Infrared studies of superconducting MgB2 thin films

Superconducting pairing and the two-gap nature of Al and C doped MgB2: screened phonon-pairing mechanism

“…Switching to the parameters related to the ions, a reasonable value of the background dielectric constant reported [19] in MgB 2 is ε ∞ ∼ = 3.5. The optic-mode frequencies, on the other hand, can be estimated in an ionic model using a value of Z = 2.…”

“…It is worth mentioning that strong couplings of the carriers with the boron vibrations are shown in the electron-tunneling spectra [9,10]. This evidence relating the coupled phononelectron excitations associated with the boron in MgB 2 together with inelastic x-ray scattering [17], electron-energyloss-spectroscopy [18] reflectance studies [19] and theoretical studies on dielectric response as function of momentum transfer [20,21] of MgB 2 , have provided the motivation for the present work. We may refer to our earlier work for cuprates, bismuthates [23] and for MgB 2 [24] in a pairing interaction mediated by charge transfer oscillations associated with phonons.…”

“…The charge fluctuation mechanism is emphasized in diboride superconductors by their strong anisotropy and by the presence of a layer stacking sequence. Inelastic x-ray scattering [7], electron-energy-loss-spectroscopy [18] and optical reflectivity measurements [19] have been successfully used to investigate the electronic structure of the MgB 2 superconductor. The energy loss spectra for low momentum transfer predict a free-electron-like plasmon just below 20 eV and low energy collective electron excitations around 3 eV.…”

Multiband and impurity effects in infrared and optical spectra of MgB2

“…Further experiments on thin films [49,50,51,52,53,54], thick films [18], single crystals [46,24] and dense polycrystals [19,55] provided much more detailed and quantitative spectral information that in general supported such assignment ( Table 1). Note that this gap value gives an anomalously small ratio 2∆/k B T c ∼ 1 − 2 as compared to the weak-coupling BCS result of 3.52.…”

“…An early far-infrared grazing-incidence reflectivity measurement by Gorshunov et al [48] on a porous polycrystalline sample of MgB 2 (T c =39 K) revealed a superconductivity-induced rise of reflectivity with a max- imum effect at about 3-4 meV, which was tentatively attributed to the lowest value of 2∆. Further experiments on thin films [49,50,51,52,53,54], thick films [18], single crystals [46,24] and dense polycrystals [19,55] provided much more detailed and quantitative spectral information that in general supported such assignment (Table 1). Note that this gap value gives an anomalously small ratio 2∆/k B T c ∼ 1 − 2 as compared to the weak-coupling BCS result of 3.52.…”