Superconducting characteristics in electron-doped layered hafnium nitride : 15N isotope effect studies

We report 15N nuclear magnetic resonance (NMR) and the nitrogen-isotope effect studies on c-axis oriented sample of Li-doped HfNCl with superconducting transition temperature Tc~25.5 K. 15N NMR Knight shift decreases toward zero below Tc, giving evidence that the pairing symmetry is an even-parity spin singlet, and that the nitrogen site plays an important role in the occurrence of superconductivity. A nitrogen isotope shift is found to be quite small, ∇Tc~0.1 K (N = 0.07±0.02), which cannot reproduce such a high Tc within the traditional BCS framework

NMR study in the superconducting silicon clathrate compound NaxBaySi46

29Si, 23Na, 137Ba, and 135Ba NMR experiments were carried out in the superconducting silicon clathrate compound NaxBaySi46 to study the electronic states above the superconducting transition temperature. We observed three distinct 29Si signals with different Knight shift (2036, 862, and 720 ppm at 90 K with a small temperature dependence of ∼10%), which were ascribed to inequivalent Si sites in the Si46 structure. The results indicate that the conduction-electron spin and/or charge density is strongly site-dependent. At all the atomic sites, the Korringa relation in NMR relaxation was observed, indicative of metallic electronic structure with moderate electronic correlation. The Na and Ba atoms are partially ionized in NaxBaySi46, which shows the noticeable difference from the system of alkali-metal-doped fullerides. The comparison with band calculations is discussed

Multiwalled carbon nanotubes grown in hydrogen atmosphere : An x-ray diffraction study

X-ray diffraction study of multiwalled carbon nanotube (MWNT) grown by arc discharge in hydrogen atmosphere is presented. It is found that the thermal-expansion coefficient along the radial direction of MWNT is widely distributed in a range from 1.6×10-5 K-1 to 2.6×10-5 K-1, indicating the existence of both of Russian doll MWNT and highly defective MWNT. Russian doll MWNT is suggested to have the outer diameter less than ∼100  Å . Thicker MWNT's are typically highly defective, and may have the jelly roll (scroll) or defective polygonal structure consisting of flat graphite domains

Thermal expansion of single-walled carbon nanotube (SWNT) bundles: X-ray diffraction studies

Thermal expansion coefficient in single-walled carbon nanotube bundles was determined as (-0.15±0.20)×10-5 (1/K) for the tube diameter and (0.75±0.25)×10-5 (1/K) for the triangular lattice constant by means of x-ray scattering between 300 K to 950 K. The value for the intertube gap was (4.2±1.4)×10-5 (1/K), which is larger than 2.6×10-5 (1/K) for the c-axis thermal expansion in graphite. The results reveal the presence of a remarkably larger lattice anharmonicity in nanotube bundles than that of graphite. The small value for the tube diameter is consistent with the seamless tube structure formed by a strong covalent bond between carbon atoms comparable to that in graphite