Design and simulation of losses in Ge/SiGe terahertz quantum cascade laser waveguides.

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1-xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 µm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 µm thick active gain region with silver metal of 21 cm-1 at 300 K reducing to 14.5 cm-1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1-xGex with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm-1 and 23.8 cm-1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings.

Investigating the CVD Synthesis of Graphene on Ge(100): toward Layer-by-Layer Growth.

Germanium is emerging as the substrate of choice for the growth of graphene in CMOS-compatible processes. For future application in next generation devices the accurate control over the properties of high-quality graphene synthesized on Ge surfaces, such as number of layers and domain size, is of paramount importance. Here we investigate the role of the process gas flows on the CVD growth of graphene on Ge(100). The quality and morphology of the deposited material is assessed by using µ-Raman spectroscopy, X-ray photoemission spectroscopy, scanning electron microscopy, and atomic force microscopy. We find that by simply varying the carbon precursor flow different growth regimes yielding to graphene nanoribbons, graphene monolayer, and graphene multilayer are established. We identify the growth conditions yielding to a layer-by-layer growth regime and report on the achievement of homogeneous monolayer graphene with an average intensity ratio of 2D and G bands in the Raman map larger than 3.

Narrow intersubband transitions in n-type Ge/SiGe multi-quantum wells: control of the terahertz absorption energy trough the temperature dependent depolarization shift.

In this paper we present a detailed study of the intersubband absorption occurring between electron states confined in strained Ge multi-quantum wells as a function of the temperature. The high structural quality of the samples is reflected by the very narrow absorption line-shape constant with temperature. We observe a temperature driven charge transfer occurring between the ground and the first excited subband which, in turn, induces a change in the depolarization shift and consequently in the energy of the absorbance peak. The experimental observations are well accounted for by a multi-valley k·p model.

Self-ordering of a Ge island single layer induced by Si overgrowth.

We provide a direct experimental proof and the related modeling of the role played by Si overgrowth in promoting the lateral ordering of Ge islands grown by chemical vapor deposition on Si(001). The deposition of silicon induces a shape transformation, from domes to truncated pyramids with a larger base, generating an array of closely spaced interacting islands. By modeling, we show that the resulting gradient in the chemical potential across the island should be the driving force for a selective flow of both Ge and Si atoms at the surface and, in turn, to a real motion of the dots, favoring the lateral order.

Commensurability and stability in nonperiodic systems.

We have investigated the response of 3D Bi(2)Sr(2)CaCu(2)O(8) vortex structures to a weak perturbation induced by 2D Fe pinning structures acting on one extremity of vortex lines. The pinning patterns were nano-engineered at the sample surface by means of either a Bitter decoration of the vortex lattice or electron-beam lithography. The commensurability conditions between 2D rigid pinning potentials and 3D elastic structures with short-range positional and long-range orientational correlation have been experimentally determined. When the 2D potential is a replica of the nonperiodic vortex structure an amplification of its interaction with the vortex structure takes place. This effect is detected only for the first matching field, becoming negligible for other matching fields. On the other hand, a periodic 2D perturbation is shown to transform the nonperiodic Bragg glass-like structure into an Abrikosov crystal with an effective Debye-Waller factor.

Natural history of hepatitis C virus carriers with persistently normal aminotransferase levels.

BACKGROUND & AIMS: Some patients with serum hepatitis C virus (HCV) have persistently normal aminotransferase (ALT) levels and are affected by cirrhosis. This study prospectively evaluated progression of the disease in a group of anti-HCV-positive patients with persistently normal ALT levels. METHODS: Thirty-seven subjects were studied. Each subject underwent liver biopsy at baseline and after 5 years of follow-up. At baseline, serum samples were tested for genotypes and HCV RNA load. ALT levels and serum HCV RNA were tested every other month and every 6 months, respectively. Patients with increased ALT were discharged from the study and treated with IFN. Five years after the end of IFN therapy, a liver biopsy was performed. RESULTS: Liver biopsy at baseline showed chronic hepatitis in 34 patients and normal histology in 3 patients, 2 of whom were negative for HCV RNA and 1 positive. HCV genotypes were distributed as follows: 2a, 56%; 1b, 41%; and 1a, 3%. At the end of 7-year follow-up, 73% of the patients still had normal ALT values. Liver histology after 5 years was comparable to that observed at entry to study. CONCLUSIONS: Most patients with persistently normal ALT serum levels have very mild chronic hepatitis. However, healthy anti-HCV-positive subjects exist. In patients with HCV-related chronic hepatitis associated with persistently normal ALT levels, the grade of disease activity does not increase over years and progression to cirrhosis is slow or absent.

Non-ulcer dyspepsia and Helicobacter pylori in type 2 diabetic patients: association with autonomic neuropathy.

Diabetic patients often suffer from symptoms arising from the gastrointestinal tract. Several factors are considered responsible for these alterations, including abnormalities of gastric motility. Recently Helicobacter pylori (HP) has been identified in a relevant aliquot of subjects with or without gastrointestinal abnormalities, but only scarce and controversial data are available on the prevalence of HP and the association between HP and chronic gastritis or peptic ulcer in diabetic patients. In addition, the possible association between alterations of gastric motility induced by autonomic neuropathy (AN) and the presence of HP has never been evaluated in diabetic subjects. In this study we document the presence of HP in the gastric biopsies of 73% out of a series of 29 patients affected by type 2 diabetes and non-ulcer dyspepsia (3 with oesophagitis, 10 with gastritis, 7 with bulbar duodenitis, and 9 with a normal endoscopy), with a significantly higher prevalence (P < 0.01) in subjects with AN (74%) than in subjects without AN (26%). Two other tests have been compared with the histological evidence of HP (used as golden standard), i.e. the urease test (CP-test) and the assay of anti-HP G-immunoglobulins, both of which were positive in a significantly (P < 0.01) higher percentage of neuropathic patients in comparison with non-neuropathic patients. The sensitivity and the specificity of the CP-test were 96% and 100%, respectively. Similarly, both the sensitivity and the specificity of the assay of IgG HP-Ab were 100%. Since patients affected by non-ulcer dyspepsia and NIDDM complicated by autonomic neuropathy are under a higher risk to be carriers of HP than non-neuropathic or non-diabetic patients. The assay of serum IgG HP-Ab could be used as a screening method, thus avoiding the more expensive and time-consuming endoscopy.

The LUMO-derived band of the [Formula: see text] phases.

The LUMO-derived band of the [Formula: see text] compound was investigated by means of x-ray and UV photoemission and photoelectric yield spectroscopies. A double-peak structure is found, with peak maxima at 1.1 eV and 0.35 eV. The relative magnitude of the two peaks is strongly temperature dependent: a large transfer of spectral weight from the lower- to the higher-binding-energy peak takes place upon cooling to 110 K. The peaks are tentatively attributed to the dimer and the polymeric phase respectively. However the behaviour as a function of temperature does not agree with the metastability of the dimer phase at room temperature. The position of the Fermi level is compatible with a correlated system on the border of the metal - insulating transition.

[Infarction of the sigmoid. Clinical considerations]. / Infarto del sigma. Considerazioni cliniche.

The authors present a case of sigma infarction. They examine the etiopathogenesis, underline the clinical and instrumental signs and discurs the vascular anatomy and diagnostic difficulties. Operation is considered to be necessary to reduce the severe of prognosis.