Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe influence of device geometry on many-body effects in quantum point contacts: Signatures of the 0.7 anomaly, exchange and Kondo
107
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The conductance of a quantum point contact (QPC) shows several features that result from many-body electron interactions. The spin degeneracy in zero magnetic field appears to be spontaneously lifted due to the so-called 0.7 anomaly. Further, the g-factor for electrons in the QPC is enhanced, and a zero-bias peak in the conductance points to similarities with transport through a Kondo impurity. We report here how these many-body effects depend on QPC geometry. We find a clear relation between the enhanced g-factor and the subband spacing in our QPCs, and can relate this to the device geometry with electrostatic modeling of the QPC potential. We also measured the zero-field energy splitting related to the 0.7 anomaly, and studied how it evolves into a splitting that is the sum of the Zeeman effect and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations and no clear dependence on QPC geometry, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting. Signatures of the Kondo effect also show no regular dependence on QPC geometry, but are possibly correlated with splitting of the 0.7 anomaly.
rate research
Read More
The spin degeneracy of the lowest subband that carries one-dimensional electron transport in quantum point contacts appears to be spontaneously lifted in zero magnetic field due to a phenomenon that is known as the 0.7 anomaly. We measured this energy splitting, and studied how it evolves into a splitting that is the sum of the Zeeman effect and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting.
Quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are investigated by low-temperature electrical conductance spectroscopy measurements. Besides one-dimensional conductance quantization in units of $2e^{2}/h$ a pronounced extra plateau is found at about $0.7(2e^{2}/h)$ which possesses the characteristic properties of the so-called 0.7 anomaly known from experiments with n-type samples. The evolution of the 0.7 plateau in high perpendicular magnetic field reveals the existence of a quasi-localized state and supports the explanation of the 0.7 anomaly based on self-consistent charge localization. These observations are robust when lateral electrical fields are applied which shift the relative position of the electron wavefunction in the quantum point contact, testifying to the intrinsic nature of the underlying physics.
A quantum point contact (QPC) is a very basic nano-electronic device: a short and narrow transport channel between two electron reservoirs. In clean channels electron transport is ballistic and the conductance $G$ is then quantised as a function of channel width with plateaus at integer multiples of $2e^2/h$ ($e$ is the electron charge and $h$ Plancks constant). This can be understood in a picture where the electron states are propagating waves, without need to account for electron-electron interactions. Quantised conductance could thus be the signature of ultimate control over nanoscale electron transport. However, even studies with the cleanest QPCs generically show significant anomalies on the quantised conductance traces and there is consensus that these result from electron many-body effects. Despite extensive experimental and theoretical studies understanding of these anomalies is an open problem. We report evidence that the many-body effects have their origin in one or more spontaneously localised states that emerge from Friedel oscillations in the QPC channel. Kondo physics will then also contribute to the formation of the many-body state with Kondo signatures that reflect the parity of the number of localised states. Evidence comes from experiments with length-tunable QPCs that show a periodic modulation of the many-body physics with Kondo signatures of alternating parity. Our results are of importance for assessing the role of QPCs in more complex hybrid devices and proposals for spintronic and quantum information applications. In addition, our results show that tunable QPCs offer a rich platform for investigating many-body effects in nanoscale systems, with the ability to probe such physics at the level of a single site.
We measure the transmission phase of a quantum point contact (QPC) at a low carrier density in which electron interaction is expected to play an important role and anomalous behaviors are observed. In the first conductance plateau, the transmission phase shifts monotonically as the carrier density is decreased by the gate voltage. When the conductance starts to decrease, in what is often called the 0.7 regime, the phase exhibits an anomalous increase compared with the noninteracting model. The observation implies an increase in the wave vector as the carrier density is decreased, suggesting a transition to a spin-incoherent Luttinger liquid.
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.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
