Anomalous Coherence Peak in the Microwave Conductivity ofc-Axis OrientedMgB2Thin Films

Abstract

Abstract----The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB 2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on BardeenCooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB 2 . Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by bandstructure calculations. In addition, the observation of a coherence peak indicates that the π-band is in the dirty limit and dominates the total conductivity of our films.

Multiband and impurity effects in infrared and optical spectra of MgB2

“…To get the full picture of the electromagnetic response of MgB 2 in the superconducting state one would have to complement the discussed spectroscopic data with the temperature dependence of σ 1 and σ 2 (usually expressed via the penetration depth λ) as measured by microwave [60,61,62] and submillimeter [63] techniques. Interestingly, the structure of the so-called coherence peak is highly anomalous for a single gap superconductor, revoking the explanation in terms of two gaps (some of these measurements are reviewed in Ref.…”

Superconductivity of Magnesium Diboride: Theoretical Aspects

“…A possible explanation could be a stronger scattering rate in the σ band at least in the thin film samples studied in Ref. [33]. To illustrate that, in Fig.…”

“…In Ref. [33] the observation of an anomalous microwave conductivity peak in MgB 2 thin films has been reported and its interpretation in terms of two gap behavior shall be discussed here. We will first start with a review of conductivity peaks in conventional superconductors as well as in high-T c cuprates and then compare them with the results in MgB 2 .…”

“…Similar observations, that larger interband pairing strengths than found by band structure calculations are needed to fit experimental data, have been made also in Refs. [33] and [34]. Possible explanations could be either that band structure calculations underestimate the interband pairing strength or that some small amount of interband impurity scattering or effects of strong electron-phonon coupling, which were neglected in the above calculations of the upper critical field, mimic a somewhat stronger interband pairing strength.…”

“…In Ref. [33] an intermediate approach was taken: the partial densities of states and the cut-off frequency were taken from band structure calculations, while the remaining parameters were adjusted to the T c of the sample and the value of the small gap, which could be extracted from the exponential fall-off of the temperature dependent change of the penetration depth. A typical temperature dependence of the two gaps found this way is shown in Fig.…”