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Probing relaxation times in graphene quantum dots.

Volk C, Neumann C, Kazarski S, Fringes S, Engels S, Haupt F, Müller A, Stampfer C - Nat Commun (2013)

Bottom Line: This is mainly due to challenges in device fabrication, in particular concerning the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times.This is achieved by an advanced device design that allows to individually tune the tunnelling barriers down to the low megahertz regime, while monitoring their asymmetry.Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60-100 ns.

View Article: PubMed Central - PubMed

Affiliation: JARA-FIT and II Institute of Physics B, RWTH Aachen, 52074 Aachen, Germany.

ABSTRACT
Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dots have been extensively investigated with respect to their excitation spectrum, spin-filling sequence and electron-hole crossover. However, their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular concerning the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here we report pulsed-gate transient current spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design that allows to individually tune the tunnelling barriers down to the low megahertz regime, while monitoring their asymmetry. Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60-100 ns.

No MeSH data available.


Related in: MedlinePlus

Graphene QD device and low-bias transport measurements.(a,b) Schematic and scanning force micrograph of the measured device. The central island is connected to source and drain electrodes by two 40 nm wide and 80 nm long constrictions, whose transparency can be tuned by the voltage applied to the nearby nanoribbons (VLG,VRG). The nanoribbon on the right hand side is also used as a charge detector for the dot. The central gate is connected to a bias-tee mixing AC and DC signals. (c) Current through the dot as a function of the left and right gate voltages VLG,VRG, recorded at a source-drain voltage VSD=−1.5 mV and VCG=0 mV. Here and in the following, the electron temperature is Te<100 mK, and the back gate voltage is VBG=34.6 V, corresponding to a Fermi level deep in the transport gap (Supplementary Fig. S1a). Different families of resonances can be identified in this measurement, which can either be attributed to the dot (features with relative lever arm 0.9; dotted line) or to localized states either in the left or right constriction (features with a relative lever arm of 5 and of 0.25, respectively; dashed lines). This indicates the possibility of tuning the transparency of the two constrictions independently, and controlling the current through the dot down to the sub-pA level by acting on VLG,VRG. (d) Simultaneous measurements of the current flowing through the dot and through the right nanoribbon as a function of VCG. The bias voltage applied to the dot and to the nanoribbon are VSD=−1.5 mV and VCD=0.2 mV, respectively. Barrier-gate voltages are VLG=0.4 V and VRG=0 V.
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f1: Graphene QD device and low-bias transport measurements.(a,b) Schematic and scanning force micrograph of the measured device. The central island is connected to source and drain electrodes by two 40 nm wide and 80 nm long constrictions, whose transparency can be tuned by the voltage applied to the nearby nanoribbons (VLG,VRG). The nanoribbon on the right hand side is also used as a charge detector for the dot. The central gate is connected to a bias-tee mixing AC and DC signals. (c) Current through the dot as a function of the left and right gate voltages VLG,VRG, recorded at a source-drain voltage VSD=−1.5 mV and VCG=0 mV. Here and in the following, the electron temperature is Te<100 mK, and the back gate voltage is VBG=34.6 V, corresponding to a Fermi level deep in the transport gap (Supplementary Fig. S1a). Different families of resonances can be identified in this measurement, which can either be attributed to the dot (features with relative lever arm 0.9; dotted line) or to localized states either in the left or right constriction (features with a relative lever arm of 5 and of 0.25, respectively; dashed lines). This indicates the possibility of tuning the transparency of the two constrictions independently, and controlling the current through the dot down to the sub-pA level by acting on VLG,VRG. (d) Simultaneous measurements of the current flowing through the dot and through the right nanoribbon as a function of VCG. The bias voltage applied to the dot and to the nanoribbon are VSD=−1.5 mV and VCD=0.2 mV, respectively. Barrier-gate voltages are VLG=0.4 V and VRG=0 V.

Mentions: Our devices consist of an etched graphene island (diameter, d=110 nm) connected to source and drain leads by long and narrow constrictions serving as tunnelling barriers (Fig. 1a). Two graphene nanoribbons, located symmetrically on each side of the island, can be used simultaneously as gates to tune the transparency of the barriers and as detectors sensitive to individual charging events in the dot2627. The electrostatic potential of the QD is controlled by a central gate, on which a bias-tee mixes alternate current (AC) and direct current (DC) signals (Fig. 1b). This allows performing pulsed-gate experiments with the same gate used for DC control. The device is tuned by a back gate in the low charge-carrier density regime, where transport is dominated by Coulomb blockade effects181920 (Supplementary Fig. S1a).


Probing relaxation times in graphene quantum dots.

Volk C, Neumann C, Kazarski S, Fringes S, Engels S, Haupt F, Müller A, Stampfer C - Nat Commun (2013)

Graphene QD device and low-bias transport measurements.(a,b) Schematic and scanning force micrograph of the measured device. The central island is connected to source and drain electrodes by two 40 nm wide and 80 nm long constrictions, whose transparency can be tuned by the voltage applied to the nearby nanoribbons (VLG,VRG). The nanoribbon on the right hand side is also used as a charge detector for the dot. The central gate is connected to a bias-tee mixing AC and DC signals. (c) Current through the dot as a function of the left and right gate voltages VLG,VRG, recorded at a source-drain voltage VSD=−1.5 mV and VCG=0 mV. Here and in the following, the electron temperature is Te<100 mK, and the back gate voltage is VBG=34.6 V, corresponding to a Fermi level deep in the transport gap (Supplementary Fig. S1a). Different families of resonances can be identified in this measurement, which can either be attributed to the dot (features with relative lever arm 0.9; dotted line) or to localized states either in the left or right constriction (features with a relative lever arm of 5 and of 0.25, respectively; dashed lines). This indicates the possibility of tuning the transparency of the two constrictions independently, and controlling the current through the dot down to the sub-pA level by acting on VLG,VRG. (d) Simultaneous measurements of the current flowing through the dot and through the right nanoribbon as a function of VCG. The bias voltage applied to the dot and to the nanoribbon are VSD=−1.5 mV and VCD=0.2 mV, respectively. Barrier-gate voltages are VLG=0.4 V and VRG=0 V.
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Related In: Results  -  Collection

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Show All Figures
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f1: Graphene QD device and low-bias transport measurements.(a,b) Schematic and scanning force micrograph of the measured device. The central island is connected to source and drain electrodes by two 40 nm wide and 80 nm long constrictions, whose transparency can be tuned by the voltage applied to the nearby nanoribbons (VLG,VRG). The nanoribbon on the right hand side is also used as a charge detector for the dot. The central gate is connected to a bias-tee mixing AC and DC signals. (c) Current through the dot as a function of the left and right gate voltages VLG,VRG, recorded at a source-drain voltage VSD=−1.5 mV and VCG=0 mV. Here and in the following, the electron temperature is Te<100 mK, and the back gate voltage is VBG=34.6 V, corresponding to a Fermi level deep in the transport gap (Supplementary Fig. S1a). Different families of resonances can be identified in this measurement, which can either be attributed to the dot (features with relative lever arm 0.9; dotted line) or to localized states either in the left or right constriction (features with a relative lever arm of 5 and of 0.25, respectively; dashed lines). This indicates the possibility of tuning the transparency of the two constrictions independently, and controlling the current through the dot down to the sub-pA level by acting on VLG,VRG. (d) Simultaneous measurements of the current flowing through the dot and through the right nanoribbon as a function of VCG. The bias voltage applied to the dot and to the nanoribbon are VSD=−1.5 mV and VCD=0.2 mV, respectively. Barrier-gate voltages are VLG=0.4 V and VRG=0 V.
Mentions: Our devices consist of an etched graphene island (diameter, d=110 nm) connected to source and drain leads by long and narrow constrictions serving as tunnelling barriers (Fig. 1a). Two graphene nanoribbons, located symmetrically on each side of the island, can be used simultaneously as gates to tune the transparency of the barriers and as detectors sensitive to individual charging events in the dot2627. The electrostatic potential of the QD is controlled by a central gate, on which a bias-tee mixes alternate current (AC) and direct current (DC) signals (Fig. 1b). This allows performing pulsed-gate experiments with the same gate used for DC control. The device is tuned by a back gate in the low charge-carrier density regime, where transport is dominated by Coulomb blockade effects181920 (Supplementary Fig. S1a).

Bottom Line: This is mainly due to challenges in device fabrication, in particular concerning the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times.This is achieved by an advanced device design that allows to individually tune the tunnelling barriers down to the low megahertz regime, while monitoring their asymmetry.Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60-100 ns.

View Article: PubMed Central - PubMed

Affiliation: JARA-FIT and II Institute of Physics B, RWTH Aachen, 52074 Aachen, Germany.

ABSTRACT
Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dots have been extensively investigated with respect to their excitation spectrum, spin-filling sequence and electron-hole crossover. However, their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular concerning the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here we report pulsed-gate transient current spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design that allows to individually tune the tunnelling barriers down to the low megahertz regime, while monitoring their asymmetry. Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60-100 ns.

No MeSH data available.


Related in: MedlinePlus