760 research outputs found
Ice Formation on Kaolinite: Insights from Molecular Dynamics Simulations
The formation of ice affects many aspects of our everyday life as well as
technologies such as cryotherapy and cryopreservation. Foreign substances
almost always aid water freezing through heterogeneous ice nucleation, but the
molecular details of this process remain largely unknown. In fact, insight into
the microscopic mechanism of ice formation on different substrates is difficult
to obtain even via state-of-the-art experimental techniques. At the same time,
atomistic simulations of heterogeneous ice nucleation frequently face
extraordinary challenges due to the complexity of the water-substrate
interaction and the long timescales that characterize nucleation events. Here,
we have investigated several aspects of molecular dynamics simulations of
heterogeneous ice nucleation considering as a prototypical ice nucleating
material the clay mineral kaolinite, which is of relevance in atmospheric
science. We show via seeded molecular dynamics simulations that ice nucleation
on the hydroxylated (001) face of kaolinite proceeds exclusively via the
formation of the hexagonal ice polytype. The critical nucleus size is two times
smaller than that obtained for homogeneous nucleation at the same supercooling.
Previous findings suggested that the flexibility of the kaolinite surface can
alter the time scale for ice nucleation within molecular dynamics simulations.
However, we here demonstrate that equally flexible (or non flexible) kaolinite
surfaces can lead to very different outcomes in terms of ice formation,
according to whether or not the surface relaxation of the clay is taken into
account. We show that very small structural changes upon relaxation
dramatically alter the ability of kaolinite to provide a template for the
formation of a hexagonal overlayer of water molecules at the water-kaolinite
interface, and that this relaxation therefore determines the nucleation ability
of this mineral
Impact of level of personality pathology on affective, behavioral, and thought problems in pregnant women during the Coronavirus Disease 2019 pandemic
Among at-risk groups for psychological distress in the context of the Coronavirus Disease 2019 (COVID-19) pandemic, pregnant women might be especially vulnerable. Identifying subgroups of pregnant women at high risk of poor adaptation might optimize clinical screening and intervention, which could, in turn, contribute to mitigating the potentially devastating effects of prenatal stress on mothers and fetus. Level of personality functioning may be a good indicator of who may be more vulnerable to distress in challenging periods like the COVID-19 pandemic, as adults with high levels of personality dysfunction may experience significant difficulties in mentalizing threatening situations. The aims of the present study are (a) to determine the impact of level of personality pathology on affective, behavioral, and thought problems in pregnant women during the COVID-19 pandemic; and (b) to test a model where mentalization of trauma mediates the impact of personality pathology on symptomatology. Data from 1,207 French-Canadian pregnant women recruited through social media during the COVID-19 pandemic were analyzed. Latent profile analysis, using the Criterion A elements of the alternative model for personality disorders (Identity, Self-Direction, Empathy, Intimacy) as latent indicators, yielded four profiles: Healthy, Mild Self-Impairment, Intimacy Impairment, and Personality Disorder. Profiles showed significant associations with diverse indicators of symptomatology. Mediation models showed both direct and indirect (through mentalization of trauma) significant associations between level of personality functioning and affective/behavioral/thought problems. Results have clinical implications on prophylactic measures for at-risk pregnant women, especially in challenging contexts such as the COVID-19 pandemic
Integral Grothendieck-Riemann-Roch theorem
We show that, in characteristic zero, the obvious integral version of the
Grothendieck-Riemann-Roch formula obtained by clearing the denominators of the
Todd and Chern characters is true (without having to divide the Chow groups by
their torsion subgroups). The proof introduces an alternative to Grothendieck's
strategy: we use resolution of singularities and the weak factorization theorem
for birational maps.Comment: 24 page
Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves
Individual self-assembled Quantum Dots and Quantum Posts are studied under
the influence of a surface acoustic wave. In optical experiments we observe an
acoustically induced switching of the occupancy of the nanostructures along
with an overall increase of the emission intensity. For Quantum Posts,
switching occurs continuously from predominantely charged excitons (dissimilar
number of electrons and holes) to neutral excitons (same number of electrons
and holes) and is independent on whether the surface acoustic wave amplitude is
increased or decreased. For quantum dots, switching is non-monotonic and shows
a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of
positively charged and neutral excitons is observed at high surface acoustic
wave amplitudes. These findings are explained by carrier trapping and
localization in the thin and disordered two-dimensional wetting layer on top of
which Quantum Dots nucleate. This limitation can be overcome for Quantum Posts
where acoustically induced charge transport is highly efficient in a wide
lateral Matrix-Quantum Well.Comment: 11 pages, 5 figure
The triconnected abstraction of process models
Companies use business process models to represent their working procedures in order to deploy services to markets, to analyze them, and to improve upon them. Competitive markets necessitate complex procedures, which lead to large process specifications with sophisticated structures. Real world process models can often incorporate hundreds of modeling constructs. While a large degree of detail complicates the comprehension of the processes, it is essential to many analysis tasks. This paper presents a technique to abstract, i.e., to simplify process models. Given a detailed model, we introduce abstraction rules which generalize process fragments in order to bring the model to a higher abstraction level. The approach is suited for the abstraction of large process specifications in order to aid model comprehension as well as decomposing problems of process model analysis. The work is based on process structure trees that have recently been introduced to the field of business process management
Electric-field-induced coherent coupling of the exciton states in a single quantum dot
The signature of coherent coupling between two quantum states is an
anticrossing in their energies as one is swept through the other. In single
semiconductor quantum dots containing an electron-hole pair the eigenstates
form a two-level system that can be used to demonstrate quantum effects in the
solid state, but in all previous work these states were independent. Here we
describe a technique to control the energetic splitting of these states using a
vertical electric field, facilitating the observation of coherent coupling
between them. Near the minimum splitting the eigenstates rotate in the plane of
the sample, being orientated at 45{\deg} when the splitting is smallest. Using
this system we show direct control over the exciton states in one quantum dot,
leading to the generation of entangled photon pairs
Double Beta Decay: Historical Review of 75 Years of Research
Main achievements during 75 years of research on double beta decay have been
reviewed. The existing experimental data have been presented and the
capabilities of the next-generation detectors have been demonstrated.Comment: 25 pages, typos adde
On-demand semiconductor single-photon source with near-unity indistinguishability
Single photon sources based on semiconductor quantum dots offer distinct
advantages for quantum information, including a scalable solid-state platform,
ultrabrightness, and interconnectivity with matter qubits. A key prerequisite
for their use in optical quantum computing and solid-state networks is a high
level of efficiency and indistinguishability. Pulsed resonance fluorescence
(RF) has been anticipated as the optimum condition for the deterministic
generation of high-quality photons with vanishing effects of dephasing. Here,
we generate pulsed RF single photons on demand from a single,
microcavity-embedded quantum dot under s-shell excitation with 3-ps laser
pulses. The pi-pulse excited RF photons have less than 0.3% background
contributions and a vanishing two-photon emission probability.
Non-postselective Hong-Ou-Mandel interference between two successively emitted
photons is observed with a visibility of 0.97(2), comparable to trapped atoms
and ions. Two single photons are further used to implement a high-fidelity
quantum controlled-NOT gate.Comment: 11 pages, 11 figure
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