Boron Planes Research Articles

Evidence of disordered local structure of ZnO buffered MgB2 films via polarized extended X-ray absorption fine structure analysis

Herein, we investigated the mechanism by which the critical temperature (Tc) of a magnesium diboride (MgB2) superconductor is altered by the buffer layer. Zinc oxide (ZnO) layers with different thicknesses (28, 80, and 134 nm) were deposited epitaxially on a Hastelloy substrate beneath a ∼ 1 μm MgB2 film. To measure the local structural distortion of the boron plane, three different angle polarizations (0°, 45°, and 90° parallel to the ab-axis) on the Mg K-edge, extended X-ray absorption fine structure (EXAFS) measurements were performed. The EXAFS analyses revealed that the ZnO buffer layer caused the boron layer to disorient, leading to buckling between the Mg and B layers, which affected the charge transfer capability in the intra-layer structure. Furthermore, the uneven distribution of B atoms amid the surrounding Mg atoms resulted in an asymmetric crystal structure, which leads to Tc fluctuation. This study reveals the relationship between modifications made to the boron plane condition of MgB2 by the ZnO buffer layer and the superconducting Tc behavior of MgB2.

Metal-Rich Metallaboranes: Synthesis, Structures and Bonding of Bi- and Trimetallic Open-Faced Cobaltaboranes

Synthesis, isolation, and structural characterization of unique metal rich diamagnetic cobaltaborane clusters are reported. They were obtained from reactions of monoborane as well as modified borohydride reagents with cobalt sources. For example, the reaction of [Cp*CoCl]2 with [LiBH4·THF] and subsequent photolysis with excess [BH3·THF] (THF = tetrahydrofuran) at room temperature afforded the 11-vertex tricobaltaborane nido-[(Cp*Co)3B8H10] (1, Cp* = η5-C5Me5). The reaction of Li[BH2S3] with the dicobaltaoctaborane(12) [(Cp*Co)2B6H10] yielded the 10-vertex nido-2,4-[(Cp*Co)2B8H12] cluster (2), extending the library of dicobaltadecaborane(14) analogues. Although cluster 1 adopts a classical 11-vertex-nido-geometry with one cobalt center and four boron atoms forming the open pentagonal face, it disobeys the Polyhedral Skeletal Electron Pair Theory (PSEPT). Compound 2 adopts a perfectly symmetrical 10-vertex-nido framework with a plane of symmetry bisecting the basal boron plane resulting in two {CoB3} units bridged at the base by two boron atoms and possesses the expected electron count. Both compounds were characterized in solution by multinuclear NMR and IR spectroscopies and by mass spectrometry. Single-crystal X-ray diffraction analyses confirmed the structures of the compounds. Additionally, density functional theory (DFT) calculations were performed in order to study and interpret the nature of bonding and electronic structures of these complexes.

Structures and Properties of CoB19 +/0/- Clusters.

A global search for the lowest energy structure of Co atom-doped boron clusters (CoB19+, CoB19, and CoB19– clusters) was conducted. The lowest energy structures of them are remarkably different from those of B20 and CoB18– clusters. CoB19+ clusters have a bowl-shaped geometry, where the Co atom is at the bottom of the bowl and is coordinated with eight B atoms. The CoB19 cluster presents seven- and eight-membered B rings. The CoB19– cluster can be viewed as a structure that evolves from a Co-doped boron plane. The coordination number of CoB19 and CoB19– clusters are 16 and 14, respectively. Several low-lying isomers have quasi-planar structures for the CoB19– cluster. Some properties including charge transformation and distribution, HOMO–LUMO gaps, molecular orbital distribution, and stability of neutral CoB19 are discussed. CoB19+ and CoB19– exhibit magnetism with a net moment of 1.0 and 0.94 μB because of odd number of electrons.

Theoretical investigation on anti-sandwich beryllium-boron clusters Be2mBn (m = 1–3): Fluxionality and multi-aromaticity

Recently, a kind of anti-sandwich beryllium doped boron clusters with middle boron ring and beryllium units on both sides have been experimentally obtained. These clusters attract our interests due to their special three layers geometry with structural fluxionality and aromaticity. Therefore, the anti-sandwich Be2mBn (m = 1–3) clusters are theoretically investigated in this paper to get a general view of this series of beryllium-boron binary systems. The results indicate that all these clusters possessed three-layer geometries after optimization, whose stability is demonstrated by diverse multi-aromaticity. Among these structures, Be4B102+ could be viewed as an analogue of naphthalene depending on its pattern of delocalized π electrons, and Be6B12 has a distorted boron plane and high fluxionality with potential applications as a 3D Wankel motor. Our study further reveals structural rules of anti-sandwich beryllium-boron binary clusters and enriches their structural diversity.

Structure and properties of MgB2 bulks: ab-initio simulations compared to experiment

Analysis of XRD patterns by Rietveld refinement has been shown that the main phase of superconducting MgB2-based bulk materials (with high level of superconducting characteristics) has AlB2 type structure and near MgB1.8-1.68O0.2-0.32 stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm2 j c (at 0 - 1 T, 20 K); up to 15 T B c2 (at 22.5 K) and Birr (at 18 K). The ab-initio simulation confirmed (1) benefits in binding energy and enthalpy of formation if stoichiometry of the solid solution is near MgB1.75O0.25; (2) energetic advantage in case if impurity oxygen present only in each second boron plane of MgB2 cell while the first boron plane of the same cell stays pristine and location of substituted oxygen atoms in the nearby positions. Besides, the results of ab-initio modeling allow explanation of the tendency towards segregation of O-impurity in MgB2 structure during synthesis or sintering, and formation of Mg-B-O inclusions or nanolayers (with MgO type of structure) which effect pinning. Calculated transition temperatures, Tc, for MgB1.75O0.25 occurred to be 23.3 K, while for MgB2 it was 21.13 K only. Experimental Tc of the bulk materials was 35.7-38.2 K.

High yield synthesis of boron-based nanosheets

ABSTRACTWe present the chemical exfoliation of magnesium diboride (MgB2) by employing organic chelating agents for synthesising boron-based nanosheets in high yield. MgB2 is a promising parent material towards realising quasi-2D forms of boron, owing to its unique layered constitution: graphenic boron planes interleaved with hexagonal arrays of Mg atoms. Chelating agents can selectively extract the interlayer Mg from MgB2 in water, to form Mg-chelant complexes. The loss of Mg results in delamination of MgB2 into boron-based nanosheets. This approach is further improved by rational choice of chelants with greater affinity for Mg and tuning the optimal pH for metal complexation, thereby leading to a better yield of nanosheets. These boron-based nanosheets have been explored for their candidacy as a new class of inorganic fillers in a polymeric matrix – polyvinyl alcohol (PVA). The mechanical and thermal properties of these boron-based nanosheet-PVA nanocomposite films have been studied.

Boron-Doping Proximity Effects on Dislocation Generation during Non-Planar MPCVD Homoepitaxial Diamond Growth.

Epitaxial lateral growth will be required if complex diamond-based device architecture, such as, for example, Metal-oxide-semiconductor Field-effect transistors (MOSFETs) or epitaxial lateral overgrowth (ELO) substrates, need to be developed for high-power applications. To this end, undoped and doped non-planar homoepitaxial diamond were overgrown on (001)-oriented diamond-patterned substrates. Defects induced by both the heavy boron doping and three-dimensional (3D) growth were studied by transmission electron microscopy (TEM). At high methane and boron concentrations, threading dislocations with Burgers vectors b = 1/6 〈211〉, b = 1/2 〈110〉, or both were observed. Their generation mechanisms were established, revealing boron proximity effects as precursors of dislocations generated in boron-doped samples and providing clues as to the different Burgers vectors. The concentration ranges of boron and methane resulting in good crystalline quality depended on the plane of growth. The microwave plasma-enhanced chemical vapour deposition (MPCVD) growth conditions and the maximum boron concentration versus plane orientation yielding a dislocation-free diamond epitaxial layer were determined.

Study of B11 and C13 NMR on doped MgB2 in the normal and in the superconducting state

We have studied carbon-doped magnesium diboride nanoparticles using $^{13}\mathrm{C}$ and $^{11}\mathrm{B}$ NMR in the normal and superconducting states. Measurements of the line shape reveal the role of carbon as a flux-pinning center and, combined with Knight shift measurements, suggest the doping procedure favors the chemical substitution scenario. We perform ab initio calculations on a structure with a single carbon-boron substitution which yield results that match the experimental data. The $^{13}\mathrm{C}$ and $^{11}\mathrm{B}$ Knight shift data are used to extract the spin susceptibility, which indicates a BCS pairing mechanism; however, we do not observe the Hebel-Slichter coherence peak from 1/${T}_{1}$ data, which we hypothesize is due to a pair-breaking mechanism present in the boron planes.

A Radical Anion of Structurally Constrained Triphenylborane

Chemical reduction of the structurally constrained triphenylborane 1 with K produced a radical anion. EPR analysis demonstrated the delocalization of the unpaired electron spin density over the entire π skeleton to a greater extent than is the case for a nonconstrained Ph3B radical anion. DFT calculations indicated that the coplanarization of the benzene rings with the boron plane and the shortening of the B–C bond lengths by the structural constraint are responsible for the spin delocalization. The geometry optimization also suggested a bowl-shaped conformation as a viable local minimum structure in addition to the planar conformation. X-ray crystal structure analysis indeed revealed the nonplanar structure of the radical anion 1•–.

Unexpectedly hard and highly stable WB3 with a noncompact structure

Tungsten borides are currently a topic of promising superhard materials, but their structures are still not fully resolved. Here, we identify a highly stable WB3 in hR24 symmetry (hR24-WB3) by first-principles calculations. It can be derived from well-known hP3-WB2 by removing one third of tungsten atoms systematically so that the remaining tungsten atoms form three cycled layers of open hexagons with each a layer displaced by one atom. Furthermore, such a noncompact and metallic system has an unexpectedly high hardness, which originates from its three-dimensional covalent framework consisting of hexagonal boron planes interconnected with strong zigzag W–B bonds.

9-[(E)-2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)ethenyl]-9H-carbazole

The title compound, C20H22BNO2, is a simple olefinic compound which carries both B and N atoms that are trans to one another. The π-conjugated system of the compound is considered to be isoelectronic with 1,3-butadiene. There are two independent mol­ecules in the asymmetric unit in which the environments around the boron atoms are essentially planar (r.m.s. deviations of 0.0032 and 0.0021 Å for the BO2C planes). The dihedral angles of the olefinic planes with the boron planes are 5.70 (11) and 9.74 (9)°, respectively, while the dihedral angles of the olefinic planes with the carbazole planes are 19.37 (3) and 10.74 (6)°. These dihedral angles are consistent with those in 9-ethenylcarbazole and an ethenylboronic ester derivative. The N—Csp 2, B—Csp 2 and C=C bond lengths suggest that the contribution of the canonical structure can be described as N+=C—C=B−.

Superconducting pairing and the two-gap nature of Al and C dopedMgB2: screened phonon-pairing mechanism

A simplified two-band model for describing the longitudinal dielectric function for a layeredMgB2 system has been developed to reveal the attractive pairing mechanismleading to the superconducting state. A model dielectric function forσ andπ bandcarriers is set up that fulfils the appropriate sum rules on the electronic and ionic polarizabilities.The value of the in-layer electron–phonon coupling strength and the Coulomb screening parameterfor σ (π) band carriers is obtained from the residues of the dielectric function. It is noticed that theσ-holes in the two-dimensional boron planes via the screened phonon coupling isresponsible for the attractive interaction and the superconductor is in the strongcoupling regime. Within this framework, the superconducting transition temperatureTc ofMgB2 is estimated as 42 K and the pressure derivative ofTc isnegative. The isotope effect, coherence length, magnetic penetration depth, and volume derivative ofTc are alsoestimated to be consistent with the earlier results. Within the two-band model of superconductivityin MgB2, we place particular emphasis on intraband channels, carriers either inσ orπ bands,as interband effects are negligible. In such a situation the superconductivity originates from eitherσ orπ band carriers. The implications of the effective interactive potential with bothσ andπ carriers and its analysis are discussed.

Superconductivity in Transition Metal Doped MoB4

Polycrystalline ingots of MoB4 have been found to be superconducting near 6 to 8 K when doped with Ti or Nb. Magnetic susceptibility measurements performed on these alloys indicate that a substantial fraction of the sam- ple volume is superconducting below the transition temper- ature, while X-ray diffraction scans provide insight into the crystallographic effects of the dopants on the layered MoB4 structure, revealing both a systematic increase of the lattice constant in the direction of the boron planes and a system- atic decrease in the inter-plane spacing as the dopant amount is increased. This structural expansion and contraction cor- responds to an increase in Tc until the dopant solubility limit is reached. These findings present a stark contrast to the ef- fects of lattice parameter modification on superconductivity in the structurally related AlB2-type alloys.

Increased Superconductivity for CNT Doped ${\hbox {MgB}}_{2}$ Sintered in 5T Pulsed Magnetic Field

The superconductivity of carbon nanotube (CNT) doped MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. Although the carbon (C) substitution for the boron (B) is increased for the sample sintered in PMF, its superconductivity is advanced comparing with the sample sintered without PMF. The improved critical transition temperature, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , as reflected by the broadened <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2g</sub> mode in the Raman spectra, because the carbon atoms are homogeneously distributed in the boron planes.

Superconductivity of various borides and the role of carbon in their high performance

The superconductivity of MgB2, Mg1−xAlxB2 andNbB2+x is compared. Thestretched c-latticeparameter (c = 3.52 Å) of MgB2 incomparison to NbB2.8 (c = 3.32 Å)and AlB2 (c = 3.25 Å) decides empirically the population of theirπ andσ bands and, asa result, their Tc values at 39 and 11 K, respectively, for the first two and no superconductivity for the latter. Besides stretchingof the c-lattice parameter not only the density of the carriers but also their signs change in these isostructuraldi-borides. The thermoelectric power of these compounds clearly demonstrates their changingπ and σ band contributions and the ensuing appearance/disappearance of superconductivity. An increasedc parameter increases the boron plane constructed hole typeσ band population and decreases the contribution from the Mg or Al plane electron typeπ band. This turns thehole type (mainly σ band conduction) MgB2 superconductor (39 K) into the electron type (mainlyπ band conduction)non-superconducting AlB2. Theimportance of hole type σ band conduction dominating the superconductivity of the various boridesis further established by the high performance of intrinsically pinnedMgB2−xCx. Our resultson MgB2 addedwith nano-diamond, nano-SiC and various organics such as glucose, PVA and adipic acid, when comparedwith MgB2−xCx, clearly demonstrate that the main role is played by C substitution at the B site in the hostMgB2 and theensuing σ plane disorder and vortex pinning. The best strategy could be to add(<10 nm)nanoparticles to MgB1.8C0.2 to ensure both extrinsic pinning by the former and intrinsic pinning by the latter.

Critical magnetic field H c2 and electron scattering in MgB2

The correlation between the upper critical field Hc2 and the residual resistivity ρ of the MgB2 compound is studied in a wide range of ρ values. The slope −dHc2/dT of the temperature dependence of Hc2 near Tc is found to increase steadily with increasing residual resistivity to 100 μΩ cm. Over the range from 0 to 50 μΩ cm, the dependence of −dHc2/dT on ρ is fitted well by a linear function, which is typical of a single-band superconductor. The fit is performed using the electronic parameters of the two bands (π and σ) that form the Fermi surface. The field Hc2 is assumed to depend on the electronic parameters of the σ band only. Using this approximation, the following quantities are estimated: the relative contribution of σ electrons to the total conductivity along the boron planes (which turns out to be about 1/2), the ratio of the mean free paths of electrons in the σ and π bands lσ/lπ ≈ 1.5, and the ratios between some other parameters describing electron scattering.

Anisotropic optical conductivity and two colors ofMgB2

We present the anisotropic optical conductivity of MgB$_{2}$ between 0.1 and 3.7 eV at room temperature obtained on single crystals of different purity by the spectroscopic ellipsometry and reflectance measurements. The bare (unscreened) plasma frequency $\omega_{p}$ is almost isotropic and equal to 6.3 eV, which contrasts some earlier reports of a very small value of $\omega_{p}$. The data suggests that the $\sigma$-bands are characterized by a stronger electron-phonon coupling $\lambda_ {tr}$ but smaller impurity scattering $\gamma_{imp}$, compared to the $\pi$-bands. The optical response along the boron planes is marked by an intense interband transition at 2.6 eV, due to which the reflectivity plasma edges along the a- and c-axes are shifted with respect to each other. As a result, the sample spectacularly changes color from a blueish-silver to the yellow as the polarization is rotated from the in-plane direction towards the c-axis. The optical spectra are in good agreement with the published {\it ab initio} calculations. The remaining discrepancies can be explained by the relative shift of $\sigma$-bands and $\pi$-bands by about 0.2 eV compared to the theoretical band structure, in agreement with the de Haas-van Alphen experiments. The widths of the Drude and the interband peaks are both very sensitive to the sample purity.

Observation of Interband Pairing Interaction in a Two-Band Superconductor:MgB2

The recently discovered anisotropic superconductor MgB2 is the first of its kind showing the intriguing properties of two-band superconductivity. By tunneling experiments using thin film tunnel junctions, electron-coupled phonon spectra were determined showing that superconductivity in MgB2 is phonon mediated. In a further analysis, which involves first principles calculations, the strongest feature in these spectra could be traced back to the key quantity of two-band superconductivity, the interband pairing interaction. For the phonons, this interaction turns out quite selective. It involves mainly low-energy optical phonon modes, where the boron atoms move perpendicular to the boron planes.

Influence of disorder on H c 2 ‐anisotropy and flux pinning in MgB 2

The upper critical field and flux pinning in MgB2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Disorder, introduced by irradiation with neutrons, enhances the upper critical field, reduces the anisotropy and drastically changes flux pinning. While the enhanced Hc2 and the reduced anisotropy generally improve the transport properties of the polycrystalline samples, the contribution of the radiation-induced defects to flux pinning is small compared to the as-grown defect structure (grain boundary pinning).

Superconducting gap structure and pinning in disorderedMgB2 films

We have performed a comparative study of two thin films of magnesium diboride(MgB2) grown by different techniques. The critical current density at different temperatures andmagnetic fields was evaluated from magnetization curves, the structure of thesuperconducting order parameter was obtained from point-contact spectroscopyand the scattering rates were evaluated by fitting the temperature dependentnormal-state resistivity to the two-band model. The films have similar criticaltemperatures close to 39 K, but the upper critical fields were different by afactor of 2 (5.2 and 2.5 T at 20 K). We have found that the film with higherHc2 also had strongerscattering in the σ band and a smaller value of the superconducting gap in this band. As the scattering in theσ band is primarily due to the defects in the boron plane, our results are consistentwith the assumption that disordering the boron planes leads to enhancedHc2 and better pinning properties in a magnetic field.