Observation of a subgap density of states in superconductor-normal metal bilayers in the Cooper limit

We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, $d_{Pb}$ and $d_{Ag}$, that are much less than the superconducting coherence length. The transition temperature, $T_c$, and energy gap, $\Delta$, in the tunneling Density of States (DOS) decrease exponentially with $d_{Ag}$ at fixed $d_{Pb}$. Simultaneously, a DOS that increases linearly from the Fermi energy grows and fills nearly 40% of the gap as $T_c$ is 1/10 of $T_c$ of bulk Pb. This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as $T_c$ goes to 0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.Comment: 5 pages and 4 figures. This version is just the same as the old version except that we try to cut the unnecessary white space in the figures and make the whole paper look more compac

Deviations from mean-field behavior in disordered nanoscale superconductor-normal-metal-superconductor arrays

We have fabricated quasi-two-dimensional arrays of nano-scale Pb grains coupled by an overlayer of Ag grains. Their temperature dependent resistive transitions follow predictions for an array of mesoscopic superconductor-normal-superconductor junctions. The decrease of their transition temperatures with Ag overlayer thickness systematically deviates from the Cooper limit theory of the proximity effect as the Pb grain size decreases. The deviations occur when the estimated number of Cooper pairs per grain is less than or equal to 1 and suggest the approach to a superconductor to metal transition.Comment: 11 pages, Pdf only, Revisions include text and figure

Static Tactile Sensing Based on Electrospun Piezoelectric Nanofiber Membrane

Here, a static tactile sensing scheme based on a piezoelectric nanofiber membrane, prepared via the electrospinning method, is presented. When the nanofiber membrane is kept under a constant vibration, an external contact onto the membrane will attenuate its vibration. By monitoring this change in the oscillation amplitude due to the physical contact via the piezoelectrically coupled voltage from the nanofiber membrane, the strength and duration of the static contact can be determined. The proof-of-concept experiment demonstrated here shows that the realization of a static tactile sensor is possible by implementing the piezoelectric nanofiber membrane as an effective sensing element

Field-Dependent Resonant Behavior of Thin Nickel Film-Coated Microcantilever

Herein we describe the vibration of a thin nickel film-coated microcantilever at resonance under an external magnetic field. The resonance frequency and the mechanical loss—experimentally observed while varying the magnetic field—closely follow the field-dependence of the magnetostriction coefficient, indicating the strong coupling between the mechanical motion and the magnetostriction through the surface stress. Comparing to the surface stress model based on uniformly distributed axial load, the magnetostriction coefficient of a nickel film has been estimated, and its value is comparable to the reported one. Our study suggests that the nature of the surface stress originating from the magnetostrictive film can govern and modulate the resonant behavior of miniaturized mechanical systems

Dynamics of a surface-modified miniaturized SiN mechanical resonator via a nanometer-scale pore array

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.101sciescopu