Using multiplexers to study the statistics of quantum phenomenon in one-dimensional wires

The quantum point contact (QPC) is a one-dimensional constriction with the differential conductance quantised in units of G_Q=2e²/h. However, the transport behaviour below the first plateau is still not fully understood, including the 0.7 anomaly and the 0.25 anomaly in the linear and non-linear transport regimes respectively. In this work, we utilise a multiplexing technique and statistically investigate the 0.7 anomaly observed on the first three plateaus respectively in 571 QPCs, fitting well the van-Hove model. The 0.7 anomaly shows the transconductance suppression due to the effective electron interactions which are modified by the local density of states (LDOS). At the maximum of LDOS, the interaction strength becomes strongest, resulting in the strongest transconductance suppression. The strongest interaction strength is determined by the ratio of transverse confinement curvature and longitudinal barrier curvature. Moreover, we realise measurements of the effective g factor (g^*) and high-field offset (\Delta E^hfo) in numerous devices in a single cooldown at T=40 mK. The statistical results show both the g^* and \Delta E^hfo increase with the potential confinement, which supports the predictions about the role of interaction strength on g^* and \Delta E^hfo in a 1D tight-binding model. We explore the origin of \Delta E^hfo and find that it is only considerable for the first plateau. Using a short and narrow QPC could result in a stronger potential confinement and thus a higher g^*, which could be beneficial for its use in spintronic applications. Last, we investigate the formation and development of the DC-bias-induced 0.75 and 0.25 anomalies for 402 QPCs. We find the anomalies evolve similarly in a magnetic field. To explain the anomaly behaviours, we propose a phenomenological DC-bias-induced spin-splitting model. In the model, with the increasing DC bias (V_DC), the 0.75 anomaly occurs first at a differential conductance of 0.75 G_Q, while the 0.25 anomaly is formed at a differential conductance of 0.5 G_Q and moves to 0.375 G_Q. The spin gap of the first subband opens to be e|V_DC|, which enables an all-electric manipulation of spin polarisation simply by applying a DC bias.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2017

Author dc:creator

• Ma, Pengcheng

Advisor dc:contributor.advisor

• Smith, Charles

• quantum point contact
• multiplexer
• the 0.7 anomaly
• 0.25 anomaly
• effective g factor

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/00c43f12-4488-409c-877c-accd5ad8dd60/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

en

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.17151

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/270301