Emergent Localization from Many-Body Physics in Clean Quantum Point Contacts

Quantized conductance in quantum point contacts (QPCs) is the signature of control over electron transport at the nanoscale. As a function of channel width the conductance then increases in steps of $G_0=2e^2/h$. However, experiments often show an additional feature with a conductance plateau near $0.7G_0$, known as the 0.7 anomaly. This has been studied since 1995 but its full understanding is still an open problem. Spontaneous localization due to many-body effects in open QPCs, and the associated Kondo effect, has emerged as a promising theory for the 0.7 anomaly [1]. This theory work predicted that the many-body effects should for certain QPC geometries not give a single localized state but a pair of localized states, but signatures of this were till now not reported. For the first time, we have fabricated length-tunable QPCs in clean semiconductors and we discovered a periodic modulation of the 0.7 anomaly as a function of length. This modulation correlates with signatures for single and paired quasi-localized states, in the form of single- and two-impurity Kondo physics. Our results demonstrate that Friedel oscillations and emergent impurity states from many-body physics are fundamental to these phenomena. [1] T. Rejec and Y. Meir, Nature 442, 900 (2006).