Resistively detected NMR line shapes in a quasi-one-dimensional electron system

Abstract

We observe variation in the resistively-detected nuclear magnetic resonance (RDNMR) lineshapes in quantum Hall breakdown. The breakdown is locally occurred in a gate-defined quantum point contact (QPC) region. Of particular interest is the observation of a dispersive lineshape occured when the bulk 2D electron gas (2DEG) is set to ν b = 2 and the QPC filling factor to the vicinity of νQPC = 1, strikingly resemble the dispersive lineshape observed on a 2D quantum Hall state. This previously unobserved lineshape in a QPC points to simultaneous occurrence of two hyperfinemediated spin flip-flop processes within the QPC. Those events give rise to two different sets of nuclei polarized in the opposite direction and positioned at a separate region with different degree of electronic spin polarization.Recent advent in NMR technique through a resistive detection (RDNMR) has made it possible to study various spin physics in a 2D quantum Hall system [1][2][3][4][5][6][7], and a quasi-1D channel [8,9]. Despite the success achieved, a certain aspect related to the origin of the RDNMR lineshape variations noted experimentally in continuous wave (cw) mode is still poorly understood. One of them involved the puzzling observation of a dispersive lineshape in the quantum Hall state, a resistance dip followed by a resistance peak resonance line with increasing radio frequency [10]. It is first reported by Desrat et al [11] in the vicinity of ν b = 1 and has been confirmed in a number of follow-up papers [7,[12][13][14][15][16][17]. Similar dispersive like lineshape has been observed as well in the vicinity of ν b = 2/9[18], ν b = 2/3, ν b = 1/3 [19], and at ν b = 2 Landau level crossing [20]. A number of appealing explanations has been put forward, but none of them provides a comprehensive explanation. Part of the reason why it still is difficult to unravel its physical origin is that we do not have a mature level of understanding about manybody 2D electronic states at the first Landau level yet, let alone their coupling to the nuclear spin. Thus, it would be highly desirable to study the lineshape variations in a platform where one can avoid such complexity.In this Rapid Communication, we resort to a quasione dimensional system in a gate-defined quantum point contact (QPC) to study various possible lineshapes including the dispersive lineshape noted experimentally in cw mode. Unlike on the 2D system, the mechanism for generation and resistive detection of nuclear spin polarization is tractable, allowing conveniently a direct interpretation of the observed lineshapes.Generation and detection of nuclear spin polarization are achieved by setting the filling factor in the bulk 2DEG to ν b = 2 and ν QPC = 1 in the QPC[21-30]. Fig. 1(a)-(b) schematically displays how the nuclear polarization affects the transmission probability through the potential barrier of the QPC. For ν QPC < 1 (the down-spin channel T ↓ does not affect the transport), the up-spin channel T ↑ sees an increase(decrease) in the barrier potential in the presence of p...

Dynamic nuclear polarization at high Landau levels in a quantum point contact

“…The layout and dimensions of the present devices displayed in Fig. 1(a) are similar to those used by Fauzi et al 8 with three independent Schottky gates to define the quantum point contact. Throughout the measurement process, we always equally biased a pair of split gates (V SG ) and set the bias voltage zero to the center gate (V CG ).…”

Dipolelike dynamical nuclear spin polarization around a quantum point contact

“…6b). Such QHE-DL have been seen too [16,53,55], but the opposite shape is perhaps more standard [47].…”

“…For the latter, we consider the QPC conductance changes upon applying NMR, socalled resistively detected NMR (RD-NMR) [12]. Variations of this setup have been already considered in experiments and theory [3,[13][14][15][16][17], focusing on various phenomena such as conductance hysteresis, nuclear-spin relaxation, the DNSP mechanism, or "dispersive" RD-NMR line shapes. We, however, feel that despite all these investigations, certain general aspects remained obscure.…”

Two-step breakdown of a local ν=1 quantum Hall state

“…The occurrence of spin-flip scattering as well as spin-conserved scattering in the QH effect breakdown has been manifested by the dynamic nuclear polarization of the host crystal in both integer and fractional regimes [6][7][8][9] . Indeed, resistively detected nuclear magnetic resonance (NMR) [10][11][12] has shown that the electron spin polarization decreases under a high bias in both mesoscopic and macroscopic systems 13 . On the other hand, inter-edge scattering generates shot noise in mesoscopic systems, from which the spin polarization of the transmitted current can be deduced under the assumption that the scattering event is stochastic, i.e., with no correlation 13 .…”