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.

According to an earlier Abrikosov model, a positive, nonsaturating, linear magnetoresistivity (LMR) is expected in clean, low-carrier-density metals when measured at very low temperatures and under very high magnetic fields. Recently, a vast class of materials were shown to exhibit extraordinary high LMR but at conditions that deviate sharply from the above-mentioned Abrikosov-type conditions. Such deviations are often considered within either classical Parish-Littlewood scenario of random-conductivity network or within a quantum scenario of small-effective mass or low carriers at tiny pockets neighboring the Fermi surface. This work reports on a manifestation of novel example of a robust, but moderate, LMR up to ∼100 K in the diamagnetic, layered, compensated, semimetallic CaAl2Si2. We carried out extensive and systematic characterization of baric and thermal evolution of LMR together with first-principles electronic structure calculations based on density functional theory. Our analyses revealed strong correlations among the main parameters of LMR and, in addition, a presence of various transition/crossover events based on which a P − T phase diagram was constructed. We discuss whether CaAl2Si2 can be classified as a quantum Abrikosov or classical Parish-Littlewood LMR system.

The full substitution of Ni by Co and Pt enclosed in the same crystalline structure ͑I4/mmm͒ of Pr-based compounds have been investigated. The structural characteristics, magnetic properties, and magnetoresistance of PrCo 2 B 2 C, PrNi 2 B 2 C, and PrPt 2 B 2 C single crystals were studied to determine the reason for the absence or presence of superconductivity in those compounds. An antiferromagnetic transition was found for PrCo 2 B 2 C at about 8.5 K, where the easy magnetization axis is along the c-crystallographic direction whereas in PrNi 2 B 2 C the easy magnetization was at the ab plane. In PrNi 2 B 2 C a small magnetization without traces of saturation is found when it is measured up to 18 Tesla of applied magnetic field. This behavior suggests a screening effect as a result of a moderate hybridization between the conduction band and Pr ions. Superconductivity occurs in PrPt 2 B 2 C at about 6 K. The Pt-Pt distance and internal bonding angles are similar to those observed in the superconducting Ni-based compounds. Magnetoresistance measurements show positive and quadratic behavior ͑⌬ / 0 ϳ aH 2 ͒ suggesting a spin fluctuation system. In the PrPt 2 B 2 C compound the electronic mean free path is smaller than the BCS coherence length ͑ Ӷ͒ suggesting a dirty type II superconductor.

Single-walled carbon nanotubes (SWNTs) have many interesting properties; they may be metallic or semiconducting depending on their diameter and helicity of the graphene sheet. Hydrostatic or quasi-hydrostatic high pressures can probe many electronic features. Resistance -temperature measurements in SWNTs from normal condition and under 0.4 GPa of quasi-hydrostatic pressures reveal a semiconducting-like behavior. From 0.5 to about 2.0 GPa the resistance changes to a Kondo-like feature due to magnetic impurities used to catalyse the nanotube formation. Above 2.0 GPa, they become metallic and at about 2.4 GPa the resistance decreases dramatically around 3 K suggesting a superconducting transition. 71.30.+h, 74.10.+v, 75.20.Hr, 74.62.Fj Two unique characteristics of carbon nanotubes are their different electronic behavior depending on diameter and helicity [1,2]. Electronic and transport properties of singlewalled carbon nanotubes (SWNTs) also differ depending on chirality [3][4][5]. Many theoretical studies indicate that the local density of electronic states changes if the graphene sheet is wrapped in a zigzag or an armchair configuration. Armchair

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