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PHYSICS PHD DISSERTATION DEFENSE: Varun Harbola

Ph.D. Candidate:  Varun Harbola

Research Advisor:  Harold Y. Hwang

Date: Friday, October 1, 2021
Time: 9:30 AM PDT


Zoom Link:  https://stanford.zoom.us/j/92598321604

Zoom Password: 112358

Title:  Nanomechanical properties of thin freestanding complex oxide membranes

Abstract: Modern oxide materials have garnered significant interest toward them, as they are a class of materials that exhibit a variety of interesting physical properties, ranging from superconductivity to magnetism to ferroelectrics to multiferroics. Modern techniques in thin film fabrication of oxides have made it possible to make freestanding membranes of oxides and their heterostructures with precise control over thickness and composition. By lifting the thin films off from substrates and placing these membranes on a suitable substrate, the membranes can be manipulated mechanically in a desired manner. This serves as a powerful tool to measure the nanomechanical properties of thin oxide membranes by making freestanding structures and probing them using calibrated nanomechanical probes such as atomic force microscopy. Since freestanding membranes are devoid of any substrate effects, it becomes straightforward to probe them directly and measure the nanomechanical properties of oxides in the sub-100 nm thickness regime, which was not possible previously. In this talk, I will discuss our efforts to measure and understand the nanomechanical properties of single crystalline oxide membranes.

 

We transfer oxide membranes onto porous SiNx membranes to form freestanding circular drumheads which are used to measure elastic and fracture properties of these oxides using atomic force microscopy. We find that the materials respond mechanically completely differently at the nanoscale compared to their bulk counterparts. We observe that nanoscale elasticity goes beyond the traditional linear and local description of elasticity and exhibits strain gradient elasticity, which stems from a strain gradient induced polarization known as flexoelectricity. Furthermore, we observe that the strain sustenance upon local loading is almost two orders of magnitude higher than the tensile limit for these materials in bulk, along with a fatigue lifetime of a billion cycles. These studies set the platform for mechanical measurements and manipulation of oxide membranes at the nanoscale.

 

October 1, 2021 - 9:30am