Searcharxiv⌕ Search

arXiv subjects

Todor M. Mishonov

Publications and source records attributed to Todor M. Mishonov.

At least 37 records · Page 2Linked to original sources
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↗
arXiv2019

Determination of Boltzmann constant by equipartition theorem for capacitors

A new experimental set-up for Boltzmann constant measurement is described. Statistically averaged square of voltage $\left $ is measured for different capacitances $C$. Boltzmann constant is determined by the equipartition theorem $C\left =k_{_\mathrm{B}}T$. For fixed capacitance, voltages could be measured for different temperatures. The set-up consists of low-noise high frequency operational amplifiers ADA4898-2. An instrumental amplifier is followed by an inverting amplifier, square of the voltage is created by an analog multiplier AD633 and finally the averaged signal is measured by a multimeter.

physics.ed-ph↗
arXiv2019

Measurement of the electron charge $q_e$ using Schottky noise. Problem of the 6-th Experimental Physics Olympiad. Sofia 8 December 2018

Several consecutive experiments are described with a printed circuit board PCB set-up, especially designed for these experiments. Doing the consecutive experimental tasks opens up possibility to determine the value of electron charge $q_e.$ The fluctuations of the voltage $U(t)$ should be measured for different illuminations of a photodiode. The voltage is amplified 1 million times $Y=10^6$. The amplified voltage $YU(t)$ is applied to the device, which gives the result of the value of the time averaged square of the voltage $U_\mathrm{S}=\left<(Y U(t))^2\right>/U_0$. This voltage $U_\mathrm{S}$ is measured with a multimeter. The series of measurements gives the possibility to determine the $q_e$ using the well known Schottky formula for the spectral density of the current noise $(I^2)_f=2q_e\left .$ For the junior high school students, the basic problem is to analyze the analog squaring. Students' work is separated and graded in four categories S, M, L, XL divided by age of students. For the last XL categories, the tasks contain problems oriented to physics university education program and include theoretical research of the PCB set-up as an engineering device. This is the problem of EPO6, December 2018 ``Day of the Charge'' considered. EPO6 is organized by Sofia branch of Union of physicists in Bulgaria in cooperation with Faculty of physics of Sofia University and Society of Physicists of Republic of Macedonia.

physics.ed-ph↗
arXiv2018

Planck's constant measurement by Landauer quantization for students laboratories

A simple experimental setup for measuring the Planck's constant, using Landauer quantization of the conductance between touching gold wires, is described. It consists of two gold wires with thickness of 0.5 mm and 1.5 cm length, and an operational amplifier. The setup costs less than $30 and can be realized in every teaching laboratory in two weeks. The usage of oscilloscope is required.

physics.ed-ph↗
arXiv2016

Measuring the speed of light with electric and magnetic pendulum

This problem was given at the Fourth Open and International Experimental Physics Olympiad (EPO4) Day of Light on 23 April 2016 in Sofia organized by the Sofia Branch of the Union of Physicists in Bulgaria and the Society of Physicists of the Republic of Macedonia, Strumica.

physics.ed-ph↗
arXiv2016

Measuring Plank constant with colour LEDs and compact disk

This problem was given on the Open Experimental Physics Olympiad (OEPO) "The day of the photon" on 25 April 2015. The Olympiad was a part of the celebration of the International Year of Light (IYL) http://www.light2015.org/Home/Event-Programme/2015/Competition/Bulgaria-Second-Experimental-Physics-Olympiad--25-April-2015-in-Sofia.-The-Day-of-the-Photon-in-the-International-Year-of-the-Light.html and was organized by the Sofia Branch of the Union of Physicist in Bulgaria and the Regional Society of Physicists of Strumica, Macedonia.

physics.ed-ph↗
arXiv2016

Heating of the solar corona by Alfven waves - self-induced opacity

Static distributions of temperature and wind velocity at the transition region are calculated within the framework of magnetohydrodynamics (MHD) of completely ionized hydrogen plasma. The numerical solution of the derived equations gives the width of the transition layer between the chromosphere and the corona as a self-induced opacity of high-frequency Alfven waves (AW). The domain wall is direct consequence of the self-consistent MHD treatment of AW propagation. The low-frequency MHD waves coming from the Sun are strongly reflected by the narrow transition layer, while the high-frequency waves are absorbed - that is why we predict considerable spectral density of the AW in the photosphere. The numerical method allows consideration of incoming AW with arbitrary spectral density. The idea that Alfven waves might heat the solar corona belongs to Alfven, we simply solved the corresponding MHD equations. The comparison of the solution to the experiment is crucial for revealing the heating mechanism.

astro-ph.SR↗
arXiv2015

Comment on "Interface tension of Bose-Einstein condensates" by Bert Van Schaeybroeck, Phys. Rev. A 78, 023624-9 (2008)

The purpose of the comment is to point out that the leading term of the Ginzburg-Landau nonanalytical correction to the interface tension of Bose-Einstein condensates with strong segregation and the surface tension of extreme type-I superconductors are described by a common coefficient derived from the universal equation for the phase boundary. The agreement between the numerical value of the coefficients gives a hint that this can be an exact result which deserves to be checked. The outcome will be of interest for physicists working in both fields.

cond-mat.quant-gas↗
arXiv2015

Capillary waves at the interface of two Bose-Einstein condensates. Long wavelengths asymptotic by trial function approach

The dispersion relation for capillary waves at the boundary of two different Bose condensates is investigated using a trial wave-function approach applied to the Gross-Pitaevskii (GP) equations. The surface tension is expressed by the parameters of the GP equations. In the long wave-length limit the usual dispersion relation is re-derived while for wavelengths comparable to the healing length we predict significant deviations from the $ω\propto k^{3/2}$ law which can be experimentally observed. We approximate the wave variables by a frozen order parameter, i.e. the wave function is frozen in the superfluid analogous to the magnetic field in highly conductive space plasmas.

cond-mat.quant-gas↗
arXiv2015

Resonance measurement of periodically driven contact potential difference

A new type of quartz resonance device for measurements of oscillations of the contact potential difference induced by modulated light is described. Special attention is devoted to the compensation of the constructive capacitance of the quartz resonator by a negative capacitance. In such a way the the quartz resonance filter is very narrow and at the same time has extremely small total bandwidth, which improves significantly the signal to noise ratio. For metals, this device gives the temperature dependence of the work function and opens perspectives for creation of imaging spectroscopy based on this temperature derivative. The proposed device can be used for creation of defectoscopy of metallic materials based on the temperature derivative of the work function.

physics.ins-det↗
arXiv2008

Plasmon spectrum of degenerated electron gas. T=0 Green function method. Detailed pedagogical derivation

Plasmon spectrum and polarization operator of 1, 2, and 3 dimensional electron gas are calculated by T=0 Green function technique. It is shown that this field theory method gives probably the simplest pedagogical derivation of the statistical problem for the response function. The explanation is complimentary to the standard courses on condensed matter and plasma physics of the level of IX volume of Landau-Lifshitz encyclopedia on theoretical physics.

cond-mat.stat-mech↗
arXiv2007

Boltzmann equation for fluctuation Cooper pairs in Lawrence-Doniach model. Possible out-of-plane negative differential conductivity

The differential conductivity for the out-of-plane transport in layered cuprates is calculated for Lawrence-Doniach model in the framework of time-dependent Ginzburg-Landau (TDGL) theory. The TDGL equation for the superconducting order parameter is solved in the presence of Langevin external noise, describing the birth of fluctuation Cooper pairs. The TDGL correlator of the superconducting order parameter is calculated in momentum representation and it is shown that the so defined number of particles obeys the Boltzmann equation. The fluctuation conductivity is given by an integral over the Josephson phase θof the particles distribution, depending on that phase, n(θ) and their velocity v(θ). It is demonstrated that in case of overcooling under T_c the transition from normal to superconducting phase while reducing the external electric field runs via annulation of the differential conductivity. The presented results can be used for analysis and experimental data processing of measurements of differential conductivity and fluctuation current in strong electric and magnetic fields. The possible usage of a negative differential conductivity (NDC) for generation of THz oscillations is shortly discussed.

cond-mat.supr-con↗
arXiv2005

Theory of terahertz electric oscillations by supercooled superconductors

We predict that below T_c a regime of negative differential conductivity (NDC) can be reached. The superconductor should be supercooled to T<T_c in the normal phase under DC voltage. In such a nonequilibrium situation the NDC of the superconductor is created by the excess conductivity of the fluctuation Cooper pairs. We propose NDC of supercooled superconductors to be used as an active medium for generation of electric oscillations. Such generators can be used in the superconducting electronics as a new type THz source of radiation. Oscillations can be modulated by the change of the bias voltage, electrostatic doping by a gate electrode when the superconductor is the channel of a field effect transistor, or by light. When small amplitude oscillations are stabilized near the critical temperature T_c the generator can be used as a bolometer. The essential for the applications NDC is predicted by the solution of the Boltzmann kinetic equation for the metastable in the normal phase Cooper pairs. Boltzmann equation for fluctuation Cooper pairs is a result of state-of-the-art application of the microscopic theory of superconductivity. Our theoretical conclusions are based on some approximations like time dependent Ginzburg-Landau theory, but nevertheless can reliably predict appearance of NDC. The maximal frequency at which superconductors can operate as generators is determined by the critical temperature \hbar omega_max ~ k_B T_c. For high-T_c superconductors this maximal frequency falls well inside the terahertz range. Technical conditions to avoid nucleation of the superconducting phase are briefly discussed. We suggest that nanostructured high-T_c superconductors patterned in a single chip can give the best technical performance of the proposed oscillator.

cond-mat.supr-con↗
arXiv2004

Two-band BCS model describes well the thermodynamics of MgB$_2$

Based on weakly-coupled anisotropic BCS theory, the dependence of energy gap and specific heat on temperature are evaluated for superconductor MgB$_2$, and the results are compared with experimental data.We show that the weak coupling anisotropic BCS theory describes thermodynamic experimental data with high precision 3-6%.

cond-mat.supr-con↗
arXiv2003

Kinetics and Boltzmann kinetic equation for fluctuation Cooper pairs

The Boltzmann equation for excess Cooper pairs above T_c is derived in the framework of the time-dependent Ginzburg-Landau (TDGL) theory using Langevin's approach of the stochastic differential equation. The Newton dynamic equation for the momentum dependent drift velocity is obtained and the effective drag force is determined by the energy dependent life time of the metastable Cooper pairs. The Newton equation gives just the Drude mobility for the fixed momentum of Cooper pairs. It is shown that the comparison with the well-known result for Aslamazov-Larkin paraconductivity and BCS treatment of the excess Hall effect can give the final determination of all the coefficients of TDGL theory. As a result the intuitive arguments used for an interpretation of the experimental data for fluctuation kinetics are successively introduced. The presented simple picture of the degenerated Bose gas in tau-approximation near to Bose-Einstein condensation temperature can be used for analysis of fluctuation conductivity for the cases of high frequency and external magnetic field for layered and bulk superconductors. The work of the Boltzmann equation is illustrated by frequency dependent Aslamazov-Larkin conductivity in nanowires, two dimensional case and the case of strong electric field where the TDGL equation is solved directly. There are also derived explicit formulas for the current in the case of arbitrary time dependence of electric field up to THz range, the distribution of fluctuation Cooper pairs for non-parabolic dispersion, the influence of the energy cut-off and the self consistent equation for the reduced temperature. The general theory is illustrated by formulae for fluctuation conductivity in nanowires and nanostructured superconductors.

cond-mat.supr-con↗
arXiv2003

Specific Heat Discontinuity in Impure Two-Band Superconductors

The Ginzburg-Landau coefficients, and the jump of the specific heat are calculated for a disordered two-band superconductor. We start with the analysis of a more general case arbitrary anisotropy. While the specific heat discontinuity at the critical temperature T_c decreases with increasing disorder, its ratio to the normal state specific heat at T_c increases and slowly converges to the isotropic value. For a strong disorder the deviation from the isotropic value is proportional to the elastic electron scattering time. In the case of a two-band superconductor we apply a simplified model of the interaction independent on momentum within a band. In the framework of this model all thermodynamic values can be found explicitly at any value of the scattering rate. This solution explains the sample dependence of the specific heat discontinuity in MgB_2 and the influence of the disorder on the critical temperature.

cond-mat.supr-con↗
arXiv2003

RKKY interaction in framework of T=0 Green function method

A methodical derivation of RKKY interaction in framework of T=0 Green function method is given in great detail. The article is complimentary to standard textbooks on the physics of magnetism and condensed matter physics. It is shown that the methods of statistical mechanics gives a standard and probably simplest derivation of the exchange interaction. A parallel with theory of plasma waves demonstrates the relation between the Fourier transformation of polarization operator of degenerate electron gas at zero frequency and the space dependence of the indirect electron exchange due to itinerant electrons.

cond-mat.stat-mech↗