On a quantum theory of superconducting glasses and other impure superconducting phases

We review and extend a previous electronic mean field theory of superconducting glass phases. These phases are defined by vanishing ensemble averaged BCS-order parameter and non-vanishing Edwards-Anderson type averages of the inhomogeneous superconducting order parameter. Solutions are worked out for the replica symmetric case, but the possibility of replica symmetry breaking and hence ergodicity breaking is also discussed in the field theory. The order parameter for Ising-like, anisotropic and isotropicXY-like superconducting glass phases are identified by their spontaneous symmetry breaking effect in the action of the discorder ensemble. The isotropicXY-like phase is found to allow superconductivity in arbitrary strong magnetic fields. Generally the results show that: the occurrence of superconducting glass phases is supported by strong local attractive electron-electron coupling together with a high probability of nonsuper-conducting areas, the vicinity of a metal-insulator transition or the presence of a magnetic field. We suggest that for strong coupling the theory is applicable to HighT _c superconductors like Ba–La–Cu–O.A first result beyond mean field approximation displays at one loop order quantum fluctuation contributions to the density of states in the superconducting glass phases. We suggest that these phases may show an infinite nonlinear dc conductivity in higher order response.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Electric field gradients in semiconducting alloys

The results of recent perturbed angular correlation (PAC) experiments in semiconducting alloys are discussed. The observed temperature dependence of the electric field gradients (EFG) differs drastically from that of methals. The variation of the conduction electron density with temperature certainly influences the EFG but can not explain in general the data. A model developed for In₂Te₅ based on the change of the local electron density is discussed in detail.     

Electron relaxation mechanisms in n-Bi-Sb semiconducting alloys

The magnetic field dependence of diffusion thermal electromotive force α₂₂(H) (?T ∥ C ₁) in degenerate n-Bi-Sb semiconducting alloys, in which only L electrons participate in transfer phenomena, had a maximum at H ∥ C ₃. The electron relaxation time was determined from the magnetic field value corresponding to this maximum. The dependences of the electron relaxation time on temperature and the concentration of alloy components and the dopant (on the concentration of electrons) were used to separate electron relaxation time components corresponding to scattering by phonons, ionized impurities, and component concentration fluctuations. The latter (“alloy”) mechanism of electron scattering by concentration fluctuations was for the first time considered for Bi-Sb alloys; its contribution was found to be comparable with those of the other scattering mechanisms. The obtained relaxation times were used to calculate theoretical magnetic field dependences of thermal electromotive force and the Nernst-Ettingshausen coefficient. The calculation results were in satisfactory agreement with experiment.     

Thermodynamic properties of ternary semiconducting alloys

Using the new information supplied by extended-x-ray-absorption-fine-structure measurements and the results of our structural model, we compute the bond probability of a few ternary semiconducting III–V and II–VI alloys as a function of temperature and composition in the framework of a modified quasi-chemical approximation. We derive the thermodynamic functions of mixing, considering both elastic and chemical contributions to the bond energies. We examine how the deviation from the full randomness affects the ordering of such alloys and we construct the phase diagrams in the region of phase separation. Possible formation of ordered compounds at low temperatures is predicted.     

On a possible superconducting phase transition in Fe2Te3

A superconducting phase transition is predicted for Fe₂Te₃ on the basis of an empirical determination of the electron-phonon coupling constant λ. This result is shown to be consistent with recent calculations of the superconducting state parameters.     

Appearance of topological phases in superconducting nanocircuits

We construct non-Abelian geometric transformations in superconducting nanocircuits, which resemble in properties the Aharonov-Bohm phase for an electron transported around a magnetic flux line. The effective magnetic fields can be strongly localized, and the path is traversed in the region where the energy separation between the states involved is at maximum, so that the adiabaticity condition is weakened. In particular, we present a scheme of topological charge pumping.     

Localized spin fluctuations in superconducting alloys

The effects due to localized spin fluctuations (LSF) in dilute superconducting alloys are investigated. Expressions for the critical temperature T_c as function of impurity concentration n_I and the specific heat jump at the transition ΔC(T_c) are obtained, both in the weak and strong magnetic limits. The results are discussed in the light of available experimental data.     

CPA treatment of superconducting alloys

A theory for superconducting alloys is formulated on the basis of the Coherent-Potential-Approximation (CPA) to treat the disorder problem. Using a simple model for the density of states, the transition temperature is calculated as a function of the concentration and compared with experiments.     

Direct laser writing of superconducting patterns into semiconducting ceramic Y-Ba-Cu-O

Investigations on the laser-induced oxidation of YBa₂Cu₃O₆ are presented. Here, the oxygen content of the material is locally increased by laser-induced heating under cw Ar⁺ or Kr⁺ laser irradiation in 02 atmosphere. The technique permits direct-writing of superconducting patterns into the semiconducting sample surface.     

Structure property correlations in superconducting Ti-Nb alloys

Titanium-rich transition metal alloys are metastable in their quenched boc β phase. The instability is relieved by low temperature structural transformations. We have investigated this in a series of Ti-Nb alloys, through the measurements of electrical resistivity (ρ), superconducting transition temperature and upper critical field. Supporting structural evidence has been obtained from transmission electron microscopy (tem) and x-ray studies. It is shown that both ρ and dρ/dT can be used as useful indices of this instability. The enhanced value of resistivity on account of the instability results in the enhancement of upper critical field as shown from dH _c2/dT measurements.     

Separation of magnetic phases in alloys

In this paper we present a study of the separation of phases in multi-phase alloys. The proposed technique is based on the hyperbolic model of magnetization. By using this model it is possible to decompose the magnetic phases of alloys and determine their magnetic properties separately. Experimental verification was carried out on a transformer-like setup, constructed from layered samples representing the various magnetic phases. The samples were constructed from elements of strongly different magnetic properties. The results given by the model are in an excellent agreement with the experimental results, giving justification for the proposed method of decomposition. The proposed method is the first step towards the recognition and the separation of magnetic constituencies of different magnetic properties in an alloy by analytical means.     

Insulating,superconducting, and large-compressibility phases in nanotube ropes

The superconducting properties of carbon nanotube ropes are studied using a new computational framework that incorporates the renormalization of intratube interactions and the effect of intertube Coulomb screening. This method allows one to study both the limits of thin and thick ropes ranging from purely one-dimensional physics to the setting of 3D Cooper-pair coherence, providing good estimates of the critical temperature as a function of the rope physical parameters. We discuss the connection of our results with recent experiments.     

Detecting non-Abelian geometric phases with superconducting nanocircuits

We propose a feasible scheme to detect the noncommutative feature of non-Abelian geometric phases (NAGP) with superconducting nanocircuits. The induced NAGP associated with two-fold degenerate states naturally arises in the chosen four-level subsystem interacting with the microwave pulses. We explicitly show the noncommutative effect of the NAGP by considering the difference between the level populations at the end of the composed evolution loops l₂l₁ and the counter-ordered one l₁l₂. The scheme opens the new possibility for detecting the fundamental characteristics of the NAGP with superconducting circuit devices.     

Composition profile in semiconducting alloys by second derivative wavelength modulation

The wavelength (λ) and composition (x) modulations are used simultaneously in order to obtain the second derivative ∂² R/∂λ∂x of the reflectivity. A great sensitivity to a small composition difference is obtained. This method is applied to the determination of the compositional profile for a ternary alloy Mg _x Zn_1−x Te withx<0.01. A good agreement is observed with the theoretical profile calculated on the basis of a regular associated solution model.