Quantum wells

 

  • Fractional quantum Hall effect in a dilute magnetic semiconductor

 

We report the observation of the fractional quantum Hall effect in the lowest Landau level of a two-dimensional electron system (2DES), residing in the diluted magnetic semiconductor Cd1−xMnxTe. The presence of magnetic impurities results in a giant Zeeman splitting leading to an unusual ordering of composite fermion Landau levels. In experiment, this results in an unconventional opening and closing of fractional gaps around the filling factor ν = 3/2 as a function of an in-plane magnetic field, i.e., of the Zeeman energy. By including the s-d exchange energy into the composite Landau level spectrum the opening and closing of the gap at filling factor 5/3 can be modeled quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES provides a means to manipulate fractional states.

C. Betthausen et al., Phys. Rev. B 90, 115302, (2014)

 

  • Fine structure of zero-mode Landau levels in HgTe/HgxCd1−xTe quantum wells

 

HgTe/HgxCd1−xTe quantum wells with the inverted band structure have been probed using far-infrared magnetospectroscopy. Realistic calculations of Landau level diagrams have been performed to identify the observed transitions. Investigations have been greatly focused on the magnetic field dependence of the peculiar pair of “zero-mode” Landau levels, which characteristically split from the upper conduction and bottom valence bands, and merge under the applied magnetic field. The observed avoided crossing of these levels is tentatively attributed to the bulk inversion asymmetry of zinc-blende compounds

M. Orlita et al., Phys. Rev. B 83, 115307, (2011)

 

  • Fractional quantum Hall effect in CdTe

 

The fractional quantum Hall (FQH) effect is reported in a high mobility CdTe quantum well at millikelvin temperatures. Fully developed FQH states are observed at filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state for which the lowest energy excitation is not a spin flip. This can be accounted for by the relatively high intrinsic Zeeman energy in this single valley two-dimensional electron gas. FQH minima are also observed in the first excited (N= 1) Landau level at filling factor 7/3 and 8/3 for intermediate temperatures. In contrast, the 5/2 FQH state remains absent down to T 10 mK.

B. Piot et al., Phys. Rev. B 82, 081307, (2010)

 

  • Electron-phonon interactions in a single modulation-doped GaInAs quantum well

 

Precise absolute far–infra-red magneto-transmission experiments have been performed under magnetic fields up to 28 T on a series of single Ga0.24In0.76As quantum wells n-type modulation doped at different levels. The transmission spectra have been simulated with a multilayer dielectric model. This allows us to extract the imaginary part of the optical response function which reveals new singular features related to electron-phonon interactions. In addition to the expected polaronic effects due to the longitudinal (LO) phonons, one observes other interactions with the transverse optical (TO) phonons and a new kind of carrier concentration dependent interaction with interface phonons. This system provides a unique opportunity to study multiple types of electron-phonon interactions in a single type of compound.