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Register a new userRecovery of the electron-phonon interaction function in superconducting tantalum ballistic contacts
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N. L. Bobrov
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The experimentally observed nonlinearities of the current-voltage characteristics (CVCs) of tantalum-based point homo- and hetero- contacts in both normal and superconducting states related to electron-phonon interaction (EPI) were analyzed. It was taken into account that additional nonlinearity of CVCs arising upon contact transition to the superconducting state (superconducting spectral component) is formed not only near the constriction in the region roughly equal to the contact diameter (as is the case for the normal state, and as predicted theoretically for the superconducting state), but also in a markedly larger region that is about the size of the coherence length. In this case, a considerable role in the formation of this superconducting component is played by nonequilibrium phonons with low group velocity, which account for the experimentally observed sharpening of the phonon peaks in the EPI spectra (the second derivatives of the CVCs) during the superconducting transition of the contacts, instead of the theoretically expected peak broadening (spreading), and for the increase in the superconducting contribution to the point contact spectrum in the low and medium energy regions. The high-energy part of the EPI spectrum changes much less significantly during the superconducting transition, which is attributable to suppression of the excess contact current by nonequilibrium quasi-particles. A detailed procedure was proposed for the recovery of the EPI spectral function from the point contact spectrum contribution (the second derivative of the CVC) that arises during the superconducting transition of one or both contacting metals.
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The recovering procedure of the electron-phonon interaction (EPI) functions from the additional nonlinearities of the current-voltage curve ($I-V$ curve) of point contacts associated with an excess current is considered. The approach proposed takes into account both inelastic scattering, which causes suppression of the excess current in the reabsorption of nonequilibrium phonons by electrons undergoing Andreev reflection (Andreev electrons), and elastic processes associated with the electron-phonon renormalization of the energy spectrum in a superconductor. The results obtained are systematically expounded for both the ballistic contacts, wherein the second derivatives of the $I-V$ curve in the normal state are proportional to the EPI functions, and inhomogeneous contacts (with dirty constrictions and clean banks), whose second derivatives in the normal state are either free of phonon singularities or weakly pronounced.
The application of inhomogeneous niobium point-contacts in the superconducting stata for reconstructing the electron-phonon interaction function is considered. The method is based on the use of the nonlinearity of current-voltage curve, which is due to the inelastic processes of suppressing excess point contact current when nonequilibrium phonons are scattered from the electrons undergoing Andreev reflection. A new model of background occurrence in point-contact Yanson spectra and some ways to correct this background are proposed.
In the ballistic regime, the transport across a normal metal (N)/superconductor (S) point-contact is dominated by a quantum process called Andreev reflection. Andreev reflection causes an enhancement of the conductance below the superconducting energy gap, and the ratio of the low-bias and the high-bias conductance cannot be greater than 2 when the superconductor is conventional in nature. In this regime, the features associated with Andreev reflection also provide energy and momentum-resolved spectroscopic information about the superconducting phase. Here we theoretically consider various types of N/S point contacts, away from the ballistic regime, and show that even when the superconductor under investigation is simple conventional in nature, depending on the shape, size and anatomy of the point contacts, a wide variety of spectral features may appear in the conductance spectra. Such features may misleadingly mimic theoretically expected signatures of exotic physical phenomena like Klein tunneling in topological superconductors, Andreev bound states in unconventional superconductors, multiband superconductivity and Majorana zero modes.
For the 40K-superconductor MgB2 we have calculated the electronic and phononic structures and the electron-phonon interaction throughout the Brillouin zone ab initio. In contrast to the isoelectronic graphite, MgB2 has holes in the bonding sigma-bands, which contribute 42 per cent to the density of states: N(0) =0.355 states/(MgB2 eV spin). The total interaction strength, lambda =0.87 and lambda,tr=0.60, is dominated by the coupling of the sigma-holes to the bond-stretching optical phonons with wavenumbers in a narrow range around 590 cm^{-1}. Like the holes, these phonons are quasi two-dimensional and have wave-vectors close to Gamma-A, where their symmetry is E. The pi-electrons contribute merely 0.25 to lambda and to lambda,tr. With Eliashberg theory we evaluate the normal-state resistivity, the density of states in the superconductor, and the B-isotope effect on Tc and Delta0, and find excellent agreement with experiments, when available. Tc=40 K is reproduced with mu*=0.10 and 2Delta0/kB Tc=3.9. MgB2 thus seems to be an intermediate-coupling e-ph pairing s-wave superconductor.
Using Ta, 2H-NbSe2 and MgB2 as an example it is shown that it is possible to reconstruct qualitatively a function of the electron-phonon interaction from point-contact spectra in a superconducting state. The limits and the restrictions of this method are also shown. The results obtained are compared with available literature data.
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