Geoff Gardner

Surface morphology evolution of m-plane (11¯00) GaN during molecular beam epitaxy growth: Impact of Ga/N ratio, miscut direction, and growth temperature

We present a systematic study of morphology evolution of [11¯00] m-plane GaN grown by plasma-assisted molecular beam epitaxy on free-standing m-plane substrates with small miscut angles towards the –c [0001¯] and +c [0001] directions under various gallium to nitrogen (Ga/N) ratios at substrate temperatures T = 720 °C and T = 740 °C. The miscut direction, Ga/N ratio, and growth temperature are all shown to have a dramatic impact on morphology. The observed dependence on miscut direction supports the notion of strong anisotropy in the gallium adatom diffusion barrier and growth kinetics. We demonstrate that precise control of Ga/N ratio and substrate temperature yields atomically smooth morphology on substrates oriented towards +c [0001] as well as the more commonly studied –c [0001¯] miscut substrates.

Quantitative analysis of the disorder broadening and the intrinsic gap for the

We report a reliable method to estimate the disorder broadening parameter from the scaling of the gaps of the even and major odd denominator fractional quantum Hall states of the second Landau level. We apply this technique to several samples of vastly different densities and grown in different molecular beam epitaxy chambers. Excellent agreement is found between the estimated intrinsic and numerically obtained energy gaps for the ν = 5/2 fractional quantum Hall state. Furthermore, we quantify the dependence of the intrinsic gap at ν = 5/2 on Landau-level mixing.

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The unusual electronic properties of graphene, which are a direct consequence of its two-dimensional honeycomb lattice, have attracted a great deal of attention in recent years. Creation of artificial lattices that re-create graphenes honeycomb topology, known as artificial graphene, can facilitate the investigation of graphenelike phenomena, such as the existence of massless Dirac fermions, in a tunable system. In this work, the authors present the fabrication of artificial graphene in an ultrahigh quality GaAs/AlGaAs quantum well, with lattice period as small as 50 nm, the smallest reported so far for this type of system. Electron-beam lithography is used to define an etch mask with honeycomb geometry on the surface of the sample, and different methodologies are compared and discussed. An optimized anisotropic reactive ion etching process is developed to transfer the pattern into the AlGaAs layer and create the artificial graphene. The achievement of such high-resolution artificial graphene should allo...

=5/2 fractional quantum Hall state

Engineered honeycomb lattices, called artificial graphene (AG), are tunable platforms for the study of novel electronic states related to Dirac physics. In this work, we report the achievement of electronic bands of the honeycomb topology with the period as low as 40 nm on the nano-patterned modulation-doped AlGaAs/GaAs quantum wells. Resonant inelastic light scattering spectra reveal peaks which are interpreted as combined electronic transitions between subbands of the quantum well confinement with a change in the AG band index. Spectra lineshapes are explained by joint density of states obtained from the calculated AG electron band structures. These results provide a basis for further advancements in AG physics.

Fabrication of artificial graphene in a GaAs quantum heterostructure

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 interactions3–6. So far, the emergence of a Dirac band structure supporting MDFs has been observed in AG using molecular5, atomic6, 7 and photonic systems8–10, 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.Advanced nanoengineering of small-period AG lattices enables the observation of a vanishing density of states that suggests the presence of massless Dirac fermions.