The binding energy of a donor impurity in a spherical GaAs–(Ga,Al)As quantum dot with parabolic confinement is calculated as a function of the radius of the quantum dot and as a function of the intensity of an applied electric field. Calculations are performed within the effective‐mass approximation and using a variational method. We have found that when the radius of the quantum dot is reduced, both the energy of the ground state in the well of GaAs and the binding energy of the impurity increase. Likewise, we have found that these energies decrease with the electric field for quantum dots of different radii when the intensity of the electric field is increased. Finally we compare our results with previous reports in quantum dots.
<p>This study was conducted in the southeast region of the Central American Pacific, an area of great oceanographic importance due to the presence of various upwelling phenomena and the direct influence of the ENSO on its waters. Its main objective was to contribute to the knowledge of the main factors that modulate the regional dynamics. We describe the geostrophic circulation and thermohaline features along two transects obtained in October 2010 and March 2011, one from Costa Rica at (84°54’ W - 9°37’ N) to the SW of Cocos Island at (88°19’ W - 3°06’ N), and the second oriented zonally across the island from (88°14’ W - 5°33’ N) to (84°33’ W - 5°33’ N). Surface temperatures ranged from 27°C to 29°C and a near isothermal layer, with an average thickness of 40 m, was apparent above the thermocline centered at 60 m. Surface salinities were between 32 and 32.8, typical values of the Tropical Surface Water. In both years, Cocos Island was located in a region of low surface salinities (~32). The salty core of the Subtropical Subsurface Water (~35) was located at an average depth of 150m. In October the circulation between Cocos Island and the continent was dominated by the presence of the North Equatorial Countercurrent (NECC), with speeds above 40 cm s<sup>-1 </sup>in the upper 50 m of the water column. No flow to the northwest near the coast that could be associated with the Costa Rica Coastal Current (CRCC) in October was observed. The Cocos Island was located in the center of a 150 m deep, 100 km diameter anticyclonic eddy, with surface speeds of 10 cm s<sup>-1</sup>and 20 cm s<sup>-1</sup>. In March the study area was again dominated by an anticyclonic cell, with eastward flow between 50 cm s<sup>-1</sup> and 60 cm s<sup>-1</sup> located between 200 km north and 100 km south of the island. The southern end of this cell, with velocities between 10 cm s<sup>-1 </sup>and 50 cm s<sup>-1</sup> to the northwest, was located 200 km south of Cocos Island. A flow to the NW near the edge of the continental shelf, consistent with the CRCC, was observed in May. Our study contributes to document the oceanography of the eastern end of the Equatorial Current System near the coast of Central America, where regional forcing modifies the zonal flow which prevails west of the study area.</p><div> </div>
A pseudohelical approximation for the calculation of the bandstructures of DNA base homostacks in B conformation is introduced. It consists of choosing a unit cell of only two nucleobases with relative parallel displacement and twist that locally mimic the helical conformation. It is tested employing the extended Hückel method with a unique Wolfsberg-Helmholtz parameter. The resulting bandgaps and ionization potential trend agree well with the ones reported in the literature employing the full screw-axis symmetry and higher levels of theory. The electron and hole effective masses extracted from the bandstructures follow the same trends as the experimentally reported mobilities
We calculate the 2p→1s-like transition energy and the spontaneous lifetime of an on-center shallow donor impurity in a spherical parabolic GaAs-(Ga,Al)As quantum dot (PQD) as a function of the radius of the structure and the strength of an applied electric field. In our calculations we use a variational method, within the effective mass and dipolar approximations. We find that the spontaneous lifetimes increase with the radius of the PQD and the applied electric field. In this direction our results revel that the electric field can be used to suppress the electron-phonon interaction driving to the increasing of the 2p-1s spontaneous life-time, showing the feasibility of using impurity states to be used in quantum computing developments.1 Introduction Semiconductor quantum dots (QD's) have been the object of major research efforts during the past two decades, due to the interesting physics they exhibit and their applicability in the construction of novel laser systems. New types of semiconductor QD's have been fabricated, which display a nearly parabolic confinement for both electrons and holes [1,2]. The physical properties of these systems have been found to strongly depend on the degree of confinement, the presence of impurities, and the strength of applied electric and magnetic fields.The impurity binding and transition energies in this type of structure under applied electric and magnetic fields have been extensively studied theoretically [3][4][5][6][7][8], but the competing roles of the fields and the confinement for the control of the spontaneous emission have not been considered so far. The possibility of controlling radiative lifetimes is a key aspect for semiconductor lasers and micro cavity light emitters. In the last years attention has focused on the enhancement and inhibition of the spontaneous recombination of excitons generated inside quantum dots positioned in a three-dimensional micro cavity and coupled to a single optical mode [9]. Recently, the control of the dynamics of the spontaneous emission from semiconductor quantum dots embedded in inverse opal photonic crystals has been considered [10], but little attention has been devoted to the role of the impurities in this connection.In particular, in this work we study the spontaneous emission lifetime of the 2p→1s-like transition of an on-center shallow donor impurity in a spherical GaAs-(Ga,Al)As quantum dot with parabolic confinement under the action of an applied electric field.
Calculations of the electronic structure of a stacked dimmer sequence from the D(GCAAACGTTTGC)2 B-DNA dodecamer resolved in a PDB file 1HQ7 are performed within density functional theory. Seeking to understand the minimum level of theory that yields a reliable description for these systems, the basis sets 6-31g*, 6-31g*+BSSE, 6-311g*, 6-311g**, 6-311++g** along with the B3LYP and PBE0 exchange-correlation functionals were employed. These results are then used to implement a one dimensional model of long stacked systems to obtain a new semiempirical method that can be employed at low computational cost.
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