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Phase diagram of the TIP4P/Ice water model by enhanced sampling simulations
No full textWe studied the phase diagram for the TIP4P/Ice water model using enhanced sampling molecular dynamics simulations. Our approach is based on the calculation of ice–liquid free energy differences from biased coexistence simulations that reversibly sample the melting and growth of layers of ice. We computed a total of 19 melting points for five different ice polymorphs, which are in excellent agreement with the melting lines obtained from the integration of the Clausius–Clapeyron equation. For proton-ordered and fully proton-disordered ice phases, the results are in very good agreement with previous calculations based on thermodynamic integration. For the partially proton-disordered ice III, we find a large increase in stability that is in line with previous observations using direct coexistence simulations for the TIP4P/2005 model. This issue highlights the robustness of the approach employed here for ice polymorphs with diverse degrees of proton disorder. Our approach is general and can be applied to the calculation of other complex phase diagrams
Obstructed Surface States as the Descriptor for Predicting Catalytic Active Sites in Inorganic Crystalline Materials
No full textThe discovery of new catalysts that are efficient and sustainable is a major research endeavor for many industrial chemical processes. This requires an understanding and determination of the catalytic origins, which remains a challenge. Here, a novel method to identify the position of active sites based on searching for crystalline symmetry‐protected obstructed atomic insulators (OAIs) that have metallic surface states is described. The obstructed Wannier charge centers (OWCCs) in OAIs are pinned by symmetries at some empty Wyckoff positions so that surfaces that accommodate these sites are guaranteed to have metallic obstructed surface states (OSSs). It is proposed and confirmed that the OSSs are the catalytic activity origins for crystalline materials. The theory on 2H‐MoTe2, 1T′‐MoTe2, and NiPS3 bulk single crystals is verified, whose active sites are consistent with the calculations. Most importantly, several high‐efficiency catalysts are successfully identified just by considering the number of OWCCs and the symmetry. Using the real‐space‐invariant theory applied to a database of 34 013 topologically trivial insulators, 1788 unique OAIs are identified, of which 465 are potential high‐performance catalysts. The new methodology will facilitate and accelerate the discovery of new catalysts for a wide range of heterogeneous redox reactions
Emergence of local irreversibility in complex interacting systems
No full textLiving systems are fundamentally irreversible, breaking detailed balance and establishing an ar-
row of time. But how does the evident arrow of time for a whole system arise from the interactions
among its multiple elements? We show that the local evidence for the arrow of time, which is the
entropy production for thermodynamic systems, can be decomposed. First, it can be split into two
components: an independent term reflecting the dynamics of individual elements and an interaction
term driven by the dependencies among elements. Adapting tools from non–equilibrium physics,
we further decompose the interaction term into contributions from pairs of elements, triplets, and
higher–order terms. We illustrate our methods on models of cellular sensing and logical computa-
tions, as well as on patterns of neural activity in the retina as it responds to visual inputs. We find
that neural activity can define the arrow of time even when the visual inputs do not, and that the
dominant contribution to this breaking of detailed balance comes from interactions among pairs of
neurons
Correlated and integrated directionality for sub-MeV solar neutrinos in Borexino
No full textLiquid scintillator detectors play a central role in the detection of neutrinos from various sources. In
particular, it is the only technique used so far for the precision spectroscopy of sub-MeV solar neutrinos, as
demonstrated by the Borexino experiment at the Gran Sasso National Laboratory in Italy. The benefit of a
high light yield, and thus a low energy threshold and a good energy resolution, comes at the cost of the
directional information featured by water Cherenkov detectors, measuring 8B solar neutrinos above a few
MeV. In this paper we provide the first directionality measurement of sub-MeV solar neutrinos which
exploits the correlation between the first few detected photons in each event and the known position of the
Sun for each event. This is also the first signature of directionality in neutrinos elastically scattering off
electrons in a liquid scintillator target. This measurement exploits the subdominant, fast Cherenkov light
emission that precedes the dominant yet slower scintillation light signal. Through this measurement, we
have also been able to extract the rate of 7Be solar neutrinos in Borexino. The demonstration of directional
sensitivity in a traditional liquid scintillator target paves the way for the possible exploitation of the
Cherenkov light signal in future kton-scale experiments using liquid scintillator targets. Directionality is
important for background suppression as well as the disentanglement of signals from various sources
A high-throughput method to deliver targeted optogenetic stimulation to moving C. elegans populations
No full textWe present a high-throughput optogenetic illumination system capable of simultaneous closed-loop light delivery to specified targets in populations of moving Caenorhabditis elegans. The instrument addresses three technical challenges: It delivers targeted illumination to specified regions of the animal’s body such as its head or tail; it automatically delivers stimuli triggered upon the animal’s behavior; and it achieves high throughput by targeting many animals simultaneously. The instrument was used to optogenetically probe the animal’s behavioral response to competing mechanosensory stimuli in the the anterior and posterior gentle touch receptor neurons. Responses to more than 43,418 stimulus events from a range of anterior–posterior intensity combinations were measured. The animal’s probability of sprinting forward in response to a mechanosensory stimulus depended on both the anterior and posterior stimulation intensity, while the probability of reversing depended primarily on the anterior stimulation intensity. We also probed the animal’s response to mechanosensory stimulation during the onset of turning, a relatively rare behavioral event, by delivering stimuli automatically when the animal began to turn. Using this closed-loop approach, over 9,700 stimulus events were delivered during turning onset at a rate of 9.2 events per worm hour, a greater than 25-fold increase in throughput compared to previous investigations. These measurements validate with greater statistical power previous findings that turning acts to gate mechanosensory evoked reversals. Compared to previous approaches, the current system offers targeted optogenetic stimulation to specific body regions or behaviors with many fold increases in throughput to better constrain quantitative models of sensorimotor processing
Prenatal development of neonatal vocalizations
No full textHuman and non-human primates produce rhythmical sounds as soon as they are born. These early vocalizations are important for soliciting the attention of caregivers. How they develop remains a mystery. The orofacial movements necessary for producing these vocalizations have distinct spatiotemporal signatures. Therefore, their development could potentially be tracked over the course of prenatal life. We densely and longitudinally sampled fetal head and orofacial movements in marmoset monkeys using ultrasound imaging. We show that orofacial movements necessary for producing rhythmical vocalizations differentiate from a larger movement pattern that includes the entire head. We also show that signature features of marmoset infant contact calls emerge prenatally as a distinct pattern of orofacial movements. Our results establish that aspects of the sensorimotor development necessary for vocalizing occur prenatally, even before the production of sound
Rapidly descending dark energy and the end of cosmic expansion
No full textAlthough the universe is expanding at an accelerating rate today, this paper presents a simple mechanism by which a dynamical form of dark energy (known as quintessence) could cause the acceleration to come to end and smoothly transition from expansion to a phase of slow contraction. That raises questions, How soon could this transition occur? And at what point would it be detectable? The conclusions are that the transition could be surprisingly soon, maybe less than 100 million y from now, and yet, for reasons described in the main text, it is not yet detectable today. The scenario is not far-fetched. In fact, it fits naturally with recent theories of cyclic cosmology and conjectures about quantum gravity
Heparan sulfate is necessary for the early formation of nascent fibronectin and collagen I fibrils at matrix assembly sites
No full textFibronectin (FN), an essential component of the extracellular matrix (ECM), is assembled via a cell-mediated process in which integrin receptors bind secreted FN and mediate its polymerization into fibrils that extend between cells, ultimately forming an insoluble matrix. Our previous work using mutant Chinese hamster ovary (CHO) cells identified the glycosaminoglycan heparan sulfate (HS) and its binding to FN as essential for the formation of insoluble FN fibrils. In this study, we investigated the contributions of HS at an early stage of the assembly process using knockdown of exostosin-1 (EXT1), one of the glycosyltransferases required for HS chain synthesis. NIH 3T3 fibroblasts with decreased EXT1 expression exhibited a significant reduction in both FN and type I collagen in the insoluble matrix. We show that FN fibril formation is initiated at matrix assembly sites, and while these sites were formed by cells with EXT1 knockdown, their growth was stunted compared with wild-type cells. The most severe defect observed was in the polymerization of nascent FN fibrils, which was reduced 2.5-fold upon EXT1 knockdown. This defect was rescued by the addition of exogenous soluble heparin chains long enough to simultaneously bind multiple FN molecules. The activity of soluble heparin in this process indicates that nascent fibril formation depends on HS more so than on the protein component of a specific HS proteoglycan. Together, our results suggest that heparin or HS is necessary for concentrating and localizing FN molecules at sites of early fibril assembly
Simons Observatory Focal-Plane Module: In-lab Testing and Characterization Program
No full textThe Simons Observatory is a ground-based cosmic microwave background instrument to be sited in the Atacama Desert in Chile. SO will deploy 60,000 transition-edge sensors (TES) bolometers in 49 separate focal-plane modules across a suite of four telescopes covering three dichroic bands termed low frequency (LF), mid-frequency and ultra-high frequency. Each MF and UHF focal-plane module packages 1720 feedhorn-coupled detectors with cryogenic components for highly multiplexed readout using microwave SQUID multiplexing. In this paper, we describe the testing program we have developed for high-throughput validation of modules after they are assembled. The validation requires measurements of the yield, saturation powers, time constants, noise properties and optical efficiencies. Additional measurements will be performed for further characterization as needed. We describe the methods developed and demonstrate preliminary results from the initial testing of a prototype module
