A combined atomic force- and tunneling microscopy system at 10mK temperature

We present the implementation of atomic force microscopy (AFM) with a self-sensing quartz tuning fork (qPlus) sensor in our ultra-low temperature scanning tunneling microscopy (STM) system. The system operates in an ultra-high vacuum (UHV) environment inside a dilution refrigerator (DR) with a base temperature of 10mK and magnetic fields up to 15T^[1]. Radio frequency (RF) filtering of all signal lines entering the UHV chamber and improved home built RF powder filters at low temperatures were implemented to produce an improved energy resolution in tunneling spectroscopy, i.e. a lower effective electron temperature. Low noise preamplifiers for the sensor deflection^[2] and the STM current signal^[3] were implemented at the 4K stage within the DR. This allows for reduced Johnson noise of the amplifier feedback resistors and a relatively short distance (1.2m) between amplifier and the mK stage and magnet where the sensor is operating. As cable capacity between sensor and amplifier adds noise, a special cable with a capacity of merely 30pF/m has been designed and implemented.

[1] Song et al., RSI 81, 121101 (2010); doi: 10.1063/1.3520482
[2] Huber and Giessibl, RSI 88, 073702 (2017); doi: 10.1063/1.4993737
[3] adapted from le Sueur and Joyez, RSI 77, 123701 (2006); doi: 10.1063/1.2400024