Physics Department Colloquia
Fall 2018 Schedule
All talks will be in the Physics Department Library (accessed from 3438 Ingersoll Hall) unless otherwise stated.
Please check here for updates to the schedule as the semester progresses.
Tuesday, Sept 25, 12:30 p.m.
Speaker: Tim Benseman, Department of Physics, Queens College, CUNY
Title: Coherent THz radiation from Bi2Sr2CaCu2O8 Josephson junctions: Recent progress and approaches to wafer-scale device production
Abstract: Stacks of intrinsic Josephson junctions in extremely anisotropic high-temperature superconductors such as Bi2Sr2CaCu2O8 are a promising compact solid-state source of coherent, continuous-wave radiation in the 'terahertz gap' range. At present, no such sources exist in the range from approximately 0.5 THz to 1.3 terahertz, and this region is of particular interest for many scientific, medical, and security-related applications.
In order to generate technologically useful levels of power from these stacks, it is necessary to obtain efficient phase-synchronized emission from the largest possible number of individual Josephson junctions. Following this approach, we have successfully increased the coherent power output from this type of device to 0.6 milliwatts at 0.51 THz, which is approximately the power level required for practical imaging applications. I will also describe recent progress towards operation of these devices at temperatures above 77 Kelvin, which enables their use with much simpler cryogenics. Compact Bi2Sr2CaCu2O8 terahertz sources also open up the possibility of integrated circuits which exploit the unique nonlinear physics of intrinsic Josephson junctions. Such electronics would allow ultrafast signal processing, for instance in high-bandwidth wireless communications at THz frequencies.
Thursday, October 11, 12:30 p.m.
Speaker: Vincent LaBella, SUNY Polytechnic Institute, Albany, NY
Title: Visualizing the Electrostatics of Material Interfaces to Nanoscale Dimensions
Abstract: PDF of abstract (with figures) is available here.
Monday, October 22, 12:30 p.m.
Speaker: Ray T. Tung,* Department of Physics, Brooklyn College, CUNY
Title: Band-Offset “Bulk-Like” Mystery Solved: Lessons From Covalent And Ionic Heterojunctions
Abstract: The insensitivity of semiconductor heterojunction band offset (BO) on the orientation and the atomic structure of the interface, combined with the observed “transitivity” of BOs among some semiconductors, led to the suggestion that the formation mechanism of BO is “bulk-like”, namely, the BO is decided by bulk properties of semiconductors. At the same time, BO’s at heterovalent interfaces are known to be tunable and non-transitive. Because the BO is a direct result of the equilibrium charge distribution at the heterojunction interface, and the formation of the latter is governed by energy minimization, the observed bulk-like behavior for some interfaces and tunability for others must both have explanations within the concept of energy minimization. In this talk, we briefly review results previously obtained from analysis of charge distribution at covalent zinc blende interfaces and describe recent DFT results from heterojunctions between lattice-matched perovskite oxides. The BO’s for perovskite oxides, which depend very significantly on the orientation and atomic structure of unrelaxed interfaces, i.e. tunable, converge sharply upon lattice relaxation, i.e. become bulk-like, to magnitudes that agree with predictions of neutral polyhedra theory (NPT), previously developed for zinc blende interfaces. The unexpected success of the NPT for both covalent and ionic heterojunctions unmasks a link between a main assumption of this theory and energy minimization in general. The seemingly bulk-like BO behavior thus appears to reflect an independence of junction electrostatics on atomic arrangement at a relaxed interface, following this simple rule on energy minimization. The tunability of BO under other circumstances identifies bonding geometries where this rule is relaxed. Because energy minimization governs the formation of all charge distributions, NPT is expected to be valuable for other types of interfaces.
*Work done in collaboration with L. Kronik, Weizmann Institute of Science
Thursday, November 8, 12:30 p.m.
Speaker: Angelo Bongiorno, College of Staten Island, CUNY
Title: TBC
Abstract: TBC
Tuesday, November 20, 12:30 p.m.
Speaker: Matthew O'Dowd, Lehman College, CUNY
Title: TBC
Abstract: TBC
Wednesday, December 3, 12:30 p.m.
Speaker: Nicolas Biais, Department of Biology, Brooklyn College, CUNY
Title: TBC
Abstract: TBC
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