31,168 research outputs found
Nonperturbative signatures in pair production for general elliptic polarization fields
The momentum signatures in nonperturbative multiphoton pair production for
general elliptic polarization electric fields are investigated by employing the
real-time Dirac-Heisenberg-Wigner formalism. For a linearly polarized electric
field we find that the positions of the nodes in momenta spectra of created
pairs depend only on the electric field frequency. The polarization of external
fields could not only change the node structures or even make the nodes
disappear but also change the thresholds of pair production. The momentum
signatures associated to the node positions in which the even-number-photon
pair creation process is forbid could be used to distinguish the orbital
angular momentum of created pairs on the momenta spectra. These distinguishable
momentum signatures could be relevant for providing the output information of
created particles and also the input information of ultrashort laser pulses.Comment: 8 pages, 4 figures, submitted to Europhysics Letter
The classical nature of nuclear spin noise near clock transitions of Bi donors in silicon
Whether a quantum bath can be approximated as classical noise is a
fundamental issue in central spin decoherence and also of practical importance
in designing noise-resilient quantum control. Spin qubits based on bismuth
donors in silicon have tunable interactions with nuclear spin baths and are
first-order insensitive to magnetic noise at so-called clock-transitions (CTs).
This system is therefore ideal for studying the quantum/classical nature of
nuclear spin baths since the qubit-bath interaction strength determines the
back-action on the baths and hence the adequacy of a classical noise model. We
develop a Gaussian noise model with noise correlations determined by quantum
calculations and compare the classical noise approximation to the full quantum
bath theory. We experimentally test our model through dynamical decoupling
sequence of up to 128 pulses, finding good agreement with simulations and
measuring electron spin coherence times approaching one second - notably using
natural silicon. Our theoretical and experimental study demonstrates that the
noise from a nuclear spin bath is analogous to classical Gaussian noise if the
back-action of the qubit on the bath is small compared to the internal bath
dynamics, as is the case close to CTs. However, far from the CTs, the
back-action of the central spin on the bath is such that the quantum model is
required to accurately model spin decoherence.Comment: 5 pages, 3 figure
Influence of Parent Concrete Properties on Compressive Strength and Chloride Diffusion Coefficient of Concrete with Strengthened Recycled Aggregates.
Parent concrete coming from a wide range of sources can result in considerable differences in the properties of recycled coarse aggregate (RCA). In this study, the RCAs were obtained by crushing the parent concrete with water-to-cement ratios (W/Cparent) of 0.4, 0.5 and 0.6, respectively, and were strengthened by carbonation and nano-silica slurry wrapping methods. It was found that when W/Cparen was 0.3, 0.4 and 0.5, respectively, compared with the mortar in the untreated RCA, the capillary porosity of the mortar in the carbonated RCA decreased by 19%, 16% and 30%, respectively; the compressive strength of concrete containing the carbonated RCA increased by 13%, 11% and 13%, respectively; the chloride diffusion coefficient of RAC (DRAC) containing the nano-SiO2 slurry-treated RCA decreased by 17%, 16% and 11%; and that of RAC containing the carbonated RCA decreased by 21%, 25% and 26%, respectively. Regardless of being strengthened or not, both DRAC and porosity of old mortar in RCAs increased with increasing W/Cparent. For different types of RCAs, DRAC increased obviously with increasing water absorption of RCA. Finally, a theoretical model of DRAC considering the water absorption of RCA was established and verified by experiments, which can be used to predict the DRAC under the influence of different factors, especially the water absorption of RCA
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Strain control of composite superconductors to prevent degradation of superconducting magnets due to a quench: I. Ag/Bi2Sr2CaCu2Ox multifilament round wires
The critical current of many practical superconductors is sensitive to strain, and this sensitivity is exacerbated during a quench that induces a peak local strain which can be fatal to superconducting magnets. Here, a new method is introduced to quantify the influence of the conductor stress and strain state during normal operation on the margin to degradation during a quench, as measured by the maximum allowable hot spot temperature T allowable, for composite wires within superconducting magnets. The first conductor examined is Ag-sheathed Bi2Sr2CaCu2Ox round wire carrying high engineering critical current density, J E, of 550 A mm-2 at 4.2 K and 15 T. The critical axial tensile stress of this conductor is determined to be 150 MPa and, in the absence of Lorentz forces, T allowable is greater than 450 K. With increasing axial tensile stress, σ a, however, T allowable decreases nonlinearly, dropping to 280 K for σ a = 120 MPa and to 160 K for σ a = 145 MPa. T allowable(σ a) is shown to be nonlinear and independent of magnetic field from 15 to 30 T. T allowable(σ a) dictates the balance between magnetic field generation, which increases with the magnet operating current and stress, and the safety margin, which decreases with decreasing T allowable, and therefore has important engineering value. It is also shown that T allowable(σ a) can be predicted accurately by a general strain model, showing that strain control is the key to preventing degradation of superconductors during a quench
Latest Observational Constraints to the Ghost Dark Energy Model by Using Markov Chain Monte Carlo Approach
Recently, the vacuum energy of the QCD ghost in a time-dependent background
is proposed as a kind of dark energy candidate to explain the acceleration of
the universe. In this model, the energy density of the dark energy is
proportional to the Hubble parameter , which is the Hawking temperature on
the Hubble horizon of the Friedmann-Robertson-Walker (FRW) universe. In this
paper, we perform a constraint on the ghost dark energy model with and without
bulk viscosity, by using the Markov Chain Monte Carlo (MCMC) method and the
combined latest observational data from the type Ia supernova compilations
including Union2.1(580) and Union2(557), cosmic microwave background, baryon
acoustic oscillation, and the observational Hubble parameter data.Comment: 12 pages, 4 figure
Uncovering many-body correlations in nanoscale nuclear spin baths by central spin decoherence
Many-body correlations can yield key insights into the nature of interacting
systems; however, detecting them is often very challenging in many-particle
physics, especially in nanoscale systems. Here, taking a phosphorus donor
electron spin in a natural-abundance 29Si nuclear spin bath as our model
system, we discover both theoretically and experimentally that many-body
correlations in nanoscale nuclear spin baths produce identifiable signatures in
the decoherence of the central spin under multiple-pulse dynamical decoupling
control. We find that when the number of decoupling -pulses is odd, central
spin decoherence is primarily driven by second-order nuclear spin correlations
(pairwise flip-flop processes). In contrast, when the number of -pulses is
even, fourth-order nuclear spin correlations (diagonal interaction renormalized
pairwise flip-flop processes) are principally responsible for the central spin
decoherence. Many-body correlations of different orders can thus be selectively
detected by central spin decoherence under different dynamical decoupling
controls, providing a useful approach to probing many-body processes in
nanoscale nuclear spin baths
Construction of a polarization insensitive lens from a quasi-isotropic metamaterial slab
We propose to employ the quasiisotropic metamaterial (QIMM) slab to construct
a polarization insensitive lens, in which both E- and H-polarized waves exhibit
the same refocusing effect. For shallow incident angles, the QIMM slab will
provide some degree of refocusing in the same manner as an isotropic negative
index material. The refocusing effect allows us to introduce the ideas of
paraxial beam focusing and phase compensation by the QIMM slab. On the basis of
angular spectrum representation, a formalism describing paraxial beams
propagating through a QIMM slab is presented. Because of the negative phase
velocity in the QIMM slab, the inverse Gouy phase shift and the negative
Rayleigh length of paraxial Gaussian beam are proposed. We find that the phase
difference caused by the Gouy phase shift in vacuum can be compensated by that
caused by the inverse Gouy phase shift in the QIMM slab. If certain matching
conditions are satisfied, the intensity and phase distributions at object plane
can be completely reconstructed at image plane. Our simulation results show
that the superlensing effect with subwavelength image resolution could be
achieved in the form of a QIMM slab.Comment: 25 pages, 8 figure
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