NEWS: The CQM Distinguished Lecture series has been established in the Fall of 2015 to bring to Stony Brook University the renown experts in the physics of quantum matter.
April – October 2018
Speaker: Jiadong Zang, Univ. of New Hampshire
Topological Spin Textures in Chiral Magnets
B-131 Physics, Stony Brook University
Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the Dzyaloshinskii-Moriya interaction, stimulates the formation of many novel topological spin textures. One typical example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability and gives out promise on future magnetic memory devise. Inspired by skyrmions, in this talk, I will give a comprehensive introduction of skyrmions and their behavior in confined geometries. I will also present three other relevant spin textures in chiral magnets. One is the target skyrmion we recently observed, both theoretically and experimentally, in ultra-small nanodisks of chiral magnets. Zero-field target skyrmions and their polarization switch will be discussed. Putting in heterostructures, we also found a new type of topological configuration dubbed the Hopfion therein. Finally, I will discuss emergent topology driven by thermal fluctuations.
An Inside View of Physical Review Family of Journals
The Physical Review journals of the APS have a long tradition of publishing important physics papers, and serving as the bedrock of physics research. In the past six years, the APS has launched four new journals to broaden this collection: Physical Review X, a highly selective Open Access journal; Physical Review Applied, dedicated to publishinghigh-quality papers that bridge the gap between engineering and physics, and between current and future technologies; Physical Review Fluids, an journal publishing innovative research that will significantly advance the fundamental understanding of fluid dynamics; and Physical Review Materials: a new broad-scope journal serving the multidisciplinary community working on materials research.
In this talk, I will give a brief overview of our journal family and the peer review process, and offer some guidelines on how to communicate effectively with editors and referees during the review process. I will also discuss our current scope and standard of published papers on condensed matter, AMO physics, and materials science. I will be available to answer questions, hear ideas, and discuss comments about the journals.
Yiming Xu received his B.Sc. from Nanjing University in China and his Ph.D. from Boston College, both in experimental condensed matter physics. Prior to joining PRX in 2014, he was a postdoctoral fellow in the Materials Sciences Division at Lawrence Berkeley National Laboratory. His research focus was on the electronic properties of strongly correlated materials.
Dr. Navaneetha Ravichandran, Boston College
Title: Phonon scattering from material boundaries and higher-order anharmonicity
Phonons, which are quantized lattice vibrations, govern the thermal and thermodynamic properties of crystalline solids. Understanding phonon properties is essential to engineer new materials for a wide variety of energy applications such as thermoelectrics, superconductors, energy storage etc., and has been a topic of intense research interest over the past several decades.
In the first part of my talk, I will describe my experimental research at Caltech to answer an important nanoscale phonon transport problem that has remained unsolved for decades: “Do THz-frequency thermal phonons reflect specularly from atomically rough surfaces, thereby preserving their phase? Or do they scatter diffusely and lose it?”. By implementing a novel non-contact optical experiment called the transient grating (TG) on suspended thin silicon (Si) membranes, and by rigorous first-principles analysis of the TG experimental data, I will show that thermal phonons are exquisitely sensitive to the surface roughness of just a few atomic planes on the Si membrane, and that our experimental and computational machinery enables us to obtain the first measurements of the specular phonon reflection probability as a spectral function of phonon wavelength.
In the second part of my talk, I will discuss my computational research at Boston College, where I am developing new first-principles tools to analyze the thermal properties of novel materials, for which the conventional phonon theory fails drastically. I will begin by describing a curious case of thermal transport in boron arsenide (BAs), where the lowest order scattering processes involving three phonons are unusually weak and four-phonon scattering due to higher-order anharmonicity affects the thermal conductivity significantly. Finally I will talk about phonons in sodium chloride (NaCl), where, once again, the conventional phonon theory fails drastically, but for a different reason: the unusually strong anharmonic bonds in NaCl. I will show that the phonons interact so strongly in NaCl that they invalidate the Peierls-Boltzmann description of phonon transport, even below half of the melting temperature. To address this issue, I have developed a new phonon renormalization approach based on many-body theory, which creates new “dressed-up” quasi-particles that interact weakly to admit the Peierls-Boltzmann treatment of heat conduction. I will show that our new phonon renormalization approach along with higher-order four-phonon scattering enables us to get good agreement with several temperature-dependent measurements of phonon dispersions, thermal expansion and thermal conductivity simultaneously.
I am originally from India. I obtained my undergraduate degree from the Indian Institute of Technology, Madras. I obtained my Masters and PhD from Caltech, working with Prof. Austin Minnich. For my PhD, I worked on experimentally investigating phonon boundary scattering in thin silicon membranes using the transient grating experiment. I am currently a postdoctoral fellow at Boston College, where I am working with Prof. David Broido on developing a rigorous predictive first-principles computational tool that simultaneously works for multiple thermal and thermodynamic properties of strongly anharmonic materials.