Toshiyuki Onogi

Theoretical Study of Ga Adsorbates around Dangling-Bond Wires on an H-Terminated Si Surface: Possibility of Atomic-Scale Ferromagnets

By using first-principles calculations within the local density functional approach, we have examined the behavior of Ga adsorbates around a dangling-bond wire, which is constructed by extracting H atoms from the H-terminated Si(100)2×1 surface. In addition to clarifying the stable atomic arrangements and the electronic structures of the Ga adsorbates, we found strong evidence of a possible ferromagnetic ground state of the Ga adsorbate wire in specific atomic arrangements. Appearance of the ferromagnetic ground state has been supported by a preliminary calculation of magnetic moments using a simple tight-binding model and the mean-field approximation. The result of this investigation is expected to provide a new way to obtain atomic-scale ferromagnets produced from only nonmagnetic elements.

Power-law dissipative behavior in high-Tc superconductor

Abstract Induced voltage V in the mixed state of high- T c superconductor ErBa 2 Cu 3 O 7-x film is measured as a function of transport current I and applied magnetic field H at 77K. Both I–V characteristics and magnetoresistance exhibit similar power-law behaviors with nonuniversal exponents in the forms of V ∞ I n ( H ) and V ∞ ( H - H 0 ) m ( I ) , respectively. Response voltages to the modulated magnetic field have anomalous field-dependences nearby H = H 0 which corresponds to the lower critical field. This power-law behavior suggests that there exists a new nonlinear dissipative phase probably due to fluxiod motion.

FERROMAGNETISM IN A HUBBARD MODEL FOR AN ATOMIC QUANTUM WIRE : A REALIZATION OF FLAT-BAND MAGNETISM FROM EVEN-MEMBERED RINGS

We have examined a Hubbard model on a chain of squares, which was proposed by Yajima et al as a model of an atomic quantum wire As/Si(100), to show that the flat-band ferromagnetism according to a kind of Mielke-Tasaki mechanism should be realized for an appropriate band filling in such a non-frustrated lattice. Reflecting the fact that the flat band is not a bottom one, the ferromagnetism vanishes, rather than intensified, as the Hubbard U is increased. The exact diagonalization method is used to show that the critical value of U is in a realistic range. We also discussed the robustness of the magnetism against the degradation of the flatness of the band.

Reduced Density of Missing-Dimer Vacancies on Tungsten-Contaminated Si(100)-(2×n) Surface by Hydrogen Termination

A Si(100) surface with missing-dimer vacancies forming (2×n) phase was prepared by tungsten deposition and the morphological change was observed by scanning tunneling microscopy when the surface was terminated by hydrogen. The density of dimer vacancies was significantly reduced by the hydrogen termination, suggesting that the density of subsurface W atoms decreased. We discuss the mechanism of this morphological change based on the traditional theory of chemisorption-induced surface segregation and on the energetic instability of W atoms buried in the subsurface of the hydrogen-terminated Si surface.

Two-stage melting of disordered vortex-line lattice in Bi2Sr2CaCu2O8+δ

Abstract We study vortex matter phase diagram of Bi2Sr2CaCu2O8+δ, by using Monte Carlo simulation based on Lawrence–Doniach model. Introducing strong point-like disorder, we find two successive phase transformations from the strongly pinned vortex glass to pancake vortex gas via an intermediate weakly pinned glass, when temperature is increased. The first one is identified as an intra-layer depinning transition as pre-melting, which subdivides the glass phase into two phases. The intermediate glass melts with inter-layer decoupling of vortex lines, implying the loss of interlayer long-range coherence. These two-phase boundaries agree with recent experimental data.

Power-law resistive behavior of two-dimensional Josephson junction array with bond defects: Dynamic simulation and its relevance to high-Tc oxides

Abstract Nonlinear electric resistivity of the two-dimensional(2D) Josephson junction array with bond defects (defect density p ) is calculated directly using the Langevin simulation technique. It is found that both the current-voltage ( I-V ) characteristics and the p-V resistive behavior at low temperatures obey power-laws with continuously varying exponents: V ∝ I α p and V ∝ p β I . Computational results are quite similar to those observed experimentally in the ion irradiation effect on high- T c oxides. The ion irradiation is thought to create random defects in the 2D intrinsic weak-link structure which is peculiar to the high- T c oxide superconductor. It is suggested that the vortex-antivortex pair excitation and its transport property in 2D media with bond disorder are crucial for the defect-dependent power-law.

Monte Carlo study of vortex-line dynamics with correlated disorders

Abstract We have studied single vortex-line dynamics with multiple point- and columnar-pins under weak magnetic fields ( H || c -axis) in a model high- T c superconductor, via Monte Carlo simulations in terms of vortex variables. Under various disorder configurations, the I – V characteristics and thermal averages of localization lenght l ⊥ of a single vortex line for various external currents are calculated by introducing the tilted potential. It is found that the non-Ohmic I – V characteristic R ∝ exp[−( E k / T )( I l / I )] appears in the case with multiple columnar-pins. We will discuss this behavior in association with tunneling via half-loop excitation.

Direct Imaging of Thermodynamic Process in Atom Migration by Using Scanning Tunneling Microscopy

We report novel imaging of surface adatom migration that allowed us to probe the impurity atoms buried in a semiconductor, by scanning tunneling microscopy (STM). We adsorbed Ga atoms onto a chemically inactive H-terminated Si(100) surface, and directly observed thermal Ga-atomic motion near 100 K. By exploiting the fact that the STM image reveals the thermodynamic distribution function of atomic trajectories, we achieved high-sensitivity detection of a positional variation in the surface potential. The position of subsurface P dopants was thereby obtained, in combination with first principles calculations of electronic states.

The effect of pointlike pinning on vortex phase diagram of Bi2Sr2CaCu2O8+δ

Abstract We numerically examine a possible vortex-matter phase diagram of Bi 2 Sr 2 CaCu 2 O 8+ δ in the presence of dense pointlike defects, by using a Monte Carlo simulation based on the Lawrence–Doniach model. Distinct phases of the Bragg glass, vortex glass, and pancake vortex gas, together with a depinning line within the glass phases, manifest themselves on the field ( B )-temperature ( T ) plane. While the locations of these phase boundaries are sensitive to pointlike pinning energy U p ( T ), we phenomenologically obtain a universal B – T phase diagram depending on a single parameter U p ( T ). We also discuss the relevance of numerical simulation to recent experiments.

Electronic and Magnetic Structures in Slab Models Including Al/Co and O/Co Interfaces

On the basis of the density-functional theory, we numerically calculated the interfacial magnetism of slab models including Al/Co and O/Co interfaces. We obtained the local magnetic moments and spin polarizations for various types of models. We found that the interfacial Al layers exhibit positive spin polarizaition in specific atomic arrangements, such as O/Al/Co. The sign is consistent with the experimental result of the ferromagnet/insulator/superconductor junction. This result suggests that in the junction Co/Al2O3/Co, the tunneling of s-character electrons in Al is favored to that of d-character electrons in Co.