Conductance Spectra Of (Nb, Pb, In)/Nbp Superconductor/Weyl Semimetal Junctions

The possibility of inducing superconductivity in type-I Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by x-ray diffraction, scanning electron microscopy and energy-dispersive x-ray spectroscopy, and angle-resolved photoelectron spectroscopy techniques and by electron transport measurements. The mobility spectrum of the carriers was determined and it was found that there are four separate sharp peaks visible, which indicates that the carriers participating in the conductance have four different (almost discrete) mobilities. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nanometer-thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces as a function of the DC bias were investigated. Upon cooling of the devices during which the metals become superconducting, all three types of junctions show conductance increase, pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is satisfactorily described by modified Blonder-Tinkham-Klapwijk model. Unfortunately, the absolute value of the conductance is much smaller than that for the bulk crystal, indicating that the transmission occurs through only a small part of the contact area. An opposite situation occurs in In-NbP junction, where we observe very high and narrow peak at zero bias. The conductance at the peak reaches the bulk value, indicating that almost the whole contact area is transmitting and, additionally, a superconducting proximity phase is formed in the material. We interpret this as a result of indium diffusion into NbP, where the metal atoms penetrate the surface barrier and form very transparent superconductor-Weyl semimetal contact inside. However, further diffusion occurring already at room temperature leads to degradation of the effect, so it is observed only in the pristine structures. Despite this, our observation directly demonstrates possibility of inducing superconductivity in a type-I Weyl semimetal.