Search results for "Tunnelling"
showing 10 items of 218 documents
Phase diagram of a model adsorbate with internal quantum states
1992
Abstract An adsorbate in the strong binding and small corrugation limit is studied. The resulting two-dimensional fluid is treated in the adiabatic approximation: the translations of the heavy molecules are treated classically, whereas the molecular quantum degrees of freedom are modelled by interacting two state tunneling systems. The complete temperature/coverage phase diagram is obtained to a high degree of precision by a new combination of finite size scaling ideas with Path Integral Monte Carlo techniques. Even this simplified adsorbate model possesses a surprisingly complex phase diagram including first- and second-order transitions as well as tricritical and triple points. We identif…
Self-trapped holes and recombination luminescence in LiBaF3 crystals
2001
Abstract We investigated electron paramagnetic resonance (EPR) angular dependencies, recombination afterglow and thermostimulated luminescence of undoped LiBaF3 crystals, X-irradiated at low temperatures. EPR parameters of the F2− molecule oriented along the [1 1 0] direction have been obtained. Based on the value of the g-shift Δg=g⊥−gII=0.02, characteristic for the VK-centres in similar perovskites, we propose that we indeed observed the VK-centres, not the H-centres. X-irradiation below 170 K results in creation of a long-time temperature-independent afterglow due to the tunnelling recombination between close electron and hole centres. The F-type electron centres and the VK as well as pr…
Localised transitions in luminescence of AlN ceramics
2014
Abstract The photoluminescence (PL) and thermoluminescence (TL) properties of AlN ceramics revealed under UV irradiation are determined mainly by oxygen-related centres, giving rise to the UV (around 3.18 eV) and the Blue (2.58 eV) bands. It was found that the UV irradiation-generated donor–acceptor pairs (DAPs), responsible for the UV emission band, are randomly distributed with regard to separation distance. Luminescence properties of AlN are interpreted basing on the model of localised recombination involving electron tunnel transitions from the excited state of D to the ground state of A, proposed by Jain et al. (2012) . The observed features of PL, afterglow and TL of AlN ceramics are …
Trap spectroscopy and tunnelling luminescence in feldspars
1998
Abstract Comparative studies of tunnel luminescence, thermally stimulated luminescence (TSL), and trap energy spectra by the fractional glow technique (FGT) have been made both with sanidine and microcline feldspars. After X-irradiation at LNT these feldspars show intense tunnelling afterglow. As the temperature is raised, several TSL glow peaks may be observed, starting at 135 K. An intense peak at 250 K is common to all four investigated microclines. In samples of sanidine, tunnelling afterglow overcomes TSL up to 250–260 K. With sanidine and microcline samples, the distributions of trap activation energies vs temperature obtained by the FGT are continuous and quasi-linear. Maxima in the …
Electronic transport in intermediate sized carbon nanotubes
2009
We have studied systematically basic transport properties of multiwalled carbon nanotubes in a relatively unexplored diameter range, corresponding to tubes just slightly larger than single- or double-walled tubes to tubes up to 17 nm in diameter. We find in all the smaller tubes a gap in the transport data which increases with decreasing tube diameter. Within the gap region of several tubes, negative differential resistance was observed at small or moderate biases and at bias values that scaled inversely with the tube diameter. For this latter type of behavior of the conductance, we tentatively propose interlayer resonant tunneling as the cause.
Morphology and magnetoresistance of Co2Cr0.6Fe0.4Al-based tunnelling junctions
2009
Some ferromagnetic Heusler compounds are theoretically predicted to be half metallic materials, i.e. to be characterized by a huge spin polarization at the Fermi energy. We investigate the correlations between junction preparation conditions, morphology and transport properties of planar MgO/Co2Cr0.6Fe0.4Al/AlOx/Co/CoOx/Pt tunnelling junctions. Epitaxial Co2Cr0.6Fe0.4Al thin films were deposited by dc and rf magnetron sputtering on different buffer layers (Cr, Fe, MgO) on MgO(1 0 0) substrates. By RHEED, LEED and in situ STM investigations different surface morphologies were observed. Atomically flat surfaces with Co2Cr0.6Fe0.4Al unit cell sized steps (B2 structure) were obtained by rf sput…
MgO Magnetic Tunnel Junction Electrical Current Sensor With Integrated Ru Thermal Sensor
2013
Full Wheatstone bridge electrical current sensor incorporating 114 MgO-based magnetic tunnel junction elements (3 × 30 μm2) connected in series was produced for improved electrical robustness. To that end, magnetic tunnel junctions with R × A ~7 KΩ μm2 tunneling magnetoresistance of 200%, were produced. The sensor was designed with an integrated Ru thin film resistive thermal detector (RTD) for temperature drift monitoring and compensation. In order to achieve a full bridge signal, a U-shaped copper trace was placed under a printed circuit board (PCB) specifically designed for this type of device. The resulting device exhibit sensitivities of 63.9 V/Oe/A in a 75 Oe linear range biased with …
Scanning tunneling microscopy and spectroscopy of Mo clusters grown on TiO2(110).
2007
Molybdenum was deposited in two steps (3 eq. ML and 1 eq. ML) on the light blue rutile TiO2 10) (1 x 1) surface at room temperature, each Mo deposition cycle being followed by an annealing up to 950-1000 K. This procedure was found to lead to formation of separated clusters having a size in very wide range (1-20 nm). Scanning tunneling microscopy showed a dependence of the cluster morphology as a function of the size. The scanning tunneling spectra of Mo clusters was studied as a function of cluster dimensions and discussed in comparison with photoelectron spectroscopy results previously obtained for homogeneous Mo films. The dI/d V curves do not display the valence band structure of deposi…
Field-induced tip–sample oxygen transfer in scanning tunneling microscopy on TiO2(110) (1 1).
2008
International audience; A study on the field-induced tip–surface oxygen transfer at room temperature and its influence on the tunneling conditions for stable STM imaging of the TiO2(110) (1 1) surface is reported. A simple model of field-induced transfer is applied to tungsten and platinum–iridium tips. The oxygen transition rates from the sample to the tip or from the tip to the sample depend on the oxygen desorption barriers formed at tunneling distance. For stable imaging the applied bias voltage has to balance the oxygen transfer probabilities in both directions. In the case of Pt/Ir tips, the tunneling conditions for images with clear evidence of bridging oxygen point defects have been…
Quantum dissipative dynamics of a bistable system in the sub-Ohmic to super-Ohmic regime
2016
We investigate the quantum dynamics of a multilevel bistable system coupled to a bosonic heat bath beyond the perturbative regime. We consider different spectral densities of the bath, in the transition from sub-Ohmic to super-Ohmic dissipation, and different cutoff frequencies. The study is carried out by using the real-time path integral approach of the Feynman-Vernon influence functional. We find that, in the crossover dynamical regime characterized by damped \emph{intrawell} oscillations and incoherent tunneling, the short time behavior and the time scales of the relaxation starting from a nonequilibrium initial condition depend nontrivially on the spectral properties of the heat bath.