Quantized Ballistic Transport of Electrons and Electron Pairs
Dimensionality has a profound effect on electron transport. When electrons are confined in two dimensions (2D), new phases such as the integer and fractional quantum Hall effect emerge. Electrons confined in one dimension (1D) lose nearly all of their recognizable features. For example, the electron spin and charge can separate and move independently of one another, and the charge itself can fractionalize. However, in 1D the conductance remains quantized in units of e2 /h. Quasi-1D transport was first reported in narrow constrictions, also known as “quantum point contacts”. The conductance through these narrow channels is given by the number of allowed transverse modes, which is tunable by an external gate. The confined regions are generally short, of the order 100−200 nm, with a channel length set by the distance between the top gate electrodes and the buried high-mobility layer.
SrTiO3-based heterointerfaces support quasi two-dimensional (2D) electron systems that are analogous to III−V semiconductor heterostructures, but also possess superconducting, magnetic, spintronic, ferroelectric, and ferroelastic degrees of freedom. Despite these rich properties, the relatively low mobilities of 2D complex-oxide interfaces appear to preclude ballistic transport in 1D.
In this article, authors have shown the quantized ballistic transport of electrons and (nonsuperconducting) electron pairs within quasi-1D structures at 2D LaAlO3/SrTiO3 interface that are created using a well-established conductive atomic-force microscope (c-AFM) lithography technique. The nature of transport ranges from truly single-mode (1D) to three-dimensional (3D), depending on the applied magnetic field and gate voltage. Quantization of the lowest e2/h plateau indicate a ballistic mean-free path lMF ∼ 20 μm, more than 2 orders of magnitude larger than for 2D LaAlO3/SrTiO3 heterostructures. Nonsuperconducting electron pairs are found to be stable in magnetic fields as high as B = 11 T and propagate ballistically with conductance quantized at 2e2/h. Theories of one-dimensional (1D) transport of interacting electron systems depend crucially on the sign of the electron−electron interaction, which may help explain the highly ballistic transport behavior. The 1D geometry yields new insights into the electronic structure of the LaAlO3/SrTiO3 system and offers a new platform for the study of strongly interacting 1D electronic systems.
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