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An ultra-low-vibration facility for housing a dilution temperature scanning tunneling microscope

University of British Columbia

This thesis details the specification, design and characterization of an ultra-low vibration facility and aspects of the design and performance of an ultra-high-vacuum dilution temperature scanning tunneling microscope (STM) housed within this facility. The basic principles of vibration isolation and STM are introduced. Existing ultra-low-vibration facilities and dilution temperature scanning tunneling microscopy experiments are reviewed. A specification for the vibration isolation performance of the facility is developed based on a simple model of the vibrational mechanics of a STM head. The experimental techniques of accelerometery and microphony are introduced. A survey of the acoustic and vibrational conditions at the site of the facility prior to its construction leads into a detailed description of the facility design. This is followed by an experimental characterization of the facility performance. Acoustic transmission functions of double-walled acoustic isolation vaults are reported; the dominant ambient sounds inside these vaults are found to coincide substantially with the acoustic modes of the vaults. Massive pneumatically supported concrete inertia blocks are found to perform approximately as ideal 2nd order damped spring mass systems below 10-15 Hz. Above these frequencies, acoustic forces are found to cause additional motion of the pneumatically supported stages. It is found that these systems must be carefully adjusted and monitored to ensure low resonant frequencies are maintained. Inertia blocks optimized for flexural resonant frequencies above 200Hz are presented; these vibrations are found to be poorly damped and to degrade isolation performance at the flexural resonance frequencies. Experiments mounted on light-load pneumatic isolators on top of the inertia blocks are found to be very susceptible to acoustic forces and as a result exhibit non-ideal isolation behavior above approximately 7 Hz. The design of a rigid STM head for use in the ultra-high-vacuum dilution refrigerator experiment is detailed and an overview of the supporting experimental system is given. The results of preliminary commissioning of the microscope are given and poorly damped vibrations of the dilution refrigerator structure at ~20 Hz are found to be the dominant contribution to the noise in the tunneling current signal when the instrument is operated at dilution temperature.

Text

2015-10-24

Attribution-NonCommercial-ShareAlike 2.5 Canada

http://hdl.handle.net/2429/54762

Engineering Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-sa/2.5/ca/