Research Group for Nanoscale Structure and Function of Advanced Materials

Group Leader : FUKUTANI Katsuyuki
Members : HIGEMOTO Wataru, FUKAYA Yuki, ITO U. Takashi , YASUDA Satoshi, YANO
Masahiro, YAMAKAWA Koichiro, TERASAWA Tomoo, UETA Hirokazu, SUZUKI Seiya,SHAMOTO Shinichi, MACHIDA Shinichi, SUZUKI Taiga

Nanoscale structures including nanoclusters, two-dimensional
materials, surfaces, interfaces, and defects reveal particularfeatures different from bulk materials. Our group fabricatesnovel nanoscale materials with synthesis techniques of chemicalvapor deposition and molecular beam epitaxy. We also elucidate
the underlying mechanisms behind the phenomena such as
Dirac states, nuclear-spin conversion, and cation-π interaction
by making full use of quantum beams, such as muons, positrons,
neutrons, IR-THz photons, and ions. In close collaboration
between the members and theoreticians, we aim to explore
novel functions in advanced nanoscale structures.
Nuclear-spin conversion analysis of ν2 + ν4 combination
band of crystalline methane in phase II
A methane molecule (CH4) has four normal vibrationalmodes named ν1, ν2, ν3, and ν4. In accordance with the total
nuclear spin I, CH4 is classified into three kinds of isomers:
ortho (I = 1), meta (I = 2), and para (I = 0). Since each isomer
occupies specific rotational states, infrared absorption
spectroscopy can be applied to the detection of interconversion
between the isomers. In this work [1], we conducted reflection–
absorption infrared spectroscopy in an ultrahigh vacuum andobserved the ν2 + ν4 combination band of crystalline CH4 inphase II, where the rotating and librating species coexist. We
analyzed the time dependence of the spectrum (see Fig. 1) due
to the nuclear-spin conversion of CH4 to update the assignmentof the rotational and librational structure of this band observed.
For instance, the absorption signals observed at 2806 and 2824
cm-1 had been assigned to the Q(1) and R(1) branches in aprevious work [2], but were found to be due to P(1) and R(0),respectively. The conversion analysis performed in our work
will also be applicable to the detailed assignments of overtonesand other combination bands.

Atomic configuration of quasicrystal bilayer graphenerevealed by positron diffraction
Geometrical consideration suggested that two hexagonallattices superimposed with the twisted angle of 30° forms adodecagonal quasicrystal that has 12-fold rotational symmetry
without translational symmetry. Graphene, a hexagonal lattice
sheet of carbon, is a good candidate material for dodecagonal
qusicrystal. Recently, the succesful fabrication of quasicrystal
bilayer graphene has been reported by using chemical vapor
deposition technique. Quasicrystal bilayer graphene has
attracted increasing attention as a two-dimensional quasicrystal
having relativistic Dirac fermions. However, the fundamental
information on atomic configuratons remained unknown. In thisstudy [3], we have investigated the atomic configuration ofquasicrystal bilayer graphene on a silicon carbide substrateusing positron diffraction method. By means of the structure
analysis based on dynamical diffraction theory, we determined
the interlayer spacing of quasicrystal bilayer graphene as 3.46 Å,
while the normal stacked bilayer graphene 3.29 Å (see Fig. 2).
We found that the twisted angle of 30° gives rise to theexpansion of the interlayer spacing. The origin of Dirac cone
replica is one of the unresolved issues in quasicrystal bilayer
graphene [4]. Our results will pave the way to understand such
peculiar electronic properties.

Fig. 2 Structure models of (a) quasicrystal bilayer graphene and (b)normal (Bernal-type) stacked bilayer graphene on silicon carbide
substrate. Grey and magenta circles denote C and Si atoms,
respectively [3].

Fig. 1 Infrared spectra of crystalline CH4 in phase II at 5.3K. The rangeof the ν2+ν4 combination band is shown. The blue dashed andred solid lines indicate the spectra just after and 187 minutes after
annealing at 38 K, respectively [1].

References[1] T. Sugimoto and K. Yamakawa, J. Chem. Phys. 154, 026101(2021).
[2] G. Baciocco et al., J. Chem. Phys. 87, 1913 (1987).
[3] Y. Fukaya et al., Phys. Rev. B 104, L180202 (2021).
[4] S. J. Ahn et al., Science 361, 782 (2018).