The amazing climbing ability of geckos has attracted the interest of
philosophers and scientists alike for centuries. However,
only in the past few years has progress been made in
understanding the ...mechanism behind this ability, which relies on submicrometre
keratin hairs covering the soles of geckos. Each hair produces a miniscule
force 10−7 N (due to van der Waals and/or capillary
interactions) but millions of hairs acting together create a formidable
adhesion of 10 N cm−2: sufficient to keep geckos
firmly on their feet, even when upside down on a glass ceiling. It is very
tempting to create a new type of adhesive by mimicking the
gecko mechanism. Here we report on a prototype of such 'gecko tape' made by
microfabrication of dense arrays of flexible plastic pillars, the geometry of
which is optimized to ensure their collective adhesion. Our approach shows a
way to manufacture self-cleaning, re-attachable dry adhesives, although
problems related to their durability and mass production are yet to be
resolved.
We report on a direct search for sub-GeV dark photons (A^{'}), which might be produced in the reaction e^{-}Z→e^{-}ZA^{'} via kinetic mixing with photons by 100 GeV electrons incident on an active ...target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75×10^{9} electrons on target. We set new limits on the γ-A^{'} mixing strength and exclude the invisible A^{'} with a mass ≲100 MeV as an explanation of the muon g_{μ}-2 anomaly.
Graphene's linear dispersion relation makes its charge carriers behave as if they were massless. However, near the Dirac point where graphene's valence and conduction bands meet, electron-electron ...interactions cause this relation to diverge, such that it becomes strongly nonlinear and the effective carrier velocity doubles. A study reports measurements of the cyclotron mass in suspended graphene for carrier concentrations n varying over three orders of magnitude.
We point out that superconducting quantum computers are prospective for the simulation of the dynamics of spin models far from equilibrium, including nonadiabatic phenomena and quenches. The ...important advantage of these machines is that they are programmable, so that different spin models can be simulated in the same chip, as well as various initial states can be encoded into it in a controllable way. This opens an opportunity to use superconducting quantum computers in studies of fundamental problems of statistical physics such as the absence or presence of thermalization in the free evolution of a closed quantum system depending on the choice of the initial state as well as on the integrability of the model. In the present paper, we performed proof-of-principle digital simulations of two spin models, which are the central spin model and the transverse-field Ising model, using 5- and 16-qubit superconducting quantum computers of the IBM Quantum Experience. We found that these devices are able to reproduce some important consequences of the symmetry of the initial state for the system’s subsequent dynamics, such as the excitation blockade. However, lengths of algorithms are currently limited due to quantum gate errors. We also discuss some heuristic methods which can be used to extract valuable information from the imperfect experimental data.
Disorder-induced Anderson localization usually causes conducting materials to become insulating at low temperature. Graphene is a notable exception. But by increasing the carrier density in one ...graphene layer, a metal-insulator transition can be induced in an isolated second layer stacked above it.
The nematic phase transition in electronic liquids, driven by Coulomb interactions, represents a new class of strongly correlated electronic ground states. We studied suspended samples of bilayer ...graphene, annealed so that it achieves very high quasiparticle mobilities (greater than 10 6 square centimers per volt-second). Bilayer graphene is a truly two-dimensional material with complex chiral electronic spectra, and the high quality of our samples allowed us to observe strong spectrum reconstructions and electron topological transitions that can be attributed to a nematic phase transition and a decrease in rotational symmetry. These results are especially surprising because no interaction effects have been observed so far in bilayer graphene in the absence of an applied magnetic field.
There is an increasing amount of literature concerning electronic properties of graphene close to the neutrality point. Many experiments continue using the two-probe geometry or invasive contacts or ...do not control samples’ macroscopic homogeneity. We believe that it is helpful to point out some problems related to such measurements. By using experimental examples, we illustrate that the charge inhomogeneity induced by spurious chemical doping or metal contacts can lead to large systematic errors in assessing graphene’s transport properties and, in particular, its minimal conductivity. The problems are most severe in the case of two-probe measurements where the contact resistance is found to strongly vary as a function of gate voltage.
In the framework of compressional magnetohydrodynamics (MHD), we numerically studied the commonly accepted presumption that the Alfvénic turbulence is generated by the collisions between ...counter-propagating Alfvén waves (AWs). In the conditions typical for the low-beta solar corona and inner solar wind, we launched two counter-propagating AWs in the three-dimensional simulation box and analyzed polarization and spectral properties of perturbations generated before and after AW collisions. The observed post-collisional perturbations have different polarizations and smaller cross-field scales than the original waves, which supports theoretical scenarios with direct turbulent cascades. However, contrary to theoretical expectations, the spectral transport is strongly suppressed at the scales satisfying the classic critical balance of incompressional MHD. Instead, a modified critical balance can be established by colliding AWs with significantly shorter perpendicular scales. We discuss consequences of these effects for the turbulence dynamics and turbulent heating of compressional plasmas. In particular, solar coronal loops can be heated by the strong turbulent cascade if the characteristic widths of the loop substructures are more than ten times smaller than the loop width. The revealed new properties of AW collisions have to be incorporated in the theoretical models of AW turbulence and related applications.
Nanostructured precursors of yttria-alumina garnet powders have been synthesized using the glycine-nitrate technique. It was shown that the main characteristics of the powders are determined by the ...glycine/nitrate (G/N) molar ratio in the initial solution and the temperature of the heat treatment of the precursors. The maximum specific surface area of the powders was obtained for the G/N ratio of 0.4. Subsequently, the powders were subjected to mechanical treatment and sintering. The effects of the treatment conditions and the amount of the sintering additive (Sc
2
O
3
) on the properties of the resulting ceramics were studied using erbium-yttrium garnet (Er
1,5
Y
1,5
Al
5
O
12
) as an example. Ceramics with a transparency of approximately 70% were obtained at 1790°С.
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