Miniaturization of the electronic circuits used in digital devices lies at the heart of current technological revolution. A promising strategy is to employ molecules as efficient building blocks, replacing transistors, resistors or contacts by objects as small as few atoms. This effort lead to the emergence of a new interdisciplinary research field of molecular electronics. We employ a combination of the density-functional theory (DFT) with Green function-based techniques to study the transport properties of molecular devices.
Published results:H. Li et al, Angew. Chem. Int. Ed. 56 14145 (2017).
Research group homepage:Computational theory of condensed matter group
Quantum dots with superconducting leads
Single-level quantum dots with local Coulomb interaction connected to superconducting BCS leads are ideal systems for studying the interplay between correlation effects and superconducting pairing. These systems can be described by a modified Anderson impurity model. We developed a simple self-consistent second-order diagrammatic perturbation technique to calculate calculate spectral and transport properties. Together with numerical renormalization group (NRG) and CT-HYB quantum Monte-Carlo methods we try to develop a consistent description of the behavior of the subgap Andreev bound states (ABS) and the Josephson current. The main focus is on the description of the single-doublet (zero-pi) quantum phase transition, marked by the crossing of ABS at the Fermi energy and an abrupt change in the Josephson current.
Published results:V. Pokorný and M. Žonda, accepted to Physica B (2017).
T. Domański, M. Žonda, V. Pokorný, G. Górski, V. Janiš, and T. Novotný,
Phys. Rev. B 95, 045104 (2017).
V. Janiš, V. Pokorný, and M. Žonda, Eur. Phys. J. B 89, 197 (2016).
M. Žonda, V. Pokorný, V. Janiš, and T. Novotný, Phys. Rev. B 93, 024523 (2016).
M. Žonda, V. Pokorný, V. Janiš, and T. Novotný, Sci. Rep. 5, 8821 (2015).
V. Pokorný, V. Janiš, T. Novotný, and M. Žonda, A. Phys. Pol. A 126, 352 (2014).
Presentations and seminars:1. 1. 2018, Universität Regensburg, Germany
29. 11. 2016, IoP CAS, Prague, Czech Republic
24. 3. 2015, IoP CAS, Prague, Czech Republic
17. 10. 2014, IIASS, Vietri sul Mare (Salerno), Italy
Thermodynamically consistent description of quantum criticality
We are developing methods based on diagrammatic perturbation techniques to study quantum criticality in interacting electron systems described by lattice models (e.g. Anderson or Hubbard model). Methods are based on the parquet construction of two-particle functions and provide a consistent description that is free of unphysical symmetry breaking and does not violate conservation laws.
Published results:P. Zalom, V. Pokorný, and V. Janiš, accepted to Physica B (2017).
V. Janiš, V. Pokorný, and A. Kauch, Phys. Rev. B 95, 165113 (2017).
V. Janiš, A. Kauch, and V. Pokorný, Phys. Rev. B 95, 045108 (2017).
V. Pokorný, M. Žonda, A. Kauch, and V. Janiš, Acta Phys. Pol. A 131, 1042 (2017).
Presentations:Poster from the 2017 summer school in Tallahassee (pdf)
Software:SPEpy - Simplified Parquet Equations in python - set of python scripts to calculate properties of the single-impurity Anderson model using the simplified parquet equation solver
Correlated materials with strong spin-orbit interactions
We use the combination of ab-initio (LDA) methods and dynamical mean-field theory (DMFT) to study materials with strong effect of spin-orbit coupling on their electronic and magnetic properties. The focus lies on transition-metal oxides with heavy ions such as iridium and osmium. We use the continuous-time quantum Monte-Carlo method in the strong-coupling limit (CT-HYB) to solve the underlying impurity problem.
Published results:K. Pajskr, P. Novák, V. Pokorný, J. Kolorenč, R. Arita, and J. Kuneš, Phys. Rev. B 93, 035129 (2016).
Correlated electrons in disordered alloys (PhD thesis)
We developed methods to calculate charge transport in disordered systems of elastically scattered electrons. These diagrammatic methods are based on an asymptotic limit to high spatial dimensions and utilize advanced diagram summation techniques on the two-particle level. They allow us to calculate electrical conductivity and diffusion coefficient for arbitrary disorder strength. The numerical calculations are performed on the disordered Anderson model and the Falicov-Kimball model.
Published results:V. Janiš and V. Pokorný, Phys. Rev. B 90, 045143 (2014).
V. Pokorný and V. Janiš, J. Phys.: Cond. Mat. 25, 175502 (2013).
V. Janiš and V. Pokorný, Ann. Phys. (Berlin) 523, 715 (2011).
V. Janiš and V. Pokorný, Phys. Rev. B 81, 165103 (2010).
PhD thesis (English, pdf, defended in 2013 at Charles University in Prague).
Presentations and seminars:24. 4. 2012, IoP CAS, Prague, Czech Republic
14. 10. 2011, IIASS, Vietri sul Mare (Salerno), Italy
Critical properties of the mixed-spin Heisenberg model (Master thesis)
We studied the critical properties of the anisotropic mixed spin-1 and spin-1/2 quantum Heisenberg model using a simple two-site mean-field approximation. We treat in detail the system on the simple cubic lattice considering both exchange anisotropy and uniaxial singe-ion anisotropy. The critical behavior is analysed and the complete phase diagrams were calculated, where aside from the typical ferromagnetic phase a new, low-temperature quantum ordered phase, is described.
Published results:A. Bobák, V. Pokorný, and J. Dely, J. Phys.: Conf. Ser. 200, 022001 (2010).
A. Bobák, V. Pokorný, and J. Dely, Physica A 388, 2157 (2009).
Master thesis (Slovak, pdf, defended in 2008 at Pavol Jozef Šafárik University in Košice).
Presentations and seminars:16. 12. 2008, IoP CAS, Prague, Czech Republic