4,875 research outputs found
Efficient single photon emission from a high-purity hexagonal boron nitride crystal
Among a variety of layered materials used as building blocks in van der Waals
heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform
for hosting optically-active defects owing to its large bandgap ( eV).
Here we study the optical response of a high-purity hBN crystal under green
laser illumination. By means of photon correlation measurements, we identify
individual defects emitting a highly photostable fluorescence under ambient
conditions. A detailed analysis of the photophysical properties reveals a high
quantum efficiency of the radiative transition, leading to a single photon
source with very high brightness. These results illustrate how the wide range
of applications offered by hBN could be further extended to photonic-based
quantum information science and metrology.Comment: 5 pages, 4 figure
Charge and Spin Density Waves observed through their spatial fluctuations by coherent and simultaneous X-ray diffraction
Spatial uctuations of spin density wave (SDW) and charge density wave (CDW)
in chromium have been compared by combining coherent and simultaneous X-ray
diffraction experiments. Despite their close relationship, spatial fluctuations
of the spin and of the charge density waves display a very different behavior:
the satellite reflection associated to the charge density displays speckles
while the spin one displays an impressive long-range order. This observation is
hardly compatible with the commonly accepted magneto-elastic origin of CDW in
chromium and is more consistent with a purely electronic scenario where CDW is
the second harmonic of SDW. A BCS model taking into account a second order
nesting predicts correctly the existence of a CDW and explains why the CDW is
more sensitive to punctual defects.Comment: 4 pages, 4 figures; Accepted in Phys. Rev.
Coherent Population Trapping with a controlled dissipation: applications in optical metrology
We analyze the properties of a pulsed Coherent Population Trapping protocol
that uses a controlled decay from the excited state in a -level
scheme. We study this problem analytically and numerically and find regimes
where narrow transmission, absorption, or fluorescence spectral lines occur. We
then look for optimal frequency measurements using these spectral features by
computing the Allan deviation in the presence of ground state decoherence and
show that the protocol is on a par with Ramsey-CPT. We discuss possible
implementations with ensembles of alkali atoms and single ions and demonstrate
that typical pulsed-CPT experiments that are realized on femto-second
time-scales can be implemented on micro-seconds time-scales using this scheme.Comment: 9 pages, 7 figure
Surface effects on the Mott-Hubbard transition in archetypal VO
We present an experimental and theoretical study exploring surface effects on
the evolution of the metal-insulator transition in the model Mott-Hubbard
compound Cr-doped VO. We find a microscopic domain formation that is
clearly affected by the surface crystallographic orientation. Using scanning
photoelectron microscopy and X-ray diffraction, we find that surface defects
act as nucleation centers for the formation of domains at the
temperature-induced isostructural transition and favor the formation of
microscopic metallic regions. A density functional theory plus dynamical mean
field theory study of different surface terminations shows that the surface
reconstruction with excess vanadyl cations leads to doped, and hence more
metallic surface states, explaining our experimental observations.Comment: 5 pages, 4 figure
Observation of correlations up to the micrometer scale in sliding charge-density waves
High-resolution coherent x-ray diffraction experiment has been performed on
the charge density wave (CDW) system KMoO. The satellite
reflection associated with the CDW has been measured with respect to external
dc currents. In the sliding regime, the satellite reflection displays
secondary satellites along the chain axis which corresponds to correlations up
to the micrometer scale. This super long range order is 1500 times larger than
the CDW period itself. This new type of electronic correlation seems inherent
to the collective dynamics of electrons in charge density wave systems. Several
scenarios are discussed.Comment: 4 pages, 3 figures Typos added, references remove
The essential role of surface pinning in the dynamics of charge density waves submitted to external dc fields
A Charge Density Wave (CDW) submitted to an electric field displays a strong
shear deformation because of pinning at the lateral surfaces of the sample.
This CDW transverse pinning was recently observed but has received little
attention from a theoretical point of view until now despite important
consequences on electrical conductivity properties. Here, we provide a
description of this phenomenon by considering a CDW submitted to an external dc
electric field and constrained by boundary conditions including both
longitudinal pinning due to electrical contacts and transverse surface pinning.
A simple formula for the CDW phase is obtained in 3D by using the Green
function and image charges method. In addition, an analytical expression of the
threshold field dependence on both length and sample cross section is obtained
by considering the phase slip process. We show that the experimental data are
well reproduced with this model and that bulk pinning can be neglected. This
study shows that the dynamical properties of CDW systems could be mainly driven
by boundary effects, despite the comparatively huge sample volumes.Comment: 7 pages of main text, 7 pages of appendices 15 figure
Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales
Femtosecond time-resolved X-ray diffraction is used to study a photo-induced
phase transition between two charge density wave (CDW) states in 1T-TaS,
namely the nearly commensurate (NC) and the incommensurate (I) CDW states.
Structural modulations associated with the NC-CDW order are found to disappear
within 400 fs. The photo-induced I-CDW phase then develops through a
nucleation/growth process which ends 100 ps after laser excitation. We
demonstrate that the newly formed I-CDW phase is fragmented into several
nanometric domains that are growing through a coarsening process. The
coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth
law, which describes the ordering kinetics in systems exhibiting a
non-conservative order parameter.Comment: 6 pages, 5 figure
Quantum Transition State Theory for proton transfer reactions in enzymes
We consider the role of quantum effects in the transfer of hyrogen-like
species in enzyme-catalysed reactions. This study is stimulated by claims that
the observed magnitude and temperature dependence of kinetic isotope effects
imply that quantum tunneling below the energy barrier associated with the
transition state significantly enhances the reaction rate in many enzymes. We
use a path integral approach which provides a general framework to understand
tunneling in a quantum system which interacts with an environment at non-zero
temperature. Here the quantum system is the active site of the enzyme and the
environment is the surrounding protein and water. Tunneling well below the
barrier only occurs for temperatures less than a temperature which is
determined by the curvature of potential energy surface near the top of the
barrier. We argue that for most enzymes this temperature is less than room
temperature. For physically reasonable parameters quantum transition state
theory gives a quantitative description of the temperature dependence and
magnitude of kinetic isotope effects for two classes of enzymes which have been
claimed to exhibit signatures of quantum tunneling. The only quantum effects
are those associated with the transition state, both reflection at the barrier
top and tunneling just below the barrier. We establish that the friction due to
the environment is weak and only slightly modifies the reaction rate.
Furthermore, at room temperature and for typical energy barriers environmental
degrees of freedom with frequencies much less than 1000 cm do not have a
significant effect on quantum corrections to the reaction rate.Comment: Aspects of the article are discussed at
condensedconcepts.blogspot.co
Three-dimensional diffraction mapping by tuning the X-ray energy
Three-dimensional reciprocal-space maps of a single SiGe island around the Si(004) Bragg peak are recorded using an energy-tuning technique with a microfocused X-ray beam with compound refractive lenses as focusing optics
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