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Single Dirac Cone Topological Surface State and Unusual Thermoelectric Property of Compounds from a New Topological Insulator Family

Description: Angle resolved photoemission spectroscopy (ARPES) study on TlBiTe2 and TlBiSe2 from a Thallium-based III-V-VI2 ternary chalcogenides family revealed a single surface Dirac cone at the center of the Brillouin zone for both compounds. For TlBiSe{sub 2}, the large bulk gap ({approx} 200meV) makes it a topological insulator with better mechanical properties than the previous binary 3D topological insualtor family. For TlBiTe{sub 2}, the observed negative bulk gap indicates it as a semi-metal, rather than a narrow gap semi-conductor as conventionally believed; this semi-metality naturally explains its mysteriously small thermoelectric figure of merit comparing to other compounds in the family. Finally, the unique band structures of TlBiTe{sub 2} also suggests it as a candidate for topological superconductors.
Date: August 18, 2011
Creator: Chen, Y
Partner: UNT Libraries Government Documents Department

Proposed Dark Current Studies at the Argonne Wakefield Accelerator Facility

Description: A study of dark currents has been initiated at the Argonne Wakefield Accelerator Facility (AWA). Emission of dark current is closely related to a breakdown. Breakdown may include several factors such as local field enhancement, explosive electron emission, Ohmic heating, tensile stress produced by electric field, and others. The AWA is building a dedicated facility to test various models for breakdown mechanisms and to determine the roles of different factors in the breakdown. An imaging system is being put together to identify single emitters on the cathode surface. This will allow us to study dark current properties in the gun. We also plan to trigger breakdown events with a high-powered laser at various wavelengths (IR to UV). Another experimental idea follows from the recent work on a Schottky-enabled photoemission in an RF photoinjector that allows us to determine in situ the field enhancement factor on a cathode surface. Monitoring the field enhancement factor before and after can shed some light on a modification of metal surface after the breakdown.
Date: January 18, 2008
Creator: Antipov, S. P.; Conde, Manoel Eduardo; Gai, Wei; Power, John Gorham; Yusof, Z. M.; Spentzouris, L. K. et al.
Partner: UNT Libraries Government Documents Department

BSCCO superconductors : hole-like fermi surface and doping dependence of the gap function.

Description: We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at ({pi},{pi}), Then, the superconducting gap along the FS is precisely determined from ARPES measurements on over-doped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k{sub x})-cos(k{sub y}) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent.
Date: August 18, 1999
Creator: Campuzano, J. C.; Ding, H.; Fretwell, H. M.; Kadowaki, K.; Kaminski, A.; Mesot, J. et al.
Partner: UNT Libraries Government Documents Department

Core-Level Satellites and Outer Core-Level Multiplet Splitting in Mn Model Compounds

Description: We report a systematic study of the Mn 2p, 3s and 3p core-level photoemission and satellite structures for Mn model compounds. Charge-transfer from the ligand state to the 3d metal state is observed and is distinguished by prominent shake-up satellites. We also observe that the Mn 3s multiplet splitting becomes smaller as the Mn oxidation state increases, and that 3s-3d electron correlation reduces the branching ratio of the 7S:5S states in the Mn 3s spectra. In addition, as the ligand electronegativity decreases, the spin state purity is lost in the 3s spectra as evidenced by peak broadening. Our results are best understood in terms of the configuration-interaction (CI) model including intrashell electron correlation, charge-transfer and final-state screening.
Date: October 18, 1999
Creator: Nelson, A.J.; Reynolds, J.G. & Roos, J.W.
Partner: UNT Libraries Government Documents Department