Polarization dependent X-ray spectra of MgB2 (original) (raw)

Electronic structure of MgB2: X-ray emission and absorption studies

Physical Review B, 2002

Measurements of x-ray emission and absorption spectra of the constituents of MgB2 are presented. The results obtained are in good agreement with calculated x-ray spectra, with dipole matrix elements taken into account. The comparison of x-ray emission spectra of graphite, AlB2, and MgB2 in the binding energy scale supports the idea of charge transfer from σ to π bands, which creates holes at the top of the bonding σ bands and drives the high-Tc superconductivity in MgB2.

Inelastic X-ray scattering and first-principles study of electron excitations in MgB2

Solid State Communications, 2009

An experimental and theoretical study of electronic excitations in MgB 2 covering the domain of large energy and momentum transfers is reported. Energy-loss spectra for several values of momentum transfers were measured in a polycrystalline sample by means of inelastic X-ray scattering spectroscopy. Ab initio calculations of the dielectric function as well as the energy-loss function were performed in the frame of the time-dependent local density approximation with inclusion of crystal local-field effects. We obtained very good agreement between the experimental and the theoretical energy dispersion of the peak maximum of the loss function. We found that crystal local-field effects are responsible for this agreement at large momenta. Fine structure observed in the measured spectra was interpreted in terms of strong interband transitions predicted by the calculations in the Γ A and Γ K directions. The theoretical dispersion of these features is in good accordance with the experimental data. Further spectral features in the measured spectra due to Mg 2s and 2p core electron excitations are also discussed.

X-ray photoelectron spectroscopy studies of MgB2 for valence state of Mg

Physica C: Superconductivity and its Applications, 2005

Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystalline MgB 2 pellets over the whole binding energy range with a view to having an idea of the charge state of Magnesium (Mg). We observe 3 distinct peaks in Mg 2p spectra at 49.3 eV (trace), 51.3 eV (major) and 54.0 eV (trace), corresponding to metallic Mg, MgB 2 and MgCO 3 or, divalent Mg species respectively. Similar trend has been noticed in Mg 2s spectra. The binding energy of Mg in MgB 2 is lower than that corresponding to Mg(2+), indicative of the fact that the charge state of Mg in MgB 2 is less than (2+). Lowering of the formal charge of Mg promotes the σ→π electron transfer in Boron (B) giving rise to holes on the top of the σ-band which are involved in coupling with B E 2g phonons for superconductivity. Through this charge transfer, Mg plays a positive role in hole superconductivity. B 1s spectra consist of 3 peaks corresponding to MgB 2 , boron and B 2 O 3. There is also evidence of MgO due to surface oxidation as seen from O 1s spectra.

SOFT X-RAY ABSORPTION SPECTROSCOPY OF THE MgB 2 BORON K EDGE IN AN MgB 2 /Mg COMPOSITE

Modern Physics Letters B, 2006

Soft X-ray absorption spectroscopy (XAS), using fluorescence yield, was used to study the boron K near edge in MgB2 superconductor. The sample consists of MgB2 crystallites randomly oriented in a magnesium matrix. Abrasion of the sample surface in vacuum provides a surface relatively free of boroncontaining impurities. The intrinsic boron K near edge spectrum of the sample at a temperature of 295 K is identified. This spectrum is then compared in detail with a spectrum calculated using the full potential linearized augmented plane wave method. Features predicted by the theory appear near the expected energies, with qualitative agreement regarding shape and intensity.

First-principles calculations of dielectric and optical properties of MgB2

Physical Review B, 2008

We report on the results of calculation of low-energy dielectric response and optical properties of MgB 2. The calculations have been performed with full inclusion of the ab initio electron band structure making use of random-phase and time-dependent local-density approximations. The role of local-field and exchangecorrelation effects in MgB 2 dielectric function is thoroughly examined. Index of refraction, extinction coefficient, and normal-incidence reflectivity exhibiting strong anisotropy were calculated for electromagnetic waves polarized along the a ‫ء‬ and c ‫ء‬ axes by using the evaluated electron-density-response functions. Our results are in good agreement with recent x-ray and optical measurements performed on MgB 2 single crystals.

Valence electron distribution in MgB2 studied by accurate diffraction measurements and first principle calculations

We use synchrotron x-ray and precision electron diffraction techniques to determine accurately the structure factors of selected reflections that are sensitive to the valence electron distribution in the superconductor MgB 2 . These values deviate significantly from calculated structure factors using the scattering factors of free atoms, but agree well with our calculated structure factors based on density functional theory. Having experimentally established the reliability of our firstprinciple calculated structure factors, we present electron density maps of the redistribution of the valence electrons that takes place when hypothetical free atoms of Mg and B in MgB 2 interact to form the real crystal.

Comparative Experimental and Density Functional Theory (DFT) Study of the Physical Properties of MgB2 and AlB2

Journal of Superconductivity and Novel Magnetism, 2011

In present study, we report an inter-comparison of various physical and electronic properties of MgB 2 and AlB 2 . In particular the results of phase formation, resistivity (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 MgB 2 shrinks in c-direction for AlB 2 with a = 3.006 Å, and c = 3.254 Å. The resistivity (T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB 2 . Superconductivity is not observed for AlB 2 . Interestingly, the sign of S(T) is +ve for MgB 2 the same is -ve for AlB 2 . This is consistent our band structure plots. We fitted the experimental specific heat of MgB 2 to Debye Einstein model and estimated the value of Debye temperature ( D ) and Sommerfeld constant ( ) for electronic specific heat. Further, from 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 MgB 2 in the strong coupling BCS category. The electronic specific heat of MgB 2 is also fitted below T c using -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 MgB 2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB 2 and hence it is not superconducting. DFT calculations demonstrated that for MgB 2 the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB 2 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 AlB 2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB 2 and AlB 2

DENSITY OF STATES, CHARGE TRANSFER, AND OPTICAL PROPERTIES OF MAGNESIUM DIBORIDE

International Journal of Modern Physics B, 2002

We performed ab-initio, local density functional calculations of the electronic structure, charge transfer, and optical properties of MgB 2 , using the LCAO formalism. The Fermi level of MgB 2 cuts through relatively narrow electron bands which have a dominant contribution from B(2p) states. There is a substantial charge transfer from magnesium to boron atoms. We found the ionic formula for this material to be Mg 1.68+ B 0.84− 2 . A clearly metallic distribution of the electronic charge density in the plane of boron atoms is interwoven with a visibly covalent one in the direction perpendicular to this plane. The calculated optical conductivities from the direct inter-band transitions exhibit a strong anisotropy between σxx(ω) or σyy(ω) and σzz(ω). Due to our application of the BZW procedure, major peaks in the density of states above the Fermi level are at markedly higher energies (1-1.5 eV) than the results of previously reported ones. A similar pattern is followed by our findings for optical conductivities.

Absence of a boron isotope effect in the magnetic penetration depth of MgB2

Physical Review B, 2004

The magnetic penetration depth λ(0) in polycrystalline MgB2 for different boron isotopes ( 10 B/ 11 B) was investigated by transverse field muon spin rotation. No boron isotope effect on the penetration depth λ(0) was found within experimental error: ∆λ(0)/λ(0) = −0.8(8)%, suggesting that MgB2 is an adiabatic superconductor. This is in contrast to the substantial oxygen isotope effect on λ(0) observed in cuprate high-temperature superconductors.