Intersubband magnetophonon resonances in quantum cascade structures

We report on our magnetotransport measurements of GaAs/GaAlAs quantum cascade structures in a magnetic field of up to 62 T. We observe novel quantum oscillations in tunneling current that are periodic in reciprocal magnetic field. We explain these oscillations as intersubband magnetophonon resonance due to electron relaxation by emission of either single optical or acoustic phonons. Our work also provides a non-optical in situ measurement of intersubband separations in quantum cascade structures.Comment: 5 pages, 4 figure

Identification of main contributions to conductivity of epitaxial InN

Complex effect of different contributions (spontaneously formed In nanoparticles, near-interface, surface and bulk layers) on electrophysical properties of InN epitaxial films is studied. Transport parameters of the surface layer are determined from the Shubnikov-de Haas oscillations measured in undoped and Mg-doped InN films at magnetic fields up to 63 T. It is shown that the In nanoparticles, near-interface and bulk layers play the dominant role in the electrical conductivity of InN, while influence of the surface layer is pronounced only in the compensated low-mobility InN:Mg films

Ultrahigh magnetic field spectroscopy reveals the band structure of the 3D topological insulator Bi2_2Se3_3

We have investigated the band structure at the $\Gamma$ point of the three-dimensional (3D) topological insulator Bi$_2$Se$_3$ using magneto-spectroscopy over a wide range of energies ($0.55-2.2$\,eV) and in ultrahigh magnetic fields up to 150\,T. At such high energies ($E>0.6$\,eV) the parabolic approximation for the massive Dirac fermions breaks down and the Landau level dispersion becomes nonlinear. At even higher energies around 0.99 and 1.6 eV, new additional strong absorptions are observed with a temperature and magnetic-field dependence which suggest that they originate from higher band gaps. Spin orbit splittings for the further lying conduction and valence bands are found to be 0.196 and 0.264 eV

Cyclotron resonance of extremely conductive 2D holes in high Ge content strained heterostructures

Cyclotron resonance has been observed in steady and pulsed magnetic fields from high conductivity holes in Ge quantum wells. The resonance positions, splittings and linewidths are compared to calculations of the hole Landau levels

Temperature-dependent magnetospectroscopy of HgTe quantum wells

We report on magnetospectroscopy of HgTe quantum wells in magnetic fields up to 45 T in temperature range from 4.2 K up to 185 K. We observe intra- and inter-band transitions from zero-mode Landau levels, which split from the bottom conduction and upper valence subbands, and merge under the applied magnetic field. To describe experimental results, realistic temperature-dependent calculations of Landau levels have been performed. We show that although our samples are topological insulators at low temperatures only, the signature of such phase persists in optical transitions at high temperatures and high magnetic fields. Our results demonstrate that temperature-dependent magnetospectroscopy is a powerful tool to discriminate trivial and topological insulator phases in HgTe quantum wells

Autocorrelation measurements of the FELBE free-electron laser and photocurrent saturation study in two-photon QWIPs

The two-photon QWIP approach involves three equidistant subbands, two of which are bound in the quantum well, and the third state is located in the continuum. The intermediate subband induces a resonantly enhanced optical nonlinearity, which is about six orders of magnitude stronger than in usual semiconductors. Temporal resolution is only limited by the sub-ps intrinsic time constants of the quantum wells, namely the intersubband relaxation time and the dephasing time of the intersubband polarization. Both properties make this device very promising for pulse diagnostics of pulsed mid-infrared lasers. We have performed autocorrelation measurements of ps optical pulses from the free-electron laser (FEL) facility FELBE at the Forschungszentrum Dresden Rossendorf. Using a rapid-scan autocorrelation scheme at a scan frequency of 20 Hz, high-quality quadratic autocorrelation traces are obtained, yielding ratios close to the theoretically expected value of 8:1 between zero delay and large delay for interferometric autocorrelation, and 3:1 for intensity autocorrelation. Thus, two-photon QWIPs provide an excellent new technique for online pulse monitoring of the FEL. In addition, we have investigated the saturation mechanism of the photocurrent signal, which is due to internal space charges generated in the detector

Spin gap state in α'-NaV2O5 studied by far infrared spectroscopy

We present far infrared transmission measurements of α'-NaV2O5 single crystals. This material is known to undergo a spin-Peierls like transition at TSP=35K. Below the transition temperature we observed together with expected new sharp phonon lines a strong gap-like depression of absorption occurring in the low energy part of a continuum likely connected with two-magnon excitations. The optical gap temperature dependence and its value closely match the spin gap values measured by neutron scattering experiments. Taking the optical gap as a fingerprint of the spin gap, we found that the relationship between the spin gap value and the dimerisation lattice parameter do not follow the scaling relation expected for the SP systems

Intersubband magnetophonon resonances in quantum cascade structures

We report on magnetotransport measurements of GaAs/GaAlAs quantum cascade structures in magnetic fields up to 62 T parallel to the current. We observe novel quantum oscillations series in tunneling current that are periodic in reciprocal magnetic field and have field positions independent of the applied bias. These oscillations are explained as intersubband magnetophonon resonance due to electron relaxation by emission of optical or acoustic phonons