EFRC Seminar Series

  • Ganesh BalasubramanianApril 10, 2015 - UC Riverside
  • Thermal transport in isotope substituted nanomaterials: From fundamentals to design
  • Professor Ganesh Balasubramanian
  • Iowa State University

Abstract: Thermal conductivity in nanomaterials such as carbon nanotubes (CNT) and graphene nanoribbons (GNR) is governed by lattice vibrations (also called phonons) and the various energy scattering phenomena associated with them. Impurities such as atomic vacancies, dopants and isotopes enhance the scattering effects, further reducing the energy transfer ability of these materials. We present results from quantum mechanical and classical molecular simulations on the effects of isotopes on the thermal conductivity of CNTs and GNRs. Strong shifts in the characteristic vibrational frequencies of the phonon modes are observed in the mass disordered structures that decrease the energy carrying capacity of the nanomaterials. Our investigations reveal that contrary to intuitive understanding the out of plane modes in a graphene sheet contribute significantly to thermal transport through them. An ordered arrangement of these isotope impurities can facilitate engineering of material systems for targeted thermal transport behavior. Results from our recent efforts at employing informatics and optimization tools show the importance of high frequency modes in the vibrational spectra towards designing mass disordered structures for desired thermal conductivities.

Bio: Ganesh Balasubramanian has been an Assistant Professor of Mechanical Engineering at Iowa State University since 2012. He received his BS degree in Mechanical Engineering from Jadavpur University, India in 2007, his PhD in Engineering Mechanics from Virginia Tech in 2011, and was a postdoctoral research associate in the Theoretical Physical Chemistry unit at Technische Universit├Ąt Darmstadt in Germany. Some of his recognitions include the 2015 ASEE/AFOSR Summer Faculty Fellowship, Miller Faculty Fellowship at Iowa State, and the Graduate Man of the Year and Liviu Librescu Scholarship at Virginia Tech.

  • Xiaoquin LiNovember 14, 2014 - UC Riverside
  • Probing Magnons and Phonons using Brillouin Light Scattering
  • Professor Xiaoqin Li
  • University of Texas-Austin

Abstract: When light interacts with quasiparticles in solids such as phonons (coordinated lattice vibrations) and magnons (coordinated spin rotations), annihilation or creation of a quasiparticle leads to inelastically scattered light at higher or lower frequency known as the Anti-Stokes and Stokes peaks in the familiar Raman scattering. The principle of Brillouin light scattering is identical to Raman scattering, except that the experimental technique is modified to monitor low frequency quasiparticles (1 GHz-150 GHz) such as magnons and acoustic phonons. I will present two examples in this talk. In the first example, we study the how spin waves (or magnon) can be amplified or attenuated by a direct current passing through a heavy metal/ferromagnetic bilayer via the spin Hall effect. In the second example, we discuss how Brillouin light scattering can be used as a temperature sensor for acoustic phonons in silicon.     

Bio: Dr. Xiaoqin (Elaine) LI obtained her Master degree in Electrical Engineering in 2002 and her PhD in physics in 2003 from the University of Michigan. After a postdoctoral fellowship at JILA, Colorado, She joined the physics department at University of Texas-Austin in 2007. Her group is currently exploring unique properties of metallic and hybrid nanostructures, spin waves in micromagnetic devices, and atomically thin semiconductors. In recent years, Dr. Li has received a number of awards which include the Sloan fellowship, the NSF CAREER award, and the Presdential Early Career Award for Scientists and Engineers (PECASE).