Superconductivity in the Nb2SnC compound (original) (raw)
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Journal of Physics and Chemistry of Solids, 1967
The system Nb~Al Sn, exhibits complete solid solubility, a positive Seebeck voltage that increases monatomically from 1.8 to over 12 |dV/°K, and a superconducting transition temperature T that has a broad minimum of T « 15.3 °K at about y = 0.5. The c c pseudobinary Nb-Sn, Sb , on the other hand, contains two phases, ■3 x-"X X each with the A15 structure, in the interval 0.3 < x < 0.8. Whereas the Seebeck voltage of the low-Sb phase is positive and has a T > 14°K, the high Sb phase has a negative Seebeck voltage and is not superconducting above 4.2°K. These properties have been interpreted with the aid of a qualitative band-structure scheme. Thus, the rapid drop in T to below 4.2°K in the system Nb^Sn, Sb is believed to be associated with the filling of 3 1-x x the bonding, interchain d bands of the Nb subarray.
Crystalline structure and the superconducting properties of NbB 2+ x
Journal of Physics: Condensed Matter, 2004
The effect of boron excess in the structure and superconducting properties of NbB 2 is reported. Rietveld refinements of the x-ray diffraction patterns indicate that boron excess induces significant changes in the Nb-B bond length, increasing the c-axis. In contrast, the B-B bond length remains essentially constant. Magnetization behaviour was studied in the temperature range from 2 to 15 K. We found that for (B/Nb) exp 2.20(2) of boron excess samples display superconductivity with a maximum T C of about 9.8 K at (B/Nb) exp = 2.34(1). High pressure measurements in samples with two different boron contents reveal that T C decreases at different ratios, dT C /d P. Superconducting parameters were determined, indicating that NbB 2+x is a type II superconductor. We correlated the change of T C with the evolution of the structural parameters and found that it coincides with theoretical predictions of band structure.
Phase Relationships, Basic Metallurgy, and Superconducting Properties of Nb3Sn and Related Compounds
Advances in Cryogenic Engineering Materials, 1982
The phase relationships and the superconducting properties of Nb 1 Sn are compared with those of other high Tc compounds crystallizing in Ehe AIS structure: Nh3Al, Nb3Ga, Nb3Ge, V3Si, V3Ga ••. Characteristic differences of these systems, i.e., the shape of the•AIS phase field, the variation of Tc with composition or with atomic ordering, are discussed. Recent methods leading to the accurate determination of the phase relationships in these systems up to 2000 °C are reviewed. The discussion is extended to the low temperature relationships in Nb3Sn and V3Si with the corresponding tetragonal modifications. Methods for observing these low temperature details and recent results about their influence on Tc are presented. In the case of Nb3Sn, the factors influencing the formation of the low temperature tetragonal phase, such as hydrostatic pressure, precompression in multifilamentary wires, or hydrogen loading, are discussed, Zusammenfassung Phasenbeziehungen, Metallurgie und supraleitende Eigenschaften von Nb~ und anderen Verbindungen desselben Typs •Die Phasenbeziehungen und die supraleitenden Eigens•chaften von Nb 3 Sn werden mit denen anderer AIS Verbindungen mit hohem Tc verglichen: Nb3Al, Nb 3 Ga, Nb3Ge, v 3 si, V3Ga, ..• Charakteristische Unterschiede in diesen Systemen betreffend das Homogenitätsgebiet der AIS Phase sowie die Änderung von Tc mit der Zusammensetzung und dem Ordnungsgrad werden diskutiert. Kürzlich entwickelte Methoden zur präzis•en Bestimmung dieser Phasendiagramme bis zu 2000 °C werden beschrieben. Die Tieftemperaturphasendiagramme von Nb3Sn und v 3 si mit der dort auftretenden tetragonalen Modifikation werden diskutiert. Die Methoden zur Untersuchung dieser Details sowie neue Resultate über deren Einfluß auf Tc werden vorgestellt. Verschiedene Faktoren wie der hydrostatische Druck, die Vorspannung in Vielkernsupraleitern oder der Wasserstoffgehalt können die Bildung der tetragonalen Phase beeinflussen und werden diskutiert.
Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric η-Carbide Nb4Rh2C1−δ
ACS Materials Au, 2021
The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Cooper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as μ 0 H Pauli ≈ 1.86[T/K]•T c for a weakcoupling Bardeen−Schrieffer−Cooper (BCS) superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy Fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic η-carbidetype compound Nb 4 Rh 2 C 1−δ , with a normalized specific heat jump of ΔC/γT c = 1.64. We find that this material has a remarkably high upper critical field of μ 0 H c2 (0) = 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of μ 0 H Pauli = 18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.
Magnetic order and crystal structure in the superconducting RNi2B2C materials
Physical Review B, 1997
Neutron-diffraction measurements have been carried out to investigate the crystal structure, magnetic structures, and magnetic phase transitions in RNi 2 B 2 C (Rϭ Y, Ce, Pr, Nd, Tb, Dy, Ho, Er, Tm, and Yb͒. The materials that order magnetically exhibit a wide variety of both commensurate and incommensurate magnetic structures, which argues strongly that the dominant exchange interactions are of the indirect Ruderman-Kittel-Kasuya-Yosida type. The Nd system exhibits a commensurate antiferromagnetic ordering at 4.8 K, with wave vector ␦ϭ(1/2,0,1/2) and moment direction along a ͑or equivalently with ␦ϭ(0,1/2,1/2) and moment direction along b in this tetragonal system͒. For Dy (T N ϭ10.6 K), Pr (T N ϭ4.0 K), and the low-temperature phase of Ho, the magnetic structure is also a commensurate antiferromagnet that consists of ferromagnetic sheets of rare-earth moments in the a-b plane, with the sheets coupled antiferromagnetically along the c axis ͓␦ϭ(0,0,1)͔. Pr is not superconducting, while for Dy (T c ϭ6 K) and Ho (T c ϭ8 K) this magnetic order coexists with superconductivity. For Ho, though, the magnetic state that initially forms at T N Ϸ8.5 K is an incommensurate spiral antiferromagnetic state along the c axis in which the direction of these ferromagnetic sheets are rotated in the a-b plane by ϳ17°from their low-temperature antiparallel configuration ͓␦ϭ(0,0,0.91)͔. The intensity for this spiral state reaches a maximum near the reentrant superconducting transition (ϳ5 K); the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. An incommensurate a-axis modulation, with ␦ϭ(0.55,0,0), is also observed above the spiral-antiferromagnetic transition, but it exists over a narrower temperature range than the spiral state, and also collapses near the reentrant superconducting transition. The Er system forms an incommensurate, transversely polarized spin-density wave ͑SDW͒ state at T N ϭ6.8 K, with ␦ϭ(0.553,0,0) and moment direction along b ͑or with ␦ along b and the moment direction along a). The SDW squares up at low T, and coexists with the superconducting state (T c ϭ11 K) over the full temperature range where magnetic order is present. Tb, which does not superconduct, orders with a very similar wave vector, but the SDW is longitudinally polarized in this case and again squares up at low T. Tm orders at T N ϭ1.5 K in a transversely polarized SDW state, but with the moments along the c axis and ␦ϭ(0.093,0.093,0). This state is coexistent with superconductivity (T c ϭ11 K). No significant magnetic moment is found to develop on the Ni site in any of the materials, and there is no magnetic ordering of any kind in the Y, Yb, or Ce materials. Profile refinements have also been carried out on these same samples to investigate the systematics of the crystallography, and the crystal structure is I4/mmm over the full range of compositions and temperatures investigated. The area of the a-b plane and the volume of the unit cell both decrease smoothly with either decreasing lanthanide radius or decreasing temperature, but the strong boron-carbon and nickel-carbon bonding then forces the c axis to expand.
Electronic and structural properties of superconducting<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline">mml:mrowmml:msubmml:mrow<mml:mi mathvariant="normal">MgBmml:mrowmml:mn2mml:mo,</mml:m...
Physical review, 2001
We report a detailed study of the electronic and structural properties of the 39K superconductor MgB 2 and of several related systems of the same family, namely Mg 0.5 Al 0.5 B 2 , BeB 2 , CaSi 2 and CaBeSi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dimensional σ-bonding bands contribute significantly at the Fermi level. Similarities and differences between MgB 2 and BeB 2 (whose superconducting properties have not been yet investigated) are analyzed in detail. Our calculations for Mg 0.5 Al 0.5 B 2 show that metal substitution cannot be fully described in a rigid band model. CaSi 2 is studied as a function of pressure, and Be substitution in the Si planes leads to a stable compound similar in many aspects to diborides.
Superconductivity in Nb-Sn Thin Films of Stoichiometric and Off-Stoichiometric Compositions
IEEE Transactions on Applied Superconductivity, 2013
Binary Nb-Sn thin film samples were fabricated and characterized in terms of their composition, morphology, and superconducting properties. Nb-Sn was magnetron-sputtered onto heated R-plane sapphire substrates at 700 • C, 800 • C, and 900 • C, using a custom-built heater assembly. Samples were cut into strips, where each strip has a unique composition. For a subset of the samples, Nb-Sn was selectively etched away at an etching rate of 6 ± 1 nm/s using an aqueous solution of 3 vol.% hydrofluoric and 19 vol.% nitric acid. The sample composition was investigated with a scanning electron microscope with an X-ray energy dispersive spectroscopy detector. Surface and cross-section morphologies were investigated using scanning electron microscopy and scanning transmission electron microscopy, revealing a dense columnar poly-crystalline grain structure. X-ray diffraction measurements indicate a highly textured film that is (100) oriented out-of-plane and random in-plane. The critical temperature T c (ranging from 9.8 to 17.9 K), critical magnetic field μ 0 H c2 (ranging from 12.5 to 31.3 T), residual resistivity ratio (RRR), and normal state resistivity ρ 0 were measured and found to be broadly consistent with literature data on bulk Nb 3 Sn.
Single crystal MgB2 with anisotropic superconducting properties
2001
The discovery of superconductor in magnesium diboride MgB2 with high Tc (39 K) has raised some challenging issues; whether this new superconductor resembles a high temperature cuprate superconductor(HTS) or a low temperature metallic superconductor; which superconducting mechanism, a phonon- mediated BCS or a hole superconducting mechanism or other new exotic mechanism may account for this superconductivity; and how about its future for applications. In order to clarify the above questions, experiments using the single crystal sample are urgently required. Here we have first succeeded in obtaining the single crystal of this new MgB2 superconductivity, and performed its electrical resistance and magnetization measurements. Their experiments show that the electronic and magnetic properties depend on the crystallographic direction. Our results indicate that the single crystal MgB2 superconductor shows anisotropic superconducting properties and thus can provide scientific basis for the ...
The NbB 2-phase revisited: Homogeneity range, defect structure, superconductivity
Acta Materialia, 2005
The discovery of superconductivity below 40 K in MgB 2 has motivated new investigations on similar compounds, especially on binary diborides. The great majority of these compounds represent the AlB 2 -type structure (P6/mmm space group, number 191) and comprise line compounds. However, among those, NbB 2 and TaB 2 are reported to present a significant homogeneity region, a value of 12 at.% being reported for the case of NbB 2 . In this work we have evaluated the homogeneity range of the NbB 2 -phase through detailed microstructural characterization of as-cast, as-cast + heat-treated and solid state sintered Nb-B alloys. Neutron diffraction experiments were performed to assess the defect mechanism responsible for accommodating the non-ideal NbB 2 stoichiometries (ideal = 66.7 at.% B). The results clearly showed that the width of the homogeneity range of this phase is nearly 5 at.%, extending from 65 at.% B (NbB 1.86 ) up to 70 at.% B (NbB 2.34 ). Rietveld refinement of the neutron intensity diffraction data indicated a random distribution of vacancies in the Nb-subnet for hyperstoichiometric NbB 2 . The occurrence of a possible Nb-vacancy ordered supercell was evaluated; however, a simple AlB 2 -type is observed throughout the entire homogeneity range. The superconducting properties of selected alloys were checked via magnetic measurements. The Nb-deficient samples were found to contain traces of a superconducting phase with T c % 3.5 K.
Superconductor Science and Technology, 2010
Carbon (C) and rare earth oxide (REO) co-doped bulk polycrystalline MgB 2 samples with nominal compositions Mg 1−y (REO) y (B 0.95 C 0.05) 2 (where y = 0.00, 0.01, 0.03, 0.05 and REO = Eu 2 O 3 or Pr 6 O 11) have been synthesized via a solid state reaction route. The XRD results reveal the presence of impurity phases EuB 6 in Eu 2 O 3 and PrB 4 and PrB 6 in Pr 6 O 11 co-doped samples along with the main hexagonal phase of MgB 2 and a small amount of MgO. The values of upper critical field (H c2) and irreversibility field (H irr), except H c2 of y = 0.05 for Eu 2 O 3 , have been found to increase at all temperatures (<T c) with increasing doping concentration of REO. Improvements in the values of critical current density (J c) at 10 K for y = 0.01 of Eu 2 O 3 and y = 0.01, 0.03 of Pr 6 O 11 co-doped samples have been observed in high fields (>6.5 T) region. At 20 K enhancement in the high field (>6 T) J c values for y = 0.01, 0.03 of Eu 2 O 3 and y = 0.01 of Pr 6 O 11 co-doped samples are also reported in the present work. The correlations between the structural characteristics and the observed superconducting properties of the co-doped samples are described and discussed in this paper.