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

Abstract

Quantum point contacts exhibit mysterious conductance anomalies in addition to well-known conductance plateaus at multiples of 2e 2 /h. These 0.7 and zero-bias anomalies have been intensively studied, but their microscopic origin in terms of many-body effects is still highly debated. Here we use the charged tip of a scanning gate microscope to tune in situ the electrostatic potential of the point contact. While sweeping the tip distance, we observe repetitive splittings of the zero-bias anomaly, correlated with simultaneous appearances of the 0.7 anomaly. We interpret this behaviour in terms of alternating equilibrium and nonequilibrium Kondo screenings of different spin states localized in the channel. These alternating Kondo effects point towards the presence of a Wigner crystal containing several charges with different parities. Indeed, simulations show that the electron density in the channel is low enough to reach one-dimensional Wigner crystallization over a size controlled by the tip position.

Scanning-gate-induced effects in nonlinear transport through nanostructures

“…Indeed, independently of the details of the model describing the system, if g(r T ) shows interference fringes, then nonlinear corrections should become dominant away from the constriction. This observation is in line with recent experimental results [10,43], where the oscillating tip-induced corrections at finite bias voltage do not decrease in magnitude with increasing tip-QPC distances when scanning in some regions of the 2DEG adjacent to the QPC. This is unusual, since an increase of V has a radically different effect from that of a temperature rise.…”

“…[10] happen for bias voltages in the scale of μV . Subtle many-body effects are beyond the scope of the present work, based on a one-particle approach yielding results on the scale of the constriction quantization energy (meV ).…”

“…While an interpretation in terms of the nonequilibrium distribution of electrons in a localized region of the 2DEG near the QPC was proposed, further experimental and theoretical work appeared necessary in order to justify the use of an effective electron temperature [8]. Working in the regime of a partially closed QPC, an oscillatory splitting of the zero-bias anomaly with tip position, correlated with simultaneous appearances of the 0.7 anomaly, has been recently reported [10]. These findings concerning an SGM setup in the regime of nonlinear transport through a QPC illustrate the need to address two related questions.…”

“…The use of a scanning tunneling microscope (STM) to obtain information about the local field was proposed 25 years ago [1,3,6], but only recently [7][8][9][10] a related technique, the scanning gate microscopy (SGM), has been applied in the nonlinear regime to study electron-electron scattering in a two-dimensional electron gas (2DEG) surrounding a quantum point contact (QPC). The SGM appears as a less invasive probe than the STM, as it consists of a charged atomic force microscope scanning over the sample and thus modifying the conductance only through a capacitive coupling to the buried 2DEG [11][12][13].…”

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

“…A number of theoretical investigations focused on the role of quantum interference [4,5], spontaneous spin polarization [1, 6,7], Wigner crystallization [8] and the Kondo effect [9][10][11]. Even very recently, two papers provided evidence for the Kondo scenario, based on the formation of a quasi-bound state at the constriction [12,13]. On the other hand, another recently proposed model involves a "van Hove ridge" in the density of states of the QPC that strongly enhances interaction effects for subopen QPCs without needing spin-polarization or quasi-bound states [14].…”