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 RN.
Distinct structures due to superconductivity (SC) appear at voltages
very close to 2D 0/e » 0.12
mV when 5W £ RN
£ 200W . The ratio 2D0
indicates a BCS type SC. At larger RN the SC anomalies
are difficult to resolve, at smaller RN the non-linearities
appear at lower voltages that tend to decrease with decreasing RN.
We found the critical current Ic of the junctions to
be equal to (in the RN» 10-40W range) or smaller than (for RN<10W) the expected dirty-limit value Ic=(1.32pD/2eRN)tanh(D/2kBT). This indicates that for contacts in the 10-40W range RN agrees reasonably well with the ideal ballistic Sharvin resistance. At large RN the supercurrent is strongly suppressed. The reduction of Ic at lower
RN 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 gPC(eU) =(a PC)2F(eU). Here (aPC)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 £RN£
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 Tc»0.4K.
Figure 1: Typical dU/dI(U) spectra at T=0.1
K. Verticalbars indicate the BCS-value for Tc=0.4K.
K. Flachbart, P. Samuely, P. Szabó
K. Gloos (Tech. Univ. Darmstadt), Y. Paderno, N. Shitsevalova (IPM UAS Kiev)