Effect of Biexciton on Cooper Pairing in High Tc Superconductors

In this paper, we have modified the Cooper theory by considering a biexcitonic attractive interaction for high T _c superconductors. We have developed an expression for (2/K _B T _c) as a function of phonon and biexciton mediated coupling constants. For the sake of simplicity, we use a two-square-well potential model (electron–phonon and electron–biexciton). The calculated values of (2/K _B T _c) show good agreement with experimental values.     

Superconductivity in β-Tin Germanium

The superconducting properties of the β-tin germanium under pressure have been investigated by the first-principles calculations based on density functional perturbation theory. The electron–phonon coupling strength as a function of pressure was calculated, and it deceases with the application of pressure. Using the calculated electron–phonon coupling strength, the superconducting transition temperature T _c has been estimated. Assuming a nominal Coulomb repulsion parameter of 0.1, the calculated T _c is in excellent agreement with experimental data. The T _c correlates with electron–phonon coupling strength, indicating phonon-mediated superconductivity in β-tin Ge phase. Our findings have great implications to other Group IVa elements.     

Phonons and Electron–Phonon Coupling in Nickel Borocarbides

We present first principles calculations of the bandstructure, the phonons and the electron–phonon interaction in superconducting YNi₂B₂C (T _c?=?15.5 K) within the framework of the local density approximation (LDA). The results are compared with those of nonsuperconducting reference systems LaNi₂B₂C and Y(Ni_.75Co_.25)₂B₂C.     

Existence of Pseudogap at the Transition Temperature of an Electron-Doped Copper–Oxide Superconductor

We present the scanning tunnelling spectroscopy (STS) studies of the energy gap (Δ) at the Fermi surface of electron-doped high-temperature (high-T _c ) cuprate superconductor Pr_0.88 LaCe _0.12 CuO _4?δ , which is commonly known as PLCCO. We found that the energy gap does not vanish at the transition temperature (T _c ). Moreover, it was observed that gap-like features are present at some locations of the sample at temperatures above T _c . The energy gap with magnitude is found consistent with d-wave symmetry. We also found the value of the ratio 2Δ/K _B T _c higher than 7, which puts electron-doped cuprates in strong coupling regime.     

Metal-insulator transition in graphite: A comparison to heterostructures with high carrier mobility

Conditions for a transition from the insulator (I) to metal (M) state in the electron system of highly oriented pyrolytic graphite (HOPG) have been studied by means of magnetotransport measurements in a broad temperature range (0.3–150 K). In magnetic fields below a certain critical value (B < B _c ≈ 0.05 T), HOPG exhibits the classical magnetoresistance, while for B > B _c, the temperature dependence of the resistance is determined by the state (insulator versus metal) of the electron system. The M-I transition in HOPG, by analogy with that in heterostructures with two-dimensional electron gas, obeys the power law T _c ∞ (B ? B _c) ^k (k = 0.25) and is related to the spin-orbit interaction of electron waves.     

Eliashberg Spectral Functions for Some High-Tc Superconductors

We used a simplified but more realistic form of a model spectral functions for some high-T _c superconductors (e.g., LSCO, YBCO, BSCCO), avoiding the elaborate process of solving the Eliashberg equations numerically. The resulting spectral functions, constructed using the phonon density of states (PDOS), obtained from INS experiments with the easy use of available expressions, reproduce the observed critical temperature, gap ratio, and several other parameters. T _c is found to have a stronger increase with the electron–phonon coupling constant λ compared to those obtained from several equations, constructed from numerical solution of the Eliashberg equations for lower λ values. The obtained functions show widely spreading spectra that inhibit lattice instability by yielding not too large values of λ (< 2.95).     

Influence of Electron–Phonon Interaction on Superconducting State Parameters of MgB2

A three-square well model is employed for the three interactions namely, electron–acoustic phonon, electron–optical phonon, and Coulomb in the calculation of superconducting transition temperature (T _c) for layered structure MgB₂. The analytical solutions for the energy gap equation allow us to understand the relative interplay of these interactions. The values of the coupling strength and of the Coulomb interaction parameter indicate that the test material is in the intermediate coupling regime. The superconducting transition temperature of MgB₂ is estimated as 41 K for λ_ac ≈ 0.3, λ_op ≈ 0.1, and μ* ≈ 0.07. We suggest from these results that both the acoustic and optical phonons within the framework of a three-square well scheme consistently explains the effective electron–electron interaction leading to superconductivity in layered structure MgB₂.     

Generalized Multiple Gap Model for the Superconductivity in MgB2

The free energy of the two-band superconductor is obtained taking into account all actual electron–phonon and Coulomb interactions. The temperature dependences of the superconducting gaps, the specific heat with its jump at T = T _c (T _c is the critical temperature) are calculated minimizing the free energy. The theory is applied to the MgB₂ superconductor.     

Effect of negative pressure on superconducting transition temperature of MgB2

Atomistic simulation has been performed with interatomic potentials to investigate the effect of negative hydrostatic pressure on the superconducting transition temperature (T_c) of MgB₂. The calculation reveals that T_c can be greatly enhanced by applying negative pressure and it can reach up to 52.2 K at ?13 GPa, about 13 K higher than that at ambient pressure. The mechanism for the enhancement of T_c is attributed to that negative pressure reduces high-frequency phonon vibrations of B and thus dramatically enhances the electron–phonon coupling. Our result seems to open up a possible way for the enhancement of T_c in MgB₂.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, ΔT _c =0.3 K). A sharp (FWHM 7.2 cm^?1 at 70 K and 10.0 cm^?1 at 110 K) 340 cm^?1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^?1 in theB _1g polarization extends down to 250 cm^?1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^?1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^?1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^?1 mode anomalies have been explained by the electronic spectral function renormalization.     

Superconducting wires of pbmo6s8

Using a gas diffusion technique, we have produced PbMo₆S₈ in the form of wires. The critical temperature of these wires is strongly dependent on the reaction conditions. Optimal conditions give a T_c of 14.4 K (ΔT_c ≈ 0.25 K). The lower limit for the critical current density at 4.2 K is 4.10⁴ A cm^?2 in a field of 4.2 T.     

Investigation of Size and Shape Dependencies of Phase Diagrams of the Ising Nanofilms and Nanotubes on the Honeycomb Lattice Using Cellular Automata Simulation Approach

Size and shape dependencies of phase diagrams of the Ising nanofilms and nanotubes on the honeycomb lattice are investigated by means of probabilistic cellular automata simulation based on Glauber algorithm. The values of reduced critical temperature, K _c = k _B T _c/J (where k _B and J are the Boltzmann constant and nearest-neighbor coupling, respectively), for both nanofilms and nanotubes, are obtained at the different sizes of the lattices and the dependency of K _c to the number of layers is studied. By increasing the number of layers K _c increases but for number of layer more than 8, the critical temperature increases very slowly. We have shown that between two isotropic nanotubes with the same number of spins, the ones with greater diameter (more spins on the edge) have larger critical temperature. For equal size of lattices, the obtained values of K _c for nanotube are greater than the nanofilm, but for large sizes, this difference disappears.     

The Phonon Modes with Strong Electron–Phonon Interactions in p- and n-Type High T c Superconductors

The phonon modes with strong electron–phonon interactions were investigated by two-phonon Raman scattering in p- and n-type high T _c superconductors. In p-type superconductors, the strong electron–phonon interaction mode changes from the breathing mode at (π, π) to the half breathing mode at (π, 0) as carrier density increases across the optimum doping in LSCO or the 60 K phase in YBCO. It is in good accordance with the change of the superconducting coherent peak position in k-space. In n-type superconductors, the strong electron–phonon interaction modes change from (0.4π, 0.4π) to (0.4π, 0) at the insulator–superconductor transition. Electron–phonon interactions play an important role in superconductivity.     

Evidence of Electron–Phonon Interaction in Al-Substituted Mg1−x Al x B2

Phonon density of states in Al substituted Mg_1?x Al _x B₂ has been studied by means of inelastic neutron scattering experiments with pulsed neutron time-of-flight technique. Phonon dynamics have a significant role for realizing superconductivity in the recent discovered superconductor MgB₂. We present a systematic investigation on generalized phonon density of states (PDOS) up to E = 120 meV on Al content of x = 0.1, 0.25, and 0.5. We have clearly observed a large effect on the PDOS by Al substitution; a large energy shift occurs around at 70 meV and 89 meV. We have also observed a visible increase of PDOS at a particular energy range below T _c. Those phonon states are related with E _2g mode to be expected to have a strong coupling with the electron states at the Γ–A region.     

Heat conduction in Ba1−xKxBiO3

We have investigated heat conduction of single crystal Ba_1?xK_xBiO₃ in the temperature range of 2–300 K and in a magnetic field of up to 6 Tesla. Temperature dependence of thermal conductivityκ(T) reveals the participation of both electrons and phonons with their relative contributions that depend critically on the potassium doping concentration. Crystals underdoped with potassium (samples with higherT _c) exhibit a strong suppression ofκ and a glass-like temperature dependence. In contrast, those with a higher potassium content (lowerT _c) show an increase as temperature decreases with a peak near 23 K. Field dependence ofκ(H) is also very sensitive to the level of potassium doping. Crystals exhibiting a large phonon contribution show an initial drop inκ(H) at low fields followed by a minimum and then a slow rise to saturation as the field increases. The initial drop is due to the additional phonon scattering by magnetic vortices as the sample enters a mixed state. The high field behavior ofκ(H), arising from a continuous break-up of Cooper pairs, exhibits scaling which suggests the presence of an unconventional superconducting gap structure in this material.     

Effects of the Substitution of 30 % Pr for La on the Magnetic Properties and Magnetocaloric Effect in LaFe 1 1 . 5 Si 1 . 5 B 0 . 2

Magnetic properties and magnetocaloric effect (MCE) in LaFe_11.5Si_1.5 B _0.2 have been investigated by substituting 30 % Pr for La. It is found that the substitution of 30 % Pr for La leads to a reduction in Curie temperature (T _C) from 200 to 190 K. The characteristic of the itinerant electron metamagnetic (IEM) transition above T _C becomes more prominent with the substitution of 30 % Pr for La. The maximum magnetic entropy change (?ΔS _M) is remarkably enhanced from 13.0 to 17.9 J/kg·K under a field change of 0–3 T, and correspondingly, the values of refrigerant capacity (RC) for LaFe_11.5Si_1.5 B _0.2 and La_0.7Pr_0.3Fe_11.5Si_1.5 B _0.2 are 167 and 224 J/kg, respectively. The large ?ΔS _M and the high RC in a low magnetic field indicate that La_0.7Pr_0.3Fe_11.5Si_1.5 B _0.2 may be a promising candidate for magnetic refrigeration material near 200 K     

Superconductivity in Doped Systems Beyond Migdal's Theorem

The basic system of equations for a theory of superconductivity in systems with a chaotically distributed paramagnetic impurity of substitution, in which Migdal theorem is violated (it cannot be supposed ω₀ ? E _F), is presented. We take into account electron–phonon and impurity diagrams as well as supplement ones corresponding to intersection of electron–phonon and electron–impurity lines. Decrease of the quantity T _C with increase of impurity concentration is shown to weaken in comparison with the case of usual superconductors. Coefficient of isotope effect α, energy gap, and order parameter at T = 0 are also calculated. Behavior of these quantities as a function of impurity concentration depends on Migdal's parameter m and transferred momentum q _c.