Shot-Noise Signatures of 0.7 Structure and Spin in a Quantum Point Contact

Abstract

We report simultaneous measurement of shot noise and dc transport in a quantum point contact as a function of source-drain bias, gate voltage, and in-plane magnetic field. Shot noise at zero field exhibits an asymmetry related to the 0.7 structure in conductance. The asymmetry in noise evolves smoothly into the symmetric signature of spin-resolved electron transmission at high field. Comparison to a phenomenological model with density-dependent level splitting yields good quantitative agreement.Shot noise, the temporal fluctuation of current resulting from the quantization of charge, is sensitive to quantum statistics, scattering and many-body effects [1,2]. Pioneering measurements [3,4,5] of shot noise in quantum point contacts (QPCs) observed the predicted [6] suppression of shot noise below the Poisson value due to Fermi statistics. In regimes where many-body effects are strong, shot noise measurements have been exploited to directly observe quasiparticle charge in strongly correlated systems [7,8,9] as well as to study coupled localized states in mesoscopic tunnel junctions [10] and cotunneling in nanotube-based quantum dots [11].Paralleling these developments, a large literature has emerged concerning the surprising appearance of an additional plateau in transport through a QPC at zero magnetic field, termed 0.7 structure. Experiment [12,13,14] and theory [15,16] suggest that 0.7 structure is a manybody spin effect. Its underlying microscopic origin, however, remains an outstanding problem in mesoscopic physics. This persistently unresolved issue is remarkable given the simplicity of the device.In this Letter, we report simultaneous measurements of the shot noise at 2 MHz and dc transport in a QPC, exploring the noise signature of the 0.7 structure and its evolution with in-plane magnetic field B . A suppression of the noise relative to that predicted by theory for spindegenerate transport [6] is observed near 0.7 × 2e 2 /h at B = 0, in agreement with results from Roche et al. [14] obtained at kHz frequencies. This suppression evolves smoothly with increasing B into the signature of spinresolved transmission. We find quantitative agreement between noise data and a phenomenological model for a density-dependent level splitting [16], with model parameters extracted solely from conductance.Measurements are performed on a gate-defined QPC fabricated on the surface of a GaAs/Al 0.3 Ga 0.7 As heterostructure grown by molecular beam epitaxy (see mi- * These authors contributed equally to this work. FIG. 1: Equivalent circuit near 2 MHz of the noise detection sys-tem measuring QPC noise by cross-correlation on two amplification channels [17]. The scanning electron micrograph shows a device of identical design to the one measured. The QPC is formed by negative voltages V g1 and V g2 applied on two facing electrostatic gates. All other gates on the device are grounded. crograph in Fig. 1). The two-dimensional electron gas 190 nm below the surface has a density of 1.7 × 10 −11 cm −2 and mobility 5.6 × 10 6 cm 2 /Vs. A...

0.7 anomaly and magnetotransport of disordered quantum wires

“…This point of 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.…”

“…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, however, bring out rather contradictory observations regarding the temperature dependence of the additional plateau. Already Thomas and coworkers [1] revealed that the 0.7 "kink" disappears when temperature is lowered, typically below few hundreds milikelvins.…”

“…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.…”

Finite shot noise and electron heating at quantized conductance in high-mobility quantum point contacts

“…Since this theory is based on the single-particle picture, additional information such as the many-body interaction may be deduced from the precise investigation of the Fano factor combined with the conductance, as we see in the shot noise study in the Kondo regime [19][20][21][22] . Actually, the shot noise of QPC was addressed by several experimental groups; the Pauli exclusion principle was first studied, and then the quantum interference 23 , the 0.7 anomaly [24][25][26] , and the spin polarization 25,27 .…”

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

“…Several models have been proposed that relate the 0.7 anomaly to a spontaneous spin splitting in zero magnetic field [5][6][7][8][9][10][11], since the 0.7 plateau evolves continuously into the spin-resolved plateau at 0.5(2e 2 /h) when an inplane magnetic field is applied. A recent theory paper [12] presented spin-density functional calculations of realistic QPC geometries that show that a localized state can exist near pinch-off in a QPC, providing a theoretical back-ground for the Kondo-like physics that was found experimentally [13].…”