Quantifying Regeneration in Dye Sensitized Solar Cells: A Step Toward Red Absorbing Dyes having Lower Energy Loss

A limiting factor on DSSC efficiency is the lower fraction of the solar spectrum that is absorbed by the dye molecules developed to this point. Dye molecules that function well in DSSCs tend to have poor or no absorption to the red of 750 nm. Extending this absorption to the red by 100 nm, without losing efficiency in other ways, would result in a significant improvement in photocurrent. This challenge has proven difficult, in large part because of one slow reaction in the electron transfer cycle of DSSCs, the regeneration reaction. Better understanding of this reaction is thus critical. The kinetics of regeneration is understudied relative to the other processes in DSSCs, this is in part because the regeneration reaction produces no, as yet detected, measurable electrical signal. It must be studied by more difficult transient absorbance (TA) techniques. The first step of this thesis focuses on isolating a reliable transient signal that reflects the regeneration reaction. This is made by upgrading the conventional TA system to also acquire transient electrical (TE) signals simultaneously (TA-TE). The system is used to characterize dye-sensitized solar cells (DSSCs) under 1 sun illumination whilst the cells are fully operational and their stability is monitored. The second step of the work consists of the development of a methodology and a kinetic model which uses the isolated regeneration signal and a range of complimentary measurements on operating cells, to determine the quantum yield and the associated intrinsic rate constants and orders of the regeneration reaction. This enabled understanding of the regeneration mechanism and its optional rate limiting steps. Finally, the use of steady state photoinduced absorption (SSPA), as a complementary or alternative tool to assess regeneration, is also questioned. SSPA is compared with the regeneration TA –TE and charge extraction measurements

Dirac parameters and topological phase diagram of Pb1-xSnxSe from magneto-spectroscopy

Pb1-xSnxSe hosts 3D massive Dirac fermions across the entire composition range for which the crystal structure is cubic. In this work, we present a comprehensive experimental mapping of the 3D band structure parameters of Pb1-xSnxSe as a function of composition and temperature. We cover a parameter space spanning the band inversion that yields its topological crystalline insulator phase. A non-closure of the energy gap is evidenced in the vicinity of this phase transition. Using magnetooptical Landau level spectroscopy, we determine the energy gap, Dirac velocity, anisotropy factor and topological character of Pb1-xSnxSe epilayers grown by molecular beam epitaxy on BaF2 (111). Our results are evidence that Pb1-xSnxSe is a model system to study topological phases and the nature of the phase transition.Comment: Submitte

Attempted density blowup in a freely cooling dilute granular gas: hydrodynamics versus molecular dynamics

It has been recently shown (Fouxon et al. 2007) that, in the framework of ideal granular hydrodynamics (IGHD), an initially smooth hydrodynamic flow of a granular gas can produce an infinite gas density in a finite time. Exact solutions that exhibit this property have been derived. Close to the singularity, the granular gas pressure is finite and almost constant. This work reports molecular dynamics (MD) simulations of a freely cooling gas of nearly elastically colliding hard disks, aimed at identifying the "attempted" density blowup regime. The initial conditions of the simulated flow mimic those of one particular solution of the IGHD equations that exhibits the density blowup. We measure the hydrodynamic fields in the MD simulations and compare them with predictions from the ideal theory. We find a remarkable quantitative agreement between the two over an extended time interval, proving the existence of the attempted blowup regime. As the attempted singularity is approached, the hydrodynamic fields, as observed in the MD simulations, deviate from the predictions of the ideal solution. To investigate the mechanism of breakdown of the ideal theory near the singularity, we extend the hydrodynamic theory by accounting separately for the gradient-dependent transport and for finite density corrections.Comment: 11 pages, 9 figures, accepted for publication on Physical Review

High-precision realization of robust quantum anomalous Hall state in a hard ferromagnetic topological insulator

The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two dimensional electron layer under a strong magnetic field. The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon as the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism.Here we report the experimental observation of the QAH state in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down to 0.00013+-0.00007h/e2 (~3.35+-1.76 ohm), Hall conductance reaching 0.9998+-0.0006e2/h and the Hall angle becoming as high as 89.993+-0.004degree at T=25mK. Further advantage of this system comes from the fact that it is a hard ferromagnet with a large coercive field (Hc>1.0T) and a relative high Curie temperature. This realization of robust QAH state in hard FMTIs is a major step towards dissipationless electronic applications without external fields.Comment: 16 pages, 4 figures, this is the final version, accepted by Nature Materials, forthcomin

Massive and massless Dirac fermions in Pb1-xSnxTe topological crystalline insulator probed by magneto-optical absorption

Dirac fermions in condensed matter physics hold great promise for novel fundamental physics, quantum devices and data storage applications. IV-VI semiconductors, in the inverted regime, have been recently shown to exhibit massless topological surface Dirac fermions protected by crystalline symmetry, as well as massive bulk Dirac fermions. Under a strong magnetic field (B), both surface and bulk states are quantized into Landau levels that disperse as B^1/2, and are thus difficult to distinguish. In this work, magneto-optical absorption is used to probe the Landau levels of high mobility Bi-doped Pb0.54Sn0.46Te topological crystalline insulator (111)-oriented films. The high mobility achieved in these thin film structures allows us to probe and distinguish the Landau levels of both surface and bulk Dirac fermions and extract valuable quantitative information about their physical properties. This work paves the way for future magnetooptical and electronic transport experiments aimed at manipulating the band topology of such materials.Comment: supplementary material included, to appear in Scientific Report

Limitations to Frechet's Metric Embedding Method

Frechet's classical isometric embedding argument has evolved to become a major tool in the study of metric spaces. An important example of a Frechet embedding is Bourgain's embedding. The authors have recently shown that for every e>0 any n-point metric space contains a subset of size at least n^(1-e) which embeds into l_2 with distortion O(\log(2/e) /e). The embedding we used is non-Frechet, and the purpose of this note is to show that this is not coincidental. Specifically, for every e>0, we construct arbitrarily large n-point metric spaces, such that the distortion of any Frechet embedding into l_p on subsets of size at least n^{1/2 + e} is \Omega((\log n)^{1/p}).Comment: 10 pages, 1 figur

Las “moscas de las flores” (Diptera: Syrphidae) en Lules, Tucumán

La familia Syrphidae es un grupo de insectos que se caracteriza por ser moscas grandes y vistosas capaces de adoptar la apariencia de abejas o avispas para defenderse de los depredadores. Los adultos generalmente se encuentran sobre las flores o en vuelo suspendido en lugares soleados, de ahí que sean llamados comúnmente “moscas de las flores” o “flower flies”. Su tamaño fluctúa de 4 a 25 mm, el cuerpo puede ser delgado o robusto y presentar coloración metálica u opaca. El abdomen es variable en forma, de ancho a muy delgado (Sarmiento Cordero et al., 2010).Fil: Maza, Noelia. Universidad Nacional de Tucumán. Facultad de Agronomía y Zootecnia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sopena, Y. N.. Universidad Nacional de Tucumán. Facultad de Agronomía y Zootecnia; ArgentinaFil: Assaf, M. J. T.. Universidad Nacional de Tucumán. Facultad de Agronomía y Zootecnia; ArgentinaFil: Paz, Miriam Rosana. Universidad Nacional de Tucumán. Facultad de Agronomía y Zootecnia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Jaime, Adriana Patricia. Universidad Nacional de Tucumán. Facultad de Agronomía y Zootecnia; Argentin

Maximum gradient embeddings and monotone clustering

Let (X,d_X) be an n-point metric space. We show that there exists a distribution D over non-contractive embeddings into trees f:X-->T such that for every x in X, the expectation with respect to D of the maximum over y in X of the ratio d_T(f(x),f(y)) / d_X(x,y) is at most C (log n)^2, where C is a universal constant. Conversely we show that the above quadratic dependence on log n cannot be improved in general. Such embeddings, which we call maximum gradient embeddings, yield a framework for the design of approximation algorithms for a wide range of clustering problems with monotone costs, including fault-tolerant versions of k-median and facility location.Comment: 25 pages, 2 figures. Final version, minor revision of the previous one. To appear in "Combinatorica