Quantum point contacts fabricated using a backgated two-dimensional electron-gas system show clear quantized features and a 0.7 anomaly in conductance. Using these density-tunable point contacts, we have studied the behavior of the 0.7 anomaly. The 0.7 step shifts down to around 0.5 as the electron density is decreased under a zero magnetic field. This suggests that electron-electron interactions play an important role in forming the conductance anomalies of quantum point contacts.

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Articles you may be interested inResistive detection of optically pumped nuclear polarization with spin phase transition peak at Landau level filling factor 2/3 Appl. Phys. Lett. 99, 112106 (2011); 10.1063/1.3640215 Features of polymer chain dynamics as revealed by intermolecular nuclear magnetic dipole-dipole interaction: Model calculations and field-cycling NMR relaxometry J. Chem. Phys. 132, 094903 (2010); 10.1063/1.3336832Temperature dependence of the resistively detected electron spin resonance signals in a wide parabolic quantum well AIP Conf.We study decoherence of nuclear spins in a GaAs quantum well structure using resistively detected nuclear magnetic resonance. The transverse decoherence time T 2 of 75 As nuclei is estimated from Rabi-type coherent oscillations as well as by using spin-echo techniques. By analyzing T 2 obtained by decoupling techniques, we extract the role of dipole-dipole interactions as sources of decoherence in GaAs. Under the condition that the device is tilted in an external magnetic field, we exhibit enhanced decoherence induced by the change in strength of the direct dipole-dipole interactions between first nearest-neighbor nuclei. The results agree well with simple numerical calculations.

We studied the receiver performance of two photoconductive antennas ͑bow tie and dipole antennas͒ fabricated on the same low-temperature-grown GaAs substrate to clarify the effect of the antenna structure and gate pulse intensity on terahertz wave detection. We observed the gate pulse intensity dependence of the temporal profiles of the terahertz waves or terahertz spectra. For both antennas, the sensitivity in the low-frequency regime ͑Ͻ0.5 THz͒ was enhanced compared to that in the high-frequency regime for large gate pulse intensities. This is because the carrier trap time increased due to the saturation of the GaAs defect levels. We also observed that the peak-to-peak amplitude of the terahertz wave detected by one antenna was not always larger than that detected by the other antenna, and the peak-to-peak amplitude of the bow tie antenna was larger ͑smaller͒ than that of the dipole antenna when the gate pulse intensity was high ͑low͒. This was explained by the gate pulse intensity dependence of the frequency-dependent detection sensitivity and also by the resonance frequency of the antenna structure.

Reflection-type degenerate pump-probe spectroscopy was performed for low-temperature grown (LT-) GaAs to study the effects of arsenic pressure during crystal growth and annealing on carrier lifetime and to ascertain the annealing dynamics. It was found that a sample grown under a high arsenic pressure has a shorter carrier lifetime for both as-grown and anneal conditions. It was also found that the carrier decay times of samples changed drastically when the annealing temperature was above 550 °C. We determined the annealing dynamics of LT-GaAs based on a model in which AsGa antisite defects trap photoexcited carriers. An Arrhenius plot of the carrier decay rate vs. annealing temperature in the high temperature regime gave an energy EPA that was different from the true activation energy. The annealing time dependences of EPA obtained by the two diffusion models (self diffusion and VGa vacancy assisted diffusion of defects) were compared with EPAs of our data and other works, which proved that the annealing dynamics of AsGa antisite defects was dominated by VGa vacancy assisted diffusion.