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Sander Mooij
Departamento de Física 
FCFM Universidad de Chile

Viernes 9 de septiembre, 16:15
Sala de seminarios, 3er piso
Departamento de Física, FCFM
Av. Blanco Encalada 2008

In the paradigm of cosmological inflation, the early universe has undergone a brief period of exponential expansion, during which quantum fluctuations of spacetime itself, and of the ``inflaton" field, provide the seeds for all structure in the universe that we observe today.

Despite a massive (ongoing) experimental effort, the standard vanilla single-field slow-roll inflation scenario has been verified observationally time and again. However, the data still leave room for sharp features (sobresaltos) in the inflationary spectra, which would rule out this simplest scenario.

In our work, we study very general actions that produce such sharp features in the power spectrum and bispectrum (two- and three-point function) of the inflationary perturbations. In particular, we identify a relation between features in the power spectrum and in the bispectrum, and another one between various shape functions for the bispectrum. Rather than analyzing the inflationary spectra separately, this provides a powerful cross-check that can confirm or rule out vast classes of models at one.

Seminarios Anteriores

Thermodynamics of information Imprimir
Juan M. R. Parrondo
Universidad Complutense de Madrid. 

Miércoles 7 de septiembre, 16:00
Sala de seminarios (lado poniente), 3er piso
Departamento de Física, FCFM
Av. Blanco Encalada 2008

Soon after the discovery of the second law of thermodynamics, Maxwell illustrated its probabilistic nature with a gedanken experiment, now known as Maxwell's demon. He argued that if an intelligent being—a demon—had information about the velocities and positions of the particles in a gas, then that demon could transfer the fast, hot particles from a cold reservoir to a hot one, in apparent violation of the second law. Maxwell's demon reveals a fundamental relationship between entropy and information. Information is a thermodynamic resource and consequently, information manipulation involves some thermodynamic cost. In this talk I will present some ideas about the physical nature of information and how can be incorporated into thermodynamics. The starting point is that information is stored in slow degrees of freedom that are out of equilibrium. Form this viewpoint, information thermodynamics is the study of a certain class of non-equilibrium states. Those states involve the coexistence of macro- or mesoscopic phases, such as in a memory that stores bits of information, and correlations, like those that come out when a "demon" measures on a system.
Microscopic theory of dense suspension flows and the vibrational modes of amorphous solids. Imprimir
Gustavo Düring
Instituto de Física
Pontificia Universidad Católica de Chile

Viernes 2 de septiembre, 16:15
Sala de seminarios, 3er piso
Departamento de Física, FCFM
Av. Blanco Encalada 2008

A fundamental and elusive question that lies in the interface of statistical physics and materials science is: how do athermal amorphous materials transition  between fluid and solid states of matter. This phenomenon, coined as jamming  displays critical behavior with diverging length scales and power law behavior of macroscopic material properties.
In my talk I will first present a microscopic theory to describe the rheology of dense amorphous materials, such as suspension flows,  close to the jamming point.
I will show that nonlocal effects associated with particle collisions along with the condition of stationarity determine several critical exponents. Next, I will show that the rheological properties  of suspension flows  are controlled by "low-energy" modes  similarly  to elastic properties in solids, suggesting  a unified framework for amorphous materials.
In the second part of my talk I will discuss the properties  of the vibrational modes of amorphous solids.  In particular I will present recent result on the low-frequency tail of the spectrum associated to low frequency quasi-localized modes. Such low-frequencies excitation has been predicted before, but never observed  in structural glasses and their effect remain unclear.
Geometric phase in liquid crystal optics Imprimir
Raouf Barboza
Departamento de Física
FCFM, Universidad de Chile
Viernes 26 de agosto, 16:15
Sala de seminarios, 3er piso
Departamento de Física, FCFM
Av. Blanco Encalada 2008
Geometric phase is not a novel concept in optics. It is known to occur when dealing with cyclic transformation of degrees of freedom of light such as: propagation direction, Rytov-Vladimirskii-Berry phase; and polarization, Pancharatnam-Berry phase. The latter case of manipulation, Pancharatnam-Berry phase, has gained ground as it is obtainable with planar spatially varying anisotropic structures such liquid crystals. Althought beeing a key concept in modern optics, the Panchanratnam-Berry phase remains an elusive concept. 
Our discussion will cover the basics of optical geometric phase in liquid crystals (nematics) with their application in singular optics, and recent results on chiral reflective surfaces made of cholesteric liquid crystals.
Universidad Adolfo Ibáñez 
Viña del Mar, Chile

Viernes 19 de agosto, 16:15
Sala de seminarios, 3er piso
Departamento de Física, FCFM
Av. Blanco Encalada 2008

The growth of plant, fungal, and bacterial cells depends critically on two processes: the deposition of new wall material at the cell surface and the mechanical deformation of this material by forces developed within the cell. To understand how these two processes contribute to cell growth, we have undertaken an experimental and theoretical investigation of tip growth morphogenesis. Tip growth is a fast and robust elongation process observed in many specialized cells such as root hairs, fungal hyphae, and pollen tubes. Our work has revealed that simple mechanical principles can explain many of the key features of tip growth. Cells are modeled as thin shells made of an inhomogeneous material. The strain rate profile predicted by the model was compared to experimental data and was shown to be surprisingly accurate. The same strain rate profile is observed in many types of tip-growing cells suggesting that the profile is a generic mechanical feature of elongating finger-like structures. We have also analyzed the relationship between the velocity of the growing tip and the force that the cell applies to penetrate the environment. Again, a simple balance of forces between the internal pressure of the cell and the resistance of the environment explains all of the main features of the cell’s response. An elegant model of tip growth morphogenesis emerges from these results. Growth appears to be essentially a mechanical process that is controlled, at the cellular level, by the internal turgor pressure of the cell and a graded incorporation of wall material.
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