Jeong-Il Kye

Ultimate Permeation Across Atomically Thin Porous Graphene

Thin and Selective Outpourings When using a membrane to separate materials, the efficiency of the separation is limited by how fast the gas or liquid passes through the membrane and by how selective it is. Thinner membranes usually allow for faster flow rates but are usually less selective. Attempting to maintain selectivity, Celebi et al. (p. 289) developed a sophisticated way to drill holes of controlled diameter in a graphene sheet about two layers thick. For such a thin membrane, the primary barriers to separation come from entrance and exit from the holes and not from the motion through the membrane. Atomically thin nanoporous graphene membranes can sustain ultimate permeation in mass transport. A two-dimensional (2D) porous layer can make an ideal membrane for separation of chemical mixtures because its infinitesimal thickness promises ultimate permeation. Graphene—with great mechanical strength, chemical stability, and inherent impermeability—offers a unique 2D system with which to realize this membrane and study the mass transport, if perforated precisely. We report highly efficient mass transfer across physically perforated double-layer graphene, having up to a few million pores with narrowly distributed diameters between less than 10 nanometers and 1 micrometer. The measured transport rates are in agreement with predictions of 2D transport theories. Attributed to its atomic thicknesses, these porous graphene membranes show permeances of gas, liquid, and water vapor far in excess of those shown by finite-thickness membranes, highlighting the ultimate permeation these 2D membranes can provide.

High critical current densities in superconducting MgB2 thin films

Superconducting MgB2 thin films were prepared on Al2O3(0001) and MgO(100) substrates. Boron thin films were deposited by the electron-beam evaporation followed by postannealing process with magnesium. Proper postannealing conditions were investigated to grow good superconducting MgB2 thin films. The x-ray diffraction patterns showed randomly oriented growth of MgB2 phase in our thin films. The surface morphology was examined by scanning electron microscope and atomic force microscope. Critical current density (JC) measured by transport method was about 107 A/cm2 at 15 K, and superconducting transition temperature (TC) was ∼39 K in the MgB2 thin films on Al2O3.

Josephson effect in MgB/sub 2//Ag/MgB/sub 2/ step-edge junctions

We have made MgB/sub 2//Ag/MgB/sub 2/ junctions using the steps formed on Al/sub 2/O/sub 3/(0001) substrates. The current-voltage characteristics of these superconductor-normal-superconductor (SNS) type Josephson junctions have shown nonhysteretic behavior with some excess current. The measured I/sub c/R/sub n/ values were 1.2 mV at 4.2 K and 0.24 mV at 20 K. We have observed the modulation of critical current under dc magnetic field as well as Shapiro steps under 20 GHz microwave irradiation.

Investigation of a magnetic flux-guide for a HTS scanning superconducting quantum interference device microscope

A magnetic flux-guide in the form of a sharp needle is known to enhance the spatial resolution of the scanning SQUID microscope. We investigated the properties of a soft ferromagnetic tip as a flux-guide by measuring the local field of the tip end. The effects of the flux-guide, such as the transmission of a magnetic signal, length dependence, and nonlinear distortion, were observed. We also present the design and construction of a HTS scanning SQUID microscope with a flux-guide and a planar gradiometer. The gradiometer with a flux-guide is an appropriate solution for improving the performance of the scanning SQUID microscope with a flux-guide as regards the prevention of external field noise without magnetic shields.

The normal-state resistivity of grain boundaries in YBa2Cu3O7−δ

Using an optimized bridge geometry, we have been able to make accurate measurements of the properties of YBa2Cu3O7−δ grain boundaries above Tc. The results show a strong dependence of the change of resistance with temperature on grain boundary angle. Analysis of our results in the context of band bending at the boundary allows us to estimate the height of the potential barrier present at the grain boundary interface.

Y-Ba-Cu-O grain boundary resistivity above and below the critical temperature

The interface resistance of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// grain boundaries has been measured in the range 5-270 K by means of a Wheatstone bridge for 24/spl deg/, 30/spl deg/ and 36/spl deg/ symmetric and 45/spl deg/ degree asymmetric, [001] tilt, thin film grain boundaries. A number of preliminary experiments have demonstrated the validity of the technique employed. Above T/sub c/, the grain boundary resistance decreases as the temperature is increased, and the temperature variation is more pronounced for higher angle boundaries. A number of possibilities for interpreting the data are considered.

Interface-modified YBCO ramp-edge Josephson junctions by deionized water

We have investigated YBa 2 Cu 3 O 7-x (YBCO) ramp-edge Josephson junctions by reacting the ramp-edge interface with deionized water. YBa 2 Cu 3 O 7-x /PrBa 2 Cu 3 O x (YBCO/PBCO) films were deposited on SrTiO 3 (100) by on-axis KrF laser deposition. After patterning the bottom YBCO/PBCO layer, the ramp edge was cleaned by Br-in-ethanol and then reacted with deionized water. The top YBCO/PBCO layer was deposited and patterned by photolithography and ion milling. We measured the current-voltage (I-V) characteristics, magnetic field modulation of the critical current and microwave response at 10 GHz. The 20-minute water-immersed junction showed resistively shunted junction (RSJ)-type I-V characteristics, while others exhibited flux-flow behaviour. The average values of I c , R n and I c R n of these RSJ-type junctions were 4 mA, 0.1 Ω and 400 μV, respectively, and Shapiro steps were fitted well by the microwave characteristic parameter Ω (= hf/2eI c R n ) = 0.18.

Effect of ion beam damage and heat treatment on interface-modified junctions

We have fabricated Josephson junctions by modifying ramp edges of the base electrodes without depositing any artificial barrier layer. We irradiated the ramp edge surfaces with ion beams and heat-treated them under various conditions prior to the deposition of counter-electrode layers. After patterning the samples into ramp edge junctions using photolithography and ion beam etching, we measured their electrical properties, such as current-voltage characteristics, magnetic field modulation of the critical current, and microwave response. Some showed resistively shunted junction (RSJ)-type current-voltage (I-V) characteristics, while others exhibited flux-flow behavior, depending on the details of interface treatment. Junctions fabricated using optimized conditions showed fairly uniform distribution of junction parameters. Their I-V curves were RSJ-type, also shown by the microwave-induced constant voltage steps. I/sub c/R/sub n/ values of typical RSJ-type junctions were about 0.07 mV at 77 K.