Oscillating Transverse Voltage in a Channel with Quantum Point Contact Voltage Probes

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

On the frequentist coverage of Bayesian credible intervals for lower bounded means

For estimating a lower bounded location or mean parameter for a symmetric and logconcave density, we investigate the frequentist performance of the $100(1-\alpha)$ Bayesian HPD credible set associated with priors which are truncations of flat priors onto the restricted parameter space. Various new properties are obtained. Namely, we identify precisely where the minimum coverage is obtained and we show that this minimum coverage is bounded between $1-\frac{3\alpha}{2}$ and $1-\frac{3\alpha}{2}+\frac{\alpha^2}{1+\alpha}$; with the lower bound $1-\frac{3\alpha}{2}$ improving (for $\alpha \leq 1/3$) on the previously established ([9]; [8]) lower bound $\frac{1-\alpha}{1+\alpha}$. Several illustrative examples are given.Comment: Published in at http://dx.doi.org/10.1214/08-EJS292 the Electronic Journal of Statistics (http://www.i-journals.org/ejs/) by the Institute of Mathematical Statistics (http://www.imstat.org

Inversion symmetry and bulk Rashba effect in methylammonium lead iodide perovskite single crystals

Methylammonium lead iodide perovskite (MAPbI_3) exhibits long charge carrier lifetimes that are linked to its high efficiency in solar cells. Yet, the mechanisms governing these unusual carrier dynamics are not completely understood. A leading hypothesis—disproved in this work—is that a large, static bulk Rashba effect slows down carrier recombination. Here, using second harmonic generation rotational anisotropy measurements on MAPbI_3 crystals, we demonstrate that the bulk structure of tetragonal MAPbI_3 is centrosymmetric with I4/mcmspace group. Our calculations show that a significant Rashba splitting in the bandstructure requires a non-centrosymmetric lead iodide framework, and that incorrect structural relaxations are responsible for the previously predicted large Rashba effect. The small Rashba splitting allows us to compute effective masses in excellent agreement with experiment. Our findings rule out the presence of a large static Rashba effect in bulk MAPbI_3, and our measurements find no evidence of dynamic Rashba effects

Spectral hole burning: examples from photosynthesis

The optical spectra of photosynthetic pigment–protein complexes usually show broad absorption bands, often consisting of a number of overlapping, ‘hidden’ bands belonging to different species. Spectral hole burning is an ideal technique to unravel the optical and dynamic properties of such hidden species. Here, the principles of spectral hole burning (HB) and the experimental set-up used in its continuous wave (CW) and time-resolved versions are described. Examples from photosynthesis studied with hole burning, obtained in our laboratory, are then presented. These examples have been classified into three groups according to the parameters that were measured: (1) hole widths as a function of temperature, (2) hole widths as a function of delay time and (3) hole depths as a function of wavelength. Two examples from light-harvesting (LH) 2 complexes of purple bacteria are given within the first group: (a) the determination of energy-transfer times from the chromophores in the B800 ring to the B850 ring, and (b) optical dephasing in the B850 absorption band. One example from photosystem II (PSII) sub-core complexes of higher plants is given within the second group: it shows that the size of the complex determines the amount of spectral diffusion measured. Within the third group, two examples from (green) plants and purple bacteria have been chosen for: (a) the identification of ‘traps’ for energy transfer in PSII sub-core complexes of green plants, and (b) the uncovering of the lowest k = 0 exciton-state distribution within the B850 band of LH2 complexes of purple bacteria. The results prove the potential of spectral hole burning measurements for getting quantitative insight into dynamic processes in photosynthetic systems at low temperature, in particular, when individual bands are hidden within broad absorption bands. Because of its high-resolution wavelength selectivity, HB is a technique that is complementary to ultrafast pump–probe methods. In this review, we have provided an extensive bibliography for the benefit of scientists who plan to make use of this valuable technique in their future research

Compositions by Louis W. Ballard

We report low-temperature results of accumulated and two-pulse picosecond photon-echo experiments on the optical transition of the nitrogen-bound exciton in GaP:N. The observed temperature dependence of the dephasing of the so-called A line at 534.95 nm can be explained in terms of (stimulated) phonon processes involving the lower B state and, at higher temperatures, the free-exciton band. The low-temperature-echo decay time of 25.5±0.5 ps is shown to be determined by the lifetime of a A state. Analysis of the factors governing the intensity of the accumulated photon echo as a function of temperature leads to the conclusion that in the B state temperature-independent excitation transfer occurs. The rate constant of this phenomenon, which we ascribe to excitation tunneling, is 10^7 s–1

Quantum Hall effect from the topological surface states of strained bulk HgTe.

We report transport studies on a three-dimensional, 70-nm-thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate. The strain induces a band gap in the otherwise semimetallic HgTe, which thus becomes a three-dimensional topological insulator. Contributions from residual bulk carriers to the transport properties of the gapped HgTe layer are negligible at mK temperatures. As a result, the sample exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states

Tunneling magnetoresistance in devices based on epitaxial NiMnSb with uniaxial anisotropy

We demonstrate tunnel magnetoresistance junctions based on a trilayer system consisting of an epitaxial NiMnSb, an aluminum oxide, and a CoFe trilayer. The junctions show a tunneling magnetoresistance of Delta R/R of 8.7% at room temperature which increases to 14.7% at 4.2 K. The layers show a clear separate switching and a small ferromagnetic coupling. A uniaxial in-plane anisotropy in the NiMnSb layer leads to different switching characteristics depending on the direction in which the magnetic field is applied, an effect which can be used for sensor applications. (c) 2006 American Institute of Physics

Circular photogalvanic effect in HgTe/CdHgTe quantum well structures

We describe the observation of the circular and linear photogalvanic effects in HgTe/CdHgTe quantum wells. The interband absorption of mid-infrared radiation as well as the intrasubband absorption of terahertz (THz) radiation in the QWs structures is shown to cause a dc electric current due to these effects. The photocurrent magnitude and direction varies with the radiation polarization state and crystallographic orientation of the substrate in a simple way that can be understood from a phenomenological theory. The observed dependences of the photocurrent on the radiation wavelength and temperature are discussed