Electronic Properties of a New Structure of BC2N and Its Intercalation with Magnesium: Possibility for a New High-Tc Superconductor

We propose a new structure for the graphite-like BC₂N, other than the one proposed in Liu et al. (Phys. Rev. B 39, 1760, 1989). We compare it with the older structure and show that it has a lower energy. The band structure calculations of a single sheet of this new structure show that it is a semiconductor with a very small band gap of 0.25 eV, whereas an AA stacking of BC₂N layers of this structure behaves like a metal. Because of the similarity of this structure with the boron layers in MgB₂, we propose to intercalate the layers of the new structure of BC₂N with the magnesium atoms to obtain Mg₂BC₂N. The band structure calculations of this new structure show an unusually large metallic density of states at the Fermi level, much higher than that of MgB₂. This leads us to expect Mg₂BC₂N to be a superconductor with a higher T _c.     

Investigation on the Structure and Superconducting Properties of Mg1 − xMxB2 (M = deficiency or Ca)

We report the preparation of Mg_{1 ? x} M _x B₂ (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg_{1 ? x} B₂, although nearly single-phase samples can be obtained for x = 0, MgB₄ coexists with the MgB₂ phase and some minor impurity phases, and the amount of MgB₄ increases with x for 0 < x ≤ 0.5. The lattice parameters a and c of MgB₂ decreases and increases, respectively, with the increase of x, and T _c also decreases. While for Mg_{1 ? x} Ca _x B₂, the superconducting transition temperature remains unchanged for x ≤ 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x ≤ 0.3 samples show that MgB₂ phase coexists with CaB₆, Mg, and MgO. With increasing x, the amount of CaB₆, Mg, and MgO increases, while the amount of MgB₂ decreases. The lattice parameters of MgB₂ phase do not show any obvious change in contrast to Mg_{1 ? x} B₂. The results were discussed by considering some possible contributions.     

Thermodynamic Properties and Thermal Stability of Magnesium Borides

Thermodynamic modeling (p = 9.8066 × 10^?2 MPa, T = 300–2500 K) is used to assess the thermal stability of MgB₁₂, Mg₂B₁₄, MgB₆, MgB₄, and MgB₂. The thermodynamic properties and functions of MgB₆ and Mg₂B₁₄ are calculated for the first time. The results are compared with earlier reported data.     

Electronic band structure and stability of fluorite-like phases in the Mg-Sn-B system

Self-consistent FLMTO-GGA calculations are used to analyze the electronic states and energy bands in cubic fluorite-like (Mg₂Sn, “Mg₂B,” Mg₂SnB_x) and hexagonal (MgB₂) phases of the Mg-Sn-B system. Their electronic structure and chemical bonding are discussed, and their equilibrium lattice parameters, bulk moduli, and the derivatives of their bulk moduli are determined. According to the calculated energies of formation, the stability of the phases in question decreases in the order MgB₂ > Mg₂Sn > “Mg₂B” > Mg₂SnB. The positive energies of formation of Mg₂SnB and “Mg₂B” suggest that the synthesis of these phases from elements (Mg, Sn, and B) under equilibrium conditions is unlikely.     

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.     

The role of the substrate surface morphology in enhancing the MgB2 superconducting temperature

We hereby report on the role of the surface morphology of various substrates in the enhancement of the superconducting critical temperature of MgB₂. MgB₂ thin layers were grown by hybrid physical–chemical vapour deposition on silicon carbide SiC substrates/fibers and several other substrates, characterized by diverse surface morphologies. By investigating the structural, morphological and transport properties of MgB₂ thin layers, the presented data show that the superconducting critical temperature T _c exceeds the bulk value only when the MgB₂ films are grown on atomically flat (0001) SiC single crystals and on MgB₂ bottom layers. These results further confirm the interpretation of the coalescence-driven tensile strain mechanism behind the enhancement of superconducting properties in MgB₂ thin films.     

Effects of Metallic Spacer in Layered Superconducting Sr2(Mg y Ti1−y )O3FeAs

The highly two-dimensional superconducting sys tem Sr₂(Mg _y Ti_1?y )O₃FeAs, recently synthesized in the range of 0.2≤y≤0.5, shows an Mg concentration-dependent T _c . Reducing the Mg concentration from y=0.5 leads to a sudden increase in T _c , with a maximum T _c ≈40 K at y=0.2. Using first principles calculations, the unsynthesized stoichiometric y=0 and the substoichiometric y=0.5 compounds have been investigated. For the 50 % Mg-doped phase (y=0.5), Sr₂(Mg _y Ti_1?y )O₃ layers are completely insulating spacers between FeAs layers, leading to the fermiology such as that found for other Fe pnictides. At y=0, representing a phase with metallic Sr₂TiO₃ layers, the Γ-centered Fe-derived Fermi surfaces (FSs) considerably shrink or disappear. Instead, three Γ-centered Ti FSs appear, and in particular two of them have similar size, like in MgB₂. Interestingly, FSs have very low Fermi velocity in large fractions: the lowest being 0.6×10⁶ cm/s. Furthermore, our fixed spin moment calculations suggest the possibility of magnetic ordering, with magnetic Ti and nearly nonmagnetic Fe ions. These results indicate a crucial role of Sr₂(Mg _y Ti_1?y )O₃ layers in this superconductivity.     

Proximity Effect of Magnesium Diboride on Single-Walled Carbon Nanotube: an Ab Initio Study

Magnesium diboride (MgB₂) superconductor with excellent physical properties continues to attract the attention of researchers since its discovery. It derives its versatility from the absence of weak links, large coherence length, and small anisotropy. On the other hand, reports of superconductivity in small-diameter single-walled carbon nanotubes (SWCNTs) suspended between superconducting contacts and proximity induced supercurrents in Ta/SWNTs/Au junctions have also aroused great interest in the scientific community. Proximity induced superconductivity in SWCNTs has opened up new frontiers of research which will lead to many novel discoveries. This paper reports ab initio investigations on the proximity effect of MgB₂ on the electronic structure of a SWCNT. Condensation of electronic states is observed in the electronic band structure of the pristine SWCNT when MgB₂ is held in proximity. An additional band gap is generated below the lowest energy state of the valence band of the pristine CNT which we suggest, is due to Cooper pair formation. This leads to the prediction that SWCNTs will show superconducting properties in proximity of MgB₂. We envision MgB₂-coated SWCNTs as a novel nanomaterial that has a combination of proximity induced superconductivity and inherently unique mechanical and optical properties of SWCNTs.     

Ionicity Versus Covalency in MgB2: The Hidden Role of Mg

The discovery of superconductivity in MgB₂ at T _c = 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 T _c 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.     

Hydrogen Storage in Partially Exfoliated Magnesium Diboride Multilayers

Metal boride nanostructures have shown significant promise for hydrogen storage applications. However, the synthesis of nanoscale metal boride particles is challenging because of their high surface energy, strong inter- and intraplanar bonding, and difficult-to-control surface termination. Here, it is demonstrated that mechanochemical exfoliation of magnesium diboride in zirconia produces 3–4 nm ultrathin MgB₂ nanosheets (multilayers) in high yield. High-pressure hydrogenation of these multilayers at 70 MPa and 330 °C followed by dehydrogenation at 390 °C reveals a hydrogen capacity of 5.1 wt%, which is ≈50 times larger than the capacity of bulk MgB₂ under the same conditions. This enhancement is attributed to the creation of defective sites by ball-milling and incomplete Mg surface coverage in MgB₂ multilayers, which disrupts the stable boron–boron ring structure. The density functional theory calculations indicate that the balance of Mg on the MgB₂ nanosheet surface changes as the material hydrogenates, as it is energetically favorable to trade a small number of Mg vacancies in Mg(BH₄)₂ for greater Mg coverage on the MgB₂ surface. The exfoliation and creation of ultrathin layers is a promising new direction for 2D metal boride/borohydride research with the potential to achieve high-capacity reversible hydrogen storage at more moderate pressures and temperatures.     

Comparative Experimental and Density Functional Theory (<Emphasis Type="Italic">DFT</Emphasis>) Study of the Physical Properties of MgB<Subscript>2</Subscript> and AlB<Subscript>2</Subscript>

In the present study, we report an intercomparison of various physical and electronic properties of MgB₂ and AlB₂. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C _P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 Å, and c=3.524 Å of MgB₂ shrinks in c-direction for AlB₂ with a=3.006 Å, and c=3.254 Å. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB₂. Superconductivity is not observed for AlB₂. Interestingly, the sign of S(T) is +ve for MgB₂ the same is ?ve for AlB₂. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB₂ to Debye–Einstein model and estimated the value of Debye temperature (Θ _D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E _F) is calculated. From the ratio of experimental N(E _F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB₂ in the strong coupling BCS category. The electronic specific heat of MgB₂ is also fitted below T _c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB₂ is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB₂, and hence it is not superconducting. DFT calculations demonstrated that for MgB₂, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB₂ is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB₂ is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB₂ and AlB₂.     

The T c Amplification by “Shape Resonance” in Doped MgB2

We have applied two channels (σ and π) superconductivity model to the Al_1?x Mg _x B₂. Using the experimental data, we have calculated the strength of the interchannel pairing due to quantum interference effects, probed by the interband coupling parameter, and the two gaps as a function of the x. While in MgB₂ the quantum interference effects gives an amplification of T _c by factor 1.5 in comparison with the dominant intra σ band single channel pairing, in AlMgB₄ the amplification is about 100, in comparison with the dominant intra π band single channel pairing.     

Effect of Cd Doping on Structure and Superconductivity in Mg0.5Cd0.5B2

The effect of Cd doping on structure and superconductivity in Mg_0.5Cd_0.5B₂ fabricated by a solid-state reaction at ambient pressure has been investigated. The resulting changes in crystal structure, superconducting transition temperature T _c and critical current density J _c are characterized by X-ray diffraction, dc magnetization, electrical resistance, and magnetic measurements. It reveals that Cd does not occupy the atomic Mg sites in the MgB₂ crystal structure, but merely reacts with Mg and forms a MgCd₃ phase. It is striking to note that although the nonsuperconducting phase MgCd₃ is as high as about 67 vol.% in Mg_0.5Cd_0.5B₂, the T _c of the doped sample drops only by about 1 K. Most important, a surprising improvement of J _c of 5.0 × 10⁵ A/cm² (5 K, 0 T) has been achieved in Mg_0.5Cd_0.5B₂. It is suggested that the improvement in J _c in Mg_0.5Cd_0.5B₂ is primarily due to pinning effects induced by MgCd₃. Also, it is thought that MgCd₃ may fill up gaps among grains in MgB₂ and produce better grain linkage, which may be another source of improvement in J _c in Mg_0.5Cd_0.5B₂.     

Electronic structures and optical properties of GaN nanotubes with MgGa–ON co-doping

Both the electronic structures and the optical properties of single-walled zigzag GaN nanotubes (NTs) with Mg_Ga–O_N co-doping are investigated using first-principles calculations. We find that the Mg_Ga–O_N defect complex can exist stably in GaN NTs. The direct band gap width of the GaN NTs can be reduced by means of the Mg_Ga–O_N co-doping. The electrons of the valence band maximum (VBM) state are localized around the N atoms bonded with the Mg atom. The imaginary part ε₂ of the complex dielectric function of GaN NTs with Mg_Ga–O_N co-doping has a sharp peak closely related to the optical transitions between the VBM and conduction band minimum states.     

Electronic structures of Mg3Pn2 (Pn=N, P, As, Sb and Bi) and Ca3N2 calculated by a first-principle pseudopotential method

Electronic structure calculations for Mg₃N₂, Mg₃P₂, Mg₃As₂ (low and high temperature modifications), Mg₃Sb₂, Mg₃Bi₂, and Ca₃N₂ have been performed. Mg₃Sb₂ is predicted to be an indirect semiconductor with the gap value of about 0.41 eV. Mg₃As₂ with a high temperature modification is also predicted to be a semiconductor with the gap value of about 1.1 eV, but the valence band maximum and the conduction band minimum of Mg₃Bi₂ contacts at Γ which would make it a semimetal. Mg₃N₂, Mg₃P₂, and Mg₃As₂ (low temperature phase) are semiconductors with the direct band gaps of 1.64 eV, 1.73 eV, and 1.57 eV, respectively. Ca₃N₂ is a semiconductor with a gap of about 1.2 eV.     

The Magnetic and Structural Properties of SiC-Doped MgB2 Bulks Prepared by the Standard Ceramic Processing

According to general formula MgB_2?x SiC _x (x=0,0.05,0.1,0.2), MgB₂ and SiC-doped bulk superconductors were prepared by the standard ceramic processing. The mixtures of the corresponding powders were sintered at 750?°C for 0.5 h under pressure of 8 bar Argon. X-ray diffraction patterns show that all the samples have MgB₂ as the main phase with a very small amount of MgO; further, with SiC-doped, the presence of Mg₂Si is also noted. The magnetization-temperature measurements showed a transition temperature of 37.5 K for the undoped sample which indicates the typical transition temperature of MgB₂. When the content of SiC increased in the sample, the transition temperatures decreased to the lower temperatures systematically. The M?CH loops measured using a VSM showed very large magnetization value at low temperature for SiC doped samples. The largest M?CH loops were taken from the sample contains 5% SiC. The critical current density of samples calculated from M?CH loops indicated a value of around 4×10⁵ A/cm², which is in good agreement with the literature.     

Thermal oxidation behavior of hexagonal BC2N

Non-isothermal thermogravimetry and differential scanning calorimetry (DSC) kinetic analysis methods were used to study the thermal oxidation behavior of hexagonal BC₂N. The results show that the oxidation of hexagonal BC₂N begins at about 900 °C under a dry air atmosphere. The apparent activation energy E_a for the oxidation of hexagonal BC₂N is calculated to be 223.57 and 231.48 kJ/mol from the Kissinger and Ozawa methods, respectively. These values are higher than those (153.24 and 161.10 kJ/mol) for carbon, which means that hexagonal BC₂N has higher thermal stability than carbon at high temperature in an oxygen-containing environment.     

Estimation of π and σ band contributions in the normal state electrical conductivity of (Bi,Pb)-2223 added MgB2 superconductors

Temperature dependence of the normal state electrical resistivity of polycrystalline MgB₂ added with 0, 0.5, 1, 3 and 5 wt. % of (Bi, Pb)-2223 (Bi_1.8Pb_0.26Sr₂Ca₂Cu₃O_10+x) superconducting powder have been investigated in the light of two band approach based on π and σ bands of MgB₂ superconductor. The scattering rates (γ_σ, γ_π) and residual resistivity (ρ_0σ, ρ_0π) of each band are estimated for the investigated samples. Our observation for pure MgB₂ shows much higher scattering rate in π bands, as compared to σ bands and hence indicates ‘dirty’ nature of the samples. However, the addition of 2223 in MgB₂ is found to enhance the scattering rate in both bands, but the enhancement is more pronounced in π bands as compared to σ bands. Contribution of each individual band towards the total electrical conductivity of 2223 added MgB₂ pellets are separated. Our analysis confirms that σ band contribution shows a small increase with 2223 addition and reaches nearly 89% for 5 wt. % 2223 added MgB₂ polycrystalline pellets. The electron–phonon coupling constant (λ) of pure and 2223 added MgB₂ pellets calculated using Mc-Millan expression is found to be nearly invariant with 2223 addition.     

Investigation on the Structure and Superconducting Properties of Mg1 ? xMxB2 (M = deficiency or Ca)

We report the preparation of Mg_{1 – x} M _x B₂ (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg_{1 – x} B₂, although nearly single-phase samples can be obtained for x = 0, MgB₄ coexists with the MgB₂ phase and some minor impurity phases, and the amount of MgB₄ increases with x for 0 < x 0.5. The lattice parameters a and c of MgB₂ decreases and increases, respectively, with the increase of x, and T _c also decreases. While for Mg_{1 – x} Ca _x B₂, the superconducting transition temperature remains unchanged for x 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x 0.3 samples show that MgB₂ phase coexists with CaB₆, Mg, and MgO. With increasing x, the amount of CaB₆, Mg, and MgO increases, while the amount of MgB₂ decreases. The lattice parameters of MgB₂ phase do not show any obvious change in contrast to Mg_{1 – x} B₂. The results were discussed by considering some possible contributions.     

A novel layer-structured PtN2: First-principles calculations

The potential structures of platinum nitride with a chemical composition of PtN₂ have been examined by utilizing a widely adopted evolutionary methodology for crystal structure prediction. Except reproducing the previously proposed phases, a Pmmm symmetric novel layer structure with a low formation enthalpy that is slightly lower than those of marcasite and CoSb₂ structures but slightly higher than that of pyrite structure has also been identified. The elastic constants and the lattice dynamical calculations show that this layer-structured PtN₂ is mechanically and dynamically stable. The calculated band structures suggest this new phase together with the simple tetragonal phase are metallic, while other phases are insulators. In addition, it has been found by the phonon spectrum calculations that the fluorite structure is dynamically unstable, although it is mechanically stable as suggested by calculated elastic constants.