Takaya Fujita

STM studies of a catalytically active p(3 × 1) PtRh(100) alloy surface

Abstract Oxygen-induced p(3 × 1) reconstruction of a Pt 0.25 Rh 0.75 (100) alloy surface was studied by scanning tunneling microscopy (STM). When a Pt-enriched (1 × 1) PtRh(100) alloy surface was exposed to O 2 , a clear p(3 × 1) LEED pattern appeared at temperatures higher than ca. 400 K, and segregation of Rh atoms was observed by STM. Once the p(3 × 1) PtRh(100) surface is established at T ≥ 400 K, a reversible interconversion, p(3 × 1)→p(1 × 1) by H 2 and p(1 × 1)→p(3 × 1) by O 2 , is brought about at room temperature. The p(3 × 1) surface showed three different STM images, depending on the tip condition, from which a model structure was deduced. The first formation of the p(3 × 1) structure requires the segregation of Rh atoms, but the interconversion between the p(3 × 1) and p(1 × 1) structures is responsible for the oxygen-induced array of Rh atoms along the 〈011tdirections.

CHARACTERIZATION OF CARRIER-TRAPPING PHENOMENA IN ULTRATHIN CHEMICAL OXIDES USING X-RAY PHOTOELECTRON SPECTROSCOPY TIME-DEPENDENT MEASUREMENTS

We report a technique to characterize carrier-trapping phenomena in SiO2 by measuring the Si 2p core-level energy of Si substrates covered with thin SiO2 layers as a function of x-ray irradiation time. It is found that the Si 2p peak energy, which corresponds to the band bending at the SiO2/Si interface, changes as the x-ray irradiation time increases. We attribute this to carrier-trapping phenomena in SiO2. By using this technique, it is found that the carrier-trapping phenomena differ remarkably among several chemical oxides. We also discuss the atomic structure of the traps that cause the trapping phenomena.

Nano-scale patterning of metal surfaces by adsorption and reaction

Nano-scale patterning of the metal surfaces was attained by the adsorption and the reaction of metal atoms. Growth of metal islands on the metal surfaces was markedly influenced by preparing the surface with preadsorption and/or by the reconstruction. Adsorption of oxygen on Cu(100) form randomly coalesced nano-size c(2 x 2)-O which undergoes the (2√2 x √2)R45° reconstruction with increasing the oxygen coverage. In contrast, N-atoms on Cu(100) make well ordered array of squared c(2 X 2)-N patches of ca. 50 A 2 . Cu atoms deposited on the nano-size c(2 X 2)-O Cu(100) form one atomic height rectangular Cu islands but Ni atoms undergo no orientational growth of Ni islands. Ni atoms deposited on the (2√2 x √2)R45°-C Cu(100) surface, however, gave an extremely anisotropic growth of Ni-wire. In contrast, when Ni atom was deposited on a Cu(100) surface having a super-grid like pattem made by the boundaries of squared c(2 x 2)-N patches, one atomic height Ni islands grew at the crossings of the super-grid. These phenomena are distinctive from the chemical reaction of surface atoms which provides various quasi-compounds. The reaction of Cu with (-Ag-O-) strings on Ag(110) provided a new quasi-compound of (-Cu-O-) on Ag(110), which undergoes a reversible reaction of (-Cu-O-)⇄ (Cu) 6 + O 2 . In addition, the (-Ag-O-) strings on Ag(110) undergo selective photo-erasing. These results suggest that chemical reaction is a promising tool for making atomic-scale pattern on the surface.

All-in-all-out magnetic domain wall conduction in a pyrochlore iridate heterointerface

Metallic surface states emerging at all-in-all-out type antiferromagnetic domain walls in bulk pyrochlore iridates have recently been observed. Such bulk crystals, however, inevitably contain huge amounts of the domains at random, and so it is quite challenging to investigate and utilize these novel surface conducting states. The results presented here are for a pyrochlore iridate heterointerface, which is realized by an advanced thin film growth technique. The authors successfully demonstrate the detection and control of a surface state linked to the antiferromagnetic domain wall, guiding further efforts for utilizing such states for surface transport in electronics and spintronics applications as well as for investigating the emergent topological transport at the interface.

STM study of preferential growth of one-dimensional nickel islands on a Cu(100)-(2)R45°-O surface

Abstract Aggregation of Cu and Ni adatoms on oxygen preadsorbed Cu(100) was studied by STM. Cu deposited on a Cu(100) c(2×2)-O surface agglomerate into anisotropic islands with a (2 2 × 2 )R45°-O structure, while Ni deposited on a c(2×2)-O surface forms isotropic Ni islands with c(2×2)-O structure. On the other hand, Ni on a (2 2 × 2 )R45°-O structure undergoes extremely anisotropic aggregation, which forms Ni-wires with a c(2×2)-O structure. Formation of Ni-wire on a (2 2 × 2 )R45°-O structure has been inferred by a preferential nucleation along the Cu atom missing trough inlaid with Ni.

All-in-all-out magnetic domain size in pyrochlore iridate thin films as probed by local magnetotransport

Pyrochlore iridates have attracted growing attention because of a theoretical prediction of a possible topological semimetal phase originating from all-in-all-out spin ordering. Related to the topological band structure, recent findings of the magnetic domain wall conduction have stimulated investigations of magnetic domain distribution in this system. Here, we investigate the size of magnetic domains in Eu2Ir2O7 single crystalline thin films by magnetoresistance (MR) using microscale Hall bars. Two distinct magnetic domains of the all-in-all-out spin structure are known to exhibit linear MR but with opposite signs, which enables us to estimate the ratio of the two domains in the patterned channel. The linear MR for 80 × 60 μm2 channel is nearly zero after zero-field cooling, suggesting random distribution of domains smaller than the channel size. In contrast, the wide distribution of the value of the linear MR is detected in 2 × 2 μm2 channel, reflecting the detectable domain size depending on each cooli...

Controlled epitaxial growth of Cu and Ni islands on oxygen-preadsorbed Cu(100) surfaces

Growth of Cu and Ni islands was studied on the two oxygen-preadsorbed Cu(100) surfaces, a Cu(100) surface covered with nanometer size c(2×2)–O domains and a reconstructed (22×2)R45°Cu(100)–O surface. Preadsorbed oxygen on Cu(100) migrates onto the epitaxially grown metal islands. Homoepitaxial growth of Cu on a nanometer-size c(2×2)–O Cu(100) surface forms monolayered rectangular Cu islands which grow in the [010] and [001] directions, where the island takes an anisotropic (22×2)R45°–O structure. In contrast, Ni atoms grow in no specific orientation on the nanometer-size c(2×2)–O Cu(100) surface and the Ni islands take a c(2×2)–O structure. Contrary to this, Ni atoms deposited on the reconstructed (22×2)R45°Cu(100)–O surface undergo specific nucleation along the Ni rows paved in Cu-missed troughs on the (22×2)R45°Cu(100)–O surface and Ni wire of nanometer width grows along the [001] or [010] direction. The shape of epitaxial islands may be governed in an energetically feasible fashion influenced by the adsorption of oxygen, but the nanometer-width Ni wire is the result of the kinetic feasibility of the specific nucleation.

Signatures of Hyperfine, Spin-Orbit, and Decoherence Effects in a Pauli Spin Blockade.

We detect in real time interdot tunneling events in a weakly coupled two-electron double quantum dot in GaAs. At finite magnetic fields, we observe two characteristic tunneling times T_{d} and T_{b}, belonging to, respectively, a direct and a blocked (spin-flip-assisted) tunneling. The latter corresponds to the lifting of a Pauli spin blockade, and the tunneling times ratio η=T_{b}/T_{d} characterizes the blockade efficiency. We find pronounced changes in the behavior of η upon increasing the magnetic field, with η increasing, saturating, and increasing again. We explain this behavior as due to the crossover of the dominant blockade-lifting mechanism from the hyperfine to spin-orbit interactions and due to a change in the contribution of the charge decoherence.

Atomic-scale fabrication of novel surfaces using chemical reactions

Abstract An idea of the chemical reconstruction caused by the formation of quasi-compounds and their self-assembly is defined. New surfaces were fabricated by combining the reaction of the quasi-compounds and their self-assembly. A quasi-compound of (CuO) strings was grown in the [110] direction on Ag(110) by the reaction of (AgO) strings with Cu atoms, which brought about a reversible reaction of (CuO)⇄(Cu)6+O2. By the reaction of (AgO) strings with CO2, a composite structure of (AgO) and (AgCO3) was established on the Ag(110) surface. When Cu atoms were vaporized on this composite surface, the (AgO) strings reacted selectively with Cu atoms, and (CuO) strings grew in the [110] direction. Interestingly, an opposite selective reaction occurred when the composite surface was scanned with a W tip coated with Cu; that is, Cu atoms react selectively with carbonate species. These phenomena suggest that the chemical reaction and the chemical transportation reaction will make realize atomic scale surface fabrication possible. The photochemical reaction of a quasi-compound was also attained on a composite surface of (AgO) and (Ag2N) strings on the Ag(110) surface, where the (AgO) strings were selectively erased by illumination.

Single photoelectron detection after selective excitation of electron heavy-hole and electron light-hole pairs in double quantum dots

We demonstrate the real-time detection of single photogenerated electrons in two different lateral double quantum dots made in AlGaAs/GaAs/AlGaAs quantum wells having a thin or a thick AlGaAs barrier layer. The observed incident laser power and photon energy dependences of the photoelectron detection efficiency both indicate that the trapped photoelectrons are, for the thin barrier sample, predominantly photogenerated in the buffer layer followed by tunneling into one of the two dots, whereas for the thick barrier sample they are directly photogenerated in the well. For the latter, single photoelectron detection after selective excitation of the heavy and light hole state in the dot is well resolved. This ensures the applicability of our quantum well-based quantum dot systems for the coherent transfer from single photon polarization to single electron spin states.