Zero-bias conductance peak

STS scanning tunneling spectroscopy ZBCP zero-bias conductance peak... 

Kashiwaya and Tanaka (Kashiwaya et al. 1994b, 1995b, 1996, Y. Tanaka and Kashiwaya 1995) have examined both theoretically and experimentally the interface effect of anisotropic superconductors, predicting the occurrence of the zero-bias conductance peak and its splitting in some cases due to the formation of Andreev bound states... 

Splitting of zero-bias conductance peak in zero magnetic field... 

Covington et al. 1996a). It was reported that the zero-bias conductance peak split when a magnetic field is applied (Lesueur et al. 1992) however, Covington et al. (1997) observed that the conductance peak split even when no magnetic field was applied, once the temperature was lowered below about 7K, as shown in fig. 31. 

The tunneling spectra depicted in fig. 19 are quite different from those in Bi2212 single crystals. The gap structure is not consistent with an isotropic BCS s-wave state, and is different from the simple d-wave picture. The multi-peak structure resembles the spectrum expected for extended s-wave, except for the large zero-bias conductivity. 

Furthermore, it was shown the unpaired spin S = 1/2, which is delocalized over the two Pc rings, still remained in the Jt-orbitals after absorption on Au(lll). Consequently, STS measurements also provided direct observation ofthe S = 1/2 radical on the TbPc2 molecules on Au(lll) whereby the indicative Kondo-peak could be switched off by tunnelling current pulses [215]. Indeed the tunnelling conductance (dl/dV) was analysed from STS experiments of TbPc2 on Au(lll) near the Fermi level showed a zero-bias peak (ZBP) in the spectra, which could be assigned as a Kondo resonance. Clear Kondo features for the molecules with 9 = 45° were observed when the tip was positioned over one ofthe lobes of TbPc2. 

Another conductivity mechanism could be suggested for LB films of this polymer with Ag+ cations. Such cations can accept or release electrons easily, so in the layer of such cations the conductivity could be caused by electron transitions between the ions with different degrees of oxidation. With tunneling microscopy an anomaly in the dl/dV(V) curves near zero bias was discovered for the LB films in Ag form with an odd number of layers there was a conductivity peak some 150-200 mV wide (Figure 7.4, Curves 1, 3) but no anomaly for these same films with an even number of layers (Figure 7.4, Curve 2). For LB films with an odd number of layers the ordered superstructure of the scale 11.5 x 11.5 x lO cm has been found in a conductivity dl/dV (x,y) measurement regime. The scale of such a structure corresponds to 3 x 2 surface reconstruction (Figure 7.5). 

As the temperature is increased from T = 29 K up to 293 K both peaks obviously shift towards the Fermi level, i.e. zero bias. This observation is in strong disagreement with a pure spin mixing behavior as proposed on the basis of PE experiments performed by Li et al. [121]. However, increasing the temperature above 293 K does not lead to a further shift of both peaks. Unfortunately, the binding energy of the occupied surface state could not be determined above T — 360 K. This is caused by the background of the differential conductance... 

While the native oxide layer on conventional metal superconductors passivates the surface, this is not the case for HTSC. A short-circuit in a native or artificial insulating layer has been considered to be responsible for the appearance of zero-bias peaks accompanied by the proximity effect. Mechanical damage at the interface may also create small particles, causing a charging effect, which then gives rise to various spurious features in the spectrum such as multi-peak conductance. Another difficulty in point-contact methods is the arbitrariness of the results due to the dependence on the contact pressure. 

The conductance peak near zero bias voltage (ZBCP) has been observed when tunneling... 

Fig. 5.3-11 Left differential conductance as a function of the applied voltage for a 49-period superlattice at 20 K. Right schematic model to explain the 48 negative peaks in the differential conductance, (a) Zero bias (b) ground-state resonanttunneling conduction (c) first field localization, where resonant tunneling between the ground state and an adjacent excited state takes place (d) expansion of the high-field region by one additional quantum well. (After [3.62])...

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