Correlation energy of an electron gas: a functional approach

Correlation effects of an electron gas in an external potential are derived using an Effective Action functional method. Corrections beyond the random phase approximation (RPA) are naturally incorporated by this method. The Effective Action functional is made to depend explicitly on two-point correlation functions. The calculation is carried out at imaginary time. For a homogeneous electron gas, we calculate the effect of exchange on the ring diagrams at zero temperature and show how to include some of the ladder diagrams. Our results agree well with known numerical calculations. We conclude by showing that this method is in fact a variant of the time dependent density functional method and suggest that it is suitable to be applied to the study of correlation effects in the non-homogeneous case.Comment: 20 figures numbered as in the tex

The structure of Green functions in quantum field theory with a general state

In quantum field theory, the Green function is usually calculated as the expectation value of the time-ordered product of fields over the vacuum. In some cases, especially in degenerate systems, expectation values over general states are required. The corresponding Green functions are essentially more complex than in the vacuum, because they cannot be written in terms of standard Feynman diagrams. Here, a method is proposed to determine the structure of these Green functions and to derive nonperturbative equations for them. The main idea is to transform the cumulants describing correlations into interaction terms.Comment: 13 pages, 6 figure

Phase transition and critical behaviour of the d=3 Gross-Neveu model

A second order phase transition for the three dimensional Gross-Neveu model is established for one fermion species N=1. This transition breaks a paritylike discrete symmetry. It constitutes its peculiar universality class with critical exponent \nu = 0.63 and scalar and fermionic anomalous dimension \eta_\sigma = 0.31 and \eta_\psi = 0.11, respectively. We also compute critical exponents for other N. Our results are based on exact renormalization group equations.Comment: 4 pages, 1 figure; v4 corresponds to the published articl

An RBC model with investment-specific technological change: lessons for Bulgaria (1999–2018)

We introduce investment-specific technological change (ISTC) into an otherwise standard real-business-cycle setup with a government sector. We calibrate the model to Bulgarian data for the period following the introduction of the currency board arrangement (1999–2018). We then investigate the quantitative importance of the ISTC process in such a model for cyclical fluctuations in Bulgaria, and compare the results to a setup where cycles are driven by shocks to total factor productivity. We find that the model with ISTC shocks matches Bulgarian data better than the standard model driven by changes to total factor productivity only. The ISTC process is thus a better candidate for a a ”technology shock generation process,” at least in Bulgaria since the 2000s

Optical precursors in transparent media

We theoretically study the linear propagation of a stepwise pulse through a dilute dispersive medium when the frequency of the optical carrier coincides with the center of a natural or electromagnetically induced transparency window of the medium (slow-light systems). We obtain fully analytical expressions of the entirety of the step response and show that, for parameters representative of real experiments, Sommerfeld-Brillouin precursors, main field and second precursors "postcursors" can be distinctly observed, all with amplitudes comparable to that of the incident step. This behavior strongly contrasts with that of the systems generally considered up to now

Degenerate Landau-Zener model: Exact analytical solution

The exact analytical solution of the degenerate Landau-Zener model, wherein two bands of degenerate energies cross in time, is presented. The solution is derived by using the Morris-Shore transformation, which reduces the fully coupled system to a set of independent nondegenerate two-state systems and a set of decoupled states. Due to the divergence of the phase of the off-diagonal element of the propagator in the original Landau-Zener model, not all transition probabilities exist for infinite time duration. In general, apart from some special cases, only the transition probabilities between states within the same degenerate set exist, but not between states of different sets. An illustration is presented for the transition between the magnetic sublevels of two atomic levels with total angular momenta J=2 and 1

Domain diversity and polarization switching in amino acid β-glycine

Piezoelectric materials based on lead zirconate titanate are widely used in sensors and actuators. However, their application is limited because of high processing temperature, brittleness, lack of conformal deposition and, more importantly, intrinsic incompatibility with biological environments. Recent studies on bioorganic piezoelectrics have demonstrated their potential in these applications, essentially due to using the same building blocks as those used by nature. In this work, we used piezoresponse force microscopy (PFM) to study the domain structures and polarization reversal in the smallest amino acid glycine, which recently attracted a lot of attention due to its strong shear piezoelectric activity. In this uniaxial ferroelectric, a diverse domain structure that includes both 180° and charged domain walls was observed, as well as domain wall kinks related to peculiar growth and crystallographic structure of this material. Local polarization switching was studied by applying a bias voltage to the PFM tip, and the possibility to control the resulting domain structure was demonstrated. This study has shown that the as-grown domain structure and changes in the electric field in glycine are qualitatively similar to those found in the uniaxial inorganic ferroelectrics. © 2019 by the authors

Approaches to resource management of a modern university in the context of digitalisation

The article considers the evolution of university models, examines in detail the model of a digital university and pays special attention to changing the paradigm of university resource management in the context of digitalization. The transition from managing university resources to managing access to them is substantiated. This problem is relevant, since the Russian economy is faced with the task of overcoming technological dependence in the current conditions of restrictions and Western sanctions. The development of universities of a new quality, integrated with the real sector of the economy, operating on the basis of digital technologies and providing competitive innovative development, could be a solution to this problem. The purpose of the article is to propose a new methodology for the development of the university and the management of its resources in the context of digitalization. In the course of the study the authors solved the following tasks: they analyzed global trends and external risks for higher education; revealed the phenomenon of the digital university; substantiated the new principles of digital university resource management that meet the requirements of the fourth industrial revolution; proposed an approach to the digital development of the university based on the harmonization of the interests and requirements of stakeholders; considered the concepts of infrastructure logic and the strategy of digital transformation of the university. The article used both general scientifc research methods and special ones: frstly, statistical analysis of indicators of global innovation trends in the feld of digitalization of education and the economy; secondly, an institutional approach aimed at identifying effective institutions (rules of the game) that can increase the overall motivation of internal and external stakeholders of the university to ensure an effective digital transformation process; thirdly, the method of multiple thematic comparisons to analyze the scientifc literature and ofcial sources on the digital development of the university and the innovative economy. As a result of the study, the authors formulated a set of recommendations for developing a strategy for the digital transformation of Russian universities based on the author’s methodology for resource management in the digital age for integration into the global innovation process and obtaining leadership positions in global university rankings