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Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots.

Marquardt B, Geller M, Baxevanis B, Pfannkuche D, Wieck AD, Reuter D, Lorke A - Nat Commun (2011)

Bottom Line: For these systems, great progress has been made in addressing spin states by optical means.The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method.An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum.

View Article: PubMed Central - PubMed

Affiliation: Fakultät für Physik and CeNIDE, Universität Duisburg-Essen, Lotharstraße 1, Duisburg 47048, Germany.

ABSTRACT
Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature.

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Schematic sample geometry and measurement technique.(a) The transistor structure, which is used to record the time-dependent tunnelling into self-assembled QDs. A voltage pulse applied to the gate electrode (b) (top) will abruptly change the carrier density in the electron channel (2DEG). The corresponding change in resistance is observed in the time-resolved measurement of the source-drain current ISD (VSD= constant) (b) (bottom). The voltage pulse will also strongly shift the chemical potential of the dots embedded in the dielectric between the gate and the 2DEG. This creates a non-equilibrium situation, in which electrons from the 2DEG can tunnel into excited QD states (right inset) (b). The time-dependent transfer of charge into the dots, QQD(t), will cause a decrease in carrier density of the 2DEG, which is detected by recording the change in source-drain current ΔISD(t) (c).
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f1: Schematic sample geometry and measurement technique.(a) The transistor structure, which is used to record the time-dependent tunnelling into self-assembled QDs. A voltage pulse applied to the gate electrode (b) (top) will abruptly change the carrier density in the electron channel (2DEG). The corresponding change in resistance is observed in the time-resolved measurement of the source-drain current ISD (VSD= constant) (b) (bottom). The voltage pulse will also strongly shift the chemical potential of the dots embedded in the dielectric between the gate and the 2DEG. This creates a non-equilibrium situation, in which electrons from the 2DEG can tunnel into excited QD states (right inset) (b). The time-dependent transfer of charge into the dots, QQD(t), will cause a decrease in carrier density of the 2DEG, which is detected by recording the change in source-drain current ΔISD(t) (c).

Mentions: The investigated sample consists of an inverted high electron mobility transistor (HEMT) with an embedded ensemble of self-assembled InAs QDs1165; see Figure 1a and the Methods section for details. As the electron channel, which supplies the carriers for the QDs, we employ a two-dimensional electron gas (2DEG)17. The tunnelling barrier between the 2DEG and QDs was chosen so that the tunnelling time is in the range of a few milliseconds. When a voltage pulse is applied to the gate, the carrier density and thus the conductivity of the 2DEG will change with a short RC time constant tRC=300 μs. At the same time, the energy levels of the dots, embedded in the dielectric of the HEMT, will shift1 and electrons will start to tunnel between the 2DEG and the dots. This increase in the number of electrons per dots will lead to a time-dependent decrease of the carrier density in the 2DEG18, which can be monitored with a high resolution by recording its conductivity. Thus, applying a constant voltage across the source and drain of the HEMT and recording the current transient ISD(t) will give a direct measure of the time-dependent tunnelling into the dots19. Because the response time of the 2DEG to an applied gate voltage pulse is much shorter than the typical tunnelling times, the present setup allows us to prepare non-equilibrium situations, in which the chemical potentials in the 2DEG and the dot layer differ greatly and tunnelling can take place over a wide range of energies (see insets in Fig. 1b). As we will show in the following section, this makes it possible to investigate excited QD states, as in n−i−n tunnelling structures2021, however, with adjustable, well-defined initial charge (zero, one or two electrons per dot).


Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots.

Marquardt B, Geller M, Baxevanis B, Pfannkuche D, Wieck AD, Reuter D, Lorke A - Nat Commun (2011)

Schematic sample geometry and measurement technique.(a) The transistor structure, which is used to record the time-dependent tunnelling into self-assembled QDs. A voltage pulse applied to the gate electrode (b) (top) will abruptly change the carrier density in the electron channel (2DEG). The corresponding change in resistance is observed in the time-resolved measurement of the source-drain current ISD (VSD= constant) (b) (bottom). The voltage pulse will also strongly shift the chemical potential of the dots embedded in the dielectric between the gate and the 2DEG. This creates a non-equilibrium situation, in which electrons from the 2DEG can tunnel into excited QD states (right inset) (b). The time-dependent transfer of charge into the dots, QQD(t), will cause a decrease in carrier density of the 2DEG, which is detected by recording the change in source-drain current ΔISD(t) (c).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3105341&req=5

f1: Schematic sample geometry and measurement technique.(a) The transistor structure, which is used to record the time-dependent tunnelling into self-assembled QDs. A voltage pulse applied to the gate electrode (b) (top) will abruptly change the carrier density in the electron channel (2DEG). The corresponding change in resistance is observed in the time-resolved measurement of the source-drain current ISD (VSD= constant) (b) (bottom). The voltage pulse will also strongly shift the chemical potential of the dots embedded in the dielectric between the gate and the 2DEG. This creates a non-equilibrium situation, in which electrons from the 2DEG can tunnel into excited QD states (right inset) (b). The time-dependent transfer of charge into the dots, QQD(t), will cause a decrease in carrier density of the 2DEG, which is detected by recording the change in source-drain current ΔISD(t) (c).
Mentions: The investigated sample consists of an inverted high electron mobility transistor (HEMT) with an embedded ensemble of self-assembled InAs QDs1165; see Figure 1a and the Methods section for details. As the electron channel, which supplies the carriers for the QDs, we employ a two-dimensional electron gas (2DEG)17. The tunnelling barrier between the 2DEG and QDs was chosen so that the tunnelling time is in the range of a few milliseconds. When a voltage pulse is applied to the gate, the carrier density and thus the conductivity of the 2DEG will change with a short RC time constant tRC=300 μs. At the same time, the energy levels of the dots, embedded in the dielectric of the HEMT, will shift1 and electrons will start to tunnel between the 2DEG and the dots. This increase in the number of electrons per dots will lead to a time-dependent decrease of the carrier density in the 2DEG18, which can be monitored with a high resolution by recording its conductivity. Thus, applying a constant voltage across the source and drain of the HEMT and recording the current transient ISD(t) will give a direct measure of the time-dependent tunnelling into the dots19. Because the response time of the 2DEG to an applied gate voltage pulse is much shorter than the typical tunnelling times, the present setup allows us to prepare non-equilibrium situations, in which the chemical potentials in the 2DEG and the dot layer differ greatly and tunnelling can take place over a wide range of energies (see insets in Fig. 1b). As we will show in the following section, this makes it possible to investigate excited QD states, as in n−i−n tunnelling structures2021, however, with adjustable, well-defined initial charge (zero, one or two electrons per dot).

Bottom Line: For these systems, great progress has been made in addressing spin states by optical means.The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method.An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum.

View Article: PubMed Central - PubMed

Affiliation: Fakultät für Physik and CeNIDE, Universität Duisburg-Essen, Lotharstraße 1, Duisburg 47048, Germany.

ABSTRACT
Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature.

Show MeSH