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Register a new userCoherent Electron Optics with Ballistically Coupled Quantum Point Contacts
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Added by
Jaan Freudenfeld M. Sc.
Publication date
2020
fields
Physics
and research's language is
English
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The realization of integrated quantum circuits requires precise on-chip control of charge carriers. Aiming at the coherent coupling of distant nanostructures at zero magnetic field, here we study the ballistic electron transport through two quantum point contacts (QPCs) in series in a three terminal configuration. We enhance the coupling between the QPCs by electrostatic focusing using a field effect lens. To study the emission and collection properties of QPCs in detail we combine the electrostatic focusing with magnetic deflection. Comparing our measurements with quantum mechanical and classical calculations we demonstrate how the coherent and ballistic dynamics depend on the details of the QPC confinement potentials.
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An unusual increase of the conductance with temperature is observed in clean quantum point contacts for conductances larger than 2e^2/h. At the same time a positive magnetoresistance arises at high temperatures. A model accounting for electron-electron interactions mediated by bound- aries (scattering on Friedel oscillations) qualitatively describes the observation. It is supported by numerical simulation at zero magnetic field.
The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts.
We present measurements of current noise in quantum point contacts as a function of source-drain bias, gate voltage, and in-plane magnetic field. At zero bias, Johnson noise provides a measure of the electron temperature. At finite bias, shot noise at zero field exhibits an asymmetry related to the 0.7 structure in conductance. The asymmetry in noise evolves smoothly into the symmetric signature of spin-resolved electron transmission at high field. Comparison to a phenomenological model with density-dependent level splitting yields quantitative agreement. Additionally, a device-specific contribution to the finite-bias noise, particularly visible on conductance plateaus (where shot noise vanishes), agrees quantitatively with a model of bias-dependent electron heating.
We investigate experimentally the influence of current flow through two independent quantum point contacts to a nearby double quantum dot realized in a GaAs-AlGaAs heterostructure. The observed current through the double quantum dot can be explained in terms of coupling to a bosonic bath. The temperature of the bath depends on the power generated by the current flow through the quantum point contact. We identify the dominant absorption and emission mechanisms in a double quantum dot as an interaction with acoustic phonons. The experiment excludes coupling of a double quantum dot to shot noise generated by quantum point contact as the dominant mechanism.
Spin-density-functional theory of quantum point contacts (QPCs) reveals the formation of a local moment with a net of one electron spin in the vicinity of the point contact - supporting the recent report of a Kondo effect in a QPC. The hybridization of the local moment to the leads decreases as the QPC becomes longer, while the onsite Coulomb-interaction energy remains almost constant.
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