0.7 anomaly and magnetotransport of disordered quantum wires

Abstract

The unexpected "0.7" plateau of conductance quantisation is usually observed for ballistic onedimensional devices. In this work we study a quasi-ballistic quantum wire, for which the disorder induced backscattering reduces the conductance quantisation steps. We find that the transmission probability resonances coexist with the anomalous plateau. The studies of these resonances as a function of the in-plane magnetic field and electron density point to the presence of spin polarisation at low carrier concentrations and constitute a method for the determination of the effective g-factor suitable for disordered quantum wires.PACS numbers: 73.63. Nm, 73.23.Ad, 72.25.Dc It is expected that quantum point contacts (QPC) and quantum wires (QW) will act as active components of future nano-electronic devices and circuits. Therefore, the renewed interest in transport and spin properties of one-dimensional (1D) systems recently takes place in the mesoscopic physics community. In those studies, special attention is directed towards the long standing problem of quantum transport -the so called "0.7 anomaly" [1] most often, but not exclusively, observed for devices fabricated on modulation doped GaAs/AlGaAs heterostructures. Usually, anomalous behavior is observed in transport data as a "kink" on the conductance G vs. the device width curve, occurring for the low carrier densities, when G ∼ 0.7 × 2e 2 /h, here −e is the electron charge and h is the Planck constant. 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]. If it is so, the 1D systems may be used as an efficient spin filter with possible practical applications. This point of * Also at Instytut Fizyki Teoretycznej, Uniwersytet Warszawski, Hoża 69, PL 00-681 Warszawa, Poland; ERATO Semiconductor Spintronics Project, Japan Science Technology Agency, 1-18 Kitamemachi, Aoba-ku Sendai, 980-0023, Japan † Also at ERATO Semiconductor Spintronics Project, Japan Science and Technology Agency, 1-18 Kitamemachi, Aoba-ku Sendai, 980-0023, Japan view is supported by magnetic focusing data obtained for the p-type device, which reveal the static spin polarisation of holes transmitted through the constriction [8].Furthermore, recent shot-noise measurements carried out for n-type QPC [9] show that distinct transport channels exist at G < 2e 2 /h = G 0 , presumably related to spin, exhibiting quite different transmission probabilities.Many experiments, howeve...

Electrostatic Gates for GaN/AlGaN Quantum Point Contacts

“…1b). Let us note the smooth edges of separating grooves with roughness which favourably compares to the results of a wet chemical etching [5,6]. Unfortunately, the dry plasma processes create depletion regions (1026) on the mesa walls, it may explain why the measured conductance G of our devices is not monotonic as a function of W lith .…”

“…Additionally, a metallic (Au) top gate is applied which covers the active region of the device and is separated from the substrate by an insulating and passivating layers of the hafnium oxide HfO 2 or Al 2 O 3 /HfO 2 . Side gates are known to produce excellent and adiabatically smooth constrictions on GaAs/AlGaAs heterostructures [5,6] and the metal gate over the active region of the device helps to symmetrize transmission coecients by screening surface states and smoothing the connement potential [7]. We believe that the combination of side/ top gates, which is for the rst time applied for GaN/AlGaN nanostructurization, can provide an additional control over one-dimensional (1D) carrier density and leakage currents in this material.…”

Non-Kondo zero-bias anomaly in quantum wires

“…20 It was also found that the 0.7 structure is not associated with backscattering of electron waves and reduction in transmission probability. 21 As more and more studies imply the Kondo effect is not linked to the 0.7 structure, this topic requires further studies.The key evidence linking the 0.7 structure and the Kondo effect, is the presence of a "zero-bias anomaly" ͑ZBA͒, or conductance peak at zero dc bias, in the region of the 0.7 structure; 6 the Kondo effect causes a similar feature in quantum dots. 3 This ZBA was found to broaden and disappear with increasing temperature, and to split in two with increasing magnetic field, both of which can be interpreted as manifestations of the Kondo effect.…”

“…20 It was also found that the 0.7 structure is not associated with backscattering of electron waves and reduction in transmission probability. 21 As more and more studies imply the Kondo effect is not linked to the 0.7 structure, this topic requires further studies.…”

Evidence for charging effects in CdTe/CdMgTe quantum point contacts

“…As a result, when electrons travel only in one direction (at finite biases), the pinning of the resonant level leads to the appearance of a plateau-like feature at G ≈ 0.25 on G(V g ) curves 21 . Such "0.25-anomaly" was measured for rather long ( L = 0.4 to 1.0 µm) GaAs quantum wires 16,22,23 , where the weakly bound states were present or were probably induced by strong bias 3,21 . A similar feature is observed also in our shorter device, as it follows from Fig.…”