arXiv subjects

Victor I. Danchev

Publications and source records attributed to Victor I. Danchev.

6 recordsLinked to original sources

arXiv2022

Constraining scalarization in scalar-Gauss-Bonnet gravity through binary pulsars

In the present paper we derive strong constrains on scalarization in scalar-Gauss-Bonnet (sGB) gravity using observations of pulsars in close binary systems. Since scalarized neutron stars carry a nonzero scalar change, they emit scalar dipole radiation while inspiraling which speeds up the orbital decay. The observations support the conjecture that such radiation is either absent or very small for the observed binary pulsars. Using this, we determine the allowed range of parameters for sGB gravity. We also transfer the derived constraints to black holes in sGB gravity. It turns out that the maximum mass of a scalarized static black hole can not exceed roughly ten to twenty solar masses, depending on the initial assumptions we make for the nuclear matter equations of state. The black hole scalar charge on the other hand can reach relatively large values that are potentially observable.

gr-qc↗

arXiv2021

Probability distribution function of crossover frequency of operational amplifiers

For the first time the cumulative distribution function of the crossover frequency of a contemporary operational amplifier ADA4898-2 is experimentally studied. Using a USB Lock-In amplifier, which allows automatic frequency sweep of the current response of a non-inverting amplifier with significant static amplification, we measure the crossover frequency of 200 samples of ADA4898-2 operational amplifiers. This new method gives a significant advantage in accuracy and speed of study of every operational amplifier. The theory we use is based on the universal relation between time dependent output and input voltages. This common relation for all operational amplifiers is applicable for frequencies much smaller than the crossover frequency and the frequencies of non-dominant poles. In other words, this approximation is adequate, when an operational amplifier is included in a circuit with significant amplification.

eess.SP↗

arXiv2020

Constraining the equation of state in modified gravity via universal relations

Modern multi-messenger astronomical observations and heavy ion experiments provide new insights into the structure of compact objects. Nevertheless, much ambiguity remains when it comes to super dense matter above the nuclear saturation density such as that found within neutron stars. This work explores equation of state (EOS)-independent universal relations between the physical parameters of static neutron stars at the maximum-mass point, previously proven for General Relativity (GR) and used to constraint EOS candidates within the GR framework. We explore 53 different EOS candidates and prove that these relations hold for scalarized neutron stars in massive scalar-tensor theories, exploring also the effect of non-baryonic EOS. We further show that the relation's fit parameters are highly dependent on the theory's parameters. On the basis of these relations multiple constraints on the EOS can be derived and it turns out that they can be significantly different than the GR ones. This demonstrates the importance of taking into account the modified gravity effects even when imposing constraints on the EOS.

gr-qc↗

arXiv2020

Slowly rotating topological neutron stars -- universal relations and epicyclic frequencies

In the modern era of abundant X-ray detections and the increasing momentum of gravitational waves astronomy, tests of General Relativity in strong field regime become increasingly feasible and their importance for probing gravity cannot be understated. To this end, we study the characteristics of slowly rotating topological neutron stars in the tensor-multi-scalar theories of gravity following the static study of this new type of compact objects by two of the authors. We explore the moment of inertia and verify that universal relations known from General Relativity hold for this new class of compact objects. Furthermore, we study the properties of their innermost stable circular orbits (ISCO) and the epicyclic frequencies due to the latter's hinted link to observational quantities such as quasi-periodic X-ray spectrum features.

gr-qc↗

arXiv2019

Electric Oscillations Generated by Fluctuation Cooper Pairs

A short review of the history and the contemporary numerical calculations of the operation of an electronic device for generation of electric oscillations by negative differential conductivity of a supercooled below the critical temperature superconductor. The superconductor is cooled below the critical temperature at applied small constant electric field, which keeps the superconductor in normal state. To simulate the device operation, beforehand analytical expressions for the conductivity of nano-structured superconductors supercooled below the critical temperature in electric field are used. The numerical analysis meant to alleviate the development of a device shows that the region of negative differential conductivity of the current voltage characteristics leads to generation of electric oscillations. The study of layered high temperature superconductors and radiated electromagnetic waves in space will be an important problem of the future space technology.

cond-mat.supr-con↗

arXiv2019

Computer Simulation of Electronic Device for Generation of Electric Oscillations by Negative Differential Conductivity of Supercooled Nanostructured Superconductors in Electric Field

We use the formerly derived explicit analytical expressions for the conductivity of nanostructured superconductors supercooled below the critical temperature in electric field. Computer simulations reveal that the negative differential conductivity region of the current-voltage characteristic leads to excitation of electric oscillations. We simulate a circuit with distributed elements as a first step to obtain gigahertz oscillations. This gives a hint that a hybrid device of nanostructured superconductors will work in terahertz frequencies. If the projected setup is successful, we consider the possibility for it to be put in a nanosatellite (such as a CubeSat). A study of high temperature superconductor layers in space vacuum and radiation would be an important technological task. The interface thermal boundary resistance is taken into account in the numerical illustrations for the work of the device.

cond-mat.supr-con↗