A. E. Kar’kin

New superconductor with a layered crystal structure: Nickel oxybismuthide LaO1−δNiBi

The results of synthesizing a new layered phase—nickel oxybismuthide LaO1−δNiBi in a series of superconducting oxypnictides—and its properties in the superconducting and normal states are reported. Although the temperature of the transition of this phase to the superconducting state, Tc ∼ 4 K, is much lower than the value Tc = 55 K reached at present in oxyarsenide SmO1−δFeAs, the similarity of the crystal structures and ρ(T) dependencies indicates that the mechanism responsible for the appearance of the semiconducting state is the same in lanthane oxybismuthide and samarium oxyarsenide.

Superconducting properties of the atomically disordered MgB2 compound

The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1×1019cm−2) on the superconducting transition temperature Tc and the upper critical field Hc2 of polycrystalline MgB2 samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature Tc decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 70°C. A weak change in the dHc2/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the “pure” limit of the theory of disordered superconductors. The suppression of Tc upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level.

Structure and properties of the intercalation compound CuxTiSe2

Compounds in the pseudobinary Cu-TiSe2 intercalation system are directly synthesized from elements. The phase diagram of the system is investigated, the solubility limit of copper is measured, and the structure of the material is determined. In the copper concentration range up to 60 mol %, single crystals are grown and the temperature dependence of the electrical resistance is measured. It is demonstrated that, in the concentration range under investigation, the intercalation of the system with copper gives rise to a set of phenomena observed upon intercalation of alkali metals.

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρxx and transverse ρxy resistivities and magnetic susceptibility χac of La0.8Sr0.2MnO3 single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρxy∼ρxx, which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χac. As a result, the curve of ρxy versus ρxx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρxx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽TC. The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials.

Relation between Tc and the defect structure of neodymium cerium cuprite Nd2−xCexCuO4−y

Neutron powder diffraction has been used to study the defect structure of neodymium cerium cuprite Nd2−xCexCuO4±y (x=0.15). It has been shown that in addition to oxygen vacancies, O2 sites in superconducting samples may also contain a small quantity of implanted oxygen atoms positioned between copper ions and neodymium/cerium, which control the electrical charge in the Cu-O planes. The oxygen distribution among crystal lattice sites in Nd2−xCexCuO4±y (x=0.15) as determined, the average charge of the copper ions was calculated by the method of valence sums, and a correlation was established between the charge of the copper-oxygen plane and Tc.

Size effect in nanocrystalline manganites La1−x A xMnO3 (A=Ag, Sr)

Nanocrystalline samples of the manganites La0.9Ag0.1MnO3, La0.7Ag0.3MnO3, and La0.7Sr0.3MnO3 were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.

Effect of pressure and anionic substitution on the electrical properties of HgTeS crystals

The resistivity ρ and the Hall constant R for the HgTe1−xSx (0.04≤x≤0.6) crystals have been investigated in the temperature range 4.2–350 K in the magnetic fields B up to 14 T. The pressure dependences of the resistivity ρ have been measured at the pressures P as high as 1 GPa at temperature T=77–300 K and magnetic field B=0–2 T. It is found that the samples with x≤0.20 exhibit a decreasing dependence ρ(T) typical of zero-gap semiconductors, whereas the samples with x≥0.27 show the dependence ρ(T) characteristic of semimetals. For the semiconducting crystals with x≈0.20 and x≈0.14, the temperature coefficient of ρ(T) changes sign at T=265 and T>300 K, respectively. Under a pressure of ≈1 GPa, the temperature of the sign inversion decreases by ≈30 K. An increase in the magnetic field B and a rise in the temperature T lead to a change in the sign of the Hall constant R for the semiconducting samples, but do not affect the electronic sign of R for the semimetallic samples. The behavior of R and ρ correlates with the thermoemf data obtained at the quasi-hydro-static pressure P up to 3 GPa. It is demonstrated that the substitution of sulfur atoms for tellurium atoms brings about an increase in the concentration of electrons and a decrease in their mobility. The transition to the wide-gap semiconductor phase is observed at P>1–1.5 GPa. The conclusion is drawn that the semimetallic crystals HgTe1−xSx with x≥0.27 and HgSe are similar in properties.

Synthesis, single-crystal growth, and superconducting properties of Fe-Se system

Single crystals FeSex were grown by the vapor-transport reaction method using a polycrystalline material, a sealed quartz tube, and iodine I2. The single crystals grown are 0.1–0.5 mm in size and are characterized by hexagonal prismatic, tetragonal prismatic, platelike square, and hexagonal faceting. Measurements of the electron-transport and magnetic properties of the FeSex single crystals indicate the existence of a superconducting transition at 24 K.

Accumulation and annealing of radiation defects under low-temperature electron and neutron irradiation of ODS steel and Fe-Cr alloys

The processes of accumulation and annealing of radiation defects at low-temperature (77 K) electron and neutron irradiation and their effect on the physicomechanical properties of Fe-Cr alloys and oxide dispersion strengthened (ODS) steel have been studied. It has been shown that the behavior of radiation defects in ODS steel and Fe-Cr alloys is qualitatively similar. Above 250 K, radiation-induced processes of the solid solution decomposition become conspicuous. These processes are much less pronounced in ODS steel because of specific features of its microstructure. Processes related to the overlapping of displacement cascades under neutron irradiation have been considered. It has been shown that, in this case, it is the increase in the size of vacancy clusters, rather than the growth of their concentration, that is prevailing. Possible mechanisms of the radiation hardening of the ODS steel and the Fe-13Cr alloy upon irradiation and subsequent annealing have been discussed.

Fast neutron bombardment induced semiconductor-metal electron transition in lead selenide

We have studied the electrical and galvanomagnetic properties of p-PbSe single crystals irradiated with fast neutrons. Changes in the temperature dependence of the resistivity and the Hall constant show evidence of the metal-semiconductor electron transition. Subsequent annealing at 350–390 K leads to a partial recovery of the initial properties and the reverse electron transition.