Interface electronic states and boundary conditions for envelope functions

The envelope-function method with generalized boundary conditions is applied to the description of localized and resonant interface states. A complete set of phenomenological conditions which restrict the form of connection rules for envelope functions is derived using the Hermiticity and symmetry requirements. Empirical coefficients in the connection rules play role of material parameters which characterize an internal structure of every particular heterointerface. As an illustration we present the derivation of the most general connection rules for the one-band effective mass and 4-band Kane models. The conditions for the existence of Tamm-like localized interface states are established. It is shown that a nontrivial form of the connection rules can also result in the formation of resonant states. The most transparent manifestation of such states is the resonant tunneling through a single-barrier heterostructure.Comment: RevTeX4, 11 pages, 5 eps figures, submitted to Phys.Rev.

Coupled free-carrier and exciton relaxation in optically excited semiconductors

The energy relaxation of coupled free-carrier and exciton populations in semiconductors after low-density ultrafast optical excitation is studied through a kinetic approach. The set of semiclassical Boltzmann equations, usually written for electron and hole populations only, is complemented by an additional equation for the exciton distribution. The equations are coupled by reaction terms describing phonon-mediated exciton binding and dissociation. All the other relevant scattering mechanisms, such as carrier-carrier, carrier-phonon, and exciton-phonon interactions, are also included. The resulting system of rate equations in reciprocal space is solved by an extended ensemble Monte Carlo method. As a first application, we show results for the dynamics of bulk GaAs in the range from 1 to ∼200 ps after photoexcitation. The build-up of an exciton population and its sensitivity to the excitation conditions are discussed in detail. As a consequence of the pronounced energy dependence of the LO-phonon-assisted transition probabilities between free-pair states and excitons, it is found that the efficiency of the exciton-formation process and the temporal evolution of the resulting population are sensitive to the excitation energy. We discuss the effects on luminescence experiments

International nosocomial infection control consortium (INICC) report, data summary of 36 countries, for 2004-2009

The results of a surveillance study conducted by the International Nosocomial Infection Control Consortium (INICC) from January 2004 through December 2009 in 422 intensive care units (ICUs) of 36 countries in Latin America, Asia, Africa, and Europe are reported. During the 6-year study period, using Centers for Disease Control and Prevention (CDC) National Healthcare Safety Network (NHSN; formerly the National Nosocomial Infection Surveillance system [NNIS]) definitions for device-associated health care-associated infections, we gathered prospective data from 313,008 patients hospitalized in the consortium's ICUs for an aggregate of 2,194,897 ICU bed-days. Despite the fact that the use of devices in the developing countries' ICUs was remarkably similar to that reported in US ICUs in the CDC's NHSN, rates of device-associated nosocomial infection were significantly higher in the ICUs of the INICC hospitals; the pooled rate of central line-associated bloodstream infection in the INICC ICUs of 6.8 per 1,000 central line-days was more than 3-fold higher than the 2.0 per 1,000 central line-days reported in comparable US ICUs. The overall rate of ventilator-associated pneumonia also was far higher (15.8 vs 3.3 per 1,000 ventilator-days), as was the rate of catheter-associated urinary tract infection (6.3 vs. 3.3 per 1,000 catheter-days). Notably, the frequencies of resistance of Pseudomonas aeruginosa isolates to imipenem (47.2% vs 23.0%), Klebsiella pneumoniae isolates to ceftazidime (76.3% vs 27.1%), Escherichia coli isolates to ceftazidime (66.7% vs 8.1%), Staphylococcus aureus isolates to methicillin (84.4% vs 56.8%), were also higher in the consortium's ICUs, and the crude unadjusted excess mortalities of device-related infections ranged from 7.3% (for catheter-associated urinary tract infection) to 15.2% (for ventilator-associated pneumonia). Copyright © 2012 by the Association for Professionals in Infection Control and Epidemiology, Inc. Published by Elsevier Inc. All rights reserved

Luminescence of Ca(NbO3)2:Pr3+: Pr3+ and self-trapped exciton emission

Photoluminescence and time resolved photoluminescence spectra of Ca(NbO3)(2) doped with Pr3+, excited under 37,000 cm(-1) (270 nm), obtained at high hydrostatic pressure up to 20 kbar applied in a sapphire anvil cells, are presented. At ambient conditions, the emission spectrum obtained in the time interval 0-1 is is dominated by spin allowed transitions from the P-3(0) state. The luminescence related to transitions from D-1(2), characterized by a decay time equal to 33 mu s, is observed when one excites directly the Pr3+ ion with 30,770 cm(-1) (325 nm) wavelength. The introduction of Pr3+ impurities in Ca(NbO3)(2) does not quench the self-trapped exciton (STE) luminescence. This luminescence, peaking at 20,000 cm(-1) (500 nm), having a decay time of 61 +/- 1 mu s, still occurs when the crystal is excited with a wavelength of 37,000 cm(-1) (270 nm) or shorter. Under such excitation a fraction of the STE luminescence is reabsorbed by Pr3+ ions; in this case the emission lifetime of the D-1(2) -> H-3(4) transition of Pr3+ is 64 +/- 3 mu s. This effect is stable also at high pressure. (C) 2010 Elsevier Ltd. All rights reserved

Luminescence of Ca(NbO3)2:Pr3+ at ambient and high hydrostatic pressure

In this contribution, photoluminescence and time-resolved photoluminescence spectra of Ca(NbO3)(2) doped with Pr3+ obtained at high hydrostatic pressure up to 72 kbar applied in a diamond anvil cell are presented. At ambient conditions, the emission spectrum obtained in the time interval 0-1 mu s is dominated by spin-allowed transitions from the P-3(0) state. On the other hand, transitions from D-1(2), characterized by a decay time equal to 30 mu s dominate the steady-state luminescence. At pressures lower than 60 kbar, the continuous wave emission spectrum consists of sharp lines peaking between 600 and 625 nm, related to the D-1(2) -> H-3(4) transition and three lines at 500, 550 and 650 nm related to emission transitions originating from the P-3(0) level of Pr3+. The emission from the D-1(2) excited state depends weakly on the pressure. Its decay time decreases from 33 mu s at ambient pressure to less than 22 Its at 68 kbar. On the other hand, the P-3(0) emission is strongly pressure dependent, At pressures of 60 kbar and higher, the Pr3+ emission intensity from the P-3(0) state decreases. This is accompanied by a strong shortening of the luminescence decay time. The observed pressure quenching of the f-f emission transitions and the concomitant lifetime shortening have been attributed to increasing crossover from the P-3(0) state of Pr3+ to a Pr3+-trapped exciton stat

Q-switched nanosecond Nd3+:Ca(NbO3)2 crystallineself-Raman laser with single-step cascade SE(\u3bbSE = 1.0615 \u3bcm of 4F3/2 \u2192 4I11/2 channel) \u2192 SRS(\u3bbSt1 = 1.1741 \u3bcm of \u3c9 SRS 48 904 cm-1 promotionvibration mode) wavelength conversion

A passively Q switched nonosecond Nd3+Ca(NbO3)(2) self-Raman laser with 0.808 mu m laser-diode pumping has been demonstrated operating by nonlinear cascaded scheme at converted wavelength of Nd3+ one-micron stimulated emission