8,222 research outputs found
Self-assembly and two-dimensional spontaneous resolution of cyano-functionalized [7]helicenes on Cu111
Birds of a feather flock together: STM and DFT studies provide the first example of spontaneous chiral resolution of a helicene on a surface. Racemic 6,13-dicyano[7]helicene forms fully segregated domains of pure enantiomers (2D conglomerate) on Cu(111). The propensity of the system to optimize intermolecular CNâ‹…â‹…â‹…HC(Ar) hydrogen bonding and CNâ‹…â‹…â‹…CN dipolar interactions translates into chiral recognition with preferential assembly of homochiral molecules
Graviton propagator from background-independent quantum gravity
We study the graviton propagator in euclidean loop quantum gravity, using the
spinfoam formalism. We use boundary-amplitude and group-field-theory
techniques, and compute one component of the propagator to first order, under a
number of approximations, obtaining the correct spacetime dependence. In the
large distance limit, the only term of the vertex amplitude that contributes is
the exponential of the Regge action: the other terms, that have raised doubts
on the physical viability of the model, are suppressed by the phase of the
vacuum state, which is determined by the extrinsic geometry of the boundary.Comment: 6 pages. Substantially revised second version. Improved boundary
state ansat
On the geometry of loop quantum gravity on a graph
We discuss the meaning of geometrical constructions associated to loop
quantum gravity states on a graph. In particular, we discuss the "twisted
geometries" and derive a simple relation between these and Regge geometries.Comment: 6 pages, 1 figure. v2: some typos corrected, references update
Inflationary Perturbations in Palatini Generalised Gravity
We examine the generation of primordial perturbations during an inflationary
epoch in generalised theories of gravity when the equations of motion are
derived using the Palatini variational principle. Both f(R) and Scalar-Tensor
theories are considered and we compare our results with those obtained under
the conventional metric formalism. Non-linear generalisations of the action
lead to different theories under the two variational choices and we obtain
distinct results for scalar and tensor spectral indices and their ratio. We
find the following general result; inflation driven solely by f(R)
modifications alone do not result in suitable curvature perturbations whilst
Scalar-Tensor theories generate nearly scalar invariant curvature perturbations
but no tensor modes.Comment: 9 page
Can lizard embryos survive climate warming? Thermal constraints on the physiology of developing Eastern fence lizards
Sublethal stressors will likely mediate an organism’s response to climate change. Mobile animals can change their behavior to maintain temperatures within preferred ranges. However, animals in immobile life stages are particularly vulnerable to warming. For example, lizard embryos experience recurrent thermal stress as they develop in shallow nests with daily temperature fluctuations. Nesting temperatures above 41.5°C can be lethal to Eastern fence lizard (Sceloporus undulatus) embryos. Although that is beyond the range experienced at current nest sites, rapid warming will push nest conditions toward critical limits. Therefore, it is important to examine the effects of sublethal high nest temperatures on developing lizards. We reared S. undulatus embryos under three thermal regimes—one to simulate contemporary nest conditions with a maximum daily temperature (Tmax) of 32.1°C, and two regimes to simulate warming scenarios in which the Tmax was raised to 35.6°C and 39.1°C. We tracked changes in physiology and survival through embryonic development, and we measured growth and metabolic rates of surviving hatchlings. Results showed reduced embryo survival and decreased hatchling growth for lizards reared under sublethal warming, suggesting that thermal constraints on the physiology of developing Eastern fence lizards may limit the persistence of the species under climate warming
The complete LQG propagator: II. Asymptotic behavior of the vertex
In a previous article we have show that there are difficulties in obtaining
the correct graviton propagator from the loop-quantum-gravity dynamics defined
by the Barrett-Crane vertex amplitude. Here we show that a vertex amplitude
that depends nontrivially on the intertwiners can yield the correct propagator.
We give an explicit example of asymptotic behavior of a vertex amplitude that
gives the correct full graviton propagator in the large distance limit.Comment: 16 page
Giant g factor tuning of long-lived electron spins in Ge
Control of electron spin coherence via external fields is fundamental in
spintronics. Its implementation demands a host material that accommodates the
highly desirable but contrasting requirements of spin robustness to relaxation
mechanisms and sizeable coupling between spin and orbital motion of charge
carriers. Here we focus on Ge, which, by matching those criteria, is rapidly
emerging as a prominent candidate for shuttling spin quantum bits in the mature
framework of Si electronics. So far, however, the intrinsic spin-dependent
phenomena of free electrons in conventional Ge/Si heterojunctions have proved
to be elusive because of epitaxy constraints and an unfavourable band
alignment. We overcome such fundamental limitations by investigating a two
dimensional electron gas (2DEG) confined in quantum wells of pure Ge grown on
SiGe-buffered Si substrates. These epitaxial systems demonstrate exceptionally
long spin relaxation and coherence times, eventually unveiling the potential of
Ge in bridging the gap between spintronic concepts and semiconductor device
physics. In particular, by tuning spin-orbit interaction via quantum
confinement we demonstrate that the electron Land\'e g factor and its
anisotropy can be engineered in our scalable and CMOS-compatible architectures
over a range previously inaccessible for Si spintronics
Stable classical structures in dissipative quantum chaotic systems
We study the stability of classical structures in chaotic systems when a
dissipative quantum evolution takes place. We consider a paradigmatic model,
the quantum baker map in contact with a heat bath at finite temperature. We
analyze the behavior of the purity, fidelity and Husimi distributions
corresponding to initial states localized on short periodic orbits (scar
functions) and map eigenstates. Scar functions, that have a fundamental role in
the semiclassical description of chaotic systems, emerge as very robust against
environmental perturbations. This is confirmed by the study of other states
localized on classical structures. Also, purity and fidelity show a
complementary behavior as decoherence measures.Comment: 4 pages, 3 figure
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