S. Yoksan

Hc2 of anisotropy two-band superconductors by Ginzburg–Landau approach

The purpose of this research is to study the upper critical field (Hc2) of two-band superconductors by two-band Ginzburg - Landau approach. The analytical formula of Hc2 included anisotropy of order parameter and anisotropy of effective-mass are found . The parameters of the upper critical field in ab-plane and c-axis can be found by fitting to the experimental data . Finally, we can find the ratio of upper critical field that temperature dependent in the range of experimental result .

Effect of density of state on isotope effect exponent of two-band superconductors

The exact formula of Tcs equation and the isotope effect exponent of two-band s-wave superconductors in weak-coupling limit are derived by considering the influence of two kinds of density of state : constant and van Hove singularity. The pairing interaction in each band consisted of 2 parts : the electron-phonon interaction and non-electron-phonon interaction are included in our model. We find that the interband interaction of electron-phonon show more effect on isotope exponent than the intraband interaction and the isotope effect exponent with constant density of state can fit to an experimental data,MgB2, and high-Tc superconductors, better than van Hove singularity density of state.

Ratio 2Δ0/kTc in a van Hove superconductor

The influence of logarithmic singularity in the density of states at the Fermi energy on the zero-temperature gap-to-Tc ratio (R) of a high-temperature superconductor is studied within the framework of the BCS theory. An exact gap-ratio integral is derived. The numerical values forR are found to contradict the recent approximate results of Getinoet al. [Phys. Rev. B48, 597 (1993)].

The ratio 2Δ0/Tc in BCS superconductivity

We show numerical calculations for the gap ratioR for a BCS superconductor with a singular density of states of the formA|ɛ|a, whereA andα are model parameters. With proper finite energy cutoffs, we find larger values forR than the recent results of Mattis and Molina.

Thermodynamic properties of a d-wave superconductor near zero temperature

On the basis of Fermi liquid theory, we study the thermodynamic properties near zero temperature of a d-wave superconductor assuming the BCS pairing model. The temperature dependence of the order parameter is derived. We then obtain an analytical expression for the free energy difference between the normal and superconducting states in the vicinity of zero temperature from the coupling constant integral. New expressions for the critical field and the specific heat in the superconducting state are given as functions of temperature. We compare our expressions with previous formulas due to others.

The Gap-to- Tc Ratio of a van Hove Superconductor

We give an analytical expression for the gap-to-Tc ratio (R) of a superconductor with a van Hove singularity in the density of states. Our calculation yields R in very good agreement with the results obtained numerically by S. Ratanaburi et al. [J. Supercond. 9, 485 (1996)].

Gap-to-T c ratio as a function of the Fermi level shift

AbstractWithin the framework of the BCS theory, the gap-to-TC ratioR=2‡0/kTc is evaluated numerically (‡0 is the energy gap atT = 0 andT> c is the critical temperature) for a superconductor with a van Hove singularity (vHs) in the density of states as a function of the shifts (δ) of the Fermi level with respect to the vHs. It is found thatR varies asymmetrically with δ and that the variations are strong near δ = 0. Our numerical calculation shows that the largest R’s occur at certain values of δ⊋0.

Reduced-Gap Ratio of High-Tc Cuprates Within the d-Wave Two-Dimensional Van Hove Scenario

The gap-to-Tc ratio (R) of high-temperature superconductors is calculated in the context of d symmetry of the superconducting order parameter and a two-dimensional Van Hove singularity in the density of states, using the BCS theory for weak coupling. Exact numerical calculation and an analytic formula for R are given. The ratios are found to be substantially larger than the BCS weak coupling limit of 3.53. The overall dependence of R on ωD/Tc, where ωD is the cutoff frequency, is given.

Thermodynamic properties of a BCS superconductor

Abstract On the basis of the Bardeen–Cooper–Schrieffer (BCS) gap equation, we derive complete expressions for the free-energy density, and heat capacity differences between the normal and superconducting states as functions of the superconducting energy gap Δ(T), Debye cut-off energy ωD and temperature T near the critical temperature Tc. We have analyzed these expressions at Tc as well as at the temperature near Tc for s and d types of pairing. The critical field and specific heat versus ωD/Tc curves show that the BCS universal values are recovered in the ωD/Tc→∞ limit. Corrections to BCSs approximate results are given as functions of experimentally measured parameters and temperature.

Effect of orthorhombic distortion and second-nearest neighbor hopping on gap-to-T cratio

Within the framework of the BCS theory, we perform a numerical study of the effect of orthorhombic distortion and second nearest neighbor hopping on the ratioR = 2δ(0)/Tc based on the van Hove singularity scenario. We find that the gap-to-Tc ratio depends on the distortion and hopping parameters, in contradiction to the recent conclusion of Sarkaret al. [Phys. Rev. B51, 12854 (1995)]. In particular, the introduction of orthorhombic distortion increases the value ofR, and with decreasing second neighbor hopping,R decreases.