Browsing by Author "Randall, J. N."
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
Item On the Effect of Local Barrier Height in Scanning Tunneling Microscopy: Measurement Methods and Control Implications(American Institute of Physics Inc, 2018-10-24) Tajaddodianfar, Farid; Moheimani, S. O. Reza; Owen, J.; Randall, J. N.; 0000-0002-6135-1993 (Tajaddodianfar, F); Tajaddodianfar, Farid; Moheimani, S. O. RezaA common cause of tip-sample crashes in a Scanning Tunneling Microscope (STM) operating in constant current mode is the poor performance of its feedback control system. We show that there is a direct link between the Local Barrier Height (LBH) and robustness of the feedback control loop. A method known as the "gap modulation method" was proposed in the early STM studies for estimating the LBH. We show that the obtained measurements are affected by controller parameters and propose an alternative method which we prove to produce LBH measurements independent of the controller dynamics. We use the obtained LBH estimation to continuously update the gains of a STM proportional-integral (PI) controller and show that while tuning the PI gains, the closed-loop system tolerates larger variations of LBH without experiencing instability. We report experimental results, conducted on two STM scanners, to establish the efficiency of the proposed PI tuning approach. Improved feedback stability is believed to help in avoiding the tip/sample crash in STMs.Item Scanning Tunneling Microscope Control: A Self-Tuning PI Controller Based on Online Local Barrier Height Estimation(Institute of Electrical and Electronics Engineers Inc.) Tajaddodianfar, F.; Moheimani, S. O. Reza; Randall, J. N.; 0000-0002-1225-4126 (Moheimani, SOR); 298210 (Moheimani, SOR); Moheimani, S. O. RezaWe identify the dynamics of a scanning tunneling microscope (STM) in closed loop and show that the plant dc gain is proportional to the square root of local barrier height (LBH), a quantum mechanical property of the sample and/or tip that affects the tunneling current. We demonstrate that during a scan, the LBH may undergo significant variations and this can adversely affect the closed-loop stability if the controller parameters remain fixed. Feedback instabilities increase the risk of tip-sample crash in STMs. In order to improve the closed-loop performance, we estimate the LBH, on the fly, and use that to adaptively tune the proportional-integral (PI) controller parameters. Experimental results obtained with the self-tuning PI controller confirm the improved STM performance compared to the conventional fixed-gain PI controller. Additional experiments confirm effectiveness of the proposed method in extending the tip lifetime by lowering the chance of a tip/sample crash. ©2018 IEEE