Superconductivity and resistance behaviour of σ-phase alloys: Nb−Rh and Ta−Rh

Superconductivity, structure and electrical resistance behaviour of -phase alloys of Nb–Rh and Ta–Rh are investigated. The Ta–Rh alloys do not become superconducting above 1.2 K. The andH _c2 (0) values of a homogeneous alloy with the composition Nb_65.2Rh_34.8 are 2.95 K, 13.9 kG/K and 23 kG, respectively, whereas for an inhomogeneous alloy with the composition Nb_63.7Rh_36.3 these values are 4.24 K, 5.5 kG/K and 14 kG, respectively. Splat quenching results in a substantial increase in the andH _c2 (0) values of the Rh-rich sample. Annealing (900°C, 100 h) of the Rh-rich sample leads only to small changes in the superconducting properties but a small amount of Nb–Rh solid solution has been formed. The electrical resistance of Nb_65.2Rh_34.8 decreases with decreasing temperature and varies asT ^0.5 between 150 and 240 K and asT between 60 and 140 K. For Ta_70.0Rh_30.0 the temperature coefficient changes to negative values below 170K. values are calculated for Nb–Rh using McMillan's formula. An estimatedT _c value of Ta–Rh is 0.2 K. TheH _c2 (0) values of Nb–Rh are in good agreement with the theoreticalH _c ^2** (0) values.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday     

Symmetry of superconducting compositions in the BaPb1−xBixO3 system

Reexamination of the symmetry of superconducting phases in the BaPb_1−xBi_xO₃ system was carried out by x-ray diffraction using a synchrotron source which offers exceptional resolution. These studies confirm our original finding of tetragonal symmetry. We believe that confusion on the symmetry has resulted because some compositions in this system are two phase mixtures.     

Temperature dependence of the electron effective mass in MnxHg1−xSe

We have investigated the temperature dependence of the electron effective mass in Mn_xHg_1–xSe crystals (0 < x 0.1) in the T=90–300 K temperature range. We have determined that the temperature-dependent changes in the band gap (_g), in the band diagram nonparabolicity, and in the conduction band carrier concentration have a strong effect on the temperature dependence of the carrier effective mass at the Fermi level m ^*=f(T).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 5–9, November, 1987.     

CP-violation effect in neutralino pair production in e+ −e− annihilation and the electric dipole moment of the electron

It is shown that in the supersymmetric extensions of the standard electroweak theory, the threshold behaviour of the cross section of production of two different neutralinos in e⁺ −e⁻ annihilation may be sensitive to the violation of CP invariance by the neutral gaugino-higgsino mass term. The corresponding CP violation effect may be substantial quite far from the threshold. A relation (although rather indirect) between the form of the behaviour of this section in the threshold region and the value of the electric dipole moment of the electron may exist.     

Inhomogeneous state of the electron system in BaPb1 − x Sb x O3 superconducting perovskites: The 207Pb NMR study

Comprehensive measurements of the ²⁰⁷Pb NMR spectra, Knight shifts ²⁰⁷ K _s , spin-lattice relaxation rate ²⁰⁷ T ₁ ^?1 , and the Pb spin echo decay ²⁰⁷ E(2t) in BaPb_{1 ? x} Sb _x O₃ superconducting oxides (x ≤ 0.33) have been performed. A considerable increase in the shift distribution width when Sb is substituted for Pb indicates the formation of the inhomogeneous state of the electron system in the oxide conduction band. The shift magnitude proportional to the density of states near the Fermi energy, ²⁰⁷ K s ∝ N(E _F ), reaches its maximum in the compounds with x = 0.18–0.25 with the maximum temperatures T _c of the superconducting transition. In the BaPb_{1 ? x} Sb _x O₃ compounds, the Korringa relation is met, indicating that all of the samples are in the metal phase. The data on the Pb spin echo decay and the relevant estimates of the indirect coupling constants of the nuclei of the nearest neighbor Pb atoms clearly demonstrate the development of the electron system state in the oxide metal phase, which is microscopically inhomogeneous over the crystal.     

Calculation of the temperature dependence of the thermal emf in amorphous alloys CaxAl1−x and AuxNi1−x

The temperature dependence of the thermal emf of Ca_xAl_1–x and Au_xNi_1–x for four different concentrations of the components of the alloys is calculated on the basis of the concept of dynamic concentrated excitations in amorphous metal systems. It is shown that increasing x from 0.15 to 0.50 in Au_xNi_1–x raises the thermal emf, and a further increase in the Au concentration from 0.50 to 0.80 lowers S(T). For Ca_xAl_1–x the dependence S(T) is calculated in the interval of Ca concentrations from 0.55 to 0.75. In this concentration interval the thermal emf decreases as x is increased. It is shown that for both types of alloys the S(T) curve bends abruptly at a temperature near 10T₀ (where T₀ is the concentration-dependent characteristic temperature of amorphous alloys separating the ranges of strong and weak scattering of electrons by dynamic concentration excitations). The so-called S(T) knee shifts toward lower temperatures when the thermal emf increases with increasing x and toward higher temperatures when S(T) decreases with increasing x. The results agree with experimental data.Institute of Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–48, August, 1994.     

The effect of the violation of Vegard's law on the optical bowing in Si1 − xGex alloys

The effect of the deviation of the lattice parameters from Vegard's rule on the optical bowing of the fundamental gap in Si_{1 − x}Ge_x semiconductor alloys is discussed. Our computations are based on the pseudopotential method. To make allowance for the chemical disorder, the virtual-crystal approximation is used, including a correction to the alloy potential. Our results show that the bowing parameter is highly improved when the lattice relaxation effect is included which indicates the importance of the lattice mismatch in the electronic band structure calculations of IV-IV semiconductor alloys.     

Effects of disorder in Mg1−xTaxB2 alloys using the coherent-potential approximation

Using the Korringa–Kohn–Rostoker coherent-potential approximation in the atomic-sphere approximation (KKR-ASA CPA) method for taking into account the effects of disorder, Gaspari–Gyorffy formalism for calculating the electron–phonon coupling constant λ, and Allen–Dynes equation for calculating T_c, we have studied the variation of T_c in Mg_1−xTa_xB₂ alloys as a function of Ta concentration. Our results show that the T_c decreases with the addition of Ta for up to 40 at.% and remains essentially zero from 60 to 80 at.% of Ta. We also find TaB₂ to be superconducting, albeit at a lower temperature. Our analysis shows that the variation in T_c in Mg_1−xTa_xB₂ is mostly dictated by the changes in the B p density of states associated with the addition of Ta.     

Temperature variation of the spin reorientation in Er2−xDyxFe14B alloys

The competition between the uniaxial magnetic anisotropy of the iron sublattice and the basal anisotropy of the rare earth sublattice in the R₂Fe₁₄B alloys, occuring for the rare earths with a positive Steven's factor (a_j> 0), leads to spin reorientation at 316 K for the Er₂Fe₁₄B compound. We have succeeded, by making a series of pseudoternaries of the type Er_2−xDy_xFe₁₄b, to establish a correlation between the spin reorientation temperature and the quantity x. Mossbauer spectra of aligned powders show a smooth decrease of the spin reorientation temperature for values of x up to 1, while for larger values spin reorientation is absent.     

Absolute composition depth-profiles in surface segregation of PtRh alloys

The absolute composition depth profiles of the first 12 atomic layers of the (001) surface and the first 9 layers of the (111) surface of a PtRh alloy have been measured. As found in earlier studies, the surface is enriched with Pt, the sub-surface layer is depleted of Pt, and the Pt concentration oscillates toward the bulk value. The detailed oscillations are, however, very different for the two surfaces. Using a simple model for the metallic bonds, Monte Carlo simulations are performed to extract a set of bond energy differences of the constituent atom pairs from the experimental data.     

Influence of electron correlations on Auger electron —and appearance potential — spectra of solids

Within the framework of the Hubbard-model the influence of electron correlations on AES and APS for a non-degenerate energy band is investigated. Both spectroscopies are determined by the same two-particle Green function which is solved by a diagrammatic vertex-correction method in the Matsubara-formalism. For empty (n=0) and for completely filled (n=2) bands the method turns out to be exact. The spectra are strongly influenced by the Coulomb interactionU/W and by the degreen of band-filling. Already very weakU/W are sufficient for a substantial deviation of APS and AES from the self-fold of the one particle density of states. For intermediate or even strongU/W(>1) the spectra consist of two parts, a relatively broad band-like region and a sharp satellite. As soon as the satellite splits off, it takes practically the whole spectral weight. In all cases the satellite has almost exactly the shape of the free density of states. The two-hole (electron) bound state, which causes the satellite, propagates virtually without scattering through the lattice. For fixedU/W the band occupationn must be below (above) a critical value to push away a satellite in APS (AES). The temperature-dependence of the spectra is non-negligible for partially filled bands (exceptionsn=0,2), being, however, qualitatively not very striking as long as a non-magnetic systems is considered.     

Renormalization of the contribution of the electron—electron interaction to the conductivity of two-dimensional electron systems

The contribution of the electron—electron interaction to the conductivity of the two-dimensional electron gas in an In_x Ga_1-x As single quantum well with different disorder strengths was experimentally studied. It is shown that the data are described well within the framework of the one-loop approximation of the renormalization group theory so long as the conductivity of the system remains higher than around 15e ²/μh.     

Spin correlations of Λ hyperons in the SELEX experiment

Two-particle correlations of the pairs of identical Λ hyperons in Σ⁻ A interactions were studied based on the statistical data obtained in the SELEX experiment performed using the Tevatron collider at FNAL in 1996–1997. The preliminary results from the investigations of spin correlations of ΛΛ pairs are presented, and the dimension of the region of ΛΛ generation is estimated. The results are in good agreement with the world data and the result obtained using correlation function analysis.     

Magnetic behaviour of alloys in the series (Fe1−xCox)2P

Magnetization and Mössbauer spectroscopic measurements are reported on five alloys in the series (Fe_1−xCo_x)₂P (corresponding to x=0.03, 0.10, 0.30, 0.50 and 0.70). All the five alloys are ferromagnetically ordered. With increasing cobalt, the Curie temperature initially increases becoming maximum for x=0.30 at 480 K and then drops. In going from x=0.10 to 0.30, when the crystalline symmetry changes from Fe₂P-like hexagonal one to Co₂P-like orthorhombic one, the magnetic moment on the pyramidal metallic site tilts off the V_zz-axis. In all the compositions, substitution of cobalt reduces the internal magnetic field at the tetrahedral site whereas that at the pyramidal site shows little change. The system with x=0.03 exhibits temporal relaxation of magnetization over a long period of time. Mössbauer spectra of this alloy exhibit a reduction in linewidth with increasing temperature from 80 to 125 to 160 K. These observations are indicative of the existence of magnetic clusters.