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Register a new userDirect Measurement of Cyclotron Coherence Times of High-Mobility Two-Dimensional Electron Gases
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We have observed long-lived (~30 ps) coherent oscillations of charge carriers due to cyclotron resonance (CR) in high-mobility two-dimensional electrons in GaAs in perpendicular magnetic fields using time-domain terahertz spectroscopy. The observed coherent oscillations were fitted well by sinusoids with exponentially-decaying amplitudes, through which we were able to provide direct and precise measures for the decay times and oscillation frequencies simultaneously. This method thus overcomes the CR saturation effect, which is known to prevent determination of true CR linewidths in high-mobility electron systems using Fourier-transform infrared spectroscopy.
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GaAs-based two-dimensional electron gases (2DEGs) show a wealth of remarkable electronic states, and serve as the basis for fast transistors, research on electrons in nanostructures, and prototypes of quantum-computing schemes. All these uses depend on the extremely low levels of disorder in GaAs 2DEGs, with low-temperature mean free paths ranging from microns to hundreds of microns. Here we study how disorder affects the spatial structure of electron transport by imaging electron flow in three different GaAs/AlGaAs 2DEGs, whose mobilities range over an order of magnitude. As expected, electrons flow along narrow branches that we find remain straight over a distance roughly proportional to the mean free path. We also observe two unanticipated phenomena in high-mobility samples. In our highest-mobility sample we observe an almost complete absence of sharp impurity or defect scattering, indicated by the complete suppression of quantum coherent interference fringes. Also, branched flow through the chaotic potential of a high-mobility sample remains stable to significant changes to the initial conditions of injected electrons.
We report an observation of magnetooscillations of the microwave power transmitted through the high mobility two-dimensional electron system hosted by a GaAs quantum well. The oscillations reflect an enhanced absorption of radiation at high harmonics of the cyclotron resonance and follow simultaneously measured microwave-induced resistance oscillations (MIRO) in the dc transport. While the relative amplitude (up to 1%) of the transmittance oscillations appears to be small, they represent a significant (>50%) modulation of the absorption coefficient. The analysis of obtained results demonstrates that the low-B decay, magnitude, and polarization dependence of the transmittance oscillations accurately follow the theory describing photon-assisted scattering between distant disorder-broadened Landau levels. The extracted sample parameters reasonably well describe the concurrently measured MIRO. Our results provide an insight into the MIRO polarization immunity problem and pave the way to probe diverse high-frequency transport properties of high-mobility systems using precise transmission measurements.
In a high mobility two-dimensional electron gas (2DEG) in a GaAs/AlGaAs quantum well we observe a strong magnetoresistance. In lowering the electron density the magnetoresistance gets more pronounced and reaches values of more than 300%. We observe that the huge magnetoresistance vanishes for increasing the temperature. An additional density dependent factor is introduced to be able to fit the parabolic magnetoresistance to the electron-electron interaction correction.
We have observed cyclotron resonance in a high-mobility GaAs/AlGaAs two-dimensional electron gas by using the techniques of terahertz time-domain spectroscopy combined with magnetic fields. From this, we calculate the real and imaginary parts of the diagonal elements of the magnetoconductivity tensor, which in turn allows us to extract the concentration, effective mass, and scattering time of the electrons in the sample. We demonstrate the utility of ultrafast terahertz spectroscopy, which can recover the true linewidth of cyclotron resonance in a high-mobility ($>{10}^{6} mathrm{cm^{2} V^{-1} s^{-1}}$) sample without being affected by the saturation effect.
The discovery of two-dimensional electron gas (2DEG) at well-defined interfaces between insulating complex oxides provides the opportunity for a new generation of all-oxide electronics. Particularly, the 2DEG at the interface between two perovskite insulators represented by the formula of ABO3, such as LaAlO3 and SrTiO3, has attracted significant attention. In recent years, progresses have been made to decipher the puzzle of the origin of interface conduction, to design new types of oxide interfaces, and to improve the interfacial carrier mobility significantly. These achievements open the door to explore fundamental as well as applied physics of complex oxides. Here, we review our recent experimental work on metallic and insulating interfaces controlled by interfacial redox reactions in SrTiO3-based heterostructures. Due to the presence of oxygen-vacancies at the SrTiO3 surface, metallic conduction can be created at room temperature in perovskite-type interfaces when the overlayer oxide ABO3 involves Al, Ti, Zr, or Hf elements at the B-sites. Furthermore, relying on interface-stabilized oxygen vacancies, we have created a new type of 2DEG at the heterointerface between SrTiO3 and a spinel {gamma}-Al2O3 epitaxial film with compatible oxygen ions sublattices. The spinel/perovskite oxide 2DEG exhibits an electron mobility exceeding 100,000 cm2V-1s-1, more than one order of magnitude higher than those of hitherto investigated perovskite-type interfaces. Our findings pave the way for design of high-mobility all-oxide electronic devices and open a route towards studies of mesoscopic physics with complex oxides.
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