Angular Dependence of Universal Conductance Fluctuations in Noble-Metal Nanowires

Abstract

We investigate fluctuations in the magnetoconductance (measured at T ഠ 50 mK) of noble-metal nanowires in a mesoscopic two-lead configuration, with lengths 500 and 1000 nm and widths w between 45 and 360 nm. We determine the dependence of the correlation field B c on the angle u between the direction of the magnetic field and the long axis of the wires. u is changed at low T continuously from 90 ± (usual geometry) to 0 ± . We present a calculation taking into account the 3D diffusive motion of the conduction electrons describing the angular dependence of B c . We compare this B c ͑u͒ dependence with conductance fluctuations observed while sweeping u at constant magnetic field. Universal conductance fluctuations (UCF) in metals are a direct manifestation of quantum interference of electron wave packets. They can be observed if the sample dimensions are smaller than or comparable to the phase coherence length l f ͑Dt f ͒ 1͞2 (D y F l͞3 is the diffusion constant and t f is the phase-breaking scattering time) for conduction electrons [1,2]. In polycrystalline metal films l f is of the order of 1 mm at very low temperatures T , 1 K, whereas the elastic mean free path l is about 10 to 50 nm. In this diffusive regime the electrical conductance G is influenced by the interference of electronic trajectories which is sensitive to small changes in the Hamiltonian. G is found to fluctuate around a mean value G as a function of an external control parameter as, for example, the Fermi energy [3], the transport voltage [4,5], the perpendicular magnetic field [1], configuration of the scattering centers [6] or, as we will show here, the angle between external magnetic field and current direction. In a mesoscopic two-lead configuration, i.e., when within the phase coherent sample volume l 3 f only two measuring probes are in contact with the mesoscopic sample, the rms amplitude of the fluctuations depends only very weakly on the sample shape (length L, width w, and thickness d) as long as the transport is diffusive and coherentThe properties of the fluctuations in all these realizations are essentially the same with the only exception being that the rms amplitude observed for fluctuations measured as a function of the magnetic field B is smaller by a factor p 2 because of time-reversal symmetry breaking [7]. An additional reduction of the rms amplitude by a factor of 2 occurs in metals with strong spin-orbit scattering. Altogether, one expects a saturation rms amplitude of magnetoconductance fluctuations for Au or Ag of rms G 1D 0.26e 2 ͞h for quasi-1D wires [9].The characteristic fluctuation period B c , the correlation field, is a measure of the area A f enclosed by typical interference paths since A f B c Cf 0 , where f 0 h͞e is the elementary flux quantum, in analogy to the wellknown Aharonov-Bohm effect in ring structures [1]. Here C is a constant of order unity [7]. B c is given by the half width at half maximum ( HWHM) of the autocorrelation function F͑DB͒ of the magnetoconductance curves. Because all coherent trajectorie...

Vectorial Control of the Spin–Orbit Interaction in Suspended InAs Nanowires

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