Contact details

Department of Theoretical Physics
Faculty of Physics
St Clement of Ohrid University at Sofia
5 J. Bourchier Boulevard
BG-1164 Sofia, Bulgaria
e-mail: tmishonov at phys dot uni-sofia dot bg
Phone: (++359 2) 8161 653
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Research experience, scientific plans and interests

My present interests are mainly focused on the theory of superconductivity. The problem of the mechanism of high-Tc superconductivity is still one of the biggest unresolved problems of the contemporary condensed matter physics. Recently I found that intra-atomic two-electron exchange amplitude Jsd between Cu 4s and Cu 3d orbitals can mediate high-Tc superconductivity and can explain the main properties of high-Tc cuprates: d-wave gap anisotropy, temperature dependence of the specific heat and penetration depth angle resolved photoemission spectra, fluctuation conductivity, correlation between the position of Cu 4s level and critical temperature Tc. In such a way, well-known intra-atomic correlation leading to two-electron exchange gives a natural explanation of the cuprate superconductivity.

The next problem on which I am working at the moment is whether the pairing Hamiltonian can explain the normal state properties and especially the anisotropy of the electron scattering rate. Further I will apply the same approach to other exotic superconductors with strongly anisotropic gap - according to my hypotheses, gap anisotropy is a simple consequence of the symmetry of tight binding Bloch functions, whereas the pairing is mediated by intra-atomic two electron exchange in ions with uncompleted shells. This programme requires as a basis a good tight binding fit of the ab initio LDA calculations and the technical part of these calculations is appropriate task to introduce students in the current scientific research.

Fluctuation phenomena in superconductors are another field of my permanent interest. Working in the past on the theory of twinning plane superconductivity I analyzed how the magnetic moment calculated in the framework of Ginzburg-Landau theory can be separated from the fluctuation magnetization of the bulk crystal. Later on, I wrote a review article on the superconducting fluctuations where convenient for experimental data processing formulae for the fluctuation magnetization and conductivity for layered superconductors such as high-Tc cuprates have been derived. My current research in this field is related to the fluctuation conductivity of superconducting nanowires. The nanowires under an external voltage are supercooled in the normal state below the critical temperature Tc. The time dependent Ginzburg-Landau theory predicts that at such non-equilibrium conditions nanowires will have negative differential conductivity. I consider as very intriguing the perspective this negative differential conductivity to be used for creation of new type terahertz oscillators because a lot of applications need effective low-power terahertz emission; the idea is already patented in Europe. The mathematical techniques and notions used for this task could be applied for approximate treatment of some problems of turbulent hydrodynamics. In collaboration with an astrophysicist and former my student I applied technique to the problem of missing viscosity of magnetized accretion disks. The purpose of this research was to understood the mechanism of shining of quasars.

Electrodynamic behavior of superconductors is the last example of my scientific interests in the physics of superconductors. Whereas the gap anisotropy reveals relative motion of electrons in the Cooper pairs in the momentum space the real space dependence of the gap is related to the wavefunction of the center of mass. Analyzing the dynamics of superfluid current I predicted Cooper pair plasmons related to motion of the center of mass. Such plasmons with frequency below the superconducting gap have very small attenuation and can be considered as quasiparticles. The existence of such plasmons was later on experimentally confirmed in thin Al films and bulk Bi2Sr2CuO8. Experimentalists from Japan, USA and France recognized the priority of my theoretical prediction. Now Cooper pair plasmons in layered superconductors are known as Josephson plasmons. I am strongly motivated to continue theoretical research in this field analyzing more sophisticated experiments related to Doppler shift of Cooper pair plasmons and determination of the superfluid drift velocity.

Another task related to electrodynamics of superconductors is the superconducting Bernoulli effect. Superconductivity as phenomenon means that below the critical temperature the static Ohmic dissipation disappears. In such a regime the energy of the superfluid is conserving and we have a nice condition to apply the Bernoulli theorem for the charged superfluid. The consequence of this general theorem is that the current creates a change of the electric potential proportional to the kinetic energy of Cooper pairs and to the square of the current density. According my theoretical consideration a systematic investigation of the Bernoulli effect in superconductors can lead to creation of Cooper pair mass spectroscopy and densitometry. I am expecting that this Bernoulli effect can be used for a contactless control of the quality at production of second-generation high-Tc superconducting cables. Using a high-tech layered structure made in Caen, France I performed perhaps the first observation of the Bernoulli effect in high-Tc superconductors. The experimental setup was completely new and designed by me in Sofia. Unfortunately the electronic equipment of my collaborators in Sofia is quite old and I wish to repeat the successful measurement in some laboratory having good traditions in the physics of superconductivity. That is why I am motivated to search for collaboration with experimentalists in order to investigate together new effects related to the electric fields in superconductors.