Physics Colloquium - Fall 2017

Speaker: Candice Hansen, Planetary Science Institute

Abstract: In 2005 the Cassini spacecraft in orbit around Saturn made the amazing discovery of a plume of water vapor and ice jets erupting from the moon Enceladus. Since then numerous observations have filled in our picture of the properties of this ocean world. Piece by piece our understanding of the scale, variability, driving source of energy, composition, and potential for habitability have emerged.

Speaker: Timm Rohwer, Massachusetts Institute of Technology Department of Physics

Abstract: This talk will combine the characterization of both, the driving mechanisms of CDW’s in solids - and the evolution of experimental techniques like trARPES and UED used to study the corresponding dynamics.

Charge Density Waves (CDW’s) are the dominant perturbation in low dimensional crystals. Furthermore, CDW phases are most often associated with superconductivity at their quantum critical points. In this talk, I will present how the correlated dynamics of CDW’s can be disentangled in the time-domain. A clear classification of the underlying driving mechanisms can be achieved by the study of the corresponding time scales of these nonthermal transitions.

Furthermore, this quest for the underlying nature of CDW’s has fostered the development and enhancement of the time-resolved probing techniques themselves. Thus, the first part will discuss the improvement of time-resolved Angle Resolved Photo-Electron Spectroscopy (trARPES). trARPES has emerged as a leading technique in identifying dynamic key properties of complex electronic systems. Ever since the initial application of High-Harmonic Generation based ARPES on CDW systems [1], we were able to improve its parameters like energy resolution and repetition rate by orders of magnitude. I will face the benefits of these enhancement and discuss the technical advances and new insights that were necessary to achieve this new level.

The second part will take a closer look on the particular system of Charge Density Waves in rare earth tri-tellurites. The combination of trARPES with Ultrafast Electron Diffraction (UED) and transient optical spectroscopy enables us to record and compare the correlated dynamics in the electronic and lattice subsystem. An unexpected recovery behavior is observed and I will show how the combination of these complementary techniques provides a more complete picture of the nonthermal transition and allows a solid conclusion of its origin.

[1] T. Rohwer et al., Nature 471, 490 (2011)

Speaker: Aleksandr Chernatynski, Missouri Science and Technology

Abstract:"Phonon Thermal Transport from First Principles" - Simulations of the thermal conductivity in dielectric materials have rapidly advanced in the last 10 years or so due to the increased computational capabilities as well as methodological developments. One of the most significant aspects is an ability to compute the thermal conductivity of essentially any crystalline material from first principles. In this presentation, I will review the history of the thermal transport simulations, outline the key aspects of the Boltzmann Transport Equation approach to the calculations of the thermal conductivity, and illustrate its applications in such diverse fields as geophysics and nuclear energetics.