Development of a scanning tunneling microscope for variable temperature electron spin resonance
SEPTEMBER 15, 2022
Jiyoon Hwang, Denis Krylov, Robbie Elbertse, Sangwon Yoon, Taehong Ahn, Jeongmin Oh, Lei Fang, Won-jun Jang, Franklin H. Cho, Andreas J. Heinrich, and Yujeong Bae
Review of Scientific Instruments (2022)
Description
The energy resolution of scanning tunneling microscopy (STM) has been improved by integrating it with electron spin resonance (ESR). In this work, we introduce our newly built STM with ESR capability and its performance at variable temperatures. The STM was integrated with a home-built Joule-Thompson (JT) refrigerator and a two-axis vector magnet. We also installed coaxial cables for the STM tip and for an antenna, which allows us to apply radio frequency (RF) signals in two different schemes and compare the resultant ESR signals side-by-side. We confirm that there is no noticeable difference in the ESR lineshape, intensity, and resonance frequency once the transfer function is well characterized, but the operational frequency window is found to be different. The antenna is more beneficial in the higher frequency range than 35 GHz, which expands the available frequency window up to 45 GHz. By taking this advantage of the antenna, we measure the ESR signals at 40 GHz at different temperatures. The ESR signals decreased at elevated temperatures according to the Boltzmann distributions. Thanks to the high stability of the system, we are able to measure very weak ESR signals, like a few fA at 10 K, which is the highest temperature for the ESR measurement to our knowledge. The discovery of ESR measurement at higher temperature will accelerate a generalized use of ESR-STM.
Abstract
Recent advances in improving the spectroscopic energy resolution in scanning tunneling microscopy (STM) have been achieved by integrating electron spin resonance (ESR) with STM. Here, we demonstrate the design and performance of a homebuilt STM capable of ESR at temperatures ranging from 1 to 10 K. The STM is incorporated with a homebuilt Joule–Thomson refrigerator and a two-axis vector magnet. Our STM design allows for the deposition of atoms and molecules directly into the cold STM, eliminating the need to extract the sample for deposition. In addition, we adopt two methods to apply radio-frequency (RF) voltages to the tunnel junction: the early design of wiring to the STM tip directly and a more recent idea to use an RF antenna. Direct comparisons of ESR results measured using the two methods and simulations of electric field distribution around the tunnel junction show that, despite their different designs and capacitive coupling to the tunnel junction, there is no discernible difference in the driving and detection of ESR. Furthermore, at a magnetic field of ∼1.6 T, we observe ESR signals (near 40 GHz) sustained up to 10 K, which is the highest temperature for ESR-STM measurement reported to date, to the best of our knowledge. Although the ESR intensity exponentially decreases with increasing temperature, our ESR-STM system with low noise at the tunnel junction allows us to measure weak ESR signals with intensities of a few fA. Our new design of an ESR-STM system, which is operational in a large frequency and temperature range, can broaden the use of ESR spectroscopy in STM and enable the simple modification of existing STM systems, which will hopefully accelerate a generalized use of ESR-STM.