[en] Low temperature molecular beam epitaxy regrowths of Ga_1-xMn_xAs (x≅0.04) diluted magnetic semiconductors on GaAs/In_1-yGa_yP/GaAs(001) and In_1-yGa_yP/GaAs(001) (y≅0.51) heterostructures prepared by metal-organic chemical vapor deposition are described. The resulting Ga_1-xMn_xAs properties are comparable to epitaxial films grown directly on GaAs (001) substrates from in situ reflection high-energy electron diffraction, x-ray diffraction, magnetometry, and transport measurements with magnetic ordering temperature of as-grown films to range between ∼50 and ∼60 K. Postgrowth low temperature annealing enhances both magnetic and transport properties. Perfect etch selectivity between Ga_1-xMn_xAs/GaAs and In_1-yGa_yP is utilized to realize suspended Ga_1-xMn_xAs/GaAs doubly clamped beam micromechanical freestanding structures

[en] We have fabricated porous miniaturized SiN resonators with various dimensions and studied their mechanical dynamics at their resonant modes. The surface modification of the resonators has been achieved by etching through a thin porous anodic aluminum oxide (AAO) mask, prepared by two-step anodization. Even though these porous resonators show well-defined Lorentzian line-shapes at their resonant modes, the corresponding fundamental flexural resonance frequencies are lower than those from typical non-porous resonators. The change in the resonance frequency is due to the presence of the pores on the surface, which reduces the effective tensile stress across the beam structure, as shown from both experimental measurements and the computational model. In addition, the observed quality factor reveals the level of dissipation originating from the surface modification. The principal dissipation mechanism is found to be gas damping in the free molecular flow regime. Based on the dissipation measurement, one can see an increase in the surface-to-mass ratio, which is responsible for the increased dissipation in the porous beam structure. The work presented here demonstrates simple integration of mechanical elements with a nanopatterning technique based on an AAO as well as the tuning of mechanics via surface modification at a small scale. Such a scheme could provide an additional degree of freedom in developing a mechanical sensing element with enhanced effective surface area. (paper)

[en] Measurement of reflected optical light from the surface of microscopic structures can offer a precise tool to investigate the oscillatory motion of micro- and nano-mechanical resonators. We demonstrate an optical investigation of multiple flexural modes of a nanomechanical resonator in thermal motion based on amplitude-modulated laser light, with higher-order resonant modes up to ∼ 17 MHz measured with this technique. Scanning focused laser light reproduces the mode shape of each flexural mode, and theoretical estimation of measurement sensitivity shows a displacement sensitivity of ∼ 10−15 m/Hz1/2. For nanomechanical devices incompatible with interferometric systems, this technique can exhibit sufficient sensitivity to explore small motional states.

[en] Electrical transport across lateral geometrical nanoconstrictions realized in 100 nm thick GaMnAs epifilms is studied. The constrictions are patterned with the aid of chemical etching techniques, as opposed to plasma-assisted methods. Transport behavior across the constrictions, where domain walls can be formed and pinned, changes from Ohmic to non-Ohmic below temperatures corresponding to epifilm T_C for junctions with high resistances. Magnetoresistance measurements across such junctions qualitatively show similar behavior to unpatterned epifilms attributable to anisotropic magnetoresistance. The experimental IV curves are in good agreement with theoretical models accounting for spin flop across a region of high resistance

[en] Measurement of reflected optical light from the surface of microscopic structures can offer a precise tool to investigate the oscillatory motion of micro- and nano-mechanical resonators. We demonstrate an optical investigation of multiple flexural modes of a nanomechanical resonator in thermal motion based on amplitude-modulated laser light, with higher-order resonant modes up to ~ 17 MHz measured with this technique. Scanning focused laser light reproduces the mode shape of each flexural mode, and theoretical estimation of measurement sensitivity shows a displacement sensitivity of ~ 10 ⁻¹⁵ m/Hz^1/2. For nanomechanical devices incompatible with interferometric systems, this technique can exhibit sufficient sensitivity to explore small motional states.

[en] We measured the inelastic electron tunneling spectroscopy (IETS) characteristics of metal-molecule-metal junctions made with alkanethiolate self-assembled monolayers. The molecular junctions were fabricated using a direct metal transfer method, which we previously reported for high-yield metal-molecule-metal junctions. The measured IETS data could be assigned to molecular vibration modes that were determined by the chemical structure of the molecules. We also observed discrepancies and device-to-device variations in the IETS data that possibly originate from defects in the molecular junctions and insulating walls introduced during the fabrication process and from the junction structure

[en] Carbon nanotubes (CNTs) have great potential in the development of high-power electron beam sources. However, for such a high-performance electronic device, the electric and thermal contact problem between the metal and CNTs must be improved. Here, we report graphene as an interfacial layer between the metal and CNTs to improve the interfacial contact. The interfacial graphene layer results in a dramatic decrease of the electrical contact resistance by an order of 2 and an increase of the interfacial thermal conductivity by 16%. Such a high improvement in the electrical and thermal interface leads to superior field emission performance with a very low turn-on field of 1.49 V μm"−"1 at 10 μA cm"−"2 and a threshold field of 2.00 V μm"−"1 at 10 mA cm"−"2, as well as the maximum current of 16 mA (current density of 2300 A cm"−"2). (paper)

[en] SrRu_1−xFe_xO_3−δ (x = 0.00, 0.05, 0.10, and 0.20) thin films were fabricated to study the intrinsic aspects of a “self spin valve”. Using epitaxial strain and high oxygen partial pressure during thin film growth, single phase thin films with negligible oxygen vacancies were successfully grown, and problems related to A-site disorder and grain boundaries were minimized. Under application of an external magnetic field of up to 9 T, the resistivity of all films decreased, resulting in large negative magnetoresistance (up to ∼14.4%), which was stronger at temperatures in the range 10–30 K. An abrupt metal-insulator transition at T∼ 43 K was found in the x = 0.20 film, which was explained using a two-fluid model related to electron–electron interactions. From the model, two fitting parameters were found to be necessary for in-situ and homogenous defects, while three or unphysical fitting parameters were necessary for ex-situ and inhomogeneous defects. - Highlights: • Growth of SrRu_1−xFe_xO_3−δ film without A-site disorder and grain boundary problem. • Intrinsic nature of “self spin valve” observed in SrRu_1−xFe_xO_3−δ film. • Two different nature of metal-insulator transition was found in SrRu_1−xFe_xO_3−δ. • Magnetoresistance up to ∼14.4% were found in SrRu_1−xFe_xO_3−δ in low temperature. • Antiferromagnetic bonding increases with Fe doping concentration.

[en] We observed bipolar switching behavior from an epitaxial strontium cobaltite film grown on a SrTiO₃ (001) substrate. The crystal structure of strontium cobaltite has been known to undergo topotactic phase transformation between two distinct phases: insulating brownmillerite (SrCoO_2.5) and conducting perovskite (SrCoO_3−δ) depending on the oxygen content. The current–voltage characteristics of the strontium cobaltite film showed that it could have a reversible insulator-to-metal transition triggered by electrical bias voltage. We propose that the resistance switching in the SrCoO_x thin film could be related to the topotactic phase transformation and the peculiar structure of SrCoO_2.5