888 research outputs found
Genesis of the Floquet Hofstadter butterfly
We investigate theoretically the spectrum of a graphene-like sample
(honeycomb lattice) subjected to a perpendicular magnetic field and irradiated
by circularly polarized light. This system is studied using the Floquet
formalism, and the resulting Hofstadter spectrum is analyzed for different
regimes of the driving frequency. For lower frequencies, resonances of various
copies of the spectrum lead to intricate formations of topological gaps. In the
Landau-level regime, new wing-like gaps emerge upon reducing the driving
frequency, thus revealing the possibility of dynamically tuning the formation
of the Hofstadter butterfly. In this regime, an effective model may be
analytically derived, which allows us to retrace the energy levels that exhibit
avoided crossings and ultimately lead to gap structures with a wing-like shape.
At high frequencies, we find that gaps open for various fluxes at , and
upon increasing the amplitude of the driving, gaps also close and reopen at
other energies. The topological invariants of these gaps are calculated and the
resulting spectrum is elucidated. We suggest opportunities for experimental
realization and discuss similarities with Landau-level structures in non-driven
systems.Comment: 8 pages, 4 figure
A Logic of Blockchain Updates
Blockchains are distributed data structures that are used to achieve
consensus in systems for cryptocurrencies (like Bitcoin) or smart contracts
(like Ethereum). Although blockchains gained a lot of popularity recently,
there is no logic-based model for blockchains available. We introduce BCL, a
dynamic logic to reason about blockchain updates, and show that BCL is sound
and complete with respect to a simple blockchain model
Stability of HEB receivers at THz frequencies
Stability of a hot-electron bolometer (HEB) heterodyne receiver was investigated at frequencies from 0.6THz to 1.9THz. The Allan variance was measured as a function of the integration time and the Allan time was obtained for HEB mixers of different size, as well as with different types of the local oscillator: FIR laser, multiplier chain, and BWO. We have found that due to stronger dependence of the mixer gain and noise vs mixer bias voltage and current the Allan time is shorter for smaller mixers. At 1.6THz the Allan time is 3 sec for 4x0.4μm^2 bolometer, and 0.15-0.2 sec for 1x0.15μm^2 bolometer. Obtained stability apears to be the same for the FIR laser and the mulitplier chain. The Allan time for smaller bolometers increases to 0.4-0.5sec at 0.6-0.7THz LO frequencies. The influence of the IF chain on the obtained results is also analyzed
Mg/Ti multilayers: structural, optical and hydrogen absorption properties
Mg-Ti alloys have uncommon optical and hydrogen absorbing properties,
originating from a "spinodal-like" microstructure with a small degree of
chemical short-range order in the atoms distribution. In the present study we
artificially engineer short-range order by depositing Pd-capped Mg/Ti
multilayers with different periodicities and characterize them both
structurally and optically. Notwithstanding the large lattice parameter
mismatch between Mg and Ti, the as-deposited metallic multilayers show good
structural coherence. Upon exposure to H2 gas a two-step hydrogenation process
occurs, with the Ti layers forming the hydride before Mg. From in-situ
measurements of the bilayer thickness L at different hydrogen pressures, we
observe large out-of-plane expansions of the Mg and Ti layers upon
hydrogenation, indicating strong plastic deformations in the films and a
consequent shortening of the coherence length. Upon unloading at room
temperature in air, hydrogen atoms remain trapped in the Ti layers due to
kinetic constraints. Such loading/unloading sequence can be explained in terms
of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by
diffusion calculations on a model multilayered systems. Absorption isotherms
measured by hydrogenography can be interpreted as a result of the elastic
clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces
Detection of OH+ and H_2O+ towards Orion KL
We report observations of the reactive molecular ions OH+, H_(2)O+, and H_(3)O+ towards Orion KL with Herschel/HIFI. All three N = 1-0 fine-structure transitions of OH+ at 909, 971, and 1033 GHz and both fine-structure components of the doublet ortho-H_(2)O+ 1_(11)–0_(00) transition at 1115 and 1139 GHz were detected; an upper limit was obtained for H_(3)O+. OH+ and H_(2)O+ are observed purely in absorption, showing a narrow component at the source velocity of 9 km s^(-1), and a broad blueshifted absorption similar to that reported recently for HF and para-H_(2)^(18)O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH+ and H_(2)O+ for the 9 km s^(-1) component of 9 ± 3 × 10^(12) cm^(-2) and 7 ± 2 × 10^(12) cm^(-2), and those in the outflow of 1.9 ± 0.7 × 10^(13) cm^(-2) and 1.0 ± 0.3 × 10^(13) cm^(-2). Upper limits of 2.4 × 10^(12) cm^(-2) and 8.7 × 10^(12) cm^(-2) were derived for the column densities of ortho and para-H_(3)O+ from transitions near 985 and 1657 GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate
Field-effect transistors assembled from functionalized carbon nanotubes
We have fabricated field effect transistors from carbon nanotubes using a
novel selective placement scheme. We use carbon nanotubes that are covalently
bound to molecules containing hydroxamic acid functionality. The functionalized
nanotubes bind strongly to basic metal oxide surfaces, but not to silicon
dioxide. Upon annealing, the functionalization is removed, restoring the
electronic properties of the nanotubes. The devices we have fabricated show
excellent electrical characteristics.Comment: 5 pages, 6 figure
Mega-electron-volt ion beam induced anisotropic plasmon resonance of silver nanocrystals in glass
30 MeV Si ion beam irradiation of silica glass containing Ag nanocrystals causes alignment of Ag nanocrystals in arrays along the ion tracks. Optical transmission measurements show a large splitting of the surface plasmon resonance bands for polarizations longitudinal and transversal to the arrays. The splitting is in qualitative agreement with a model for near-field electromagnetic plasmon coupling within the arrays. Resonance shifts as large as 1.5 eV are observed, well into the near-infrared.
Sediment compaction rates and subsidence in deltaic plains : numerical constraints and stratigraphic influences
This paper is not subject to U.S. copyright. The definitive version was published in Basin Research 19 (2007): 19-31, doi:10.1111/j.1365-2117.2006.00310.x.Natural sediment compaction in deltaic plains influences subsidence rates and the evolution of deltaic morphology. Determining compaction rates requires detailed knowledge of subsurface geotechnical properties and depositional history, neither of which is often readily available. To overcome this lack of knowledge, we numerically forward model the incremental sedimentation and compaction of stochastically generated stratigraphies with geotechnical properties typical of modern depositional environments in the Mississippi River delta plain. Using a Monte Carlo approach, the range of probable compaction rates for stratigraphies with compacted thicknesses <150 m and accumulation times <20 kyr. varies, but maximum values rarely exceed a few mm yr-1. The fastest compacting stratigraphies are composed primarily of peat and bar sand, whereas the slowest compacting stratigraphies are composed of prodelta mud and natural levee deposits. These results suggest that compaction rates can significantly influence vertical and lateral stratigraphic trends during deltaic evolution
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