Temperature dependence of the E2h phonon mode of wurtzite GaN/AlN quantum dots

Raman scattering has been used to study the temperature dependence of the frequency and linewidth of the E2h phonon mode of GaN/AlN quantum dot stacks grown on 6H-SiC. The evolution of the nonpolar phonon mode was analyzed in the temperature range from 80 to 655 K for both quantum dots and barrier materials. The experimental results are interpreted by comparison with a model that takes into account symmetric phonon decay and the different thermal expansions of the constituents of the heterostructure. We find a small increase in the anharmonic parameters of the phonon modes in the heterostructure with respect to [email protected] [email protected] [email protected]

Ginzburg-Landau Expansion in Non-Fermi Liquid Superconductors: Effect of the Mass Renormalization Factor

We reconsider the Ginzburg-Landau expansion for the case of a non-Fermi liquid superconductor. We obtain analytical results for the Ginzburg-Landau functional in the critical region around the superconducting phase transition, T <= T_c, in two special limits of the model, i.e., the spin-charge separation case and the anomalous Fermi liquid case. For both cases, in the presence of a mass renormalization factor, we derived the form and the specific dependence of the coherence length, penetration depth, specific heat jump at the critical point, and the magnetic upper critical field. For both limits the obtained results reduce to the usual BCS results for a two dimensional s-wave superconductor. We compare our results with recent and relevant theoretical work. The results for a d--wave symmetry order parameter do not change qualitatively the results presented in this paper. Only numerical factors appear additionally in our expressions.Comment: accepted for publication in Physical Review

Optimal control of high-harmonic generation by intense few-cycle pulses

At the core of attosecond science lies the ability to generate laser pulses of subfemtosecond duration. In tabletop devices the process relies on high-harmonic generation, where a major challenge is to obtain high yields and high cutoff energies required for the generation of attosecond pulses. We develop a computational method that can simultaneously resolve these issues by optimizing the driving pulses using quantum optimal control theory. Our target functional, an integral over the harmonic yield over a desired energy range, leads to a remarkable cutoff extension and yield enhancement for a one-dimensional model H atom. The physical enhancement process is shown to be twofold: the cutoff extension is of classical origin, whereas the yield enhancement arises from increased tunneling probability. The scheme is directly applicable to more realistic models and, within straightforward refinements, also to experimental verification.This work was supported by the Academy of Finland; COST Action CM1204 (XLIC); the European Community’s FP7 through the CRONOS project, Grant No. 280879; the European Research Council Advanced Grant DYNamo (Grant No. ERC-2010-AdG-267374); Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT578-13); Spanish Grant No. FIS2010-21282-C02-01; and the University of Zaragoza (Project No. UZ2012-CIE-06).Peer Reviewe

Optimal control of high-harmonic generation by intense few-cycle pulses

At the core of attosecond science lies the ability to generate laser pulses of subfemtosecond duration. In tabletop devices the process relies on high-harmonic generation, where a major challenge is to obtain high yields and high cutoff energies required for the generation of attosecond pulses. We develop a computational method that can simultaneously resolve these issues by optimizing the driving pulses using quantum optimal control theory. Our target functional, an integral over the harmonic yield over a desired energy range, leads to a remarkable cutoff extension and yield enhancement for a one-dimensional model H atom. The physical enhancement process is shown to be twofold: the cutoff extension is of classical origin, whereas the yield enhancement arises from increased tunneling probability. The scheme is directly applicable to more realistic models and, within straightforward refinements, also to experimental verification

Shaped electric fields for fast optimal manipulation of electron spin and position in a double quantum dot

We use quantum optimal control theory algorithms to design external electric fields that drive the coupled spin and orbital dynamics of an electron in a double quantum dot, subject to the spin-orbit coupling and Zeeman magnetic fields. We obtain time profiles of multifrequency electric field pulses which increase the rate of spin-flip transitions by several orders of magnitude in comparison with monochromatic fields, where the spin Rabi oscillations were predicted to be very slow. This precise (with fidelity higher than 1×10-4) and fast (at the time scale of the order of 0.1 ns, comparable with the Zeeman spin rotation and the interdot tunneling time) simultaneous control of the spin and position is achieved while keeping the electron in the four lowest tunneling- and Zeeman-split levels through the duration of the pulse. The proposed algorithms suggest effective applications in spintronics and quantum information devices

Nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy: The effect of temperature

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