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Through the femtosecond-time-resolved study of photoacoustic pulse generation in ultra-thin gold nanofilms, we observed a time delay of 0.5–0.7 ps in the formation of thermal-expansion pulses after photoexcitation. Our observation indicates that lattice anharmonicity dominates over hot electron pressure in the thermal expansion of Au nanofilms under ultrashort-pulsed photoexcitation.

We investigate the effects of surface tunneling on electrostatics and transport properties of two-dimensional electron gases (2DEGs) in undoped Si/SiGe heterostructures with different 2DEG depths. By varying the gate voltage, four stages of density-mobility dependence are identified with two density saturation regimes observed, which confirms that the system transitions between equilibrium and nonequilibrium. Mobility is enhanced with an increasing density at low biases and, counterintuitively, with a decreasing density at high biases as well. The density saturation and mobility enhancement can be semiquantitatively explained by a surface tunneling model in combination with a bilayer screening theory.

We investigate DC characteristics of a two-dimensional electron gas (2DEG) in an undoped Si/SiGe heterostructure and its temperature dependence. An insulated-gate field-effect transistor was fabricated, and transfer characteristics were measured at 4 K–300 K. At low temperatures (T 45 K, the current saturation and negative transconductance disappear and the device acts as a normal transistor.