Emi Minamitani

Structure of Silicene Grown on Ag(111)

The structure of silicene, the two-dimensional honeycomb sheet of Si, grown on Ag(111) was investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) combined with density functional theory (DFT) calculation. Two atomic arrangements of honeycomb configuration were found by STM, which are confirmed by LEED and DFT calculations; one is 4×4 and the other is √13×√13 R13.9°. In the 4×4 structure, the honeycomb lattice remains with six atoms displaced vertically, whereas the √13×√13 R13.9° takes the regularly buckled honeycomb geometry.

Electronic decoupling by h-BN layer between silicene and Cu(111): A DFT-based analysis

Geometric and electronic structures of silicene on Cu(111) covered with a monolayer of hexagonal boron nitride (h-BN) were investigated by ab initio density functional theory calculations. We found that a silicene with a regularly buckled configuration is stabilized on h-BN layer stacking commensurately to the Cu(111) substrate. The electronic band structure projected to Si 3pz orbital clearly shows a band crossing similar to a Dirac cone emerging in the band structure of freestanding buckled silicene. This is in contrast to the silicene on Cu(111), in which the Dirac fermion features disappear entirely due to the strong interactions at the interface. These examples demonstrate that the h-BN monolayer effectively prevents silicene from interacting with the underlying Cu(111) substrate and that the h-BN monolayer on Cu(111) is a promising candidate for use as a substrate on which to realize silicene hosting the Dirac fermion features.

Electronic Structure of the 4 × 4 Silicene Monolayer On Semi-Infinite Ag(111)

The electronic structure of the 4 × 4 silicene monolayer on a semi-infinite Ag(111) substrate is calculated within density functional theory by using the embedded Green’s function technique. The present calculation confirms the conclusion of previous studies that the two-dimensional (2D) Dirac bands do not exist on this surface as a result of the symmetry breaking and strong orbital hybridizations between the Si π and Ag sp states. In addition, by making use of the advantage of the semi-infinite calculation in which the energy continuum of the bulk Ag bands is fully reproduced, we investigate details of the silicene-induced electronic states, including not only their energy dispersion with 2D wave vector but also their spectral shape as a function of energy at each .

Controlling orbital-selective Kondo effects in a single molecule through coordination chemistry

Iron(II) phthalocyanine (FePc) molecule causes novel Kondo effects derived from the unique electronic structure of multi-spins and multi-orbitals when attached to Au(111). Two unpaired electrons in the d(z)(2) and the degenerate dπ orbitals are screened stepwise, resulting in spin and spin+orbital Kondo effects, respectively. We investigated the impact on the Kondo effects of the coordination of CO and NO molecules to the Fe(2+) ion as chemical stimuli by using scanning tunneling microscopy (STM) and density functional theory calculations. The impacts of the two diatomic molecules are different from each other as a result of the different electronic configurations. The coordination of CO converts the spin state from triplet to singlet, and then the Kondo effects completely disappear. In contrast, an unpaired electron survives in the molecular orbital composed of Fe d(z)(2) and NO 5σ and 2π* orbitals for the coordination of NO, causing a sharp Kondo resonance. The isotropic magnetic response of the peak indicates the origin is the spin Kondo effect. The diatomic molecules attached to the Fe(2+) ion were easily detached by applying a pulsed voltage at the STM junction. These results demonstrate that the single molecule chemistry enables us to switch and control the spin and the many-body quantum states reversibly.

Single-molecule quantum dot as a Kondo simulator

Structural flexibility of molecule-based systems is key to realizing the novel functionalities. Tuning the structure in the atomic scale enables us to manipulate the quantum state in the molecule-based system. Here we present the reversible Hamiltonian manipulation in a single-molecule quantum dot consisting of an iron phthalocyanine molecule attached to an Au electrode and a scanning tunnelling microscope tip. We precisely controlled the position of Fe2+ ion in the molecular cage by using the tip, and tuned the Kondo coupling between the molecular spins and the Au electrode. Then, we realized the crossover between the strong-coupling Kondo regime and the weak-coupling regime governed by spin–orbit interaction in the molecule. The results open an avenue to simulate low-energy quantum many-body physics and quantum phase transition through the molecular flexibility.

Visualizing Type-II Weyl Points in Tungsten Ditelluride by Quasiparticle Interference

Weyl semimetals (WSMs) are classified into two types, type I and II, according to the topology of the Weyl point, where the electron and hole pockets touch each other. Tungsten ditelluride (WTe2) has garnered a great deal of attention as a strong candidate to be a type-II WSM. However, the Weyl points for WTe2 are located above the Fermi level, which has prevented us from identifying the locations and the connection to the Fermi arc surface states by using angle-resolved photoemission spectroscopy. Here, we present experimental proof that WTe2 is a type-II WSM. We measured energy-dependent quasiparticle interference patterns with a cryogenic scanning tunneling microscope, revealing the position of the Weyl point and its connection with the Fermi arc surface states, in agreement with prior theoretical predictions. Our results provide an answer to this crucial question and stimulate further exploration of the characteristics of WSMs.

Study of Li atom diffusion in amorphous Li3PO4 with neural network potential

To clarify atomic diffusion in amorphous materials, which is important in novel information and energy devices, theoretical methods having both reliability and computational speed are eagerly anticipated. In the present study, we applied neural network (NN) potentials, a recently developed machine learning technique, to the study of atom diffusion in amorphous materials, using Li3PO4 as a benchmark material. The NN potential was used together with the nudged elastic band, kinetic Monte Carlo, and molecular dynamics methods to characterize Li vacancy diffusion behavior in the amorphous Li3PO4 model. By comparing these results with corresponding DFT calculations, we found that the average error of the NN potential is 0.048 eV in calculating energy barriers of diffusion paths, and 0.041 eV in diffusion activation energy. Moreover, the diffusion coefficients obtained from molecular dynamics are always consistent with those from ab initio molecular dynamics simulation, while the computation speed of the NN potential is 3-4 orders of magnitude faster than DFT. Lastly, the structure of amorphous Li3PO4 and the ion transport properties in it were studied with the NN potential using a large supercell model containing more than 1000 atoms. The formation of P2O7 units was observed, which is consistent with the experimental characterization. The Li diffusion activation energy was estimated to be 0.55 eV, which agrees well with the experimental measurements.

Scanning tunnelling spectroscopy of superconductivity on surfaces of LiTi 2 O 4 (111) thin films

Unique superconductivity at surfaces/interfaces, as exemplified by LaAlO3/SrTiO3 interfaces, and the high transition temperature in ultrathin FeSe films, have triggered intense debates on how superconductivity is affected in atomic and electronic reconstructions. The surface of superconducting cubic spinel oxide LiTi2O4 is another interesting system because its inherent surface electronic and atomic reconstructions add complexity to superconducting properties. Investigations of such surfaces are hampered by the lack of single crystals or high-quality thin films. Here, using low-temperature scanning tunnelling microscopy and spectroscopy, we report an unexpected small superconducting energy gap and a long coherence length on the surface of LiTi2O4(111) epitaxial thin films. Furthermore, we find that a pseudogap opening at the Fermi energy modifies the surface superconductivity. Our results open an avenue for exploring anomalous superconductivity on the surface of cubic transition-metal oxides, where the electronic states are spontaneously modulated involving rich many-body interactions.

Theoretical prediction of phonon-mediated superconductivity with T c ≈ 25 K in Li-intercalated hexagonal boron nitride bilayer

A superconducting transition temperature (T c) up to 25 K in the Li-intercalated bilayer of hexagonal boron nitride (h-BN) is predicted according to ab-initio calculations. A T c higher than that of metal-intercalated graphene (MIG) is ascribed to the characteristic spatial distribution of electronic states near the Fermi level, which is distinctly different from that in MIG. In the Li-intercalated bilayer h-BN, the breaking of the symmetrical restriction and the increase in the overlap between the charge density and the Li in-plane motion enhance the electron–phonon coupling. Our results provide a new design guideline for two-dimensional superconductors based on intercalated layered materials.

Combined scanning tunneling microscopy and high-resolution electron energy loss spectroscopy study on the adsorption state of CO on Ag(001).

The adsorption site and vibrational energies of CO on a clean Ag(001) surface were determined using scanning tunneling microscopy, inelastic electron tunneling spectroscopy with a scanning tunneling microscope, and high-resolution electron energy loss spectroscopy. The CO molecules were found to adsorb on the atop site of the Ag(001) surface, which was similar to their adsorption on the Cu(001) surface. The vibrational energy of the CO internal stretching mode was found to be 263 meV, which is only 3 meV less than that of CO in the gas phase. This result indicates that the CO molecules chemisorb very weakly on the Ag(001) surface.