Research projects
Molecular electronics

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 densityfunctional theory (DFT) with Green functionbased 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

Singlelevel 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 selfconsistent secondorder diagrammatic perturbation technique to calculate calculate spectral and transport properties. Together with numerical renormalization group (NRG) and CTHYB quantum MonteCarlo 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 singledoublet (zeropi) 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). 
Selected presentations and seminars:
12. 3. 2018, DPG Spring Meeting, Berlin, Germany
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 
Software:
 SQUAD  SUperconducting QUAntum Dot  set of python scripts to calculate properties of these systems using the diagrammatic second order perturbation method
 SQUADCTHYB  a TRIQSbased code to calculate properties of these systems using the CTHYB quantum MonteCarlo method
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 twoparticle 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 singleimpurity Anderson model using the simplified parquet equation solver
Correlated materials with strong spinorbit interactions

We use the combination of abinitio (LDA) methods and dynamical meanfield theory (DMFT) to study materials with strong effect of spinorbit coupling on their electronic and magnetic properties. The focus lies on transitionmetal oxides with heavy ions such as iridium and osmium. We use the continuoustime quantum MonteCarlo method in the strongcoupling limit (CTHYB) 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 twoparticle 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 FalicovKimball 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 mixedspin Heisenberg model (Master thesis)

We studied the critical properties of the anisotropic mixed spin1 and spin1/2 quantum Heisenberg model using a simple twosite meanfield approximation. We treat in detail the system on the simple cubic lattice considering both exchange anisotropy and uniaxial singeion anisotropy. The critical behavior is analysed and the complete phase diagrams were calculated, where aside from the typical ferromagnetic phase a new, lowtemperature 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