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During my research career, I have accumulated a strong background for instrumentations and operation of ultrahigh vacuum (UHV) low temperature (LT) scanning tunneling microscopy and spectroscopy (STM/S) and attained various knowledges on physical and chemical properties of nano-size materials based on STM/S researches. STM/S is a powerful method to study nanometer-scale structures with high spatial and energy resolutions. I have developed a strong research foundation that will help to ensure my research activities as a scientist.
2004~2011, Ph. D. period
During my thesis periods, I have focused on the study of local electronic structures of organometallic systems adsorbed on metal surfaces using STM. First of all, I spent a few years in constructing two variable-temperature (4.2K ~ 300K) UHV-STM systems, which were really time- and labor-consuming and nerve-fraying but gave me strong instrumental skill and mental toughness throughout the construction period. After finishing the construction I performed STM/S measurements of porphyrin molecules that contain a magnetic Co atom, Co-porphyrin (CoP), adsorbed on Au surfaces. One of the main subjects was the Kondo effect in the molecule, which is induced by spin exchange interactions between the local spin of Co atom and conductive electrons on the metallic substrate. By observing a strong resonance state (Kondo resonance) near the Fermi level and taking a spatial distribution of the Kondo resonance using a spatially-resolved STS technique, I found that the resonance extends differently along the two highly symmetric directions of the molecule (Fig.1). The anisotropic spatial distribution of the Kondo resonance was explained in terms of different local molecule-substrate interactions (hybridization) along the two directions.
2011, a postdoctoral fellow in Seoul, Korea
As a postdoctoral researcher in Korea university, I was mainly interested in the interaction of the magnetic atom (cobalt) of the CoP molecule with various gas molecules (NO, O2, CO) and their influences on the Kondo resonance. These reactions modify the electronic and spin structures of CoP molecules due to unusual coordination around the Co atoms, and thus so-called chemically-controlled Kondo manipulations occurs. In STS measurements, I found that the Kondo resonance disappeared after forming a complex with the gas molecules as the chemical reactions quenched the net spin of CoP. In addition, I also investigated a bond breaking between the Co atom and an adsorbed gas molecule by injecting electrons and holes from a STM tip. For instance, a NO molecule can be completely removed from the molecule by the injection in a certain range of the bias voltage (Fig. 2). Based on our experimental results and theoretical supports, we could understand the mechanisms of STM-induced bond breaking at the diatomic-molecule-metal atom in terms of resonant tunneling to the molecular orbitals and their inelastic tunneling process.
2012~2016, a project researcher in Kashiwa, Japan
After joining Prof. Yukio Hasegawa’s group at ISSP, the University of Tokyo in Japan, I have my research subjects changed to electronic structures of nano-sized superconducting materials, in particular Pb-based nano-structures formed on semiconducting Si and Ge substrates. Using tunneling spectroscopy, one can measure the superconducting gap, and from the width and depth of the gap one can characterize the nature of the superconductivity of the local area just below the STM tip. Using a 3He-cooled low-temperature STM, I have studied a proximity effect at superconductor/normal metal (S/N) interfaces using Pb island structures as a superconductor and a Pb-induced reconstructed surface structure (SIC) as a metallic layer. From the spectroscopic mapping taken around the Pb-based S/N interface, I observed the gap at the Fermi energy, reminiscent of the superconducting gap, propagating into the metal region (proximity effect) and its depth decaying with the distance from the interface. One of the curious things I observed is that the propagation of the gap is terminated by the steps of the substrate (Fig. 3). Moreover, I observed that enhancement of the gap depth in the area between the interface and the step edge. The observed local enhancement of the proximity effect is probably explained in terms of phase-conserving electron reflection at the step edge. I am now collaborating with theoretical groups so that our results will be explained quantitatively based on a quasi-classical description using the Usadel theory.
As an extended subject from the studying about S/N proximity effect in nanometer scales, I studied another exotic proximity effect at the interface between superconductor and ferromagnetic materials (S/F). When a superconductor comes in a good contact to the ferromagnet, the Cooper pairs can penetrate into the ferromagnet side, and the superconducting-like gap or pairing amplitude decays with oscillatory behaviors (it’s different from usual S/N proximity effect which showed just decaying without any oscillation). This oscillatory behavior of pair amplitude is called FFLO-like state, which is similar mechanism to the case of heavy fermionic superconductor like CeCoIn5. To realize this FFLO-like state at the S/F interface, we studied interface between Pb (superconductor) and Mn5Ge3 (ferromagnet) by using local tunneling spectroscopy. Unfortunately, we couldn’t observe any oscillatory behaviors at the S/F interface because of poor interface transparency (or high conductivity mismatch between those two materials). But, we could observe anomalous inverse proximity effect on the Pb-film caused by embedded Mn5Ge3 islands. From our spatially-resolved local tunneling spectra, superconductivity of Pb film was suppressed (inverse proximity effect) by Mn5Ge3 and the spatial distribution of suppressions showed anisotropic extensions (triangular shaped extensions) from the location of Mn5Ge3 . By considering the pairing symmetry and the shape of Fermi surface for Pb, it might be understood that the observed anisotropic inverse proximity effect is a reflection of Fermi surface for Pb(111) and its anisotropic pairing mechanism, which is deviated from ideal s-wave pairings.
In addition to the proximity effect subjects, I successfully detected the Josephson current by precise positioning a superconducting tip in contact with an atomic lattice of the superconducting Pb(111) and measured the electrical conductance at the zero-bias voltage. In fact, I measured the conductance while decreasing the tip-sample gap distances, and observed evolutions of not only normal state conductance but also the Josephson current during the approach. Interestingly, the conductance measurement revealed that the amount of the Josephson current depends on the local atomic configuration of the junction; the Josephson current measured in a configuration of the tip-apex atom touching at a 3-fold hollow site of the Pb island surface is larger than that measured on an on-top site.
The spatial mapping of the Josephson current taken in the atomic-contact regime, in fact, shows clear modulation rendering atomic sites of the surface. The observed spatial variation of the Josephson current can be explained in terms of local variations in the transmission probabilities of the conductance channel at the atomic point contact regime.
Next, I have changed the subject from superconducting Pb to the strong correlated material, CeCoIn5. CeCoIn5 is known as a heavy fermion compound with the highest superconducting transition temperature TC=2.3 K among heavy fermion superconductors. Several STM works on CeCoIn5 successfully visualized the formation of heavy quasiparticles and nodal superconductivity with dx2-y2 symmetry at low temperature. We studied superconductivity of CeCoIn5 at the low temperature (~0.5K) with extreme tunneling conditions. We observed Ce atoms and large variation in the in-gap RDOS taken at the two different atomic sites, Ce and In, by precise STM measurements with extremely small tip-sample distances. The site-dependent in-gap RDOS is smeared out in fields and disappears above the critical field indicating that is intrinsic to the superconductivity. We discussed our results based on the context of multi-gap superconductivity of this compound. Another curious point what we observed is that we could see unusual atomic structures on Co-plane (alternating dumbbell-like structures) in CeCoIn5, which is deviated from its crystal structure. Based on our current and temperature dependent STM images and theoretical supports, it might be understood in terms of surface induced ordered states in CeCoIn5 (with Dr. Y. Yoshida).
I was involved in the project for the thin-film superconducting surface, which shows superconducting nature together with Rashba-type spin splitting nature. The Rashba effect, which resides in spin splitting of two-dimensional electronic states due to the spin-orbit interaction arising from space-inversion asymmetry, has been found to be especially pronounced in Bi and Tl monolayers on a Si(111) surface. In such noncentrosymmetric superconductors due to Rashba-type spin splitting, the spin-orbit interaction can serve a mixing of spin-singlet and spin triplet pairing components and a topologically nontrivial superconducting phase. Motivated from recent surface-transport study on Tl/Pb-√3/Si(111), for that system our idea is to reveal how singlet-triplet mixing component contribute to the local density of states in tunneling spectroscopy and its spatial distribution. In our temperature and magnetic field dependent tunneling spectra, a superconducting gap was clearly observed upto T=3.1 K and B=2.2 T. Additionally, For 0.1T<B<1T, we could visualize the vortex structure on the terrace with coherence length of 40 nm. Different from the conventional superconductor, superconducting gap feature was survived even at the vortex core, which we are speculating that triplet component contribution to the local density of states. (with Prof. Shuji Hasegawa and Prof. A. Saranin's group)
2016~present, postdoctoral research fellow in Hamburg, Germany
I'm currently working on the project: "superconductivity and magnetism hybrid system" at University of Hamburg as a postdoctoral research associate, the goal of this project is the experimental realization of model-type systems which can serve as a platform for designing Majorana states. A magnetic nanowire on the surface of a spin-orbit coupled s-wave superconductor is a fascinating platform, which has been proposed for observing the emergence of zero-energy Majorana bound states at the ends of the wires. Evidences for topologically non-trivial end-states were experimentally found for self-assembled ferromagnetic Fe nanowires on superconducting Pb(110) substrates by using scanning tunneling microscopy and spectroscopy (STM/S). However, self-assembled nanowires of Fe on Pb surfaces have unavoidable limitations, such as (1) intermixing of atomic species of the nanowire and the substrate during the annealing process, and (2) uncontrolled length and orientation of the nanowires.Within this project, we will fabricate 1D atomic chains from individual magnetic adatoms on superconducting substrates by utilizing STM at T=350 mK. Spin-polarized STM (SP-STM) measurements will be performed in order to determine the spin texture of the 1D magnetic chains.
Contributed works
Superconductivity of In:Si(111) : Josephson vortices (with Dr. S. Yoshizawa, Dr. T. Uchihashi@NIMS)
Ge/ZrB2 on Ge(111) : Germanine (with Dr. A Fleurence@JAIST)
Sn on B:Si(111) : Reconstruted Sn structures (with Prof. H. Weitering@The Univ. of Tennessee)
WSe2 and TaxW1-xSe2 : exploring single layer (with Prof. Iwasa@Univ. of Tokyo)
Break junction (with Prof. A.Sakai@Kyoto University)
2D Boron sheet on Ag(111) (with Dr. Baojie Feng@Univ. of Tokyo)
Superconducting thin film - (Tl,Pb)/Si(111) : Rashiba superconductor (with Prof. Shuji Hasegawa's group@Univ of Tokyo)
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