Tunable Valley Splitting and Bipolar Operation in Graphene Quantum Dots

Abstract

Quantum states in graphene are 2-fold degenerate in spins, and 2-fold in valleys. Both degrees of freedom can be utilized for qubit preparations. In our bilayer graphene quantum dots, we demonstrate that the valley g-factor g v, defined analogously to the spin g-factor g s for valley splitting in a perpendicular magnetic field, is tunable by over a factor of 4 from 20 to 90, by gate voltage adjustments only. Larger g v results from larger electronic dot sizes, determined from the charging energy. On our versatile device, bipolar operation, charging our quantum dot with charge carriers of the same or the opposite polarity as the leads, can be performed. Dots of both polarities are tunable to the first charge carrier, such that the transition from an electron to a hole dot by the action of the plunger gate can be observed. Addition of gates easily extends the system to host tunable double dots.

Spin-valley coupling in single-electron bilayer graphene quantum dots

“…4 c, d). The values of g v are similar to those reported in previous studies of similar BLG QDs and compatible with theoretical calculations 6 , 23 . Considering the same geometry of both QDs and the similar voltages applied to GL and GR, it is reasonable to assume that both QDs have very similar valley g -factors.…”

“…QDs are created using three layers of top gates, following previous studies of gate-defined BLG QDs 5 , 6 , 9 , 10 , 22 , 23 . A band gap is opened by applying an out-of-plane displacement field 24 , 25 with the help of the SG ( V SG = 1.73 V) and BG ( V BG = −1.56 V), while the Fermi energy ( E F ) is tuned into the band gap.…”

Kondo effect and spin–orbit coupling in graphene quantum dots

“…Improvements in the fabrication of nanostructures [15][16][17] in 2D materials pave the way to reveal Kondo physics in quantum dots electrostatically defined in a flat bilayer graphene sheet with small spin-orbit coupling. In addition an unusual two-hole triplet ground state 15 , and an exceptional tunability of tunnel rates, dot size and valley magnetic moment 18 are present. Measuring the Kondo resonance in different magnetic fields, allows to identify a clear level scheme for the first two charge carriers loaded into the dot as well as a spin orbit splitting of 80 μeV.…”

Single-shot readout in graphene quantum dots

“…In addition, the 2D nature of graphene allows for much smaller and possibly more strongly coupled quantum devices [16]. Furthermore, bilayer graphene quantum dots (QDs) offer the flexibility of bipolar operation [17]. Compared to the mature Si-based technology, the development of quantum devices in graphene is in its infancy.…”

“…Compared to the mature Si-based technology, the development of quantum devices in graphene is in its infancy. Recent advances in the controllability of individual states in single QDs [17][18][19][20] and double QDs [21,22], as well as the implementation of charge detection [23], enable the realization of spin qubits based on electrostatically defined QDs in bilayer graphene. Major milestones such as qubit manipulation and detection have yet to be achieved to unlock the qubit potential of graphene.…”