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Frontier of Low-Temperature Scanning Tunnelling Microscopy and Future Perspective Free

Since the invention of scanning tunnelling microscope (STM) in 1982, this experimental technique has been increasingly applied to a wide range of research fields. In particular, STM is recognized as a powerful tool to study various physical phenomena occurring at low temperatures due to unique capabilities of combining topographic imaging with atomic-scale spatial resolution and spectroscopic imaging with high energy resolution. In this talk, recent research topics studied using low temperature STMs are briefly reviewed as selected below and lastly future prospects for this technique foreseen from the recent trend will be discussed.

Identifying Majorana fermions in topological superconductors is one of key issues in the low temperature STM research field. Since Majorana fermions are expected to exist at edge states and vortex cores, low temperature STM can play a decisive role for this purpose. Many claims have been constantly reported but distinguishing between Majorana and ordinary quasiparticle bound states in a vortex core remained unclear because the measurement temperature was not sufficiently low. Machida et al. recently developed a dilution-refrigerator based STM working below 90 mK and successfully identified the difference between Majorana and ordinary quasiparticle bound states in a vortex core [1]. This study will initiate researchers to pursue STM measurements at ultra-low temperature.

Twisted bilayer graphene is a new stimulating topic because the electronic correlation can be tuned by the rotational angle between two stacking graphene sheets and at a specific angle (θ ~ 1.1˚) where the electronic correlation is greatly enhanced, a similar phase diagram as high temperature superconductors is discovered by tuning the chemical potential using a gate voltage [2]. Although the sample preparation was challenging for STM measurements, several groups successfully revealed direct evidences of unusual strong electron-electron interactions as a function of carrier density [3,4,5]. STM measurements at lower temperatures below a superconducting transition temperature (T_c ~ 1 K) are expected to be conducted in the near future.

Combining STM with optical techniques is another active field and enables us to resolve various optical phenomena with nanometer-scale spatial precision. For instance, tip-enhanced Raman spectroscopy with sub-molecular spatial resolution has been recently used to detect molecular vibrations as a finger-print for chemical identification [6]. Scanning tunnelling luminescence spectroscopy can visualize fundamental optical properties including emission and absorption spectra of a single molecule and the energy transfer between two molecules [7]. Time-resolved STM realized by the combination of STM and the optical pump-probe technique makes it possible to explore ultrafast transient carrier and spin dynamics of various functional materials [8].

Considering the recent trend including the topics described above, it is reasonable to expect that ultra-low temperature STM combined with superconducting (vector) magnets applicable for thin film samples with gating will play an important role in the condensed matter physics. The combination of STM and the optical techniques will be further advanced and recently developed systems such as THz-STM and time-resolved scanning multiprobe systems will be available for low temperature measurements in the near future. Another future direction of the instrumental development demanded from the market is a cryogen-free system because of the recent world-wide shortage of helium. Such cryogen-free systems are commercially available but the base temperatures of those systems are relatively high and the vibration noise level needs to be further suppressed. Thus, a highly stable cryogen-free STM working at a lower temperature is expected to be available and might become a standard system in the future, if successfully developed.

1. T Machida et al., Nat. Mater. 18 (2019) 811-816.

2. Y Cao et al. Nature 556 (2019) 43-50.

3. Y Xie et al. Nature 572 (2019) 101-105.

4. A Kerelsky et al. Nature 572 (2019) 95-100.

5. Y Jiang et al. Nature 572 (2019) 215-219.

6. R Zhang et al. Nature 498 (2013) 82-86.

7. H Imada et al. Nature 538 (2016) 364-367.

8. Y Terada et al. Nat. Photon. 4 (2010) 869-879.