Two-Gap Superconductivity in Niobium Carbide-Coated Single-Walled Carbon Nanotubes: A First-Principles Study

Superconductivity in carbon nanotubes is attracting worldwide attention because of the reported high superconducting transition temperature in small-diameter single-walled carbon nanotubes (SWCNTs). However, it is well known that superconductivity in low-dimensional (quasi-1D) systems is not so common due to low density of states (DOS), strong quantum fluctuations and other phenomena in such systems. In this paper, we present theoretical investigations of the proximity effect of superconducting niobium carbide on single-walled carbon nanotube using density functional theory (DFT). The relaxed structure shows that Nb atoms are held around the SWCNT, forming a layer through weak van der Waals’ forces. The stability of the structure has been confirmed by Hirshfeld analysis and Mullikan population analysis as well. The study of the electronic band structure of the pristine and modified SWCNT shows a fascinating condensation of electronic states and a striking shift in the Fermi level. Further, two additional band gaps have appeared below the valence band suggesting some kind of pairing mechanism being operational. This indicates the possibility of superconducting behaviour in SWCNT in proximity of niobium carbide. The relaxed structure thus envisions the feasibility and stability of NbC-coated SWCNTs which will have superconducting properties as well as the remarkable mechanical and optical properties of SWCNTs. This prediction seeks interest of the researchers to try and develop such a novel nanomaterial which, if realized, will prove to be highly significant for many technological applications.     

Comparative analysis of electronic properties of tin,gallium, and bismuth chalcogenide-filled single-walled carbon nanotubes

In the present work, the channels of single-walled carbon nanotubes (SWCNTs) were filled with tin sulfide (SnS), gallium telluride (GaTe), and bismuth selenide (Bi₂Se₃). The successful encapsulation of the compounds was proven by high-resolution transmission electron microscopy. The electronic properties of the filled SWCNTs were studied by optical absorption spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It was found that the embedded metal chalcogenides have different influence on the electronic properties of the nanotubes. The incorporation of tin sulfide into the SWCNTs does not result in sufficient changes in the electronic structure of SWCNTs, except for a minor influence on metallic nanotubes. The filling of SWCNTs with gallium telluride causes the charge transfer from the SWCNT walls to the encapsulated compound due to acceptor doping of the nanotubes. The insertion of bismuth selenide inside the SWCNT channels does not lead to the modification of the electronic properties of nanotubes.     

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.     

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.     

A Description of MgB2 Superconductivity Including σ–π Bands Coupling

A two-band scheme of the MgB₂ superconductivity has been developed. The pairing channels incorporate σ-intraband electron–phonon attraction besides the Coulombic repulsion, together with pair transfer between effective σ- and π-bands. Various MgB₂ superconducting characteristics calculated using a plausible parameter set (which is narrower than the amount of considered properties) agree on a quantitative level with the observations.     

Influence of Electron–Phonon Interaction on Superconducting State Parameters of MgB2

A three-square well model is employed for the three interactions namely, electron–acoustic phonon, electron–optical phonon, and Coulomb in the calculation of superconducting transition temperature (T _c) for layered structure MgB₂. The analytical solutions for the energy gap equation allow us to understand the relative interplay of these interactions. The values of the coupling strength and of the Coulomb interaction parameter indicate that the test material is in the intermediate coupling regime. The superconducting transition temperature of MgB₂ is estimated as 41 K for λ_ac ≈ 0.3, λ_op ≈ 0.1, and μ* ≈ 0.07. We suggest from these results that both the acoustic and optical phonons within the framework of a three-square well scheme consistently explains the effective electron–electron interaction leading to superconductivity in layered structure MgB₂.     

Fabrication and Properties of MgB2 Coated Superconducting Tapes

We present the formation of MgB₂ coatings by simple and novel aerosol deposition technique which has a potential to escalate towards the fabrication of long superconducting tapes. The thin MgB₂ coatings were produced by using pre-synthesized MgB₂ powder. The ability of this technique to form a precursor powder in a thin film form has greatly reduced the intricacies involved in the synthesis of MgB₂ by other techniques like hybrid physical chemical vapor deposition etc. The as-synthesized thin films were characterized by the x-ray diffraction technique to study the structural properties. The thin films were found to be x-ray amorphous in nature depicting the formation of frustrated structure which showed a superconducting transition onset at around 36 K.     

Effect of Rare-Earth Oxides Doping on the Superconductivity and Flux Pinning of MgB<Subscript>2</Subscript> Superconductor

A series of rare-earth-oxide-doped MgB₂ bulks are prepared by in situ solid-state reaction with Pr₆O₁₁, CeO₂, Lu₂O₃, and Ho₂O₃ as the dopants. The superconducting properties are investigated and compared with the nano-Fe₃O₄ doped MgB₂. It is found that different from doping ferromagnetic nano-Fe₃O₄ which drastically suppresses superconductivity of MgB₂, doping the rare-earth-oxides has little effect on superconductivity of MgB₂ although most rare-earth elements have strong magnetic moment. In addition, some boride impurities formed during the reaction between rare-earth oxides and boron can work as effective pinning centers and significantly improve J _c and H _irr of MgB₂ when these fine nanoboride precipitates (<20 nm) are embedded into the MgB₂ intragrains.     

Attractive Vortex Interaction and the Intermediate-Mixed State of?Superconductors

The magnetic vortices in superconductors usually repel each other. Several cases are discussed when the vortex interaction has an attractive tail, and thus a minimum, leading to vortex clusters and chains. Decoration pictures then typically look like in the intermediate state of type-I superconductors, showing lamellae or islands of Meissner state or surrounded by Meissner state, but with the normal regions filled with Abrikosov vortices that are typical for type-II superconductors in the mixed state. Such intermediate-mixed state was observed and investigated in detail in pure Nb, TaN and other materials 40 years ago; last year it was possibly also observed in MgB₂, where it was called ??a totally new state?? and ascribed to the existence of two superconducting electron bands, one of type-I and one of type-II. The complicated electronic structure of MgB₂ and its consequences for superconductivity and vortices are discussed. It is shown that for the real superconductor MgB₂ which possesses a single transition temperature, the assumption of two independent order parameters with separate penetration depths and separate coherence lengths is unphysical.     

Magneto-Optical Imaging of Superconductors for Liquid Hydrogen Applications

Restricted deposits of fossil fuels and ecological problems created by their extensive use require a transition to renewable energy resources and clean fuel free from emissions of CO₂. This fuel is likely to be liquid hydrogen. An important feature of liquid hydrogen is that it allows wide use of superconductivity. Superconductors provide compactness, high efficiency, savings in energy and a range of new applications not possible with other materials. The benefits of superconductivity justify use of low temperatures and facilitate development of fossil-free energy economy. The widespread use of superconductors requires a simple and reliable technique to monitor their properties. Magneto-optical imaging (MOI) is currently the only direct technique allowing visualization of the superconducting properties of materials. We report the application of this technique to key superconducting materials suitable for the hydrogen economy: MgB₂ and high temperature superconductors (HTS) in bulk and thin-film form. The study shows that the MOI technique is well suited to the study of these materials. It demonstrates the advantage of HTS at liquid hydrogen temperatures and emphasizes the benefits of MgB₂, in particular.     

Anisotropic Upper Critical Field in Single Crystal MgB2

Recent experiments on MgB₂ single crystals appear to indicate that superconductivity in this material is described not only by two superconducting order parameters attached to the band and the band, respectively, but also these two order parameters have different anisotropy. More explicitly the anisotropy of H _c2(t) requires an oblate order parameter in momentum space attached to the a band while H _c1(t) is described by a prolate order parameter, attached to the band. Therefore the vortex state in MgB₂ should be described by an interplay of these two superconducting order parameters.     

Electroless deposition of superconducting MgB2 films on various substrates

The superconducting properties of magnesium diboride (MgB₂) films prepared by electroless deposition on various substrates including silver, gold and silicon are reported. In this study, MgB₂ films were fabricated on silver, gold, and silicon using an electroless plating technique, while controlling the redox potential to improve the deposition quality. The structure, morphology, and superconducting properties of the samples were investigated using X-ray diffraction, magnetometry, scanning electron microscopy, and Raman spectroscopy. X-ray diffraction and Raman spectroscopy confirmed that the films are polycrystalline MgB₂ but also contain some impurity phases. All the MgB₂ films show superconducting transitions near 39 K, the value for bulk MgB₂, with the superconducting volume fraction ranging from approximately 1 to 2%. We find a strong dependence of film quality with the oxidation potential of the bath.     

Superconducting Bands Stabilizing Superconductivity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> and MgB<Subscript>2</Subscript>

It is shown that the ceramic superconductor YBa₂Cu₃O₇ as well as the superconducting intermetallic compound MgB₂ possesses a narrow, partly filled “superconducting band” with Wannier functions of special symmetry in their band structures. This result corroborates previous observations about the band structures of numerous superconductors and non-superconductors showing that evidently superconductivity is always connected with such superconducting bands. These findings are interpreted in the framework of a nonadiabatic extension of the Heisenberg model. Within this new group-theoretical model of correlated systems, Cooper pairs are stabilized by a nonadiabatic mechanism of constraining forces effective in narrow superconducting bands. The formation of Cooper pairs in a superconducting band is mediated by the energetically lowest boson excitations in the considered material that carry the crystal-spin angular momentum 1⋅ℏ. These crystal-spin-1 bosons are proposed to determine whether the material is a conventional low-T _c or a high-T _c superconductor. This interpretation provides the electron–phonon mechanism that enters the BCS theory in conventional superconductors.     

Selective Growth of Metal‐Free Metallic and Semiconducting Single‐Wall Carbon Nanotubes

A major obstacle for the use of single‐wall carbon nanotubes (SWCNTs) in electronic devices is their mixture of different types of electrical conductivity that strongly depends on their helical structure. The existence of metal impurities as a residue of a metallic growth catalyst may also lower the performance of SWCNT‐based devices. Here, it is shown that by using silicon oxide (SiO_x) nanoparticles as a catalyst, metal‐free semiconducting and metallic SWCNTs can be selectively synthesized by the chemical vapor deposition of ethanol. It is found that control over the nanoparticle size and the content of oxygen in the SiO_x catalyst plays a key role in the selective growth of SWCNTs. Furthermore, by using the as‐grown semiconducting and metallic SWCNTs as the channel material and source/drain electrodes, respectively, all‐SWCNT thin‐film transistors are fabricated to demonstrate the remarkable potential of these SWCNTs for electronic devices.     

Plasma arc Synthesis of Nano-Boron Towards the Synthesis of MgB2 Superconductors

Since the discovery of superconductivity in the tempting binary intermetallic compound MgB₂, the solid-state synthesis technique is highly dominated by the usage of amorphous boron as one of the precursor powders. The formation of MgB₂ phase proceeds through the diffusion of Mg into B powder mainly driven by the low melting point of Mg as compared to B. Once the nucleation is achieved, the progress of polycrystalline MgB₂ phase occurs due to the out diffusion of boron through the MgB₂ layer and by the inward diffusion of Mg. This growth is impeded due to the presence of certain oxide phases or formation of Mg deficient phases. It is speculated that the probability for the inclusion of Mg(B)–O phases is higher for crystalline boron precursor as compared to the amorphous B. Thus, the use of nanosized amorphous boron may lead to larger nucleation centers, smaller grain size and consequently higher packing density in the polycrystalline MgB₂, which will in turn provide optimum superconducting properties. Hence an attempt to synthesize amorphous nano-boron powders is presented. Plasma arc discharge technique was successfully employed to produce nano-boron powder. The XPS analysis was carried out to inveterate the formation of boron. The as-synthesized powder had a uniform average particle size distribution of around 20 nm as confirmed by TEM measurements. The selected area electron diffraction pattern composed of diffused ring clearly depicts the amorphous nature of boron powder.     

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.     

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.     

Superconductivity in Fe and As Based Compounds: A Bridge Between MgB<Subscript>2</Subscript> and Cuprates

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multigap superconductivity, it is shown that these systems strongly resemble MgB₂, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similarly to MgB₂ and in contrast to cuprates, the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps.