Scanning gate imaging of quantum point contacts and the origin of the 0.7 anomaly

Abstract

The origin of the anomalous transport feature appearing at conductance G ≈ 0.7 x (2e 2 /h) in quasi-1D ballistic devices − the so-called 0.7 anomaly − represents a long standing puzzle. Several mechanisms were proposed to explain it, but a general consensus has not been achieved. Proposed explanations are based on quantum interference, Kondo effect, Wigner crystallization, and more. A key open issue is whether point defects that can occur in these low-dimensional devices are the physical cause behind this conductance anomaly. Here we adopt a scanning gate microscopy technique to map individual impurity positions in several quasi-1D constrictions and correlate these with conductance characteristics. Our data demonstrate that the 0.7 anomaly can be observed irrespective of the presence of localized defects, and we conclude that the 0.7 anomaly is a fundamental property of low-dimensional systems.

Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy

“…This local potential change induces electron backscattering towards the QPC, which can be used to image single-particle phenomena such as wave-function quantization in the channel 19 , branched flow in the disorder potential 20 , interference patterns induced by the tip [21][22][23] , or to investigate electron-electron interactions inside 24 or outside 25 the QPC. This movable gate can also be used to tune in situ the saddle potential of the QPC, in a more flexible and less invasive way than fixed surface gates, and probe intrinsic properties of the QPC such as the 0.7 anomaly 26,27 .…”

Spin and valley degrees of freedom in a bilayer graphene quantum point contact: Zeeman splitting and interaction effects

“…As mentioned in the introduction, the 0.7 feature in the QPC conductance is seen as an additional kink below the lowest plateau sitting at around 0.7 × 2 e 2 h [41] for systems without additional degeneracies of the spectrum. It has been observed and extensively studied in quantum point contacts in GaAs/AlGaAs heterostructures for both electrons [41,43,47,55,57,[73][74][75][76][77][78][79][80][81][82][83], and holes [84][85][86][87][88][89][90]; signatures of the anomaly were also observed in Si/SiGe heterostructures [91].…”

“…[41] and, while it is fair to say that there is still no commonly accepted theory explaining all aspects of the anomaly, there are several microscopic theories that are capable of capturing some salient features of the phenomenon. These theories invoke various distinct physical mechanisms driven by electron-electron correlations, such as variants of the Kondo effect [42][43][44][45][46][47], Wigner crys- tallization [48][49][50], and other interaction-based mechanisms [51][52][53][54][55][56][57]. In particular, there are studies investigating the influence of the QPC barrier on electron-electron interaction effects perturbatively.…”

Ballistic One-Dimensional Holes with Strong g-Factor Anisotropy in Germanium

“…Discovered and widely studied in quantum point contacts defined in high-mobility two-dimensional electron systems, 30-35 and more recently observed also in semiconductor nanowires, 36,37 the interpretation of this phenomenon remains somewhat debated. [38][39][40][41][42] To further confirm the 1D nature of the observed conductance quantization, we present in Figs. 2 (a)-(c) waterfall plots of the non-linear G(V ds ) at three different perpendicular magnetic fields (B = 0, 0.3, and 0.5 T, respectively) for device D1.…”