New Type of Electron Nuclear-Spin Interaction from Resistively Detected NMR in the Fractional Quantum Hall Effect Regime

Abstract

Two dimensional electron gases in narrow GaAs quantum wells show huge longitudinal resistance (HLR) values at certain fractional filling factors. Applying an RF field with frequencies corresponding to the nuclear spin splittings of 69 Ga, 71 Ga and 75 As leads to a substantial decreases of the HLR establishing a novel type of resistively detected NMR. These resonances are split into four sub lines each. Neither the number of sub lines nor the size of the splitting can be explained by established interaction mechanisms.Two-dimensional electron gases (2DEGs) with very high mobilities of the electrons can be formed in quantum wells and heterostructures based on the GaAs/Al x Ga 1−x As system. If such a 2DEG is subjected to an intense perpendicular magnetic field at very low temperatures, it shows the integer [1] and the fractional [2] quantum Hall effects at integer and fractional filling factors of one or more Landau levels. The signature of both types of quantum Hall effects is the quantization of the Hall resistance and the vanishing of the longitudinal resistance. Recently, however, huge longitudinal resistance maxima (HLR) have been observed at fractional filling factors between 1 2 and 1 [3]. The HLR is only found in samples which have a reduced well thickness (15 nm, [4]) as compared to the conventional ones. As an example, figure 1 shows longitudinal resistance measurements on a sample similar to the one used in [3] for two different carrier densities (dotted and dashed line) at a temperature of 0.35 K. Here, the magnetic field is swept at a rate of 0.7 T/min and the applied source drain current is 100 nA. The width of the sample is 80 µm and the voltage probes are 80 µm apart. For both carrier densities a very regular behavior is seen. At integer filling factors the resistance vanishes completely and at filling factor ν = 2 3 one finds a clear minimum. However, if the sweep rate of the magnetic field is drastically reduced to 0.002 T/min, a huge maximum in the longitudinal resistance (solid lines) is observed at ν = 2 3 for both carrier densities. The size of the HLR is maximal at a current density of approximately 0.6 mA/m. The HLR vanishes in tilted magnetic fields, indicating that the electron spin polarization plays an important role for the HLR. Similar maxima are also reported at other fractional filling factors [3], but in this paper we want to concentrate on the HLR at ν = 2 3 at 0.35K. The HLR develops with a time constant of about 15 min. These very long times are typical for relaxation effects of the nuclear spin system [5,6]. The only direct way to demonstrate an involvement of the nuclear spins in the HLR is a nuclear magnetic resonance (NMR) [7][8][9][10] experiment, because it allows direct modification of the nuclear polarization. In this Letter we report on experiments where radio frequency is irradiated on a sample in the HLR state and a drastic reduction of the resistance values is observed whenever the nuclei are in resonance. This is to our knowledge the clearest form of a resistively ...

Evidence for Skyrmion Crystallization from NMR Relaxation Experiments

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Layer-charge instability in unbalanced bilayer systems in the quantum Hall regime

“…They are common magnetic materials, and often indicate a non-equilibrium situation associated with a phase transition and the presence of domains [1]. Recently, hysteresis has also been reported in various two-dimensional (2D) carrier systems in semiconductor structures at low temperatures and high magnetic fields [2,3,4,5,6,7]. In these cases, magneto-resistance (ρ xx ) hysteresis appears in the quantum Hall (QH) regime when two Landau levels (LLs) with opposite spin are brought into coincidence.…”

Measurements of composite fermion masses from the spin polarization of two-dimensional electrons in the region 1<ν<2

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