The effect of tunneling on the transport properties of quantum Hall double layers in the regime of the excitonic condensate at total filling factor one is studied in counterflow experiments. If the tunnel current I is smaller than a critical I C , tunneling is large and is effectively shorting the two layers. For I > I C tunneling becomes negligible. Surprisingly, the transition between the two tunneling regimes has only a minor impact on the features of the filling-factor one state as observed in magneto-transport, but at currents exceeding I C the resistance along the layers increases rapidly.

We report the observation of inverse-magnetic-field-periodic, radiation-induced magnetoresistance oscillations in GaAs/AlGaAs heterostructures prepared in W. Wegscheider's group, compare their characteristics with similar oscillations in V. Umansky's material, and describe the lineshape variation vs. the radiation power, P , in the two systems. We find that the radiation-induced oscillatory ∆Rxx, in both materials, can be described by ∆Rxx = −Aexp(−λ/B)sin(2πF/B), where A is the amplitude, λ is the damping parameter, and F is the oscillation frequency. Both λ and F turn out to be insensitive to P . On the other hand, A grows nonlinearly with P .

Microwave transmission of a coplanar microresonator deposited on a sample surface over a two-dimensional electron system has been studied. The transmission signal reveals a series of resonances corresponding to the excitation of hybrid cavity plasmon-photon modes, and ultrastrong plasmon-photon coupling has been realized. The hybridization frequency (Rabi frequency) is shown to be anomalously larger than the frequencies of unperturbed modes and it reaches values of up to 25 GHz. The effect of electron density and magnetic field on the excitation spectrum of cavity polariton has been investigated.

We present low temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide bandgap GaN/AlGaN system. We observed pronounced anti-localization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is αso ∼ 6× 10 −13 eVm, while the conduction band spin-orbit splitting energy amounts to ∆so ∼ 0.3meV at ne=1×10 16 m −2 .GaN has emerged as a leading material for a variety of new device applications, ranging from solid-state, ultra-violet optical sources to high power electronics [1]. While the performance of many devices fabricated from GaN has been stunning, several fundamental physical processes remain to be understood. A prime example is spin-orbit coupling in GaN and its heterostructures. The burgeoning field of spintronics has invigorated the study of spin-orbit coupling in semiconducting materials [2,3]. To date, much experimental effort has been devoted to narrow bandgap material systems (InAs, InGaAs, GaAs, etc) as spin-orbit coupling is expected to be strong in these systems. Conversely, far less experimental effort has been directed toward wide bandgap systems like GaN in which spin-orbit effects are predicted to be suppressed by the large fundamental bandgap, E g , and reduced spinorbit splitting, ∆ 0 , of the valence band at zone center. Indeed, in the k·p formalism [4], the bare Rashba spinorbit coupling constant for electrons, α 0 , scales as: α 0 ∼ ∆ 0 /E 2 g . As the value of ∆ 0 for GaAs exceeds that of GaN by a factor of 30, it is reasonable to suspect that spin splitting of the conduction band in GaN-based heterostructures would be insignificant compared to GaAs and other narrow gap heterostructures.Spin-orbit coupling for conduction band electrons in bulk GaN was considered by Krishnamurthy [5] who calculated that the spin relaxation times in bulk GaN should exceed the spin relaxation time in GaAs by three orders of magnitude, thus making GaN an excellent candidate for transport of spin polarized currents over macroscopic distances. However, in Ref.[5] GaN was assumed to have the zinc-blende lattice structure. GaN is typically grown in the more stable wurtzite phase. It is known that the symmetry of the underlying crystal has a profound impact on spin-orbit induced splittings in the conduction band [6,7]. While the work of Ref.[5] is suggestive, very few experimental results for bulk wurtzite GaN have been reported [8] and the impact of spin-orbit coupling on transport in wurtzite GaN/AlGaN heterostructures remains an open question. A few preliminary experiments have considered spin-orbit coupling for the twodimensional electron gas (2DEG) [9,10,11,12] in a narrow paramet...

Unipolar intersubband lasers like quantum cascade laser structures might be realized not only in III-V semiconductors but also in Si/SiGe multiple layer structures since no optical transitions across the indirect band gap are involved. We report on well-defined intersubband electroluminescence emission of Si/SiGe quantum cascade structures with different active quantum wells parameters. The complex valence band structure and a nonradiative relaxation rate of about 400 fs were calculated by multiband k•p formalism including Si/Ge segregation effects. The observed spectral shift of the electroluminescence peak from 146 to 159 meV is described well by quantum confinement of the two lowest heavy hole subbands. The electroluminescence observed reveals transverse magnetic polarization, a spectral line shape that changes with the direction of the current, and low-energy line broadening with an increase in temperature and current. All these features are described well by the k•p model calculation.