Superconducting properties of MgB2 from first principles (original) (raw)
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Multi-gap superconductivity in MgB2: Magneto-Raman spectroscopy
Physica C: Superconductivity, 2007
Electronic Raman scattering studies on MgB2 single crystals as a function of excitation and polarization have revealed three distinct superconducting features: a clean gap below 37 cm −1 and two coherence peaks at 109 cm −1 and 78 cm −1 which we identify as the superconducting gaps in π-and σ-bands and as the Leggett's collective mode arising from the fluctuation in the relative phase between two superconducting condensates residing on corresponding bands. The temperature and field dependencies of the superconducting features have been established. A phononic Raman scattering study of the E2g boron stretching mode anharmonicity and of superconductivity induced self-energy effects is presented. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ∼ 2.5% hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.
Electronic structure and superconductivity of MgB2
Bulletin of Materials Science, 2003
We have calculated the electronic structure of Eu for the bcc, hcp, and fcc crystal structures for volumes near equilibrium up to a calculated 90 GPa pressure using the augmented-plane wave method in the local-density approximation. The frozen-core approximation was used with a semi-empirical shift of the f-states energies in the radial Schrödinger equation to move the occupied 4f valence states below the Γ 1 energy and into the core. This shift of the highly localized f-states yields the correct europium phase ordering with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the bcc and hcp structures are also found to agree with and follow a T c trend similar to recent measurement by Debessai et al. 1
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...
Physical Review B, 2002
We report on a detailed first-principles local-density-functional investigation of the structural, electronic and dynamical properties of a MgB 2 free surface in both unreconstructed ideal terminations and in the case of Na coverage. Our study shows that in the case of a B-terminated surface the density of states at the Fermi level is significantly larger than in the bulk, a behavior that might lead to an enhancement of superconducting properties. We find the Mg-terminated surface to be the most stable structure in the whole range admitted by the chemical potentials, in agreement with very recent experimental results. The Na-covered surface is just slightly less stable than the Mg-terminated one in B-rich conditions and cannot be ruled out as a potentially relevant metastable structure. We rationalize the thermodynamical trends and the dynamical properties of the surfaces considered in terms of the B-state filling; in particular, we find that the larger the filling the more stable the surface and the higher the frequency of the vibrational modes localized at the crystal surface.
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.
Strong coupling and double-gap density of states in superconducting MgB 2
Europhysics Letters (EPL), 2002
Using scanning tunneling spectroscopy at T = 4.2 K, we perform simultaneously the topographic imaging and the quasiparticle density of states (DOS) mapping in granular MgB2. We observe a new type of spectrum, showing a pronounced double gap, with the magnitudes of ∆S = 3.9 meV and ∆L = 7.5 meV, i.e. well below and well above the BCS limit. The largest gap value gives the ratio 2∆L/kBTc = 4.5, which implies strong electron-phonon coupling. Other superconducting regions are found to have a characteristic BCS-shaped DOS. However, the variation of the spectral shape and lower gap widths, from 2.0 meV to 6.5 meV, indicate the importance of surface inhomogeneity and proximity effects in previously published tunneling data. Our finding gives no evidence for any important gap anisotropy. Instead, it strongly supports the multiple gap scenario in MgB2 in the clean limit, and the single gap scenario in the dirty limit.
Density functional theory for superconductors: applications to MgB 2 and solids under pressure
To extend the applicability of density functional theory for superconductors (SCDFT) to systems with significant particle-hole asymmetry, we construct a new exchange-correlation kernel entering the gap equation. We show that the kernel is numerically stable and does not diverge even in the low temperature limit. Solving the gap equation for model systems with the present kernel analytically and numerically, we find that the asymmetric component of electronic density of states, which has not been considered with the previous kernel, systematically decreases transition temperature (Tc). We present a case where the decrease of Tc amounts to several tens of percent.
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.