Tunneling resistance

Fig. 2. Schematic diagram of the tunnel gap between sample and tip, with the extension of the electric double layers indicated by the outer Helmholtz plane(OHP). (a) No tip interaction at large tip-sample separation, (b) Overlap of the electric double layers at a distance s = 0.6 nm, which can be achieved by conventional imaging conditions (e.g., Uj = 50 mV It = 2 nA Rt = 2.5 x 107 Q). Inset Dependence of the tunnel gap s on the tunnel resistance Rt for a tunnel barrier of 1.5 eV.

FIG. 9. The tunnel resistance as a function of tip movement is shown in (a). Positive z displacement corresponds to a decreasing tunnel junction width, and the jump in conductance at is associated with point contact. The inset figure shows the degree of reproducibility associated with the experiment which employed an Ir tip and substrate. The gradient associated with the attractive force between the two electrodes was measured simultaneously as shown in (b). (From Ref. 66.)... 

STM images of molecules are often a sensitive function of the tunnel conductance. For example, when examining TMPyP on Au( 111 )-I, the iodine underlayer was imaged at a tunnel resistance of 5 X 10 ohms while the electron density of TMPyP was observed at 8 X 10 ohms [484], Similar effects have been noted for protoporphyrins [488] and purines [489,490] adsorbed on graphite. 

Fig. 7. Simulated results with a clean tungsten tip (a), (b), and comparison between simulated contours with a clean and oxygen-contaminated tip (c). The shapes change when the tunneling resistance is changed from 300 (a) to 30 (b) MSI. The contrast is reversed when the tip is contaminated by oxygen (c), top and bottom frames.

Fig. 2. Tip height curves during manipulation of (a) a Cu-atom, (b, c) a Pb-atom, (d) a CO molecule and (e—g) a Pb-dimer along step edges on Cu(211). The tip is moved from left to right and respective tunneling resistances are indicated. The vertical dotted lines correspond to fee sites next to the step edge. The initial sites of the manipulated particles are indicated. Notice that in the attractive manipulation modes (a,b,e,f,g pulling and c sliding) the particles first hop towards the tip and then follow it, whereas in the repulsive mode (d pushing) the particle performs hops away from the tip [4] (image supplied by L. Bartels).

Let us consider the tunnel resistance of a % = 1 nm-broad vacuum gap between two gold electrodes. For the Fermi energy Ep 8 eV and the work function ll 5 eV, the dominating electron wave function exponentially decreases in vacuum with the rate 7 = s/2m W/h 1.3A-1, where m is the electron mass. The enormous resistance Rinm = e-2 X1 /go 2.5 x 1015 il prohibits electron transport in vacuum in the absence of a quantum dot. 

It was found that the apparent corrugation is dependent mainly upon the tunneling resistance, which is defined by the ratio of bias voltage to tunneling current and which is expected to have a maximum of about 0.3 nm on a freshly cleaved surface. The surface of graphite has a layer structure in which each layer is composed of carbon atoms in a honeycomb arrangement. The distances between the carbon atoms were found to be 0.246 mn (2.46 A) and 0.142 nm (1.42 A). Adjacent layers were separated by 0.335 nm. 

The ZT- bias curves are almost symmetric and linear at low bias voltages (Fig. 23A). The estimated tunneling resistance of the Au-tip/BPn/Au(lll)-(1x1) junction follows the same trend as discussed in the analysis of the distance tunneling characteristics. The current increases exponentially at higher bias voltages. The experimental data can be qualitatively represented by the Simmons tunneling model [187] in - 0.500 V < Fbias < 0.500 V. 

Figure 10.10 shows an example of a measurement of the energy gap by a tunneling experiment The measurement gives the tunnel resistance between two crystals of /S-(BEDT-TTF)2 13 in the superconducting state. The tunnel barrier here is the vacuum between the two minimally separated crystals [7]. The plot of d V/dl, the dif-... 

Fig. 10.10 The differential tunnel resistance dV/d/of (BEDT-TTF)2l3 in the superconducting state at various temperatures T. 4Aj is the full width at d V/dl = 0 and is a measure of the band gap at the given temperature T (see text).

Mine fan system The wind adit section S.(m2) Wind speed V (m/s) Air volume Q (mVs) Mine resistance Adit wind resistance h.( Pa) K=ht//i X100 (%) Wind tunnel resist- ance Rt (N. sVm )... 

The source and drain tunnel resistances and capacitances are assumed equal, the charging energy is 100 k T. For gate charge Qc = 0, the single-electron transistor is in the Coulomb blockade regime, and the l-Vcurve exhibits the... 

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