shot noise in mesoscopic conductors - Low Temperature Laboratory

112 Ya.M. Blanter, M. Bu( ttiker / Physics Reports 336 (2000) 1}166
Fig. 31. Voltage dependence of the Fano factor (202) for the same set of parameters as Fig. 30 (solid line); Fano factor (78)
for a charge neutral quantum well (dashed line).
Poissonian. Jahan and Anwar [236], who also found super-Poissonian noise enhancement,
included self-consistent e!ect at the level of the stationary transmission probabilities, but also did
not take partition noise into account. As we already mentioned, an explanation of the super-
Poissonian noise in terms of the potential #uctuations was given by Iannaccone et al. [240], and
the analytical theory of this enhancement, identifying the relevant energy scales, was proposed in
Refs. [241,226].
Experimentally, enhancement of noise in quantum wells, as the voltage approaches the range of
negative di!erential resistance, was observed already in the early experiments by Li et al. [64] and
by Brown [231]. The super-Poissonian shot noise in the negative di!erential resistance range was
observed by Iannaccone et al. [240]. Kuznetsov et al. [232] have presented a detailed investigation
of the noise oscillations from sub-Poissonian to super-Poissonian values of a resonant quantum
well in the presence of a parallel magnetic "eld. The magnetic "eld leads to multiple voltage ranges
of negative di!erential resistance and permits a clear demonstration of the e!ect. Their results are
shown in Fig. 32.
To conclude this section, we discuss the following issue. To obtain the super-Poissonian noise
enhancement, we needed multi-stable behavior of the I}< curve; in turn, the multi-stability in
quantum wells is induced by charging e!ects. It is easy to see, however, that the charging (or,
generally, interaction) e!ects are not required to cause the multi-stability. Thus, if instead of
a voltage-controlled experiment, we discuss a current-controlled experiment, the I}< characteristics
for the uncharged quantum well (Fig. 30) are multi-stable for any external current. For the
case of an arbitrary load line there typically exists a "nite range of external parameters where
multi-stable behavior is developed. Furthermore, the quantum wells are not the only systems with
multi-stability; as one well-known example we mention Esaki diodes, where the multi-stability is
caused by the structure of the energy bands [243].