Planar Tunneling into SmB₆ Using 2^nd Harmonic Detection in Low Applied Magnetic Fields

Planar tunneling into self-oxidized samarium hexaboride (SmB₆) using lead (Pb) as a counter-electrode has revealed Dirac cones which do not persist throughout the entire bulk insulating gap, along with features which are asymmetric with respect to tunneling conductance bias [1]. We have used 2^nd harmonic detection which increases the resolution and sensitivity of measurement. Additionally, we substitute junction materials to test the intrinsic nature of the reported features.

We fabricate tunnel junctions by plasma oxidizing SmB₆ to create the tunnel barrier and then deposit aluminum (Al) as the counter-electrode. With this new counter-electrode material we no longer need to apply a magnetic field to drive the Pb counter-electrode normal (~0.2T), allowing us to study the low-field spectra. Previously, Pb was used because of its well-known superconducting density of states, thus providing a measure of the junction quality. With our experience of tunneling into SmB₆ we use the quality of the SmB₆ tunneling density of states as a powerful diagnostic of junction quality. These data are analyzed by considering spin fluctuations and other magnetic excitations within the SmB₆ [2,3].

[1] W K Park et al., PNAS, 113, 6599 (2016)

[2] P K Biswas et al., Phys. Rev. B 95, 020410 (20147)

[3] G A Kapilevich et al, Phys. Rev. B 92, 085133 (2015)