Research Group for Surface and Interface Science

Group Leader : FUKUTANI Katsuyuki
Members : FUKAYA Yuki, HIGEMOTO Wataru, YASUDA Satoshi, ITO U. Takashi, UETA
Hirokazu, YAMAKAWA Koichiro, FUJIHALA Masayoshi, YANO Masahiro, TERASAWA
Tomo-o, SUZUKI Seiya, MACHIDA Shin-ichi

Surfaces and interfaces of materials including lowdimensional (D) materials such as layered materials (2D),
atomic chains (1D), and clusters (0D), exhibit intriguing
properties different from the bulk materials owing to thebreaking of spatial inversion symmetry. The objective of ourgroup is to explore exotic low-D materials/surfaces andhydrogen dynamics towards the development of novel
functionalities at surfaces and interfaces by taking advantage ofadvanced quantum beams such as muons, positrons, neutrons,
IR-THz photons, and ions.
In this report, we show two topics, orbital hybridization
between graphene and Au surface [1] and weakly interacting
quantum spin chain antiferromagnet [2], in addition to theresearch on hydrogen/deuterium separation with a graphenesheet [3] (Research Highlights, separate page).
Discovery of Orbital Hybridization between Graphene andHex–Au(001) Reconstructed Surface
Graphene, a monolayer honeycomb lattice of carbon atoms,
has extraordinary electronic properties such as ultrahigh carrier
mobility and quantum Hall effect at room temperature. Au, afamiliar noble metal, has a large spin-orbit interaction. Theinterface between graphene and Au is expected to haveapplications in spintronics devices. However, the mechanism ofwhen and how Au modifies the electronic properties ofgraphene has not been clarified because the arrangement of Au
atoms at the graphene-Au interfaces was often unknown. In thisstudy, graphene was grown on the Hex–Au(001) reconstructedsurface, which has quasi-one-dimensional (1D) periodicity and awell-defined atom arrangement [1]. The angle-resolved
photoemission spectroscopy (ARPES) and the density
functional theory (DFT) calculation revealed the band structureof graphene on the Hex–Au(001) surface, which governs theelectronic properties of graphene. Previously, it was assumed
that the quasi-1D periodicity of the Hex–Au(001) would affectthe band structure of graphene without chemical bonding. In thisstudy, the effect of quasi-1D periodicity was not observed tomodify the band structure of graphene, as revealed by ARPESand DFT. Instead, a bandgap of 0.2 eV was observed at thecrossing point of the graphene π and Au 6sp orbitals, indicating
the hybridization of these orbitals. In other words, graphene andAu formed chemical bonds Through comparison with previous
reports on graphene-Au interfaces, the epitaxial relation
between graphene and Au and an isolated electronic structure ofAu at the interface are found essential for the hybridization. Theelucidation of the mechanism of hybridization between Au andgraphene in this study will lead to the method to pass thespontaneous spin polarization of Au to graphene throughchemical bonding. This phenomenon is expected to be utilizedin spintronics devices, a research field such as next-generationenergy-saving integrated circuits that utilize spin.
Magnetic gap excitations in weakly interacting quantum
spin chain antiferromagnet KCuPO4·H2O

The S = 1/2 Heisenberg linear chain antiferromagnet is thesimplest spin model; nevertheless it serves as a platform forvarious quantum many-body phenomena. In real linear chain
magnets, often long-range magnetic order at finite temperatures
is induced by weak interchain coupling. However, adimensional crossover behavior can be observed by the inelastic
neutron scattering (INS) measurement in a weakly interacting
quantum spin chain antiferromagnet. In the INS spectrum, it isexpected to observe gapped magnon excitations at lower
energies and multiple magnon continuum excitations at higher
energies.
We present KCuPO4·H2O as a new weakly interacting
quantum spin chain antiferromagnet [2]. The intrachaininteraction J and interchain interaction |J’| are estimated to be
172 K and 4.25 K, indicating that this compound has“moderately weak interchain interactions” suitable forinvestigation of a dimensional crossover phenomenon. A longrange antiferromagnetic order with an ordered moment 0.31 μB
per spin occurs at TN = 11.7 K. Both above and below TN, at
higher energies, the INS excitation is characterized by a two
spinon continuum (Fig. 1(a)). At lower energies, below TN, aspin gap is observed in the dispersive excitations (Fig. 1(b)).
Our neutron scattering measurements reveal that KCuPO4·H2O
is a better candidate for a weakly interacting quantum spin chain
system than previous linear chain antiferromagnets.

Fig. 1 (a) INS spectra at 4 K were observed with an incident neutronenergy of 15.16 meV. The superimposed grey solid lines indicate
the lower and upper energy boundaries of the continuum. (b)Energy dependence of the scattering integrated over Q in therange 0.85 Å−1 < Q < 1.05 Å−1, measured at 4, 8, and 15 K.

References[1] T. Terasawa et al., Phys. Rev. Materials 7, 014002 (2023).
[2] M. Fujihala et al., Phys. Rev. B 107, 054435 (2023).
[3] S. Yasuda et al., ACS NANO 16, 14362 (2022).