Correlation between Superconducting Carrier Density and Transition Temperature in NbB 2+ x (original) (raw)

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.

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.

Superconductivity in the Nb2SnC compound

Solid State Communications, 2006

Nb 2 SnC is a member of the large family of lamellar materials that crystallize in the hexagonal structure with space group P6 3 /mmc which are isomorphs with Cr 2 AlC, also named H-phase. In spite of the great number of compounds which belong to this family, the superconductivity has been reported only for two cases: Mo 2 GaC and Nb 2 SC. In this work we show that superconductivity can be observed in Nb 2 SnC depending on the synthesis method used. The quality of the superconductor is strongly dependent of the synthesis method and the optimal results were reached for samples synthesized at 2.5 GPa and 523 ± 50ºC. This sample showed a critical temperature close to 7.8K, revealed from magnetization and transport measurement, the highest critical temperature reported up to now for an Hphase.

The role of boron lattice expansion in superconducting diborides

Intermetallics, 2003

Lattice structure of the diborides has been studied to explore a possible relation between high T c superconductivity and expansion of the boron plane. High purity Mg 1Àx A x B 2 diborides have been prepared by direct reaction of the elements and the lattice parameters are determined by X-ray diffraction, providing a measure to the strain E of the B-B distance. The results show that the superconductivity in these intermetallics appears in a critical region of tensile strain and charge density in the boron plane.

High Tc superconductivity at a critical strain and charge density in diborides

2001

We report variation of lattice structure in diborides (AB2) with different A atoms to identify the special case of MgB2 showing high Tc superconductivity. High purity MgB2 (Tc~39 K) has been prepared by direct reaction of the elements and the lattice parameters are measured by x-ray diffraction determining the strain e of the B-B distance. The results show that the superconductivity in these intermetallics appear near a critical point in a two variables (strain and charge density) phase diagram at (ec, rc).

On the possibility of superconductivity at higher temperatures in sp-valent diborides

2001

Superconducting transition temperatures (T c 's) of MgB 2 , Mg 1−x Ca x B 2 , Mg 1−x Na x B 2 , and Mg 1−x Al x B 2 are studied within the McMillan approximation using electronic and structural information obtained from density functional theory within the generalized gradient approximation. The density of states and T c of MgB 2 are both shown to be extremely sensitive to volume; in fact the density of states around the Fermi level is found to rise with increasing volume because of a prominent van Hove peak. Doping the Mg sublattice with small amounts of either Ca, which substantially increases the unit cell volume, or Na, which removes an electron from the unit cell while likewise increasing its volume, shifts the Fermi level toward the peak and thus both types of doping are predicted to enhance T c ; in Mg 1−x Al x B 2 , however, the combined effects of the additional electron and decreasing average unit cell volume are shown to decrease T c with increasing Al concentration, consistent with recent experiments.

Superconducting Properties of MgB2 from First Principles

Physical Review Letters, 2005

Solid MgB$_2$ has rather interesting and technologically important properties, such as a very high superconducting transition temperature. Focusing on this compound, we report the first non-trivial application of a novel density-functional-type theory for superconductors, recently proposed by the authors. Without invoking any adjustable parameters, we obtain the transition temperature, the gaps, and the specific heat of MgB$_2$ in very good agreement with experiment. Moreover, our calculations show how the Coulomb interaction acts differently on s and p states, thereby stabilizing the observed superconducting phase.