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Register a new userCombining the advantages of superconducting MgB2 and CaC6 in one material: suggestions from first-principles calculations
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We show that a recently predicted layered phase of lithium monoboride, Li2B2, combines the key mechanism for strong electron-phonon coupling in MgB2 (i.e., interaction of covalent B sigma bands with B bond-stretching modes) with the dominant coupling mechanism in CaC6 (i.e., interaction of free-electron-like interlayer states with soft intercalant modes). Yet, surprisingly, the electron-phonon coupling in Li2B2 is calculated to be weaker than in either MgB2 or CaC6. We demonstrate that this is due to the accidental absence of B pi states at the Fermi level in Li2B2. In MgB2, the pi electrons play an indirect but important role in strengthening the coupling of sigma electrons. Doping Li2B2 to restore pi electrons at the Fermi level is expected to lead to a new superconductor that could surpass MgB2 in Tc.
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Very recently (November, 2010, PRB, 82, 180520R) the first 122-like ternary superconductor KxFe2Se2 with enhanced TC ~ 31K has been discovered. This finding has stimulated much activity in search of related materials and triggered the intense studies of their properties. Indeed already in 2010-2011 the superconductivity (TC ~ 27-33K) was also found in the series of new synthesized 122 phases such as CsxFe2Se2, RbxFe2Se2, (TlK)xFeySe2 etc. which have formed today the new family of superconducting iron-based materials without toxic As. Here, using the ab initio FLAPW-GGA method we have predicted for the first time the elastic properties for KFe2Se2 and discussed their interplay with inter-atomic bonding for this system. Our data reveal that the examined phase is relatively soft material. In addition, this system is mechanically stable, adopts considerable elastic anisotropy, and demonstrates brittleness. These conclusions agree with the bonding picture for KFe2Se2, where the inter-atomic bonding is highly anisotropic and includes ionic, covalent and metallic contributions.
In this work, global search for crystal structures of ternary Mg-Sc-H hydrides (Mg$_x$Sc$_y$H$_z$) under high pressure ($100 le P le 200$ GPa) were performed using the evolutionary algorithm and first-principles calculations. Based on them, we computed the thermodynamic convex hull and pressure-dependent phase diagram of Mg$_x$Sc$_y$H$_z$ for $z/(x+y) < 4$. We have identified the stable crystal structures of four thermodynamically stable compounds with the higher hydrogen content, i.e., $Rbar{3}m$-MgScH$_{6}$, $C2/m$-Mg$_{2}$ScH$_{10}$, $Immm$-MgSc$_{2}$H$_{9}$ and $Pmbar{3}m$-Mg(ScH$_{4}$)$_{3}$. Their superconducting transition temperatures were computationally predicted by the McMillan-Allen-Dynes formula combined with first-principles phonon calculations. They were found to exhibit superconductivity; among them, $Rbar{3}m$-MgScH$_{6}$ was predicted to have the highest $T_{c}$ (i.e. 23.34 K) at 200 GPa.
By means of synchrotron X-ray diffraction, we studied the effect of high pressure, P, up to 13 GPa on the room temperature crystal structure of superconducting CaC6. In this P range, no change of the pristine space group symmetry, textit{R=3m}, is found. However, at 9 GPa, i.e. close to the critical value at which a large T_c reduction was reported recently, we observed a compressibility jump concomitant to a large broadening of Bragg peaks. The reversibility of both effects upon depressurization and symmetry arguments give evidence of an order-disorder phase transition of second order, presumably associated with the Ca sublattice, which provides a full account for the above Tc reduction.
This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since early 1950s, but recently discovered to be superconductive at a remarkably high critical temperature Tc=40K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, transparency of grain boundaries to current promises that MgB2 will be a good material for both large scale applications and electronic devices. During the last seven month, MgB2 has been fabricated in various forms, bulk, single crystals, thin films, tapes and wires. The largest critical current densities >10MA/cm2 and critical fields 40T are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is still to be resolved, a wide range of values being reported, between 1.2 and 9. Also there is no consensus about the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, being the tip of an iceberg who awaits to be discovered. Up to date MgB2 holds the record of the highest Tc in its class. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materials, several compounds being already announced to become superconductive: TaB2, BeB2.75, C-S composites, and the elemental B under pressure.
The HfV$_2$Ga$_4$ compound was recently reported to exhibit unusual bulk superconducting properties, with the possibility of multiband behavior. To gain insight into its properties, we performed ab-initio electronic structure calculations based on the Density Functional Theory (DFT). Our results show that the density of states at the Fermi energy is mainly composed by V--$d$ states. The McMillan formula predicts a superconducting critical temperature ($T_{c}$) of approximately $3.9,$K, in excellent agreement with the experimental value at $4.1,$K, indicating that superconductivity in this new compound may be explained by the electron-phonon mechanism. Calculated valence charge density maps clearly show directional bonding between Hf and V atoms with 1D highly populated V-chains, and some ionic character between Hf--Ga and V--Ga bonds. Finally, we have shown that there are electrons occupying two distinct bands at the Fermi level, with different characters, which supports experimental indications of possible multiband superconductivity. Based on the results, we propose the study of a related compound, ScV$_2$Ga$_4$, showing that it has similar electronic properties, but probably with a higher $T_c$ than HfV$_2$Ga$_4$.
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