Relation between the 0.7 anomaly and the Kondo effect: Geometric crossover between a quantum point contact and a Kondo quantum dot

Abstract

Quantum point contacts (QPCs) and quantum dots (QDs), two elementary building blocks of semiconducting nanodevices, both exhibit famously anomalous conductance features: the 0.7-anomaly in the former case, the Kondo effect in the latter. For both the 0.7-anomaly and the Kondo effect, the conductance shows a remarkably similar low-energy dependence on temperature T , source-drain voltage V sd and magnetic field B. In a recent publication [F. Bauer et al., Nature, 501, 73 (2013)], we argued that the reason for these similarities is that both a QPC and a Kondo QD (KQD) feature spin fluctuations that are induced by the sample geometry, confined in a small spatial regime, and enhanced by interactions. Here we further explore this notion experimentally and theoretically by studying the geometric crossover between a QD and a QPC, focussing on the B-field dependence of the conductance. We introduce a one-dimensional model with local interactions that reproduces the essential features of the experiments, including a smooth transition between a KQD and a QPC with 0.7-anomaly. We find that in both cases the anomalously strong negative magnetoconductance goes hand in hand with strongly enhanced local spin fluctuations. Our experimental observations include, in addition to the Kondo effect in a QD and the 0.7-anomaly in a QPC, Fano interference effects in a regime of coexistence between QD and QPC physics, and Fabry-Perot-type resonances on the conductance plateaus of a clean QPC. We argue that Fabry-Perot-type resonances occur generically if the electrostatic potential of the QPC generates a flatter-than-parabolic barrier top.

Influence of assisted hopping interaction on the linear conductance of quantum dot

“…For larger values of B we obtain the plateau of m(B) corresponding to the saturated polarization. A similar dependence of magnetization on the external magnetic field was reported by Heyder and coauthors [76] for SIAM model and for Kondo quantum dot. The dot energy (ε d ) dependence of the linear conductance for several values of magnetic field strength and for T = 0 is presented in Fig.…”

Microwave-induced direct spin-flip transitions in mesoscopic Pd/Co heterojunctions

“…The first one is a zero-bias anomaly (ZBA), which is present, when B≠0 and remains approximately constant when |B| is increased. Different explanations have been proposed for the zero-bias conductance peak close to the 0.7 G 0 anomaly including the formation of a spontaneous spinpolarisation [49,51], quasi-bound states leading to a Kondo impurity [48,50,52] and ferromagnetic spincoupling [53]. We attribute the ZBA to a local magnetic impurity that develops in atomic contacts of some transition metal elements, even in clean contacts, as a result of the reduced dimensionality [54,55].…”

Conductance anomalies in quantum point contacts and 1D wires

“…However, this precondition has been put into question, particularly for clean QPCs. More recent works following this type of argument have appeared [11][12][13]19].…”

“…Since the 0.7 structure is observed to disappear with lower temperatures they ascribed this behaviour to the Kondo effect (in addition to the presence of a zero-bias conductance peak). Further investigations suggested that the 0.7 anomaly and ZBA share certain similarities with the Kondo effect in quantum dots [12,13].…”

“…In the current literature both 0.7 and ZBA features have been explained [11,12] by either: amplification of interaction effects when a smeared van-Hove singularity of the local density of states (DOS) (at the bottom of the lowest 1D sub-band) crosses the chemical potential; or by an emergent localized state contributing to the formation of a collective (many-body) state arising from the Kondo effect in electron transport through the localized state. Another work, [13], develops a similar idea based on Kondo physics.…”