Blocking-state influence on shot noise and conductance in quantum dots

Abstract

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. arXiv:1801.00286v1 [cond-mat.mes-hall]

Strongly Correlated Charge Transport in Silicon Metal-Oxide-Semiconductor Field-Effect Transistor Quantum Dots

“…In refs. [16,17], the structure of the excited states suggest that the enhanced shot noise is due to COSET processes; however, no clear modulations in the Fano factor, such as the ones predicted in refs. [10,12] that discriminate between the mechanisms described above, have been observed so far.…”

“…Inelastic cotunneling events will then randomly switch the quantum dot between states with different (albeit small) conductances, yielding super-Poissonian fluctuations [10,12]. This latter regime occurs if ∆ * < E c 3, where ∆ * is the excited state energy and E c the charging energy [10,12,17,19,21,23]. Previous experimental works reported the observation of super-Poissonian noise in carbon nanotubes [8,17] and in GaAs/AlGaAs [16] quantum dots in the inelastic cotunneling regime.…”

“…Coulomb-blockaded quantum dots are very rich systems, as they can not only present Poissonian and sub-Poissonian transport regimes [1,4], but also, depending on their internal structure, strongly correlated transport with super-Poissonian current fluctuations [5][6][7][8][9][10][11][12][13][14][15][16][17]. The latter regime corresponds to non-Markovian transport processes where the transfer of a charge across the dot changes its state, thereby influencing the next transfer event [5].…”

“…This can happen in the sequential tunneling regime if several levels of a dot, with markedly different couplings to the leads, participate to transport. In that case the current across the quantum dot shows random telegraph signal features [15], yielding strongly enhanced fluctuations [6,[13][14][15]17]. Inelastic cotunneling [18] is also expected to enhance the current fluctuations, as it is, by definition, a charge transfer process that changes the state of the quantum dot [5].…”

“…N ⋆ ↔ N + 1) transition involving N ⋆ sits in the bias window, allowing direct transport through the dot after the cotunneling event [19,20]. This process, usually referred to as cotunneling-assisted tunneling (CAST [10,21], or COSET [12,17,22,23]), is depicted as dashed red arrows in Fig. 1.…”