Physics (Internal)

Diamond Nanoelectromechanical Resonators: Dissipation and Superconductivity

This event is part of the PhD Final Oral Exams.

Examining Committee:  Raj Mohanty, Steve Ahlen, Robert Carey, Anders Sandvik, 
Shyam Erramilli, William Skocpol

Abstract:
Nanoelectromechanical systems (NEMS) have become a viable commercial
technology and are becoming more and more prevalent in research
applications. Through miniaturization, the mechanical response to external
sources becomes ever more sensitive. This transduction, coupled to an
electrical readout circuit, results in unprecedented sensitivity.

This thesis examines dissipation in diamond NEMS resonators in the MHz to
GHz range.  NCD (Nano-crystalline diamond) has extraordinary properties
that make it an intriguing material to study. To begin with, the
mechanical hardness allows for a boost in resonance frequency, but beyond
that, boron-doped diamond also shows extraordinary electrical behavior.

Although scaling benefits speed and sensitivity, dissipation increases
dramatically with miniaturization, negating some of the gains in
sensitivity. The dissipative mechanisms at play in the MHz range are
identified at high temperatures. It is found that extrinsic dissipation
mechanisms, mainly circuit and clamping losses, can limit the quality
factor (inverse of the dissipation). Furthermore, due to the high
surface-to-volume ratio of NEMS, surface defects become significant at the
nano-scale.  For the first time, quantum dissipation due to assisted
phonon tunneling of two level systems is observed in diamond NEMS
resonators at millikelvin temperatures. Through scaling, it is shown that
the low temperature behavior is universal for a broad range of MHz
resonators, including silicon and gallium arsenide, as well as graphene
and carbon-nanotubes.

Beyond the mechanical response, the superconducting properties of highly
boron-doped diamond (BDD) are studied.  It is found that the critical
temperature of 3.3 K for the thin-film is maintained at the nano-scale.
The high critical field, on the order of 3 T for thin-films, is strongly
suppressed, already at the micro-scale. The zero resistance state is
compromised with fields as low as 0.1 T for submicron wide constrictions.
It is known that the superconducting state will couple to the strain
field. Here, the piezoresistive detection technique is developed for BDD
structures in the MHz range at room as well as cryogenic temperatures.
This serves as a framework for future studies of strain-superconductivity
coupling.