First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism (original) (raw)
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Physical review letters, 2001
First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).
Pseudopotential approach to superconductivity in MgB 2
2010
Superconductivity in MgB 2 has been re-examined in BCS-Eliashberg framework by employing Mc-Millan's [Phys Rev, 167 (1968) 331] T c -equation and form factors of MgB 2 computed from the form factors of component metals (Model-I). The empty core model pseudopotential due to Ashcroft [Phys Lett, 23 (1966) 48 ] and random phase approximation form of dielectric screening due to Gellmann and Brueckner [Phys Rev, 106 (1958) 364] are used in the present work. An excellent agreement between the present values and other theoretically computed values of T c and with the relevant experimental data for MgB 2 confirms the validity of the present approach. The explicit dependence of λ and T c on the isotopic masses of Mg and B, as revealed from the present work, confirms the role of lattice vibrations in the superconducting behaviour of MgB 2 and the high value of T c in it may be attributed to the phonon mediated e-e interaction coupled with higher values of phonon frequencies due to light m...
Strong electron-phonon coupling in superconducting MgB 2 : A specific heat study
Physical Review B, 2001
We report on measurements of the specific heat of the recently discovered superconductor MgB2 in the temperature range between 3 and 220 K. Based on a modified Debye-Einstein model, we have achieved a rather accurate account of the lattice contribution to the specific heat, which allows us to separate the electronic contribution from the total measured specific heat. From our result for the electronic specific heat, we estimate the electron-phonon coupling constant λ to be of the order of 2, significantly enhanced compared to common weak-coupling values ≤ 0.4. Our data also indicate that the electronic specific heat in the superconducting state of MgB2 can be accounted for by a conventional, s-wave type 74.25.Kc The recent discovery of superconductivity in MgB 2 below T c ≈ 39 K [1] has caused a remarkable excitement in the solid-state physics community. Critical temperatures of this magnitude inevitably raise the question whether mechanisms other than the common electron-phonon interaction are responsible for the transition. In their very recent work, Bud'ko et al.
Superconducting Properties of MgB2 from First Principles
Physical Review Letters, 2005
Solid MgB$_2$ has rather interesting and technologically important properties, such as a very high superconducting transition temperature. Focusing on this compound, we report the first non-trivial application of a novel density-functional-type theory for superconductors, recently proposed by the authors. Without invoking any adjustable parameters, we obtain the transition temperature, the gaps, and the specific heat of MgB$_2$ in very good agreement with experiment. Moreover, our calculations show how the Coulomb interaction acts differently on s and p states, thereby stabilizing the observed superconducting phase.
Superconductivity of Metallic Boron in MgB2
Physical Review Letters, 2001
Boron in MgB2 forms layers of honeycomb lattices with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizeable electron-phonon coupling. Using the rigid atomic sphere approximation and an analogy to Al, we estimate the coupling constant λ to be of order 1. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and 700 cm −1 , this produces a high critical temperature, consistent with recent experiments. Thus MgB2 can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen.
Physica C-superconductivity and Its Applications, 2010
Here, we report the efficient and feasible analytical method for the generalized Bloch–Gruneisen law in association with Debye temperature and various temperatures range in terms of incomplete gamma function. In addition, our results are in agreement with previous reports as shown in this letter. Bloch–Gruneisen function describes the contribution of electron–phonon interaction to the results of temperature dependence behavior of resistivity for integer and noninteger values of index m. In conclusion, the algorithm is constructed in Fortran 90 language for replicate the variation of temperature dependence of resistivity for pristine MgB2 sample. Moreover, the comparison of numerical results with the proposed method reveals the validity and precision of the method.
Two-Band Eliashberg Theory in Doped MgB _{2}$$ : Experimental T c and Superconductive Gaps
Journal of Superconductivity, 2005
The variation of the critical temperature T c and of the superconductive gaps as functions of doping (Al, C) in the diboride MgB 2 has been studied in the framework of the two-band Eliashberg theory and traditional phonon coupling mechanism. We have solved the two-band Eliashberg equations using first-principle calculations or simple assumptions for the variation of the relevant physical quantities. We have found that the experimental T c curves can be exactly explained only if the Coulomb pseudopotential changes with x by tuning the Fermi level toward the σ band edge. We also found that a small amount of impurities changes the structural properties of the material, so we cannot treat the Mg 1−x Al x B 2 and MgB 2−x C x systems as a contamination with Al or C of MgB 2 , but as new materials. Finally, we compare the predictions of our theory with the available experimental data.
Small Fermi energy and phonon anharmonicity in MgB2 and related compounds
Physical Review B, 2002
The remarkable anharmonicity of the E2g phonon in MgB2 has been suggested in literature to play a primary role in its superconducting pairing. We investigate, by means of LDA calculations, the microscopic origin of such an anharmonicity in MgB2, AlB2, and in heavily hole-doped graphite. We find that the anharmonic character of the E2g phonon is essentially driven by the small Fermi energy of the σ holes. We present a simple analytic model which allows us to understand in microscopic terms the role of the small Fermi energy and of the electronic structure. The relation between anharmonicity and nonadiabaticity is pointed out and discussed in relation to various materials.
Small Fermi energy and phonon anharmonicity in MgB_2
APS March Meeting Abstracts, 2002
The remarkable anharmonicity of the E2g phonon in MgB2 has been suggested in literature to play a primary role in its superconducting pairing. We investigate, by means of LDA calculations, the microscopic origin of such an anharmonicity in MgB2, AlB2, and in heavily hole-doped graphite. We find that the anharmonic character of the E2g phonon is essentially driven by the small Fermi energy of the σ holes. We present a simple analytic model which allows us to understand in microscopic terms the role of the small Fermi energy and of the electronic structure. The relation between anharmonicity and nonadiabaticity is pointed out and discussed in relation to various materials.