Superconductivity of boron-doped diamond, reported recently at Tc=4 K, is investigated exploiting its electronic and vibrational analogies to MgB2. The deformation potential of the hole states arising from the C-C bond stretch mode is 60% larger than the corresponding quantity in MgB2 that drives its high Tc, leading to very large electron-phonon matrix elements. The calculated coupling strength λ ≈ 0.5 leads to Tc in the 5-10 K range and makes phonon coupling the likely mechanism. Higher doping should increase Tc somewhat, but effects of three dimensionality primarily on the density of states keep doped diamond from having a Tc closer to that of MgB2.PACS numbers: 71.20.Be, 71.20.Eh, 71.27.+a Discovery of new types of superconducting materials has accelerated since the discovery of the high temperature superconductors, with a recent breakthrough being the discovery of superconductivity at T c = 40 K in the simple (structurally and electronically) compound MgB 2 .[1] The origin of its remarkable superconductivity is now understood to arise from charge carriers doped (in this case, self-doped) into very strongly bonding states that in turn respond very sensitively to the bond-stretching vibrational modes.[2] The strong B-B bonds in the graphitic layers of MgB 2 make it appear near optimal, although graphite itself and diamond are materials that have even stronger bonds. Of these two, only diamond has bonding states that can conceivably become conducting through hole-doping. While the study of B as a hole dopant in diamond has a long history, there have been recent developments due to the ability to dope diamond films more heavily (to and beyond the 10 20 cm −3 range) than was possible previously. In spite of its growing importance, and unlike the situation for donors, [6] there has been little theoretical work on the acceptor state (such as determining its spatial extent) beyond obtaining the structural and vibrational properties of the isolated B impurity. Given these indications of degenerate behavior even below 2.5% doping, in this paper we adopt the viewpoint that the majority fraction of the hole carriers resides in states overlapping the diamond valence band, and behave primarily as degenerate valence band holes. Boeri et al.[11] have taken a similar viewpoint, and two supercell calculations [12,13] have verified this degenerate metal picture. The distinctly different low-concentration, nonmetallic limit has also been suggested. [14] We investigate the magnitude and effect of hole-phonon coupling analogously to what has been found to drive superconductivity in MgB 2 , and present evidence that at hole-doping levels similar to that reported, the hole -bond-stretch coupling is surprisingly strong and makes phonon exchange a prime candidate for the mechanism of pairing. In the case that such coupling is strong, it can be verified by spectroscopy of the Raman-active bond stretch mode. In fact, Ekimov et al. report [4] a Raman spectrum in which the sharp diamond peak at 1332 cm −1 has vanished, leaving spec...

The strength and effect of Coulomb correlations in the (superconducting when hydrated) x ≈1/3 and "enhanced" x ≈2/3 regimes of NaxCoO2 are evaluated using the correlated band theory LDA+U (local density approximation of Hubbard U) method. Our results, neglecting quantum fluctuations, are: (1) allowing only ferromagnetic order, there is a critical Uc = 3 eV, above which charge disproportionation occurs for both x=1/3 and x=2/3, (2) allowing antiferromagnetic order at x=1/3, Uc drops to 1 eV for disproportionation, (3) disproportionation and gap opening occur simultaneously, (4) in a Co 3+ -Co 4+ ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below Uc ferromagnetism is favored. Comparison of the calculated Fermi level density of states compared to reported linear specific heat coefficients indicates enhancement of the order of five for x ∼0.7, but negligible enhancement for x ∼0.3. This trend is consistent with strong magnetic behavior and local moments (Curie-Weiss susceptibility) for x >0.5 while there no magnetic behavior or local moments reported for x <0.5. We suggest that the phase diagram is characterized by a crossover from effective single-band character with U >> W for x > 0.5 into a three-band regime for x < 0.5, where U → U ef f ≤ U/ √ 3 ∼ W and correlation effects are substantially reduced.

To study the microscopic electronic and magnetic interactions in the substoichiometric iron chalcogenide FeSe1−x which is observed to superconduct at x ≈ 1 8 up to Tc=27 K, we use first principles methods to study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a ferrimagnetic cluster of eight Fe atoms, which for the ordered x= 1 8 alloy leads to half metallic conduction. Similar magnetic clusters are obtained for FeTe1−x and for BaFe2As2 with an As vacancy, although neither of these are half metallic. Based on fixed spin density results, we suggest the low energy excitations in FeSe1−x are antiparamagnon-like with short correlation length.