233 research outputs found
Coherent x-ray wavefront reconstruction of a partially illuminated Fresnel zone plate
International audienceA detailed characterization of the coherent x-ray wavefront produced by a partially illuminated Fresnel zone plate is presented. We show, by numerical and experimental approaches, how the beam size and the focal depth are strongly influenced by the illumination conditions, while the phase of the focal spot remains constant. These results confirm that the partial illumination can be used for coherent diffraction experiments. Finally, we demonstrate the possibility of reconstructing the complex-valued illumination function by simple measurement of the far field intensity in the specific case of partial illumination
Effect of the quantum well thickness on the performance of InGaN photovoltaic cells
International audienceWe report on the influence of the quantum well thickness on the effective band gap and conversion efficiency of In0.12Ga0.88N/GaN multiple quantum well solar cells. The band-to-band transition can be redshifted from 395 to 474 nm by increasing the well thickness from 1.3 to 5.4 nm, as demonstrated by cathodoluminescence measurements. However, the redshift of the absorption edge is much less pronounced in absorption: in thicker wells, transitions to higher energy levels dominate. Besides, partial strain relaxation in thicker wells leads to the formation of defects, hence degrading the overall solar cell performance. InGaN alloys are considered as promising candidates for high-efficiency photovoltaic devices [1-4] since their band gap spans almost the whole solar spectrum from 0.7 eV (InN) to 3.4 eV (GaN). This makes theoretically possible the development of all-InGaN multijunction solar cells with a freely customizable number of junctions to enhance the overall efficiency. However, the large lattice mismatch between GaN and InN has led several groups to study the possibility of hybrid integration, combining an InGaN cell in a tandem device with silicon [5,6] or other non-III-nitride [7] photovoltaic cells. The difficulty of growing high-quality InGaN layers increases with the In content. Reports of InGaN-based junctions with an In mole fraction exceeding 0.3 are rare [1]; the best external quantum efficiencies (EQEs) exceeding 0.7 are obtained at around 400 nm and quickly drop for longer wavelengths [8-10]. The main challenges are the large dislocation density and In-clustering, caused by the strong tendency to phase separation during growth. Absorbing layers in the form of a multiple quantum well (MQW) structure are often used to delay strain relaxation. Furthermore, the quantum confined Stark effect (QCSE) associated to the strong piezoelectric fields in the InGaN/GaN system [11] offers the possibility to tune the effective band gap of the structure by adjusting the quantum well (QW) and barrier thickness (tQW and tB, respectively). The effect of tuning tB in InGaN/GaN MQW photovoltaic devices has been studied by Wierer et al. [12] and Watanabe et al. [13]. According to their results, the absorption cutoff of the solar cells redshifts with decreasing tB. However, this does not always translate in enhanced overall cell efficiency, since the short circuit current density (Jsc) and open circuit voltage (Voc) also depend on tB. In this paper, we focus on the influence of the QW thickness on the effective band gap of the junction and its impact on the overall cell efficiency. We experimentally demonstrate that the band-to-band transition in InGaN QWs can be significantly redshifted in larger QWs. However, this redshift appears linked to a dramatic enlargement of the Stokes shift, so that increasing the tQW above a few nm is n
Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging
Coherent diffraction imaging (CDI) on Bragg reflections is a promising
technique for the study of three-dimensional (3D) composition and strain fields
in nanostructures, which can be recovered directly from the coherent
diffraction data recorded on single objects. In this article we report results
obtained for single homogeneous and heterogeneous nanowires with a diameter
smaller than 100 nm, for which we used CDI to retrieve information about
deformation and faults existing in these wires. The article also discusses the
influence of stacking faults, which can create artefacts during the
reconstruction of the nanowire shape and deformation.Comment: 18 pages, 6 figures Submitted to New Journal of Physic
Quantum Impurity Entanglement
Entanglement in J_1-J_2, S=1/2 quantum spin chains with an impurity is
studied using analytic methods as well as large scale numerical density matrix
renormalization group methods. The entanglement is investigated in terms of the
von Neumann entropy, S=-Tr rho_A log rho_A, for a sub-system A of size r of the
chain. The impurity contribution to the uniform part of the entanglement
entropy, S_{imp}, is defined and analyzed in detail in both the gapless, J_2 <=
J_2^c, as well as the dimerized phase, J_2>J_2^c, of the model. This quantum
impurity model is in the universality class of the single channel Kondo model
and it is shown that in a quite universal way the presence of the impurity in
the gapless phase, J_2 <= J_2^c, gives rise to a large length scale, xi_K,
associated with the screening of the impurity, the size of the Kondo screening
cloud. The universality of Kondo physics then implies scaling of the form
S_{imp}(r/xi_K,r/R) for a system of size R. Numerical results are presented
clearly demonstrating this scaling. At the critical point, J_2^c, an analytic
Fermi liquid picture is developed and analytic results are obtained both at T=0
and T>0. In the dimerized phase an appealing picure of the entanglement is
developed in terms of a thin soliton (TS) ansatz and the notions of impurity
valence bonds (IVB) and single particle entanglement (SPE) are introduced. The
TS-ansatz permits a variational calculation of the complete entanglement in the
dimerized phase that appears to be exact in the thermodynamic limit at the
Majumdar-Ghosh point, J_2=J_1/2, and surprisingly precise even close to the
critical point J_2^c. In appendices the relation between the finite temperature
entanglement entropy, S(T), and the thermal entropy, S_{th}(T), is discussed
and and calculated at the MG-point using the TS-ansatz.Comment: 62 pages, 27 figures, JSTAT macro
Wafer-scale selective area growth of GaN hexagonal prismatic nanostructures on c-sapphire substrate
Selective area growth of GaN nanostructures has been performed on full 2"
c-sapphire substrates using Si3N4 mask patterned by nanoimprint lithography
(array of 400 nm diameter circular holes). A new process has been developed to
improve the homogeneity of the nucleation selectivity of c-oriented hexagonal
prismatic nanostructures at high temperature (1040\circ C). It consists of an
initial GaN nucleation step at 950 \circ C followed by ammonia annealing before
high temperature growth. Structural analyses show that GaN nanostructures are
grown in epitaxy with c-sapphire with lateral overgrowths on the mask. Strain
and dislocations are observed at the interface due to the large GaN/sapphire
lattice mismatch in contrast with the high quality of the relaxed crystals in
the lateral overgrowth area. A cathodoluminescence study as a function of the
GaN nanostructure size confirms these observations: the lateral overgrowth of
GaN nanostructures has a low defect density and exhibits a stronger near band
edge (NBE) emission than the crystal in direct epitaxy with sapphire. The shift
of the NBE positions versus nanostructure size can be mainly attributed to a
combination of compressive strain and silicon doping coming from surface mask
diffusion
Protein Kinase A Regulates Molecular Chaperone Transcription and Protein Aggregation
Heat shock factor 1 (HSF1) regulates one of the major pathways of protein quality control and is essential for deterrence of protein-folding disorders, particularly in neuronal cells. However, HSF1 activity declines with age, a change that may open the door to progression of neurodegenerative disorders such as Huntington's disease. We have investigated mechanisms of HSF1 regulation that may become compromised with age. HSF1 binds stably to the catalytic domain of protein kinase A (PKAcα) and becomes phosphorylated on at least one regulatory serine residue (S320). We show here that PKA is essential for effective transcription of HSP genes by HSF1. PKA triggers a cascade involving HSF1 binding to the histone acetylase p300 and positive translation elongation factor 1 (p-TEFb) and phosphorylation of the c-terminal domain of RNA polymerase II, a key mechanism in the downstream steps of HSF1-mediated transcription. This cascade appears to play a key role in protein quality control in neuronal cells expressing aggregation-prone proteins with long poly-glutamine (poly-Q) tracts. Such proteins formed inclusion bodies that could be resolved by HSF1 activation during heat shock. Resolution of the inclusions was inhibited by knockdown of HSF1, PKAcα, or the pTEFb component CDK9, indicating a key role for the HSF1-PKA cascade in protein quality control
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