Interaction effects in a one-dimensional constriction

Abstract

We have investigated the transport properties of one-dimensional (1D) constrictions defined by split-gates in high quality GaAs/AlGaAs heterostructures. In addition to the usual quantized conductance plateaus, the equilibrium conductance shows a structure close to 0.7(2e 2 /h), and in consolidating our previous work [K. J. Thomas et al., Phys. Rev. Lett. 77, 135 (1996)] this 0.7 structure has been investigated in a wide range of samples as a function of temperature, carrier density, in-plane magnetic field B and source-drain voltage V sd . We show that the 0.7 structure is not due to transmission or resonance effects, nor does it arise from the asymmetry of the heterojunction in the growth direction. All the 1D subbands show Zeeman splitting at high B , and in the wide channel limit the g-factor is | g |≈ 0.4, close to that of bulk GaAs. As the channel is progressively narrowed we measure an exchange-enhanced g-factor. The measurements establish that the 0.7 structure is related to spin, and that electron-electron interactions become important for the last few conducting 1D subbands.

Microscopic origin of the ‘0.7-anomaly’ in quantum point contacts

“…At temperatures around 1K, measured linear conductance curves of most clean QPCs show a pronounced shoulder at around 0.7G Q , as shown in figure 2.5 a green line, and in figure 1.2 a (see also Thomas et al, 1998). This shoulder gradually weakens if temperature is lowered.…”

Anomalous spin-dependent behavior of one-dimensional subbands

“…Our new findings also provide an explanation for one of the most characteristic features of the 0.7 Structure, which has, to our knowledge, not been explained by any model or theory so far -the question of how the 0.7 Structure can survive to such high temperatures, when all other conductance features have disappeared [4].…”

“…Using DC-bias spectroscopy [3], we show that as spin-down (lower energy) subbands begin to populate, they abruptly drop in energy by as much as 0.5 meV, within a vanishingly small range of gate-voltage. These results also cast doubt on basic assumptions which are often made when analysing quantum-wires -such as the idea that electron density is proportional or equivalent to gate-voltage, and the validity of an 'exchange-enhanced g-factor' in quantum wires [4].Our new findings also provide an explanation for one of the most characteristic features of the 0.7 Structure, which has, to our knowledge, not been explained by any model or theory so far -the question of how the 0.7 Structure can survive to such high temperatures, when all other conductance features have disappeared [4].Our samples consist of split-gate devices [5] defined by electron-beam lithography on a Hall bar etched from a GaAs/Al x Ga 1−x As heterostructure. All the samples used in this work have a length of 0.4 µm and a width of 0.7 µm.…”

“…In fact, it is the riser up to the 0.7 Structure that is almost invariant with temperature, which gives the impression that the 0.7 Structure does not experience significant thermal smearingat high T , above this riser, there are no other features in the conductance (see for example ref. [4], fig. 3(a)).…”

Conductance anomalies in quantum point contacts and 1D wires

“…• Thomas et al [8] originally identified an anomalous structure in low-temperature conductance below the first quantized step in GaAs QPCs at about G 0.7 0 . It has been argued that this perceived anomaly is an intrinsic property caused by some many-body effect independent of the details of the material system.…”