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Monolayer graphene aligned with hexagonal boron nitride ($h\text{\ensuremath{-}}\mathrm{BN}$) develops a gap at the charge neutrality point (CNP). This gap has previously been extensively studied by electrical transport through thermal activation measurements. Here, we report the determination of the gap size at the CNP of $\mathrm{graphene}/h\text{\ensuremath{-}}\mathrm{BN}$ superlattice through photocurrent spectroscopy study. We demonstrate two distinct measurement approaches to extract the gap size. A maximum of $\ensuremath{\sim}14\text{ }\text{ }\mathrm{meV}$ gap is observed for devices with a twist angle of less than 1\ifmmode^\circ\else\textdegree\fi{}. This value is significantly smaller than that obtained from thermal activation measurements, yet larger than the theoretically predicted single-particle gap. Our results suggest that lattice relaxation and moderate electron-electron interaction effects may enhance the CNP gap in $\mathrm{graphene}/h\text{\ensuremath{-}}\mathrm{BN}$ superlattice.

Monolayer graphene aligned with hexagonal boron nitride (h-BN) develops a gap at the charge neutrality point (CNP). This gap has previously been extensively studied by electrical transport through thermal activation measurements. Here, we report the determination of the gap size at the CNP of graphene/h-BN superlattice through photocurrent spectroscopy study. We demonstrate two distinct measurement approaches to extract the gap size. A maximum of ∼14 meV gap is observed for devices with a twist angle of less than 1°. This value is significantly smaller than that obtained from thermal activation measurements, yet larger than the theoretically predicted single-particle gap. Our results suggest that lattice relaxation and moderate electron-electron interaction effects may enhance the CNP gap in graphene/h-BN superlattice.

We address the effects of disorder on the conducting properties of metal and semiconducting carbon nanotubes. Experimentally, the mean free path is found to be much larger in metallic tubes than in doped semiconducting tubes. We show that this result can be understood theoretically if the disorder potential is long-ranged. The effects of a pseudospin index that describes the internal sublattice structure of the states lead to a suppression of scattering in metallic tubes, but not in semiconducting tubes. This conclusion is supported by tight-binding calculations., Comment: four pages

Ultrafast photocurrent measurements are performed on individual carbon nanotube p-i-n photodiodes. The photocurrent response to subpicosecond pulses separated by a variable time delayt shows strong photocurrent suppression when two pulses overlap (� t ¼ 0). The picosecond-scale decay time of photocurrent suppression scales inversely with the applied bias VSD, and is twice as long for photon energy above the second subband E22 as compared to lower energy. The observed photocurrent behavior is well described by an escape time model that accounts for carrier effective mass.

We have measured dc transport through a GaAs/AlGaAs quantum dot in the presence of a microwave signal of frequency $f$. We find features related to the photon energy $\mathrm{hf}$ whose positions in gate voltage are independent of the microwave power buy vary linearly with frequency. The measurements demonstrate photon-assisted tunneling in the mesoscopic regime. A comparison is made with a model that extends Coulomb blockade theory to include photon-assisted tunneling.

Semiconducting single-walled carbon nanotubes are studied in the diffusive transport regime. The peak mobility is found to scale with the square of the nanotube diameter and inversely with temperature. The maximum conductance, corrected for the contacts, is linear in the diameter and inverse temperature. These results are in good agreement with theoretical predictions for acoustic phonon scattering in combination with the unusual band structure of nanotubes. These measurements set the upper bound for the performance of nanotube transistors operating in the diffusive regime.

We use an Atomic Force Microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p-region, with no Schottky barrier. In the n-region large contact resistances were found which dominate the transport properties., 4 pages, 5 figures

We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a bandgap in a metallic NT and modify the bandgap in a semiconducting NT. Theoretical work predicts that bandgap changes can range between +100meV and -100meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range., Comment: 5 pages, 5 figures; minor changes and corrections

We have used quantum point contacts (QPCs) to locally create and probe dynamic nuclear polarization (DNP) in GaAs heterostructures in the quantum Hall regime. DNP is created via scattering between spin-polarized Landau level electrons and the Ga and As nuclear spins, and it leads to hysteresis in the dc transport characteristics. The nuclear origin of this hysteresis is demonstrated by nuclear magnetic resonance (NMR). Our results show that QPCs can be used to create and probe local nuclear spin populations, opening up new possibilities for mesoscopic NMR experiments.

Transport measurements of a Coulomb island, a semiconductor dot small enough that Coulomb interactions dominate transport, are presented. At moderate magnetic fields (B=2−4T) the amplitude and position of the Coulomb-regulated conductance peaks show distinct periodic structure as a function of B. This structure is shown to result from the B dependence of the quantized single-particle energy states on the island. Analysis of successive peaks is used to map out the single-particle level spectrum of the island as a function of B