Publications

2024

• Inelastic tunneling through normal and superconducting junctions in the presence of a photonic bath within the Lindbladian formalism
  Ádám Bácsi, Rok Žitko
  Phys. Rev. A 110, 012224 (2024) (PDF)
  We reveal relations between the P(E) theory for inelastic tunneling and the Lindbladian quantum master equation approach. As a demonstration, the Lindbladian approach is then applied to calculate the heat dissipation in the junction. For superconducting contacts with particle-hole asymmetry in the normal-state DOS, we find that coherence-factor corrections enter the expression for I(V) in terms of the P(E) function.
• Higher order spin interactions mediated by the substrate
  Szczepan Głodzik and Rok Žitko
  Phys. Rev. B 110, 045414 (2024) (PDF)
  We discuss the significance of fourth order exchange coupling (ring exchange) on the spin alignment tendencies for four impurities coupled to a bath of itinerant electrons in a square lattice.
• Generalized transmon Hamiltonian for Andreev spin qubits
  Rok Žitko, Luka Pavešič
  Phys. Rev. B 109, 155164 (2024) (PDF)
  We present a method for solving the problem of an interacting quantum dot embedded in a Josephson junction between two superconductors with finite charging energy described by the transmon (Cooper pair box) Hamiltonian. The method can be directly applied for modelling Andreev spin qubits embedded in transmon circuits in all parameter regimes, for describing time-dependent processes, and for the calculation of transition matrix elements for microwave-driven transmon, spin-flip and mixed transitions that involve coupling to charge or current degree of freedom.
• Correlation between two distant quasiparticles in separate superconducting islands mediated by a single spin
  Juan Carlos Estrada Saldaña, Alexandros Vekris, Luka Pavešič, Rok Žitko, Kasper Grove-Rasmussen, Jesper Nygård
  Nat. Commun. 15, 3465 (2024) (PDF)
  We discuss how a single quantum dot can be YSR screened by two quasiparticles, one in each of two superconductors. This allows to generate ferromagnetic correlations between fermionic excitations over a micrometer distance. Such alternating chains of quantum dots and superconducting islands represent a new platform for controllable large-scale spin coupling.
• Strong-coupling theory of quantum-dot Josephson junctions: Role of a residual quasiparticle
  Luka Pavešič, Ramón Aguado, Rok Žitko
  Phys. Rev. B 109, 125131 (2024) (PDF)
  We present the theory of a quantum dot Josephson junction (Anderson impurity model coupled to two superconducting baths with a phase difference) in the strong-coupling limit which explains the changing nature of the doublet states as a function of coupling strength and the phase differences, as well as the origin of the doublet chimney at φ=π.
• True random number generators with flicker noise: stochastic model, min-entropy calculation and online test
  Rok Žitko
  Research square preprint (PDF)
  It is shown that it is possible to build a random number generator for cryptographic use in the presence of strong flicker (1/f, pink) noise. An online test based on measuring Allan variance is proposed.
• Auxiliary master equation approach to the Anderson-Holstein impurity problem out of equilibrium
  Daniel Werner, Rok Žitko, and Enrico Arrigoni
  Phys. Rev. B 109, 075156 (2024) (PDF)
  The problem of an impurity with local electron-electron and electron-phonon embedded between two conduction leads with a voltage bias is solved using auxiliary master equation approach. The method is applicable to the regime of sizeable interactions at high bias voltage.
• Mbit/s-range alkali vapour spin noise quantum random number generators
  Matija Koterle, Samo Beguš, Jure Pirman, Tadej Mežnaršič, Katja Gosar, Erik Zupanič, Rok Žitko & Peter Jeglič
  EPJ Quantum Technol. 11, 11 (2024) (PDF) [open access]
  Alkali vapour spin-noise quantum random number generator can reach 1Mbit/s generation rates by increasing the scattering rate by removing the coating, increasing temperature and introducing a magnetic-field gradient, and by using an improved bit generation protocol.

2023

• Richardson model with complex level structure and spin-orbit coupling for hybrid superconducting islands: Stepwise suppression of pairing and magnetic pinning
  Juan Carlos Estrada Saldaña, Luka Pavešič, Alexandros Vekris, Kasper Grove-Rasmussen, Jesper Nygård, Rok Žitko
  Phys. Rev. B 108, 224507 (2023) (PDF)
  We discuss Coulomb spectroscopy results of quantum dots coupled to superconducting islands in hybrid semi-super nanowire that show equal spacing as a function of gate voltage applied to the island. We propose a model involving Andreev levels in the semiconducting core of the hybrid wire that switch step-by-step to spin polarized state with increasing magnetic field, with the even-odd oscillations being washed out through spin-orbit coupling.
• Exchange interaction between two quantum dots coupled through a superconducting island
  Ádám Bácsi, Luka Pavešić, Rok Žitko
  Phys. Rev. B 108, 115160 (2023) (PDF)
  We study the inter-dot exchange coupling in a QD-SI-QD device (QD=quantum dot, SI=superconducting island). We find qualitative difference between simplified flat-band and full-model calculations. This indicates that zero-bandwidth models can produce misleading results.
• Spectroscopy of spin-split Andreev levels in a quantum dot with superconducting leads
  Arno Bargerbos, Marta Pita-Vidal, Rok Žitko, Lukas J. Splitthoff, Lukas Grünhaupt, Jaap J. Wesdorp, Yu Liu, Leo P. Kouwenhoven, Ramón Aguado, Christian Kraglund Andersen, Angela Kou, Bernard van Heck
  Phys. Rev. Lett. 131, 0970101 (2023) (PDF) (Supplementary information)
  The spin-orbit coupling in hybrid super-semi transmon device leads to spin splitting of the doublet subgap states. Spin-flip excitations can be performed using all-electrical control.
• Impurity Knight shift in quantum dot Josephson junctions
  Luka Pavešić, Marta Pita-Vidal, Arno Bargerbos, Rok Žitko
  SciPost Phys. 15, 070 (2023) (PDF) (Data & notebooks)
  We discuss the coupling between the spin degree of freedom of a quasiparticle trapped in a QD Josephson junction and the supercurrent, which manifests as a phase-dependent renormalization of the Zeeman splitting. This coupling could find wide application in superconducting spintronics.
• Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit
  Marta Pita-Vidal, Arno Bargerbos, Rok Žitko, Lukas J. Splitthoff, Lukas Grünhaupt, Jaap J. Wesdorp, Yu Liu, Leo P. Kouwenhoven, Ramón Aguado, Bernard van Heck, Angela Kou, Christian Kraglund Andersen
  Nat. Phys. 19, 1110 (2023) (PDF) (self-archived PDF)
  Andreev spin qubit is implemented using the spin-split doublet ground state of an electrostatically-defined quantum dot Josephson junction with large charging energy. This spin qubit is coherently coupled with a superconducting transmon qubit.
• Topological quantum phase transition of nickelocene on Cu(100)
  G. G. Blesio, R. Žitko, L. O. Manuel, A. A. Aligia
  SciPost Phys. 14, 042 (2023) (PDF)
  We show that recent STM spectra of nickelocene adsorbed on the surface Cu(100) reveal the existence of a topological quantum phase transition between two distinct Fermi liquid ground states, which are of Landau and non-Landau type, the latter with a non-trivial value of the Luttinger integral.
• Incommensurable matter-wave jets in quasi-1D geometry
  Tadej Mežnaršič, Rok Žitko, Katja Gosar, Katja Arh, Matevž Jug, Erik Zupanič, Peter Jeglič
  arXiv:2303.07004 (PDF)
  We present the discovery of matter-wave jets with velocities in the golden ratio emitted from a Bose-Einstein condensate. Using numerical simulations we also predict the formation of a wide jet velocity distribution resembling the generation of supercontinua in nonlinear optical media. The incommensurable jets are precursors of the supercontinuum formation.
• Quantum capacitance of a superconducting subgap state in an electrostatically floating dot-island
  Filip K. Malinowski, R. K. Rupesh, Luka Pavešić, Zoltán Guba, Damaz de Jong, Lin Han, Christian G. Prosko, Michael Chan, Yu Liu, Peter Krogstrup, András Pályi, Rok Žitko, Jonne V. Koski
  arXiv:2210.01519 (PDF)
  RF reflectometry is used to characterize the subgap states in a device consisting of a quantum dot coupled to a superconducting island. The device as a whole is electrostatically floating, so that the total electron number remains constant on a long time scale, providing access to ground states that cannot be reached in electrically grounded devices. Capacitance loading of the resonator can be strongly reduced by thermal excitations.

2022

• Spin-orbital Yu-Shiba-Rusinov states in single Kondo molecular magnet
  Hui-Nan Xia, Emi Minamitani, Rok Žitko, Zhen-Yu Liu, Xin Liao, Min Cai, Zi-Heng Ling, Wen-Hao Zhang, Svetlana Klyatskaya, Mario Ruben, Ying-Shuang Fu
  Nat. Commun. 13, 6388 (2022) (PDF) [open access]
  Tb₂Pc₃ phthalocyanine-based triple-decker dinuclear sandwich-type terbium complexes on Pb(111) superconducting surface have two-pairs of Yu-Shiba-Rusinov states delocalized over the molecule. One level corresponds to a well-defined local moment, another to a valence fluctuating orbital.
• Singlet-Doublet Transitions of a Quantum Dot Josephson Junction Detected in a Transmon Circuit
  Arno Bargerbos, Marta Pita-Vidal, Rok Žitko, Jesús Ávila, Lukas J. Splitthoff, Lukas Grünhaupt, Jaap J. Wesdorp, Christian K. Andersen, Yu Liu, Leo P. Kouwenhoven, Ramón Aguado, Angela Kou, and Bernard van Heck
  PRX Quantum 3, 030311 (2022) (PDF) (Supplementary) (Josephson potentials) [open access]
  Microwave spectroscopy of a hybrid superconductor-semiconductor transmon device is used to probe the ground-state parity of the Josephson junction quantum dot, in agreement with predictions of a single-impurity Anderson model with superconducting leads. Quasiparticle dynamics can be resolved accross the parity phase boundaries.
• Yu-Shiba-Rusinov states, BCS-BEC crossover, and exact solution in the flat-band limit
  Rok Žitko and Luka Pavešić
  Phys. Rev. B 106, 024513 (2022) (PDF)
  We study the dependence of the YSR states as a function of the bandwidth in the superconductor. As the bandwidth shrinks, the renormalization processes that lead to Kondo temperature T_K=exp(-1/ρJ) and superconducting gap Delta=exp(-1/ρg) are both suppressed. The dependence of the YSR states on the bandwidth is weak. Furthermore, we demonstrate an equivalence between the flatband limit of the Richardson model and the zero-bandwidth limit of the BCS Hamiltonian. These results shed light on the success of simple approximation schemes to the problem of a magnetic impurity in a superconducting bath.
• Excitations in a superconducting Coulombic energy gap
  Juan Carlos Estrada Saldaña, Alexandros Vekris, Luka Pavešić, Peter Krogstrup, Rok Žitko, Kasper Grove-Rasmussen & Jesper Nygård
  Nat. Commun. 13, 2243 (2022) (PDF) (Supplementary) [open access]
  We show how the Coulomb repulsion on the superconducting island changes the nature of the singlet sub-gap state and breaks the electron-hole energy symmetry of the excitations.
• Qubit based on spin-singlet Yu-Shiba-Rusinov states
  Luka Pavešić and Rok Žitko
  Phys. Rev. B 105, 075129 (2022) (PDF)
  We explore the possibility of using a device that consists of a quantum dot in a junction between two superconducting islands as a qubit based on spin-singlet Yu-Shiba-Rusinov subgap states. Such a qubit is insensitive to magnetic field fluctuations and can be operated at gate voltage sweep spot where it is also resiliant to electric field fluctuations.
• Nobelova nagrada za fiziko 2022 za poskuse s kvantno prepletenimi delci
  R. Žitko
  Alternator, 31/2022 (PDF)
  (Slovenian only) O Nobelovi nagradi za fiziko 2022 za pionirske poskuse s kvantno prepletenimi delci (dokaz kršitve Bellove neenačbe, kvantna teleportacija, izmenjava prepletenosti).
• Stanje na področju generatorjev naključnih števil
  Matjaž Depolli, Peter Jeglič, Roman Novak, Erik Zupanič, Rok Žitko
  CRP V1-2119-P1 (PDF)
  (Slovenian only) Poročilo o stanju na področju generatorjev naključnih števil, namenjeno razvijalcem programske in strojne opreme, administratorjem računalniških sistemov, strokovnjakom za varnost, uporabnikom naključnih števil v znanstvene namene in delavcem v loteriji.

2021

• Subgap states in superconducting islands
  Luka Pavešić, Daniel Bauernfeind, and Rok Žitko
  Phys. Rev. B 104, L241409 (2021) (PDF) (suppl)
  We introduce a quantum impurity model for the situation where the bath is a superconducting island with finite charging energy. Such a bath is interacting, thus the conventional impurity solvers are not applicable. We show that the Hamiltonian admits a matrix-product-operator representation in terms of small matrices which can be solved using the density-matrix renormalization group (DMRG). We use this methodology we systematically study the subgap states for different ratios of charging energy to superconducting gap. We uncover the existence of screened subgap states where the local moment is reduced both by charge transfer and exchange interaction. Such states interpolate between the Yu-Shina-Rusinov states and the Coulobic subgap states. Subgap states exist also for odd occupancy of the superconducting island.
• Universal Magnetic Oscillations of dc Conductivity in the Incoherent Regime of Correlated Systems
  Jakša Vučičević, Rok Žitko
  Phys. Rev. Lett 127, 196601 (2021) (PDF) Editor's Suggestion Featured in Physics
  Interactions induce quantum oscillations at high temperatures with a frequency that is independent of the electron density. These oscillations could provide information about the scattering mechanisms and dominant interactions in strongly correlated materials. The finding is in excellent qualitative agreement with the recent experiments on graphene superlattices. We elucidate the key roles of the off-diagonal elements of the current vertex and the incoherence of electronic states, and explain the trends with respect to temperature and doping.
• Electrical conductivity in the Hubbard model: Orbital effects of magnetic field
  Jakša Vučičević, Rok Žitko
  Phys. Rev. B 104, 205101 (2021) (PDF) Editor's Suggestion Featured in Physics [open access]
  We present the formalism for calculating conductivity in the presence of perpendicular magnetic field, focusing on orbital effects. We prove that the Kubo bubble preserves gauge and translational invariance, and that in the DMFT the vertex corrections cancel regardless of the magnetic field. We present the DMFT results for the spectral function and both the longitudinal and Hall conductivities in several regimes of parameters. We analyze thoroughly the quantum oscillations of the longitudinal conductivity and identify a high-frequency oscillation component, arising as a combined effect of scattering and temperature.
• Iron phthalocyanine on Au(111) is a “non-Landau” Fermi liquid
  R. Žitko, G. G. Blesio, L. O. Manuel, A. A. Aligia
  Nat. Commun. 12, 6027 (2021) (PDF) (Supplementary) Featured in Editor's Highlights [open access]
  We show that iron phthalocyanine molecule on the surface of gold has a ground state that is a non-Landau Fermi liquid in that it is not adiabatically conncected with the non-interacting limit. In this work we extend the theory of topologically nontrivial local Fermi liquids to multi-orbital problems with magnetic fields, which allowed us to make this inference.
• Correlation of Yu–Shiba–Rusinov States and Kondo Resonances in Artificial Spin Arrays on an s-Wave Superconductor
  Anand Kamlapure, Lasse Cornils, Rok Žitko, Maria Valentyuk, Roberto Mozara, Saurabh Pradhan, Jonas Fransson, Alexander I. Lichtenstein, Jens Wiebe, Roland Wiesendanger
  Nano Lett. 21, 6748 (2021) PDF
  Fe adatom interacting with assemblies of intestitial magnetic atoms on a superconducting surface Ta(100) shows an evolution of the in-gap states with the same phenomenology as the two-impurity Kondo problem. We interpret this to be an effect of the formation of a collective spin degree of freedom with increasing magnetic alignment of interstitials.
• Single-shot Stern-Gerlach magnetic gradiometer with an expanding cloud of cold cesium atoms
  Katja Gosar, Tina Arh, Tadej Mežnaršič, Ivan Kvasič, Dušan Ponikvar, Tomaž Apih, Rainer Kaltenbaek, Rok Žitko, Erik Zupanič, Samo Beguš, Peter Jeglič
  Phys. Rev. A 103, 022611 (2021) (PDF)
  We demonstrate a gradiometer using a single image of an expanding atomic Cs cloud with the phase difference imprinted along the cloud. The resolution of our single-shot gradiometer is not limited by thermal motion of atoms and has an estimated absolute accuracy below ±0.2 mG/cm (±20 nT/cm).
• Coupled Yu-Shiba-Rusinov States Induced by a Many-Body Molecular Spin on a Superconductor
  Carmen Rubio-Verdú, Javier Zaldívar, Rok Žitko, Jose Ignacio Pascual
  Phys. Rev. Lett 126, 017001 (2021) (PDF)
  Iron porphyrin molecules on Pb(111) have a S=1 state ground state due to Hund's-like exchange coupling between two orbitals. The orbitals have opposite potential scattering, yielding two sub-gap states with particular spatial patterns showing particle-hole asymmetry inversion across the molecule.

2020

• Two-impurity Yu-Shiba-Rusinov states in coupled quantum dots
  J. C. Estrada Saldaña, A. Vekris, R. Žitko, G. Steffensen, P. Krogstrup, J. Paaske, K. Grove-Rasmussen, and J. Nygård
  Phys. Rev. B 102, 195143 (2020) (PDF)
  We demonstrate the superconducting analog of the two-impurity Kondo problem. In addition to antiferromagnetic spin-singet and Kondo cloud spin-singlet, its phase diagram also includes doublet Yu-Shiba-Rusinov regimes where a single local moment is screened.
• Anisotropic Kondo screening induced by spin-orbit coupling in quantum wires
  E. Vernek, G. B. Martins, and R. Žitko
  Phys. Rev. B 102, 155114 (2020) (PDF)
  We study an Anderson impurity coupled to a 1D wire with Rashba and Dresselhaus spin-orbit coupling (SOC) with numerical renormalization group (NRG) using no approximations to the hybridisation function. Fingerprints of anisotropy in the wire are revealed through the anisotropic response to external magnetic field. Maximal Kondo peak suppression is found for a field oriented along the effective SOC field axis.
• Charge transport in the Hubbard model at high temperatures: Triangular versus square lattice
  A. Vranić, J. Vučičević, J. Kokalj, J. Skolimowski, R. Žitko, J. Mravlje, and D. Tanasković
  Phys. Rev. B 102, 115142 (2020) (PDF)
  Vertex corrections to optical conductvity, which are significant on the square lattice even at high temperatures, are found to contribute less on the triangular lattice.
• Erasing odd-parity states in semiconductor quantum dots coupled to superconductors
  Z. Su, R. Žitko, P. Zhang, H. Wu, D. Car, S. R. Plissard, S. Gazibegovic, G. Badawy, M. Hocevar, J. Chen, E. P. A. M. Bakkers, S. M. Frolov
  Phys. Rev. B 101, 235315 (2020) (PDF)
  We assess to what degree the erasure of odd-parity regimes in open quantum dots coincides at low and high biases.
• Emission of correlated jets from a driven matter-wave soliton in a quasi-one-dimensional geometry
  Tadej Mežnaršič, Rok Žitko, Tina Arh, Katja Gosar, Erik Zupanič, and Peter Jeglič
  Phys. Rev. A 101, 031601(R) (2020) (PDF) (GPE solver)
  Ejection of correlated atom jets from a soliton can be triggered by modulating the interaction strength. Number correlations beyond mean-field are demonstrated.

2019

• Conductivity in the Square Lattice Hubbard Model at High Temperatures: Importance of Vertex Corrections
  J. Vučičević, J. Kokalj, R. Žitko, N. Wentzell, D. Tanasković, and J. Mravlje
  Phys. Rev. Lett. 123, 036601 (2019) (PDF) (suppl)
  We show that while the self-energy is practically local at high temperatures, the vertex corrections to conductivity remain rather important. At high temperatures, the finite-size effects are small even at the latice size 4x4 and the Lanczos diagonlization result is close to the exact result in the thermodynamic limit.
• Charge localization and reentrant superconductivity in a quasi-ballistic InAs nanowire coupled to superconductors
  J. C. Estrada Saldaña, R. Žitko, J. P. Cleuziou, E. J. H. Lee, V. Zannier, D. Ercolani, L. Sorba, R. Aguado and S. De Franceschi
  Sci. Adv. 5, eaav1235 (2019) PDF [open access]
  We explore a nanowire at low subband filling in the presence of superconducting proximity effect and a magnetic field, and show that the plateau of ballistic conductance actually hides the presence of Coulomb blockade effect and Kondo physics.
• Observation of Coexistence of Yu-Shiba-Rusinov States and Spin-Flip Excitations
  Shawulienu Kezilebieke, Rok Žitko, Marc Dvorak, Teemu Ojanen, Peter Liljeroth
  Nano Lett. 19, 4614 (2019) PDF
  The cross-over from the weak impurity-substrate hybridisation with inelastic spin-flip excitations outside the gap to strong hybridisation with Yu-Shiba-Rusinov peaks inside the gap is smooth, with a parameter range where both spectral features are visible at the same time.
• Kondo screening in a charge-insulating spinon metal
  M. Gomilšek, R. Žitko, M. Klanjšek, M. Pregelj, C. Baines, Y. Li, Q. M. Zhang, A. Zorko
  Nat. Phys. 15, 754 (2019) (self-archived PDF)
  We report on the first experimental observation of the Kondo screening by chargeless quasiparticles in a charge-insulating quantum spin liquid, where spinon excitations forming a Fermi surface take the role of conduction electrons.
• Cesium bright matter-wave solitons and soliton trains
  Tadej Mežnaršič, Tina Arh, Jure Brence, Jaka Pišljar, Katja Gosar, Žiga Gosar, Rok Žitko, Erik Zupanič, and Peter Jeglič
  Phys. Rev. A 99, 033625 (2019) (PDF)
  The production of bright solitons and soliton trains in cesium BEC is demonstrated. BEC was also observed to fragment outside confinement, in free space.
• Mottness collapse without metallisation in the domain walls of triangular-lattice Mott insulator 1T-TaS₂
  J. Skolimowski, Ya. Gerasimenko, R. Žitko
  Phys. Rev. Lett. 122, 036802 (2019) (PDF) (suppl)
  Electronic state of a free-standing domain wall in 1T-TaS₂ can be explained as a would-be metallic state resulting from the Mottness collapse: the full metalization is preempted by the splitting of the quasiparticle band into bonding and antibonding subbands separated by a band gap. This picture is consistent with the experimental tunneling spectra.
• Comparative study of nonequilibrium insulator-to-metal transitions in electron-phonon systems
  Sharareh Sayyad, Rok Žitko, Hugo U. R. Strand, Philipp Werner, and Denis Golež
  Phys. Rev. B 99, 045118 (2019) (PDF)
  Different impurity solvers are compared for their ability to describe the equilibrium and nonequilibrium properties of electron-phonon systems described by the Hubbard-Holstein model using the dynamical mean-field theory.
• Numerical renormalization group
  Rok Žitko
  NRG UFU/2019 Advanced Studies School (PDF)
  A brief overview of the numerical renormalization group and its "NRG Ljubljana" implementation. Prepared for the NRG UFU/2019 Advanced Studies School, Uberlandia, Brazil.

2018

• Supercurrent in a Double Quantum Dot
  J. C. Estrada Saldaña, A. Vekris, G. Steffensen, R. Žitko, P. Krogstrup, J. Paaske, K. Grove-Rasmussen, and J. Nygård
  Phys. Rev. Lett. 121, 257701 (2018) (PDF) (suppl)
  We demonstrate the Josephson effect in a serial double quantum dot defined in a nanowire with epitaxial superconducting leads. The consistency between experiment and theory indicates that our device is a faithful realization of the two-impurity Anderson model.
• Aharonov-Bohm and Aharonov-Casher effects in a double quantum dot Josephson junction
  Damian Tomaszewski, Piotr Busz, Rosa López, Rok Žitko, Minchul Lee, and Jan Martinek
  Phys. Rev. B 98, 174504 (2018) (PDF)
  Cooper pair splitting can be made perfectly efficient by the addition of quantum dots in a nanostructure where two parallel nanowires bridge two superconductors. The Aharaonov-Casher phase is different for non-spin-flip and spin-flip processes.
• Reversal of particle-hole scattering-rate asymmetry in the Anderson impurity model
  R. Žitko, H. R. Krishnamurthy, and B. Sriram Shastry
  Phys. Rev. B 98, 161121(R) (2018) (PDF)
  In the single-impurity Anderson model we identify a line in the (U-n) parameter plane where the asymmetry of the scattering-rate becomes zero. This line, defined through impurity dynamics, traces out the cross-over between the weak-coupling and strong-coupling regimes.
• Yu–Shiba–Rusinov screening of spins in double quantum dots
  K. Grove-Rasmussen, G. Steffensen, A. Jellinggaard, M.H. Madsen, R. Žitko, J. Paaske, J. Nygård
  Nat. Commun. 9, 2376 (2018) (PDF) (Supplementary) [open access]
  Highly-tunable double quantum dot (DQD) is coupled to a superconductor and characterized via tunneling spectroscopy using a further normal-state electrode. It is shown how the honeycomb pattern characteristic of DQDs with normal-state leads is replaced by lines of zero-energy Yu-Shina-Rusinov states that delineate the different ground states of the system.
• Aharonov-Bohm and Aharonov-Casher effects for local and nonlocal Cooper pairs
  Damian Tomaszewski, Piotr Busz, Rosa López, Rok Žitko, Minchul Lee, Jan Martinek
  Phys. Rev. B 97, 214506 (2018) (PDF)
  In geometries allowing for both local and non-local (split) Cooper-pair Josephson current, both Aharonov-Bohm (AB) and Aharonov-Casher (AC) effects can occur. For the AC effect, a Cooper pair must be split and nonlocally preserve its entangled singlet state, and each electron of the pair has to experience a different Rashba phase.
• Spin-1 two-impurity Kondo problem on a lattice
  A.Allerdt, R. Žitko, A. E. Feiguin
  Phys. Rev. B 97, 045103 (2018) (PDF)
  Two-impurity spin-1 model is studied on a square lattice for different inter-impurity separations using the DMRG. When the impurities are underscreened, the dangling spin-1/2 degrees of freedom generate the inter-impurity entanglement. Kondo screening and RKKY spin alignment coexist.
• Quantum impurity models for magnetic adsorbates on superconductor surfaces
  R. Žitko
  Physica B 536, 230 (2018)
  Classical and quantum impurity models for magnetic adosrbates on superconductors differ in a number of qualitative respects: number and degeneracies of the sub-gap states, presence of symmetry breaking, and the effect of the external magnetic field. The high-spin adsorbates with Ising (easy-axis) magnetic anisotropy show features that are different from both isotropic quantum impurities and classical impurities.
• Superconducting quantum dot and the sub-gap states
  R. Žitko
  Proc. SPIE 10732, Spintronics XI, 107321N (20 September 2018) (PDF)
  Brief review of the physics of quantum impurities coupled to superconducting contacts.
• Na poti do kvantne prevlade
  R. Žitko
  Gea, april 2018
  (Slovenian only) O kvantnem računalništvu in ostalih prihajajočih kvantnih tehnologijah.
• Evropa želi postati velesila v kvantnih tehnologijah. Kaj pa Slovenija?
  Peter Jeglič, Dragan Mihailović, Anton Ramšak, Denis Arčon, Rok Žitko
  DELO, 24. november (2018)
  

2017

• Field-Induced Instability of a Gapless Spin Liquid with a Spinon Fermi Surface
  M. Gomilšek, M. Klanjšek, R. Žitko, M. Pregelj, F. Bert, P. Mendels, Y. Li, Q. M. Zhang, and A. Zorko
  Phys. Rev. Lett. 119, 137205 (2017) (PDF)
  The quantum spin liquid state in Zn brochantite undergoes a transition in an arbitrarily-small applied magnetic field to a different spin liquid state with a full or a partial gap, likely due to spinon pairing.
• Strange metal from Gutzwiller correlations in infinite dimensions: Transverse transport, optical response, and rise of two relaxation rates
  Wenxin Ding, Rok Žitko, and B. Sriram Shastry
  Phys. Rev. B 96, 115153 (2017) (PDF)
  We study the transverse transport and optical response in the infinite-U Hubbard model in the low-temperature regimes. We extract the longitudinal and transverse relaxation rates. A single rate controls both longitudinal and transverse transport in the lowest-temperature Fermi-liquid regime, however in the strange-metal regime two different relaxation rates emerge due to the dynamical particle-hole asymmetry of the Dyson self-energy.
• A high-temperature quantum spin liquid with polaron spins
  Martin Klanjšek, Andrej Zorko, Rok Žitko, Jernej Mravlje, Zvonko Jagličić, Pabitra Kumar Biswas, Peter Prelovšek, Dragan Mihailovic, Denis Arčon
  Nat. Phys. 13, 1130 (2017) (self-archived PDF)
  The local moments in TaS₂ do not order down to the mK range, as evidenced by NQR and muSR measurements. Between 55K and 200K, the T² relaxation dynamics are observed, while below 55K a new gapless state with reduced density of spin excitations is observed.
• Spin-orbit coupling in three-orbital Kanamori impurity model and its relevance for transition-metal oxides
  Alen Horvat, Rok Žitko, and Jernej Mravlje
  Phys. Rev. B 96, 085122 (2017) (PDF)
  The spin-orbit coupling (SOC) in the three-orbital impurity model with Kanamori interaction becomes effective if it is larger in strength than the orbital Kondo temperature. Depending on the sign of SOC the system then behaves as either van Vleck nonmagnetic impurity or a J=2 impurity. We find that irrespective of the strength of the SOC, the system always has a Fermi liquid ground state.
• Strange metal from Gutzwiller correlations in infinite dimensions
  Wenxin Ding, Rok Žitko, Peizhi Mai, Edward Perepelitsky, and B. Sriram Shastry
  Phys. Rev. B 96, 054114 (2017) (PDF)
  We examine the two lowest temperature regimes (Gutzwiller correlated Fermi liquid and Gutzwiller correlated Fermi liquid) in the U=inf d=inf Hubbard model. Most quantities (thermodynamic and transport properties) show a change of behavior at the cross-over between these two regimes. The temperature dependence of resistivity in this temperature range simply follows the temperature dependence of zero-frequency imaginary part of the self-energy, which starts to deviate from its low-temperature asymptotic behavior on the scale of the Fermi-liquid temperature.
• Nonperturbative effects and indirect exchange interaction between quantum impurities on metallic (111) surfaces
  A. Allerdt, R. Žitko, and A. E. Feiguin
  Phys. Rev. B 95, 235416 (2017) (PDF)
  We study the exchange interaction between two impurities on the (111) surface which hosts surface states using the density matrix renormalization group, using a representation which allows obtaining high accuracy results in the thermodynamic limit. Ferromagnetism is never developped, except at short distances.
• Scaling of subgap excitations in a superconductor-semiconductor nanowire quantum dot
  Eduardo J. H. Lee, Xiaocheng Jiang, Rok Žitko, Ramón Aguado, Charles M. Lieber, and Silvano De Franceschi
  Phys. Rev. B 95, 180502(R) (2017) (PDF)
  Editor's Suggestion
  We study the position of the subgap Andreev level excitations in a hybrid superconductor-semiconductor device and analyse it in terms of a simple Anderson impurity model with superconducting leads. The model parameters are extracted by a careful analysis of the linear conductance in the normal-state (taking into account temperature and magnetic field effects), and then used as the input in a calculation of the superconducting sub-gap spectra. We find excellent agreement between theory and experiments for the scaling of the level vs. T_K/Delta ratio, where T_K is the Kondo temperature and Delta the BCS gap.
• Non-Fermi-liquid behavior in quantum impurity models with superconducting channels
  R. Žitko and M. Fabrizio
  Phys. Rev. B 95, 085121 (2017) (PDF)
  In the limit of small superconducting gap in the bath, the sub-gap spectrum of overscreened Kondo models shows universal features characteristic of the fixed point of the normal-state problem. These effects could be detected in hybrid devices with mesoscopic superconducting islands.
• Magnetic impurities in spin-split superconductors
  W.-V. van Gerven Oei, D. Tanasković, R. Žitko
  Phys. Rev. B 95, 085115 (2017) (PDF)
  We study the sub-gap (Shiba) state of classical and quantum impurities in a superconductor with Zeeman splitting of Bogoliubov quasiparticle states. For classical impurities, this results in a trivial shift of the Shiba state. For quantum impurities, we find a non-monotonic dependence of the singlet-doublet quantum phase transition line in the phase diagram in the (gap,field) plane.
• Fine structure of the spectra of the Kondo lattice model: Two-site cellular dynamical mean-field theory study
  Žiga Osolin, Rok Žitko
  Phys. Rev. B 95, 035107 (2017) (PDF)
  Cellular DMFT calculation is performed using the NRG method as the impurity solver for the Kondo lattice model, focusing on the fine structure of the spectra in the antiferromagnetic phase. The discretization is performed in the matrix space. We discuss the limitations of the method due to significant computational demands and agressive state trucation.
• Nobelova nagrada za fiziko 2016
  Rok Žitko
  Obzornik za matematiko in fiziko 64, 10 (2017) (preprint)
  (Slovenian only) O Nobelovi nagradi D. Thoulessu, D. Haldanu in M. Kosterlitzu za vpeljavo konceptov iz topologije za razlago osnovnih lasnosti različnih faz kondenzirane snovi ter faznih prehodov med njimi.

2016

• Transport and optical conductivity in the Hubbard model: A high-temperature expansion perspective
  Edward Perepelitsky, Andrew Galatas, Jernej Mravlje, Rok Žitko, Ehsan Khatami, B. Sriram Shastry, and Antoine Georges
  Phys. Rev. B 94, 235115 (2016) (PDF)
  Spectral moments of the dynamical response functions of the Hubbard moment are derived through high-temperatures series expansions, then these functions are reconstructed. Excellent agreement is found with DMFT results obtained using state-of-the-art numerical methods in a surprisingly wide temperature range. High-T resistivity is linear without saturation as a result of saturating scattering rate.
• Low-energy physics of three-orbital impurity model with Kanamori interaction
  Alen Horvat, Rok Žitko, Jernej Mravlje
  Phys. Rev. B 94, 165140 (2016) (PDF)
  The three-orbital impurity model with Kanamori interaction, appropriate for Hunds metals, maps to a Kondo model with spin, orbital and quadrupole (as well as mixed) coupling terms. We study the degree of the separation between the spin and orbital degrees of freedom. We find slow screening of the spin degrees of freedom. At low energies, the SU(3) orbital symmetry is restored.
• Kondo effect at low electron density and high particle-hole asymmetry in 1D, 2D, and 3D
  Rok Žitko, Alen Horvat
  Phys. Rev. B 94, 125138 (2016) (PDF)
  We discuss the relation between the impurity binding energy and the Kondo temperature. We discuss the effects of the band-edge van Hove singularities on the Kondo scale, finding strong departures in 1D and 3D cases due to the nontrivial role of the renormalization of the potential scattering.
• Spectral properties of Shiba subgap states at finite temperatures
  Rok Žitko
  Phys. Rev. B 93, 195125 (2016) (PDF)
  We study the temperature dependence of the sub-gap part of the spectral function of the Anderson impurity model with superconducting bath. We find a background continuum at finite temperatures due to inelastic exchange scattering of thermally excited Bogoliubov quasiparticles. In charge-fluctuation regime with singlet ground state, we detect an additional sub-gap resonance just below the gap-edge. It appears to be related to scattering on the thermally excited doublet sub-gap state.
• Metal-to-insulator crossover in alkali doped zeolite
  Mutsuo Igarashi, Peter Jeglič, Andraž Krajnc, Rok Žitko, Takehito Nakano, Yasuo Nozue, Denis Arčon
  Sci. Rep. 6, 18682 (2016) (PDF) [open access]
  NMR investigation of LSX zeolites reveals a doping-dependent cross-over from an insulating to a metal state. In the metal state, Korringa behaviour is found for the nuclear spin-lattice relaxation time.
• Evropa z milijardami poganja kvantno revolucijo. Slovenije ni zraven
  Peter Jeglič, Dragan Mihailović, Tomaž Prosen, Anton Ramšak, Rok Žitko
  DELO, 27. maj (2016)
  

2015

• Z₂ gauge theory description of the Mott transition in infinite dimensions
  Rok Žitko, Michele Fabrizio
  Phys. Rev. B 91, 245130 (2015) (PDF)
  Infinite-dimensional Hubbard model maps exactly onto a model of free fermions coupled in a Z₂ gauge-invariant manner to auxiliary spin variables in a transverse field, thereby separating charge and spin degrees of freedom. The Mott transition translates into spontaneous breaking of the local Z₂ gauge symmetry, which is not forbidden in finite dimensions and is not an artifact of the mean-field decomposition. This endows the Mott transition of an order parameter.
• Numerical subgap spectroscopy of double quantum dots coupled to superconductors
  Rok Žitko
  Phys. Rev. B 91, 165116 (2015) (PDF) (suppl)
  Sub-gap spectra are analyzed in the DQD system coupled to superconducting leads in an extended range of parameters, including the valence-fluctuation and empty orbital regimes. We determine conditions for observing the doublet state Shiba state splitting, and for observing both singlet and triplet Shiba states. Inter-dot capacitive coupling is also considered: it may lead to cross-over at half-filling and to phase transitions close to quarter-filling. Finally, for two impurities coupled solely by the exchange interaction, we find a quantum phase transition between two singlet states.
• Repulsive versus attractive Hubbard model: Transport properties and spin-lattice relaxation rate
  Rok Žitko, Žiga Osolin, Peter Jeglič
  Phys. Rev. B 91, 155111 (2015) (PDF)
  We compare the properties of repulsive and attractive Hubbard model in the paramagnetic state for generic filling using the DMFT. In the attractive model, the particle-hole asymmetry of the scattering rate is opposite to that in the repulsive case, leading to a reduction of the slope of the Seebeck coefficient at low temperatures. We point out the difference in Seebeck coefficient between the positive-U model at half filling in magnetic field and negative-U model away from half-filling, arising from the partial p-h mapping.
• Fine structure of spectra in the antiferromagnetic phase of the Kondo lattice model
  Žiga Osolin, Thomas Pruschke, and Rok Žitko
  Phys. Rev. B 91, 075105 (2015) (PDF)
  Correlated Kondo systems in the large Fermi surface regime with Neel antiferromagnetic order have inner structure in the resulting renormalized bands. These peaks/dips are due to an order-parameter-dependent shift of the resonances in the self-energy and non-equal Weiss fields in the c and f bands.
• Shiba states and zero-bias anomalies in the hybrid normal-superconductor Anderson model
  Rok Žitko, Jong Soo Lim, Rosa Lopez, Ramon Aguado
  Phys. Rev. B 91, 045441 (2015) (PDF)
  We study the sub-gap spectra of a quantum impurity coupled to one superconducting and one normal-state lead (tunneling probe). We show under which conditions a zero-bias anomaly appears either due to the presence of the normal-state lead or due to Zeeman splitting of the doublet Shiba state. The calculations are performed for arbitrary ratio Delta/T_K, thus Kondo correlations are properly taken into account.

2014

• Entanglement switching via the Kondo effect in triple quantum dots
  S. B. Tooski, B. R. Bulka, R. Zitko, A. Ramsak
  Eur. Phys. J. B 87, 145 (2014) (PDF)
  Entanglement properties of a triple quantum dot system are studied with the focus on the switching between separable and entangled state due to Kondo singlet formation.
• Effect of assisted hopping on thermopower in an interacting quantum dot
  S. B. Tooski, A. Ramsak, B. R. Bulka, R. Zitko
  New J. Phys. 16, 055001 (2014) (PDF) [open access]
  Assisted hopping (occupancy-dependence of the tunneling amplitudes) can lead to anomalies in the mixed-valence regime which, in turn, are associated with very high Seebeck coefficient.

2013

• Extremely correlated Fermi liquid theory meets dynamical mean-field theory: Analytical insights into the doping-driven Mott transition
  R. Žitko, D. Hansen, E. Perepelitsky, J. Mravlje, A. Georges, B. S. Shastry
  Phys. Rev. B 88, 235132 (2013) (PDF) DMFT(NRG) code used in this paper
  We analyze the doped Mott insulator using DMFT(NRG) and ECFL. The ECFL Ansatz has a suitable form to well describe the features of the self-energy, in particular the asymmetry between particle and hole excitations, even when symmetric (Fermi-liquid-like) auxiliary self-energies are used as the input to the ECFL formalism.
• Spin thermopower in the overscreened Kondo model
  R. Žitko, J. Mravlje, A. Ramšak, T. Rejec
  New J. Phys. 15, 105023 (2013) (PDF) NRG scripts used in this paper [open access]
  We study the spin thermopower in various members in the family of Kondo models, focusing on the two-channel spin-1/2 case, which exhibits non-Fermi-liquid ground state. We show that the critical exponent can be deduced from the magnetic-field and temperature dependence of the spin Seebeck coefficient.
• Lifshitz phase transitions in the ferromagnetic regime of the Kondo lattice model
  Denis Golež, Rok Žitko
  Phys. Rev. B 88, 054431 (2013) (PDF) (suppl) DMFT(NRG) code used in this paper
  We establish the phase diagram of the topologically different ferromagnetic phases of spin-1/2 and spin-1 Kondo lattice models using the DMFT and static mean-field approaches. We determine the "ferromagnetic Doniach diagram".
• Tunneling spectroscopy of a single quantum dot coupled to a superconductor: From Kondo ridge to Andreev bound states
  J.-D. Pillet, P. Joyez, Rok Žitko, M. F. Goffman
  Phys. Rev. B 88, 045101 (2013) (PDF)
  Described in Physics Viewpoint PRB Kaleidoscope
  The evolution of the Kondo resonance towards Andreev bound states is observed in a carbon nanotube quantum dot coupled to a contact in either normal or superconducting state. The results are in quantitative agreement with a single-level Anderson model.
• Padé approximant approach for obtaining finite-temperature spectral functions of quantum impurity models using the numerical renormalization group technique
  Žiga Osolin, Rok Žitko
  Phys. Rev. B 87, 245135 (2013) (PDF)
  We present a new method for obtaining finite-temperature spectral functions of impurity models using the NRG, based on performing Pade continuation to the real frequency axis of raw data accumulated on the imaginary axis. The method provides improved results for spectral functions on frequency scales of order T.
• Unconventional Superconductivity from Local Spin Fluctuations in the Kondo Lattice
  Oliver Bodensiek, Rok Žitko, Matthias Vojta, Mark Jarrell, Thomas Pruschke
  Phys. Rev. Lett 110, 146406 (2013) (PDF)
  We find a singlet s-wave superconducting solution for the Kondo lattice model within the single-site DMFT. We argue the pairing is due to strong local spin fluctuations.
• Non-Drude universal scaling laws for the optical response of local Fermi liquids
  C. Berthod, J. Mravlje, X. Deng, R. Žitko, D. van der Marel, A. Georges
  Phys. Rev. B 87, 115109 (2013) (PDF)
  Editor's Suggestion
  Optical response of a Fermi liquid is computed, assuming a local self-energy with FL scaling form. The optical conductivity shows a non-Drude "foot".
• How Bad Metals Turn Good: Spectroscopic Signatures of Resilient Quasiparticles
  X. Deng, J. Mravlje, R. Žitko, M. Ferrero, G. Kotliar, A. Georges
  Phys. Rev. Lett 110, 086401 (2013) (PDF) (suppl)
  In hole-doped Mott insulators, resilient quasiparticles remain well defined above the Fermi temperature and dominate transport in the intermediate regime up to the Mott-Ioffe-Regel regime.
• SU(3) Kondo effect in spinless triple quantum dots
  R. Lopez, T. Rejec, J. Martinek, R. Žitko
  Phys. Rev. B 87, 035135 (2013) (PDF)
  We study the Kondo physics in the system of three-spin polarized quantum dots, which provide a realization of the SU(3) Kondo model. We discuss the stability of the SU(3) Kondo effect with respect to different perturbation.

2012

• Vibrational Andreev bound states in magnetic molecules
  D. Golez, J. Bonca, R. Žitko
  Phys. Rev. B 86, 085142 (2012) (PDF)
  A deformable magnetic molecule adsorbed on a superconducting surface is predicted to exhibit sub-peaks to the main sub-gap Anderson bound state (Shiba state). We discuss the phonon hardening effects due to anharmonic potential for the vibrational mode which is in competition with the phonon softening due to electron-phonon coupling.
• Tunable Kondo Effect in a Double Quantum Dot Coupled to Ferromagnetic Contacts
  Rok Žitko, Jong Soo Lim, Rosa Lopez, Jan Martinek, Pascal Simon
  Phys. Rev. Lett 108, 166605 (2012) (PDF)
  Phase diagram for a DQD coupled to spin-polarized contacts is established. The two-impurity Kondo cross-over is generalized to the (p,t) plane, where p is polarization and t the inter-dot tunneling. The spin-current can be controlled and reversed using electrostatic gates alone.
• Spin thermopower in interacting quantum dots
  T. Rejec, R. Žitko, J. Mravlje, A. Ramsak
  Phys. Rev. B 85, 085117 (2012) (PDF)
  We study the spin thermopower of a quantum dot in a magnetic field: the temperature difference applied across the dot drives a pure spin current without accompanying charge current.
• Ground state of the parallel double quantum dot system
  R. Žitko, J. Mravlje, K. Haule
  Phys. Rev. Lett 108, 066602 (2012) (PDF)
  We show that CT-QMC and NRG produce consistent results for the low-temperature behavior of parallel double quantum dot (singular Fermi liquid state), which are in disagreement with Bethe Ansatz calculations. We argue that the origin of the unconventional behavior is the odd-symmetry "dark state" on the dots.

2011

• Kondo effect in the presence of Rashba spin-orbit interaction
  R. Žitko, J. Bonca
  Phys. Rev. B 84, 193411 (2011) (PDF)
  We study the Anderson model for an impurity hybridizing with a 2D electron gas with Rashba spin-orbit interaction. The spectral function exhibits an additional peak at the bottom of the band which becomes more pronounced due to diverging effective DOS. The Kondo temperature varies only little (non-exponentially).
• Kondo resonance lineshape of magnetic adatoms on decoupling layers
  R. Žitko
  Phys. Rev. B 84, 195116 (2011) (PDF)
  Excellent numerical agreement between the theoretical Kondo line-shape and the experimental results for the Ti/CuN/Cu(100) zero-bias anomaly is found, including even the asymptotic logarithmic tails. No potential scattering needs to be included in the impurity model: the valence of the impurity atom is well defined. The extraction of the g factor is also discussed: two interpretations are possible.
• Quantum impurity coupled to Majorana edge fermions
  R. Žitko, P. Simon
  Phys. Rev. B 84, 195310 (2011) (PDF)
  Anderson impurity that hybridizes with the sub-gap states of a helical topological superconductor (a pair of counterpropagating Majorana fermion edge channels) either behaves as a magnetic impurity with residual ln 2 impurity entropy and anisotropic magnetic susceptibility, or as a non-magnetic impurity with weak diamagnetic contribution to the total magnetic susceptibility of the system.
• Hydrogenation properties of theTiBx structures
  R. Žitko, H.J.P. Van Midden, E. Zupanic, A. Prodan, S.S. Makridis, D. Niarchos, A.K. Stubos
  Int. J. of Hydrogen Energy 36, 12268 (2011)
  Random structure search using the DFT has been performed for hydrogenated TiB2, TiB and Ti2B titanium borate structures.
• Quantitative determination of the discretization and truncation errors in numerical renormalization-group calculations of spectral functions
  R. Žitko
  Phys. Rev. B 84, 085142 (2011) (PDF)
  Errors in the spectral-function calculations using NRG can be quantitatively estimated. Application to magnetic-field splitting of the Kondo resonance.
• SNEG - Mathematica package for symbolic calculations with second-quantization-operator expressions
  R. Žitko
  Comp. Phys. Comm. 182, 2259 (2011) (PDF) [SNEG download page]
  Description of the philosphy and design choices behind SNEG, a package for symbolic computation with second-quantization operators.
• Detection of Majorana edge states in topological superconductors through non-Fermi-liquid effects induced in an interacting quantum dot
  R. Žitko
  Phys. Rev. B 83, 195137 (2011) (PDF)
  The presence of the continuum of Majorana fermion edge states along the perimeter of a chiral topological superconductor can be probed using an interacting quantum dot coupled to three terminals: the lead supporting the Majorana edge states and two spin-polarized (ferromagnetic) measurement leads.
• Low-energy properties of the Kondo lattice model
  O. Bodensiek, R. Žitko, R. Peters, T. Pruschke
  J. Phys.: Condens. Matter 23, 094212 (2011) (PDF)
  The heavy-fermion physics in the Kondo lattice model is studied within the DMFT using the NRG as the solver. The cross-over between large and small Fermi surface is discussed, as well as the effect of phonons.
• Magnetic anisotropy effects on quantum impurities in superconducting host
  Rok Žitko, Oliver Bodensiek, Thomas Pruschke
  Phys. Rev. B 83, 054512 (2011) (PDF)
  Magnetic anisotropy leads to the splitting of sub-gap states localized on magnetic impurities coupled to a superconducting host. This splitting may have already been experimentally resolved in high-resolution STM studies of Mn and Cr on superconducting Pb. We also discuss the effect of the external magnetic field on the sub-gap states, and the sub-gap spectrum of the two-impurity model, where two different singlet states, a pair of doublet states, and a triplet state compete for becoming the ground state of the system.

2010

• Josephson current in strongly correlated double quantum dots
  Rok Žitko, Minchul Lee, Rosa Lopez, Ramon Aguado, Mahn-Soo Choi
  Phys. Rev. Lett. 105, 116803 (2010) (PDF)
  Phase diagram of 0- and π-junction regimes of Josephson supercurrent is established for a double quantum dot in contact with two superconducting leads. An island of π-phase appears in the range where all three competing interactions (superexchange, Kondo screening, superconductivity) are equally strong. The π phases are associated with the doublet Shiba state becoming the ground state.
• Pinning of adsorbed cobalt atoms by defects embedded in the copper (111) surface
  Erik Zupanic, Rok Žitko, Herman J. P. van Midden, Igor Musevic, Albert Prodan
  Phys. Rev. Lett. 104, 196102 (2010) (PDF)
  Substitutional Ag defects in the top-most atomic layer of the Cu(111) act as pinning centers for Co adatoms without affecting their electronic nor magnetic properties (the Kondo resonance remains essentially the same). The Ag impurities are hardly detectable in topographic imaging as they produce no standing-wave pattern and the dI/dV recorded above them is hardly different from that of the clean surface.
• Charge density wave ordering in NbSe3: possible models and the experimental evidence
  A Prodan, H J. P. van Midden, R Žitko, E Zupanic, J C Bennett, H Boehm
  Solid state Comm. 150, 2134 (2010) (PDF)
  Discussion of charge density wave (CDW) modes in NbSe3. It is pointed out that the standard model with two incommensurate modulations localized on different prismatic columns does not account for the totality of experimental results.
• Quantum impurity on the surface of a topological insulator
  R. Žitko
  Phys. Rev. B 81, 241414(R) (2010) (PDF)
  The Hamiltonian for a quantum magnetic impurity on the surface of a topological insulator can be mapped to the conventional pseudo-gap Anderson impurity model. This implies that the magnetic moment is fully Kondo screened, thus at low enough temperature a single magnetic dopant is effectively non-magnetic and does not break the time-reversal-invariance (TRI) protection of the surface states.
• Many-particle effects in adsorbed magnetic atoms with easy-axis anisotropy: the case of Fe on CuN/Cu(100) surface
  Rok Žitko, Thomas Pruschke
  New J. Phys. 12, 063040 (2010) (PDF) [open access]
  A Kondo effect is predicted for Fe on CuN/Cu(100) surfaces for a magnetic field in the 8-9T applied along the "hollow direction". We also point out that studies of Kondo features provide information about the spin relaxation times of the magnetic impurities. On the technical side, this work describes the implementation for computing spin susceptibilities using the NRG, for cases where the system has no conserved spin quantum numbers.
• Structural and Electronic Properties of the Hydrogenated ZrCr2 Laves Phases
  H. J. P. van Midden, A. Prodan, E. Zupanic, R. Žitko, S. S. Makridis, A. K. Stubos
  J. Phys. Chem. C 114, 4221-4227 (2010)
  Two interstitial H atoms in close proximity in ZrCr2 host repel each other.
• Fano-Kondo effect in side-coupled double quantum dots at finite temperatures and the importance of the two-stage Kondo screening
  Rok Žitko
  Phys. Rev. B 81, 115316 (2010) (PDF)
  The antiresonance in the transport experiments in side-coupled double quantum dot may be explained in terms of the two-stage Kondo screening with the experimental temperature between the two different Kondo scales.
• Kondo screening in high-spin side-coupled two-impurity clusters
  Rok Žitko
  J. Phys.: Condens. Matter 22, 026002 (2010) (PDF)
  The two-stage Kondo screening phenomenology is generalized to a pair of side-coupled Kondo impurities with large spin. Both antiferromagnetic and ferromagnetic (as in Hund's rule) exchange coupling between the spins is analysed.
• The Two-impurity Anderson Model Revisited: Competition between Kondo Effect and Reservoir-mediated Superexchange in Double Quantum Dots
  Minchul Lee, Mahn-Soo Choi, Rosa Lopez, Ramon Aguado, Jan Martinek, Rok Žitko
  Phys. Rev. B 81, 121311(R) (2010) (PDF)
  Editor's Suggestion
  A conduction-band mediated superexchange mechanism leads to an effective antiferromagnetic interaction between the two impurity spins in the infinite-U limit, thereby significantly modifying the transport properties of the system. This work points out the deficiencies of static mean-field description of inter-impurity coupling.
• Superconductivity in the Kondo lattice model
  O. Bodensiek, R. Žitko, Th. Pruschke
  J. Phys.: Conf. Ser. 200 012162 (2010) (PDF)
  DMFT(NRG) study of the Kondo lattice model with attractive interaction between the conduction-band electrons.
• Magnetization Curves for Anisotropic Magnetic Impurities Adsorbed on a Normal Metal Substrate
  R. Žitko
  Physical properties of nanosystes, pp 247-257 (PDF)
  NRG study of magnetization curves Sz(B,T) for magnetic impurities. Deviations from ideal paramagnetic behavior is discussed.

2009

• Van Hove singularities in the paramagnetic phase of the Hubbard model: a DMFT sutdy
  R. Žitko, J. onca, Th. Pruschke
  Phys. Rev. B 80, 245112 (2009) (PDF)
  Electron-electron interactions smooth out van Hove singularities in the spectral functions of the Hubbard model, however the logarithmic singularity (for 2D square lattices) pinned at the Fermi level survives. In this latter case, the magnetic-field-induced metal-insulator-transition falls in a different universality class as for the case with featureless density of states, as there is no field-induced localization of quasiparticles. 3D cubic and a DOS with power-law divergence are also considered.
• Charge Density Waves in Pure and Intercalated Nb3Te4
  A. Prodan, Herman J. P. van Midden, S. Sturm, R. Žitko, E. Zupanic, J. Craig Bennett, R. MacDougall, P. Starowicz
  Solid State Sciences 11, 1556 (2009)
  Intercalation of Tl and In in Nb3Te4 results in a flattening of the Fermi surfaces and the formation of charge density waves (CDW) is enhanced.
• Low-temperature Scanning Tunneling Microscopy and Spectroscopy of Noble-metal Surfaces
  E. Zupanic, R. Žitko, H. J. P. van Midden, I. Musevic, A. Prodan
  Croat. Chem. Acta 82, 485 (2009) (PDF)
  We describe some results obtained with the low-temperature STM built at the Jozef Stefan Institute. The instrument has high mechanical stability, allowing for improved spectroscopic measurements and performing atomic manipulation with single atoms.
• Convergence acceleration and stabilization for dynamical-mean-field-theory calculations
  R. Žitko
  Phys. Rev. B 80, 125125 (2009) (PDF)
  Using advanced mixing techniques (modified Broyden's method) the convergence to a self-consistent solution in DMFT calculations can be significantly accelerated (or even made possible in some extreme parameter regimes). In addition, unstable and meta-stable solutions of DMFT equations can be obtained.
• NRG calculations of the ground-state energy: application to the correlation effects in the adsorption of magnetic impurities on metal surfaces
  R. Žitko
  Phys. Rev. B 79, 233105 (2009) (PDF)
  The binding energy of the single-impurity Anderson model is computed to high accuracy. The Kondo energy is only a small contribution to the total correlation energy (total binding energy minus Hartree-Fock binding energy), which is order Γ², rather than order T_K. This work shows that the NRG can be applied to calculate the ground state energies of quantum impurity models with an excellent accuracy, sufficient to extract various quantities of physical interest (susceptibilities, etc.).
• Adaptive logarithmic discretization for numerical renormalization group methods
  R. Žitko
  Comp. Phys. Comm. 180, 1271 (2009) (PDF)
  NRG discretization scheme with adaptable discretization-mesh points reduces artefacts which result from very low density of states over extended energy intervals.
• Splitting of the Kondo resonance in anisotropic magnetic impurities on surfaces
  R. Žitko, R. Peters, T. Pruschke
  New J. Phys. 11, 053003 (2009) (PDF) [open access]
  Kondo resonance splitting in anisotropic Kondo models which map on an effective spin-1/2 Kondo model can be described using effective g-factors determined by projection of the spin-S operators on the spin-1/2 subspace, multiplied by some prefactor which depends logarithmically on the B/T_K ratio and takes into account the Kondo physics. This fully accounts for the experimental results by A. Otte et al. [Nat. Phys. 4, 847], while properly taking into account the Kondo physics and the associated renormalization of the peak splitting.
• Energy resolution and discretization artefacts in the numerical renormalization group
  R. Žitko, T. Pruschke
  Phys. Rev. B 79, 085106 (2009) (PDF)
  We show how to compute spectral functions of quantum imputity models with high resolution using the numerical renormalization group techniques. Sample high-resolution results for the Anderson and Anderson-Holstein impurity models and for Hubbard model (within DMFT). This work also introduces a new discretization technique which largely eliminates the discretization artefacts of all previous approaches.
• Anomaly in the heat capacity of Kondo superconductors
  R. Žitko, T. Pruschke
  Phys. Rev. B 79, 012507 (2009) (PDF)
  Magnetic impurities doped into a superconductor are predicted to exhibit negative contribution to the heat capacity of the system in some temperature range. The effect is large when the Kondo temperature T_K is similar to the critical temperature T_C.
• Quantum Impurity Physics in Coupled Quantum Dots
  R. Žitko, J. Bonča
  Book chapter, Encyclopedia of Complexity and System Science, Springer, Berlin (2009).(PDF)
  Introduction to the quantum impurity physics in coupled quantum dots described by the multi-impurity Anderson models.

2008

• Properties of anisotropic magnetic impurities on surfaces
  R. Žitko, R. Peters, T. Pruschke
  Phys. Rev. B 78, 224404 (2008). (PDF)
  We study the Kondo model with XXZ anisotropy as a model for anisotropic magnetic impurities on surfaces. Both easy-axis and hard-axis cases are considered. For hard-axis half-integer spin cases (such as S=3/2 for Co adatoms on CuN/Cu), we point out a strong enhancement of the Kondo temperature due to effective doubling of the transverse component of the Kondo couping; without this exponential enhancement, T_K would be unmeasurably low.
• Numerical renormalization group study of two-channel three-impurity triangular clusters
  R. Žitko, J. Bonča
  Phys. Rev. B 77, 245112 (2008). J. Bonča (PDF)
  Editor's Suggestion
  Phase diagram of triangular three-impurity clusters is determined. Two-stage Kondo screening is a relevant concept even for cross-overs between non-Fermi liquid fixed point. We find such a direct transition between two-impuriy and two-channel Kondo model fixed points. We point out that in problems with exponentially low energy scales, it is dangerous to study transport at "zero temperature".
• Vibrational effects on low-temperature properties of molecular conductors
  J. Mravlje, A. Ramsak, R. Žitko
  Physica B 403, 1484 (2008). (PDF)
  In the Anderson model with additional phonon-assisted coupling to the odd conduction channel, the zero-temperature conductance is found to be qualitatively the same as in the model with no coupling to the vibrational mode. An unstable two-channel Kondo model fixed point is found at the point where the couplings match.

2007

• Quantum phase transitions in systems of parallel quantum dots
  R. Žitko, J. Bonča
  Phys. Rev. B 76, 241305(R) (2007). (PDF)
  Conductance through clusters of parallel quantum dots as a function of the gate voltage exhibits discontinuities related to two types of quantum phase transitions: 1) magnetic transitions from ferromagnetic to antiferromagnetic impurity spin correlations, 2) transitions due to abrupt change in average impurity occupancy. These effects arise from the interplay of strong-correlation effects and decoupling of certain combinations of states from the continuum (the Dicke effect).
• Fermi-liquid versus non-Fermi-liquid behavior in triple quantum dots
  R. Žitko, J. Bonča
  Phys. Rev. Lett. 98, 047203 (2007). (PDF)
  The conductance through linear trimer of quantum dots in the particle-hole symmetric case is unitary at zero temperature. For weak inter-dot coupling, the Kondo screening occurs in two stages. For a range of parameters, the two-channel Kondo non-Fermi liquid (NFL) fixed point is approached at finite temperatures; we discuss the transport of this NFL behaviour.
• Correlation effects in side-coupled quantum dots
  R. Žitko, J. Bonča
  J. Phys.: Condens. Matter 19 255205 (2007).
  Review of two-stage Kondo effect in side-coupled double quantum dot, demonstration of three-stage Kondo effect in side-coupled triple quantum dot and a generalization to multi-stage Kondo effect in more general systems. Study of simultaneous spin and charge Kondo effect in side-coupled configuration.

2006

• Spin-charge separation and simultaneous spin and charge Kondo effect
  R. Žitko, J. Bonča
  Phys. Rev. B 74, 224411 (2006). (PDF)
  Due to the spin-charge separation, a single conduction channel may screen both spin and charge (isospin) degrees of freedom: spin and charge Kondo effects may occur simultaneously and independently. Such effect might occur in specifically tuned single molecule transistors.
• Spin qubits in double quantum dots - entanglement versus the Kondo effect
  A. Ramšak, J. Mravlje, R. Žitko, J. Bonča
  Phys. Rev. B 74, 241305(R) (2006). (PDF)
  The entanglement (concurrence) between two localized spins in a double quantum dot has universal behaviour as a function of inter-dot coupling, magnetic field and temperature for different topologies of the coupling to the leads.
• Multi-impurity Anderson model for quantum dots coupled in parallel
  R. Žitko, J. Bonča
  Phys. Rev. B 74, 045312 (2006). (PDF)
  If several Anderson impurities couple to the same orbital of the conduction band, the inter-impurity RKKY interaction is ferromagnetic. The spins align at low temperatures and the systems undergoes S=N/2 Kondo screening, where N is the number of impurities. An analytic estimate is provided for the RKKY exchange constant. Perturbation terms and induced quantum phase transitions are studied.
• Kondo effect in triple quantum dot
  R. Žitko, J. Bonča, A. Ramšak, T. Rejec
  Phys. Rev. B 73, 153307 (2006). (PDF)
  A phase diagram for the triple quantum dot is established. At strong inter-dot coupling the electrons consecutively fill molecular orbitals and the Kondo effect emerges for odd number of electrons. At weak coupling, the electrons tend to form rigid antiferromagnetic spin chains or undergo two-stage Kondo screening.
• Enhanced conductance through side-coupled double quantum dots
  R. Žitko, J. Bonča
  Phys. Rev. B 73, 035332 (2006). (PDF)
  Side-coupled double quantum dot has two Kondo plateaus of high conductance when the inter-dot coupling is strong. For weak inter-dot coupling the Kondo screening occurs in two stages and the conductance is high when the temperature is between the upper and lower Kondo temperature. The density matrix numerical renormalization group in the basis of good quantum numbers (Q,S) is described, where Q is the total charge and S the total spin.
• Tunneling Microscope as Nanotechnologic and Analytical Tool
  R. Žitko, I. Muševič
  Obz. mat. fiz., 24, 65 (2005).

Before 2006

• Microscopic mechanisms of dephasing due to electron-electron interactions
  R. Žitko, J. Bonča
  Phys. Rev. B 68, 085313 (2003). (PDF)
  A method for calculating the scattering of a single electron impinging into a region of high electron-electron interaction where additional electrons are bound. Application to the Aharonov-Bohm ring where dephasing is studied.

PhD dissertation

• Many-particle effects in resonant tunneling of electrons through nanostructures (PDF, 8.8 MB)
  Rok Žitko, PhD dissertation, University of Ljubljana, Faculty of Mathematics and Physics (2007).