Persistence of the 0.7 anomaly of quantum point contacts in high magnetic fields

Abstract

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-tosample 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.

0.7 anomaly and magnetotransport of disordered quantum wires

“…Thus, the magnetic field evolution of transmission resonances can be applied to the determination of the g * factor of 1D systems, which is expected to be enhanced by the exchange interaction [2,4]. For our sample we obtained |g * | = 1.2 ± 0.1 (as compared to 0.44 in bulk GaAs), which is in very good agreement with recent data for ballistic QPCs [6]. The advantage of the proposed method is that it can be applied for long, quasi-ballistic quantum wires.…”

“…The origin of this effect is currently under active debate since this anomaly seems to be an universal, but still unexplained feature of one-dimensional mesoscopic transport. Experimentally, the magnetic field dependence of the additional plateau is common for all studied systems -by applying a parallel in-plane field the 0.7 feature evolves gradually towards 0.5 × 2e 2 /h conductance step, when only one spinpolarised level is occupied [1,2,3,4,5,6]. Therefore, it has been suggested that such an anomalous plateau is due to spontaneous spin polarisation of one-dimensional electron liquid, caused by exchange interactions among carriers in the constricted geometry of the device [2,7].…”

“…Nevertheless, the Kondotype features appear to be typical for the point contacts only, i.e. for devices for which L/W ∼ 1, where L and W are the physical length and width of the conducting 27003-p1 channel, respectively [6,11,12]. On the contrary, for longer quantum wires, when L/W 1, the 0.7 anomaly becomes even more pronounced, when the temperature is lowered and zero-bias anomaly is not observed [13][14][15].…”

The Influence of Device Geometry on Many-Body Effects in Quantum Point Contacts: Signatures of the 0.7 Anomaly, Exchange and Kondo

“…This analysis points to the conclusion that the spontaneous energy splitting of the 0.7 anomaly is dominated by the same effect that causes the high-field offset ∆E hf o . As we discussed, this is probably an exchange contribution [30]. The error bar that we attribute to these values includes an error from the transconductance peak-fitting, one from the conversion of gate voltage to energy scale, and an error due to scatter in the ∆E datapoints as a function of B.…”