Effect of two-level systems on the spectral density of universal conductance fluctuations in doped Si

Abstract

We have studied the power spectral density [S(f)=γ/f^α] of universal conductance fluctuations (UCF's) in heavily doped single crystals of Si, when the scatterers themselves act as the primary source of dephasing. We observed that the scatterers, with internal dynamics like two-level-systems, produce a significant, temperature-dependent reduction in the spectral slope α when T≲10 K, as compared to the bare 1/f (α≈1) spectrum at higher temperatures. It is further shown that an upper cutoff frequency (f_m) in the UCF spectrum is necessary in order to restrict the magnitude of conductance fluctuations, <(δG_φ)²>, per phase coherent region (L_φ³) to <(δG_φ)²>½≲e²/h. We find that f_m≈τ_D⁻¹, where τ_D=L²/D, is the time scale of the diffusive motion of the electron along the active length (L) of the sample (D is the electron diffusivity).