Phonon singularities on volt-ampere curves of niobium point contacts

Abstract

The volt-ampere curves and their second derivatives were studied for niobium point contacts at low temperatures in the voltage range corresponding to the characteristic phonon energies. It was found that while for the dirty contacts in the normal state no PC spectra of phonons could be detected, in the superconducting state there were singularities in the I-V curves corresponding to maxima either in the first or in the second derivatives. The singularities observed were due to the energy dependence of the excess current. We suppose that the origin of these singularities is due to the inelastic transitions of electrons between chemical potentials of Cooper pairs at both sides of the contact, which differ in energy by $eV$. These transitions are possible if $\xi \left(0 \right)\gtrsim d \sim{{\Lambda}_{\varepsilon}}$ ($\xi (0)$ being the coherence length, $d$, the contact diameter, ${{\Lambda}_{\varepsilon}}={{({{l}_{i}}\cdot {{l}_{\varepsilon}})}^{1/2}}$, where ${{l}_{i}}$ and ${{l}_{\varepsilon}}$ being the elastic and inelastic electron mean free paths, respectively).Comment: 5 pages, 3 figure

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

“…Phonon peaks are smeared: while the first peak is located at approximately 16 meV, which corresponds to the position of the maximum on the EPI tunneling function, the high-frequency peak is shifted to the region of higher energies, from 22 to 25 meV. Such behavior previously observed for some spectra in the normal state [24,25] was associated with inhomogeneity of the material in the region where the 𝐼 − 𝑉 curve nonlinearity is formed, in particular, with the presence of impurities in the form of conducting niobium oxides.…”

“…Several hundred contacts have been explored, the overwhelming majority of whose spectra correspond to the dirty limit. Only a few point-contact spectra in the normal state exhibited the presence of phonon singularities, and their quality was rather low [24,25]. It is all the more interesting to confirm the feasibility of reconstructing the EPI function using inhomogeneous point contacts.…”

“…This surge corresponds to a stepwise decrease in the excess current due to a sharp decrease in the superconducting gap in the near-contact region. The reason for this behavior of the gap is that the critical density of nonequilibrium quasiparticles is attained in the near-contact region [13][14][15]. Since the energy relaxation length in the contacts under consideration is much greater than their diameter, most of the electrons lose energy in the banks, being scattered on nonequilibrium phonons.…”

“…Kulik [7]. Experimental studies, which showed the transformation of point-contact spectra in the transition of contacts to the superconducting state, appeared almost at the same time (see, for example, [8][9][10][12][13][14][15][16]). However, the first experimental study in which the EPI function was reconstructed based on the difference between the second derivatives of the I-V curve of a point contact in the superconducting and normal states (below, for brevity, using the superconducting addition to the spectrum) was published almost 30 years later, in 2012 [17].…”

Electron Transport in Magnetic Quantum Point Contacts

“…The propagation of electron waves through such a junction reveals information on its chemical nature and electronic structure as well as mechanical properties [1]. This method has been applied to study metallic conductors [2][3][4], superconductors [5][6][7][8] and individual molecules [9][10][11][12] as well as carbon nanotubes [13] among others to understand multiple phenomena that can be observed solely at the nanoscale, including conductance quantization, the Coulomb blockade and the Andreev reflection.…”