Quantum control and design at the atomic scale: Superconducting thin films and molecular complexes

Quantum materials at the atomic scale offer a versatile platform for exploring novel physical phenomena and engineering emergent quantum states. The quantum behavior of a system arises from its fundamental building blocks, requiring a deep understanding to harness its potential for advancing quantum devices and technologies. In this talk, I focuss on designing and tuning quantum systems on surfaces, ranging from single molecules (focus II) to solid-state materials (focus I), and investigating their quantum properties by using low-temperature scanning tunneling microscopy (STM).

In my first research focus, my work explores superconducting thin films, particularly iron-based superconductors at the two-dimensional (2D) limit. New physics often emerges when two energy or length scales become comparable. Based on this consideration and its shallow Fermi pockets, we choose FeSe as a candidate system to tune the critical energy scales, including the superconducting gap , the Fermi energy E_F and the Zeeman energy . FeSe thin films were grown on graphene layers on SiC substrate and investigated by ultra-low temperature STM (50 mK). The variable work function of graphene layers with different thickness and stacking orders allow precise tuning of E_F and of FeSe films. The STM is equipped with two magnets: a rotatable split-pair with in-plane field up to 7 T in all directions and a solenoid with out-of-plane field up to 15 T, which enables the exploration of quantum states with unprecedented opportunities [1-3]. Using this setup, we tune the system to enter the BCS-BEC crossover regime [3], where E_F~1 -- a state rarely achieved in solid-state systems. Subsequently, when becomes comparable to , the system undergoes a transition from a BCS to a Sarma state [4], a rare and highly sought-after superconducting phase.

In my second research focus, we address the design and control of atomic-scale spin structures, a key challenge for advancing spin-based quantum technologies and quantum information. Using electron spin resonance scanning tunneling microscopy (ESR-STM), we showcase a strategy to tailor the quantum properties of molecular spins and probe the magnetic properties on both metallic and insulating surfaces. Employing both single atoms and molecules, we show by using two different molecules (Cu(dbm)₂ and FePc) that Fe atoms preferentially attach underneath their benzene ligands. This results in an organometallic half-sandwich complex that subsequently shows a Kondo feature on Ag(001) [5]. On insulating MgO layers, we construct Fe(C₆H₆)-FePc complexes via tip-assisted assembly. The resulting magnetic system constitutes a mixed-spin (1, 1/2) quantum ferrimagnet, where strong exchange coupling between two spins leads to a well-separated two-level system [6]. This new correlated ground state, which we utilize for coherent control, offers protection against inelastic scattering with tunneling electron spins. This strikingly increases the spin lifetime (>1.5 μs) compared to ordinary atomic and molecular quantum systems on surface. Moreover, we show by building dimers of ferrimagnets that their spins can be coupled, providing a path to realize larger structures.

References

[1] W. Huang, et al., Phys. Rev. B, 103, 09245 (2021). Link

[2] W. Huang, et al., Nano letters, doi.org/10.1021/acs.nanolett.4c04461 (2024). Link

[3] H. Lin*, W. Huang*, et al., Phys. Rev. B, 107, 104517 (2023). Link

[4] W. Huang*, Y. Yin*, et al., Nature, In revision, arxiv. 2412,15850. Link

[5] W. Huang, et al., ACS nano, 19,1,1190 (2025). Link

[6] W. Huang, et al., Nat. Commun. In revision, arxiv.2410.18563. Link