232,002 research outputs found
Water activity in lamellar stacks of lipid bilayers: "Hydration forces" revisited
Water activity and its relationship with interactions stabilising lamellar
stacks of mixed lipid bilayers in their fluid state are investigated by means
of osmotic pressure measurements coupled with small-angle x-ray scattering. The
(electrically-neutral) bilayers are composed of a mixture in various
proportions of lecithin, a zwitterionic phospholipid, and Simulsol, a non-ionic
cosurfactant with an ethoxylated polar head. For highly dehydrated samples the
osmotic pressure profile always exhibits the "classical" exponential decay as
hydration increases but, depending on Simulsol to lecithin ratio, it becomes
either of the "bound" or "unbound" types for more water-swollen systems. A
simple thermodynamic model is used for interpreting the results without
resorting to the celebrated but elusive "hydration forces"Comment: 24 pages, 12 figures. Accepted for publication in The European
Physical Journal
Quantitative scattering theory of near-field response for 1D polaritonic structures
Scattering-type scanning near-field optical microscopy is a powerful imaging
technique for studying materials beyond the diffraction limit. However,
interpreting near-field measurements poses challenges in mapping the response
of polaritonic structures to meaningful physical properties. To address this,
we propose a theory based on the transfer matrix method to simulate the
near-field response of 1D polaritonic structures. Our approach provides a
computationally efficient and accurate analytical theory, relating the
near-field response to well-defined physical properties. This work enhances the
understanding of near-field images and complex polaritonic phenomena. Finally,
this scattering theory can extend to other systems like atoms or nanoparticles
near a waveguide
Supramolecular Engineering of Alkylated, Fluorinated, and Mixed Amphiphiles
The rational design of perfluorinated amphiphiles to control the supramolecular aggregation in an aqueous medium is still a key challenge for the engineering of supramolecular architectures. Here, the synthesis and physical properties of six novel non-ionic amphiphiles are presented. The effect of mixed alkylated and perfluorinated segments in a single amphiphile is also studied and compared with only alkylated and perfluorinated units. To explore their morphological behavior in an aqueous medium, dynamic light scattering (DLS) and cryogenic transmission electron microscopy/electron microscopy (cryo-TEM/EM) measurements are used. The assembly mechanisms with theoretical investigations are further confirmed, using the Martini model to perform large-scale coarse-grained molecular dynamics simulations. These novel synthesized amphiphiles offer a greater and more systematic understanding of how perfluorinated systems assemble in an aqueous medium and suggest new directions for rational designing of new amphiphilic systems and interpreting their assembly process
Using think-aloud interviews to characterize model-based reasoning in electronics for a laboratory course assessment
Models of physical systems are used to explain and predict experimental
results and observations. The Modeling Framework for Experimental Physics
describes the process by which physicists revise their models to account for
the newly acquired observations, or change their apparatus to better represent
their models when they encounter discrepancies between actual and expected
behavior of a system. While modeling is a nationally recognized learning
outcome for undergraduate physics lab courses, no assessments of students'
model-based reasoning exist for upper-division labs. As part of a larger effort
to create two assessments of students' modeling abilities, we used the Modeling
Framework to develop and code think-aloud problem-solving activities centered
on investigating an inverting amplifier circuit. This study is the second phase
of a multiphase assessment instrument development process. Here, we focus on
characterizing the range of modeling pathways students employ while
interpreting the output signal of a circuit functioning far outside its
recommended operation range. We end by discussing four outcomes of this work:
(1) Students engaged in all modeling subtasks, and they spent the most time
making measurements, making comparisons, and enacting revisions; (2) Each
subtask occurred in close temporal proximity to all over subtasks; (3)
Sometimes, students propose causes that do not follow logically from observed
discrepancies; (4) Similarly, students often rely on their experiential
knowledge and enact revisions that do not follow logically from articulated
proposed causes.Comment: 18 pages, 5 figure
Andreev bound states versus Majorana bound states in quantum dot-nanowire-superconductor hybrid structures: Trivial versus topological zero-bias conductance peaks
Motivated by an important recent experiment [Deng et al., Science 354, 1557
(2016)], we theoretically consider the interplay between Andreev bound
states(ABSs) and Majorana bound states(MBSs) in quantum dot-nanowire
semiconductor systems with proximity-induced superconductivity(SC), spin-orbit
coupling and Zeeman splitting. The dot induces ABSs in the SC nanowire which
show complex behavior as a function of Zeeman splitting and chemical potential,
and the specific question is whether two such ABSs can come together forming a
topological MBS. We consider physical situations involving the dot being
non-SC, SC, or partially SC. We find that the ABSs indeed tend to coalesce
together producing near-zero-energy midgap states as Zeeman splitting and/or
chemical potential are increased, but this mostly happens in the
non-topological regime although there are situations where the ABSs could come
together forming a topological MBS. The two scenarios(two ABSs forming a
near-zero-energy non-topological ABS or a zero-energy topological MBS) are
difficult to distinguish by tunneling conductance spectroscopy due to
essentially the same signatures. Theoretically we distinguish them by knowing
the critical Zeeman splitting for the topological quantum phase transition or
by calculating the topological visibility. We find that the "sticking together"
propensity of ABSs to produce a zero-energy midgap state is generic in class D
systems, and by itself says nothing about the topological nature of the
underlying SC nanowire. One must use caution in interpreting tunneling
conductance measurements where the midgap sticking-together behavior of ABSs
cannot be construed as definitive evidence for topological SC with non-Abelian
MBSs. We also suggest some experimental techniques for distinguishing between
trivial and topological ZBCPs.Comment: 32 pages, 29 figure
- …