Zero-bias anomaly in quantum wires

Abstract

Undoped GaAs/AlGaAs heterostructures have been used to fabricate quantum wires in which the average impurity separation is greater than the device size. We compare the behavior of the Zero-Bias Anomaly against predictions from Kondo and spin polarization models. Both theories display shortcomings, the most dramatic of which are the linear electron-density dependence of the Zero-Bias Anomaly spin-splitting at fixed magnetic field B and the suppression of the Zeeman effect at pinch-off.PACS numbers: 72.10. Fk, 72.25.Dc, 73.21.Hb, 73.23.Ad Split gates [1] can be used to restrict transport from a two-dimensional electron gas (2DEG) to a ballistic onedimensional (1D) channel. This results in the quantization of the differential conductance G = dI/dV sd in units of G 0 = 2e 2 /h at zero magnetic field [2,3]. A shoulder on the riser of the first quantized plateau, the "0.7 anomaly" or "0.7 structure" [4], is not completely understood but generally acknowledged to result from electronelectron interactions. Although spin polarization models [5,6,7,8,9,10] and 1D Kondo physics models [11,12,13] can describe many experiments, neither can explain all phenomena associated with the 0.7 structure. One example is the so-called zero-bias anomaly (ZBA): a peak in G centered at V sd = 0 for G < 2e 2 /h when sweeping sourcedrain bias V sd at a fixed gate voltage V gate at low temperature T . Spin polarization models cannot alone predict its occurrence in quantum wires, although an embedded impurity near or in the 1D channel could produce a ZBA via the 0D Kondo effect [14,15,16,17]. On the other hand, in 1D Kondo physics models, a bound state forms when G < G 0 . In this context, a resonance observed by a non-invasive detector capacitively coupled to a quantum wire at threshold [18] as well as a triple-peaked structure in G at fixed V gate below the 0.7 structure [19] are consistent with the presence of a localized state in 1D channels.Systematically studying the ZBA in modulation-doped 2DEGs has proven difficult because of the large variability of its characteristics from device to device [20,21], probably due to the randomly fluctuating background potential caused by the ionized dopants, significant even with the use of large (≥75 nm) spacer layers. This disorder is so pervasive that one can be led to wonder whether the ZBA always results from interactions between conduction electrons and a random localized state near the 1D channel. However, disorder can be dramatically reduced in undoped GaAs/AlGaAs heterostructures where an external electric field (via a voltage V top on a metal top gate) electrostatically induces the 2DEG [22,23]. Figure 1(a) shows the advantages of this technique, particularly at low carrier densities (see also Fig. 3 in Ref. [22]), a regime most relevant for the ZBA. In this Letter, we report on the study of the ZBA in ten quantum wires fabricated in undoped GaAs/AlGaAs heterostructures. We demonstrate that an unsplit ZBA does not result from interactions between conduction electrons and a random localiz...

Zero-bias anomaly of quantum point contacts in the low-conductance limit

“…15. For even lower conductances, however, ∆ pp saturated to a value that did not change down to pinchoff (see, e.g., Fig.…”

“…At high conductance, the ZBP is often attributed to Kondo effect screening of an impurity that is formed self-consistently in the QPC. 7,13 Point contact ZBPs typically persist down to much lower conductance, 15 and it is tempting to attribute the same mechanism to the formation of ZBPs through the full range 0 < G < 2e 2 /h. As pointed out in Refs.…”

“…The magnitude of the splitting, ∆ pp , was typically between 3-5E z , 7,15 where E z = |gµ B B || | is the Zeeman energy using the bulk GaAs g-factor, g = −0.44. We compare ∆ pp to Zeeman rather than orbital energy scales because the magnetic fields were applied in the sample plane, causing relatively minor orbital effects.…”

Scanning gate imaging of quantum point contacts and the origin of the 0.7 anomaly

“…In our experiment, the thermal length ℓth = shifting measurements found in literature [16,24,26] it clearly demonstrates that no impurity is present at short distance from the channel. A smaller steepness of the risers between plateaus is observed at certain directions, probably as a result of a smoother confining potential.…”

“…In particular, a plateau appearing at G ≈ 0.7 G0 is considered to be the genuine 0.7 anomaly if, in each and every single trace measured at different value of gate imbalance, it stays at the same conductance value [16]. Any movement to higher/lower conductance or the appearing of new features is judged to stem from impurity [11,24,26]. Such a technique is, however, not conclusive since it can detect only defects located at short distance from the constriction.…”