V. Filippov

Electrical resistivity and superconductivity of LaB6 and LuB12

Abstract The electrical resistivity of LaB 6 and LuB 12 single crystals with high residual resistivity ratios has been measured down to 1.6 K. The temperature dependence below 30 K, caused by phonon scattering, shows for both materials a T n behaviour with n = 4.2 ± 0.1 for LaB 6 and n = 5.2 ± 0.1 for LuB 12 . From susceptibility measurements the superconducting transition temperature of LuB 12 was determined to be 0.44 K, for the LaB 6 sample with residual resistivity ratio of 160 no transition into the superconducting state was observed down to 5 mK.

Tunneling and magnetic characteristics of superconducting ZrB12 single crystals

Bulk and surface properties of high-quality single crystals of zirconium dodecaboride have been studied in the temperature range from 4.5 K up to the superconducting transition temperature which is found to be nearly 6.06 K. Scanning tunnelling spectroscopy data, together with dc and ac magnetization measurements, are consistent with the conventional s-wave pairing scenario, whereas they disagree in estimates of the electron-phonon coupling strength. We explain the divergence, supposing a great difference between the surface and bulk superconducting characteristics of the compound. This assertion is supported by our findings of a non-linear magnetic response to an amplitude-modulated alternating magnetic field, testifying to the presence of surface superconductivity in the ZrB

Some peculiarities of structure formation in eutectic d‐ and f‐transition metals boride alloys

_{12}

The peculiarities of the structure formation in directionally crystallized eutectics EuB6-MeB2

samples at dc fields exceeding the thermodynamic critical field.

Anomalous electron-phonon coupling probed on the surface of superconductor ZrB12

There are developed the physical principles of creation of the composite materials based on transitions and rare earth metals boride phases, in which are combined the advantages of ceramics (high hardness, strength) simultaneously with very high fracture toughness and heat resistance that is not peculiar to ceramics materials.The basis of constructuion of such compositions is the combination of phases having one common element boron and others with strong difference in electron structure (for instance, d‐ and f‐ metals). Due to the fundamental differences in electron structure, electron cores symmetry, and symmetry of crystal structure of d‐ and f‐ metals borides the mutual solubility of such materials is negligible.It was shown that quasibinary boride systems, particularly lanthanum hexaboride with d‐transition metals diborides are characterized by eutectic type of constitutional diagram.There were determined the phases relations in series pseudobinary systems of borides, corresponding to the eutectic co...

CONTACT INTERACTION OF LANTHANUM HEXABORIDE WITH METALLIC MELTS

Abstract The possibility of producing fiber-strengthened eutectic composites in situ in the quasibinary EuB6–MeB2 (Me–Zr, Hf, Sc) alloys is shown. By directional crystallization of the eutectic composition perfect real structures on the base of uniformly distributed MeB2 fibers in the EuB6 matrix may be formed.

Specific heat, magnetic susceptibility, resistivity and thermal expansion of the superconductor ZrB12

Magnetization measurements under hydrostatic pressure up to 10.5 kbar in zirconium dodeca- boride ZrB12 superconductor (Tc ≃ 6.0 K at p = 0) were carried out. A negative pressure effect on Tc with dTc/dp = −0.0225(3) K/kbar was observed. The electron-phonon coupling constant �el ph decreases with increasing pressure with dlnel ph/dp ≃ −0.20%/kbar. The magnetic field penetration depthwas studied in the Meissner state and, therefore, probes only the surface of the sample. The absolute values ofand the superconducting energy gap at ambient pressure and zero temperature were found to be �(0) =140(30) nm and �0 =1.251(9) meV, respectively. �0 scales linearly with Tc as 2�0/kBTc = 4.79(1). The studies of the pressure effect onreveal that � 2 increases with pressure with dln� 2 (0)/dp = 0.60(23) %/kbar. This effect can not be explained within the framework of conventional adiabatic electron-phonon pairing, suggesting that close to the surface, an unconventional non-adiabatic character of the electron-phonon coupling takes place.

Peculiarities in the Raman spectra of ZrB12 and LuB12 single crystals

According to the current views, wetting of the solids by metallic melts is determined by the intensity of the chemical reaction which takes place at the contact boundaries [3]. in this work, we examined wetting of lanthanum hexaboride by metals of the iron group, platinum and palladium, as well as copper, gold, germanium, and tin. ~ The selected metals are characterized by different intensities of interaction with the components of the solid phase lanthanum and boron. The metals of the iron group interact with lanthanum, and in the liquid and solid states are characterized by high solubility of boron in them with the formation of chemical compounds. Gold, copper, tin, and germanium do not interact with boron, whereas the binding energy of these metals with lanthanum increases in the sequence Cu-Au-Sn-Ge. The wetting effect was examined by the sessile droplet method in vacuum using the method described in [3]. The substrate was represented by single-crystal lanthanum hexaboride produced by zone melting. The surface of the substrate was polished to the tenth surface grade finish. Melts were produced from V-3 grade copper, iron (purity of the main element 999), semiconductor purity germanium, palladium ~j,~ platinum ~),~ as well as carbonyl iron, cobalt and nickel, premelted in vacuum by electron beam heating. We determined the variation of the contact wetting angle with time for several isothermal holding periods, and also inspected the variation of the diameter of the wetting droplet. The contact interaction was examined using metaiiographic and x-ray spectrum microanaiysis methods. Sections for examination were cut out in the direction normal to the wetting plane. The data on wetting of lanthanum hexaboride are presented in Table i. The temperature of formation of the liquid phase of all metals of the iron group and platinoids in heating contact with lanthanum hexaboride is considerably lower than the melting point of the pure metals, in melts of cobalt and nickel the liquid phase wets the substrate with the angle close to zero. For iron this angle equals 30 ~ . The wetting angles of the melts of tin, germanium, gold, and copper greatly decrease with increasing temperature and no contact melting is detected. The results show that the metals of the iron group intensively interact *A. V. Gorbach took part in the investigations.