Resource
2024 EN
Leandro Beraldo e Silva · Monica Valluri · Eugene Vasiliev
+3 more
A tracer sample in a gravitational potential, starting from a generic initialcondition, phase-mixes towards a stationary state. This evolution isaccompanied by an entropy increase, and the final state is characterized by adistribution function (DF) that depends only on integrals of motion (Jeanstheorem). We present a method to constrain a gravitational potential where asample is stationary by minimizing the entropy the sample would have if it wereallowed to phase-mix in trial potentials. This method avoids assuming a knownDF, and is applicable to any sets of integrals. We provide expressions for theentropy of DFs depending on energy, $f(E)$, energy and angular momentum,$f(E,L)$, or three actions, $f(\vec{J})$, and investigate the bias andfluctuations in their estimates. We show that the method correctly recovers thepotential parameters for spherical and axisymmetric models. We also present amethodology to characterize the posterior probability distribution of theparameters with an Approximate Bayesian Computation, indicating a pathway forapplication to observational data. Using $N=10^4$ tracers with$20\%$-uncertainties in the 6D coordinates, we recover the flattening parameter$q$ of an axisymmetric potential with $\sigma_q/q\sim 10\%$.
Resource
2024 EN
Carina Killian · Philipp Blumer · Paolo Crivelli
+9 more
A low energy particle confined by a horizontal reflective surface and gravitysettles in gravitationally bound quantum states. These gravitational quantumstates (GQS) were so far only observed with neutrons. However, the existence ofGQS is predicted also for atoms. The GRASIAN collaboration pursues the firstobservation of GQS of atoms, using a cryogenic hydrogen beam. This endeavor ismotivated by the higher densities, which can be expected from hydrogen comparedto neutrons, the easier access, the fact that GQS were never observed withatoms and the accessibility to hypothetical short range interactions. Inaddition to enabling gravitational quantum spectroscopy, such a cryogenichydrogen beam with very low vertical velocity components - a few cm s$^{-1}$,can be used for precision optical and microwave spectroscopy. In this article,we report on our methods developed to reduce background and to detect atomswith a low horizontal velocity, which are needed for such an experiment. Ourrecent measurement results on the collimation of the hydrogen beam to 2 mm, thereduction of background and improvement of signal-to-noise and finally ourfirst detection of atoms with velocities < 72 m s$^{-1}$ are presented.Furthermore, we show calculations, estimating the feasibility of the plannedexperiment and simulations which confirm that we can select vertical velocitycomponents in the order of cm s$^{-1}$.
Resource
2024 EN
Andrey Babichev · Ivan Makhov · Natalia Kryzhanovskaya
+12 more
A comprehensive numerical modelling of microcavity parameters for micropillarlasers with optical pumping was presented. The structure with a hybriddielectric-semiconductor top mirror has a significantly higher calculatedquality-factor (~65000 for 5 $\mu$m pillar) due to better vertical modeconfinement. The minimum laser threshold (~370 $\mu$W for 5 $\mu$m pillar)coincided with a temperature of 130 K, which is close to zero gain to cavitydetuning. Lasing up to 220 K was demonstrated with a laser threshold of about2.2 mW.
Resource
2024 EN
Behzad Tahmasebzadeh · Andrew Lapeer · Eugene Vasiliev
+3 more
Due to observational challenges, the mass function of black holes (BH) atlower masses is poorly constrained in the local universe. Understanding theoccupation fraction of BHs in low-mass galaxies is crucial for constraining theorigins of supermassive BH seeds. Compact stellar systems (CSSs), includingultra-compact dwarf galaxies (UCDs) and compact elliptical galaxies (cEs), arepotential intermediate-mass BH hosts. Despite the difficulties posed by theirlimited spheres of influence, stellar dynamical modeling has been effective inestimating central BH masses in CSSs. Some CSSs may harbor a BH constituting upto 20% of their host stellar mass, while others might not have a central BH. Insupport of our ongoing efforts to determine the BH masses in select CSSs in theVirgo cluster using JWST/NIRSpec IFU observations and orbit-superpositiondynamical models, we create mock kinematic data mimicking the characteristicsof observed cEs/UCDs in the Virgo cluster with different BH masses. We thenconstruct a series of dynamical models using the orbit-superposition codeFORSTAND and explore the accuracy of recovering the BH mass. We find that themass of BHs comprising 1% or more of the total host stellar mass can beaccurately determined through kinematic maps featuring higher-order velocitymoments. We also assess how BH mass measurement is affected by deprojectionmethods, regularization factors, anisotropy parameters, orbit initialconditions, the absence of higher-order velocity moments, spatial resolution,and the signal-to-noise ratio.
Resource
2024 EN
Vasiliy Alekseev · Ilya Lukashevich · Ilia Zharikov
+1 more
Deep neural network models have a complex architecture and areoverparameterized. The number of parameters is more than the whole dataset,which is highly resource-consuming. This complicates their application andlimits its usage on different devices. Reduction in the number of networkparameters helps to reduce the size of the model, but at the same time,thoughtlessly applied, can lead to a deterioration in the quality of thenetwork. One way to reduce the number of model parameters is matrixdecomposition, where a matrix is represented as a product of smaller matrices.In this paper, we propose a new way of applying the matrix decomposition withrespect to the weights of convolutional layers. The essence of the method is totrain not all convolutions, but only the subset of convolutions (basisconvolutions), and represent the rest as linear combinations of the basis ones.Experiments on models from the ResNet family and the CIFAR-10 datasetdemonstrate that basis convolutions can not only reduce the size of the modelbut also accelerate the forward and backward passes of the network. Anothercontribution of this work is that we propose a fast method for selecting asubset of network layers in which the use of matrix decomposition does notdegrade the quality of the final model.
Resource
2024 EN
Farid Bikmukhametov · Lana Glazko · Yaroslav Muravev
+4 more
Acoustic black holes represent a special class of metastructures allowingefficient absorption based on the slow sound principle. The decrease of thewave speed is associated with the spatial variation of acoustic impedance,while the absorption properties are linked to thermoviscous losses induced bythe local resonances of the structure. While most of the developments in thefield of sonic black holes are dedicated to one-dimensional structures, thecurrent study is concerned with their two-dimensional counterparts. It is shownthat the change of the dimensionality results in the change of noise insulationmechanism, which relies on the opening of band-gaps rather then thermoviscouslosses. The formation of band-gaps is associated with the strong couplingbetween the resonators constituting the considered structures. Numerically andexperimentally it is shown than the structure is characterized by broadstop-bands in transmission spectra, while the air flow propagation is stillallowed. In particular, a realistic application scenario is considered, inwhich the acoustic noise and the air flow are generated by a fan embedded intoa ventilation duct. The obtained results pave the way towards the developmentof next-level ventilated metamaterials for efficient noise control.
Resource
2024 EN
G. V. Sotnikov · A. V. Vasiliev · I. V. Beznosenko
+3 more
A comparative analysis of two types of dielectric laser accelerators (DLA)based on periodic (grating) and flat dielectric structures to accelerateelectrons in the energy range from 300 keV to 3 GeV is presented. The mainattention is paid to the conditions, efficiency and restrictions of eachacceleration method, as well as the influence of laser radiation parameters onelectron acceleration processes. Single and double (both grating and flat)dielectric structures and their impact on acceleration are considered. For thestudy of two types of quartz DLA, the Ti:Sa laser system with a generation bandwidth 790-810 nm (FWHM), the laser electric field 6 GeV/m are used. The studyshowed that a flat dielectric structure provides more effective acceleration ina wide range of energies, especially with a symmetrical geometry (doublestructures), compared with the periodic structure. If we consider only aperiodic structure, then with the selected symmetrical geometry, for the ultrarelativistic electrons, it demonstrates the acceleration rate two times ofmagnitude more than for single configuration. However, the use of a one-sidedperiodic structure turns out to be preferable for accelerating electrons withmoderate energies, ~0.5-0.9 MeV, where the acceleration rate in a one-sidedconfiguration is higher than in a symmetric (double) periodic structure. Thespace-time distributions of laser-excited electromagnetic fields in theaccelerating channel and their influence on the electron beam is analyzed also.The advantage of a flat structure over a periodic one, which arises due to thedesign features of the corresponding dielectric accelerators, is discussed.
Resource
2024 EN
Myung-Hwan Whangbo · Hyun-Joo Koo · Reinhard K. Kremer
+1 more
To search for a conceptual picture describing the magnetization plateauphenomenon, we surveyed the crystal structures and the spin lattices of thosemagnets exhibiting plateaus in their magnetization vs. magnetic field curves byprobing the three questions: (a) why only certain magnets exhibit magnetizationplateaus, (b) why there occur several different types of magnetizationplateaus, and (c) what controls the widths of magnetization plateaus. We showthat the answers to these questions lie in how the magnets under field absorbZeeman energy hence changing their magnetic structures. The magnetic structureof a magnet insulator is commonly described in terms of its spin lattice, whichrequires the determination of the spin exchanges nonnegligible strengthsbetween the magnetic ions. Our work strongly suggests that a magnet undermagnetic field partitions its spin lattice into antiferromagnetic (AFM) orferrimagnetic fragments by breaking its weak magnetic bonds. Our supposition ofthe field-induced partitioning of spin lattice into magnetic fragments issupported by the anisotropic magnetization plateaus of Ising magnets and by thehighly anisotropic width of the 1/3-magnetization plateau in azurite. Theanswers to the three questions (a) - (c) emerge naturally by analyzing howthese fragments are formed under magnetic field.
Resource
2024 EN
A. V. Artemyev · D. Mourenas · X. -J. Zhang
+8 more
The dynamics of the Earth's outer radiation belt, filled by energeticelectron fluxes, is largely controlled by electron resonant interactions withelectromagnetic whistler-mode waves. The most coherent and intense wavesresonantly interact with electrons nonlinearly, and the observable effects ofsuch nonlinear interactions cannot be described within the frame of classicalquasi-linear models. This paper provides an overview of the current stage ofthe theory of nonlinear resonant interactions and discusses different possibleapproaches for incorporating these nonlinear interactions into global radiationbelt simulations. We focused on observational properties of whistler-mode wavesand theoretical aspects of electron nonlinear resonant interactions betweensuch waves and energetic electrons.
Journals
2023 EN
Murtazoev Alisher F. · Lyssenko Konstantin A. · Markina Maria M.
+3 more
A new family of compounds A Cu 7 TeO 4 (SO 4 ) 5 Cl ( A =Na, K, Rb, Cs) isostructural to mineral Nabokoite (K species) was synthesized by solid state and gas transport reactions in sealed ampoules and characterized in measurements of magnetization and specific heat in a wide temperature range. These complex compounds are of the utmost interest as a testing playground to study the properties of quasi‐two‐dimensional magnets with a square kagome lattice geometry. A quantum ground state of such a corner‐sharing network is a spin liquid. Unlike idealized grid analyzed in numerous models, the square kagome lattice in nabokoites is wavy and distorted being composed of versatile triangles. Moreover, it contains “excessive” decorating magnetic ions, which makes magnetism of these objects even more complicated. The interaction of these decorating ions through virtual excitations of the square kagome lattice is accompanied by the formation of a long‐range magnetic order coexisting with the spin liquid.