MAGNETIC-FIELD CONTROL OF AN ELECTRON TUNNEL CURRENT THROUGH A MOLECULAR WIRE

The influence of an external magnetic field on the elastic interelectr ode tunneling mediated by a molecular wire is studied theoretically fo r the case in which the wire contains paramagnetic ions. A spin-filtra tion effect for incoming electrons and a spin-polarization effect for outgoing electrons can be demonstrated for a wire including a single p aramagnetic ion as well as a pair of identical antiferromagnetically c oupled ions. It is assumed that each paramagnetic ion reduces its spin in the electronic ground-state from S to S-1/2 if the transferred ele ctron forms an intermediate bound state with the ion. Just such a spin reduction results in a transfer which is spin forbidden for spin-down electrons along the predominant tunneling channels. These channels ar e characterized by the lowest possible spin-projections of the paramag netic ions. The spin reduction also determines the magnetic field depe ndence of the transfer rate in a specific manner. In the case of two p aramagnetic ions the combined action of the magnetic field and the exc hange interaction between the ions is responsible for a step-like depe ndence of the tunnel current on the magnetic field. The exact dependen ce of the interelectrode current on the magnetic field-strength at a l ow temperature is derived in using Wigner's 6j-symbols methods. The sp ecific spin-polarization effect observed earlier in metal-ferromagneti c insulator-vacuum experiments on electron tunneling is explained in t he framework of a spin-filtration effect at which the predominant tunn eling channels are responsible for a tunnel interelectrode current. (C ) 1998 American Institute of Physics.