180 research outputs found

    Adsorption of Hexacontane on Hexagonal Boron Nitride

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    © 2018 American Chemical Society. We have investigated the adsorption of hexacontane (C60H122) on hexagonal boron nitride (hBN) using atomic force microscopy (AFM). The molecules can be deposited either by sublimation or from solution and form lamellar rows with dimensions of the order of 0.1-1 μm in three different rotational domains. High-resolution AFM images reveal that, similar to alkanes on graphite, the molecules are adsorbed parallel to the lattice vectors of hBN and we show using molecular mechanics that this corresponds to the lowest energy configuration. Lamellar rows with the same periodicity are observed even when several layers of hexacontane are deposited, although there is some orientational disorder in these multilayers. We also observe heat-induced modification of hexacontane, including recrystallization. We compare our results with recent X-ray studies of alkane adsorption on hBN and discuss the possible role of alkanes on steering molecular self-assembly on hBN

    Harmonic Content of Strain-induced Potential Modulation in Unidirectional Lateral Superlattices

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    Detailed analysis of the commensurability oscillation (CO) has been performed on unidirectional lateral superlattices with periods ranging from a=92 to 184 nm. Fourier analysis reveals the second (and the third) harmonics along with the fundamental oscillation for a>=138 nm (184 nm) at low-enough temperature, evincing the presence of corresponding harmonics in the profile of the potential modulation. The harmonics manifest themselves in CO with demagnified amplitude due to the low-pass filtering action of the thermal damping factor; with a suitable consideration of the damping effect, the harmonics of the modulation potential are found to have the amplitudes V_2 and V_3 up to roughly 30% of that of the fundamental component V_1, despite the small ratio of the period a to the depth d = 99 nm of the two-dimensional electron gas (2DEG) from the surface. The dependence of V_n on a indicates that the fundamental component originates at the surface, while the higher harmonics arise from the effect of the strain that penetrates down into subsurface. The manipulation of high harmonics thus provides a useful technique to introduce small length-scale modulation into high-mobility 2DEGs located deep inside the wafer.Comment: 9 pages, 5 figure

    Weiss Oscillations in Surface Acoustic Wave Propagation

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    The interaction of a surface acoustic wave (SAW) with a a two-dimensional electron gas in a periodic electric potential and a classical magnetic field is considered. We calculate the attenuation of the SAW and its velocity change and show that these quantities exhibit Weiss oscillations.Comment: 4 pages REVTEX, 2 figures included as eps file

    Magnetoresistance of a two-dimensional electron gas with spatially periodic lateral modulations: Exact consequences of Boltzmann's equation

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    On the basis of Boltzmann's equation, and including anisotropic scattering in the collision operator, we investigate the effect of one-dimensional superlattices on two-dimensional electron systems. In addition to superlattices defined by static electric and magnetic fields, we consider mobility superlattices describing a spatially modulated density of scattering centers. We prove that magnetic and electric superlattices in xx-direction affect only the resistivity component ρxx\rho_{xx} if the mobility is homogeneous, whereas a mobility lattice in xx-direction in the absence of electric and magnetic modulations affects only ρyy\rho_{yy}. Solving Boltzmann's equation numerically, we calculate the positive magnetoresistance in weak magnetic fields and the Weiss oscillations in stronger fields within a unified approach.Comment: submitted to PR

    Planar cyclotron motion in unidirectional superlattices defined by strong magnetic and electric fields: Traces of classical orbits in the energy spectrum

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    We compare the quantum and the classical description of the two-dimensional motion of electrons subjected to a perpendicular magnetic field and a one-dimensional lateral superlattice defined by spatially periodic magnetic and electric fields of large amplitudes. We explain in detail the complicated energy spectra, consisting of superimposed branches of strong and of weak dispersion, by the correspondence between the respective eigenstates and the ``channeled'' and ``drifting'' orbits of the classical description.Comment: 11 pages, 11 figures, to appear in Physical Review

    Magnetotunneling as a Probe of Luttinger-Liquid Behavior

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    A novel method for detecting Luttinger-liquid behavior is proposed. The idea is to measure the tunneling conductance between a quantum wire and a parallel two-dimensional electron system as a function of both the potential difference between them, VV, and an in-plane magnetic field, BB. We show that the two-parameter dependence on BB and VV allows for a determination of the characteristic dependence on wave vector qq and frequency ω\omega of the {\it spectral function}, ALL(q,ω)A_{\rm LL}(q,\omega), of the quantum wire. In particular, the separation of spin and charge in the Luttinger liquid should manifest itself as singularities in the II-VV-characteristic. The experimental feasibility of the proposal is discussed.Comment: Accepted for publication in Phys. Rev. Let

    Anisotropic scattering and quantum magnetoresistivities of a periodically modulated 2D electron gas

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    We calculate the longitudinal conductivities of a two-dimensional noninteracting electron gas in a uniform magnetic field and a lateral electric or magnetic periodic modulation in one spatial direction, in the quantum regime. We consider the effects of the electron-impurity scattering anisotropy through the vertex corrections on the Kubo formula, which are calculated with the Bethe-Salpeter equation, in the self-consistent Born approximation. We find that due to the scattering anisotropy the band conductivity increases, and the scattering conductivities decrease and become anisotropic. Our results are in qualitative agreement with recent experiments.Comment: 19 pages, 8 figures, Revtex, to appear in Phys. Rev.

    Room temperature electroluminescence from mechanically formed van der Waals III–VI homojunctions and heterojunctions

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    Room temperature electroluminescence from semiconductor junctions is demonstrated. The junctions are fabricated by the exfoliation and direct mechanical adhesion of InSe and GaSe van der Waals layered crystals. Homojunction diodes formed from layers of p- and n-type InSe exhibit electroluminescence at energies close to the bandgap energy of InSe (Eg= 1.26 eV). In contrast, heterojunction diodes formed by combining layers of p-type GaSe and n-type InSe emit photons at lower energies, which is attributed to the generation of spatially indirect excitons and a staggered valence band lineup for the holes at the GaSe/InSe interface. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct-bandgap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near-infrared to the visible spectrum

    Resonant tunnelling into the two-dimensional subbands of InSe layers

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    Two-dimensional (2D) van der Waals (vdW) crystals have attracted considerable interest for digital electronics beyond Si-based complementary metal oxide semiconductor technologies. Despite the transformative success of Si-based devices, there are limits to their miniaturization and functionalities. Here we realize a resonant tunnelling transistor (RTT) based on a 2D InSe layer sandwiched between two multi-layered graphene (MLG) electrodes. In the RTT the energy of the quantum-confined 2D subbands of InSe can be tuned by the thickness of the InSe layer. By applying a voltage across the two MLG electrodes, which serve as the source and drain electrodes to the InSe, the chemical potential in the source can be tuned in and out of resonance with a given 2D subband, leading to multiple regions of negative differential conductance that can be additionally tuned by electrostatic gating. This work demonstrates the potential of InSe and InSe-based RTTs for applications in quantum electronics.

    Absence of long-range ordered reconstruction on the GaAs(311)A surface

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    We have investigated the decapped GaAs(311)A surface using both scanning tunneling microscopy and synchrotron-radiation photoemission. While our data are in broad agreement with the structural model of GaAs(311)A proposed in a recent study [Wassermeier et al., Phys. Rev. B 51, 14 721 (1995)], we find considerable differences in the surface order. In particular, the As dimer rows are unbroken over much shorter length scales and are highly kinked. We observe a correspondingly lower degree of anisotropy in the surface roughness than that previously reported. An (8×1) reconstruction was not observed. An analysis of As 3d and Ga 3d core-level photoemission spectra suggests that surface As atoms are in only one bonding configuration while surface Ga adopts two different bonding states. We discuss possible origins for the core-level spectra surface components
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