L. M. Xu

Reverse-bias-induced bipolar resistance switching in Pt∕TiO2∕SrTi0.99Nb0.01O3∕Pt devices

Stoichiometric single-crystalline TiO2 thin films were grown on SrTi0.99Nb0.01O3 (Nb:STO) substrates by oxygen plasma-assisted molecular beam epitaxy. The Pt∕TiO2∕Nb:STO∕Pt devices showed extremely weak resistance switching hysteresis without applying reverse bias. However, when the reverse bias increased above −2V, the hysteresis became more and more prominent. Further, it was found that the low (high) resistance state can be set by applying sufficient reverse (forward) bias. The origin of the reverse-bias-induced bipolar switching behavior should be attributed to the modulation of Schottky-like barrier width by electrochemical migration of oxygen vacancies.

Nonadiabatic fiber taper-based Mach-Zehnder interferometer for refractive index sensing

A Mach-Zehnder interferometer based on a nonadiabatic fiber taper is proposed for refractive index (RI) sensing. Only the first order cladding guided mode is excited in the down-taper. The radius of the fiber core in the interference region is too thin (<1.64 μm) to confine the fundamental mode in the fiber core, the fundamental core guided mode is converted to the fundamental cladding guided mode and interference occurs between it and the first order cladding guided mode. The sensitivity of the sensor is 980 nm per refractive index unit for the surrounding refractive index ranging from 1.332 to 1.392.

N-derived signals in the x-ray photoelectron spectra of N-doped anatase TiO2

The plasma-assisted molecular beam epitaxial anatase TiO2−xNx (x<0.3) films were chosen to investigate the N-derived variation in the x-ray photoelectron spectroscopy (XPS). With increasing nitrogen concentration, the small chemical shift and the shoulder of the main peak emerge in the N 1s, O 1s, or Ti 2p3/2 core level spectra. In combination with the experimental results obtained from x-ray diffraction and atomic force microscopy, the variations in the spectra could be ascribed to the microstructural distortion. This distortion, induced by the N3− substitution for lattice O2−, could slightly decrease the average ionicity of the Ti–O (or N) bonds. In addition, the other N 1s features (at 399.8 and 401.8 eV) and the oxygen vacancy are also discussed. For the N-doped TiO2, this work introduces a correlation between the microstructural properties and the XPS signals.

Gold nanorod-induced localized surface plasmon for microparticle aggregation

We investigate a technique for microparticle aggregation via optical fiber-excited localized surface plasmon resonance of gold nanorods, which were deposited on the designated surface of a tapered optical fiber (TOF). Through the enhanced photothermal effect, a microbubble forms on the TOF at an input optical power of 14 mW. Marangoni convection transports the microparticles dispersed in water to the bubble, where they accumulate on the gold nanorod surface because of strong optical gradient force.

Semicircular fiber-based high-sensitivity displacement sensor

A semicircular fiber (SCF) was fabricated by flame-heated treatment for displacement sensing application. By launching a laser beam into the SCF, interference was created at the output port of the structure due to fiber-bending-induced birefringence. Experiments indicate that the resonant wavelength was shifted to shorter wavelengths with increasing transverse displacement. Displacement sensitivity increases with the decrease of the bending radius of the SCF. A high sensitivity of 1100 nm/mm was obtained for the SCF with a bending radius of 0.57 mm.

Room-temperature ferromagnetism in (Mn, N)-codoped TiO2 films grown by plasma assisted molecular beam epitaxy

(Mn, N)-codoped TiO2 films have been fabricated by plasma assisted molecular beam epitaxy. X-ray diffraction and x-ray photoelectron spectroscopy studies indicate the incorporation of both Mn and N ions into the TiO2 lattice but without Mn metal clusters. Ferromagnetism with clear hysteresis at 300 K is observed in (Mn, N)-codoped TiO2 films, whereas the films without nitrogen show paramagnetic behavior. It is demonstrated experimentally that the intentional nitrogen doping plays an important role in realizing dominant ferromagnetic ordering in Mn-doped TiO2 films.

Bloch-point-mediated magnetic antivortex core reversal triggered by sudden excitation of a suprathreshold spin-polarized current

We report on spin dynamics of single magnetic antivortices driven by sudden excitation of a spin-polarized direct current. Using micromagnetic simulations, we find that there is a critical current density where spin dynamics transition occurs. Above the critical value, the core is switched on a time scale of ∼200 ps through two Bloch points injection after fully suppressed by a quadruple-vortices array, not through the well-known process involving an antivortex-vortex pair creation and annihilation. As the current density decreases, the time required to switch the core increases. The state after full relaxation depends on the current density (the excitation parameter).

Surface reconstruction evolution and anatase formation in the process of oxidation of titanium nitride film

Titanium nitride film was grown on MgO(001) substrate by plasma-assisted molecular beam epitaxy and then oxidized by oxygen plasma. Reflection high-energy electron diffraction (RHEED) was employed to in situ monitor the process of growth and oxidation. After the TiN film was oxidized for a moment, spots among main streaks were observed in RHEED pattern, which should be attributed to the isolated surface reconstruction domains disorderedly distributing on flat surface. Subsequently, the spots gradually evolved to streaks so that more clear RHEED patterns of (2×1) surface reconstruction were observed. It was argued that the disordered and isolated reconstruction domains congregated to large domains or even perfect reconstruction surface with oxidation time evolving. After oxidation, a series of characterization methods were applied to study the TiO2 phase, which consistently confirmed that the phase of oxidized titanium nitride is anatase but not rutile.