IPBPub Institute of Physics Belgrade Publication Repository
Not a member yet
50 research outputs found
Sort by
Analysis of Worldwide Time-Series Data Reveals Some Universal Patterns of Evolution of the SARS-CoV-2 Pandemic
Get PDFPredicting the evolution of the current epidemic depends significantly on understanding the nature of the underlying stochastic processes. To unravel the global features of these processes, we analyse the world data of SARS-CoV-2 infection events, scrutinising two 8-month periods associated with the epidemic’s outbreak and initial immunisation phase. Based on the correlation-network mapping, K-means clustering, and multifractal time series analysis, our results reveal several universal patterns of infection dynamics, suggesting potential predominant drivers of the pandemic. More precisely, the Laplacian eigenvectors localisation has revealed robust communities of different countries and regions that break into clusters according to similar profiles of infection fluctuations. Apart from quantitative measures, the immunisation phase differs significantly from the epidemic outbreak by the countries and regions constituting each cluster. While the similarity grouping possesses some regional components, the appearance of large clusters spanning different geographic locations is persevering. Furthermore, characteristic cyclic trends are related to these clusters; they dominate large temporal fluctuations of infection evolution, which are prominent in the immunisation phase. Meanwhile, persistent fluctuations around the local trend occur in intervals smaller than 14 days. These results provide a basis for further research into the interplay between biological and social factors as the primary cause of infection cycles and a better understanding of the impact of socio-economical and environmental factors at different phases of the pandemic
Spectral and thermodynamic properties of the Holstein polaron: Hierarchical equations of motion approach
No full textWe develop a hierarchical equations of motion (HEOM) approach to compute real-time single-particle correlation functions and thermodynamic properties of the Holstein model at finite temperature. We exploit the conservation of the total momentum of the system to formulate the momentum-space HEOM whose dynamical variables explicitly keep track of momentum exchanges between the electron and phonons. Our symmetry-adapted HEOM enable us to overcome the numerical instabilities inherent to the commonly used real-space HEOM. The HEOM method is then used to study the spectral function and thermodynamic quantities of chains containing up to ten sites. The HEOM results compare favorably to existing literature. To provide an independent assessment of the HEOM approach and to gain insight into the importance of finite-size effects, we devise a quantum Monte Carlo (QMC) procedure to evaluate finite-temperature single-particle correlation functions in imaginary time and apply it to chains containing up to twenty sites. QMC results reveal that finite-size effects are quite weak, so that the results on 5 to 10-site chains, depending on the parameter regime, are representative of larger systems. A detailed comparison between the HEOM and QMC data place our HEOM method among reliable methods to compute real-time finite-temperature correlation functions in parameter regimes ranging from low- to high-temperature, and weak- to strong-coupling regime
Cold atoms in space: community workshop summary and proposed road-map
Get PDFWe summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to exploit the full potential of cold atoms in space. Finally, we present a first draft of a possible road-map for achieving these goals, that we propose for discussion by the interested cold atom, Earth Observation, fundamental physics and other prospective scientific user communities, together with the European Space Agency (ESA) and national space and research funding agencies
Vertical evolution of exocometary gas – I. How vertical diffusion shortens the CO lifetime
No full textBright debris discs can contain large amounts of CO gas. This gas was thought to be a protoplanetary remnant until it was recently shown that it could be released in collisions of volatile-rich solids. As CO is released, interstellar UV radiation photodissociates CO producing CI, which can shield CO allowing a large CO mass to accumulate. However, this picture was challenged because CI is inefficient at shielding if CO and CI are vertically mixed. Here, we study for the first time the vertical evolution of gas to determine how vertical mixing affects the efficiency of shielding by CI. We present a 1D model that accounts for gas release, photodissociation, ionization, viscous evolution, and vertical mixing due to turbulent diffusion. We find that if the gas surface density is high and the vertical diffusion weak (αv/α < [H/r]2) CO photodissociates high above the mid-plane, forming an optically thick CI layer that shields the CO underneath. Conversely, if diffusion is strong (αv/α > [H/r]2) CI and CO become well mixed, shortening the CO lifetime. Moreover, diffusion could also limit the amount of dust settling. High-resolution ALMA observations could resolve the vertical distribution of CO and CI, and thus constrain vertical mixing and the efficiency of CI shielding. We also find that the CO and CI scale heights may not be good probes of the mean molecular weight, and thus composition, of the gas. Finally, we show that if mixing is strong the CO lifetime might not be long enough for CO to spread interior to the planetesimal belt where gas is produced
Universal Magnetic Oscillations of dc Conductivity in the Incoherent Regime of Correlated Systems
Get PDFUsing the dynamical mean field theory we investigate the magnetic field dependence of dc conductivity in the Hubbard model on the square lattice, fully taking into account the orbital effects of the field introduced via the Peierls substitution. In addition to the conventional Shubnikov-de Haas quantum oscillations, associated with the coherent cyclotron motion of quasiparticles and the presence of a welldefined Fermi surface, we find an additional oscillatory component with a higher frequency that corresponds to the total area of the Brillouin zone. These paradigm-breaking oscillations appear at elevated temperature. This finding is in excellent qualitative agreement with the recent experiments on graphene superlattices. We elucidate the key roles of the off-diagonal elements of the current vertex and the incoherence of electronic states, and explain the trends with respect to temperature and doping
Microscopic derivation of Dirac composite fermion theory: Aspects of noncommutativity and pairing instabilities
Get PDFBuilding on previous work [N. Read, Phys. Rev. B 58, 16262 (1998); Z. Dong and T. Senthil, Phys. Rev. B 102, 205126 (2020)] on the system of bosons at filling factor nu = 1, we derive the Dirac composite fermion theory for a half-filled Landau level from first principles and apply the Hartree-Fock approach in a preferred representation. On the basis of the microscopic formulation, in the long-wavelength limit, we propose a noncommutative field-theoretical description, which in a commutative limit reproduces the Son's theory, with additional terms that may be expected on physical grounds. The microscopic representation of the problem is also used to discuss pairing instabilities of composite fermions. We find that a presence of a particle-hole symmetry breaking leads to a weak (BCS) coupling p-wave pairing in the lowest Landau level, and strong coupling p-wave pairing in the second Landau level that occurs in a band with nearly flat dispersion, a third power function of momentum
Fragmentation in trader preferences among multiple markets: market coexistence versus single market dominance
Get PDFTechnological advancement has led to an increase in the number and type of trading venues and a diversification of goods traded. These changes have re-emphasized the importance of understanding the effects of market competition: does proliferation of trading venues and increased competition lead to dominance of a single market or coexistence of multiple markets? In this paper, we address these questions in a stylized model of zero-intelligence traders who make repeated decisions at which of three available markets to trade. We analyse the model numerically and analytically and find that the traders’ decision parameters—memory length and how strongly decisions are based on past success—make the key difference between consolidated and fragmented steady states of the population of traders. All three markets coexist with equal shares of traders only when either learning is too weak and traders choose randomly, or when markets are identical. In the latter case, the population of traders fragments across the markets. With different markets, we note that market dominance is the more typical scenario. Overall we show that, contrary to previous research emphasizing the role of traders’ heterogeneity, market coexistence can emerge simply as a consequence of co-adaptation of an initially homogeneous population of traders.</jats:p
Particle-hole Pfaffian intracorrelations and intercorrelations in the quantum Hall bilayer
Get PDFParticle-hole (PH) Pfaffian topological phases may exist in a uniform system due to strong Landau level (LL) mixing according to theoretical predictions based on the Son-Dirac composite fermion theory. Numerical investigations in the presence of large LL mixing are limited due to numerical complexities, when taking into account at least one more LL. Because of this, we apply the same field theoretical approach to the quantum Hall bilayer at total filling factor equal to one, for which many numerical studies exist. The most advanced in Zhu et al. [Zheng Zhu, Liang Fu, D. N. Sheng, Phys. Rev. Lett. 119, 177601 (2017)] predicts an intermediate phase (for intermediate distances between layers) with an even-odd effect. According to our approach, the intermediate phase represents a mixed negative-flux p-wave pairing i.e., coexisting intra (PH Pfaffian in each layer) and inter (a la PH Pfaffian) pairing correlations. This again underlines a necessity for strong entanglement with additional degrees of freedom, i.e., at least one more (additional) LL in the search for a stable PH Pfaffian phase in a single layer. Based on the analogy with the bilayer physics, we propose a PH Pfaffian wave function that resides in two LLs
Electrical conductivity in the Hubbard model: Orbital effects of magnetic field
Get PDFCalculation of conductivity in the Hubbard model is a challenging task. Recent years have seen much progress in this respect and numerically exact solutions are now possible in certain regimes. In this paper we discuss the calculation of conductivity for the square-lattice Hubbard model in the presence of a perpendicular magnetic field, focusing on orbital effects. We present the relevant formalism in all detail and in full generality, and then discuss the simplifications that arise at the level of the dynamical mean field theory (DMFT). We prove that the Kubo bubble preserves gauge and translational invariance, and that in the DMFT the vertex corrections cancel regardless of the magnetic field. We present the DMFT results for the spectral function and both the longitudinal and Hall conductivities in several regimes of parameters. We analyze thoroughly the quantum oscillations of the longitudinal conductivity and identify a high-frequency oscillation component, arising as a combined effect of scattering and temperature, in line with recent experimental observations in moire systems
Calculations of electron mobility in II-VI semiconductors
Get PDFElectron mobility in the conduction band of II-VI semiconductors ZnSe, CdTe, ZnTe, and CdSe was studied. Temperature dependence of mobility was calculated using the methodology based on density functional theory calculations of the electronic states, phonon modes, and electron-phonon coupling constants, along with Fourier-Wannier procedure for interpolation to a dense grid in momentum space. The mobilities obtained from calculations within generalized gradient approximation of density functional theory overestimate the experimental mobility several times. The calculation that used improved electronic band structure and high-frequency dielectric constants obtained using a hybrid functional lead to a very good agreement with experimental mobilities for most of the materials studied. It was also found that the Frohlich model provides a reasonably good estimate of mobilities around room temperature where longitudinal optical phonons provide the dominant scattering mechanism, as expected for these direct gap materials where all relevant electronic states are in the vicinity of the Gamma point. The results indicate as well that the long-ranged part of electron-phonon interaction fully determines the electron mobility in the materials studied. For this reason, the approach where only this part of electron-phonon interaction is calculated using the relevant analytical formulas allows for accurate calculation of mobility without the use of the interpolation procedure for electron-phonon coupling constants