Quantum dots (QDs) investigated through electron transport measurements often
exhibit varying, state-dependent tunnel couplings to the leads. Under specific
conditions, weakly coupled states can result in a strong suppression of the
electrical current and they are correspondingly called blocking states. Using
the combination of conductance and shot noise measurements, we investigate
blocking states in carbon nanotube (CNT) QDs. We report negative differential
conductance and super-Poissonian noise. The enhanced noise is the signature of
electron bunching, which originates from random switches between the strongly
and weakly conducting states of the QD. Negative differential conductance
appears here when the blocking state is an excited state. In this case, at the
threshold voltage where the blocking state becomes populated, the current is
reduced. Using a master equation approach, we provide numerical simulations
reproducing both the conductance and the shot noise pattern observed in our
measurements.Comment: 10 pages, 7 figure