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Observation of Dirac bands in artificial graphene in small-period nanopatterned GaAs quantum wells


Charge carriers in graphene behave like massless Dirac fermions (MDFs) with linear energy-momentum dispersion1, 2, providing a condensed-matter platform for studying quasiparticles with relativistic-like features. Artificial graphene (AG)—a structure with an artificial honeycomb lattice—exhibits novel phenomena due to the tunable interplay between topology and quasiparticle interactions3,4,5,6. So far, the emergence of a Dirac band structure supporting MDFs has been observed in AG using molecular5, atomic6, 7 and photonic systems8,9,10, including those with semiconductor microcavities11. Here, we report the realization of an AG that has a band structure with vanishing density of states consistent with the presence of MDFs. This observation is enabled by a very small lattice constant (a = 50 nm) of the nanofabricated AG patterns superimposed on a two-dimensional electron gas hosted by a high-quality GaAs quantum well. Resonant inelastic light-scattering spectra reveal low-lying transitions that are not present in the unpatterned GaAs quantum well. These excitations reveal the energy dependence of the joint density of states for AG band transitions. Fermi level tuning through the Dirac point results in a collapse of the density of states at low transition energy, suggesting the emergence of the MDF linear dispersion in the AG.

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Fig. 1: Principle of the realization of AG in a modulation-doped Al1–x Ga x As/GaAs QW.
Fig. 2: Nanofabrication of AG pattern on a modulation-doped Al0.1Ga0.9As/GaAs QW sample.
Fig. 3: Spectra of inter-AG-band transitions in sample A.
Fig. 4: Dependence of RILS on incident photon energy in sample A.
Fig. 5: Spectra of inter-AG-band transitions in sample B with a smaller Fermi energy than that in sample A.

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The work at Columbia University was supported by grant DE-SC0010695 from the US Department of Energy Office of Science, Division of Materials Sciences and Engineering, and by the National Science Foundation, Division of Materials Research, under award DMR-1306976. The growth of GaAs/AlGaAs QWs at Purdue University was supported by grant DE-SC0006671 from the US Department of Energy Office of Science, Division of Materials Sciences and Engineering. The growth of GaAs/AlGaAs QWs at Princeton University was supported by the Gordon and Betty Moore Foundation under award GMBF-2719 and by the National Science Foundation, Division of Materials Research, under award DMR-0819860. V.P. acknowledges the European Graphene Flagship (contract no. CNECT-ICT-604391) for financial support and the Italian Ministry of Research (MIUR) through the program ‘Progetti Premiali 2012’ – Project ‘ABNANOTECH’. The authors thank G.P. Watson for technical assistance with the ICP-RIE etching and A. Levy and F. Qiao for discussions.

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S.W. performed ICP-RIE processing and optical experiments. S.W. and L.D. performed numerical calculations. S.W., L.D., Y.Y.K. and A.P. analysed the data. D.S. and S.W. fabricated the AG lattices. G.C.G., M.J.M., K.W. and L.N.P. fabricated the QW samples. S.W., D.S., L.D., Y.Y.K., S.J.W. and A.P. co-wrote the paper with input from other authors. V.P., S.J.W. and A.P. conceived the experiments and supervised the project.

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Correspondence to Lingjie Du.

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Wang, S., Scarabelli, D., Du, L. et al. Observation of Dirac bands in artificial graphene in small-period nanopatterned GaAs quantum wells. Nature Nanotech 13, 29–33 (2018).

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