Diffusion of Photoexcited Holes in a Viscous Electron Fluid.

The diffusion of photogenerated holes is studied in a high-mobility mesoscopic GaAs channel where electrons exhibit hydrodynamic properties. It is shown that the injection of holes into such an electron system leads to the formation of a hydrodynamic three-component mixture consisting of electrons and photogenerated heavy and light holes. The obtained results are analyzed within the framework of ambipolar diffusion, which reveals characteristics of a viscous flow. Both hole types exhibit similar hydrodynamic characteristics. In such a way the diffusion lengths, ambipolar diffusion coefficient, and the effective viscosity of the electron-hole system are determined.

Stokes flow around an obstacle in viscous two-dimensional electron liquid.

The electronic analog of the Poiseuille flow is the transport in a narrow channel with disordered edges that scatter electrons in a diffuse way. In the hydrodynamic regime, the resistivity decreases with temperature, referred to as the Gurzhi effect, distinct from conventional Ohmic behaviour. We studied experimentally an electronic analog of the Stokes flow around a disc immersed in a two-dimensional viscous liquid. The circle obstacle results in an additive contribution to resistivity. If specular boundary conditions apply, it is no longer possible to detect Poiseuille type flow and the Gurzhi effect. However, in flow through a channel with a circular obstacle, the resistivity decreases with temperature. By tuning the temperature, we observed the transport signatures of the ballistic and hydrodynamic regimes on the length scale of disc size. Our experimental results confirm theoretical predictions.

Monolithically integrated single quantum dots coupled to bowtie nanoantennas.

Deterministically integrating semiconductor quantum emitters with plasmonic nano-devices paves the way towards chip-scale integrable, true nanoscale quantum photonic technologies. For this purpose, stable and bright semiconductor emitters are needed, which moreover allow for CMOS-compatibility and optical activity in the telecommunication band. Here, we demonstrate strongly enhanced light-matter coupling of single near-surface (< 10 nm) InAs quantum dots monolithically integrated into electromagnetic hot-spots of sub-wavelength sized metal nanoantennas. The antenna strongly enhances the emission intensity of single quantum dots by up to ~ 16×, an effect accompanied by an up to 3.4× Purcell-enhanced spontaneous emission rate. Moreover, the emission is strongly polarised along the antenna axis with degrees of linear polarisation up to ~ 85 %. The results unambiguously demonstrate a pronounced coupling of individual quantum dots to state-of-the-art nanoantennas. Our work provides new perspectives for the realisation of quantum plasmonic sensors, step-changing photovoltaic devices, bright and ultrafast quantum light sources and efficient nano-lasers.

Piezoelectric Electromechanical Coupling in Nanomechanical Resonators with a Two-Dimensional Electron Gas.

The electrical response of a two-dimensional electron gas to vibrations of a nanomechanical cantilever containing it is studied. Vibrations of perpendicularly oriented cantilevers are experimentally shown to oppositely change the conductivity near their bases. This indicates the piezoelectric nature of electromechanical coupling. A physical model is developed, which quantitatively explains the experiment. It shows that the main origin of the conductivity change is a rapid change in the mechanical stress on the boundary between suspended and nonsuspended areas, rather than the stress itself.

Excitonic spin-splitting in quantum wells with a tilted magnetic field.

This work aims to investigate the effects of magnetic field strength and direction on the electronic properties and optical response of GaAs/AlGaAs-based heterostructures. An investigation of the excitonic spin-splitting of a disordered multiple quantum well embedded in a wide parabolic quantum well is presented. The results for polarization-resolved photoluminescence show that the magnetic field dependencies of the excitonic spin-splitting and photoluminescence linewidth are crucially sensitive to magnetic field orientation. Our experimental results are in good agreement with the calculated Zeeman splitting obtained by the Luttinger model, which predicts a hybridization of the spin character of states in the valence band under tilted magnetic fields.

Microwave-Induced Magneto-Oscillations and Signatures of Zero-Resistance States in Phonon-Drag Voltage in Two-Dimensional Electron Systems.

We observe the phonon-drag voltage oscillations correlating with the resistance oscillations under microwave irradiation in a two-dimensional electron gas in perpendicular magnetic field. This phenomenon is explained by the influence of dissipative resistivity modified by microwaves on the phonon-drag voltage perpendicular to the phonon flux. When the lowest-order resistance minima evolve into zero-resistance states, the phonon-drag voltage demonstrates sharp features suggesting that current domains associated with these states can exist in the absence of external dc driving.

Circularly polarized photoluminescence as a probe of density of states in GaAs/AlGaAs quantum Hall bilayers.

Polarized magnetophotoluminescence is employed to study the energies and occupancies of four lowest Landau levels in a couple quantum Hall GaAs/AlGaAs double quantum well. As a result, a magnetic field-induced redistribution of charge over the Landau levels manifesting to the continuous formation of the charge density wave and direct evidence for the symmetric-antisymmetric gap shrinkage at ν=3 are found. The observed interlayer charge exchange causes depolarization of the ferromagnetic ground state.

Microwave zero-resistance states in a bilayer electron system.

Magnetotransport measurements on a high-mobility electron bilayer system formed in a wide GaAs quantum well reveal vanishing dissipative resistance under continuous microwave irradiation. Profound zero-resistance states (ZRS) appear even in the presence of additional intersubband scattering of electrons. We study the dependence of photoresistance on frequency, microwave power, and temperature. Experimental results are compared with a theory demonstrating that the conditions for absolute negative resistivity correlate with the appearance of ZRS.

Zero-differential resistance state of two-dimensional electron systems in strong magnetic fields.

We report the observation of a zero-differential resistance state (ZDRS) in response to a direct current above a threshold value I>I th applied to a two-dimensional system of electrons at low temperatures in a strong magnetic field. Entry into the ZDRS, which is not observable above several Kelvins, is accompanied by a sharp dip in the differential resistance. Additional analysis reveals an instability of the electrons for I>I th and an inhomogeneous, nonstationary pattern of the electric current. We suggest that the dominant mechanism leading to the new electron state is a redistribution of electrons in energy space induced by the direct current.

Reentrant quantum Hall effect and anisotropic transport in a bilayer system at high filling factors.

We report on the measurements of the quantum Hall effect states in double quantum well structures at the filling factors nu=4N+1 and nu=4N+3, where N is the Landau index number, in the presence of the in-plane magnetic field. The quantum Hall states at these filling factors vanish and reappear several times and exhibit anisotropy. Repeated reentrance of the transport gap occurs due to the periodic vanishing of the tunneling amplitude in the presence of the in-plane field. Anisotropy demonstrates the existence of the stripes in the ground states.

Non-classical light emission from a single electrically driven quantum dot.

Easy to handle light sources with non-classical emission features are strongly demanded in the growing field of quantum communication. We report on single-photon emission from an electrically pumped quantum dot with unmatched spectral purity, making spatial or spectral filtering dispensable.