Phys838C

Calendar
Physics 838 Seminar
Date
09.24.2018 4:00 pm - 5:00 pm
Location
John S. Toll Rm 1201

Description

Speaker: Daniel Campbell

Title: Pressure-Resistant Kondo Insulating Behavior in Ce3Bi4Pt3

Abstract: In Kondo insulators, the energy gap comes about not from simple band structure, but rather the interactions between conduction electrons and localized electrons in an outer valence shell. Their properties can often be tuned by external parameters. In Ce3Bi4Pt3, previous studies have demonstrated gap closer using high magnetic fields or chemical substitution. This talk will show results from a diamond anvil cell (DAC) study of Ce3Bi4Pt3 to pressures above 40 GPa. Measurements of crystal structure, cerium valence, and electrical resistance all indicate that Kondo insulating behavior continues and is even enhanced at high pressure. This is in contrast to materials like SmB6, CeRhSb, and CeNiSn, where a few GPa is enough to induce metallicity. Measurement techniques in DACs and potential explanations for these observations will also be discussed.

Speaker: Bakhrom Oripov

Title: SRF Material Science with Magnetic Microwave Microscopy


Abstract:
Superconducting Radio Frequency (SRF) cavities are being widely used in new generations of
particle accelerators. Currently the microwave performance of this cavities is limited by the localized
features/defects present in the inner surface of finished SRF cavities. In order to reach as high an
accelerating gradient as possible, these defects have to be studied and appropriate treatment methods
needs to be identified. A novel near-field magnetic field microwave microscope was successfully built to
identify and study this defects. Our microscope can measure local nonlinear response (3 rd harmonic
voltage V 3f ) from Nb bulk and thin film superconductor surfaces under conditions approaching B surface ~
600mT and frequencies in the multi-GHz range. Using our probe, we performed local microwave
measurements of V 3f (T,H rf ) and its dependence on temperature and rf input power. We observed that
below a temperature dependent onset rf field amplitude H 0 (T), V 3f (T,H) is below our noise floor. For
fields H rf > H 0 (T) there exists relatively high nonlinear response signal with periodic dips at H rf = H 1 (T),
H 2 (T), H 3 (T)… Similar behavior is observed in both bulk Nb and thin film Nb samples. As temperature
approaches T c , H 0 (T) goes to zero. Numerical calculations based on Resistively and Capacitively Shunted Junctions (RCSJ) model were performed that exhibit similar behavior. Thus, the periodicity of V 3f as function of H rf can be attributed to the existence of JJ’s on the surface. This confirms the existence of the weak-links on the surface of Nb and our ability to identify them.

Upcoming Events

19 Nov
Phys838C
11.19.2018 4:00 pm - 5:00 pm
26 Nov
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11.26.2018 4:00 pm - 5:00 pm
29 Nov
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3 Dec
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CNAM COLLOQUIUM: Monica Allen, UC San Diego
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