 A cm −2 at 20 T and 4.2 K, was obtained for the Nb 3Ge tape with a Nb 3Ge layer thickness of 11 μm per side, prepared at the Nb Ge ratio in gas phase of 3.6 and at the substrate temperature of 834°C. A 17 m long Nb 3Ge tape was prepared uniformly at a tape pull rate of 4 cm min −1. The utilization efficiency of Nb and Ge particles as starting materials was 42%. Merits and demerits of resistively heating the substrate tape in the CVD process for the fabrication of long Nb 3Ge tapes are presented. Possible improvements on our method are also described.

Summary Abstract: Preparation and surface study of high‐Tc superconducting Nb–Ge film

Summary Abstract: Preparation and surface study of high‐<i>T</i>_<i>c</i> superconducting Nb–Ge film

High Field Properties of Superconducting Nb3Ge Films Prepared by Chemical Vapor Deposition

The superconducting critical current densities Jc of Nb3Ge films prepared by chemical vapor deposition have been investigated in magnetic fields parallel and perpendicular to the film plane up to above 20 T. The Jc and its field dependence are strongly influenced by any change in the microstructure of films deposited at different deposition temperatures Td. Films deposited at a low Td of 800°C exhibit a high Jc below 20 T and show a slight difference between parallel and perpendicular field dependences of Jc. As Td is increased to 900°C, the Jc of films is appreciably lowered over the entire field range and the difference between two field dependences of Jc is remarkably enlarged. The upper critical field Hc2 of these films has been obtained by extrapolating the Kramer plot of Jc1/2H1/4 versus H to J1/2cH1/4=0, and were estimated to range from 28 to 31 T at 4.2 K.

Fluctuation and Percolation Effect of Superconductivity in Nb3Ge Films

Electrical resistivity ρ (T) of R.F. sputter-deposited Nb3Ge films has been investigated in detail in the neighborhood of the transition temperature. Upon cooling from room temperature, many samples exhibited a logarithmic type of increase in ρ(T) before undergoing the transition to superconductivity. These films have very large resistivity in the normal state and show fairly high onset temperature Tc-on>25 K. The films with rather small resistivity exhibited a metalic behaivior in the normal state. But it has been found that the fluctuation effect on the electrical conductivity starts from fairly higher temperature than usual onset temperature. These experimental results have been qualitatively compared with the theory of superconductivity fluctuation.

Upper critical fields of superconducting Nb3Ge films

The upper critical field Hc2 (4.2 K) of superconducting Nb3Ge films prepared by sputtering and chemical vapor deposition (CVD) has been measured in a pulsed field. The Hc2 of these films is generally related to their superconducting critical temperature Tc. Sputtered films exhibit a linear increase of Hc2 from 25.4 to 33.3 T for a Tc range 18.6–21.3 K. Although the data of CVD-prepared films are scattered due to nonuniformity in the films, a similar dependence of Hc2 on Tc has been found.

Electron tunneling into A15 and B1 superconductors

We have prepared high quality tunnel junctions on thin films of Nb 3Sn, Nb 3Ge, Nb 3Al and TiN with artificial tunnel barriers of Al-oxide and AlZr-oxide. The Nb 3Sn/AlZr-oxide/Pb tunnel junctions prepared on Sn-deficient material showed no measurable proximity effect. Whereas the tunnel spectra of Sn deficient A15 NbSn could be satisfactorily analyzed by the standard Rowell-McMillan inversion procedure, the reduced density of states of the stoichiometric samples exhibited characteristic deformations in form of an overswing at energies far above the highest phonon energy. These deformations can be fitted by proximity analysis but it is shown that they can also be referred to the influence of an electronic density of states which is strongly structured in the range of phonon energies near the Fermi level. A very similar behavior is found for Nb 3Ge and to a smaller extent for Nb 3Al. Furthermore first tunneling results are presented for the B1 superconductors NbN and TiN. For NbN the Eliashberg function in the energy region of the acoustic phonons was determined and an upper limit for the coupling strength of the optical phonons is estimated. For TiN we show second derivative data. The energetic location of the minima in these curves agrees well with the position of maxima of the photon density of states.

Dimensional cross-over in collective flux pinning

We report a new effect regarding the collective flux pinning in sputtered amorphous Nb 3Ge films. Thick films ( d = 5−8 μm) show the two dimensional collective pinning (2DCP) at low fields and all temperatures. Below 0.7 T c, however, an extremely sharp and large peak develops in the macroscopic pinning force at fields around 0.75 B c2. It is argued that this peak is not the one usually observed in 2DCP, caused by a structural transition in the vortex lattice. Strong evidence is found that the onset of this peak is triggered by a dimensional cross-over in the collective pinning (2D to 3D) expected when the longitudinal correlation length L c becomes smaller than d/2. A discrepancy with the 3D expression of the Larkin-Ovchinnikov theory for L c nevertheless remains.

Effects of deposition parameters on the synthesis of Nb 3Ge in the CVD process

The effects of deposition parameters on the synthesis of Nb 3Ge in the chemical vapour deposition process have been studied. The chlorides, NbCl 5 and GeCl 4, prepared by in situ chlorination were used. The Nb-Ge film was deposited on a Hastelloy-X tape which moved through a tubular reaction furnace and was heated by direct current Joule heating. Film deposition was carried out under carefully controlled conditions; in particular, temperature distribution of the reactive gas along its flow and that of the substrate tape in the longitudinal direction were accurately controlled. Most of the films prepared were composed of A15 Nb 3Ge compound with a small quantity of hexagonal and/or tetragonal Nb 5Ge 3 phases. Those films showed onset T c of ≈20 K. Uniform films in layer thickness, compound compositions, and superconducting critical temperature were obtained with good reproducibility by precise control of the deposition parameters.

Study of the correlation between the structure and critical temperatureT c of sputtered Nb-Ge films

Nb-Ge layers with continuous change of chemical and phase composition were prepared by the d.c. sputtering method. The dependence of critical temperatureTc on phase composition, Ge-content, lattice imperfections and composition irregularities were studied. Films with highTc contain beside the A-15 Nb3Ge phase also the hexagonal and tetragonal modification of the Nb5Ge3 phase. Correlation betweenTc and Nb3Ge phase composition determined from the lattice parameter was found. In samples with highestTc the lattice parametera0=0·5135 nm corresponding to 22–23 at.% of germanium was determined.