2009, Brünig, R., Mensel, K., Kunze, R., Schmidt, H.

Surface acoustic wave (SAW) actuators are driven by a high frequency signal. The frequency range for an ideal SAW-generation is usually very narrow banded and may shift depending on various environmental conditions. We present a new electronic device which self-aligns to the optimal excitation frequency within a wide range. Any kind of SAW-actuator can be used. The device continuously scans a certain frequency range and characterizes the SAW-component. The ideal excitation frequency is then determined and used to drive the SAW-device. In case of changes like loading conditions or temperature variations the device automatically readjusts to the optimal frequency and prevents possible damage of the device or actuator in case of an error. © 2009.

2007, Hafenbrak, R., Ulrich, S.M., Michler, P., Wang, L., Rastelli, A., Schmidt, O.G.

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

2001, Geupel, S., Zahn, G., Paufler, P., Graw, G.

B14Ho4Ni, tetragonal, P4/mnc (No. 128), a = 7.2097(8) Å, c = 7.4587(9) Å, V = 387.7 Å3, Z = 2, Rgt(F) = 0.049, wRref(F2) = 0.087, T = 300 K.

2000, Geupel, S., Belger, A., Paufler, P., Graw, G.

BHoNi4, hexagonal, P6/mmm (No. 191), a = 4.9696(4) Å, c = 6.9419(5) Å, V= 148.5 Å3, Z= 2, ρm = 9.13(1) g·cm-3, R(P) = 0.072, wR(P) = 0.099, R(I) = 0.065, T= 300 K.

2008, Wang, L., Rastelli, A., Kiravittaya, S., Atkinson, P., Ding, F., Bof Bufon, C.C., Hermannstädter, C., Witzany, M., Beirne, G.J., Michler, P., Schmidt, O.G.

We report on the fabrication, detailed characterization and modeling of lateral InGaAs quantum dot molecules (QDMs) embedded in a GaAs matrix and we discuss strategies to fully control their spatial configuration and electronic properties. The three-dimensional morphology of encapsulated QDMs was revealed by selective wet chemical etching of the GaAs top capping layer and subsequent imaging by atomic force microscopy (AFM). The AFM investigation showed that different overgrowth procedures have a profound consequence on the QDM height and shape. QDMs partially capped and annealed in situ for micro- photoluminescence spectroscopy consist of shallow but well-defined quantum dots (QDs) in contrast to misleading results usually provided by surface morphology measurements when they are buried by a thin GaAs layer. This uncapping approach is crucial for determining the QDM structural parameters, which are required for modeling the system. A single-band effective-mass approximation is employed to calculate the confined electron and heavy-hole energy levels, taking the geometry and structural information extracted from the uncapping experiments as inputs. The calculated transition energy of the single QDM shows good agreement with the experimentally observed values. By decreasing the edge-to-edge distance between the two QDs within a QDM, a splitting of the electron (hole) wavefunction into symmetric and antisymmetric states is observed, indicating the presence of lateral coupling. Site control of such lateral QDMs obtained by growth on a pre-patterned substrate, combined with a technology to fabricate gate structures at well-defined positions with respect to the QDMs, could lead to deterministically controlled devices based on QDMs. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

2009, Rößler, S., Harikrishnan, S., Naveen Kumar, C.M., Bhat, H.L., Elizabeth, S., Rößler, U.K., Steglich, F., Wirth, S.

We report on the magnetic and electrical properties of Pr 0.6Sr 0.4MnO 3 single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T C301 K and a first-order structural transition at T S64 K. At T S, the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M-H) curves below T S are anomalous in that the virgin curve lies outside the subsequent M-H loops. The hysteretic structural transition at T S as well as the irreversible magnetization processes below T S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.

2000, Geupel, S., Zahn, G., Paufler, P., Graw, G.

B6Ho2Ni3, orthorhombic, Cmmm (No. 65), a = 7.6865(9) Å, b = 8.6679(9) Å, c = 3.4742(4) Å, V = 231.5 Å3, Z = 2, Rgt(F) = 0.021, wRref(F2) = 0.048, T= 300 K.

2000, Geupel, S., Zahn, G., Paufler, P., Graw, G.

BCHoNi, tetragonal, P4/nmm (No. 129), a =3.5621(5) Å, c = 7.556(2) Å, V = 95.9 Å3, Z = 2, Rgt(F) = 0.030, wRref(F2) = 0.076, T= 300 K.

2009, Oliveira, R.M., Ueda, M., Silva, L.L.G., Reuther, H., Lepienski, C.M.

A nitriding process based on two distinct nitrogen glow discharge modes, with sample temperatures ranging from 380 °Cto480°C, was employed to treat the surface of austenitic stainless steel (SS 304). The temperature is controlled exclusively by switching the operation conditions of the discharges. First mode of operation is the conventional one, named cathodic, which runs at higher pressure values (1 mbar) in comparison to the second mode, named anodic, which runs at the pressure range of 10-3 -10-2 mbar. Cathodic mode is used to quickly heat the sample holder, by the high ion flux. On the other hand, in the anodic mode, due to the lower operation pressure, higher effective ion acceleration takes place, which allows deeper ion implantation into the sample surface. This hybrid process was thoroughly explored regarding the duty cycle and conditions of operation, to achieve optimal performance of the treatments, which led to the attainment of surface hardness for samples of AISI SS 304 as high as 20 GPa and improvements including higher elastic modulus and resistance against corrosion. Detailed comparison among samples treated by this process with others treated by conventional method was done using nanoindentation, Auger Electron Spectroscopy (AES) and corrosion resistance testing.