Research

Interesting phenomena on delafossite oxide surfaces

Delafossite oxides ABO₂ represent a special material class for their innate layered structure and potential for functionality tuning through selection of A- and B-site cations. One most widely studied material among the family is PdCoO₂. In the bulk, its Pd layers host highly conductive electrons, while the CoO₂ octahedra layers are band-insulating. When exposed to the vacuum, the CoO₂ surface layer shows giant Rashba-like spin-split surface states with energies dominated by inversion symmetry breaking.

Our study shows that quasiparticle scattering here follows pure spin-selection rules, instead of spin-orbit selection rules obeyed by other conventional Rashba systems, and finds that the coherence length of the quasiparticles is appreciably longer than their spin precession length, suggesting that this system can be promising for realizing spin transport manipulation at the nanometer length scale. See link to the paper for more info.

Manipulation of Properties of Quantum Matter

Uniaxial pressure is an effective tool that readily breaks the innate symmetry of the materials of study and in turn changes their properties. Famous examples of study include strain-induced (i) tripling of superconducting T_c of putative p-type superconductor Sr₂RuO₄ [Steppke et al., Science (2017)] and (ii) emergence of 3D CDW order in cuprates [Kim et al., Science (2018)].

We’re the few who pioneered the use of uniaxial pressure with STM to study strain-driven phenomena at the atomic length scale. Our observations so far include (i) strain-induced emergence of a CDW order and change in superconductivity in LiFeAs [Yim et al., Nat. Commun. (2018)], and (ii) strain-driven change in magnetic order in iron-based antiferromagnet Fe_1+xTe [Yim et al., Nano Letters (2021)].

Kinetic stabilization of 1D surface electronic states near the twin boundaries of non-centrosymmetric BiPd

The search for 1D topologically-protected electronic states has become an important research goal for condensed matter physics mainly owing to their potential use for hosting Majorana fermions. In this work we show the formation of 1D electronic states at surface twin boundaries of the non-centrosymmetric material BiPd. Our work therefore demonstrates a novel route towards designing 1D electronic states with strong spin-orbit coupling.

Related link(s):
Yim et al., Phys. Rev. Lett. 121, 206401 (2018)

Origin and characteristics of the excess electrons in TiO₂

TiO₂ is a technologically relevant material that has been used in applications including water-gas shift reaction, heterogeneous catalysis, gas-sensing etc. TiO₂ is a wide gap (~ 3 eV) insulator. When TiO₂ is chemically reduced, excess electrons are generated, with the latter found to play a key role in some chemical reactions taking place on the surface of TiO₂. Through experiments, we have discovered that oxygen vacancies, a common type of point-defects type on the TiO₂ surface, are the major source of the excess electrons [Yim et al., Phys. Rev. Lett (2010)]. We have also shown their behavior as small polarons [Yim et al., Phys. Rev. Lett. (2016)], and their affinity to adsorbates [Yim et al., J. Phys. Chem. Lett. (2018)].