The optical properties of virtual bound state alloys

Optical data on dilute CuNi, AuNi and AgPd alloys are examined in terms of the Anderson model. The results for AuNi and AgPd appear to be consistent with the model while those for CuNi are not. A consistent interpretation of the data is obtained when the effects of the s-d hybridization of the copper conduction states on the Anderson mixing matrix element, V_kd, are considered. The results suggest a rapid increase in V_kd as the energy ?_k approaches the copper d bands.     

Specific heat jump for superconducting virtual bound state alloys

Using Anderson model in the non-magnetic limit, we have calculated the jump in specific heat at T_c, (ΔC), for superconducting virtual bound state alloys. It is found that the normalized (ΔC) vs normalized T_c curves deviate considerably from the BCS value. A more significant result of the present study is that the initial slope of such a curve has a maximum value of 3.638. It is the highest value reported in literature for the impurity problem and is much larger than obtained even in a large pair breaking situation like Kondo effect (2.481).     

Superconducting state parameters of binary alloys

The theoretical investigation of the superconducting state parameters (SSP) viz. electron–phonon coupling strength $\lambda$, Coulomb pseudopotential μ^*, transition temperature T_C, isotope effect exponent α and effective interaction strength N_OV of large numbers of binary alloys have been made extensively in the present work using a model potential formalism for the first time with pseudo-atom-alloy (PAA) model. A considerable influence of various exchange and correlation functions on $\lambda$ and μ^* is found from the present study. The present results of the SSP are found in qualitative agreement with the available experimental data wherever exist.     

X-ray photoelectron spectroscopy of core states in some magnetic virtual bound state alloys

The X-ray photoelectron spectra of the 2p levels of Mn in AgMn 20 at%, Fe in AuFe 20 at%, RhFe 25 at% and 20 at% and Co in RhCo 20 at% have been measured. The Mn and Fe spectra show features which are attributed to excitation of electrons from the 3d to higher energy states during photoemission, while the Co spectra do not.     

Superconducting state parameters of indium-based binary alloys

Our well-recognized pseudopotential is used to investigate the superconducting state parameters viz: electron-phonon coupling strength λ. Coulomb pseudopotential μ*, transition temperature T _c, isotope effective exponent α and interaction strength N ₀ V for the In_1−x Zn _x and In_1−x Sn _x binary alloys. We have incorporated six different types of local field correction functions, proposed by Hartree, Taylor, Vashistha-Singwi, Ichimaru-Utsumi, Farid et al and Sarkar et al to show the effect of exchange and correlation on the aforesaid properties. Very strong influence of the various exchange and correlation functions is concluded from the present study. The comparison with other such theoretical values is encouraging, which confirms the applicability of our model potential in explaining the superconducting state parameters of binary mixture.     

Superconducting state in strong coupling

We consider the behavior of the critical temperature in the limit of strong electron-phonon coupling. The peculiar behavior of the order parameter in this limit allows one to evaluate T_c analytically and directly from Eliashberg's equation. We also consider the effect of Coulomb interaction and the contribution of low phonon modes, the role of lattice instabilities and the relative contribution of phonon and non-phonon interactions.     

Superconducting intermediate state in cadmium

Perpendicular magnetic critical fields for cadmium foils were measured to thicknesses below the temperature dependent coherence length. The presence of supercooling indicates first-order thermodynamic transitions for all samples. A modification of Davies' model leads to a good fit of the experimental data.     

Superconducting properties of ion implantedIn Mn alloys

Annealed Indium films have been implanted with 150 keV-Mn ions at low temperatures. TheT _c -depression and the low temperature part of the resistivity exhibit the Kondo effect, the Kondo temperature being strongly influenced by lattice defects. The lattice disorder has been increased by pre-implantation of In ions. The influence of lattice disorder on the magnetic properties ofIn Mn alloys is explained by applying the strong coupling theory of superconductors.It is found that the main effect results from a shift of the Fermi energy relative to the virtual bound state, due to a variation of the effective massm ₀·(1+).     

Superconducting properties of amorphous transition metal alloys

We have co-sputtered amorphous films of several Mo and W-based superconducting alloys. Measurements of T_c, ($\text{d}\text{H}{}_{\mathrm{c2}}\text{d}\text{T}$)_Tc and the normal state resistivity near T_c were made on a number of these alloys. Our results and other data from the literature are studied to examine the correlation between T_c and the dressed density of states at the Fermi level.     

Superconducting and normal state properties of dilute alloys of LaSn3 containing Ce impurities

The dependence of the superconducting transition temperature T_c on Ce impurity concentration and the specific heat jump at T_c as a function of T_c are reported for the system ($\text{La}$Ce)Sn₃. The experimental results are analyzed using a theory due to Kaiser concerning the effect on superconductivity of nonmagnetic localized resonant impurity states This analysis yields values for the intraatomic Coulomb repulsion parameter U_eff and the Ce local density of states at the Fermi level N_? (E_F). The results of low temperature normal state heat capacity and magnetic susceptibility measurements which give independent estimates of N_? (E_F) are also reported. A large pressure dependence of the T_c of ($\text{La}$Ce)Sn₃ alloys was observed for pressures up to 20 kbar. This behavior is similar to that previously observed in several other superconducting matrix-Ce impurity systems in which the Ce solute 4f electron shell undergoes a continuous-pressure induced demagnetization.     

The positron-helium bound state

A variational calculation with a trial wave function including up to 17 parameters has been used to investigate the existence of a positron-helium bound state. The resulting binding energy was found to be greater than the helium binding energy, and thus it is believed that the positron does not get bound to a helium atom. The calculations provide also values of ground state helium energy and the corresponding wave functions.     

Changes of the lattice parameter in Al 3d alloys due to the virtual bound state

The measurements of the lattice parameter of $\text{Al}$Sc alloys indicate that the change in the lattice parameters of $\text{Al}$ 3d alloys is influenced both by the density of states and the atomic volume effects. Using an interpolation method we subtract from our experimental data the effect of the atomic volume and compare the remaining change in the lattice parameter, with the virtual bound state theory. Good agreement between the theory and experiment is found.     

Virtual bound states in AuTi alloys

We have measured the optical absorption of dilute AuTi alloys for photon energies from 0.1 to 4.5 eV. The extra absorption we observe below the Au interband threshold can be accounted for if the Ti impurities form virtual bound states with half-width at half height δ ～ 0.6 ± 0.2 eV centered E_d ～ 0.5 ± 0.2 eV above the Fermi level and containing 2.7 ± 0.3 electrons.     

Superconducting state parameters of binary metallic glasses

Ashcroft’s empty core (EMC) model potential is used to study the superconducting state parameters (SSPs) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C , isotope effect exponent αand effective interaction strength N _O V of some binary metallic glasses based on the superconducting (S), conditional superconducting (S’) and non-superconducting (NS) elements of the periodic table. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used for the first time with EMC potential in the present investigation to study the screening influence on the aforesaid properties. The T _C obtained from the H-local field correction function are in excellent agreement with available theoretical or experimental data. In the present computation, the use of the pseudo-alloy-atom model (PAA) was proposed and found successful. Present work results are in qualitative agreement with such earlier reported experimental values which confirm the superconducting phase in all metallic glasses. A strong dependency of the SSPs of the metallic glasses on the valence ‘Z’ is identified.      

Second bound state of PsH

The existence of a second bound state of PsH that is electronically stable and also stable against positron annihilation by the normal 2gamma and 3gamma processes is demonstrated by explicit calculation. The state can be found in the ;{2,4}S;{o} symmetries with the two electrons in a spin-triplet state. The binding energy against dissociation into the H(2p)+Ps(2p) channel was 7.03 x 10;(-4) hartree. The dominant decay mode of the states will be radiative decay into a configuration that autoionizes or undergoes positron annihilation. The NaPs system of the same symmetry is also electronically stable with a binding energy of 1.514 x 10;(-3) hartree with respect to the Na(3p)+Ps(2p) channel.     

Chevrel phases: Superconducting and normal state properties

The many unusual properties of the ternary molybdenum chalcogenides of the type MMo₆X₈ (M=Metal, X=Chalcogen) are reviewed. Special emphasis is put on the superconducting properties like critical temperature, critical field and the problem of coexistence of magnetism and superconductivity.