Moh’d Rezeq

Field regulation of single-molecule conductivity by a charged surface atom

Electrical transport through molecules has been much studied since it was proposed that individual molecules might behave like basic electronic devices, and intriguing single-molecule electronic effects have been demonstrated. But because transport properties are sensitive to structural variations on the atomic scale, further progress calls for detailed knowledge of how the functional properties of molecules depend on structural features. The characterization of two-terminal structures has become increasingly robust and reproducible, and for some systems detailed structural characterization of molecules on electrodes or insulators is available. Here we present scanning tunnelling microscopy observations and classical electrostatic and quantum mechanical modelling results that show that the electrostatic field emanating from a fixed point charge regulates the conductivity of nearby substrate-bound molecules. We find that the onset of molecular conduction is shifted by changing the charge state of a silicon surface atom, or by varying the spatial relationship between the molecule and that charged centre. Because the shifting results in conductivity changes of substantial magnitude, these effects are easily observed at room temperature.

Tungsten nanotip fabrication by spatially controlled field-assisted reaction with nitrogen

In this report we present a straightforward new technique for fabricating nanotips. This approach is based on spatially controlling the reaction of nitrogen gas with the surface atoms of a tungsten tip in a field ion microscope (FIM). Confining this field-assisted etching reaction to the shank has enabled us to produce single-atom tips with an apex radius far sharper than the nominal 10 nm radius of curvature tips we start with. Tip sharpening is evidenced in several ways. The FIM imaging voltage drops dramatically from, typically, 4.4 to 1.6 kV. Nanotip formation is also evident from the increase in the FIM magnification and the decrease in the apex area, which are monitored throughout the experiment. A subsequent field evaporation allows the nanotip to be sequentially deconstructed to further describe the extraordinary sharp tip that was formed. We also demonstrate the utility of these nanotips for the scanning tunneling microscope.

Multiple atomic scale solid surface interconnects for atom circuits and molecule logic gates

The scientific and technical challenges involved in building the planar electrical connection of an atomic scale circuit to N electrodes (N > 2) are discussed. The practical, laboratory scale approach explored today to assemble a multi-access atomic scale precision interconnection machine is presented. Depending on the surface electronic properties of the targeted substrates, two types of machines are considered: on moderate surface band gap materials, scanning tunneling microscopy can be combined with scanning electron microscopy to provide an efficient navigation system, while on wide surface band gap materials, atomic force microscopy can be used in conjunction with optical microscopy. The size of the planar part of the circuit should be minimized on moderate band gap surfaces to avoid current leakage, while this requirement does not apply to wide band gap surfaces. These constraints impose different methods of connection, which are thoroughly discussed, in particular regarding the recent progress in single atom and molecule manipulations on a surface.

Generation of quasi-reversibility in a commercial Bi:2223/Ag tape by vortex shaking with varying orthogonal magnetic fields

Abstract The magnetizations 〈 M ⊥ 〉 of strips of BiSCCO tapes are seen to evolve towards a common diamagnetic 〈 M ⊥ 〉 versus H ⊥ curve, hence display quasi-reversibility, from the application of a few cycles of a magnetic field H ∥ directed along their length, when previously magnetized, (i) to critical paramagnetic or diamagnetic states along the major hysteresis loop by an excursion of a transverse magnetic H ⊥ piercing the flat surfaces and now held fixed or, (ii) by field cooling in static H ⊥ . The remanent magnetic moment along H ∥ when the cycling ceases is made to vanish by a simple demagnetization procedure which homogenizes the flux trapped along H ∥ .

Enhancement of the Meissner effect upon the warming of fast field cooled weak-linked granular high Tc superconductors: a simple model

Several workers have observed that, after fast field cooling of weak-linked granular high Tc type II superconductors, the diamagnetic magnetization will increase and trace a valley, hence a peak of enhanced Meissner effect, as the temperature approaches Tc during slow warming to the normal state with the applied field Ha kept fixed throughout the temperature excursion. We present a simple extension to interconnected granular systems of the established model for isotropic single crystals which qualitatively and quantitatively reproduces our observations of this phenomenon over a large range of Ha, the extensive measurements of Jung et al on YBCO samples and the results of Hyun on compacted Nb3Sn polycrystals.

Enhancement of the Meissner effect in single crystals of hysteretic type II superconductors by thermal cycling

Abstract The established model, which accounts for the magnetothermal behaviour of isotropic type II superconductors during slow cooling and warming below T c in a static magnetic field, is extended to reproduce the observations of Hyun [Phys. Rev. B 48 (1993) 1244] of the enhancement of the magnitude of the diamagnetic magnetization M g of Nb 3 Sn single crystals by thermal cycling between T 1 and T 2 , where T 1 ≪ T 2 T c . The model also predicts that the thermal cycling between chosen T 1 and T 2 will lead to final closed hysteresis loops of M g versus T . The structure of the final hysteresis loops and the corresponding sequences of configurations of magnetic flux prescribed by the model are also described.

Examination of the peak in dICO/dlnt in weak-linked high Tc superconductors caused by trapped flux

Altshuler et al discovered that dICO/dlnt, the rate of increase of the critical current IC with time in polycrystalline high TC samples, traced a peak when measured versus HM, the amplitude of the sweep of the flux trapping magnetic field. We show that the sharp peak in dICO/dlnt which their model generates arises from a special feature of the formulae they use to describe IC versus HM. Pursuing an extension of these formulae, and exploiting a Brandt–Indenbom formula for the return field of the magnetized grains, we (i) reproduce observations of Altshuler et al, Batista-Leyva et al and a family of curves of dICO/dlnt reported by Cobas et al, and (ii) estimate the return fields. We also explore the peak structure of dICO/dlnt versus HM generated by using two well known empirical expressions for IC(H), and the Brandt–Indenbom formula.