Bulletin of the American Physical Society
83rd Annual Meeting of the APS Southeastern Section
Volume 61, Number 19
Thursday–Saturday, November 10–12, 2016; Charlottesville, Virginia
Session A4: Condesned Matter Physics at ORNL and the National High Magnetic Field Laboratory |
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Chair: Giti Khodparast, Virginia Tech University Room: Preston Room |
Thursday, November 10, 2016 8:30AM - 9:00AM |
A4.00001: Microscopic structure of Er-optical centers in GaN epilayers by high magnetic fields Invited Speaker: Vinh Nguyen The incorporation of rare earth into wide bandgap semiconductors are of significant interest for optoelectronic devices, because of their temperature independent, atomic-like and stable emission together with the optical and electrical excitation. Er doped GaN materials have attracted much attention due to their capability to provide highly thermal stable optical emission in the technologically important as well as eye-safer at 1540 nm wavelength window. In spite of impressive developments in this area, the GaN:Er system remains poorly understood and even controversial in regard to the microscopic structure of optical Er centers and the relevant energy transfer mechanisms, which constitutes a barrier to further increases of device emission efficiency and thermal stability. The most straightforward approach to monitoring the microscopic structure of a luminescence center is direct detection via magneto-optical measurements of the main features of the emission spectrum. We have reported for the first time a successful observation and analysis of the Zeeman effect on the 1540 nm photoluminescence band in Er-doped GaN material grown by the metal-organic chemical vapor deposition in magnetic fields up to 17 T. The magnetic field induced splitting is observed for all the main lines of the Er photoluminescence spectrum. The angular dependence of the Zeeman splitting is measured in main crystallographic planes of the sample. The g-tensor of the ground and the first excited states is experimentally determined. The magneto-optical measurements and the temperature dependence of the PL spectroscopy show that our GaN:Er samples have two optical centers and they can be excited selectively under the resonant and non-resonant (band-to-band) excitations. [Preview Abstract] |
Thursday, November 10, 2016 9:00AM - 9:30AM |
A4.00002: Evidence for impact ionization in vanadium dioxide Invited Speaker: Stephen McGill Pump-probe optical spectroscopy was used to investigate charge carrier multiplication via the process of impact ionization in the M1 insulating phase of VO$_{\mathrm{2}}$. The film was excited by optical pump pulses with energy both above and below twice the band gap energy and observed with two different probe wavelengths, and the reflectivities of the sample were then compared. We observed an enhancement of the reflectivity for the higher energy pump pulses near zero delay compared to the reflectivity for the lower energy pump pulses for both probe wavelengths. Additionally, we identified multiple timescales within the charge dynamics and observed a significant change in the dynamics between the two pump wavelengths for one of these timescales. This research was funded by NSF DMR-1229217. [Preview Abstract] |
Thursday, November 10, 2016 9:30AM - 10:00AM |
A4.00003: Probing Giant Magnetic Anisotropies in Molecular Nanomagnets Using Very High-Field EPR Invited Speaker: Stephen Hill Most Electron Paramagnetic Resonance (EPR) research is performed at the X-Band frequency of 9.5 GHz. Specialized commercial instruments exist at K- (25 GHz), Q- (35 GHz) and W-Band (95 GHz), operating to magnetic fields of 6 T. The EPR facilities at the National High Magnetic Field Laboratory (MagLab) in Florida, offer scientists from all over the world opportunities to use several home-built, high-field/high-frequency EPR instruments with continuous coverage from ~10 GHz to 1 THz [1]. Magnets are also available providing magnetic fields up to 45 T --- roughly one million times the earth's magnetic field. EPR performed at these extremes offers tremendous advantages for problems spanning diverse research fields from condensed matter physics, to chemistry, to biology. After a brief overview of the MagLab EPR facility, the remainder of the talk will focus on molecular nanomagnets --- molecules that contain either a single magnetic ion, or multiple exchange-coupled ions that possess a well-defined collective magnetic moment (or spin). These molecules are of interest in terms of their potential use as memory elements in both classical and quantum information processing devices [1]. Results obtained from EPR will be highlighted, emphasizing discoveries that have contributed to a shift away from the study of large clusters to simpler molecules containing highly anisotropic magnetic ions such as lanthanides or transition metals with unquenched orbital moments. In particular, certain transition metals residing in high-symmetry coordination environments can experience orbitally degenerate ground states and very strong first-order spin-orbit coupling. The resulting giant magnetic anisotropies associated with such species have been measured using very high-field (up to 35 T) EPR [2,3]. [1] Hill et al., Struct. Bond. \textbf{164}, 231-292 (2015). [2] Marriott et al., Chem. Sci. \textbf{6}, 6823-6828 (2015). [3] Ruamps et al., J. Am. Chem. Soc. \textbf{135}, 3017-3026 (2013). [Preview Abstract] |
Thursday, November 10, 2016 10:00AM - 10:30AM |
A4.00004: Probing Electronic Properties of Defects and Boundaries in Low-Dimensional Materials. Invited Speaker: An-Ping Li Electronic properties are the key to the novel applications of low-dimensional materials in electronic and energy technologies. Due to the restricted dimensionality, one distinctive character of these low-D systems is that the electronic properties are critically dependent on the atomic scale heterogeneities introduced by defects and boundaries. Therefore, an important aspect of research is to examine the role of defects and boundaries, particularly the correlation between their structures and electronic properties. Here I will introduce our recent results on the study of defects and boundaries in low-D materials to illustrate how defects and boundaries can largely determine the physical properties by dictating the electron scattering, transport, and excitation processes. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. [Preview Abstract] |
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