Superconducting state parameters of metallic glass Mg70Zn30 using linearized screened pseudopotential

A linearized form for the screened form factors of electron-phonon interaction in the metallic glass Mg₇₀Zn₃₀ is applied for the first time to predict the superconducting state parameters viz. electron-phonon coupling strength (λ), Coulomb pseudopotential (μ*), transition temperature (T _c), isotope effect exponent (α) and the interaction strength (N ₀ V). Computed results agree with the experimental data available in the literature.     

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.      

带间耦合相互作用对MgB_2超导体同位素效应指数的影响

在弱耦合极限下考虑带间相互作用,得到了二带s-波超导体临界温度Tc的公式及同位素效应指数的公式。在此模型中,耦合相互作用在每一带中包括两部分:电子-声子相互作用和非电子-声子相互作用,当Tc≈40K时,在弱耦合机制中得到MgB2的同位素效应指数αB=0.3,并且带间的电子-声子相互作用比非电子-声子作用对同位素效应指数的影响大。     

On the composition-dependent isotope effect of HTSC in the two-band model

The oxygen isotope effect in high-T_C superconductors has been investigated on the basis of a two-band model where superconducting phase transition is induced by interband (mainly Coulombic) interaction. The isotope shift of T_C appears due to the dependence of averaged interband electronphonon coupling constant on oxygen mass. This coupling has a repulsive nature and gives a relatively small contribution to the total interaction inducing superconductivity. The calculated isotope effect exponent depending on the carrier concentration has been compared with the experimental one as a function of x for La_2-xSr_xCuO₄.     

Superconducting state parameters of Cu C Zr100−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.     

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.     

Constraints on the mechanism of superconductivity for MgB2 from Tc and the total isotope effect

We examine the possible electron-phonon spectra that produce both T_c=39 K and an isotope coefficient β=0.32±0.01, with Eliashberg theory. We assess the viability of the conventional electron-phonon mechanism in light of these results, compared with ab initio calculations of the electron-phonon spectrum. Comparisons are made with similar considerations for low T_c materials.     

Superconducting state parameters of La<Subscript>100-<Emphasis Type="Italic">C</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">C</Emphasis></Subscript> binary metallic glasses

The theoretical investigations 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 _O V of six binary La_100-C Ga _C (C = 16, 20, 22, 24, 26 and 28 at. %) metallic glasses have been reported using Ashcroft’s empty core (EMC) model potential for the first time. 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 in the present 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 _O V show weak dependences on the local field correction functions. The T _C obtained from H-local field correction function are found in qualitative agreement with available experimental data and show almost linear nature with the concentration (C) of ‘Ga’ element. A linear T _C equation is proposed by fitting the present outcomes for H-local field correction function, which is in conformity with other results for the experimental data. Also, the present results are found to be in qualitative agreement with other such earlier reported data, which confirms the superconducting phase in the metallic glasses.      

On the role of the Coulomb interaction in the mechanism of d-wave Cooper pairing of charge carriers in high-T c superconductors

The question of the effect of the structure of the anisotropic quasi-two-dimensional electron spectrum of high-T _c superconductors on the character of the screening of the Coulomb interaction and the symmetry of the superconducting order parameter is studied. Calculations of the polarization operator of electrons are performed on the basis of the single-particle band spectrum extracted from angle-resolved photoemission spectroscopy data. It is shown that the static screened Coulomb repulsion has a minimum at small momentum transfers. This corresponds to an effective electron-electron attraction in the -wave channel of Cooper pairing of the charge carriers on account of their interaction with the long-wavelength charge-density fluctuations. This attraction together with the anisotropic electron-phonon interaction increase the critical superconducting transition temperature T _c with increasing hole density and can give quite high values of T _c while at the same time suppressing the isotope effect, in qualitative agreement with the experimental data for underdoped hole-type cuprate metal-oxide compounds. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 703–710 (25 May 1999)     

Magnetic-field-induced displacive phase transition in IV–VI compounds

The present work calculates the structural phase transition temperature (T_c) in a system with interband electron-phonon interaction in a strong magnetic field, assuming the temperature dependence of the soft-mode frequency to be due to anharmonicities. The results of the calculation are compared with the experiment on Pb_1?xGe_xTe.     

Theoretical analysis of the thermal conductivity of high-temperature superconductors

Summary The thermal conductivity of YBa₂Cu₃O_7−δ high-T _c superconductor is analysed self-consistently on both normal and superconducting states on the base of the Bardeen-Rickayzen-Tewordt extended theory to take into account the effects of magnetic field and superconducting fluctuations. It is shown that experimental data are in a quantitative agreement with theory even if the number and variation intervals of adjustable parameters are substantially reduced in comparison with previous works. Phonon relaxation rates due to different mechanisms of phonon scattering as well as the parameters of electron-phonon interaction are estimated. It is shown that thermal conductivity in YBa₂Cu₃O_7−δ is consistent with the BCS model with intermediate electron-phonon coupling λ=1–3 the phonon-electron and electron-phonon relaxation times near critical temperature are evaluated to be 10⁻¹⁰s and 10⁻¹²s, respectively.     

Interband transitions and electronic Raman continuum in systems with strong inelastic scattering: Applications to YBa2Cu3O7-δ

We consider a model for the electronic Raman continuum which takes into account strong inelastic scattering and interband transitions. Calculations are based on four-vertex Raman scattering diagrams (Kawabata formalism) within the RPA for Coulomb interaction and the ladder diagram Bethe-Salpeter equation for the vertex. We apply this method to an analysis of the nature of the electronic Raman continuum in the normal state of the high-T _c superconductor YBa₂Cu₃O₇. In numerical calculations we take into account all the self-energy effects and make simulations for vertex corrections assuming that inelastic scattering is due to electron-phonon interaction. Theab-plane polarized continuum contains a large contribution from interband processes and does not depend strongly on temperature and inelastic scattering strength. The in-plane anisotropy is determined by interband transitions rather than by anisotropy of the Fermi surface. The ZZ continuum contains only weak contribution from interband transitions. It can be crudely described within a single band model with inelastic scattering and is strongly dependent on the relaxation rates of inelastic scattering. The nature of the oxygen-deficiency dependence of the Raman spectra is also commented upon.     

Criterion for superconductivity in highT c oxidic compounds

A criterion for superconductivity in highT _c oxidic compounds is proposed based on a short range cooperative electron-phonon coupling and the Darwin interaction for Cooper pairing. It is shown that the small polaron stabilization energy (ε _p) should be less than about 0.04 eV and the system should have a metal ion for which Darwin interaction energy is large.     

On the Tc of dilute alloys

The equation for the critical temperature T_c, of a dilute superconducting alloy due to Markowitz and Kadanoff (MK) is generalized to include renormalization effects due to the electron-phonon interaction. Such corrections constitute a 50% effect in weak coupling superconductors like aluminum while in strong coupling systems like lead this correction gives a factor of 2.5. The mean square anisotropy parameter appearing in the T_c equation is also generalized to remove the separability assumption of the electron-phonon interaction. Some consequences of these two corrections to the analysis and systematization of data for dilute superconducting alloys is discussed.     

Solutions of Eliashberg equations for an electron-phonon coupling with a cutoff

In this paper we discuss the Eliashberg equations for the case of an electron-phonon coupling with an energy cutoff. This cutoff is imposed either for the energy difference by means of a strip function, or for both energies, with a Cooper-like expression. The strip function cutoff requires explicit calculation of not only the frequency renormalization functionZ but also the energy renormalizationX. The physical origin of such cutoffs might lie in the very strong electron-electron interaction which seems typical for highT _c superconductivity. If such cutoffs are admitted, the hypothesis thatT _c is caused at least in part by a strong electron-phonon interaction can be reconsidered. We find that the combination of strong coupling and low-energy cutoff could produce highT _c with only small isotope effect and with little damping or pulling of the phonon modes. Correlation with other physical properties, such as specific heat, is reexamined in view to estimate the coupling constant . Some objections to the model using strong electron phonon interaction are removed and better agreement with observed properties is obtained     

Anharmonic oxygen vibration and mode softening in Tl2Ba2Ca2Cu3O10 superconductor

The softening of the IR active 300 cm⁻¹ phonon mode in Tl-2223 superconductor, aroundT _c has been explained by considering a sixth order polarization potential at the off-center oxygen ion site in the Tl-O plane, and by using a nonlinear lattice dynamical theory. The present theory explains, more or less satisfactorily, the unusual temperature dependence of oxygen ion vibration and frequency shifts at higher temperature. The existence of strong nonlinear electron-phonon interaction atT _c has been emphasized.     

Giant magneto-optical response of ferromagnetic EuB<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Intercalation of polyatomic molecules into a superconductor can drastically affect the properties of the compound. A mechanism responsible for a large increase in T _c for such systems is proposed. It explains the recent remarkable observation of high T _c superconductivity in the hole-doped C₆₀/CHX₃ (X≡Cl, Br) compounds and the large shift in their T _c upon Cl↦Br substitution. The increase in T _c is due to contribution to the pairing arising from the interaction of electrons with the vibrational manifold of the molecule. The proposed mechanism opens up the possibility to observe a site-selective isotope effect. We also suggest that intercalating CHI₃ would further increase the critical temperature to T _c≃ 140 K. Received 23 January 2002     

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.      

Study of superconducting state parameters of amorphous metals by a pseudopotential theory

The theoretical computation 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 _O V of some monovalent (Cu and Au), divalent (Ca, Sr, Ba, αHg, βHg and Ra) and polyvalent (Lu, Rh, Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Ac, Th, Hf, Ru, Os, Ir, V, Ta, Pa, Cr, Mo, U, Re, Np and Pu) amorphous metals based on the different groups of the periodic table have been carried out for the first time using the well known Ashcroft’s empty core (EMC) model pseudopotential. Herein, we have employed five different types of local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) to study the exchange and correlation effects on the present investigations. A very strong influence of all the exchange and correlation functions have been observed in the present study. Our results are in fair agreement with documented theoretical as well as experimental data. A strong dependency of the SSP of amorphous metals on the valency Z was found.      

Calculations of the superconducting isotope effect

The strong-coupling theory of superconductivity is used, without simplification, to compute the isotope effect exponent β in T_c ∝ M^?β for several metals. A comparison is made with the values obtained using two approximate equations for T_c.