224 research outputs found
The simulation of ionospheric conditions for space vehicles
Plasma wind tunnel to simulate ionospheric conditions for space vehicle
Anisotropic Etching of Graphite and Graphene in a Remote Hydrogen Plasma
We investigate the etching of a pure hydrogen plasma on graphite samples and
graphene flakes on SiO and hexagonal Boron-Nitride (hBN) substrates. The
pressure and distance dependence of the graphite exposure experiments reveals
the existence of two distinct plasma regimes: the direct and the remote plasma
regime. Graphite surfaces exposed directly to the hydrogen plasma exhibit
numerous etch pits of various size and depth, indicating continuous defect
creation throughout the etching process. In contrast, anisotropic etching
forming regular and symmetric hexagons starting only from preexisting defects
and edges is seen in the remote plasma regime, where the sample is located
downstream, outside of the glowing plasma. This regime is possible in a narrow
window of parameters where essentially all ions have already recombined, yet a
flux of H-radicals performing anisotropic etching is still present. At the
required process pressures, the radicals can recombine only on surfaces, not in
the gas itself. Thus, the tube material needs to exhibit a sufficiently low H
radical recombination coefficient, such a found for quartz or pyrex. In the
remote regime, we investigate the etching of single layer and bilayer graphene
on SiO and hBN substrates. We find isotropic etching for single layer
graphene on SiO, whereas we observe highly anisotropic etching for graphene
on a hBN substrate. For bilayer graphene, anisotropic etching is observed on
both substrates. Finally, we demonstrate the use of artificial defects to
create well defined graphene nanostructures with clean crystallographic edges.Comment: 7 pages, 4 color figure
Influence of magnetic impurities on the heat capacity of nuclear spins
It is found that in a wide range of temperatures and magnetic fields even a
small concentration of magnetic impurities in a sample leads to a
temperature dependence of the nuclear heat capacity. This effect is related to
a nuclear-spin polarization by the magnetic impurities. The parameter that
controls the theory turns out not to be the impurity concentration
but instead the quantity , where and are
the magnetic moments of an electron and a nucleus, respectively. The ratio of
and is of order of
Theory of Interplay of Nuclear Magnetism and Superconductivity in AuIn2
The recently reported coexistence of a magnetic order, with the critical
temperature T_M=35 \mu*K, and superconductivity, with the critical temperature
T_S=207 m*K, in AuIn_2 is studied theoretically. It is shown that
superconducting (S) electrons and localized nuclear magnetic moments (LM's)
interact dominantly via the contact hyperfine (EX) interaction, giving rise to
a spiral (or domain-like) magnetic order in superconducting phase. The
electromagnetic interaction between LM's and S electrons is small compared to
the EX one giving minor contribution to the formation of the oscillatory
magnetic order. In clean samples (l>\xi_0) of AuIn the oscillatory magnetic
order should produce a line of nodes in the quasiparticle spectrum of S
electrons giving rise to the power law behavior. The critical field H_c(T=0) in
the coexistence phase is reduced by factor two with respect to its bare value.Comment: 4 pages with 2 PS figures, RevTeX, submitted to Physical Review B -
Rapid Communication
Neutron dose rate at the SwissFEL injector test facility: first measurements
At the Paul Scherrer Institute, the new SwissFEL Free Electron Laser facility is currently in the design phase. It is foreseen to accelerate electrons up to a maximum energy of 7 GeV with a pulsed time structure. An injector test facility is operated at a maximum energy of 300 MeV and serves as the principal test and demonstration plant for the SwissFEL project. Secondary radiation is created in unavoidable interactions of the primary beam with beamline components. The resulting ambient dose-equivalent rate due to neutrons was measured along the beamline with different commercially available survey instruments. The present study compares the readings of these neutron detectors (one of them is specifically designed for measurements in pulsed fields). The experiments were carried out in both, a normal and a diagnostic mode of operation of the injecto
Energy- and flux-budget (EFB) turbulence closure model for the stably stratified flows. Part I: Steady-state, homogeneous regimes
We propose a new turbulence closure model based on the budget equations for
the key second moments: turbulent kinetic and potential energies: TKE and TPE
(comprising the turbulent total energy: TTE = TKE + TPE) and vertical turbulent
fluxes of momentum and buoyancy (proportional to potential temperature).
Besides the concept of TTE, we take into account the non-gradient correction to
the traditional buoyancy flux formulation. The proposed model grants the
existence of turbulence at any gradient Richardson number, Ri. Instead of its
critical value separating - as usually assumed - the turbulent and the laminar
regimes, it reveals a transition interval, 0.1< Ri <1, which separates two
regimes of essentially different nature but both turbulent: strong turbulence
at Ri<<1; and weak turbulence, capable of transporting momentum but much less
efficient in transporting heat, at Ri>1. Predictions from this model are
consistent with available data from atmospheric and lab experiments, direct
numerical simulation (DNS) and large-eddy simulation (LES).Comment: 40 pages, 6 figures, Boundary-layer Meteorology, resubmitted, revised
versio
Щодо утворення сімейств атомарних радіальних базисних функцій
Наведено схему побудови сімейств атомарних радіальних базисних функцій, які є нескінченно диференційовними фінітними розв'язками функціонально-диференціальних рівнянь, породжених операторами Лапласа та Гельмгольца.The scheme of building a family of atomic radial basis functions which are infinitely differentiable finite solutions of the functional-differential equations containing the Laplace and Helmholtz operators is introduced
Strong spin-orbit interaction and -factor renormalization of hole spins in Ge/Si nanowire quantum dots
The spin-orbit interaction lies at the heart of quantum computation with spin
qubits, research on topologically non-trivial states, and various applications
in spintronics. Hole spins in Ge/Si core/shell nanowires experience a
spin-orbit interaction that has been predicted to be both strong and
electrically tunable, making them a particularly promising platform for
research in these fields. We experimentally determine the strength of
spin-orbit interaction of hole spins confined to a double quantum dot in a
Ge/Si nanowire by measuring spin-mixing transitions inside a regime of
spin-blockaded transport. We find a remarkably short spin-orbit length of
65 nm, comparable to the quantum dot length and the interdot distance. We
additionally observe a large orbital effect of the applied magnetic field on
the hole states, resulting in a large magnetic field dependence of the
spin-mixing transition energies. Strikingly, together with these orbital
effects, the strong spin-orbit interaction causes a significant enhancement of
the -factor with magnetic field.The large spin-orbit interaction strength
demonstrated is consistent with the predicted direct Rashba spin-orbit
interaction in this material system and is expected to enable ultrafast Rabi
oscillations of spin qubits and efficient qubit-qubit interactions, as well as
provide a platform suitable for studying Majorana zero modes
The regulatory subunit of PKA-I remains partially structured and undergoes β-aggregation upon thermal denaturation
Background: The regulatory subunit (R) of cAMP-dependent protein kinase (PKA) is a modular flexible protein that responds with large conformational changes to the binding of the effector cAMP. Considering its highly dynamic nature, the protein is rather stable. We studied the thermal denaturation of full-length RIα and a truncated RIα(92-381) that contains the tandem cyclic nucleotide binding (CNB) domains A and B. Methodology/Principal Findings: As revealed by circular dichroism (CD) and differential scanning calorimetry, both RIα proteins contain significant residual structure in the heat-denatured state. As evidenced by CD, the predominantly α-helical spectrum at 25°C with double negative peaks at 209 and 222 nm changes to a spectrum with a single negative peak at 212-216 nm, characteristic of β-structure. A similar α→β transition occurs at higher temperature in the presence of cAMP. Thioflavin T fluorescence and atomic force microscopy studies support the notion that the structural transition is associated with cross-β-intermolecular aggregation and formation of non-fibrillar oligomers. Conclusions/Significance: Thermal denaturation of RIα leads to partial loss of native packing with exposure of aggregation-prone motifs, such as the B' helices in the phosphate-binding cassettes of both CNB domains. The topology of the β-sandwiches in these domains favors inter-molecular β-aggregation, which is suppressed in the ligand-bound states of RIα under physiological conditions. Moreover, our results reveal that the CNB domains persist as structural cores through heat-denaturation. © 2011 Dao et al
A Fluorescence-Based High-Throughput Assay for the Discovery of Exchange Protein Directly Activated by Cyclic AMP (EPAC) Antagonists
Background: The discovery, more than ten years ago, of exchange proteins directly activated by cAMP (EPAC) as a new family of intracellular cAMP receptors revolutionized the cAMP signaling research field. Extensive studies have revealed that the cAMP signaling network is much more complex and dynamic as many cAMP-related cellular processes, previously thought to be controlled by protein kinase A, are found to be also mediated by EPAC proteins. Although there have been many important discoveries in the roles of EPACs greater understanding of their physiological function in cAMP-mediated signaling is impeded by the absence of EPAC-specific antagonist. Methodology/Principal Findings: To overcome this deficit, we have developed a fluorescence-based high throughput assay for screening EPAC specific antagonists. Our assay is highly reproducible and simple to perform using the ‘‘mix and measure’ ’ format. A pilot screening using the NCI-DTP diversity set library led to the identification of small chemical compounds capable of specifically inhibiting cAMP-induced EPAC activation while not affecting PKA activity. Conclusions/Significance: Our study establishes a robust high throughput screening assay that can be effectively applied for the discovery of EPAC-specific antagonists, which may provide valuable pharmacological tools for elucidating th
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