Surface and interface play a key role in recent applied researches since dimension and size of objects reach down to nanometer range and even to atomic scale. This special issue aims at promoting mutual understanding in the fields of surface science, materials science, and devices application toward interdisciplinary research. For this purpose, we here feature interdisciplinary surface science researches presented in the symposium of JPS 2021 Autumn Meeting, The Physical Society of Japan on September 20, 2021.

Low-temperature GaAs-based photoconductive antenna emits terahertz radiation when hit by femtosecond laser at 1.55 µm pump pulse. The nonlinear photocurrent at low laser power suggests a below band gap excitation due to two-step photon absorption mediated by mid-gap states. Using density functional theory with modified Becke-Johnson (mBJ) and local density approximation (LDA) for exchange-correlation, we confirm the mid-gap states and its origin. Due to broken inversion symmetry of GaAs, we introduced spin-orbit effects using projector augmented wave (PAW) method. Using the above methods, we were able to show that As antisite defect is the origin of the mid-gap state with energy level corresponding to 1.55 µm (～0.8 eV) sub-band gap excitation. In this article, we introduce our computationally-guided probing of such bulk defect in GaAs(110) surface, using Tersoff-Hamann theory-based STM simulation. The theoretically obtained STM images and the calculated local density of states are verified by experimental STM and STS.

Nitrogen-doped carbons are plausible candidates for alternative catalysts of platinum-based catalysts for oxygen reduction reaction (ORR). Although nitrogen-doped carbons are highly active in alkaline media, the activities critically decrease under acidic conditions in fuel cells. To overcome the problem, we have investigated the reaction mechanism based on the electronic structures by using model catalysts. On the active site of pyridinic nitrogen (pyri-N), we have reported that O₂＋pyri-NH^＋＋e^－→O_2a＋pyri-NH governs the ORR activities, where the adsorbed O₂ make a bond with an electron doped into π* orbital of the carbons. In the elementary step, the hydration of pyri-NH^＋ decreases the redox potential and resultantly the ORR activity. Therefore, we designed a nitrogen-doped graphene catalysts with higher hydrophobicity to prevent the hydration. To compensate the decreased conductivity, we introduced a proton-conductive particles. Thus, the compatible hydrophobicity and proton conductivity achieved the highest activity as a metal-free ORR catalyst.

Twisted bilayer graphene with the magic-angle 1.1° exhibited superconductivity due to the large density of states at the energy of the flat band. Generally, experiments on twisted bilayer graphene have been performed using micrometer-scale samples. For device applications, fabrication of large-scale twisted bilayer graphene is required. In this paper, I report on the fabrication of the millimeter-scale magic-angle twisted bilayer graphene sample and the observation of its flat band using angle-resolved photoemission spectroscopy measurements.

Topics related with multiferroic interfaces are discussed from the viewpoint of first-principles electron theory. For the adsorption of 3d transition metals on BaTiO₃(001) surfaces, the negatively charged TiO₂-terminated surface accommodates transition metals at hollow sites, while the positively charged BaO-terminated surface has transition metals as protrusions. Adsorbed transition metals modify the work function of the substrate remarkably. Furthermore, the feasibility of antisite defects strongly affects the crystal growth of Heusler alloys. For multiferroic interfaces of Heusler alloys on BaTiO₃(001), the insertion of a transition-metal atomic layer strongly prevents Si, a multiferroicity killer, from being exposed to interfaces. The magnetoelectric coupling can be enhanced by tuning the composition of the inserted transition-metal atomic layer, where the filling of 3d bands is of importance. Due to strong exchange interaction within the ferromagnetic phase, the control of the magnetization direction can be achieved by the interface magnetic anisotropy.

It has been reported that the introduction of ferromagnetism by various ways into topological insulators (TIs) can induce concerted effects, resulting in various novel magnetic phenomena such as quantum anomalous Hall effect and skyrmions. In addition, recently, efforts to increase the observation temperature of quantum phenomena by enhancing the crystal quality and improving electronic coherence have been pursued with great enthusiasm. Self-assembled regular alloy crystals such as Mn(Bi,Sb)₂Se(Te)₄ are called “intrinsic” ferromagnetic TIs that have recently attracted much attention, and we have observed skyrmions in sandwich structures based on these crystals. Furthermore, we have newly observed the ferromagnetic proximity effect even at room temperature at the heterojunction interface between Fe and topological crystalline insulator SnTe by polarized neutron reflectometry. As an extension of these efforts, it is hoped that the unique properties of TIs will be utilized to develop next-generation robust devices for such as spintronics and quantum computing.

The spin-charge current interconversion based on the direct and inverse Edelstein effects at the interface has attracted much interest due to its potential for high efficiency. However, the physical parameters that define the direct effect (charge-to-spin) and inverse effect (spin-to-charge) had not been clarified. We developed a phenomenological model that connects the direct and inverse Edelstein effects. Our model implies a trade-off relation between the conversion coefficients for the direct and inverse effects. Thus, a large conversion coefficient for the inverse effect does not necessarily bring a large conversion coefficient for the direct effect. Instead of these coefficients, we propose a figure of merit of Edelstein effects, which consists of two factors ; one of them represents the magnitude of the spin-orbit coupling, and the other represents the strength of the hybridization between bulk and interface states. We discuss how to create the interface that exhibits efficient conversion through Edelstein effects.

We review our recent study on the electronic states of a kagome superconductor CsV₃Sb₅ using angle-resolved photoemission spectroscopy with microfocused beam. The result shows polar nature of the cleaved surface and provides key insights into the origin of peculiar charge density wave at the surface of CsV₃Sb₅.

Frequent earthquakes in Japan can be problematic for vibration-sensitive measurements. For example, in scanning tunneling microscope (STM) experiments, seismic vibrations cause noises in the data and sometimes lead to a tip crash against the sample surface. The risk of tip crash rises during spectroscopic imaging measurements using STM, where the feedback control is disabled for most of the long measurement time. Here, we have developed a system to prevent tip crashes by retracting the tip in response to the earthquake early warning of the Japan Meteorological Agency. The technique utilizes a built-in function of a commercial STM controller, and an earthquake monitoring service offered by the National Research Institute for Earth Science and Disaster Prevention. We present data to demonstrate how the system works at an earthquake. Our work provides hints for conducting vibration-sensitive measurements in earthquake-prone countries.

0.2% Be-Cu material (BeCu) has excellent properties such as 13 times higher thermal conductivity, 1/7 of low heat radiation and 1/10 of outgassing compared to stainless steel material (SUS). Main applications were limited to vacuum components due to high material cost and non-weldability. The minimal fab system is promising as a high-mix, low-volume production scheme by using 1/2” wafers. In this scheme, small vacuum chambers were used and the condition of the vacuum chamber surface affect the wafer process quality. We fabricated the O-ring sealed chamber of the minimal sputtering equipment with BeCu and made low outgassing, temperature uniformity and the quality stability of the deposited thin films clear compared with the chamber with SUS.