S. Ito’s research while affiliated with Tohoku University and other places

The two-dimensional layered compound PdCoO2 is one of the most conductive oxides, providing an intriguing research arena opened by the long mean free path and the very high mobility of ∼51000cm2/Vs. These properties turn PdCoO2 into a candidate material for nanoscale quantum devices. By exploring universal conductance fluctuations originating in nanoscale PdCoO2 Hall-bar devices, we determined the phase coherence length of electron transport in c-axis oriented PdCoO2 thin films to equal ∼100 nm. The weak temperature dependence of the measured phase coherence length suggests that defect scattering at twin boundaries in the PdCoO2 thin film governs phase breaking. These results suggest that phase coherent devices can be achieved by realizing the devices smaller than the size of twin domains, via refined microfabrication and suppression of twin boundaries.

By considering the crystal symmetry of the Dirac semimetal candidate SrMnBi2, it is expected that a substrate based on a square lattice is preferred to form a single-domain growth of a thin film. In this study, however, we observed different schemes of interface formation of SrMnBi2 using molecular-beam epitaxy on two oxide substrates of SrTiO3(001) and LaAlO3(001), both of which are often applied to the growth of the films with square lattices. Although antiphase domains appear in the SrMnBi2 film on SrTiO3(001), a single domain develops on LaAlO3(001) with an abrupt interface. The distinct difference indicates that the surface of the LaAlO3(001) substrate plays a crucial role in the selection of the initial growth plane. Judging from the abrupt interface image in scanning transmission electron microscopy and the four-fold symmetric in-plane x-ray diffraction pattern representing the orientation relationship of SrMnBi2 film [110]//LaAlO3 [100], the polar surface termination with (AlO2)⁻ or (LaO)⁺ probably promotes the interface formation of the ionic Sr or Bi plane on the surface, respectively. According to the semimetallic electronic structure of SrMnBi2, the electrical transport properties of the films can be consistently evaluated by the two-carrier model with high-mobility electrons and low-mobility holes. Our demonstration of the single-domain growth of the Dirac semimetal provides a key technique toward the future engineering of heterostructures composed of topological materials.

We report the lateral and vertical electrical conduction properties of PdCrO2 thin films grown on insulating Al2O3 (001) and conducting β-Ga2O3 ( 2 ¯ 01 ) substrates. The c-axis oriented PdCrO2 films on the both substrates showed metallic temperature dependence of in-plane resistivity down to 2 K. In PdCrO2/β-Ga2O3 vertical devices, rectifying current density–voltage (J–V) characteristics revealed the formation of a Schottky barrier at the PdCrO2/β-Ga2O3 interface. The Schottky barrier height (SBH) of 1.2–1.8 eV, evaluated by J–V characteristics, is significantly larger than 0.8 eV expected from the usual Mott–Schottky relation based on the electron affinity of β-Ga2O3 (4.0 eV) and the work function of PdCrO2 (4.8 eV) determined by ultraviolet photoelectron spectroscopy. The enhanced SBH at the PdCrO2/β-Ga2O3 interface indicates the existence of interface dipoles, as in the case of PdCoO2/β-Ga2O3. Besides, we observed a large difference of the SBH between the J–V measurements (1.2–1.8 eV) and capacitance measurements (2.0–2.1 eV). While the SBH is definitely enhanced by the interface dipole effect, the level of enhancement at the PdCrO2/β-Ga2O3 interface is rather inhomogeneous, different from that at the PdCoO2/β-Ga2O3. In fact, two typical types of interfaces were found by a high-angle annular dark-field scanning transmission electron microscope, which would be the origin of the inhomogeneous SBH. Further understanding of the interface formation between delafossite oxides and β-Ga2O3 ( 2 ¯ 01 ) will improve the performance of Ga2O3 Schottky junctions as a power diode available at high temperatures.

The two-dimensional layered compound PdCoO2 is one of the best oxide conductors, providing an intriguing research arena opened by the long mean free path and the very high mobility of ~ 51000 cm2/Vs. These properties turn PdCoO2 into a candidate material for nanoscale quantum devices. By exploring universal conductance fluctuations originating at nanoscale PdCoO2 Hall-bar devices, we determined the phase coherence length of electron transport in c-axis oriented PdCoO2 thin films to equal ~ 100 nm. The weak temperature dependence of the measured phase coherence length suggests that defect scattering at twin boundaries in the PdCoO2 thin film governs phase breaking. These results suggest that phase coherent devices can be achieved by realizing the devices smaller than the size of twin domains, via refined microfabrication and suppression of twin boundaries.

We revealed the electrical transport through surface ferromagnetic states of a nonmagnetic metal PdCoO2. Electronic reconstruction at the Pd-terminated surface of PdCoO2 induces Stoner-like ferromagnetic states, which could lead to spin-related phenomena among the highly conducting electrons in PdCoO2. Fabricating a series of nanometer-thick PdCoO2 thin films, we detected a surface-magnetization-driven anomalous Hall effect via systematic thickness- and termination-dependent measurements. Besides, we discuss that finite magnetic moments in electron doped CoO2 triangular lattices may have given rise to additional unconventional Hall resistance.

High-temperature operation of semiconductor devices is widely demanded for switching/sensing purposes in automobiles, plants, and aerospace applications. As alternatives to conventional Si-based Schottky diodes usable only at 200°C or less, Schottky interfaces based on wide-bandgap semiconductors have been extensively studied to realize a large Schottky barrier height that makes high-temperature operation possible. Here, we report a unique crystalline Schottky interface composed of a wide-gap semiconductor β-Ga 2 O 3 and a layered metal PdCoO 2 . At the thermally stable all-oxide interface, the polar layered structure of PdCoO 2 generates electric dipoles, realizing a large Schottky barrier height of ~1.8 eV, well beyond the 0.7 eV expected from the basal Schottky-Mott relation. Because of the naturally formed homogeneous electric dipoles, this junction achieved current rectification with a large on/off ratio approaching 10 ⁸ even at a high temperature of 350°C. The exceptional performance of the PdCoO 2 /β-Ga 2 O 3 Schottky diodes makes power/sensing devices possible for extreme environments.

We revealed the electrical transport through surface ferromagnetic states of a nonmagnetic metal PdCoO2. Electronic reconstruction at the Pd-terminated surface of PdCoO2 induces Stoner-like ferromagnetic states, which could lead to spin-related phenomena among the highly conducting electrons in PdCoO2. Fabricating a series of nanometer-thick PdCoO2 thin films, we detected a surface-magnetization-driven anomalous Hall effect via systematic thickness- and termination-dependent measurements. Besides, we discuss that finite magnetic moments in electron doped CoO2 triangular lattices may have given rise to additional unconventional Hall resistance.