Enhancement of the Binding Energy of Charged Excitons in Disordered Quantum Wires

Negatively and positively charged excitons are identified in the spatially-resolved photoluminescence spectra of quantum wires. We demonstrate that charged excitons are weakly localized in disordered quantum wires. As a consequence, the enhancement of the "binding energy" of a charged exciton is caused, for a significant part, by the recoil energy transferred to the remaining charged carrier during its radiative recombination. We discover that the Coulomb correlation energy is not the sole origin of the "binding energy", in contrast to charged excitons confined in quantum dots.Comment: 4 Fig

Two-Stage Rotational Disordering of a Molecular Crystal Surface: C60

We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C$_{60}$ fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low temperature ordered $(2 \times 2)$ state, and a high temperature $(1 \times 1)$ disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.Comment: 4 pages, 2 figure

Nueva sonda TDR para la medida de la conductividad eléctrica de la solución del suelo

Esta comunicación presenta una sonda TDR (WECP) para la medida de la conductividad eléctrica de la solución del suelo (σw), que consiste en una sonda TDR trifilar (10 cm longitud) insertadas en 14 discos cerámicos. La σw se estima a partir de la humedad y la conductividad eléctrica del conjunto de discos cerámicos. La WECP fue calibrada y testada en laboratorio con suelo franco y en condiciones de campo con diferentes soluciones de KCl. La σw estimada con WECP en laboratorio fue comparada con las obtenidas a partir de la solución de drenaje. El experimento de campo comparó σw estimados con WECP con los medidos con lisímetros de tensión. En ambos experimentos se observó una buena y significativa correlación (R2 ≥ 0,97; P < 0,001; Coeficiente de eficeincia Nash-Sutdiffe > 0,96) entre los valores de σw estimados con WECP y los obtenidos en laboratorio y campo a partir de la solución de drenajeEsta comunicación presenta una sonda TDR (WECP) para la medida de la conductividad eléctrica de la solución del suelo (σw), que consiste en una sonda TDR trifilar (10 cm longitud) insertadas en 14 discos cerámicos. La σw se estima a partir de la humedad y la conductividad eléctrica del conjunto de discos cerámicos. La WECP fue calibrada y testada en laboratorio con suelo franco y en condiciones de campo con diferentes soluciones de KCl. La σw estimada con WECP en laboratorio fue comparada con las obtenidas a partir de la solución de drenaje. El experimento de campo comparó σw estimados con WECP con los medidos con lisímetros de tensión. En ambos experimentos se observó una buena y significativa correlación (R2 ≥ 0,97; P < 0,001; Coeficiente de eficeincia Nash-Sutdiffe > 0,96) entre los valores de σw estimados con WECP y los obtenidos en laboratorio y campo a partir de la solución de drenajePublishe

CT angiography, MR angiography and rotational digital subtraction angiography for volumetric assessment of intracranial aneurysms. An experimental study

The purpose of our experimental study was to assess the accuracy and precision of CT angiography (CTA), MR angiography (MRA) and rotational digital subtraction angiography (DSA) for measuring the volume of an in vitro aneurysm model. A rigid model of the anterior cerebral circulation harbouring an anterior communicating aneurysm was connected to a pulsatile circuit. It was studied using unenhanced 3D time-of-flight MRA, contrast-enhanced CTA and rotational DSA angiography. The source images were then postprocessed on dedicated workstations to calculate the volume of the aneurysm. CTA was more accurate than MRA (P=0.0019). Rotational DSA was more accurate than CTA, although the difference did not reach statistical significance (P=0.1605), and significantly more accurate than MRA (P<0.00001). CTA was more precise than MRA (P=0.12), although this did not reach statistical significance. Rotational DSA can be part of the diagnosis, treatment planning and support endovascular treatment of intracranial aneurysms. The emerging endovascular treatment techniques which consist of using liquid polymers as implants to exclude aneurysms from arterial circulation would certainly benefit from this precise measurement of the volume of aneurysm

VSP Traveltime Inversion: Near-Surface Issues

P-wave velocity information obtained from vertical seismic profiles (VSPs) can be useful in imaging subsurface structure, either by directly detecting changes in the subsurface or as an aid to the interpretation of seismic reflection data. In the shallow subsurface, P-wave velocity can change by nearly an order of magnitude over a short distance, so curved rays are needed to accurately model VSP traveltimes. We used a curved-ray inversion to estimate the velocity profile and the discrepancy principle to estimate the data noise level and to choose the optimum regularization parameter. The curved-ray routine performed better than a straight-ray inversion for synthetic models containing high-velocity contrasts. The application of the inversion to field data produced a velocity model that agreed well with prior information. These results show that curved-ray inversion should be used to obtain velocity information from VSPs in the shallow subsurface

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Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors

The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF)$_2$X and the spin density wave phase in (TMTSF)$_2$X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments on (TMTTF)$_2$Br and (TMTTF)$_2$SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)$_2$Ag is predicted to have a similar antiferromagnetic spin structure.Comment: 8 pages, LaTeX, 4 figures, uses jpsj.sty, to be published in J. Phys. Soc. Jpn. 66 No. 5 (1997

Carbon Nanotubes Synthesized in Channels of Alpo4-5 Single Crystals : First X-Ray Scattering Investigations

Following the synthesis of aligned single-wall carbon nanotubes in the channels of AlPO4-5 zeolite single crystals, we present the first X-ray diffraction and diffuse scattering results. They can be analysed in terms of a partial filling of the zeolite channels by nanotubes with diameter around 4A. The possible selection of only one type of nanotube during the synthesis, due to the constraints imposed by the zeolite host, is discussed.Comment: to appear in Solid State Com

New Superhard Phases for 3D C60-based Fullerites

We have explored new possible phases of 3D C60-based fullerites using semiempirical potentials and ab-initio density functional methods. We have found three closely related structures - two body centered orthorhombic and one body centered cubic - having 52, 56 and 60 tetracoordinated atoms per molecule. These 3D polymers result in semiconductors with bulk moduli near 300 GPa, and shear moduli around 240 GPa, which make them good candidates for new low density superhard materials.Comment: To be published in Physical Review Letter

Coexistent State of Charge Density Wave and Spin Density Wave in One-Dimensional Quarter Filled Band Systems under Magnetic Fields

We theoretically study how the coexistent state of the charge density wave and the spin density wave in the one-dimensional quarter filled band is enhanced by magnetic fields. We found that when the correlation between electrons is strong the spin density wave state is suppressed under high magnetic fields, whereas the charge density wave state still remains. This will be observed in experiments such as the X-ray measurement.Comment: 7 pages, 15 figure