Tunneling through rare earth trihydroxide barriers

Symmetric tunneling junctions with 4000 Å thick Pb electrodes and polycrystalline insulating barriers of Lu(OH)₃, Er(OH)₃, and Ho(OH)₃ have been fabricated. In bulk, these three rare earth trihydroxides RE(OH)₃ are nonmagnetic, antiferromagnetic (T_N<1.1 K), and ferromagnetic (T_c=2.54 K), respectively. A three-fold splitting of the conductance peak at the gap was observed only in junctions with Ho(OH)₃ barriers. The gap peak of junctions with Er(OH)₃ barriers was broadened significantly relative to that of junctions with Lu(OH)₃ barriers. From measurements of the temperature and magnetic field dependence of the tunneling conductance it is argued that the splitting in junctions with Ho(OH)₃ barriers is consistent with the existence of a peak in the electronic density of states at an energy below that of the gap of each of the electrodes. This peak is believed to be the signature of a bound state near the barrier resulting from the exchange coupling between the electron spins of the superconductor and the localized spins of the barrier.