Donor states in modulation-doped Si/SiGe heterostructures

We present a unified approach for calculating the properties of shallow donors inside or outside heterostructure quantum wells. The method allows us to obtain not only the binding energies of all localized states of any symmetry, but also the energy width of the resonant states which may appear when a localized state becomes degenerate with the continuous quantum well subbands. The approach is non-variational, and we are therefore also able to evaluate the wave functions. This is used to calculate the optical absorption spectrum, which is strongly non-isotropic due to the selection rules. The results obtained from calculations for Si/Si$_{1-x}$Ge$_x$ quantum wells allow us to present the general behavior of the impurity states, as the donor position is varied from the center of the well to deep inside the barrier. The influence on the donor ground state from both the central-cell effect and the strain arising from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure

A Search for Photons with Energies Above 2 x 1017 eV Using Hybrid Data from the Low-Energy Extensions of the Pierre Auger Observatory

Ultra-high-energy photons with energies exceeding 10¹⁷ eV offer a wealth of connections to different aspects of cosmic-ray astrophysics as well as to gamma-ray and neutrino astronomy. The recent observations of photons with energies in the 10¹⁵ eV range further motivate searches for even higher-energy photons. In this paper, we present a search for photons with energies exceeding 2 × 10¹⁷ eV using about 5.5 yr of hybrid data from the low-energy extensions of the Pierre Auger Observatory. The upper limits on the integral photon flux derived here are the most stringent ones to date in the energy region between 10¹⁷ and 10¹⁸ eV.P. Abreu ... J. M. Albury ... J. A. Bellido ... R. W. Clay ... B. R. Dawson, J. A. Day ... T. D. Grubb ... V. M. Harvey ... G. C. Hill ... B. C. Manning ... T. Sudholz ... et al. (The Pierre Auger Collaboration

Biotic degradation at night, abiotic degradation at day: positive feedbacks on litter decomposition in drylands

The arid and semi-arid drylands of the world are increasingly recognized for their role in the terrestrial net carbon dioxide (CO) uptake, which depends largely on plant litter decomposition and the subsequent release of CO back to the atmosphere. Observed decomposition rates in drylands are higher than predictions by biogeochemical models, which are traditionally based on microbial (biotic) degradation enabled by precipitation as the main mechanism of litter decomposition. Consequently, recent research in drylands has focused on abiotic mechanisms, mainly photochemical and thermal degradation, but they only partly explain litter decomposition under dry conditions, suggesting the operation of an additional mechanism. Here we show that in the absence of precipitation, absorption of dew and water vapor by litter in the field enables microbial degradation at night. By experimentally manipulating solar irradiance and nighttime air humidity, we estimated that most of the litter CO efflux and decay occurring in the dry season was due to nighttime microbial degradation, with considerable additional contributions from photochemical and thermal degradation during the daytime. In a complementary study, at three sites across the Mediterranean Basin, litter CO efflux was largely explained by litter moisture driving microbial degradation and ultraviolet radiation driving photodegradation. We further observed mutual enhancement of microbial activity and photodegradation at a daily scale. Identifying the interplay of decay mechanisms enhances our understanding of carbon turnover in drylands, which should improve the predictions of the long-term trend of global carbon sequestration.This research project was financially supported by ... and the Spanish Ministry of Economy and Competitiveness (project CGL2011-24748/PHOTODEG).Peer Reviewe

Nipple-sparing mastectomy for breast cancer and risk reduction : oncologic or technical problem?

Background: We evaluated the risks and benefits of nipple-sparing mastectomy in a multiinstitutional experience in the settings of risk-reducing surgery and breast cancer treatment. Study Design: We analyzed data on 123 patients who had undergone nipple-sparing mastectomy with breast reconstruction for prophylaxis (n = 55), treatment of breast cancer (n = 41), or both (n = 27) at four large centers. Results: Median patient age was 45 years (range 22 to 70 years). There were 192 procedures (69 bilateral, 54 unilateral). Forty-four patients had invasive cancer; 20 had ductal carcinoma in situ (DCIS); 4 had phyllodes tumor. In all of these patients, the nipple tissue was cancer free on pathologic review. Median followup was 24.6 months (range 2.0 to 570.4 months). Local recurrence developed in two patients: one had DCIS in the upper-outer quadrant, with 71.8 months of followup; the other's cancer was invasive, in the upper-outer quadrant, with 6 months of followup. Distant metastasis developed in a third patient, who died 50 months after the procedure. Breast cancer developed in two patients after prophylactic mastectomy: one in the upper-outer quadrant at 61.8 months; one in the axillary tail at 24.4 months. No patients had recurrences in the nipple-areolar complex. Necrosis of the nipple was reported in 22 of 192 patients (11%) and it was judged minimal (less than one-third total skin of nipple) in 13 of 22 patients (59%). Overall cosmesis was judged by the patient and surgeon as good to excellent in the majority of patients. Level of satisfaction with cosmetic results was similar between prophylactic and treatment patients. Conclusions: The risk of local relapse was very low in our series of nipple-sparing mastectomies performed for DCIS or invasive cancer. Nipple-sparing mastectomy in the risk-reducing and breast cancer-treatment settings may be feasible in selected patients and should be the subject of additional prospective clinical trials