Colóquios e Seminários

Journal club: “All electron topological insulator in InAs double wells”

Topological Insulators (TI) were initially theoretically predicted in graphene and in inverted band-gap HgTe-based quantum wells. A year later, in 2007, the quantum spin hall effect (QSHE) was observed experimentally in HgTe quantum wells. More recently a QSHE system based on InAs/InSb electron/hole double quantum wells was proposed, and subsequently observed experimentally. Although much progress has been made in these systems, clear observation of edge state transport is still lacking, due to background bulk conductance.

Here we show that electrons in ordinary III-V semiconductor double wells with an in-plane modulating periodic potential and spin-orbit interaction are tunable TIs. The essential TI ingredients, namely, band inversion and the opening of an overall bulk gap in the spectrum arise, respectively, from (i) the combined effect of the double well even-odd state splitting together with the superlattice potential and (ii) the interband Rashba spin-orbit coupling. We will introduce the origins of the interband spin-orbit coupling and using the k.p-like approach we will derive an effective Bernevig-Hughes-Zhang model. In addition to the analytical work we perform numerical calculations of the bands and explicitly verify the bulk-edge correspondence by considering a strip configuration and determining not only the bulk bands but also the edge states and their Dirac-like spectrum in the topological phase. Finally we will consider the effects of the intraband spin-orbit coupling, and the breaking of inversion symmetry due to the periodic superlattice potential, and discuss possible experimental implementation.

Café com física: “Bosons in Optical Lattices”

Spinless bosons in optical lattices reveal a generic quantum phase transition once the depth of the potential wells is tuned. When the on-site interaction energy is small compared to the hopping energy, the ground state is superfluid, as the bosons are delocalized and phase coherent over the whole lattice. In the opposite limit, where the on-site interaction energy dominates over the hopping energy, the ground state is a Mott insulator, as each boson is trapped in one of the respective potential minima.

In order to describe both thermodynamic and dynamic properties of this quantum phase transition we developed a Ginzburg-Landau theory [1,2]. To this end we started from the microscopic Bose-Hubbard model, applied diagrammatic techniques within a systematic strong-coupling expansion, and calculated the underlying effective action. Already the first beyond mean-field order exhibits for the boundary of the quantum phase transition in a three-dimensional cubic lattice a relative error of less than 3% when compared with most recent Quantum Monte Carlo simulations [1]. Higher orders turn out to be so accurate that they even allow for the calculation of critical exponents [3]. Furthermore, the Ginzburg-Landau theory yields excitation spectra both in the Mott and the superfluid phase, which agree qualitatively with recent experiments [4].

Finally, we discuss three intriguing examples how the quantum phase transition of bosons in optical lattices can be tuned. In the first example we consider a spinor Bose gas loaded into a three-dimensional cubic optical lattice, where the different superfluid phases of spin-1 bosons are tunable due to the presence of an external magnetic field [5]. Then we deal with interacting bosons in an optical lattice with a periodic modulation of the s-wave scattering length, so the location of the quantum phase boundary turns out to depend quite sensitively on both driving amplitude and frequency [6]. Afterwards, we study the Bose-Hubbard model for the optical Kagome superlattice, where the delicate interplay between onsite repulsion and artificial symmetry breaking yields an anisotropic superfluid density, whose directional dependence is tunable by several system parameters [7,8].

[1] F.E.A. dos Santos and A. Pelster, Phys. Rev. A 79, 013614 (2009) [2] B. Bradlyn, F.E.A. dos Santos, and A. Pelster, Phys. Rev. A 79, 01361 (2009) [3] D. Hinrichs, A. Pelster, and M. Holthaus, Applied Physics B 113, 57 (2013) [4] T.D. Grass, F.E.A. dos Santos, and A. Pelster, Phys. Rev. A 84, 013613 (2011) [5] M. Mobarak and A. Pelster, Laser Phys. Lett. 10, 115501 (2013) [6] T. Wang, X.-F. Zhang, F.E.A. dos Santos, S. Eggert, and A. Pelster, Phys. Rev. A 90, 013633 (2014) [7] T. Wang, X.-F. Zhang, S. Eggert, and A. Pelster, Phys. Rev. A 87, 063615 (2013) [8] X.-F. Zhang, T. Wang, S. Eggert, and A. Pelster, Phys. Rev. B 92, 014512 (2015)

Seminário especial: “Developing tools for algal biotechnology”

Alison Smith´s key publications

1. Grant MAA, Kazamia E, Cicuta P, Smith AG (2014) Direct exchange of vitamin B12 is demonstrated by modelling the growth dynamics of algal-bacterial coculture. ISME J doi: 10.1038/ismej.2014.9.

2. Davey MP, Horst I, Duong GH, Tomsett EV, Litvinenko A, Howe CJ, Smith AG (2014) Triacylglyceride production and autophagous responses in Chlamydomonas reinhardtii depend on resource allocation and carbon source. Eukaryotic Cell 13: 392-400; doi:10.1128/EC.00178-13

3. Kazamia E, Aldridge DA and Smith AG (2012) Synthetic ecology- a way forward for sustainable algal biofuel production? J Biotechnol 162: 163-169

4. Stephenson AL, Kazamia E, Dennis JS, Howe CJ, Scott SA and Smith AG (2010) Life-cycle assessment of potential algal biodiesel production in the United Kingdom: comparison of raceways and air-lift tubular bioreactors. Energy & Fuels 24: 4062–4077

Journal club: “Chiral Majorana edge states in HgTe quantum wells”

HgTe-based quantum wells (QWs) recently attracted a lot of attention for the realization of a two-dimensional topological insulator with protected helical edge states. Another class of topological systems are topological superconductors (TSCs) with Majorana edge states. In this paper, we show how proximity induced s-wave superconductivity in the bulk of HgTe-QWs and in the presence of a Zeeman field can exhibit a TSC with chiral Majorana edge states. We calculate the topological invariants and the corresponding Majorana edge states explicitly within a four-band model accounting for inversion symmetry breaking terms due to the Rashba spin–orbit coupling and bulk inversion asymmetry present in these QWs.

L. Weithofer and P. Recher

New Journal of Physics 15 (2013) 085008 (19pp)

Seminário do Grupo de Óptica: “Desenvolvimento de um sistema portátil de espectroscopia”

A análise de espectros e tempos de vida de fluorescência em tecidos biológicos vem sendo apresentada como uma técnica com grande potencial para a caracterização tecidual com finalidade diagnóstica. Além de medidas rápidas e não-invasivas, ela permite uma avaliação do tecido in situ, sem a necessidade de remoção e processamento da amostra biológica. No caso de lesões cutâneas, podem haver lesões benignas com aspectos clínicos semelhantes aos de malignas, o que torna o diagnóstico clínico mais complexo. O prognóstico do paciente, assim como a modalidade terapêutica, depende do tipo histológico e da evolução da lesão. Nesse contexto, técnicas auxiliares de diagnóstico são de grande relevância para melhorar o planejamento e o sucesso do tratamento. Nesta primeira etapa do projeto, um dos primeiros sistemas portáteis de espectroscopia de tempo de vida de fluorescência do mundo foi desenvolvido e caracterizado. Esse sistema foi adaptado de um sistema comercial de bancada para uma maleta metálica, de modo a facilitar o transporte e viabilizar medidas clínicas. Também serão apresentados alguns dos resultados preliminares de diferenciação entre áreas da pele expostas e não-expostas ao sol em voluntários jovens, nos quais não se esperava uma grande diferenciação.

Café com física: “MERDA – the fingerprint of life history”

The principle is well established in particle physics that, intrinsic in every particle, is information that is influenced by the physical nature of the particle, as well as its history. In a multi-particle system, every particle represents an information-rich entity that contributes in some definable way to the properties of the entire system. This same lens can be used to view microbes that inhabit the human body. All environmentally exposed surfaces of animals are saturated with microbes. The gut houses the vast majority (about 1014) of microbes associated with humans, because it is rich in the elements that support microbial life – high water and nutritional content. Like physical particles, each microbe in the gut ecosystem possesses information – there is a history as to how it got there, it has specific properties that are allowing it to stay and even proliferate there, etc.. Microbes of the gut are not a random assemblage. There are about 1000 different abundant microbial species in the typical gut, and many more species occurring in low numbers along the very long tail of the decay curve descending from 1014. Many of the most abundant microbes in the gut are initially aquired when the largely sterile baby transits the birth canal. Other usually rarer microbes derive from the diet, transmission between individuals, and the environment. Some may become long time residents of the gut, and others may just pass through. Since bacteria have the ability to replicate in the gut, the relative representation of a particular type of bacterium is defined by properties including metabolic capabilities of that microbe, and their compatibility with the environmental conditions within the gut. Environmental conditions within the gut are determined by such factors as diet, supportive or antagonistic factors produced by other resident microbes, and supportive and antagonistic factors secreted into the gut by the host. Host determinants of the microbial population composition and structure include rate of mucous secretion, rate of secretion of antimicrobial defense factors in the mucous, pH changes along the length of the gut, glucose and other nutrient content of the host secretions into the gut, temperature, water content, and many other factors. Factors that influence what the host secretes into the gut include age and state of development of the host immune system, sex of the host, diseases of the host (e.g., gut inflammation, diabetes, cystic fibrosis, etc.), relative activity of the host, and other factors that affect gut contraction rates and transit time. Other factors that influence the population structure within the gut include the rate of nutrient consumption by the host in relationship to the ability of that host to digest and absorb the nutritional content, such that excess consumption leads to a surplus of nutrients in the gut available to the microbial community. The main point is that the composition of gut microbes is highly unique to each person or animal, and depends on the specific properties of both the microbe as well as the host. Each microbe within the population conveys information about the history of the animal (such as the identity of its mother), about its diet and rate of consumption/over consumption, about its health, about its history of antibiotic use or intestinal disease, and many other traits. That is, gut flora are a highly specific historic and metabolic fingerprint of each animal. With the development of “Next Generation” DNA sequencing, we now have the ability to identify and enumerate about 99.9999% of the microbes in the gut of each animal. Based on a growing body of data, we are just beginning to develop the bioinformatic tools and rules necessary to correlate the microbial content of the gut with health. In the near future, analysis of gut flora (feces) will be a relatively painless and rapid means for comprehensively understanding the health and natural history of humans and animals. The objective is to reach a point where we understand the information content of each of the 1014 particles that constitute the human gut community.

Café com física: “MERDA – the fingerprint of life history”

The principle is well established in particle physics that, intrinsic in every particle, is information that is influenced by the physical nature of the particle, as well as its history. In a multi-particle system, every particle represents an information-rich entity that contributes in some definable way to the properties of the entire system. This same lens can be used to view microbes that inhabit the human body. All environmentally exposed surfaces of animals are saturated with microbes. The gut houses the vast majority (about 1014) of microbes associated with humans, because it is rich in the elements that support microbial life – high water and nutritional content. Like physical particles, each microbe in the gut ecosystem possesses information – there is a history as to how it got there, it has specific properties that are allowing it to stay and even proliferate there, etc.. Microbes of the gut are not a random assemblage. There are about 1000 different abundant microbial species in the typical gut, and many more species occurring in low numbers along the very long tail of the decay curve descending from 1014. Many of the most abundant microbes in the gut are initially aquired when the largely sterile baby transits the birth canal. Other usually rarer microbes derive from the diet, transmission between individuals, and the environment. Some may become long time residents of the gut, and others may just pass through. Since bacteria have the ability to replicate in the gut, the relative representation of a particular type of bacterium is defined by properties including metabolic capabilities of that microbe, and their compatibility with the environmental conditions within the gut. Environmental conditions within the gut are determined by such factors as diet, supportive or antagonistic factors produced by other resident microbes, and supportive and antagonistic factors secreted into the gut by the host. Host determinants of the microbial population composition and structure include rate of mucous secretion, rate of secretion of antimicrobial defense factors in the mucous, pH changes along the length of the gut, glucose and other nutrient content of the host secretions into the gut, temperature, water content, and many other factors. Factors that influence what the host secretes into the gut include age and state of development of the host immune system, sex of the host, diseases of the host (e.g., gut inflammation, diabetes, cystic fibrosis, etc.), relative activity of the host, and other factors that affect gut contraction rates and transit time. Other factors that influence the population structure within the gut include the rate of nutrient consumption by the host in relationship to the ability of that host to digest and absorb the nutritional content, such that excess consumption leads to a surplus of nutrients in the gut available to the microbial community. The main point is that the composition of gut microbes is highly unique to each person or animal, and depends on the specific properties of both the microbe as well as the host. Each microbe within the population conveys information about the history of the animal (such as the identity of its mother), about its diet and rate of consumption/over consumption, about its health, about its history of antibiotic use or intestinal disease, and many other traits. That is, gut flora are a highly specific historic and metabolic fingerprint of each animal. With the development of “Next Generation” DNA sequencing, we now have the ability to identify and enumerate about 99.9999% of the microbes in the gut of each animal. Based on a growing body of data, we are just beginning to develop the bioinformatic tools and rules necessary to correlate the microbial content of the gut with health. In the near future, analysis of gut flora (feces) will be a relatively painless and rapid means for comprehensively understanding the health and natural history of humans and animals. The objective is to reach a point where we understand the information content of each of the 1014 particles that constitute the human gut community.

Seminário especial: “Technology developments in Cryo-TEM as well as applications”

During this seminar, the following topics will be dealt with:

• Overview of recent technology developments in Cryo-TEM allowing for an efficient workflow for high resolution protein structure determination

• New contrast enhancements mechanisms for cryo-TEM: the FEI Volta Phase Plate

• Addressing biological questions with cryo-TEM applications

• Utilizing Cryo-TEM as a national resource at a synchrotron for protein structure determination

Seminário do Grupo de Óptica: “Avaliação da Terapia Fotodinâmica em Células de Melanoma Murino”

De acordo com os últimos dados do INCA (Instituto Nacional do Câncer), 6.000 novos casos de câncer melanoma surgiram em 2014. Este é o tipo de câncer mais perigoso, porque possui uma alta taxa de metástase levando em muitos casos ao óbito. Atualmente, o melanoma é resistente à maioria dos tratamentos quimioterápicos e radioterápicos e por isso é estudado novos métodos de tratamentos. Nesse contexto, a terapia fotodinâmica é uma alternativa ao tratamento com o a vantagem de não ser invasiva. Um dos grandes desafios para terapia é superar a alta produção de melanina pelos melanócitos, que impede a penetração da luz e, portanto, a ativação do fotosensibilizador. Neste estudo foi investigada a morte celular do melanoma murino em modelo de monocamada através da avaliação de viabilidade celular usando diferentes fluências de luz. Além disso, foi observada a captação do fotossensibilizador, Photodithazine (PDZ), pelas células melanóticas em diferentes tempos, e a sua co-localização com organelas celulares. Assim buscamos um melhor entendimento do mecanismo de atuação deste fotosensibilizador e a sua ação contra o câncer melanoma.

Seminário do Grupo de Óptica: “Entanglement and exotic superfluidity”

Exotic superfluids are characterized by the coexistence of superconductivity and magnetism. In fermionic systems superfluidity is achieved by pairing of particles while magnetization is induced by an imbalanced population of two species of fermions. This exotic coexistence, first investigated by Fulde and Ferrell and Larkin and Ovchinnikov (FFLO), is predicted to show distinct properties in comparison to a conventional BCS superfluid. Experimentally, the FFLOstate has been investigated in cold atoms experiments by several groups (Ketterle, Hulet, Törmä). From the theoretical point of view, the Hubbard model has been used to investigate the FFLO properties and to predict the regimes where the FFLO state appears. In this talk we will discuss theoretical results for the 1D Hubbard lattice with spinimbalanced populations. In particular, we have obtained an analytical expression for the critical polarization below which the FFLO state emerges and depicted the phase diagram as a function of interaction and particle density. We have also investigated i) the effects of the experimental harmonic trap, ii) the local magnetization, iii)the behavior of entanglement, and iv) the spinflip channels which favor the pairing protection. We find that beyond the FFLOstate regime the spinflip processes trigger a breaking pairs avalanche, leading the system to the normal (non superfluid) polarized phase.

Café com física: “Infinite-noise criticality”

The contact process model of nonequilibrium phase transitions into absorbing states are widely used to study turbulent liquid crystals, driven suspensions, superconducting vortex dynamics, bacteria colony biofilms among other systems. We study this transition in the presence of a time-varying environmental noise and show that such temporal disorder gives rise to a distinct class of exotic “infinite-noise” critical points at which the effective noise amplitude diverges on long time scales. This leads to enormous density fluctuations characterized by an infinitely broad probability distribution at criticality.

Seminário especial: “Tsunami, polímeros e quarks”

Os modelos mais realistas dos fenômenos naturais envolvem equações diferenciais com termos não lineares, dispersivos e dissipativos. Em geral, essas contribuições tornam o estudo desses modelos muito mais difícil. Entretanto, existem combinações muito específicas dessas propriedades que produzem um tipo de solução conhecida como soliton.

Nesse seminário, faremos uma discussão introdutória sobre o assunto, e discutiremos suas aplicações que vão desde o estudo de materiais magnéticos até o desdobramento de proteínas, comportamento de polímeros e o entendimento da força nuclear forte com os chamados solitons topológicos.

Journal club: “The astonishing blow-up of an elastic soap bubble”

The beauty of soap bubbles has long fascinated children and adults, and a quantitative model for explaining their burst was first developed by Lord Rayleigh in 1891. Slow-motion imaging of the rupture of soap bubbles generally shows the edges of liquid films retracting at a constant speed (known as the Taylor-Culick velocity), which is determined by momentum balance for the liquid rim collected at the film edge. In this talk we will report on our investigation[1] of soap bubbles formed from simple solutions of a cationic surfactant (cetyltrimethylammonium bromide – CTAB) and sodium salicylate, which have a viscolestic behavior due to the formation of “polymer-like” surfactant aggregates known as wormlike micelles. We demonstrate that these elastic bubbles collapse at a velocity up to thirty times higher than the Taylor-Culick limit, which has never been surpassed. This can be explained by a simple model that includes the elastic energy stored in the liquid film while the bubble is inflated, which is released when the film is ruptured, yielding an additional driving force for film retraction (besides surface tension). This new mechanism for the bursting of elastic liquid bubbles may have important implications to the breakup of viscoelastic sprays in industrial applications.

[1] E. Sabadini et al., Langmuir 30, 727 (2014).

Seminário do Grupo de Óptica: “Nova concepção para a esterilização fotônica, química e térmica”

A contaminação é um problema que envolve principalmente procedimentos médicos, farmacêuticos e odontológicos. Para o controle de agentes microbianos são utilizadas técnicas físicas ou químicas. Porém os métodos tradicionais de descontaminação apresentam limitações como o tipo de material a ser autoclavado, no caso do calor úmido, e a insegurança da efetividade por agentes químicos. Apesar desses problemas serem bem estudados ao longo dos últimos anos, a viabilidade funcional desses métodos ainda não atingiu os patamares desejáveis pela saúde pública. Portanto, o desenvolvimento de um equipamento que agregue de forma sinergistica técnicas seguras e rápidas, em um só equipamento de descontaminação, é muito importante neste contexto. Uma opção para este problema foi o desenvolvimento de uma autoclave multifuncional que utiliza agentes de termocoagulação, ozônio e UV de forma separada ou conjugada. Logo após todo o processo de desenvolvimento da engenharia, fabricação e montagem do protótipo e com o objetivo de validar sua eficiência, realizou-se testes iniciais para a definição de protocolos de utilização para os procedimentos de esterilização para comprovar a eficácia da configuração em forma de uma autoclave horizontal de 21 L a ser amplamente utilizada em consultórios odontológicos, estéticos, fisioterapêuticos e médicos.

Café com Física: “Homeostase em circuitos nervosos, assinaturas de neurônios e modelos simplificados

Em pequenos circuitos neurais, especializados na produção de padrões motores rítmicos de maneira autônoma, existem padrões qualitativamente semelhantes em animais e até em espécies diferentes – as assinaturas neurais. Iremos mostrar como uma abordagem de sistemas dinâmicos pode ser usada para entendermos o comportamento elétrico dos neurônios e identificar qual o tipo de neurônio que produziu determinada assinatura, indicando qual a família de modelos matemáticos mais adequada para representar este neurônio.

Café com Física: “Homeostase em circuitos nervosos”

Em pequenos circuitos neurais, especializados na produção de padrões motores rítmicos de maneira autônoma, existem padrões qualitativamente semelhantes em animais e até em espécies diferentes – as assinaturas neurais. Iremos mostrar como uma abordagem de sistemas dinâmicos pode ser usada para entendermos o comportamento elétrico dos neurônios e identificar qual o tipo de neurônio que produziu determinada assinatura, indicando qual a família de modelos matemáticos mais adequada para representar este neurônio.

Colloquium diei: “Experiência de aprendizado ativo no IF”

Nos últimos anos, novos resultados em áreas de pesquisa distintas como cognição, educação, neurociências e psicologia reforçaram e validaram iniciativas de “aprendizagem ativa”, que buscam reforçar, em sala de aula, o protagonismo do estudante. Em física, essas iniciativas remontam a meados dos anos 80, com a publicação dos primeiros estudos sistemáticos mostrando duas coisas que muitos de nós, como professores, intuímos: (i) há pouca correlação entre a competência na solução de problemas acadêmicos e a real compreensão dos conceitos subjacentes por parte de alunos que cursam disciplinas tradicionais e (ii) a eficiência das aulas expositivas é baixa em suprir essas deficiências.

Ainda, em uma época onde há informação em abundância e ao alcance de todos em qualquer celular, o interesse dos alunos por palestras somente diminui, obrigando-nos a repensar o papel das aulas. Entre as propostas de aprendizagem ativa surgidas como alternativa destacam-se a “instrução pelos pares”, desenvolvida por Eric Mazur em Harvard, e metodologias como o SCALE-UP (“student-centered active learning environment with upside-down pedagogies”), desenvolvida por Robert Beicher na Universidade Estadual da Carolina do Norte, e sua variante TEAL (“technology-enhanced active learning”), adotada no MIT e em Yale. Apresentaremos um panorama dessas propostas, e discutiremos a experiência que vem sendo realizada no IF/USP com a adoção e adaptação do método SCALE-UP nas turmas de física 1.

Seminário do Grupo de Óptica: “Avaliação do efeito fotodinâmico”

Esse projeto é uma das possibilidades de otimizar a Terapia Fotodinâmica (TFD) tópica, tratamento muito usado para diversas doenças de pele. O estudo foi feito in vivo, em pele suína, e foram testadas cinco composições de mistura com ALA (ácido aminolevulínico) e MAL (ácido metil aminolevulínico), além do ALA e MAL separados. Aplicamos cada mistura no dorso do animal e para a cinética monitoramos durante cinco horas a produção da protoporfirina IX (PpIX) através da técnica de imagem de fluorescência de campo amplo (excitação 400-450 nm) e espectroscopia de fluorescência (excitação 532 nm). Também observamos a produção da PpIX em três horas (tempo de incubação), momento em que aplicamos a TFD para cada mistura. Analisamos o dano fotodinâmico superficial pós-tratamento e o dano nas camadas mais profundas da pele através da análise histológica após 24 e 48 horas pós-TFD.