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Shot-noise scanning tunnelling microscopy


Many interesting properties in condensed matter systems, including high temperature superconductivity, originate from interactions between electrons. Since these interaction usually take place on short timescales, a measurement of time-averaged properties is often missing crucial information. In order to gain access to interactions and electron dynamics, researchers at the LPS and LPN have developed circuitry that enables measurements of current fluctuations with atomic scale spatial precision using a home-built scanning tunnelling microscope.

Shot-noise scanning tunnelling microscopy. (a) Home-built microscope with circuitry implemented, allowing us to (b) measure shot-noise at low temperature, (c) the electron temperature and (d) differential conductance at 1MHz.

Conventional scanning tunneling microscopes (STMs) can resolve the atomic and electronic structure of conducting materials by measuring the low frequency tunneling current between a tip and a sample. The main challenge to finite frequency measurements of the current fluctuations, which contains information about electron dynamics, with an STM is that cable capacitances become shorts to ground and/or convert voltage noise from room temperature amplifiers into current noise. To overcome this limitation, researchers at the LPS developed a low temperature LCcable circuit with a low noise, cryogenic high electron mobility transistor that was developed researchers at the LPN, and implemented this into the scanning tunnelling microscope that was designed and built at the LPS. This allows for measurements of current fluctuations, also called shot-noise, with a high accuracy at the resonant frequency of the LCcable circuit, which was chosen to be at 1 MHz. In addition to probing electron dynamics and interactions, shot-noise measurements can directly determine the electron temperature, and the circuitry can be used to perform measurements of the time-averaged current with increased signal-to-noise. Since the current itself is atomically resolved in the STM, the current fluctuations are too, giving a unique atomic scale insight into correlations and dynamics.

Reference

Atomic scale shot-noise using cryogenic MHz circuitry
F. Massee, Q. Dong, A. Cavanna, Y. Jin and M. Aprili
Review of Scientific Instruments 89, 093708 (2018)
doi:10.1063/1.5043261

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Freek Massee