No abstract

No abstract

The effect of radiation-induced disordering in a nuclear reactor (fast neutrons fluence Φ = 5 · 10 19 cm 2 , Tirr = 340 K) on resistivity ρ, superconducting transition temperature TC and upper critical field HC 2 of polycrystalline MgCNi3 samples was investigated. It was found that TC decreases under irradiation from 6.5 to 2.9 K and completely recovers after annealing at 600 • C. Temperature dependences ρ(T ) are characteristic of compounds with strong electron-phonon interaction. The dHC 2 /dT behaviour testifies to a considerable decrease in density of electronic state at Fermi level N (EF ) in the course of disordering.Radiation-induced disordering caused by irradiation with high-energy particles is a unique method of investigating the properties of superconducting and normal states of ordered crystals [1,2]. Even in broad-band metals, such as intermetallic compounds with A15 structure, long-range ordering loss leads to considerable rearrangement of the electronic spectrum, resulting in disappearance of individual features of the electronic structure. Disordering causes decrease in densities at Fermi level N (E F ) and respective noticeable drop of T C in compounds with high initial N (E F ) (Nb 3 Sn or V 3 Si), and considerable (from 1.5 to 7 K) rise of T C in compounds with low N (E F ) and T C due to growth of N (E F ) (Mo 3 Si and Mo 3 Ge) [3,4,5]. In type HTSC compounds, disordering leads to more significant changes in properties: fast and complete T C degradation is accompanied with N (E F ) decrease and metal-insulator transition [2]. Thus investigation of response of a system to radiation-induced disordering serves as a kind of a test to reveal the characteristic features of its electron states. It was shown in recent papers [6,7] that T C drop from 38 to 5 K observed at MgB 2 under radiation-induced disordering is connected mainly with considerable drop of N (E F ), similar to Nb 3 Sn or V 3 Si compounds. In our investigation, we concentrated on the effect of disordering on the properties of superconducting compound MgCNi 3 (T C ∼ 8 K) with perovskite cubic structure of type SrTiO 3 , unconventional for intermetallides [8]. Our interest in this system was explained by the fact that its ground state is close to ferromagnetic due to the presence of a narrow peak in N (E) located 45 meV below the Fermi level [9]. This allowed us to regard it as a candidate for an unconventional (possibly triplet) superconductivity, similar to Sr 2 RuO 4 compound. It is known that in Sr 2 RuO 4 , as distinct from conventional superconducting compounds (intermetallides), T C undergoes anomalously strong suppression even under a slight disorder [10]. In MgCNi 3 , maximum T C is achieved at excess of carbon content only (nominal composition MgC 1.5 Ni 3 ), even though, according to neutron diffraction study, the actual composition is closer to Mg 0.96 CNi 3 , and excess carbon occupies the region between sample grain boundaries [11].In the sample preparation, fine powders Mg, C and Ni with purity better than 99.5% were used...

NMR line shift and nuclear spin-lattice relaxation rate T −1 1 of 11 B and 25 Mg were measured in superconducting MgB2 (T ons c =38K) structurally disordered by nuclear reactor neutrons up to the fluence of thermal neutrons Φ = 1 • 10 19 cm −2 . The temperature of superconducting transition was shifted down to T ons c,irrad =7K under irradiation. The change due irradiation in the partial electron density of states (DOS) at the Fermi energy of boron and magnesium were traced by taking into account that T −1 1 of 11 B and 25 Mg are determined by hyperfine magnetic interactions with carriers. It was revealed that electronic states near EF of Mg are influenced negligibly by irradiation whereas partial DOS of the B 2px,y states reduces greatly in irradiated MgB2. Acording to the Mc Millan formula, NMR data show that critical temperature decreases in irradiated MgB2 mainly due to reduction in the partial DOS of p states of boron.

In this paper, the influences of fast neutron bombardment, high pressure and chemical substitution on the electronic properties of PbSe single crystals are studied. For the first time in p-PbSe an electronic transition has been established of 'metal-semiconductor' type accompanied by an increase of resistivity of several orders of magnitude under the action of fast neutron bombardment. A similar increase in electrical resistance R was observed also under application of high pressure P above ∼3-4 GPa. The last increase was associated with the phase transformation from the NaCl-lattice to the GeS-lattice, which was seen also in sharp jumps of thermopower S. From the R(P) and S(P) dependences a linear decrease has been found of pressure of the above phase transition with increase of Sn content for n-Pb 1−x Sn x Se (x = 0.06, 0.08, 0.125) compounds.