What lurks below the last plateau: experimental studies of the 0.7 × 2e²/hconductance anomaly in one-dimensional systems

Abstract

The integer quantised conductance of one-dimensional electron systems is a well-understood effect of quantum confinement. A number of fractionally quantised plateaus are also commonly observed. They are attributed to many-body effects, but their precise origin is still a matter of debate, having attracted considerable interest over the past 15 years. This review reports on experimental studies of fractionally quantised plateaus in semiconductor quantum point contacts and quantum wires, focusing on the 0.7 × 2e(2)/h conductance anomaly, its analogues at higher conductances and the zero-bias peak observed in the dc source-drain bias for conductances less than 2e(2)/h.

Hysteresis in the conductance of asymmetrically biased GaAs quantum point contacts with in-plane side gates

“…Thomas et al were the first to suggest that the 0.7G 0 conductance anomaly was an indirect evidence of a spontaneous ferromagnetic spin polarization in the narrow portion of the QPC. Many theories of conductance anomalies have now been developed on that assumption, including a spontaneous spin polarization in the narrow portion of the QPC due to the exchange-correlation interaction [13][14][15], the formation of quasi-bound states [16], and the Kondo effect [17], among others [10][11][12]. There is now an increasing number of reports showing that the number and location of conductance anomalies can be controlled by creating an asymmetry in the QPC's electrostatic confining potential [18][19][20][21].…”

Intrinsic bistability in quantum point contacts with in-plane side gates

“…Over the last ten years, there have been many experimental reports of anomalies in the quantized conductance of QPCs and there is a growing consensus that these conductance anomalies are indirect evidence for the onset of spin polarization in the QPC's channel [1]. These anomalies include conductance plateaus around 0.5x2e 2 /h and 0.7x2e 2 /h [1].…”

“…These anomalies include conductance plateaus around 0.5x2e 2 /h and 0.7x2e 2 /h [1]. There have been some recent reports that the number and location of these conductance anomalies can be tuned by deliberately introducing a broken symmetry in the QPC's electrostatic confining potential [2][3][4][5].…”

“…The NDR regions in the forward and reverse sweeps are linked to multiple reflections between the edges of the narrow portion of the QPC [11,12]. There is an emerging consensus in the spin origin of the conductance anomalies observed in QPCs of various geometries [1]. Many theories of conductance anomalies have been proposed based on that premise, including a spontaneous spin polarization in the narrow portion of the QPC resulting from the exchange-correlation interaction [16,17,23], the formation of quasibound states in the narrow portion of the QPC [24], and some based on the Kondo effect [25].…”

Conductance anomalies in quantum point contacts and 1D wires

“…In section 5 we conclude our study with a critical discussion study of the presumed universality of the so-called conductance anomalies, arguing that they do not meet any reasonable measure of what a truly generic phenomenon should be. In doing so we refute claims [14,15] that these anomalies are fundamentally intrinsic to 1D electronic transport. They are, by contrast, material specific not at all 'universal'.…”