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Sumit KUMAR presents

Low-temperature microwave optomechanics: anomalous force noise
and optomechanically induced transparency

Monday 13rd of september 2021 at 10.00 am

Link : https://univ-grenoble-alpes-fr.zoom.us/j/99517287976?pwd=RE5qdVdhdVJYRXZiVFFQWEZDck9JQT09

The presentation will be in English.


Abstract :

Most of the terrestrial matter is made of glassy or amorphous materials. The low-temperature properties of these materials, particularly below 1 kelvin, are intriguing. For instance, the internal friction of the amorphous materials does not show any dependence on temperature below 1 kelvin and at lower temperatures drops off with 𝑇³ dependence. With few exceptions, the internal friction ( in the regime where it does not depend on 𝑇 ) of almost all amorphous materials measured to date also shows quantitative similarity within a factor of 20. The internal friction along with other low-temperature properties like thermal conductivity, change in the relative speed of sound, etc. can be explained in the framework of the TTLS model developed independently by Philips [1] and Anderson et al. [2]. However many experimental findings show deviations from the predictions of the TTLS model. The microscopic nature of individual TLS also remains elusive to us. NEMS (nano-electromechanical systems) are the relevant entities to further test the validity of the TTLS model suggested by Leggett et al.[3]

This thesis concerns with the measurement of the position of the doubly clamped nanobeam coupled to superconducting microwave cavity using the principles of cavity optomechanics. The measurement of thermal motion of the nanobeam below 𝑇 < 200 mK is marred by the anomalous force noise seen in the output power from the cavity which is not consistent with the optomechanical theory, where 𝑇 is the temperature of the sample. We will show a detailed analysis of the statistics of the anomalous force noise called “spikes” and will try to give a plausible reason for the same. Further many amplifiers and notch filters can be made from optomechanical systems. They rely on optomechanically induced transparency (OMIT) and absorption (OMIA). We will investigate our results on OMIT and OMIA covering a large parameter space than has been explored in previous works. We will also talk about the dual-chip technique developed in our lab to measure the mechanical characteristics of nanobeams made of bare silicon nitride. This will allow us to probe whether the thin layer of metal on our nanobeam is responsible for spikes in our previous measurements. Also measuring the nanobeam made of bare silicon nitride will give us statistics to test the predictions of the TTLS model.
[1] William A Phillips. Tunneling states in amorphous solids. Journal of low-temperature physics, 7(3):351–360, 1972.
[2] P W Anderson, Bertrand I Halperin, and C M Varma. Anomalous low-temperature thermal properties of glasses and spin glasses. Philosophical Magazine, 25(1):1–9, 1972.
[3] Anthony J Leggett and Dervis C Vural. “tunneling two-level systems” model of the low-temperature properties of glasses: Are “smoking-gun” tests possible? The Journal of Physical Chemistry B, 117(42):12966–12971, 2013.

Thesis Directors: Andrew Fefferman and Eddy Collin.