D.-J. Kang

Fabrication of a nanoelectromechanical switch using a suspended carbon nanotube

Fabrication and characterization of a nanoelectromechanical switching device consisting of a suspended multiwalled carbon nanotube and self-aligned electrodes is reported. The device has a triode structure and is designed so that a suspended carbon nanotube is mechanically switched to one of two self-aligned electrodes by repulsive electrostatic forces between the nanotube and the other self-aligned electrode. Carbon nanotubes are dispersed on an SiO2 coated Si wafer and their locations recorded using a scanning electron microscope mapping process. Contact electrodes and self-aligned deflection electrodes are formed by a process comprising electron beam lithography, metallic thin film deposition, and lift-off. The electrical measurements show well-defined ON and OFF states with change of gate voltage. The measured threshold voltage for electromechanical switching is ∼3.6V.

Planar superconductor-normal-superconductor Josephson junctions in MgB2

We report the successful creation of planar MgB2 junctions by localized ion damage in thin (100 nm) films of MgB2 on sapphire by milling a 50 nm trench with a focused-ion beam across tracks of widths between 1 and 5 μm. When the depth of the trench is between 70% and 80% of the film thickness, devices show critical currents (IC) for temperatures below 25 K. The IC of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (ICRN) is remarkably high, implying a potential for very-high-frequency applications.

Resistance of a domain wall in La0.7Ca0.3MnO3

Although colossal magnetoresistance (CMR) materials exhibit large changes in electrical resistance (up to 106%), large magnetic fields (several tesla) must be applied. To obtain a sizeable low-field effect (<102% in several millitesla), it is necessary to incorporate structural discontinuities such as grain boundaries, or other types of interfaces. The potential for applications, however, remains limited because structural discontinuities increase electrical resistance by several orders of magnitude and hence create noise. Moreover, it has proven to be difficult to fabricate structural discontinuities reproducibly. We have attempted to investigate discontinuities that are purely magnetic via transport measurements through a precisely controlled number of magnetic domain walls of known area in thin film devices of the ferromagnetic CMR perovskite La0.7Ca0.3MnO3. A sharp low-field switching seen below ∼110 K is ascribed to the formation of a precise number of magnetic domain walls, each with resistance-area...

Fabrication of nanoscale heterostructure devices with a focused ion beam microscope

A focused ion beam (FIB) microscope has been used to fabricate junctions with dimensions in the range 100–5000 nm by three-dimensional etching. We have applied this process to a variety of structures, including current-perpendicular-to-plane giant-magnetoresistive multilayer devices, superconductor–metal–superconductor Josephson junctions, where the metal is Mo, Co, or a CuxNi1−x alloy, and GaN light-emitting diodes. In addition, Tl2Ba2CaCu2O8 intrinsic Josephson junctions were also fabricated and characterized. The flexibility of the FIB technique allowed junctions of many different materials and heterostructures to be fabricated with the same process.

Ultraviolet–visible near-field microscopy of phase-separated blends of polyfluorene-based conjugated semiconductors

We have used fluorescence scanning near-field microscopy to characterize polymer blends for electroluminescent applications, and thereby identify compositional nonhomogeneities. In particular, we have focused on the binary system constituted by poly(9,9′-dioctylfluorenealt-benzothiadiazole) and poly(9,9′-dioctylfluorene) (PFO), known to give efficiencies of up to 22 cd/A in light-emitting devices with suitable electrodes. Our primary aim was the assignment of the morphological features revealed in shear-force and atomic-force images of spin-coated films, and suggestive of phase separation on a 300-nm-length scale. From analysis of the fluorescence images (325 and 488 nm excitation), and quantitative correlation of optical and topographic data, we identify the raised features with PFO-rich regions. However, the limited variation in fluorescence intensity reveals a high extent of mixing within each phase on the length scale accessible in our experiment, approximately 100 nm for our focused-ion-beam-processe...

Pulsed laser deposition of epitaxial YBa2Cu3O7−y / oxide multilayers onto textured NiFe substrates for coated conductor applications

Pulsed laser depositions of double-buffer and triple-buffer YBa2Cu3O7−y (YBCO)/Y2O3(YSZ)/CeO2 heterostructures have been performed in situ onto commercially available biaxially textured NiFe 50%/50% tape. The deposition in the forming gas (4% H2/Ar) from a CeO2 target and the deposition in vacuum from a CeO2:Pd composite target have been explored as two possible routes for cube-on-cube growth of the first buffer layer. The influence of the critical processing parameters on the texture is investigated and some of the issues involved in the reduction of NiO (111) and the formation of cube-on-cube NiO (200) growth are discussed. X-ray diffraction has been used for texture evaluation of the substrate and subsequent deposited layers. The substrate–buffer interface region has been studied by focused ion beam cross section electron microscopy. Both the buffers and YBCO layers show biaxial alignment with ω and scans having optimum YBCO full width at half maximum (FWHM) values of 4.3° and 8.8°, respectively. The morphology has been characterized using atomic force microscopy and scanning electron microscopy. The value of Tc (onset) has been measured at 90 K (ΔTc = 10 K). The critical current density, Jc, has been measured by transport measurements and magnetic measurements performed in a dc SQUID magnetometer.

Realization and properties of YBa2Cu3O7−δ Josephson junctions by metal masked ion damage technique

We have developed a simple process to fabricate high-TC Josephson junctions by a combination of focused ion beam milling and 100 keV H2+ ion implantation. The resistively shunted junction-like current–voltage characteristics were observed in the temperature range of 48 to 4.2 K. The devices showed clear dc and ac Josephson effects. This technique is very promising in terms of simplicity and flexibility of fabrication and has potential for high-density integration.

Shape dependent thermal effects in apertured fiber probes for scanning near-field optical microscopy

Metal-coated, “pulled,” and conically shaped fiber probes used in scanning near-field optical microscopy (SNOM) typically undergo a thermal expansion when injected with laser light, due to partial energy absorption by the metallic film. Here, we report investigations into the thermal behavior of fiber probes produced by selective chemical etching that in our experience provide high light throughputs (10−3–10−4 vs 10−6 for the pulled fibers). Unexpectedly, we find a shortening of such probes in response to “high-power” laser injection (>1mW). Thermal stress due to prolonged high-power laser injection (∼9mW at 325nm; compared to powers <1mW often used in SNOM experiments) determines permanent alterations of the probes, after which their thermomechanical behavior reverts to the commonly observed elongation in response to laser injection. Scanning electron microscopy after high-power irradiation on such probes shows partial detachment of the metallic coating near the fiber termination. This, however, does not...

Disorder-induced collapse of the electron-phonon coupling in MgB 2 observed by Raman spectroscopy

The Raman spectrum of the superconductor

Strain control of superlattice implies weak charge-lattice coupling in La 0.5 Ca 0.5 Mn O 3

{\mathrm{MgB}}_{2}