Momentum Space Research Articles

Quasi-One-Dimensional Spin Transport in Altermagnetic Z^{3} Nodal Net Metals.

In three dimensions, quasi-one-dimensional (Q1D) transport has traditionally been associated with systems featuring a Q1D chain structure. Here, based on first-principle calculations, we go beyond this understanding to show that the Q1D transport can also be realized in certain three-dimensional (3D) altermagnetic (AM) metals with a topological nodal net in momentum space but lacking Q1D chain structure in real space, including the existing compounds β-Fe_{2}(PO_{4})O, Co_{2}(PO_{4})O, and LiTi_{2}O_{4}. These materials exhibit an AM ground state and feature an ideal crossed Z^{3} Weyl nodal line in each spin channel around Fermi level, formed by three straight and flat nodal lines traversing the entire Brillouin zone. These nodal lines eventually lead to an AM Z^{3} nodal net. Surprisingly, the electronic conductivity σ_{xx} in these topological nodal net metals is dozens of times larger than σ_{yy} and σ_{zz} in the up-spin channel, while σ_{yy} dominates transport in the down-spin channel. This suggests a distinctive Q1D transport signature in each spin channel, and the principal moving directions for the two spin channels are orthogonal, resulting in Q1D direction-dependent spin transport. This novel phenomenon cannot be found in both conventional 3D bulk materials and Q1D chain materials. In particular, the Q1D spin transport gradually disappears as the Fermi energy moves away from the nodal net, further confirming its topological origin. Our Letter not only enhances the comprehension of topological physics in altermagnets but also opens a new direction for the exploration of topological spintronics.

Doubly special relativity as a nonlocal quantum field theory

In this work, we explore the quantum theories of the free massive scalar, the massive fermionic, and the electromagnetic fields in a doubly special relativity scenario. This construction is based on a geometrical interpretation of the kinematics of this kind of theory. In order to describe the modified actions, we find that a higher (indeed infinite) derivative field theory is needed, from which the deformed kinematics can be read. From our construction, we are able to restrict the possible models of doubly special relativity to those that preserve linear Lorentz invariance. We quantize the theories and also obtain a deformed version of the Maxwell equations. We analyze the electromagnetic vector potential for both an electric pointlike source and magnetic dipole. We observe that the electric and magnetic fields do not diverge at the origin for some models described with an anti–de Sitter momentum space, but they do for a de Sitter space in both problems. Published by the American Physical Society 2024

On the timescales in the chaotic dynamics of a 4D symplectic map.

In this work, we investigate different timescales of chaotic dynamics in a multi-parametric 4D symplectic map. We compute the Lyapunov time and a macroscopic timescale, the instability time, for a wide range of values of the system's parameters and many different ensembles of initial conditions in resonant domains. The instability time is obtained by plain numerical simulations and by its estimates from the diffusion time, which we derive in three different ways: through a normal and an anomalous diffusion law and by the Shannon entropy, whose formulation is briefly revisited. A discussion about which of the four approaches provide reliable values of the timescale for a macroscopic instability is addressed. The relationship between the Lyapunov time and the instability time is revisited and studied for this particular system where in some cases, an exponential or polynomial law has been observed. The main conclusion of the present research is that only when the dynamical system behaves as a nearly ergodic one such relationship arises and the Lyapunov and instability times are global timescales, independent of the position in phase space. When stability regions prevent the free diffusion, no correlations between both timescales are observed, they are local and depend on both the position in phase space and the perturbation strength. In any case, the instability time largely exceeds the Lyapunov time. Thus, when the system is far from nearly ergodic, the timescale for predictable dynamics is given by the instability time, being the Lyapunov time its lower bound.

Contributions of tactile information to the sense of agency and its metacognitive representations.

Despite the ubiquitous presence of tactile information in our daily activities, studies of how we experience agency of our actions have rarely relied on manipulated visuo-tactile feedback. Instead, what is often manipulated are the distal (and arbitrarily associated) consequences of our actions. The few studies that did investigate whether tactile information contributes to the experience of agency have been limited to the binary assessment of tactile feedback about the outcome of an action being present or absent. Here, we went beyond the coarse comparison of agency with versus without tactile feedback and introduced instead an experimental manipulation where we could control the amount of mismatch between predictions and observations. Participants (N = 40) reached with their right hand toward a ridged plate with a specific orientation and saw online feedback that could match or differ from their action in one of three ways: the physical plate's orientation, the action's timing, or the hand's position in space. Absolute subjective ratings revealed that an increased mismatch in tactile information led to a diminished sense of agency, similar to what has been reported for spatial and temporal mismatches. Further, estimations of metacognitive efficiency revealed similar Mratios in identifying visuo-tactile violation predictions as compared to visuo-temporal violations (but lower than visuospatial). These findings emphasize the importance of tactile information in shaping our experience of acting voluntarily and show how this important component can be experimentally probed. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Extracting parity-violating gravitational waves from projected tidal force tensor in three dimensions

Gravitational waves (GWs) may be produced by various mechanisms in the early universe. In particular, if parity is violated, it may lead to the production of parity-violating GWs. In this paper, we focus on GWs on the scale of the large-scale structure. Since GWs induce tidal deformations of the shape of galaxies, one can extract such GW signals by observing images of galaxies in galaxy surveys. Conventionally the detection of such signals is discussed by considering the three-dimensional power spectra of the E/B-modes. Here, we develop a complementary new technique to estimate the contribution of GWs to the tidal force tensor field projected on the celestial sphere, which is a directly observable quantity. We introduce two two-dimensional vector fields constructed by taking the divergence and curl of the projected tidal field in three dimensions. Their auto-correlation functions naturally contain contributions of the scalar-type tidal field. However, we find that the divergence of the curl of the projected tidal field, which is a pseudo-scalar quantity, is free from the scalar contribution and thus enables us to extract GW signals. We also find that we can detect parity-violating signals in the GWs by observing the nonzero cross-correlation between the divergence of the projected tidal field and the curl of it. It roughly corresponds to measuring the cross-power spectrum of E and B-modes, but these are complementary to each other in the sense that our estimator can be naturally defined locally in position space. Finally we present expressions of the correlation functions in the form of Fourier integrals, and discuss the properties of the kernels specific to the GW case, which we call the overlap reduction function, borrowing the terminology used in the pulsar timing array experiments.

A nonequilibrium statistical mechanics derivation of the hydrodynamic equations of simple fluids using a noncanonical form of the Poisson bracket

The continuum level hydrodynamic equations of a simple fluid were derived by Irving and Kirkwood directly from discrete particle dynamics using statistical mechanics almost 75 years ago. Their elegant derivation demonstrated the fundamental molecular basis of macroscopic fluid flow and culminated in molecular expressions for the stress tensor and heat current density that have since been employed in countless molecular simulations to date. In this article, an alternative derivation is presented, which leads to more general expressions for the fundamental transport relationships and which arrives at them in a more straightforward chain of consistency that ensues directly from the Principle of Least Action. The main point of departure from the Irving–Kirkwood derivation is the application of a transformation mapping of the total momentum of each individual particle onto the sum of its peculiar momentum and its momentum relative to the local velocity field. This mapping provides a phase-space distribution function applicable in the space of particle positions and peculiar momentum, from which a noncanonical Poisson bracket can be derived in terms of the same set of microscopic variables. For a given dynamic variable, expressed in terms of particle positions and peculiar momenta, the expectation value of the noncanonical Poisson bracket of the dynamic variable is shown to correspond to the evolution equation of the expectation value of the dynamic variable. This allows for a direct derivation of all macroscopic density evolution equations (mass, momentum, and energy density fields) using a systematic procedure free of assumptions concerning the macroscopic state of the system. Furthermore, an explicit expression of the time evolution of the entropy density at the hydrodynamic level is derived following the same procedure. Finally, in the limit of short-range interparticle interactions, a molecular-based expression for the local stress tensor as properly defined from continuum mechanics is developed at the hydrodynamic level that elucidates the continuum mechanics connection of the general stress expression of Irving and Kirkwood.

Positional diffusion: Graph-based diffusion models for set ordering

Positional reasoning is the process of ordering an unsorted set of parts into a consistent structure. To address this problem, we present Positional Diffusion, a plug-and-play graph formulation with Diffusion Probabilistic Models. Using a diffusion process, we add Gaussian noise to the set elements’ position and map them to a random position in a continuous space. Positional Diffusion learns to reverse the noising process and recover the original positions through an Attention-based Graph Neural Network. To evaluate our method, we conduct extensive experiments on three different tasks and seven datasets, comparing our approach against the state-of-the-art methods for visual puzzle-solving, sentence ordering, and room arrangement, demonstrating that our method outperforms long-lasting research on puzzle solving with up to +17% compared to the second-best deep learning method, and performs on par against the state-of-the-art methods on sentence ordering and room rearrangement. Our work highlights the suitability of diffusion models for ordering problems and proposes a novel formulation and method for solving various ordering tasks. We release our code at https://github.com/IIT-PAVIS/Positional_Diffusion.

Cultural Aspects of the Transformation of t he Term Podcasting

This article presents the results of a study of the transformation of the very concept of podcasting in domestic scientifi c discourse, in the Russian media fi eld from a cultural point of view. Podcasting is a form of transmission of mainly audio or video materials (VODcasts) to the Internet, which has existed in the world for about twenty years. Today, podcasts are associated not only with spoken audio content, but also with video interviews, and even with voice messages on the social media Telegram. Moreover, a podcast can be understood as almost any work in the digital media space, even text. This work was carried out in 2023 as part of a comprehensive study of the transformation of the audiovisual environment on new technological platforms, which has been carried out by the Department of Television and Radio Broadcasting of the Faculty of Journalism of Moscow State University since 2017. The purpose of this stage was to determine what is currently understood by the word podcasting and an attempt to capture its distinctive format features, cultural aspects of the evolution of the concept. The methods used were conceptual and contextual types of analysis. The cognitive approach made it possible to consider the verbalized key concepts of scientifi c discourse. The defi nitions given to podcasting by domestic and foreign researchers, practitioners, market participants in the process of development and formation of the industry, as well as a control group of listeners were analyzed. The authors of this article make an attempt to give the most relevant cultural interpretation of the concept of podcasting. The main conclusion is that Russian-language podcasting, for a number of reasons, is developing along a trajectory that is largely different from the world one, in particular the American one. But its development is complicated by a number of factors, including geopolitical and cultural ones. The concept of “podcasting” will most likely continue its evolutionary movement in search of a more solid position in the sociocultural space. Keywords: podcasting, media system, audiovisual content, social media, broadcasting, formats, genres

Cloud–Cloud Collision: Formation of Hub-filament Systems and Associated Gas Kinematics. Mass-collecting Cone—A New Signature of Cloud–Cloud Collision

Massive star-forming regions (MSFRs) are commonly associated with hub-filament systems (HFSs) and sites of cloud–cloud collision (CCC). Recent observational studies of some MSFRs suggest a possible connection between CCC and the formation of HFSs. To understand this connection, we analyzed the magnetohydrodynamic simulation data from Inoue et al. This simulation involves the collision of a spherical turbulent molecular cloud with a plane-parallel sea of dense molecular gas at a relative velocity of about 10 km s−1. Following the collision, the turbulent and nonuniform cloud undergoes shock compression, rapidly developing filamentary structures within the compressed layer. We found that CCC can lead to the formation of HFSs, which is the combined effect of turbulence, shock compression, magnetic field, and gravity. The collision between the cloud components shapes the filaments into a cone and drives inward flows among them. These inward flows merge at the vertex of the cone, rapidly accumulating high-density gas, which can lead to the formation of massive star(s). The cone acts as a mass-collecting machine, involving a nongravitational early process of filament formation, followed by gravitational gas attraction to finalize the HFS. The gas distribution in the position–velocity (PV) and position–position spaces highlights the challenges in detecting two cloud components and confirming their complementary distribution if the colliding clouds have a large size difference. However, such CCC events can be confirmed by the PV diagrams presenting gas flow toward the vertex of the cone, which hosts gravitationally collapsing high-density objects, and by the magnetic field morphology curved toward the direction of the collision.

Currents in celestial CFT

In this review, we discuss currents in celestial CFT and the consistency of their naïve symmetry algebras. In particular, we study in detail the Jacobi identity and the double residue condition for soft insertions, hard momentum space insertions, and hard celestial insertions. In the latter case, we introduce the notion of a “hard current” in CFT and work through examples in the 2D critical Ising model. The current algebra of hard insertions in pure Einstein gravity is a slight conceptual generalization of the familiar [Formula: see text]-wedge current algebra. We also review branch cut terms in the celestial OPE, which indicate new primary content and were previously missed until recently. We work through an explicit toy example illustrating the mechanism by which such branch cut terms can arise. These branch cut terms prevent a symmetry interpretation but are fully compatible with a consistent OPE.

Self-induced Bose glass phase in quantum quasicrystals

We study the emergence of Bose glass phases in self sustained bosonic quasicrystals induced by a pair interaction between particles of Lifshitz–Petrich type. By using a mean-field variational method designed in momentum space as well as Gross–Pitaevskii simulations we determine the phase diagram of the model. The study of the local and global superfluid fraction allows the identification of supersolid, super quasicrystal, Bose glass and insulating phases. The Bose glass phase emerges as a quasicrystal phase in which the global superfluidity is essentially zero, while the local superfluidity remains finite in certain ring structures of the quasicrystalline pattern. Furthermore, we perform continuous space Path Integral Monte Carlo simulations for a case in which the interaction between particles stabilizes a quasicrystal phase. Our results show that as the strength of the interaction between particles is increased the system undergoes a sequence of states consistent with the super quasicrystal, Bose glass, and quasicrystal insulator thermodynamic phases.

Systematic Local Simulations of Fast Neutrino Flavor Conversions with Scattering Effects

Abstract We investigate the dynamics of fast neutrino flavor conversions (FFCs) in the one-dimensional (1D) and zero-dimensional (0D) models, in which spatial advection is considered and ignored, respectively. In this study, we employ snapshots obtained by our self-consistent, realistic Boltzmann-neutrino-radiation-hydrodynamics simulations. We show that the FFC growth rate is considerably larger in the 1D model than in the 0D model, as expected from the previous linear analysis results. We find that the momentum space dimension does not significantly influence the neutrino transition probability in 1D models. On the other hand, in the 0D model without collisions, the FFC depends on the momentum space, and the azimuthal angle dependence breaks the periodicity of the FFC. Our study demonstrates that collisional instability can lead to further flavor conversions on a long timescale in 1D models after the asymptotic state of FFC has been reached. Such an effect should be taken into consideration when the fast and collisional flavor instabilities coexist.

Magnetic alginate microrobots with dual-motion patterns through centrifugally driven flow control

Mobile microrobots have gained increasing attention in biomedical applications because they can be precisely actuated to targeted positions in a tiny space. However, their use in biomedical applications is hindered by the costly and complicated fabrication method. Herein, a facile fabrication method is proposed to produce magnetic alginate microrobots with adjustable dimensions, including teardrop and tadpole shapes, via tunable centrifugally-driven flows. The formation of these microrobots is interpreted by finite element analysis, revealing that the transition between the dripping and jetting regimes of the flow alters the microrobot's shape. The dimensions of the microrobots are quantitatively analyzed based on the flow's extrusion velocity, controlled by nozzle diameters and revolution speeds. Incorporating magnetic nanoparticles into the alginate-based hydrogel enables the microrobots to exhibit distinct motion patterns under a magnetic field. The teardrop-like microrobot can reach a maximum rolling velocity of approximately 2.7 body length s−1 at 2 Hz, while the maximum stick-slip velocity of the tadpole-like microrobot reaches about 0.42 body length s−1 at 5 Hz, comparable to the existing bioinspired magnetic microrobots. These two motion patterns allow the microrobot to overcome obstacles and navigate in vertically constrained environments, respectively. Last, an ultrasound imaging system is deployed to monitor the locomotion and degradation of the microrobots, showing their potential for targeted drug delivery applications.

Lung volumes by CT and plethysmography: are we measuring the same? - CanCOLD study data

BackgroundMeasurement of lung volumes forms an integral part of pulmonary function testing. Lung volumes determined by CT scans are well established, but the comparability to other methods like plethysmography in large cohorts remains in question.MethodsCanCOLD is a prospective longitudinal cohort study from Canada, including three matched groups of individuals with COPD I-II, smokers at risk and healthy controls. All participants underwent lung volume measurement by plethysmography and CT, using inspiratory and expiratory imaging. We compared total lung capacity (TLC) and residual volume (RV) in the different cohorts between plethysmography and CT.ResultsData from 1235 (518 females) individuals were analysed. Baseline characteristics were comparable in all three groups. Significant differences between CT and plethysmography could be observed in all groups, with consistently higher TLC and lower RV by plethysmography, respectively. Correlation was strong for TLC between the methods of measurement with a very stable bias of about 1.68 liters for all groups, but the correlation was only low/moderate for RV. Variability of differences seemed to be higher for RV. No correction for supine position or dead air space was used for CT based measurements.ConclusionMeasurement of TLC and RV by plethysmography yields higher and lower values than by CT, respectively, so results of the different methods should not be used interchangeably.

Topological Surface State Evolution in Bi2Se3 via Surface Etching.

Topological insulators are materials that have an insulating bulk interior while maintaining gapless boundary states against back scattering. Bi2Se3 is a prototypical topological insulator with a Dirac-cone surface state around Γ. Here, we present a controlled methodology to gradually remove Se atoms from the surface Se-Bi-Se-Bi-Se quintuple layers, eventually forming bilayer-Bi on top of the quintuple bulk. Our method allows us to track the topological surface state and confirm its robustness throughout the surface modification. Importantly, we report a relocation of the topological Dirac cone in both real space and momentum space as the top surface layer transitions from quintuple Se-Bi-Se-Bi-Se to bilayer-Bi. Additionally, charge transfer among the different surface layers is identified. Our study provides a precise method to manipulate surface configurations, allowing for the fine-tuning of the topological surface states in Bi2Se3, which represents a significant advancement toward nanoengineering of topological states.

Cosmological correlators for Bogoliubov initial states

We consider late-time correlators in de Sitter (dS) space for initial states related to the Bunch-Davies vacuum by a Bogoliubov transformation. We propose to study such late-time correlators by reformulating them in the familiar language of Witten diagrams in Euclidean anti-de Sitter space (EAdS), showing that they can be perturbatively re-cast in terms of corresponding dS boundary correlators in the Bunch-Davies vacuum and in turn, Witten diagrams in EAdS. Unlike the standard relationship between late-time correlators in the Bunch-Davies vacuum and EAdS Witten diagrams, this involves points on the upper and lower sheet of the EAdS hyperboloid which account for antipodal singularities of the two-point functions. Such Bogoliubov states include an infinite one parameter family of de Sitter invariant vacua as a special case, where the late-time correlators are constrained by conformal Ward identities. In momentum space, it is well known that their late-time correlators exhibit singularities in collinear (“folded”) momentum configurations. We give a position space interpretation of such solutions to the conformal Ward identities, where in embedding space they can be generated from the solution without collinear singularities by application of the antipodal map. We also discuss the operator product expansion (OPE) limit of late-time correlators in a generic dS invariant vacuum. Many results are derived using the Mellin space representation of late-time correlators, which in this work we extend to accommodate generic dS invariant vacua.

Effects of warming and parasitism on root traits and the root economics space

Abstract Plants infected by parasitic species might be further stressed by climate warming as both factors can influence the host plant's nutrient acquisition and growth. The root economics space, defined by root functional traits, reflects a plant's nutrient acquisition strategy. However, the combined effects of warming and parasitism on root functional traits and their positions within the root economics space have not been extensively studies. We grew soybean plants in pot cultures outdoor in the absence or presence of parasitism by Cuscuta gronovii and with or without simulated climate warming using infrared heater lamps. We measured various root functional traits of soybean, including root morphological traits, symbiosis with arbuscular mycorrhizal fungi (AMF) and rhizobia and enzyme activity. Correlations among these traits were analysed, and principal component analysis was used to determine the position of the plants in the root economics space. Parasitism significantly reduced AMF colonization rate, nodule biomass, root tissue density (RTD) and phosphatase activity in the rhizosheath, while increasing root nitrogen concentration (RN). Nodule biomass was positively correlated with AMF colonization, indicating an ‘outsourcing’ strategy for nutrient foraging. Parasitized soybeans occupied a trait space with higher RN and lower RTD, indicative of a ‘fast’ nutrient acquisition strategy. Warming did not significantly affect root functional traits or the plants' positions in the root economics space, regardless of parasitism presence. This study is the first to demonstrate that rhizobial colonization, similar to AMF colonization, is part of the ‘outsourcing’ strategy and that parasitism shifts host plants from a ‘slow’ to a ‘fast’ nutrient uptake strategy. These findings provide new insights into plant below‐ground strategies under warming and parasitism. Read the free Plain Language Summary for this article on the Journal blog.

MonoVisual3DFilter: 3D tomatoes’ localisation with monocular cameras using histogram filters

Abstract Performing tasks in agriculture, such as fruit monitoring or harvesting, requires perceiving the objects’ spatial position. RGB-D cameras are limited under open-field environments due to lightning interferences. So, in this study, we state to answer the research question: “How can we use and control monocular sensors to perceive objects’ position in the 3D task space?” Towards this aim, we approached histogram filters (Bayesian discrete filters) to estimate the position of tomatoes in the tomato plant through the algorithm MonoVisual3DFilter. Two kernel filters were studied: the square kernel and the Gaussian kernel. The implemented algorithm was essayed in simulation, with and without Gaussian noise and random noise, and in a testbed at laboratory conditions. The algorithm reported a mean absolute error lower than 10 mm in simulation and 20 mm in the testbed at laboratory conditions with an assessing distance of about 0.5 m. So, the results are viable for real environments and should be improved at closer distances.

Numerical indication that center vortices drive dynamical mass generation in QCD

The first calculation of the response of the momentum space quark propagator to center vortices in the ground state fields of QCD is presented. Center vortices are identified on 2+1-flavor dynamical gauge fields with mπ≃156 MeV to obtain the vortex-removed and vortex-only quark propagator. Dynamical mass generation is found to vanish upon vortex removal, while the vortex-only field is able to generate dynamical mass. These new signatures strengthen the lattice QCD evidence indicating that center vortices underpin both dynamical chiral symmetry breaking and quark confinement. Published by the American Physical Society 2024

Dynamic protected states in the non-Hermitian system

The non-Hermitian skin effect and nonreciprocal behavior are sensitive to the boundary conditions, which are unique features of non-Hermitian systems. In such systems, eigenenergies can become complex, and all eigenstates tend to localize at the boundary, a phenomenon that contrasts with Hermitian topologies. In this work, we theoretically study the dynamic behavior of the propagation of Gaussian wavepackets inside a non-Hermitian lattice and analyze the self-acceleration process of bulk state or Gaussian wavepackets toward the system’s boundary. The initial wavepackets will not only propagate toward the side where the eigenstates are localized, but also their momentum will approach to a specific value. This value corresponds to the maximum imaginary components of the energy dispersion. In addition, if the wavepackets in the momentum space cover this specific momentum, they will eventually exhibit exponentially increasing amplitudes with time evolution, maintaining the dynamic protected condition for an extended period of time until they approach the boundary. We also take two widely used toy models as examples in one and two dimensions to verify the correspondence of the non-Hermitian skin effect and the dynamic protected state.