Valmir Antonio Chitta

The study of electron hydrodynamics provides a powerful framework for understanding transport in ultraclean conductors, yet experimental evidence has thus far been largely restricted to the linear response regime. Here, we report the direct observation of a strongly nonlinear transport regime in a high-mobility two-dimensional electron system. By engineering devices in a Venturi-shaped (wedge) geometry specifically designed to enhance convective nonlinearities, we uncover a pronounced nonlinear voltage response and a large diodicity in the current-voltage characteristics. Our experimental findings show quantitative agreement with a theoretical model that attributes the observed nonlinearity to the convective acceleration of the electron fluid, analogous to the Bernoulli effect. These results provide compelling evidence for the applicability of the hydrodynamic framework to electron transport in two dimensions and open avenues for exploring nonlinear and preturbulent phenomena in solid-state systems.

We revisited the influence of electron-electron scattering on the resistivity of a two-dimensional system with a linear spectrum. In conventional systems with a parabolic spectrum, where umklapp scattering is either prohibited or ineffective due to the small Fermi surface, particle-particle scattering does not contribute to conductivity because it does not change the total momentum. However, within the framework of the Boltzmann kinetic model, we demonstrate that electron-electron scattering in Dirac systems can significantly contribute to conductivity, producing distinct temperature-dependent corrections: a T^{4} behavior at low temperatures and a T^{2} dependence at moderate temperatures. While the predicted T^{4} scaling is not observed experimentally-likely suppressed by dominant weak localization effects-the T^{2} scaling is clearly confirmed in our measurements. Specifically, temperature-dependent resistivity data from a gapless single-valley HgTe quantum well exhibit T^{2} corrections, which align well with theoretical predictions. Thus, we challenge the paradigm that the T^{2} term in resistivity is absent in single-band 2D metals.

We present a simple semiclassical model to sustain that in europium chalcogenides (EuX), Faraday rotation (FR) in the transparency gap is proportional to the magnetization of the sample, irrespective of the material’s magnetic phase, temperature, or applied magnetic field. The model is validated by FR and magnetization measurements in EuSe in the temperature interval 1.7–300 K, covering all EuSe magnetic phases (paramagnetic, antiferromagnetic type I or type II, ferrimagnetic, and ferromagnetic). Furthermore, by combining the semiclassical model with the explicit electronic energy structure of EuX, the proportionality coefficient between magnetization and FR is shown to be dependent only on the wavelength and the bandgap. Due to its simplicity, the model has didactic value; moreover, it provides a working tool for converting FR into magnetization in EuX. The possible extension of the model to other intrinsic magnetic semiconductors is discussed.