Ivan Janeček

Theoretical modeling of postionization fragmentation of rare-gas trimer cations

The dynamics of ionic rare-gas trimers (Ar(3) (+), Kr(3) (+), and Xe(3) (+)) produced by a sudden ionization of neutral precursors is investigated theoretically with a hybrid classical-quantum method for solving the equations of motion governed by a Hamiltonian obtained from a previously tested diatomics-in-molecules model. Initial conditions are selected with Monte Carlo sampling. Two possibilities for generating the initial electronic state are considered: diabatic (local) and adiabatic (delocalized). The dynamics generally leads to fragmentation, producing either monomer ions or dimer ions in a relatively short time; however, a large number of long-lived metastable trimer ions are also seen in some cases. We have analyzed the dynamics with respect to the fraction of monomer ions produced, the distribution of the kinetic energy of the products, and the distribution of fragmentation times of the trimers. Initial diabatic ionization is associated with much faster fragmentation than adiabatic ionization. Spin-orbit coupling plays an important role in the fragmentation dynamics.

Postionization fragmentation of rare-gas trimers revisited with new theoretical approaches

A new theoretical approach is presented for the general treatment of nonadiabatic hybrid dynamics (mixing classical and quantum approach) and applied to the postionization of rare-gas trimers. There was an important disagreement between trajectory surface hopping (TSH) or mean field (MF) approaches and the experimental results; noteworthy, with the new method qualitative and almost quantitative agreement is found for the fragmentation ratios of ionic monomers and dimers. For the first time in the theory as in the experiment, the dimers prevail for argon while monomers strongly dominate for the heavier rare gases, krypton and xenon. A new compromise between MF and TSH approaches is proposed and the new method is found quite robust with results not too sensitive to various possible implementations.

Intrinsic pinning and guided motion of vortices in high-Tc superconductors☆

Abstract The anisotropy of the vortex motion has been studied on textured samples of Bi based superconductors. For some orientation of the magnetic field and transport current with respect to the preferential orientation of grains a Hall (transverse) voltage even in a magnetic field has been observed in the mixed state in the low-temperature range. The observed dependences have been discussed as a result of the guided motion of vortices due to intrinsic pinning due to the layered structure of the material under study. The increase of the temperature suppresses the anisotropy of the pinning and even the voltage disappears.

Reciprocity theorem in high-temperature superconductors

Abstract This article is devoted to the problem of the validity of the reciprocity theorem in high-temperature superconductors (HTSC). The violation of the reciprocity theorem in zero external magnetic fields has been studied. Experimental data obtained for two different superconducting materials BiSrCaCuO and YBaCuO are presented. Results show that the basic form of the reciprocity theorem (without consideration of any additional anisotropy) is not valid near the critical temperature. We assume that the breakdown of the reciprocity theorem is connected with the existence of an extraordinary transverse electric field originating from additional anisotropy and that a more general form of the reciprocity relations should be valid. However, the origin of this anisotropy is not clear yet. We suggest that the vortex–antivortex dynamics model taking into account vortex guiding can be responsible for the observed effect. Also the explanation based on weak P and T symmetry breaking in HTSC, which is supported by the observation of the spontaneous magnetisation, cannot be excluded.This article is devoted to the problem of the validity of the reciprocity theorem in high-temperature superconductors (HTSC). The violation of the reciprocity theorem in zero external magnetic fields has been studied. Experimental data obtained for two different superconducting materials: BiSrCaCuO and YBaCuO are presented. Results show that the basic form of the reciprocity theorem (without consideration of any additional anisotropy) is not valid near the critical temperature. We assume that the reciprocity theorem breaking is connected with the existence of an extraordinary transverse electric field originated from additional anisotropy and more general form of the reciprocity relations should be valid. However, the origin of this anisotropy is not clear yet. We suggest that the vortex-antivortex dynamics model taking into account vortex guiding can be responsible for the observed effect. Also the explanation based on weak P and T symmetry breaking in HTSC which is supported by the observation of the spontaneous magnetisation can not be excluded.

Structural and transport properties of YBa2Cu3−yCoyO7−x solid solutions

Abstract The X-ray study of YBa 2 Cu 3- y Co y O 7 solid solutions showed that in addition to the orthorhombic-tetragonal transition existing in the compositional range of 0.0 y y >0.4. A model based on an arrangement of microwatwins, formed owing to a tendency of cobalt ions to be six-fold coordinated, was proposed to explain the observed structural changes. A comparison of the oxygen stoichiometry and Hall charge carriers density data showed that this tendency is decisive for the charge distribution in the (Me(1)O) sublattice. Besides the geometrical arrangement, the quality of the bonds and consequent character of the transport properties is influenced by a more ionic character of cobalt ions in comparison with copper ones.

Multiscale approach combining nonadiabatic dynamics with long-time radiative and non-radiative decay: Dissociative ionization of heavy rare-gas tetramers revisited

A multiscale approach is proposed to address short-time nonadiabatic dynamics and long-time decay. We show the role of both radiative and non-radiative processes in cluster decay mechanisms on examples of rare-gas cluster fragmentation after electron impact ionization. Nonadiabatic molecular dynamics is used as an efficient tool for theoretical study on femto- and picosecond scales and a multiscale approach based on kinetic rates of radiative as well as non-radiative transitions, both considered as parallel reaction channels, is used for the analysis of the long-time system relaxation spanning times over microseconds to infinity. While the radiative processes are typically slow, the system relaxation through non-radiative electronic transitions connected with electron-nuclear interchange of energy may, on the other hand, significantly vary in kinetic rates according to kinetic couplings between relevant adiabatic states. While the predictions of picosecond molecular dynamics themselves fail, the results of the multiscale model for the electron-impact post-ionization fragmentation of krypton and xenon tetramers are in agreement with experiment, namely, in leading to the conclusion that charged monomers prevail. More specifically, on microsecond and longer scales, mainly slow radiative processes are substantial for krypton cluster decay, while for xenon the radiative and slow non-radiative processes compete. In general, the role of slow decay processes through non-radiative transitions is comparable with the role of radiative decay mechanism. The novel multiscale model substantially improves theoretical predictions for the xenon tetramer decay and also further improves the good agreement between theory and experiment we reached previously for krypton.

Multiscale non-adiabatic dynamics with radiative decay, case study on the post-ionization fragmentation of rare-gas tetramers

A novel multiscale method introducing radiative decay in realistic simulations of complex systems involving metastable states is proposed in order to address the competition between non-radiative and radiative processes. Specific implementation of the proposed method is used in a case study of the post-ionization fragmentation of heavier rare-gas tetramers considered for very long times. The present multiscale approach allows to take into account the non-adiabatic dynamics typically in tens of picoseconds as well as radiative decay typically in the microsecond regime. Agreement with experimental findings that mainly monomer ion fragments are produced is found here for the first time for Kr4+ and to some extent also for Xe4+.

The hall effect anomaly in single phase Bi based ceramics

Abstract The temperature dependence of the Hall effect and resistivity have been measured in magnetic field up to 4T. At the temperatures near Tc anomalous behaviour of Hall effect has been observed. The temperature dependence below Tc is more complicated than that in the Y based system. The measured magnetic field dependence of this anomaly for the field in the range from 1T to 4T shows the opposite tendency in comparison with the behaviour below 1T, i.e. increase with decreasing magnetic field. Results are discussed in the framework of recent theories.

TRANSPORT AND MAGNETIC PROPERTIES OF Bi BASED MULTIPHASE SUPERCONDUCTORS

A series of (Bi,Pb)SrCuCaO samples with varying volume fraction of the 2223 phase (determined by X-ray diffraction) has been prepared. Temperature dependence of the resistance, Hall effect and magnetic susceptibility has been measured. Neither susceptibility (both dc and ac) nor electrical resistivity in magnetic field reveal the presence of the low temperature (2212) phase. On the other hand the Hall voltage in the mixed state is very sensitive to the presence of different phases. Obtained results have been discussed within the frame of the effective medium approach and related to the growth mechanism of the 2223 phase.

Mixed state Hall effect in the multiphase superconductors

Abstract Hall effect below T c in multiphase superconductors has been studied on Bi-based superconductors. Samples with different relative content of 2212 and 2223 phase have been prepared. The phase content has been verified by X-ray diffraction. Results show that while resistance and ac susceptibility is almost insensitive to the content of 2212 phase, the qualitative behavior of the Hall resistance is strongly influenced by the presence of both phases. Theoretical calculation of Hall resistance has been made based on effective medium approximation and compared with experimental results.