RF Excitation of the STM IMSURE Fellow Eudean

RF Excitation of the STM IM-SURE Fellow: Eudean Sun, UC Berkeley, EECS Graduate Students: Joonhee Lee, Xiuwen Tu Faculty Mentor: Dr. Wilson Ho

Scanning Tunneling Microscope n n Angstrom Resolution Microscope Tunneling Current

Quantum Tunneling JT – tunneling current VT – DC bias s – tip-sample distance Order of magnitude change in JT for every angstrom change in s.

Scanning n Feedback loop ¨ Piezoelectric drives to position sample n Ceramic materials that distort with voltage for high-precision positioning n Maintain constant tunneling current by adjusting tip-sample distance

RF Excitation via a Coil n n 800 MHz – 2. 0 GHz, 100 MHz steps Resonance

Experimental Data

The Model n n Solid. Works, Auto. CAD, Ansoft’s HFSS – High Frequency Structure Simulator Finite Element Method, like FEMLAB

The Model cont’d n Parts: ¨ Inner radiation shield ¨ Sample holder ¨ Sample ¨ RF coil ¨ Crosspiece ¨ Tip n Excitation ¨ 2 m. A n current Solution ¨ Frequency sweep ¨ Fields along polylines

Preliminary Results n Resonance due to radiation shield

Making a Better Model n Added parts: ¨ Outer shield ¨ Coaxial ¨ Rails cable / Grabber

Making a Better Model cont’d n New excitation ¨ Wave n port Finer mesh

Making a Better Model cont’d n Rebuilt all parts in HFSS ¨ Transferring between Solid. Works/Auto. CAD and HFSS was inconsistent n Refined solution setup ¨ Added parametric analysis to plot E field across sample for four different tip-sample distances: 1 e-6, 1 e-5, 1 e-4, 1 e-3 in. ¨ Increased data points across polylines to 10, 000 for plotting fields.

The New Model…in Color

Results n Frequency Sweep

Results cont’d n Parametric Analysis

Results cont’d

Results cont’d

Results cont’d n Field plots

Results cont’d

Problems n Resonance in simulation at 1. 2 GHz, 1. 4 GHz, 2. 0 GHz primarily ¨ Resonance in experiment at 800 MHz, 1. 2 GHz, 1. 3 GHz, 2. 0 GHz primarily Parametric analysis shows large E field differences between 1 e-3 in, 1 e-4 in, and 1 e-5 in, but not a big jump between 1 e-5 in and 1 e-6 in. n “Out of memory” n

Potential Fixes Better geometry n Finer meshes n

Limitations n Model complexity ¨ Can’t include everything, but what parts will affect resonance most? n Computer speed ¨ 2. 66 GHz Pentium 4 – 20 hours to complete one analysis n RAM ¨ 1 GB physical + 4 GB virtual memory ¨ “Out of memory”, literally, when finer meshes applied

Acknowledgements Dr. Wilson Ho n Joonhee Lee n Xiuwen Tu n IMSURE Program n