Tue17Apr20184:00 pmLewis Hall 101
Colloquium: Presentations by Graduate Students
Mukunda Acharya, Khagendra Adhikari, and Xudong Fan
Department of Physics and Astronomy
University of Mississippi
Presentations by Graduate Students
"Sound Speed Profiles of the Global Ocean calculated from Physical Oceanographic Data" by Mukunda Acharya
Abstract: Sound speed profiles in the global ocean are useful for modeling of sound propagation over the global oceans. This talk presents the sound speed profiles calculated from physical oceanographic data that were collected in the world ocean circulation experiment. They are data of conductivity, temperature, and pressure, taken from multiple cruises with a typical spacing of 60 km along the route of each cruise and consisting of an elaborate series of zonal (east-west) and meridional (north-south) coast-to-coast cruises across the global oceans. Nearly 8000 sound speed profiles were mapped out that were then used to determine the dependence of sound speed minimum and depth for those minima on the longitude and latitude. Based on the characterization, practical formulas of the dependence were given.
"Deforming the Fredkin Spin Chain Away from its Frustration-free Point" by Khagendra Adhikari
Abstract: The Fredkin model describes a spin-half chain segment subject to three-body, correlated-exchange interactions and twisted boundary conditions. The model is frustration-free, and its ground state wave function is known exactly. Its low-energy physics is that of a strong xy ferromagnet with gapless excitations and an unusually large dynamical exponent. We study a generalized spin chain model that includes the Fredkin model as a special tuning point and otherwise interpolates between the conventional ferromagnetic and antiferromagnetic quantum Heisenberg models. We solve for the low-lying states, using exact diagonalization and density-matrix renormalization group calculations, in order to track the properties of the system as it is tuned away from the Fredkin point. We identify a zero-temperature phase diagram with multiple transitions and unexpected ordered phases. The Fredkin ground state turns out to be particularly brittle, unstable to even infinitesimal antiferromagnetic frustration. We remark on the existence of an “anti-Fredkin” point at which all the contributing spin configurations have a spin structure exactly opposite to those in the Fredkin ground state.
"Enhancing Sound Emission by Using Subwavelength Metacavities" by Xudong Fan
Abstract: Efficient directional emission of sound waves is critical in imaging and communication, yet is held back by the inefficient emission of a small source. A change in the surrounding environment of an acoustic source can lead to enhanced emission and mode conversion. This talk will present frames of enclosing small sound sources in subwavelength meta-cavities to achieve sound enhancement and conversion with high efficiency. The enhancement is an analog of modifying spontaneous emission rate of a quantum source in a resonant cavity. The enhancement by a subwavelength meta-cavity offers a practical path toward miniaturization in applications that demand efficient emission, such as sonar, loudspeakers, or ultrasound transducers.