Inelastic quantum transport in superlattices: success and failure of the Boltzmann equation

Electrical transport in semiconductor superlattices is studied within a fully self-consistent quantum transport model based on nonequilibrium Green functions, including phonon and impurity scattering. We compute both the drift velocity-field relation and the momentum distribution function covering the whole field range from linear response to negative differential conductivity. The quantum results are compared with the respective results obtained from a Monte Carlo solution of the Boltzmann equation. Our analysis thus sets the limits of validity for the semiclassical theory in a nonlinear transport situation in the presence of inelastic scattering.Comment: final version with minor changes, to appear in Physical Review Letters, sceduled tentatively for July, 26 (1999

Tunneling magneto thermo power in magnetic tunnel junction nanopillars

We study the tunneling magneto thermo power (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by a micropatterned electric heater line. Thermo power voltages up to a few tens of \muV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence with the applied temperature gradient. The thermo power signal varies by up to 10 \muV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100 \muV/K and a large TMTP change of the tunnel junction of up to 90%.Comment: Revised version containing additional data and analyis. 13 pages, 3 figure

Социально ориентированные программы, KIDS SAVE LIVES (Дети спасают жизни), World Restart a Heart (Перезапусти сердце) и другие кампании для повышения выживаемости при внебольничной остановке сердца

Sudden out-of-hospital cardiac arrest (OHCA) is the third leading cause of death in industrialized nations.While educating lay bystanders in CPR is clearly the most important way to increase survival in this third leading cause of death and the community programs are helpful, it is less easy to reach the entire population if programs are not obligatory. Therefore experts see the need for obligatory inclusion of CPR education in schools. And this is exactly our clear expectation to all politicians involved in health care and school politics worldwide. Until this is reality in all countries, we invite everyone to help to convince the responsible politicians and to take part in campaigns like KIDS SAVE LIVES and WRAH.Внезапная внебольничная остановка сердца (ВОС) — третья по частоте причина смерти в развитых странах.Обучение основам СЛР лиц без медицинского образования — наиболее важная стратегия повышения выживаемости при остановке сердца. Программы, включающие такое обучение, весьма эффективны, однако полного охвата населения достичь очень трудно вследствие их необязательного статуса. В связи с этим, по мнению экспертов, СЛР необходимо ввести в школьную программу в качестве обязательного предмета. Именно этого мы ждем от политиков, ответственных за систему здравоохранения и образование, во всех странах. Пока наша цель не достигнута, мы призываем всех помочь нам убедить ответственных лиц и принять участие в таких кампаниях, как KIDS SAVE LIVES и World Restart a Heart

Determination of spin-dependent Seebeck coefficients of CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars

We investigate the spin-dependent Seebeck coefficient and the tunneling magneto thermopower of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) in the presence of thermal gradients across the MTJ. Thermal gradients are generated by an electric heater on top of the nanopillars. The thermo power voltage across the MTJ is found to scale linearly with the heating power and reveals similar field dependence as the tunnel magnetoresistance. The amplitude of the thermal gradient is derived from calibration measurements in combination with finite element simulations of the heat flux. Based on this, large spin-dependent Seebeck coefficients of the order of (240 \pm 110) \muV/K are derived. From additional measurements on MTJs after dielectric breakdown, a tunneling magneto thermopower up to 90% can be derived for 1.5 nm MgO based MTJ nanopillars

Biased quasi ballistic spin torque magnetization reversal

We explore the fundamental time limit of ultra fast spin torque induced magnetization reversal of a magnetic memory cell. Spin torque precession during a spin torque current pulse and free precessional magnetization ringing after spin torque pulse excitation is detected by time resolved magneto transport. Adapting the duration of the spin torque excitation pulse to the spin torque precession period allows suppression of the magnetization ringing and thus coherent control of the final orientation of the magnetization. In the presence of a hard axis bias field such coherent control enables an optimum ultra fast, quasi ballistic spin torque magnetization reversal by a single precessional turn directly from the initial to the reversed equilibrium state.Comment: 13 pages 3 Figure

Production and optical properties of liquid scintillator for the JSNS2^{2} experiment

The JSNS$^{2}$ (J-PARC Sterile Neutrino Search at J-PARC Spallation Neutron Source) experiment will search for neutrino oscillations over a 24 m short baseline at J-PARC. The JSNS$^{2}$ inner detector will be filled with 17 tons of gadolinium-loaded liquid scintillator (LS) with an additional 31 tons of unloaded LS in the intermediate $\gamma$-catcher and outer veto volumes. JSNS$^{2}$ has chosen Linear Alkyl Benzene (LAB) as an organic solvent because of its chemical properties. The unloaded LS was produced at a refurbished facility, originally used for scintillator production by the RENO experiment. JSNS$^{2}$ plans to use ISO tanks for the storage and transportation of the LS. In this paper, we describe the LS production, and present measurements of its optical properties and long term stability. Our measurements show that storing the LS in ISO tanks does not result in degradation of its optical properties.Comment: 7 pages, 4 figures

Intersubband gain in a Bloch oscillator and Quantum cascade laser

The link between the inversion gain of quantum cascade structures and the Bloch gain in periodic superlattices is presented. The proposed theoretical model based on the density matrix formalism is able to treat the gain mechanism of the Bloch oscillator and Quantum cascade laser on the same footing by taking into account in-plane momentum relaxation. The model predicts a dispersive contribution in addition to the (usual) population-inversion-dependent intersubband gain in quantum cascade structures and - in the absence of inversion - provides the quantum mechanical description for the dispersive gain in superlattices. It corroborates the predictions of the semi-classical miniband picture, according to which gain is predicted for photon energies lower than the Bloch oscillation frequency, whereas net absorption is expected at higher photon energies, as a description which is valid in the high-temperature limit. A red-shift of the amplified emission with respect to the resonant transition energy results from the dispersive gain contribution in any intersubband transition, for which the population inversion is small.Comment: 10 pages, 6 figure

Elastic constants of nematic liquid crystals of uniaxial symmetry

We study in detail the influence of molecular interactions on the Frank elastic constants of uniaxial nematic liquid crystals composed of molecules of cylindrical symmetry. A brief summary of the status of theoretical development for the elastic constants of nematics is presented. Considering a pair potential having both repulsive and attractive parts numerical calculations are reported for three systems MBBA, PAA and 8OCB. For these systems the length-to-width ratio ${x_0}$ is estimated from the experimentally proposed structure of the molecules. The repulsive interaction is represented by a repulsion between hard ellipsoids of revolution (HER) and the attractive potential is represented by the quadrupole and dispersion interactions. From the numerical results we observe that in the density range of nematics the contribution of the quadrupole and dispersion interactions are small as compared to the repulsive HER interaction. The inclusion of attractive interaction reduces the values of elastic constants ratios. The temperature variation of elastic constants ratios are reported and compared with the experimental values. A reasonably good agreement between theory and experiment is observed