Shuiquan Deng

Superconductivity and Chemical Bonding in Mercury

Band structure and Fermi surface calculations were performed for the metal mercury, for which superconductivity was discovered 87 years ago. The electronic properties of mercury were analyzed to find the origin of singlet electron pair formation and condensation which leads to superconductivity.

Flat band-steep band scenario and superconductivity - the case of calcium

Abstract Calcium is chosen as a test case for the validity of the ‘flat band/steep band’ scenario in superconductivity. Its electronic structure is reexamined by means of the self-consistent all-electron full potential LMTO and LMTO-ASA method based on the local-density approximation (LDA). The lattice properties and the electron-phonon interactions are calculated from first principles based on the linear-response theory in the LMTO frame. Our results are in reasonable agreement with experiment. Some new observations on the electronic structure close to the Fermi level are made based on the comparison between the theoretical and experimental results. Calcium does not become and according to our results should not become superconducting.

Superconductivity in MgB2: A Case Study of the “Flat Band–Steep Band” Scenario

The electronic structure of MgB2 is analyzed in terms of the “flat band–steep band” scenario for superconductivity. Good agreement between the Fermi surfaces obtained from de Haas–van Alphen measurements and from TB-LMTO and FP-LMTO calculations was achieved. The decomposed electron–phonon coupling λ(q) reveals a pronounced peak-like structure along the Γ–A and Γ–M directions.

Calcium d States: Chemical Bonding of CaC6

The recent discovery of massless Dirac fermions in single layers of graphite (graphene) and superconductivity in graphite intercalation compounds (GICs) such as CaC6 at unexpectedly high temperatures stimulated intensive research to explore the nature and interactions of charge carriers in these low-dimensional systems. The early prediction of free-electron-like interlayer electronic states in graphite and the importance of the interlayer states for the formation of GICs, verified experimentally, are regaining considerable interest. Based on calculations on the structure of YbC6 Cs+nyi et al. [5] also characterized a similar quasi-free electron band in GIC and argued about its relevance for superconductivity. However, the proposed quasi-free-electron picture and the implied pairing mechanism are at odds with experiments, particularly the large Ca isotope effect. Mazin correctly predicted the interlayer band in CaC6 as originating from calcium, but the chemical bonding remains unresolved. Recent theoretical work based on the correct structure of CaC6 does not address the nature of chemical bonding. Here we report our studies on CaC6 using the full-potential linear muffin-tin orbital (FP-LMTO) method, semi-empirical extended H9ckel (EH) method, and the tight-binding (TB) model. Figure 1 shows the R3̄m rhombohedral structure of CaC6. The structure exhibits AaAbAcAa... stacking of graphene sheets (A) and metal-atom layers (a, b, c) in such a way that the metal atoms center the hexagons of the graphene sheets. In contrast, the structure of YbC6, which contains analogous graphene and metal-atom layers, has the stacking sequence AaAbAa... Our first-principles FP-LMTO calculations for CaC6 [11] resulted in C 2s and 2ps bands far below the Fermi energy, as expected. The Ca valence bands lie close to the Fermi level. The calculated band structure along four symmetry lines together with the total density of states (DOS) and four partial DOSs are shown in Figure 2.

SUPRALEITUNG UND CHEMISCHE BINDUNG IN QUECKSILBER

Kondensierte Singulett-Elektronenpaare mussen vorliegen, damit Quecksilber supraleitend wird. Diese und andere Zusammenhange zwischen Supraleitung und chemischer Bindung wurden jetzt an dem Metall berechnet, an dem die Supraleitung auch entdeckt wurde.

Ionicity Versus Covalency in MgB2: The Hidden Role of Mg

The discovery of superconductivity in MgB2 at Tc = 39 K has initiated great efforts to identify its microscopic origin. A superconducting two-gap structure has been firmly established experimentally. However, the importance of the smaller gap which is essential in pushing Tc above the BCS value of a one-gap superconductor has mostly been ignored. Our results based on ab initio calculations and a newly introduced k, j-dependent bonding indicator show that Mg contributes significantly to states from which the small gap originates. In addition, it is revealed that Mg—B covalent bonding is an important factor in limiting the substitution of Mg in MgB2.

Electronic structure and lattice dynamics of NaFeAs

The similarity of the electronic structures of NaFeAs and other Fe pnictides has been demonstrated on the basis of first-principle calculations. The global double-degeneracy of electronic bands along X-M and R-A direction indicates the instability of Fe pnictides and is explained on the basis of a tight-binding model. The de Haas-van Alphen parameters for the Fermi surface (FS) of NaFeAs have been calculated. A

The origin of a flat band

\mathbf{Q}_{M}=(1/2,1/2,0)

Another view of electron-phonon coupling revealing peak-like structures in q-space

spin density wave (SDW) instead of a charge density wave (CDW) ground state is predicted based on the calculated generalized susceptibility

A “Flat/Steep Band” Scenario in Momentum Space

\chi(\mathbf{q})