Andrei S. Mishchenko

Distribution of Localized States from Fine Analysis of Electron Spin Resonance Spectra in Organic Transistors

We developed a novel method for obtaining the distribution of trapped carriers over their degree of localization in organic transistors, based on the fine analysis of electron spin resonance spectra at low enough temperatures where all carriers are localized. To apply the method to pentacene thin-film transistors, we proved through continuous wave saturation experiments that all carriers are localized at below 50 K. We analyzed the spectra at 20 K and found that the major groups of traps comprise localized states having wave functions spanning around 1.5 and 5 molecules and a continuous distribution of states with spatial extent in the range between 6 and 20 molecules.

Quasidegenerate Self-Trapping in One-Dimensional Charge Transfer Exciton

The self-trapping by the nondiagonal particle-phonon interaction between two quasidegenerate energy levels of the excitonic system is studied. We propose this is realized in the charge-transfer exciton, where the directions of the polarization give the quasidegeneracy. It is shown that this mechanism, unlike the conventional diagonal one, allows a coexistence and resonance of the free and self-trapped states even in one-dimensional systems and a quantitative theory for the optical properties (light absorption and time-resolved luminescence) of the resonating states is presented. This theory gives a consistent resolution for the long-standing puzzles in quasi-one-dimensional compound A-PMDA.

Theory of ferromagnetism in Ca(1-x)La(x)B(6).

Novel ferromagnetism in Ca(1-x)La(x)B(6) is studied in terms of the Ginzburg-Landau theory for excitonic-order parameters, taking into account symmetry of the wave functions. We found that the minima of the free energy break both inversion and time-reversal symmetries, while the product of these two remains preserved. This explains various novelties of the ferromagnetism and predicts a number of magnetic properties, including the magnetoelectric effect, which can be tested experimentally.

Exact treatment of exciton-polaron formation by diagrammatic Monte Carlo simulations.

We develop an approximation-free diagrammatic Monte Carlo technique to study fermionic particles interacting with each other simultaneously through both an attractive Coulomb potential and bosonic excitations of the underlying medium. Exemplarily we apply the method to the long-standing exciton-polaron problem and present numerically exact results for the wave function, ground-state energy, binding energy and effective mass of this quasiparticle. Focusing on the electron-hole pair bound-state formation, we discuss various limiting cases of a generic exciton-polaron model. The frequently used instantaneous approximation to the retarded interaction due to the exchange of phonons is found to be of very limited applicability.

Crystalline fields in systems with exchange and magnetoelastic interaction

The influence of the Kondo effect on the state of the crystalline field in the localized-moment regime is studied. It is shown that the exchange interaction leads to an anomalous temperature dependence of the intensity of the inelastic magnetic scattering of neutrons. An unconventional theory is constructed for the magnetoelastic interaction induced by Kondo scattering. The proposed model is in quantitative agreement with the results of neutron and thermodynamic investigations of the compound CeAl3.

Theory of Excitation Spectra of Electron–Phonon Coupled Systems

We present recent advances in understanding of the ground and excited states of the electron–phonon coupled systems obtained by novel methods of Diagrammatic Monte Carlo and Stochastic Optimization...

Left-Handed Spin Wave Excitation in Ferromagnet

The spin wave dynamics in metallic ferromagnet is studied theoretically taking into account the relativistic spin–orbit interaction. It is found that its quantum dynamics is seriously influenced by...

Quasielastic magnetic scattering of neutrons by heavy-fermion systems

A theory of quasielastic scattering by a spin liquid with resonating valence bonds is constructed. It is demonstrated that the dependence of the scattering cross section of the spin liquid on the energy transfer has the same shape as the quasielastic peak observed experimentally in heavy-fermion systems. It is shown that a consequence of fact that the excitations of the spin-liquid state obey Fermi statistics is that the total quasielastic scattering cross section oscillates as a function of the momentum transfer.

Lattice distortion and ferromagnetism in CaB6

Abstract Novel ferromagnetism in Ca 1− x La x B 6 is studied in terms of the Ginzburg–Landau theory for excitonic order parameters, taking into account symmetry of the wavefunctions. We can predict a number of magnetic properties, i.e. antiferromagnetism and its magnetic structure, magneto-electric (ME) effect. They can be verified experimentally. This ME effect can also explain various novelties of the compound. Similarity with ferromagnetic CaB 2 C 2 is also discussed.