Point-contact spectroscopy of the phononic mechanism of superconductivity in YB₆

Abstract

Lortz et al. [Phys. Rev. B 73, 024512 (2006)] have utilized specific heat and resistivity measurements as "thermal spectroscopies" to deconvolve the spectrum of the electron-phonon interaction in YB6 assuming a major role of the low frequency phonon mode in mediating superconductivity. Here, we present direct point-contact spectroscopy studies of the superconducting interaction in this system. As a result the normalized superconducting gap reveals a strong coupling with 2∆/kBTc = 4 and moreover the spectra contain nonlinearities typical of the electron-phonon interaction at energies around 8 meV. The in-magnetic-field measurements evidence that the phonon features found in the second derivative of the current-voltage characteristics are due to the energy dependence of the superconducting energy gap as their energy position shrinks equally as the gap is closing. This is a direct proof that the superconducting coupling in the system is due to the low energy Einstein-like phonon mode associated with the yttrium ion vibrations in a perfect agreement with determinations from bulk measurements.

Lattice instability and enhancement of superconductivity in YB6

“…T the low temperature heat capacity of investigated YB 6 crystals. The received value Θ E ≈ 97.2 K is consistent with the results of point-contact [29] and tunneling [30] spectroscopy measurements (ω E ∼ 8 meV), and with inelastic neutron scattering (ω E ∼ 10 meV) [46] and Raman scattering (ω E ∼ 70 cm −1 ) [47] data. The value Θ D ≈ 1160 K coincides with the results of heat capacity calculations for LaB 6 [41]- [44].…”

“…5] and the associated problem with the determination of the Sommerfeld coefficient, the estimation of γ was obtained from the relation γT 2 c /H 2 cm (0) = const [3]. The ratio of 2∆/k B T c ≈ 3.9÷4 found in this study for all studied YB 6 samples coincides with results obtained both in [3], with the heat capacity analysis of [29,30] and with the pointcontact and tunnel spectra of [29,30], and it significantly exceeds the value of 3.52 of the BCS model. It is also worth noting that the smaller values of 2∆/k B T c ≈ 3.8 found in [31] and 3.6 in [32] from ultra-high resolution photoemission spectra at 5 K and tunneling spectra at 4.3 K, respectively, obviously may be attributed to gap ∆(T ) lowering at about 5 K [close to T c ≈ 7 K] in comparison with ∆(0).…”

“…where Φ 0 denotes the flux quantum, the Ginzburg-Landau-Maki parameter κ 1 (T ) [28] (Fig.12), the superconducting gap ∆(0), the coherence length ξ(0) and the penetration depth λ(0) = κ 1 (0)/ξ(0) [ Fig.5] and the associated problem with the determination of the Sommerfeld coefficient, the estimation of γ was obtained from the relation γT 2 c /H 2 cm (0) = const [3]. The ratio of 2∆/k B T c ≈ 3.9÷4 found in this study for all studied YB 6 samples coincides with results obtained both in [3], with the heat capacity analysis of [29,30] and with the pointcontact and tunnel spectra of [29,30], and it significantly The analysis of magnetization was carried out based on formulas which are well-known from the Abrikosov theory of type-II superconductivity [33]…”

First Principles Study on the Electronic Structure and Optical Properties of La Doped YB6 Crystals

“…Among them, yttrium hexaboride (YB 6 ), an extremely hard, refractory and stable rare-earth hexaboride, has been also applied as solar radiation shielding materials in windows for its high optical absorption coefficient in near-infrared (NIR) range (around 1000 m) and ultraviolet (UV) range (around 350 nm), and high transmittance in visible region. To our knowledge, a lot of researches have been conducted on thermoelectricity [6], super-conductivity [7] and optical performance [8,9] of YB 6 . Even though electronic structure and optical properties of pure YB 6 have been reported by Xiao [8,9], there are no relevant researches on La doped YB 6 .…”

Influence of Pressure on Superconductivity in YB_{6}

“…In recent years some of the properties of YB 6 have been intensely investigated, such as the specic heat, resistivity, magnetic susceptibility and thermal expansion [2], optical properties [3], the electronic band structure [4,5] and point-contact spectra [6]. From these studies we can conclude that YB 6 is a conventional type-II BCS superconductor, where the strong coupling in Cooper pairs with 2∆/k B T c ≈ 4.0 is mediated by the phonon mode of Y atoms located at ≈ 8 meV.…”