Electronic Excited States in Bilayer Graphene Double Quantum Dots

Abstract

We report tunneling spectroscopy experiments on a bilayer graphene double quantum dot device that can be tuned by all-graphene lateral gates. The diameter of the two quantum dots are around 50 nm and the constrictions acting as tunneling barriers are 30 nm in width. The double quantum dot features addition energies on the order of 20 meV. Charge stability diagrams allow us to study the tunable interdot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points over a large energy range. The obtained constant level spacing of 1.75 meV over a wide energy range is in good agreement with the expected single-particle energy spacing in bilayer graphene quantum dots. Finally, we investigate the evolution of the electronic excited states in a parallel magnetic field.Keywords: graphene, bilayer graphene, quantum dot, double quantum dot, excited states Graphene quantum dots (QDs) are interesting candidates for spin qubits with long coherence times [1]. The suppressed hyperfine interaction and weak spin-orbit coupling [2, 3] make graphene and flat carbon structures in general, promising for future quantum information technology [4]. Significant progress has been made recently in the fabrication and understanding of graphene quantum devices. A "paper-cutting" technique enables the fabrication of graphene nanoribbons [5][6][7][8][9][10][11][12][13], quantum dots [14][15][16][17][18][19], and double quantum dot devices [20][21][22][23], where a disorder-induced energy gap allows confinement of individual carriers in graphene. These devices allowed the experimental investigation of excited states [16,21], spin states [19] and the electron-hole crossover [17]. However, all of these studies were based on single-layer graphene and showed a number of device limitations related to the presence of disorder, vibrational excitations and to the fact that the missing band gap makes it difficult to realize soft confinement potentials and "well-behaving" tunneling barriers. In particular, it has been shown that intrinsic ripples and corrugations in single-layer graphene can lead to unintended vibrational degrees of freedom [24] and to a coherent electron-vibron coupling in graphene QDs [25]. Bilayer graphene is a promising candidate to overcome some of these limitations. In particular it allows to open a band gap by an out-of-plane electric field [26][27][28], which may enable a soft confinement potential and may reduce the influence of localized edge states. More importantly, it has been shown that ripples and substrate-induced disorder are reduced in bilayer graphene [29], which increases the mechanical stability and suppresses unwanted vibrational modes.Here, we present a bilayer graphene double quantum dot (DQD) device, with a number of lateral gates. These local gates allow to tune transport from hole to electron dominated regimes and they enable to access different device configurations. We focus on the DQD configuration and show characteristic honeycomb-like charge stability dia...

Single-electron transport in graphene-like nanostructures

“…Graphene has been predicted to be particularly suitable for preparing spin-based qubits because of its weak spin-orbit interaction and hyper-fine effect [83], which should lead to a long spin relaxation time. However, although the 0D states in a GQD have shown the ability to store spin (see section 3.2.3), spin-related transport phenomena such as the Kondo-effect [115] and spin blockade have thus far not been observed in GQDs [86,87,[116][117][118][119][120][121]. It has been reported that the spin relaxation time in monolayer graphene ranges from 100 ps to 2 ns, significantly shorter than theoretically predicted [98-100, 122, 123].…”

“…Although attempts to probe the spin dynamics in such a system have failed, the control of confined charges in GDQDs can still be achieved. These include gate-tunable interdot coupling [86,87,119,126] and charge pumping [88], which are discussed in the following sections.…”

Magnetic-field-induced charge redistribution in disordered graphene double quantum dots

“…1(e). The difference in charging energies reflects the fact that the sizes of the dots are not equal and can be justified if the tunnel barriers defined by local disorder potential modify the size of the GQDs [7,13]. Within this picture, electrons from the source reservoir enter through a large localized state in QD 1 (E C1 is small) to a small localized state in QD 2 (E C2 is large) and then exit through the drain reservoir.…”

“…For example, the Fock-Darwin spectrum in the few-electron regime [5] and many-electron regime [11] as well as the Zeeman splitting of spin states [12] in a graphene single quantum dot have been studied. On the other hand, although it is well known that electron transport through graphene nanostructures is strongly affected by electron/hole puddles induced by potential fluctuations [7,13,14], detailed experimental and theoretical studies are lacking to address this issue in GQD transport. In this paper, we study the effect of disorder by investigating the transport properties of a large GDQD device at magnetic fields in which Landau levels (LLs) are expected to form.…”

Modeling charge relaxation in graphene quantum dots induced by electron-phonon interaction

“…Instead, the most viable way to confine charge carriers in graphene nowadays is based on etching nanostructures into graphene flakes [15]. So far, Coulomb blockade [16][17][18][19], excited states [20][21][22], charge sensing [23,24], and spin-filling sequences [25] have been studied in detail in etched graphene quantum dot devices. More recently, also charge pumps [26] and charge relaxation times of excited states [27] in graphene quantum dots have been investigated experimentally.…”