Ultraclean metal-like conductivity in semimetallic WP2
Ultraclean metals show high conductivity with a high number of charge carriers, whereas semiconductors and semimetals with low charge carriers normally show a low conductivity. This scenario in semimetals can be changed if one can protect the carriers from scattering.
In a recent study, scientists from the Max Planck Institute for Chemical Physics of Solids in Dresden, in collaboration with High Field Magnet Laboratory (HFML-EMFL), Netherlands; Dresden High Magnetic Field Laboratory (HLD-EMFL) and Paul Scherrer Institute, Switzerland show extremely large conductivity in a semimetal, WP2. The conductivity of ~ 3 x 108 W-1cm-1 in WP2 at 2 K is comparable to highly conducting metals like potassium and copper of the similar purity.
The authors identified two major reasons for diminished scattering events in WP2 and the sister compound MoP2. First, these compounds contain robust Weyl points (difficult to annihilate), which means that backscattering events of charge carriers are less possible compared to conventional metals. Second, the hydrodynamic effect at low temperatures ensures that the charge carriers are invisible to certain lattice defects and pass through them without getting scattered because they travel rather like a fluid. The consequence in WP2 is a very large carrier mobility (4 x 106 cm2/Vs) and spectacular sub-millimetre mean free path (~ 106 unit cells of WP2). The mean free path is the average distance an electron can travel without getting scattered. Because of the semimetallic nature of WP2 and MoP2, number of electrons and holes are almost equal, which additionally provide for a highly sensitive resistivity (or conductivity) towards the applied magnetic field which is otherwise not possible in a conventional metal with a single type of carriers. Hence, we observe a record breaking conductivity and magnetoresistance (change in resistivity, 200 million % at 63 T field) present together in a compound, WP2.