Ref HAL: hal-04279663_v1
Ref Arxiv: 2310.16770
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé:

Large groups of active cilia collectively beat in a fluid medium as metachronal waves, essential for some microorganisms motility and for flow generation in mucociliary clearance. Several models can predict the emergence of metachronal waves, but what controls the properties of metachronal waves is still unclear. Here, we investigate numerically a simple model for cilia in the presence of noise on regular lattices in one- and two-dimensions. We characterize the wave using spatial correlation and the frequency of collective beating. Our results clearly show that the viscosity of the fluid medium does not affect the wavelength; the activity of the cilia does. These numerical results are supported by a dimensional analysis, which is expected to be robust against the model for active force generation, unless surrounding fluid influences the cilia activity. Interestingly, enhancement of cilia activity increases the wavelength and decreases the beating frequency, keeping the wave velocity almost unchanged. These results might have significance in understanding paramecium locomotion and mucociliary clearance diseases.

Commentaires: 6 pages, 5 figures

Ref Arxiv: 2311.04969
Ref. & Cit.: NASA ADS
Résumé:

We compute instanton corrections to the partition function of sine-Liouville (SL) theory, which provides a worldsheet description of two-dimensional string theory in a non-trivial tachyon background. We derive these corrections using a matrix model formulation based on a chiral representation of matrix quantum mechanics and using string theory methods. In both cases we restrict to the leading and subleading orders in the string coupling expansion. Then the CFT technique is used to compute two orders of the expansion in the SL perturbation parameter $\lambda$, while the matrix model gives results which are non-perturbative in $\lambda$. The matrix model results perfectly match those of string theory in the small $\lambda$ expansion. We also generalize our findings to the case of perturbation by several tachyon vertex operators carrying different momenta, and obtain interesting analytic predictions for the disk two-point and annulus one-point functions with ZZ boundary condition.

Commentaires: 47+30 pages

Ref HAL: hal-04260533_v1
DOI: 10.1088/2515-7639/ad06cc
Exporter : BibTex | endNote
Résumé:

Soft materials are usually defined as materials made of mesoscopic entities, often self-organized, sensitive to thermal fluctuations and to weak perturbations. Archetypal examples are colloids, polymers, amphiphiles, liquid crystals, foams. The importance of soft materials in everyday commodity products, as well as in technological applications, is enormous, and controlling or improving their properties is the focus of many efforts. From a fundamental perspective, the possibility of manipulating soft material properties, by tuning interactions between constituents and by applying external perturbations, gives rise to an almost unlimited variety in physical properties. Together with the relative ease to observe and characterize them, this renders soft matter systems powerful model systems to investigate statistical physics phenomena, many of them relevant as well to hard condensed matter systems. Understanding the emerging properties from mesoscale constituents still poses enormous challenges, which have stimulated a wealth of new experimental approaches, including the synthesis of new systems with, e.g., tailored self-assembling properties, or novel experimental techniques in imaging, scattering or rheology. Theoretical and numerical methods, and coarse-grained models, have become central to predict physical properties of soft materials, while computational approaches that also use machine learning tools are playing a progressively major role in many investigations. This roadmap paper intends to give a broad overview of recent and possible future activities in the field of soft materials, with experts covering various developments and challenges in material synthesis and characterization, instrumental, simulation and theoretical methods as well as general concepts.

Ref HAL: hal-04258308_v1
Ref Arxiv: 2304.00492
DOI: 10.1038/s41524-023-01111-7
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé:

The boron-vacancy spin defect (V − B) in hexagonal boron nitride (hBN) has a great potential as a quantum sensor in a two-dimensional material that can directly probe various external perturbations in atomic-scale proximity to the quantum sensing layer. Here, we apply first principles calculations to determine the coupling of the V − B electronic spin to strain and electric fields. Our work unravels the interplay between local piezoelectric and elastic effects contributing to the final response to the electric fields. The theoretical predictions are then used to analyse optically detected magnetic resonance (ODMR) spectra recorded on hBN crystals containing different densities of V − B centres. We prove that the orthorhombic zero-field splitting parameter results from local electric fields produced by surrounding charge defects. By providing calculations of the spin-strain and spin-electric field couplings, this work paves the way towards applications of V − B centres for quantitative electric field imaging and quantum sensing under pressure.

Ref HAL: hal-04258293_v1
Ref Arxiv: 2307.06774
DOI: 10.1103/PhysRevLett.131.126901
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote

Ref HAL: hal-04258279_v1
Ref Arxiv: 2307.06730
DOI: 10.1063/5.0167480
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé:

Despite the considerable interest for antiferromagnets that appeared with the perspective of using them for spintronics, their experimental study, including the imaging of antiferromagnetic textures, remains a challenge. To address this issue, quantum sensors, and, in particular, the nitrogen-vacancy (NV) defects in diamond have become a widespread technical solution. We review here the recent applications of single NV centers to study a large variety of antiferromagnetic materials, from quantitative imaging of antiferromagnetic domains and non-collinear states, to the detection of spin waves confined in antiferromagnetic textures and the non-perturbative measurement of spin transport properties. We conclude with recent developments improving further the magnetic sensitivity of scanning NV microscopy, opening the way to detailed investigations of the internal texture of antiferromagnetic objects.

Ref HAL: hal-04252827_v1
DOI: 10.48550/arXiv.2307.00636
Exporter : BibTex | endNote
Résumé:

The cooperative binding of molecular agents onto a substrate is pervasive in living systems. To study whether a system shows cooperativity, one can rely on a fluctuation analysis of quantities such as the number of substrate-bound units and the residence time in an occupancy state. Since the relative standard deviation from the statistical mean monotonically decreases with the number of binding sites, these techniques are only suitable for small enough systems, such as those implicated in stochastic processes inside cells. Here, we present a general-purpose grand canonical Hamiltonian description of a small one-dimensional (1D) lattice gas with either nearest-neighbor or long-range interactions as prototypical examples of cooperativity-influenced adsorption processes. First, we elucidate how the strength and sign of the interaction potential between neighboring bound particles on the lattice determine the intensity of the fluctuations of the mean occupancy. We then employ this relationship to compare the theoretical predictions of our model to data from single molecule experiments on bacterial flagellar motors (BFM) of E. coli. In this way, we find evidence that cooperativity controls the mechano-sensitive dynamical assembly of the torque-generating units, the so-called stator units, onto the BFM. Finally, we estimate the stator-stator interaction potential and attempt to quantify the adaptability of the BFM.