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Localized electrons subject to applied magnetic fields can restart to propagate freely through the lattice in delocalized magnetic Bloch states (MBSs) when the lattice periodicity is commensurate with the magnetic length. Twisted graphene superlattices with moir\'e wavelength tunability enable experimental access to the unique delocalization in a controllable fashion. Here we report the observation and characterization of high-temperature Brown-Zak (BZ) oscillations which come in two types, 1/B and B periodicity, originating from the generation of integer and fractional MBSs, in the twisted bilayer and trilayer graphene superlattices, respectively. Coexisting periodic-in-1/B oscillations assigned to different moir\'e wavelengths, are dramatically observed in small-angle twisted bilayer graphene, which may arise from angle-disorder-induced in-plane heteromoir\'e superlattices. Moreover, the vertical stacking of heteromoir\'e supercells in double-twisted trilayer graphene results in a mega-sized superlattice. The exotic superlattice contributes to the periodic-in-B oscillation and dominates the magnetic Bloch transport.

Using wide spectral range in situ spectroscopic ellipsometry with systematic ultra high vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS$_2$ isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A ($1.92$ eV) and B ($2.02$ eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the bandstructure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This work unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS$_2$., Comment: Accepted in Physical Review Letters; Main Article: 6 pages, 4 figures; Supplemental Material: 6 pages, 7 figures, 10 tables

Vertically aligned, beaded zinc germinate (Zn2GeO4)/ zinc oxide (ZnO) hybrid nanowire arrays were successfully synthesized by a vapor-solid process via a catalyst-free approach. The as-synthesized products were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with an energy-dispersive X-ray spectrometer. TEM studies revealed the beaded microstructures of the Zn2GeO4/ZnO nanowire. Furthermore, Zn2GeO4 nanotubes were synthesized after wet etching treatments on Zn2GeO4/ZnO hybrid nanowires.

We demonstrate that a direct focused laser beam irradiation is able to achieve localized modification on GeSe2 nanostructures (NSs) film. Using scanning focused laser beam setup, micropatterns on GeSe2 NSs film are created directly on the substrate. Controlled structural and chemical changes of the NSs are achieved by varying laser power and treatment environment. The laser modified GeSe2 NSs exhibit distinct optical, electrical and optoelectrical properties. Detailed characterization is carried out and the possible mechanisms for the laser induced changes are discussed. The laser modified NSs film shows superior photoconductivity properties as compared to the pristine nanostructure film. The construction of micropatterns with improved functionality could prove to be useful in miniature optoelectrical devices.

Single crystal GeSe2 nanobelts (NBs) were successfully grown using chemical vapor deposition techniques. The morphology and structure of the nanostructures were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and Raman spectroscopy. Electronic transport properties, photoconductive characteristics, and temperature-dependent electronic characteristics were examined on devices made of individual GeSe2 nanobelt. Localized photoconductivity study shows that the large photoresponse of the device primarily occurs at the metal-NB contact regions. In addition, the electrically Schottky nature of nanobelt-Au contact and p-type conductivity nature of GeSe2 nanobelt are extracted from the current-voltage characteristics and spatially resolved photocurrent measurements. The high sensitivity and quick photoresponse in the visible wavelength range indicate potential applications of individual GeSe2 nanobelt devices in realizing optoelectronic switches.

We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the I-V curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination at a wavelength of 808 nm. The significant photoresponse and good responsitivity suggests its potential applications as photodetectors.

We examine metastable configurations of a two-dimensional system of interacting particles on a quenched random potential landscape and ask how the configurational pair correlation function is related to the particle interactions and the statistical properties of the potential landscape. Understanding this relation facilitates quantitative studies of magnetic flux line interactions in type II superconductors, using structural information available from Lorentz microscope images or Bitter decorations. Previous work by some of us supported the conjecture that the relationship between pair correlations and interactions in pinned flux line ensembles is analogous to the corresponding relationship in the theory of simple liquids. The present paper aims at a more thorough understanding of this relation. We report the results of numerical simulations and present a theory for the low density behavior of the pair correlation function which agrees well with our simulations and captures features observed in experiments. In particular, we find that the resulting description goes beyond the conjectured classical liquid type relation and we remark on the differences., Comment: 7 pages, 6 figures. See also http://rainbow.uchicago.edu/~grier/