(Português) Programa de Pós-Graduação em Física da UFSC, Florianópolis

EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

Excitações de spin em sistemas nanoestruturados   

Prof. Dr. Antonio Tavares Costa Junior – UFF/IF

 Resumen:

Em sistemas ferromagnéticos as excitações de spin determinam várias propriedades importantes do sistema, tais como a relaxação da magnetização e a absorção de energia. Também estão associadas a fenômenos com grande potencial de aplicação tecnológica, como a geração de correntes puras de spin e a reversão do estado de magnetização de domínios magnéticos. Em metais as excitações de spin têm caráter bastante diverso daquelas presentes em isolantes magnéticos. Nesse seminário apresentarei a abordagem que desenvolvemos para o cálculo de excitações de spin em sistemas magnéticos nanoscópicos em contato com substratos metálicos não magnéticos. Ela incorpora naturalmente o caráter metálico do sistema e é capaz de descrever, em pé de igualdade, excitações em sistemas tão diversos quanto filmes ultrafinos e átomos adsorvidos a superfícies.

Fecha: 29 de mayo de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

 Buracos negros ativos, inativos e múltiplos: um recenseamento  

João Evangelista Steiner – IAG/USP

 Resumen:

Buracos negros supermassivos parecem residir na maioria das galáxias de alta massa e em uma fração das galáxias de baixa massa. Ao capturar gás, esses objetos tornam-se luminosos e são chamados de núcleos ativos. Parte significativa dos núcleos de galáxias no Universo Local não mostra atividade nuclear, podendo ter buracos negros inativos (sem captura de gás), mais difíceis de serem detectados.

Estamos conduzindo um recenseamento dos núcleos de todas as galáxias brilhantes (B <12 mag) do Hemisfério Sul, utilizando espectroscopia de campo integral nos telescópios Gemini, para estudar, com precisão inédita, suas propriedades nucleares e circumnucleares. Os resultados preliminares indicam que: 1) Em galáxias massivas, núcleos ativos são ao menos duas vezes mais frequentes do que indicavam pesquisas anteriores; 2) Há uma fração significativa de objetos ativos de muito baixa luminosidade; 3) Encontramos núcleos fora do centro ou com estrutura múltipla com frequência inesperada; 4) Gás ionizado ou neutro está presente no centro de 97% das galáxias massivas.

Fecha: 15 de mayo de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

 Busca da Janela Conforme  

Tiago José Nunes da Silva – University of Groningen, RUG, Holanda

Resumen:

Teorias de Calibre Não-Abelianas com um grande número de sabores exibem restauração de simetria conforme antes que liberdade assintótica seja perdida. Um dos cenários que tenta explicar como simetria conforme é restaurada é o cenário da Janela Conforme. Neste seminário eu irei introduzir o tema e reportar resultados recentes obtidos utilizando métodos de teoria de calibre na rede, um método não-perturbativo robusto para o estudo numérico de teorias de calibre.

Fecha: 8 de Mayo de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

  Effect of singlet triplet recycling in the charge transfer state manifold and molecular geometry on thermally activated delayed fluorescence  

Prof. Andy P. Monkman

Dept. of Physics, Durham University, South Road Durham, UK

Resumen:

Figure 1: Temperature dependent delayed fluorescence measurements of a near zero exchange energy ICT TADF

Detailed photophysical measurements of intramolecular charge transfer states have been made both in solution and solid state¹. Temperature dependent emission and delayed emission are used to map the energy levels involved in molecule decay, and through detailed kinetic modelling of the thermally activated processes observed, true electron exchange energies and other energy barriers of the systems determined.

For specific donor acceptor molecular configurations, the CT singlet and triplet states are found to be the lowest lying excited states of the molecule with very small electron exchange energies = kT. In these cases the decay kinetics of the molecules become significantly different to normal molecules, and the effect of rapid recycling between CT singlet and triplet states is seen to greatly extend the lifetime of the ‘excited state’ and yield non-exponential decay. Quantum yields increase markedly, even though the intersystem crossing rate is fast, ? 10⁹ s^-1. The decay kinetics is found to be very sensitive to both temperature and sample inhomogeneity², see figure 1. Temperature dependent delayed emission measurements reveal very different time domain behaviour and the effects of ICT emitter inhomogeneity is revealed. Clear evidence will be given to show that TADF reaches 100% efficiency at harvesting triplet states^1,3, and device having > 15% EQE discussed.

 We will then go on to show the results for an ICT molecule with highly controlled structure i.e. the donor and acceptor fragments are held rigidly orthogonal. In this molecule the CT states can clearly be seen to be the lowest energy states of the molecule with a very small exchange energy (singlet triplet gap). Here we find that there are substantial differences between optical and electroluminescent photophysics resulting in device being far more efficient than is suggested by the molecules PLQY. For the first time we can show that the PLQY of the emitter is not the figure of merit to use in a device, because the excited states are formed in a different way within the device which avoids a major excited state quenching mechanism. This will be discussed in a new molecule that has a PLQY of 30% but gives devices having >19% EQE.

 TADF thus not only enables 100% of triplet states to be harvested, the charge transfer states which give rise to emission and TADF also enable more efficient singlet emission than ‘PLQY’ would lead us to believe.

 References

 1    V. Jankus, C. J. Chiang, F. Dias, and A. P. Monkman, Adv Mater 25, 1455 (2013).

2    F. B. Dias, K. N. Bourdakos, V. Jankus, K. C. Moss, K. T. Kamtekar, V. Bhalla, J. Santos, M. R. Bryce, and A. P. Monkman, Adv Mater 25, 3707 (2013).

3    D. Graves, V. Jankus, F. B. Dias, and A. Monkman, Adv Funct Mater 24, 2343 (2014).

4      Vygintas Jankus, Przemyslaw Data, David Graves, Callum McGuinness, Jose Santos, Martin R. Bryce, F. B. Dias, and A. P. Monkman, Advanced Functional Materials 24, 6178. (2014).

Fecha: 24 de abril de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

  Detection of quantum entanglement: experiments with spatially entangled photon pairs  

Stephen Patrick Walborn

Instituto de Física – UFRJ

 Resumen:

 The twin photons produced by spontaneous parametric down-conversion have been an exceptional tool for the study of entanglement and quantum information. The spatial correlation of these photons was one of the first properties to be studied. Nevertheless, the convenient and efficient detection of quantum entanglement in the spatial variables is still an active area of research, with possible applications in quantum cryptography and communication. Here we present several experiments performed at the Quantum Optics Laboratory at the Instituto de Física, Universidade Federal do Rio de Janeiro. We present several novel experimental methods to detect the entanglement in the transverse spatial variables of photon pairs. Both methods use spatial light modulators to imprint an appropriate phase profile on the down-converted photons. Our first method uses this phase profile to measure the spatial correlation functions directly. With these, we can evaluate known entanglement witnesses in a more efficient fashion, without the need for a position-dependent detection system. Our second method uses complementary masks to discretize the detection system, again allowing for a more efficient identification of entanglement via a novel entanglement witness.

Fecha: 17 de abril de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

 EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

  O Lado Escuro do Universo   

Rogerio Rosenfeld – Instituto de Física Teórica / UNESP

Resumen:

Sabemos hoje que apenas 5% do Universo é composto de átomos. Nesse colóquio vou descrever brevemente o que pensamos que são os 95% restantes: matéria escura e energia escura.

Fecha: 10 de abril de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

 EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

O Centenário da Relatividade Geral  

Prof. Dr. Victor de Oliveira Rivelles

Instituto de Física – USP

 Resumen:

Apresentaremos um panorama geral de uma das teorias que revolucionaram a física do século XX. Veremos o que levou Einstein a formular a relatividade geral, as mudanças de paradigmas que gerou, seus efeitos mais importantes, dentre eles ondas gravitacionais e buracos negros e como levou ao nascimento de um novo ramo da ciência, a cosmologia.

Fecha: 13 de marzo de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

Quantum dipolar gases for quantum magnetism  

Laurent Vernac

Laboratoire de Physique des Lasers / Université Paris 13

 Resumen:

 Bose Einstein Condensates (BEC) have been obtained with ultra cold atoms in 1995. In these quantum gases all atoms are in the same quantum state, which gives them unique properties in terms of coherence, and allows for observation of matter interference, superfluidity, or transition from superfluid to Mott insulator states in an optical lattice. I will present experimental results obtained in our group with chromium atoms. Due to the high spin (S=3) in the ground state of chromium, there are large magnetic dipole-dipole interactions (DDIs) between the atoms, which are long range, and anisotropic. They confer specific properties to chromium BECs. For example, the excitation spectrum depends on the direction of the magnetic field: a Cr BEC can be seen as an anisotropic superfluid [1]. DDIs allow as well the change of the magnetization of the sample, which leads to spontaneous depolarization of the BEC at low magnetic fields, and multi-components BECs [2,3]. The main part of my talk will be devoted to the results we have obtained in a 3D optical lattice. We have studied spin exchange dynamics due to DDIs in the Mott insulator state, with one or two atoms per lattice site [4]. Our system is well described by an effective XXZ spin Hamiltonian, with Ising and exchange terms. We thus obtain an interesting system for quantum simulation, which realizes a spin model used in condensed matter physics, with true spin-spin interactions. As correlations build up between the few thousands of atoms loaded in the lattice, our system should reveal as well the existence of entanglement between many particles, through the finding of a relevant experimental entanglement witness. I will conclude by perspectives offered by our recent production of a chromium quantum gas with the fermionic isotope (Fermi Sea) [5].

REFS.:     

[1] Anisotropic Excitation Spectrum of a Dipolar Quantum Bose Gas G. Bismut et al., Phys. Rev. Lett. 109, 155302 (2012)

[2] Spontaneous demagnetization of a dipolar spinor Bose gas atultra-low magnetic field

B. Pasquiou, Phys. Rev. Lett. 106, 255303 (2011)

[3] Thermodynamics of a Bose-Einstein condensate with free magnetization B. Pasquiou et al., Phys. Rev. Lett., 108, 045307 (2012)

[4] Nonequilibrium Quantum Magnetism in a Dipolar Lattice Gas A. de Paz et al., Phys. Rev. Lett., 111, 185305 (2013)

[5] Chromium Dipolar Fermi SeaB. Naylor et al., Phys. Rev. A 91, 011603(R) (2015)

Fecha: 6 de marzo de 2015 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

Correlating Structural, Electronic and Ionic Diffusion Properties of Nanomaterials for Energy Storage  

Profª Drª Corinne Arrouvel / UFSCar – Sorocaba

Resumen:

The quest of new materials at the nanoscale is an important topic in the modern society. The industrial interest is to improve the energy conversion, the storage capacity and the kinetic of the electrochemical energy in high-technology devices. Materials should be in addition low cost, stable under a wide range of temperature and pressure conditions and safe. This talk will introduce basic concepts of electrochemistry, of bulk and surface properties in order to explain the difference of energy carrier transport mechanisms in rechargeable lithium batteries, capacitors, solid oxide fuel cells and solar cells. Combining computational and experimental techniques, we show that some key aspects are based on the search of optimized size and morphology of their nanocomponents. Anisotropic energy pathways and crystal shape will be therefore highlighted.

Fecha: 5 de dezembro de 2014 – (viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min

 EL PROGRAMA DE POSGRADO EN FÍSICA invita a todos para lo seminario:

Nonperturbative QCD effects in forward scattering at LHC  

Prof. Emerson Gustavo de Souza Luna – IF/UFRGS

Resumen:

We study infrared contributions to semihard parton-parton interactions by considering an effective charge whose finite infrared behavior is constrained by a dynamical mass scale. Using an eikonal QCD-based model in order to connect this semihard parton-level dynamics to the hadron-hadron scattering, we obtain predictions for the proton-proton ($pp$) and antiproton-proton ($\bar{p}p$) total cross sections, $\sigma_{tot}^{pp,\bar{p}p}$, as well as for the ratios of the real to imaginary part of the forward scattering amplitude, $\rho^{pp,\bar{p}p}$. We discuss the theoretical aspects of this formalism and consider the phenomenological implications of a class of energy-dependent form factors in the high-energy behavior of the forward amplitude. Our results, obtained using a group of updated sets of parton distributions, are consistent with the recent data from the TOTEM experiment.

Fecha: 28 de noviembre de 2014 – (Viernes) – Lugar: Sala 212 Auditorio del Departamento de Física – Horário: 10h15min