7,281 research outputs found
Features in the ion emission of Cu, Al, and C plasmas produced by ultrafast laser ablation
© 2015 AIP Publishing LLC. The bi-modal nature of charge integrated ion kinetic energy distributions, which result from ultrafast laser produced plasmas, is discussed in this paper. A negatively biased Faraday cup was used as a charge collector to measure ion distributions from three different solid targets that had been irradiated with an ultrafast laser in the fluence range 0.1-1 J/cm2. A bi-modal time of flight distribution is found for all three targets (C, Al, and Cu). In the case of the metallic targets (Al and Cu), high- and low-kinetic energy peaks exhibit quite different dependencies on laser fluence, whereas for the semi-metallic target (C), both peaks scale similarly with ultrafast laser fluence. The results are discussed within the framework of a one dimensional capacitor model resulting in ion acceleration
Structure and Composition of Pollen Grains of Vernal Plants
Among the first morphological studies of pollen were those of Malpighi (1686) and of Grew (1675). While numerous articles on their structure and composition of pollen have been contributed, the amount of data available in application to several problems involved in such study is yet somewhat fragmentar
NMR Time Reversal Experiments in Highly Polarised Liquid 3He-4He Mixtures
Long-range magnetic interactions in highly magnetised liquids
(laser-polarised 3He-4He dilute mixtures at 1 K in our experiment) introduce a
significant non-linear and non-local contribution to the evolution of nuclear
magnetisation that leads to instabilities during free precession. We recently
demonstrated that a multi-echo NMR sequence, based on the magic sandwich pulse
scheme developed for solid-state NMR, can be used to stabilise the
magnetisation against the effect of distant dipolar fields. Here, we report
investigations of echo attenuation in an applied field gradient that show the
potential of this NMR sequence for spin diffusion measurements at high
magnetisation densities.Comment: Accepted for publication in the Journal of Low Temperature Physic
No Evidence for Orbital Loop Currents in Charge Ordered YBaCuO from Polarized Neutron Diffraction
It has been proposed that the pseudogap state of underdoped cuprate
superconductors may be due to a transition to a phase which has circulating
currents within each unit cell. Here, we use polarized neutron diffraction to
search for the corresponding orbital moments in two samples of underdoped
YBaCuO with doping levels and 0.123. In contrast to
some other reports using polarized neutrons, but in agreement with nuclear
magnetic resonance and muon spin rotation measurements, we find no evidence for
the appearance of magnetic order below 300 K. Thus, our experiment suggests
that such order is not an intrinsic property of high-quality cuprate
superconductor single crystals. Our results provide an upper bound for a
possible orbital loop moment which depends on the pattern of currents within
the unit cell. For example, for the CC- pattern proposed by Varma,
we find that the ordered moment per current loop is less than 0.013 for
.Comment: Comments in arXiv:1710.08173v1 fully addresse
Convergence Conditions for Random Quantum Circuits
Efficient methods for generating pseudo-randomly distributed unitary
operators are needed for the practical application of Haar distributed random
operators in quantum communication and noise estimation protocols. We develop a
theoretical framework for analyzing pseudo-random ensembles generated through a
random circuit composition. We prove that the measure over random circuits
converges exponentially (with increasing circuit length) to the uniform (Haar)
measure on the unitary group and describe how the rate of convergence may be
calculated for specific applications.Comment: 4 pages (revtex), comments welcome. v2: reference added, title
changed; v3: published version, minor changes, references update
Relativistically covariant state-dependent cloning of photons
The influence of the relativistic covariance requirement on the optimality of
the symmetric state-dependent 1 -> 2 cloning machine is studied. Namely, given
a photonic qubit whose basis is formed from the momentum-helicity eigenstates,
the change to the optimal cloning fidelity is calculated taking into account
the Lorentz covariance unitarily represented by Wigner's little group. To
pinpoint some of the interesting results, we found states for which the optimal
fidelity of the cloning process drops to 2/3 which corresponds to the fidelity
of the optimal classical cloner. Also, an implication for the security of the
BB84 protocol is analyzed.Comment: corrected, rewritten and accepted in PR
Bolometric technique for high-resolution broadband microwave spectroscopy of ultra-low-loss samples
A novel low temperature bolometric method has been devised and implemented
for high-precision measurements of the microwave surface resistance of small
single-crystal platelet samples having very low absorption, as a continuous
function of frequency. The key to the success of this non-resonant method is
the in-situ use of a normal metal reference sample that calibrates the absolute
rf field strength. The sample temperature can be controlled independently of
the 1.2 K liquid helium bath, allowing for measurements of the temperature
evolution of the absorption. However, the instrument's sensitivity decreases at
higher temperatures, placing a limit on the useful temperature range. Using
this method, the minimum detectable power at 1.3 K is 1.5 pW, corresponding to
a surface resistance sensitivity of 1 for a typical 1
mm1 mm platelet sample.Comment: 13 pages, 12 figures, submitted to Review of Scientific Instrument
Superconductivity induced by spark erosion in ZrZn2
We show that the superconductivity observed recently in the weak itinerant
ferromagnet ZrZn2 [C. Pfleiderer et al., Nature (London) 412, 58 (2001)] is due
to remnants of a superconducting layer induced by spark erosion. Results of
resistivity, susceptibility, specific heat and surface analysis measurements on
high-quality ZrZn2 crystals show that cutting by spark erosion leaves a
superconducting surface layer. The resistive superconducting transition is
destroyed by chemically etching a layer of 5 microns from the sample. No
signature of superconductivity is observed in rho(T) of etched samples at the
lowest current density measured, J=675 Am-2, and at T < 45 mK. EDX analysis
shows that spark-eroded surfaces are strongly Zn depleted. The simplest
explanation of our results is that the superconductivity results from an alloy
with higher Zr content than ZrZn2.Comment: Final published versio
Competing charge, spin, and superconducting orders in underdoped YBa2Cu3Oy
To explore the doping dependence of the recently discovered charge density
wave (CDW) order in YBa2Cu3Oy, we present a bulk-sensitive high-energy x-ray
study for several oxygen concentrations, including strongly underdoped
YBa2Cu3O6.44. Combined with previous data around the so-called 1/8 doping, we
show that bulk CDW order exists at least for hole concentrations (p) in the
CuO2 planes of 0.078 <~ p <~ 0.132. This implies that CDW order exists in close
vicinity to the quantum critical point for spin density wave (SDW) order. In
contrast to the pseudogap temperature T*, the onset temperature of CDW order
decreases with underdoping to T_CDW ~ 90K in YBa2Cu3O6.44. Together with a
weakened order parameter this suggests a competition between CDW and SDW
orders. In addition, the CDW order in YBa2Cu3O6.44 shows the same type of
competition with superconductivity as a function of temperature and magnetic
field as samples closer to p = 1/8. At low p the CDW incommensurability
continues the previously reported linear increasing trend with underdoping. In
the entire doping range the in-plane correlation length of the CDW order in
b-axis direction depends only very weakly on the hole concentration, and
appears independent of the type and correlation length of the oxygen-chain
order. The onset temperature of the CDW order is remarkably close to a
temperature T^\dagger that marks the maximum of 1/(T_1T) in planar 63^Cu
NQR/NMR experiments, potentially indicating a response of the spin dynamics to
the formation of the CDW. Our discussion of these findings includes a detailed
comparison to the charge stripe order in La2-xBaxCuO4.Comment: 11 pages, 5 figure
- …