Juhee Yang

Formation of Co nanoclusters in epitaxial Ti0.96Co0.04O2 thin films and their ferromagnetism

Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior was changed from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped TiO2 films.

Observation of an Exceptional Point in a Chaotic Optical Microcavity

We present spectroscopic observation of an exceptional point or the transition point between mode crossing and avoided mode crossing of neighboring quasieigenmodes in a chaotic optical microcavity of a large size parameter. The transition to the avoided mode crossing was impeded until the degree of deformation exceeded a threshold deformation owing to the systems openness also enhanced by the shape deformation. As a result, a singular topology was observed around the exceptional point on the eigenfrequency surfaces, resulting in fundamental inconsistency in mode labeling.

Investigations on the nature of observed ferromagnetism and possible spin polarization in Co-doped anatase TiO2 thin films

High-quality epitaxial thin films of Co-doped anatase TiO2 (Co:TiO2) were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior altered from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like pattern. Electrical conductivity and saturation magnetization increased, seemingly consistent with the picture of carrier-induced ferromagnetism. However, we also found that the spatial distribution of Co ions became highly nonuniform and the chemical state of Co ions changed from ionic to metallic. All of these PO2 dependences, even including the transport and the magnetic properties, can be explained in terms of the formation of cobalt clusters, whose existence was clearly demonstrated by transmission–electron–microscope studies. Our work clearly indicates that the cobalt clustering will res...

Kinetic roughening of ion-sputtered Pd(001) surface: beyond the Kuramoto-Sivashinsky model.

We investigate the kinetic roughening of Ar+ ion-sputtered Pd(001) surface both experimentally and theoretically. In situ real-time x-ray reflectivity and in situ scanning tunneling microscopy show that nanoscale adatom islands form and grow with increasing sputter time t. Surface roughness W(t) and lateral correlation length xi(t) follow the scaling laws W(t) approximately t(beta) and xi(t) approximately t(1/z) with the exponents beta approximately 0.20 and 1/z approximately 0.20, for an ion beam energy epsilon=0.5 keV, which is inconsistent with the prediction of the Kuramoto-Sivashinsky (KS) model. We thereby extend the KS model by applying the coarse-grained continuum approach of the Sigmund theory to the order of O(inverted Delta(4),h(2)), where h is the surface height, and derive a new term of the form inverted Delta(2)(inverted Delta h)(2) which plays a decisive role in describing the observed morphological evolution of the sputtered surface.

Surface versus bulk characterizations of electronic inhomogeneity in a VO2 thin film

thin film, theMIT should occur during the percolation process. We also used surface-sensitive scanning tunneling spectros-copy STS to investigate the microscopic evolution of the MIT near the surface. Similar to the XRD results,STS maps revealed a systematic decrease in the metallic phase as temperature decreased. However, this rate ofchange was much slower than the rate observed with XRD, indicating that the electronic inhomogeneity nearthe surface differs greatly from that inside the film. We investigated several possible origins of this discrepancyand postulated that the variety in the strain states near the surface plays an important role in the broad MITobserved using STS. We also explored the possible involvement of such strain effects in other correlatedelectron oxide systems with strong electron-lattice interactions.DOI: 10.1103/PhysRevB.76.075118 PACS number s : 71.30. h, 68.47.Gh, 71.27. a

Superparamagnetism in Co-ion-implanted anatase TiO2 thin films and effects of postannealing

We observed superparamagnetism in Co-ion-implanted anatase TiO2 thin films on SrTiO3 (001) substrates. After postannealing under oxygen pressure of 1.0×10−6 Torr, the blocking temperature increased with increasing annealing temperature. Eventually, the 600 °C-annealed sample became ferromagnetic at room temperature. These phenomena can be explained by the formation of Co (or ferromagnetic Co compound) particles and their growth during postannealing, as directly observed with transmission electron microscopy. High oxygen pressure during annealing reduced the size of the clusters and saturation magnetization moment, which indicates that cobalt clustering should be the main cause of the room-temperature ferromagnetism in the Co-ion-implanted films.

Development of a deformation-tunable quadrupolar microcavity

We have developed a technique for realizing a two-dimensional quadrupolar microcavity with its deformation variable from 0% to 20% continuously. We employed a microjet ejected from a noncircular orifice in order to generate a stationary column with modulated quadrupolar deformation in its cross section. Wavelength redshifts of low-order cavity modes due to shape deformation were measured and were found to be in good agreement with the wave calculation for the same deformation, indicating that the observed deformation is quadrupolar in nature.

Chaos-assisted nonresonant optical pumping of quadrupole-deformed microlasers

Efficient nonresonant optical pumping of a high-Q scar mode in a two-dimensional quadrupole-deformed microlaser has been demonstrated based on ray and wave chaos. Threefold enhancement in the lasing power was achieved at a properly chosen pumping angle. The experimental result is consistent with ray tracing and wave overlap integral calculations.

Enhanced nonresonant optical pumping based on turnstile transport in a chaotic microcavity laser

We report efficient nonresonant optical pumping based on turnstile transport in a chaotic microcavity laser. We observed more than tenfold enhancement in the pumping efficiency at a particular pumping angle and at a particular boundary position of the microcavity while these angle and position are consistent with the turnstile transport mechanism in chaotic ray dynamics. The pumping efficiency distribution resembles that of the output emission for the same cavity, supporting the concept of time-reversed ray transport.

Surface structures of a Co-doped anatase TiO2 (001) film investigated by scanning tunneling microscopy

The surface structure of an anatase Ti0.94Co0.06O2 (001) film, grown epitaxially on a Nb-doped SrTiO3 (001) substrate, was investigated using in situ scanning tunneling microscopy. For the as-grown film, a (1×n) (n=3, 4, 5, and 6) reconstructed surface was observed that shows (n−2) faint rows between adjacent bright rows. After annealing at 650 °C, nanoparticles appeared, mostly on the step edges. From the I–V curves measured by scanning tunneling spectroscopy, the tunneling gap of Co:TiO2 was estimated to be about 3.0 eV, consistent with the band gap (Eg=3.2 eV) of pure anatase TiO2. However, on nanoparticles, the I–V curve showed a much smaller gap, suggesting that the particle must be different from TiO2.