Magnetic-flux-induced conductance steps in microwires

Abstract

We predict a new type of steplike fluctuations in the magnetoconductance of three-dimensional ballistic microwires. These mesoscopic fluctuations, which appear on a scale corresponding to a small fraction of the quantum unit of magnetic flux 40=he/e, are a novel manifestation of the Aharonov-Bohm effect. The sharp conductance steps are caused by the shift of electronic levels through the Fermi level in a magnetic field. The flux-induced steps should be observable in, e.g. , submicrometer Bi whiskers at temperatures of order 0.1 K. Sample-characteristic fluctuations are predicted to appear in fields as low as a few gauss.In this paper we present a theory for the magnetoconductance of a three-dimensional (3D) microwire suspended between macroscopic leads. For a ballistic wire we predict sample-characteristic fluctuations in the form of sharp steps in the trace of the conductance as a function of a weak magnetic field. These mesoscopic conductance fiuctuations appear on a new scale corresponding to a longitudinal magnetic Aux through the wire of only a small fraction of the quantum unit @0=he/e. The sharp conductance steps are caused by the energy shifts of electronic levels in a magnetic field; the new scale is set by the fIux change needed to shift energy levels a distance of the order of the average spacing between quantized transverse energy levels in the 3D wire, EF j(kFa) (EF, k~i s the Fermi energy and wave vector; a is the radius of the wire). These shifts result in charge carrying singleparticle modes in the wire being "switched" on or off as energy levels move across the Fermi level and get (de)populated.The small magnetic fiux scale is characteristic for a wirea solid cylinderand does not appear in the ballistic hollow cylinders first studied. This is because the average spacing between transverse modes is larger by a factor kFa in such a geometry, which makes the corresponding magnetic scale coincide with the scale 4&o of the usual Aharonov-Bohm (AB) oscillations.The concept of a quantized conductance that changes in a steplike manner with the number of current-carrying modes is familiar from studies of quantum ballistic transport in constrained 2D electron gas systems (for a review see, e.g. , Ref. 2). In such systems a current can be forced to Aow between two large 2D reservoirs through a microconstriction defined by the electrostatic field of a split gate. For a smooth enough constriction adiabatic charge transport occurs in an integer number of effectively onedimensional modes that each contribute a quantum unit, G0=2e /h, to the total conductance. Without a magnetic field the number of active modes and hence the conductance is determined by the minimum width of the constriction, which can be conveniently controlled by a gate voltage.The effect of a magnetic field on the conductance of a microconstriction is also known.Because of the 2D character of the transport process, a magnetic field has a substantial effect only if it is so strong that the radius of V 2 jI 2' L Z, B FIG. 1. A ballistic microwir...

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