RESUMEN
The Coulomb scattering dynamics in graphene in energetic proximity to the Dirac point is investigated by polarization resolved pump-probe spectroscopy and microscopic theory. Collinear Coulomb scattering rapidly thermalizes the carrier distribution in k directions pointing radially away from the Dirac point. Our study reveals, however, that, in almost intrinsic graphene, full thermalization in all directions relying on noncollinear scattering is much slower. For low photon energies, carrier-optical-phonon processes are strongly suppressed and Coulomb mediated noncollinear scattering is remarkably slow, namely on a ps time scale. This effect is very promising for infrared and THz devices based on hot carrier effects.
RESUMEN
We report on the first direct experimental observation of carrier multiplication in graphene reaching a multiplication factor of up to 2 and persisting on a picoseconds time scale. Exploiting multicolor pump-probe measurement techniques, the excited nonequilibrium carrier distribution is retrieved on an ultrafast time scale. This provides access to the temporal evolution of the optically excited carrier density and thus allows quantitative conclusions on possible carrier multiplication. Microscopic time- and momentum-resolved calculations on the ultrafast relaxation dynamics of optically excited carriers confirm the observation of carrier multiplication under corresponding experimental conditions, suggesting graphene as a promising material for novel high-efficiency photodetection devices.
RESUMEN
We present the results of pumpprobe experiments on multilayer graphene samples performed in a wide spectral range, namely from the near infrared (photon energy 1.5 eV) to the terahertz (photon energy 8 meV) spectral range. In the near infrared, exciting carriers and probing at higher photon energies provides direct evidence for a hot carrier distribution. Furthermore, spectroscopic signatures of the highly doped graphene layers at the interface to SiC are observed in the near-infrared range. In the mid-infrared range, the various relaxation mechanisms, in particular scattering via optical phonons and Auger-type processes, are identified by comparing the experimental results to microscopic modeling. Changes from induced transmission to induced absorption are attributed to probing above or below the Fermi edge of the graphene layers. This effect occurs for certain photon energies in the near-infrared range, where it is related to highly doped graphene layers at the interface to SiC, and in the far-infrared range for the quasi-intrinsic graphene layers. In addition to the relaxation dynamics, the saturation of pump-induced bleaching of graphene is studied. Here a quadratic dependence of the saturation fluence on the pump photon energy in the infrared spectral range is revealed.
RESUMEN
The carrier dynamics in optically excited graphene is determined by an interplay between the Coulomb- and phonon-induced scattering processes. The pump fluence of the excitation pulse plays a crucial role: for high fluences the ultrafast relaxation dynamics is determined by carriercarrier scattering, whereas for low fluences the carrierphonon relaxation channels are predominant. On the basis of a microscopic approach, we shed light on the influence of the pump fluence on the temporal evolution of carrier and phonon occupations as well as on the many-particle dephasing of the microscopic polarization. The new insights obtained contribute to a better understanding of the ultrafast carrier dynamics in graphene and can guide future pumpprobe experiments.
RESUMEN
We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.
RESUMEN
Three types of hemoglobins exist in higher plants, symbiotic, non-symbiotic, and truncated hemoglobins. Symbiotic (class II) hemoglobins play a role in oxygen supply to intracellular nitrogen-fixing symbionts in legume root nodules, and in one case ( Parasponia Sp.), a non-symbiotic (class I) hemoglobin has been recruited for this function. Here we report the induction of a host gene, dgtrHB1, encoding a truncated hemoglobin in Frankia-induced nodules of the actinorhizal plant Datisca glomerata. Induction takes place specifically in cells infected by the microsymbiont, prior to the onset of bacterial nitrogen fixation. A bacterial gene (Frankia trHBO) encoding a truncated hemoglobin with O (2)-binding kinetics suitable for the facilitation of O (2) diffusion ( ) is also expressed in symbiosis. Nodule oximetry confirms the presence of a molecule that binds oxygen reversibly in D. glomerata nodules, but indicates a low overall hemoglobin concentration suggesting a local function. Frankia trHbO is likely to be responsible for this activity. The function of the D. glomerata truncated hemoglobin is unknown; a possible role in nitric oxide detoxification is suggested.
Asunto(s)
Proteínas Bacterianas/metabolismo , Hemoglobinas/genética , Magnoliopsida/metabolismo , Proteínas de Plantas/genética , Nódulos de las Raíces de las Plantas/metabolismo , Secuencia de Aminoácidos , Proteínas Bacterianas/química , Proteínas Bacterianas/genética , Clonación Molecular , Secuencia Conservada , ADN Complementario/química , ADN Complementario/metabolismo , Frankia/genética , Frankia/metabolismo , Frankia/fisiología , Regulación de la Expresión Génica de las Plantas , Hemoglobinas/metabolismo , Magnoliopsida/genética , Magnoliopsida/microbiología , Fijación del Nitrógeno , Oxígeno/metabolismo , Proteínas de Plantas/análisis , Proteínas de Plantas/metabolismo , Reacción en Cadena de la Polimerasa , Nódulos de las Raíces de las Plantas/genética , Nódulos de las Raíces de las Plantas/microbiología , SimbiosisRESUMEN
INTRODUCTION: Using screws for a better primary and secondary fixation has been discussed in the literature of the recent years, although the principle of press fit has been explained as the principle of a push-button. The authors wanted to compare their own results in patients using X-rays from the postoperative course to decide if it is really safer to use screws or not. MATERIALS AND METHODS: Two hundred and twenty one consecutive, not selected patients were treated with total hip arthroplasty using a cementless cup type Duraloc. They were followed up over a time of 5 years for radiological changes of the bony acetabulum around the cup ("acetabular zones" according to De Lee and Charnley). One hundred and ten cups were additionally fixed with one to three screws at the upper part in region C1, 101 cups were implanted without the use of screws. RESULTS: Periprosthetic changes visible on the radiographs (immediate postoperative gaps, subsequent sclerosis and radiolucent lines) were assessed according to the time of their appearance. A gap in zone C2 was seen more often immediately after operation within the group without screws (17.8% vs. 7.3%) and disappeared within the following 25 months. Within the group with screws significantly more often a gap was found in zone C1 and C3 compared to the group without screws (7.3% vs. 1.9%). In the group with screws a sclerosis of the bone in zone C1 developed in 32.7% mostly within the first 5 months. It was followed by a radiolucent line in zone C3 in 28.2% mainly within 5-25 months postoperatively and in zone C1 in 20.9% within 16 months. Significantly less seldom were these phenomena seen at the cups without screw fixation. There was no correlation between lateral or medial positioning and deviations from the ideal inclination to the bony changes around the cups. No signs of loosening occurred in either group. CONCLUSION: Less radiological changes around the cup if no screws were used and no disadvantages within this group led to the conclusion that an additional screw fixation in principle is not necessary in press-fit cups.
Asunto(s)
Acetábulo , Artroplastia de Reemplazo de Cadera/métodos , Prótesis de Cadera , Inestabilidad de la Articulación/cirugía , Falla de Prótesis , Anciano , Artroplastia de Reemplazo de Cadera/efectos adversos , Cementos para Huesos , Tornillos Óseos , Femenino , Cabeza Femoral/cirugía , Estudios de Seguimiento , Fracturas de Cadera/diagnóstico por imagen , Fracturas de Cadera/cirugía , Humanos , Inestabilidad de la Articulación/etiología , Masculino , Persona de Mediana Edad , Evaluación de Necesidades , Osteoartritis de la Cadera/diagnóstico por imagen , Osteoartritis de la Cadera/cirugía , Dimensión del Dolor , Probabilidad , Diseño de Prótesis , Radiografía , Rango del Movimiento Articular/fisiología , Sistema de Registros , Reoperación , Estudios Retrospectivos , Medición de Riesgo , Estadísticas no Paramétricas , Resultado del TratamientoRESUMEN
INTRODUCTION: We present radiological results regarding the development of stress shielding around the stem of the AML hip endoprosthesis and discuss the clinical significance of it and preventive measures against stress shielding. MATERIAL AND METHODS: 391 patients (216 female and 122 male) with an AML-hip-endoprosthesis between 1991 and 1992 were followed clinically and radiologically during 5 years postoperatively. It was searched for a statistical correlation of stress shielding with a pre-existing osteoporosis of the proximal femur (Barnett-Nordin index), the diameter of the stem, a varus position of the stem and thigh pain. RESULTS: Stress shielding occurred in only 13.5 % of all patients. Atrophy of the proximal femur was detectable within 1 year, in 5 cases already after 3 weeks. Distal sclerosis reached its maximum after 1-2 years. Remarkably, women showed a tendency to more frequent stress shielding. In women more often than in men, a pre-existing osteoporosis of the proximal femur occurred. Cases with osteoporosis of the proximal femur experienced more often stress shielding. The same was seen in prostheses with diameters >13.5 mm. A slight varus position of the stem also favoured stress shielding, but it indicated an incomplete canal fit of the stems, too. Patients with stress shielding more often were suffered from thigh pain. CONCLUSION: Stress shielding is a result of the discrepancy between the elasticity of the bony femur and the stiffness of the stem. The AML-hip-prosthesis rarely shows this phenomenon. An osteoporosis of the proximal femur (coupled with an thinning of the cortex and generating higher elasticity of the bone), stems with higher diameters (what means higher stiffness) and incomplete canal fit more likely predisposed patients to stress shielding. To prevent stress shielding, the AML prosthesis should not be implanted in cases with known severe osteoporosis of the proximal femur. This prosthesis should be taken preferable in cases with diameters of the femoral canal smaller than 15 mm. Fully canal fit has to be achieved. The frequency of thigh pain, which occurs more often in patients with stress shielding, could be reduced by considering those factors.
Asunto(s)
Artroplastia de Reemplazo de Cadera , Fémur/diagnóstico por imagen , Prótesis de Cadera , Femenino , Estudios de Seguimiento , Humanos , Masculino , Persona de Mediana Edad , Osteoartritis de la Cadera/diagnóstico por imagen , Osteoartritis de la Cadera/cirugía , Radiografía , Estrés MecánicoRESUMEN
A nodule-specific 53-kDa protein (GmNOD53b) of the symbiosome membrane from soybean was isolated and its LysC digestion products were microsequenced. cDNA clones of this novel nodulin, obtained from cDNA library screening with an RT-PCR (reverse-transcriptase polymerase chain reaction)-generated hybridization probe exhibited no homology to proteins identified so far. The expression of GmNOD53b coincides with the onset of nitrogen fixation. Therefore, it is a late nodulin. Among other changes, the GmNOD53b is significantly reduced in nodules infected with the Bradyrhizobium japonicum mutant 184 on the protein level as well as on the level of mRNA expression, compared with the wild-type infected nodules. The reduction of GmNOD53b mRNA is related to an inactivation of the sipF gene in B. japonicum 184, coding for a functionally active signal peptidase.