Adjustable mode coupling in spatially coincident one-dimensional electron systems

Quantum point contacts (QPCs) fabricated from a 30 nm wide square quantum well (QW) with two occupied two-dimensional (2D) subbands possess two distinct one-dimensional (1D) energy spectra, referred to as vertical modes. Mode coupling can be observed for 1D confining potentials with a non-harmonic component. We investigate the relative shift of the two 1D subband ladders by varying the geometric width of the QPCs, the effective doping and in-plane applied magnetic fields. It is shown that spatially coincident 1D electron systems can be classified by the difference in threshold voltages of the vertical mode contributions to the quantized conductance. The QPCs under investigation indicate that for a given QW the strength of the lateral nearly harmonic confining potential dominantly determines the spectra of mode coupling. Perturbations always present in experiments, as atomic scale fluctuations in the doping layer or confining barrier roughness on a nanometer scale, do not substantially influence the mode coupling scheme.