7,289 research outputs found
The role of the pion pair term in the theory of the weak axial meson exchange currents
The structure of the weak axial pion exchange current is discussed in various
models. It is shown how the interplay of the chiral invariance and the double
counting problem restricts uniquely the form of the pion potential term, in the
case when the nuclear dynamics is described by the Schroedinger equation with
the static nucleon-nucleon potential.Comment: 17 pages, 1 figure, 2 tables, stylistic changes of the tex
1.57 μm InGaAsP/InP surface emitting lasers by angled focus ion beam etching
The characteristics of 1.57 μm InGaAsP/InP surface emitting lasers based on an in-plan ridged structure and 45° beam deflectors defined by angled focused ion beam (FIB) etching are reported. With an externally integrated beam deflector, threshold currents and emission spectra identical to conventional edge emitting lasers are achieved. These results show that FIB etching is a very promising technique for the definition of high quality mirrors and beam deflectors on semiconductor heterostructures for a variety of integrated optoelectronic devices
Ultranarrow conducting channels defined in GaAs-AlGaAs by low-energy ion damage
We have laterally patterned the narrowest conducting wires of two-dimensional electron gas (2DEG) material reported to date. The depletion induced by low-energy ion etching of GaAs-AlGaAs 2DEG structures was used to define narrow conducting channels. We employed high voltage electron beam lithography to create a range of channel geometries with widths as small as 75 nm. Using ion beam assisted etching by Cl2 gas and Ar ions with energies as low as 150 eV, conducting channels were defined by etching only through the thin GaAs cap layer. This slight etching is sufficient to entirely deplete the underlying material without necessitating exposure of the sidewalls that results in long lateral depletion lengths. At 4.2 K, without illumination, our narrowest wires retain a carrier density and mobility at least as high as that of the bulk 2DEG and exhibit quantized Hall effects. Aharonov–Bohm oscillations are seen in rings defined by this controlled etch-damage patterning. This patterning technique holds promise for creating one-dimensional conducting wires of even smaller sizes
Thermoelectric properties of Zn_5Sb_4In_(2-δ)(δ=0.15)
The polymorphic intermetallic compound Zn_5Sb_4In_(2−δ) (δ = 0.15(3)) shows promising thermoelectric properties at low temperatures, approaching a figure of merit ZT of 0.3 at 300 K. However, thermopower and electrical resistivity changes discontinuously at around 220 K. Measurement of the specific heat locates the previously unknown temperature of the order-disorder phase transition at around 180 K. Investigation of the charge carrier concentration and mobility by Hall measurements and infrared reflection spectroscopy indicate a mixed conduction behavior and the activation of charge carriers at temperatures above 220 K. Zn_5Sb_4In_(2−δ) has a low thermal stability, and at temperatures above 470 K samples decompose into a mixture of Zn, InSb, and Zn_4Sb_3
Size tunable visible and near-infrared photoluminescence from vertically etched silicon quantum dots
Corrugated etching techniques were used to fabricate size-tunable silicon quantum dots that luminesce under photoexcitation, tunable over the visible and near infrared. By using the fidelity of lithographic patterning and strain limited, self-terminating oxidation, uniform arrays of pillar containing stacked quantum dots as small as 2 nm were patterned. Furthermore, an array of pillars, with multiple similar sized quantum dots on each pillar, was fabricated and tested. The photoluminescence displayed a multiple, closely peaked emission spectra corresponding to quantum dots with a narrow size distribution. Similar structures can provide quantum confinement effects for future nanophotonic and nanoelectronic devices
Recognizing Emotions in a Foreign Language
Expressions of basic emotions (joy, sadness, anger, fear, disgust) can be recognized pan-culturally from the face and it is assumed that these emotions can be recognized from a speaker's voice, regardless of an individual's culture or linguistic ability. Here, we compared how monolingual speakers of Argentine Spanish recognize basic emotions from pseudo-utterances ("nonsense speech") produced in their native language and in three foreign languages (English, German, Arabic). Results indicated that vocal expressions of basic emotions could be decoded in each language condition at accuracy levels exceeding chance, although Spanish listeners performed significantly better overall in their native language ("in-group advantage"). Our findings argue that the ability to understand vocally-expressed emotions in speech is partly independent of linguistic ability and involves universal principles, although this ability is also shaped by linguistic and cultural variables
Bioactivation of N-nitrosomethylbenzylamine and N-nitrosomethyl-amylamine in oesophageal papillomas
Oesophageal papillomas were induced in male F344 rats by continuous exposure to N-nitrosomethylbeazylamine (NMBZA) and N-nitrosomethyl(2-methylbutyl)ainine in the drinking water at concentrations of 10 and 19.5 p.p.m. respectively. After 81-141 days animals received a single i.p. chasing dose of NMIBZA (0.1 mmol/kg), [14C-methyl]NMBZA or N-nitroso[14C-methyl]amylamine and were killed 6 h later. Induced papillomas (3-9 per animal) were analysed by autoradlography and by immunohistochemistry using a polyclonal antibody to O6-methyldeoxyguanosine Both techniques revealed the presence of high levels of alkylation products in all papillomas investigated. Immunohistochemical staining of O6-methyldeoxyguanosine was largely restricted to nudei of the basal layer and of epithelial cells with incipient keratinization. These findings demonstrate that NMBZA and N-nitrosomethylamylamine and probably related methyl alkylnitrosamines are effectively bioactivated in premalignant lesions, indicating that during chronic exposure papifiomas can acquire additional mutations that are likely to play a major role in tumour progressio
Virtual Compton scattering off the nucleon at low energies
We investigate the low-energy behavior of the four-point Green's function
describing virtual Compton scattering off the nucleon.
Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive
the leading terms of an expansion of the operator in the four-momenta and
of the initial and final photon, respectively.
The model-independent result is expressed in terms of the electromagnetic
form factors of the free nucleon, i.e., on-shell information which one obtains
from electron-nucleon scattering experiments.
Model-dependent terms appear in the operator at ,
whereas the orders and are
contained in the low-energy theorem for , i.e., no new
parameters appear.
We discuss the leading terms of the matrix element and comment on the use of
on-shell equivalent electromagnetic vertices in the calculation of ``Born
terms'' for virtual Compton scattering.Comment: 26 pages, RevTex, to appear in Phys. Rev.
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