Point-contact spectroscopy of LuB₁₂

    We have investigated LuB₁₂ point-contacts between 0.1 K to 4.2 K using spear-anvil type technique and mechanically controllable break junctions. Figure 3 shows typical dU/dI(U) spectra at 0.1 K for various normal state resistances R_N.
Distinct structures due to superconductivity (SC) appear at voltages very close to 2D ₀/e » 0.12 mV when 5W Ł R_N Ł 200W . The ratio 2D₀ /k_BT_c» 3.5 indicates a BCS type SC. At larger R_N the SC anomalies are difficult to resolve, at smaller R_N the non-linearities appear at lower voltages that tend to decrease with decreasing R_N. We found the critical current I_c of the junctions to be equal to (in the R_N» 10-40W range) or smaller than (for R_N<10W) the expected dirty-limit value I_c=(1.32pD/2eR_N)tanh(D/2k_BT). This indicates that for contacts in the 10-40W range R_N agrees reasonably well with the ideal ballistic Sharvin resistance. At large R_N the supercurrent is strongly suppressed. The reduction of I_c at lower R_N can, like the SC energy gap, originate from the suppression of SC in the contact region by the self-magnetic field, from local heating, or from deviations from Sharvin's relation due to a reduced electronic mean free path.
    In the normal state the second derivative d²U/dI²(U) of the current-voltage characteristic of a ballistic (l > a) PC is proportional to the electron-phonon interaction (EPI) spectral function g_PC(eU) =(a _PC)²F(eU). Here (a_PC)² represents the average EPI matrix element with kinetic restrictions imposed by the contact geometry, and F(eU) is the phonon density of states.
    We have recorded d²U/dI²(U) spectra of LuB₁₂ junctions with 1W ŁR_NŁ 150W. Contacts with low resistance (spear-anvil setup) were measured at 4.2K in liquid ⁴He, contacts with high resistance (break-junction setup) in the dilution refrigerator at T»0.5 K. The spectra show characteristic peaks at certain energies which can be attributed to various phonon modes. Whereas the low-resistance contacts had peaks at ~ 14meV, ~ 24meV, ~ 29meV, and ~ 38meV, the high-resistance contacts had only a peak at a low energy of ~ 2.5meV and one at ~ 14meV. We have currently no explanation for this discrepancy.
    The spectra of low-resistance contacts are in agreement with phonon modes received from neutron scattering experiments, and a rough comparison can be made also with the phonon modes of LaB₆, which is used as a reference material for hexaborides, and is both metallic and non-magnetic. Its phonon modes are at 12.2-15.1meV, 24.5-26.6meV, 37-40.8meV, and 49.6-53.8meV. The position of LuB₁₂ modes at ~ 14meV, ~ 24meV, and ~ 38meV in low-resistance PCs seem to show similarity between the phonon modes of both compounds. However, based on similarity of the phonon modes it is not clear why LaB₆ is down to 10 mK non-superconducting and LuB₁₂ exhibits a superconducting transition at T_c»0.4K.

Figure 1: Typical dU/dI(U) spectra at T=0.1 K. Verticalbars indicate the BCS-value for T_c=0.4K.

K. Flachbart, P. Samuely, P. Szabó
K. Gloos (Tech. Univ. Darmstadt), Y. Paderno, N. Shitsevalova (IPM UAS Kiev)