Study of the Electron-Phonon Interaction in Metal DiboridesMeB₂(Me=Zr, Nb, Ta, Mg) by Point-Contact Spectroscopy

Abstract

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB2 (Me=Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter λ < ∼ 0.1 were estimated. The data are consistent with the measured surface phonon dispersion curves. The low λ value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB2 films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB2 a two-band model is exploited and the proximity effect in the k-space is suggested.

Theoretical study of electron-phonon interaction inZrB2andTaB2

“…20 In addition, the calculated Eliashberg function for ZrB 2 agrees well with the corresponding Eliashberg function obtained from pointcontact spectroscopy. 6,7 However, our calculations predict TaB 2 to superconduct with a T c ϳ12 K but so far no superconductivity 3 has been found ͑except for Ref. 1͒.…”

“…In particular, the strength of the electronphonon interaction in ZrB 2 and TaB 2 , as reflected in the average electron-phonon coupling constant , can be used to predict the possibility of superconductivity in the diborides. Experiments, based on point-contact spectroscopy, 6,7 indicate to be less than 0.1 for both ZrB 2 and TaB 2 , and thus preclude the possibility of superconductivity in these diborides. Here, we like to point out that the experimental determination of , as carried out in Refs.…”

Study of the new superconducting compound Zr0.97V0.03B2 with point-contact spectroscopy

“…In this work we focus on the study of the induced superconductivity in ZrB 2 by replacing Zr by small quantities of Vanadium. The highest transition temperature obtained at date was T C ≈ 9.3 K, obtained with a substitution of 3 to 4% of Vanadium following a procedure similar to the described by [21][22][23][24][25][26]. Our resulting composition was Zr 0.97 V 0.03 B 2 .…”

Spectroscopy of electron-phonon interaction of superconducting point contacts: experimental aspects

“…Contacts (Fig. 10a) were created by touching the 2 film with a bulk silver counter-electrode [32][33][34]. The film was highly oriented along the -axis directed perpendicular to the surface of the 2 3 substrate.…”

“…Finally, Fig. 11 displays the EPI function obtained by averaging over these five contacts in comparison with the point-contact functions for 𝑍𝑟𝐵 2 , which belongs to the same crystallographic group [33]. It can be seen that the functions are rather similar in shape and position of phonon singularities, which are located for the averaged spectrum at energies 3÷5 meV lower than those for 𝑍𝑟𝐵 2 , with the exception of the position of the first maximum, which is shifted to higher energies by ∼ 16 meV.…”