Gate-Defined Electron–Hole Double Dots in Bilayer Graphene

Abstract

We present gate-controlled single-, double-, and triple-dot operation in electrostatically gapped bilayer graphene. Thanks to the recent advancements in sample fabrication, which include the encapsulation of bilayer graphene in hexagonal boron nitride and the use of graphite gates, it has become possible to electrostatically confine carriers in bilayer graphene and to completely pinch-off current through quantum dot devices. Here, we discuss the operation and characterization of electron-hole double dots. We show a remarkable degree of control of our device, which allows the implementation of two different gate-defined electron-hole double-dot systems with very similar energy scales. In the single-dot regime, we extract excited state energies and investigate their evolution in a parallel magnetic field, which is in agreement with a Zeeman-spin-splitting expected for a g-factor of 2.

Single-Electron Double Quantum Dots in Bilayer Graphene

“…Major achievements include the implementation of charge detection [15,16], the observation of spin-states [12] and the measurement of charge relaxation times [17]. However, the influence of disorder, in particular edge disorder [18,19], prevented a precise control of the number of charge carriers on individual QDs making spin-qubit implementation impossible.The advancements in ultra-clean van der Waals heterostructures and in particular the use of local graphite gates allowed for the development of electrostatically defined bilayer graphene (BLG) quantum point contacts [20-23], quantum dots [24-26] and double quantum dots (DQDs) [27,28]. While single-electron and hole occupation has been demonstrated recently for individual QDs [24], the number of charge carriers in DQDs could not be controlled yet [27,28].…”

“…The absence of excited state transitions at the (0, 0) → (1, 1) triple point is in agreement with spin and valley conserving interdot tunneling processes.The studied device consists of a BLG flake encapsulated in two hexagonal boron nitride (hBN) crystals, fabricated by mechanical exfoliation and a dry van-der-Waals pick-up technique [29,30]. The heterostructure is placed on a graphite flake, acting as a back gate [27]. On top of the stack, Cr/Au split gates are used to define a one-dimensional (1D) channel with an approximate width of 50 nm between the source and drain contacts.…”

“…For details of the fabrication process see Ref. [27]. All measurements are performed in a 3 He/ 4 He dilution refrigerator at a base temperature of around 10 mK using a combination of DC-measurements and standard low-frequency lock-in techniques.We use the back gate and the split gates to apply an out-of-plane electrostatic displacement field in order to open a band gap in the BLG below the split gate area [31].…”

Single-Carrier Transport in Graphene/hBN Superlattices

“…[13][14][15][16][17][18][19][20][21][22][23][24][25] To form a potential barrier for graphene QDs, geometric patterning has been performed on nanostructures by etching [13][14][15][16][17][18][19] or applying a perpendicular electric field to bilayer graphene (BLG). [20][21][22][23][24][25] For etching, the graphene channel must be narrowed to a few tens of nanometers to obtain a sizable gap. [13][14][15][16][17][18]26 Consequently, the carrier transport property is dominated by the unintentional carrier localization induced by edge disorders, leading to an unstable dot configuration.…”

“…On the other hand, with recent progress in fabrication technologies such as hBN encapsulation and the fabrication of a graphite back gate 3 through an all-dry transfer process, 27 high-quality BLG-based QD devices defined by electrostatically-induced potential barrier have been achieved under a perpendicular electric field. [22][23][24][25] In this work, to study the single-carrier transport in graphene superlattices, we fabricated a BLG/hBN moiré superlattice-based QD device (Figure 1c-e), in which the double dots are defined 3 by geometric patterning and local gating with the contribution of the superlattice potential. Note that widths of constrictions and diameters of the dots were slightly large compared to the previous graphene QD devices, [13][14][15][16][17][18] for avoiding strong potential fluctuation which leads to unstable dot configuration.…”