The phase diagram of the superconducting state of superconductor-band-antiferromagnet superlattices

The formation of the superconducting phase in short-period proximity-effect layered superlattices of the superconductor-band-antiferromagnetic-metal (SC/AF) type is studied. The exact solution of the Usadel equations is used to discuss the possibility of formation in such structures of a ground state in which the order parameters of the adjacent superconducting layers have opposite signs (the “π-phase”). The dependence of the superconducting transition temperature and the upper critical field normal to the layers on the lattice period, the intensity of magnetic interaction in the antiferromagnetic layer, and the state of the interface between the layers is examined. It is found that there exists a nonlinear dependence of the conditions for the appearance of the superconducting state in a layered SC/AF system on the system’s parameters. Finally, the conditions for the appearance of the superconducting phase in proximity-effect superlattices consisting of a superconductor with nonmagnetic, ferromagnetic, and antiferromagnetic metals are compared. Zh. éksp. Teor. Fiz. 111, 547–561 (February 1997)     

Interplay of superconductivity and magnetism in presence of inter sub-lattice effect in cuprates

In the present communication, we report a model Hamiltonian to study the interplay between the two long range orders of anti-ferromagnetism (AFM) and superconductivity (SC) in cuprate superconductors in presence of the intersite pairing effect. The BCS type but non-phonon pairing mechanism is considered among the electrons of two equivalent ‘Cu’ sites. The pairing among the electrons of two nearest neighbour non-equivalent ‘Cu’ sites is included in the Hamiltonian and its effect on the interplay of SC and AFM is investigated. The Hamiltonian is solved by the Green’s function method and the corresponding gap equations are calculated and solved selfconsistently. The influence of model parameters like AFM coupling (λ), SC couling (λ ₁) and the coupling (λ ₂) for intersite superconducting interactions on the gaps (SC and AFM) are studied numerically and the results are reported.     

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi_3.9Pb_0.1Sr₃Ca₃Cu₄O _x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E _g) has also been made from the linear fit ofα ² vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.     

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.      

X-ray microanalysis of quaternary semiconductor solid solutions and its application to the (SnTe-SnSe):In system

A reliable technique of local chemical characterization of multicomponent semiconductor solid solutions has been developed, and the possibility of its application to the SnTe-SnSe quaternary solid solutions doped with 16 at.% In verified. The behavior of the electrical resistivity of samples of these solid solutions at low temperatures, 0.4–4.2 K, has been studied. The critical temperature T _c and the second critical magnetic field H _c2 of the superconducting transition and their dependences on the solid-solution composition have been determined. The superconducting transition at T _c≈2–3 K is due to hole filling of the In-impurity resonance states, and the observed variation of the superconducting transition parameters with increasing Se content in the solid solution is related to the extrema in the valence band and the In band of resonance states shifting with respect to one another. Fiz. Tverd. Tela (St. Petersburg) 41, 612–617 (April 1999)     

Parameter-dependent optical nutation in PbSe/CdSe/ZnS quantum-dot

The eigenenergies and eigenfunctions of the spherical PbSe/CdSe/ZnS quantum-dot with the core-shell-shell structure and the electric dipole moment between the a _1s state (the lowest state of the system) and b _1p state (the lowest p state above the first potential) have been calculated by using the rotational wave approximation and the effective mass approximation. The optical nutation signal induced by the transition between these two energy levels has been calculated based on the optical Bloch equations. In particular, the influence of the core’s radius increasing and the CdSe shell’s thickness increasing has been investigated respectively. It is shown from the numerical calculation results that the optical nutation signals are dependent on the size and the structure of the quantum-dot. Moreover, the quantum size-dependent Rabi frequency has been analyzed.     

Supercooling in a system of two superconductors

Supercooling in the transition of a type I superconductor to the superconducting state in contact with another superconductor whose critical temperature is higher has been measured. Using aluminum as a test material, it has been demonstrated that at temperatures below the critical temperature T _c and magnetic fields below the critical field H _c(T), aluminum remains in a metastable normal state, in spite of its contact with another superconductor. This means that it is not possible to generate a thermodynamic instability in a superconductor’s electronic system through the “proximity effect” with another superconductor whose critical temperature is higher. This experimental observation demonstrates a radical difference between surface superconductivity, which certainly generates instability in normal electronic states, and superconductivity induced by the proximity effect near a junction with another superconductor. Zh. éksp. Teor. Fiz. 112, 1119–1131 (September 1997)     

Insulator-metal transition in the single-crystal high-T c superconductor Bi-2201

Variations in the temperature behavior of resistivity, ρ(T), in the ab plane of the anisotropic single-crystal high-T _c superconductor BiSrCuO (2201 phase) have been observed at the insulator-metal (IM) transition. At low temperatures, as one approaches the transition, the Mott relation for two dimensions, ln ρ ∝ T ^−1/3, changes to ln ρ ∝ T ^−1/2, which corresponds to hopping conduction with a Coulomb gap in the density of states. Negative temperature slopes were revealed in the samples near the transition. Estimates suggest that superconductivity in these samples sets in from the Anderson insulator state. The behavior of the width of the superconducting transition and of the temperature of its onset, T _con, at the IM transition has been studied from measurements of the ac magnetic susceptibility. It is shown that in the vicinity of the IM transition the superconducting transition becomes broader, and the onset of the transition T _con shifts toward higher temperatures. This behavior is attributed to nonuniform superconductivity resulting from formation in the crystal of superconducting droplets with different values of T _c , which is caused by fluctuations in the local density of states due to the inherent disorder in the crystal. In these conditions, superconductivity has a percolation character. Fiz. Tverd. Tela (St. Petersburg) 40, 1190–1194 (July 1998)     

Superconductivity in the Ti-D system under pressure

The pressure dependence of the superconducting transition temperature in TiD_0.74 has been measured up to 30 GPa in a diamond high-pressure chamber. It is found that the deuteride TiD_0.74 becomes a superconductor at pressures corresponding to the transition to the high-pressure ζ phase, with a transition temperature that increases from 4.17 to 4.43 K in the interval P=14–30 GPa. The value extrapolated to atmospheric pressure T _c (0)=4.0 K is significantly lower than the superconducting transition temperature (T _c =5.0 K) measured earlier in the metastable state obtained by quenching TiD_0.74 under pressure. It is assumed that the significant difference of the extrapolated value from the superconducting transition temperature in the metastable state after quenching under pressure is caused by a phase transition on the path from the stability region of the ζ phase under pressure to the region of the metastable state at atmospheric pressure. Fiz. Tverd. Tela (St. Petersburg) 40, 2153–2155 (December 1998)     

Transport properties of NdBa2Cu3O6+x ceramics at the edge of the superconducting region upon a decrease in carrier density as a result of oxygen disordering in the Cu-O x planes

An initially nonsuperconducting ceramic sample with the composition NdBa₂Cu₃O_6+x is brought, by means of pressure and quenching, to a state with a high carrier density and a superconducting transition, after which it is returned to the initial state by gradual annealing in several steps. The evolution of the magnetoresistance of the sample showed that even in the most resistive state realized in the experiment the superconducting interaction influences the resistance of the sample at fields all the way up to 5–6 T. In an 8 T field the change in resistance in this state in the temperature interval from 0.4 K to 20 K is described well by a logarithmic law ΔR∝ logT. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 475–480 (25 March 1997)     

Superconducting state parameters of CuCZr100-C metallic glasses

The theoretical investigation of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _c, isotope effect exponent α and effective interaction strength N ₀ V of ten Cu _C Zr_100−C metallic glasses have been reported using Ashcroft’s empty core (EMC) model potential. Three local field correction functions proposed by Hartree (H), Taylor (T) and Ichimaru-Utsumi (IU) are used in the current investigation to study the screening influence on the aforesaid properties. It is observed that the electron-phonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N ₀ V show weak dependences on local field correction functions. The T _c obtained from IU-local field correction function are found an excellent agreement with available theoretical or experimental data. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirms the superconducting phase in metallic glasses.      

Pressure dependence of the superconducting state parameters of metallic glass superconductor Mg70Zn30

Explicit expressions have been derived for the volume dependence of electron-phonon coupling strength (λ) and the Coulomb pseudopotential (μ^*) considering the variation of Fermi momentum (κ _F) and Debye temperature (θ _D) with volume. Ashcroft’s model pseudopotential and RPA form of dielectric screening have been used for obtaining pressure dependence of transition temperature (T _C) and the logarithmic volume derivative (Φ) of the effective interaction strength (N ₀ V) for metallic glass superconductor Mg₇₀Zn₃₀. It has been observed that T _C of the metallic glass Mg₇₀Zn₃₀ decreases rapidly with increase of pressure and the superconducting phase disappears at about 30% decrease of volume, for which the μ^* curve shows a minimum and an elbow is formed in the Φ graph.     

DSC and elastic moduli studies on tellurite-vanadate glasses containing antimony oxide

xSb₂O₃-40TeO₂-(60 − x) V₂O₅ glasses with 0 ≤ x ≤ 10 (in mol%) have been prepared by rapid- melt quenching method. DSC curves of these ternary glasses have been investigated. The glass transition properties that have been measured and reported in this paper, include the glass transition temperature (T _g ), glass transition width (ΔT _g ), heat capacity change at glass transition (ΔC _P ) and fragility (F). Thermal stability, Poisson’s ratio, fragility and glass forming tendency of these glasses have been estimated, to determine relationship between chemical composition and the thermal stability or to interpret the structure of glass. In addition, Makishima and Makenzie’s theory was applied for determination of Young’s modulus, bulk modulus and shear modulus, indicating a strong relation between elastic properties and structure of glass. Generally, results of this work show that glass with x = 0 has the highest shear, bulk and Young’s moduli which make it as suitable candidate for the manufacture of strong glass fibers in technological applications; but it should be mentioned that glass with x = 8 has higher handling temperature and super resistance against thermal attack.     

Concentration dependences of the elastic constants of the two-dimensional graphene-silicene system

The concentration dependences of the elastic constants of the two-dimensional Si _x C_{1 − x} system have been investigated with the use of the Harrison bonding-orbital method and the Keating model. The central and non-central force constants and the Grüneisen parameter have been considered by means of the bonding-orbital method. All quantities under consideration have been shown to exhibit a nonlinear behavior during the transition from graphene to silicene. A nontrivial role of the short-range repulsion has been discussed. The second-order and third-order elastic constants, the pressure dependences of the second-order elastic constants, as well as the Poisson’s ratio and Young’s modulus have been investigated in the Keating model. It has been found that the elastic constants and Young’s modulus change almost linearly upon the transition from graphene to silicene, whereas the other quantities under consideration exhibit nonlinearity.     

Anomalous behavior of the structural parameters of YBa2Cu4O8 single crystals at the transition to the superconducting state

High-precision x-ray crystallographic studies of YBa₂Cu₄O₈ single crystals (T _c =70 K) are performed at eight temperatures in the interval 20–295 K. It is found that a number of structural parameters exhibit anomalous behavior near the superconducting transition of the crystal. A characteristic effect near the phase transition is the displacement of the O1 bridge atom that joins the Cu1 atom of the cuprate chain to the Cu2 atom of the cuprate plane. The shift of this oxygen toward the Cu2 atom is indicative of a change in the Cu2-O1 chemical bond and of charge transfer to the cuprate plane in the process of the transition of the crystal to the superconducting state. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 502–506 (10 October 1997)     

Disordering effects in superconductors with anisotropic pairing: From Cooper pairs to compact bosons

In the weak-coupling BCS-theory approximation, normal impurities do not influence the superconducting transition temperature T _c in the case of isotropic s pairing. In the case of d pairing they result in a rapid destruction of the superconducting state. This is at variance with many experiments on the disordering of high-T _c superconductors, assuming that d pairing is realized in them. As the interelectronic attraction in a Cooper pair increases, the system transforms continuously from a BCS-type superconductor with “loose” pairs to a picture of superconductivity of “compact,” strongly coupled bosons. Near such a transition substantial deviations can be expected from the universal disorder dependence of T _c , as determined by the Abrikosov-Gor’kov equation, and T _c becomes more stable against disordering. Since high-T _c super-conducting systems fall into the transitional region from BCS-type pairs to compact bosons, these results can explain their relative stability against disordering. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 258–262 (10 February 1997)     

Cu-NQR study of magnetism and superconductivity in Ce1+xCu2+ySi2 under pressure

In order to investigate magnetic properties in the vicinity of the magnetic to superconducting (SC) phase boundary, we have performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce_1+xCu_2+ySi₂ polycrystals: Ce_0.99Cu_2.02Si₂ (denoted as Ce 0.99) exhibits the anomalous ground state so-called A phase dominated by critical magnetic correlations and Ce_0.975Cu₂Si₂ (Ce 0.975) shows a static magnetic ordering at ambient P. With increasing P which enhances the Kondo temperature, the fraction of the A phase becomes markedly suppressed, and the SC fraction increases correspondingly, although T_c stays nearly constant in the measured pressure range, P≤14.4 kbar. This shows that the P-induced transition from the A to the SC phases is of a first-order type, without passing through a quantum (T→0) phase transition. However, no superconductivity has been attained in Ce 0.975 up to 19.4 kbar. It is considered that the P-induced superconductivity is sensitive to the sample quality. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistive transitions in Nb/Cu0.41Ni0.59/Nb trilayers

The superconducting phase transition in Nb/Cu_0.41Ni_0.59/Nb trilayers, with superconducting (S) Nb and ferromagnetic (F) Cu_0.41Ni_0.59, has been experimentally studied as a function of the F-layer thickness by measuring the temperature dependence of the electrical resistance R(T). It is shown that the shape and the width of the R(T) curves depends on the Cu_0.41Ni_0.59 thickness, in particular in the regime where π is the coupling between the S layers, which can be expected. To explain the data, we developed a qualitative model which makes the interconnection between the superconducting phase transition and the 0 to π transition in SFS structures are more evident. The text was submitted by the authors in English.     

Antiferromagnetism and superconductivity in a model with extended pairing interactions

The competition between antiferromagnetism and the d + id superconducting state is studied in a model with near and next near neighbour interactions in the absence of any on-site repulsion. A mean field study shows that it is possible to have simultaneous occurrence of an antiferromagnetic and a singlet d + id superconducting state in this model. In addition, such a coexistence generates a triplet d + id superconducting order parameter with centre of mass momentum Q = (π,π) dynamically having the same orbital symmetry as the singlet superconductor. Inclusion of next nearest neighbour hopping in the band stabilises the d_xy superconducting state away from half filling, the topology of the phase diagram, though, remains similar to the near neighbour model. In view of the very recent observation of a broad region of coexistence of antiferromagnetic and unconventional superconducting states in organic superconductors, the possibility of observation of the triplet state has been outlined. Received 30 November 2000 and Received in final form 27 March 2001     

Zero-resistance state and specific-heat anomaly at 110 K in the Y1.2Ba0.8CuO4−y superconductor

Summary The transition to the zero-resistance state at 110 K has been observed in multiphasic samples of Y_1.2Ba_0.8CuO_4−y nominal composition. The intrinsic nature of this high-temperature superconducting state is confirmed by the jump atT _c in the specific-heat curve.