5,237 research outputs found
Photo-induced Modulation of Ferroelectric Polarization in Multiferroic TbMnO3
No full textTbMnO3 is a multiferroic material with a complex magnetic structure at low temperature, giving rise to spin ordering induced ferroelectricity. We show that this material exhibits a photo-induced transient current below the ferroelectric transition temperature. The result is interpreted in terms of a photo-induced polarization modulation generating the transient pyroelectric current detected by its associated transient voltage. The experiments demonstrate a relatively simple technique to study the ferroelectric polarization modulation in multiferroic materials.
Time-Dependent (Current) Density Functional Theory for Periodic Systems
No full textIn this article we review time-dependent density functional theory for calculating the static and frequency-dependent dielectric function ε(ω) of nonmetallic crystals. We show that a real-space description becomes feasible for solids by using a combination of a lattice-periodic (microscopic) scalar potential with a uniform (macroscopic) electric field for the description of the effective one-electron system. We treat the time-dependent fields as perturbations in a periodic structure calculation. The induced density and microscopic potential can be obtained self-consistently for fixed macroscopic field by using linear response theory in which Coulomb interactions and exchange-correlation effects are included. The dielectric function can then be obtained from the induced current. We obtained ε(ω) for a wide variety of nonmetallic crystals within the adiabatic local density approximation (ALDA) in good agreement with experiment. In particular in the low-frequency range no adjustment of the band gap obtained within the local density approximation (LDA) seems to be necessary. Relativistic effects on the dielectric response have been found to be important for a few semimetals that have inverted bandstructures within the LDA. Exchange-correlation effects beyond the ALDA have been treated by a polarization-dependent functional for the effective electric field, with improved dielectric functions as result.
Role of wave functions in electromagnetism: RAS from GaAs (110)
No full textWe have calculated the reflectance anisotropy for the GaAs (110) surface using the discrete cellular method. This method extends the range of application of standard discrete dipole calculations by incorporating nonlocal polarizabilitites. The method adds a second quantum mechanical channel of nonlocality, which turns out to be necessary and yields very good agreement between theory and experiment.
Computer Simulation of Quantum Phenomena in Nano-Scale Devices
No full textThis paper reviews the general concepts for building algorithms to solve the time-dependent Schrödinger equation and to discuss ways of turning these concepts into unconditionally stable, accurate and efficient simulation algorithms. Applications to focussed electron emission from nano-scale sources, mesoscopic normal-metal-superconductor devices, and charged-particle interferometry, combining features of both the Aharonov-Bohm and Hanbury Brown-Twiss experiment illustrate the power and flexibility of the simulation method.
High current density nanofilament cathodes for microwave amplifiers
No full textWe study high current density nanofilament cathodes for microwave amplifiers. Two different types of aligned nanofilament array have been studied: first, metallic nanowires grown by electrodeposition into nanoporous templates at very low temperature (T<100°C) on a silicon wafer; second, carbon nanotubes/nanofibers (CNs) grown by catalytic plasma enhanced chemical vapour deposition. The fabrication process and the field emission properties of these two types of cathodes will be presented. Presently, the best results are obtained with CN cathodes. Arrays of 5.8 μm height and 50 nm diameter CNs exhibit geometrical enhancement factor (h/r) of 240±7.5%. Moreover, currents close to 100 μA per emitter have been measured using a scanning anode field emission microscope. Due to these properties, 0.5 × 0.5 mm2 arrays emit a 2 mA current corresponding to 0.8 A/cm2, in DC mode. The use of these cold cathodes in microwave triodes delivering 10 to 50 W at 30 GHz will be discussed for future telecommunication applications.
Temperature Effects on Crack Tip Plasticity in Polymers
No full textThe crack tip fields in amorphous glassy polymers are distinctly different from that in metals, and are associated with the initiation and propagation of shear bands. To study the effect of temperature on the near-tip behaviour for a stationary mode I crack, computations at low loading rates are confronted with adiabatic computations at high loading rates. It is shown that self-heating can raise the temperature in SAN sufficiently high locally to reach the glass transition temperature.
Spin transport in metal and oxide devices at the nanoscale
No full textHere we discuss a non-destructive technique that characterizes spin and charge transport at the nanometer scale, across buried layers and interfaces, in magnetic memory elements as used in spin transfer torque based Magnetic Random Access Memory (STT-MRAM). While probing in the current-perpendicular-to-plane direction, this method enables quantification of essential spin transport parameters as length and time scale, spin polarization in buried layers and interfaces, visualization of domain wall evolution across buried interfaces, besides investigating the homogeneity of transport, at the nanoscale, in spintronics devices.
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