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Publications
2024
 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
timedependent processes, and for the calculation of transition matrix
elements for microwavedriven transmon, spinflip 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 GroveRasmussen, 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 largescale spin coupling.
 Strongcoupling theory of quantumdot 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
strongcoupling 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.
 Lindbladian approach to P(E) theory
Ádám Bácsi, Rok Žitko
arXiv:2403.04441
(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.
 True random number generators with flicker noise: stochastic model,
minentropy 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 AndersonHolstein 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 electronelectron and electronphonon
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/srange 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 spinnoise quantum random number generator can reach 1Mbit/s
generation rates by increasing the scattering rate by removing the coating,
increasing temperature and introducing a magneticfield gradient, and
by using an improved bit generation protocol.
2023
 Richardson model with complex level structure and spinorbit coupling
for hybrid superconducting islands: Stepwise suppression of pairing and
magnetic pinning
Juan Carlos Estrada Saldaña, Luka Pavešič, Alexandros Vekris,
Kasper GroveRasmussen, 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 semisuper 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 stepbystep to spin polarized state with increasing magnetic
field, with the evenodd oscillations being washed out through spinorbit
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 interdot exchange coupling in a QDSIQD device (QD=quantum dot,
SI=superconducting island). We find qualitative difference between
simplified flatband and fullmodel calculations. This indicates that
zerobandwidth models can produce misleading results.
 Spectroscopy of spinsplit Andreev levels in a quantum dot with
superconducting leads
Arno Bargerbos, Marta PitaVidal, 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 spinorbit coupling in hybrid supersemi transmon device leads to
spin splitting of the doublet subgap states. Spinflip excitations can
be performed using allelectrical control.
 Impurity Knight shift in quantum dot Josephson junctions
Luka Pavešić, Marta PitaVidal, 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 phasedependent 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 PitaVidal, 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)
(selfarchived PDF)
Andreev spin qubit is implemented using the spinsplit doublet ground state
of an electrostaticallydefined 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 nonLandau type, the latter with a
nontrivial value of the Luttinger integral.
 Incommensurable matterwave jets in quasi1D geometry
Tadej Mežnaršič, Rok Žitko, Katja Gosar, Katja Arh, Matevž Jug, Erik
Zupanič, Peter Jeglič
arXiv:2303.07004
(PDF)
We present the discovery of matterwave jets with velocities in the golden
ratio emitted from a BoseEinstein 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 dotisland
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
 Spinorbital YuShibaRusinov states in single Kondo molecular magnet
HuiNan Xia, Emi Minamitani, Rok Žitko, ZhenYu Liu, Xin Liao, Min Cai,
ZiHeng Ling, WenHao Zhang, Svetlana Klyatskaya, Mario Ruben, YingShuang Fu
Nat. Commun. 13, 6388 (2022)
(PDF)
[open access]
Tb_{2}Pc_{3} phthalocyaninebased tripledecker dinuclear
sandwichtype terbium complexes on Pb(111) superconducting surface
have twopairs of YuShibaRusinov states delocalized over the molecule. One
level corresponds to a welldefined local moment, another to a valence
fluctuating orbital.
 SingletDoublet Transitions of a Quantum Dot Josephson Junction
Detected in a Transmon Circuit
Arno Bargerbos, Marta PitaVidal, 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 superconductorsemiconductor transmon
device is used to probe the groundstate parity of the Josephson junction quantum dot, in
agreement with predictions of a singleimpurity Anderson model with
superconducting leads. Quasiparticle dynamics can be resolved accross the
parity phase boundaries.
 YuShibaRusinov states, BCSBEC crossover, and exact solution in the
flatband 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 zerobandwidth 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 GroveRasmussen & 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 subgap state and breaks the electronhole energy
symmetry of the excitations.
 Qubit based on spinsinglet YuShibaRusinov 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
spinsinglet YuShibaRusinov 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 V12119P1
(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 matrixproductoperator representation in
terms of small matrices which can be solved using the densitymatrix
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 YuShinaRusinov 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 offdiagonal 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 highfrequency oscillation component, arising as a combined
effect of scattering and temperature.
 Iron phthalocyanine on Au(111) is a “nonLandau” 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 nonLandau Fermi liquid in that it is not
adiabatically conncected with the noninteracting limit. In this
work we extend the theory of topologically nontrivial local Fermi liquids
to multiorbital 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 sWave 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 ingap
states with the same phenomenology as the twoimpurity 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.
 Singleshot SternGerlach 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 singleshot gradiometer is not limited by thermal motion of atoms and has an estimated absolute accuracy below ±0.2 mG/cm (±20 nT/cm).
 Coupled YuShibaRusinov States Induced by a ManyBody Molecular Spin
on a Superconductor
Carmen RubioVerdú, 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'slike exchange coupling between two orbitals. The orbitals have
opposite potential scattering, yielding two subgap states with particular
spatial patterns showing particlehole asymmetry inversion across the
molecule.
2020
 Twoimpurity YuShibaRusinov states in coupled quantum dots
J. C. Estrada Saldaña, A. Vekris, R. Žitko, G. Steffensen, P. Krogstrup, J.
Paaske, K. GroveRasmussen, and J. Nygård
Phys. Rev. B 102, 195143 (2020)
(PDF)
We demonstrate the superconducting analog of the twoimpurity Kondo problem.
In addition to antiferromagnetic spinsinget and Kondo cloud spinsinglet,
its phase diagram also includes doublet YuShibaRusinov regimes where a
single local moment is screened.
 Anisotropic Kondo screening induced by spinorbit 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
spinorbit 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 oddparity 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 oddparity regimes in open quantum
dots coincides at low and high biases.
 Emission of correlated jets from a driven matterwave soliton in a
quasionedimensional 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 meanfield
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 selfenergy is practically local at high temperatures,
the vertex corrections to conductivity remain rather important. At high
temperatures, the finitesize 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
quasiballistic 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 YuShibaRusinov States and SpinFlip
Excitations
Shawulienu Kezilebieke, Rok Žitko, Marc Dvorak, Teemu Ojanen, Peter Liljeroth
Nano Lett. 19, 4614 (2019)
PDF
The crossover from the weak impuritysubstrate hybridisation with inelastic
spinflip excitations outside the gap to strong hybridisation with
YuShibaRusinov peaks inside the gap is smooth, with a parameter range
where both spectral features are visible at the same time.
 Kondo screening in a chargeinsulating spinon metal
M. Gomilšek, R. Žitko, M. Klanjšek, M. Pregelj, C. Baines, Y. Li, Q. M.
Zhang, A. Zorko
Nat. Phys. 15, 754 (2019)
(selfarchived PDF)
We report on the first experimental observation of the Kondo screening
by chargeless quasiparticles in a chargeinsulating quantum spin liquid,
where spinon excitations forming a Fermi surface take the role of conduction
electrons.
 Cesium bright matterwave 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
triangularlattice Mott insulator 1TTaS_{2}
J. Skolimowski, Ya. Gerasimenko, R. Žitko
Phys. Rev. Lett. 122, 036802 (2019)
(PDF)
(suppl)
Electronic state of a freestanding domain wall in 1TTaS_{2} can be
explained as a wouldbe 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 insulatortometal transitions in
electronphonon 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 electronphonon systems
described by the HubbardHolstein model using the dynamical meanfield 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. GroveRasmussen, 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 twoimpurity Anderson model.
 AharonovBohm and AharonovCasher 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 AharaonovCasher
phase is different for nonspinflip and spinflip processes.
 Reversal of particlehole scatteringrate 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 singleimpurity Anderson model we identify a line in the (Un) parameter plane where the asymmetry of the scatteringrate becomes zero. This line, defined through impurity dynamics, traces out the crossover between the weakcoupling and strongcoupling regimes.
 Yu–Shiba–Rusinov screening of spins in double quantum dots
K. GroveRasmussen, G. Steffensen, A. Jellinggaard, M.H. Madsen, R. Žitko, J. Paaske, J. Nygård
Nat. Commun. 9, 2376 (2018)
(PDF)
(Supplementary) [open access]
Highlytunable double quantum dot (DQD) is coupled to a superconductor and characterized
via tunneling spectroscopy using a further normalstate electrode. It is shown
how the honeycomb pattern characteristic of DQDs with normalstate leads is replaced
by lines of zeroenergy YuShinaRusinov states that delineate the different ground
states of the system.
 AharonovBohm and AharonovCasher 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 nonlocal (split)
Cooperpair Josephson current, both AharonovBohm (AB) and
AharonovCasher (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.
 Spin1 twoimpurity Kondo problem on a lattice
A.Allerdt, R. Žitko, A. E. Feiguin
Phys. Rev. B 97, 045103 (2018)
(PDF)
Twoimpurity spin1 model is studied on a square lattice for different
interimpurity separations using the DMRG. When the impurities are
underscreened, the dangling spin1/2 degrees of freedom generate the
interimpurity 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 subgap states, presence of symmetry breaking, and
the effect of the external magnetic field. The highspin adsorbates
with Ising (easyaxis) magnetic anisotropy show features that are
different from both isotropic quantum impurities and classical
impurities.
 Superconducting quantum dot and the subgap 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
 FieldInduced 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 arbitrarilysmall 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 infiniteU Hubbard model in the lowtemperature regimes. We extract the longitudinal and transverse relaxation rates. A single rate controls both longitudinal and transverse transport in the lowesttemperature Fermiliquid regime, however in the strangemetal regime two different relaxation rates emerge due to the dynamical particlehole asymmetry of the Dyson selfenergy.
 A hightemperature 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) (selfarchived PDF)
The local moments in TaS_{2} do not order down to the mK range, as evidenced
by NQR and muSR measurements. Between 55K and 200K, the T^{2} relaxation dynamics
are observed, while below 55K a new gapless state with reduced density of spin excitations is
observed.
 Spinorbit coupling in threeorbital Kanamori impurity model and its relevance for transitionmetal oxides
Alen Horvat, Rok Žitko, and Jernej Mravlje
Phys. Rev. B 96, 085122 (2017)
(PDF)
The spinorbit coupling (SOC) in the threeorbital 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 crossover between these
two regimes. The temperature dependence of resistivity in this
temperature range simply follows the temperature dependence of
zerofrequency imaginary part of the selfenergy, which starts
to deviate from its lowtemperature asymptotic behavior on
the scale of the Fermiliquid 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 superconductorsemiconductor 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 superconductorsemiconductor 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 normalstate (taking into account temperature and magnetic field effects), and then used as the input in a calculation of the superconducting subgap 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.
 NonFermiliquid 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 subgap spectrum of overscreened Kondo models shows universal features
characteristic of the fixed point of the normalstate problem. These effects could be detected in hybrid devices with mesoscopic
superconducting islands.
 Magnetic impurities in spinsplit superconductors
W.V. van Gerven Oei, D. Tanasković, R. Žitko
Phys. Rev. B 95, 085115 (2017)
(PDF)
We study the subgap (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 nonmonotonic dependence of the singletdoublet
quantum phase transition line in the phase diagram in the (gap,field) plane.
 Fine structure of the spectra of the Kondo lattice model: Twosite cellular dynamical meanfield 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 hightemperature 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 hightemperatures series expansions, then these functions are reconstructed. Excellent
agreement is found with DMFT results obtained using stateoftheart numerical methods
in a surprisingly wide temperature range. HighT resistivity is linear without saturation
as a result of saturating scattering rate.
 Lowenergy physics of threeorbital impurity model with Kanamori interaction
Alen Horvat, Rok Žitko, Jernej Mravlje
Phys. Rev. B 94, 165140 (2016)
(PDF)
The threeorbital 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 particlehole 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 bandedge 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 subgap 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 chargefluctuation regime with singlet ground state,
we detect an additional
subgap resonance just below the gapedge. It appears to be related to scattering
on the thermally excited doublet subgap state.
 Metaltoinsulator 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 dopingdependent crossover from an insulating to a metal state. In the metal state, Korringa behaviour is found for the nuclear spinlattice 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
 Z_{2} gauge theory description of the Mott transition in infinite dimensions
Rok Žitko, Michele Fabrizio
Phys. Rev. B 91, 245130 (2015)
(PDF)
Infinitedimensional Hubbard model maps exactly onto a model of free fermions coupled in a Z_{2}
gaugeinvariant 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 Z_{2} gauge symmetry, which is not forbidden in finite dimensions and
is not an artifact of the meanfield 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)
Subgap spectra are analyzed in the DQD system coupled to
superconducting leads in an extended range of parameters, including
the valencefluctuation and empty orbital regimes. We determine conditions
for observing the doublet state Shiba state splitting, and for observing
both singlet and triplet Shiba states. Interdot capacitive coupling is
also considered: it may lead to crossover at halffilling and to phase
transitions close to quarterfilling. 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 spinlattice 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 particlehole 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 positiveU model at half filling in magnetic field and
negativeU model away from halffilling, arising from the partial ph 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
orderparameterdependent shift of the resonances in the selfenergy
and nonequal Weiss fields in the c and f bands.

Shiba states and zerobias anomalies in the hybrid normalsuperconductor Anderson model
Rok Žitko, Jong Soo Lim, Rosa Lopez, Ramon Aguado
Phys. Rev. B 91, 045441 (2015)
(PDF)
We study the subgap spectra of a quantum impurity coupled to one superconducting
and one normalstate lead (tunneling probe). We show under which conditions a zerobias
anomaly appears either due to the presence of the normalstate 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 (occupancydependence of the tunneling amplitudes)
can lead to anomalies in the mixedvalence regime which, in turn,
are associated with very high Seebeck coefficient.
2013

Extremely correlated Fermi liquid theory meets dynamical meanfield
theory: Analytical insights into the dopingdriven 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 selfenergy, in particular the asymmetry between particle and hole
excitations, even when symmetric (Fermiliquidlike) auxiliary
selfenergies 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 twochannel spin1/2 case, which exhibits
nonFermiliquid ground state. We show that the critical exponent
can be deduced from the magneticfield 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 spin1/2 and spin1 Kondo lattice models
using the DMFT and static meanfield 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 singlelevel Anderson model.

Padé approximant approach for obtaining finitetemperature 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 finitetemperature 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 swave superconducting solution for the Kondo
lattice model within the singlesite DMFT. We argue the pairing is
due to strong local spin fluctuations.
 NonDrude 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
selfenergy with FL scaling form. The optical conductivity shows a
nonDrude "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 holedoped Mott insulators, resilient quasiparticles remain
well defined above the Fermi temperature and dominate transport
in the intermediate regime up to the MottIoffeRegel 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 threespin 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 subpeaks to the main subgap 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 electronphonon 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 spinpolarized contacts is
established. The twoimpurity Kondo crossover is generalized to the
(p,t) plane, where p is polarization and t the interdot tunneling.
The spincurrent 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 CTQMC and NRG produce consistent results for the
lowtemperature 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 oddsymmetry "dark state" on the dots.
2011
 Kondo effect in the presence of Rashba spinorbit 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 spinorbit 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 (nonexponentially).
 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 lineshape
and the experimental results for the Ti/CuN/Cu(100) zerobias 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 subgap 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
nonmagnetic 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 renormalizationgroup calculations of spectral
functions
R. Žitko
Phys. Rev. B 84, 085142 (2011)
(PDF)
Errors in the spectralfunction calculations using NRG can be
quantitatively estimated. Application to magneticfield splitting
of the Kondo resonance.
 SNEG  Mathematica package for symbolic calculations with
secondquantizationoperator 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 secondquantization operators.
 Detection of Majorana edge states in topological
superconductors through nonFermiliquid 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
spinpolarized (ferromagnetic) measurement leads.
 Lowenergy properties of the Kondo lattice model
O. Bodensiek, R. Žitko, R. Peters, T. Pruschke
J. Phys.: Condens. Matter 23, 094212 (2011)
(PDF)
The heavyfermion physics in the Kondo lattice model is studied within
the DMFT using the NRG as the solver. The crossover 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 subgap states localized
on magnetic impurities coupled to a superconducting host. This
splitting may have already been experimentally resolved in
highresolution STM studies of Mn and Cr on superconducting Pb.
We also discuss the effect of the external magnetic field on the subgap
states, and the subgap spectrum of the twoimpurity 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, MahnSoo Choi
Phys. Rev. Lett. 105, 116803 (2010)
(PDF)
Phase diagram of 0 and πjunction regimes of Josephson supercurrent
is established for a double quantum dot in contact with two
superconducting leads. An island of πphase 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 topmost 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 standingwave 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 pseudogap Anderson impurity model. This implies
that the magnetic moment is fully Kondo screened, thus at low enough
temperature a single magnetic dopant is effectively nonmagnetic
and does not break the timereversalinvariance (TRI) protection
of the surface states.
 Manyparticle effects in adsorbed magnetic atoms with easyaxis
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 89T 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, 42214227 (2010)
Two interstitial H atoms in close proximity in ZrCr2 host repel each
other.
 FanoKondo effect in sidecoupled double quantum dots at finite
temperatures and the importance of the twostage Kondo screening
Rok Žitko
Phys. Rev. B 81, 115316 (2010)
(PDF)
The antiresonance in the transport experiments in sidecoupled double
quantum dot may be explained in terms of the twostage Kondo screening
with the experimental temperature between the two different Kondo
scales.
 Kondo screening in highspin sidecoupled twoimpurity
clusters
Rok Žitko
J. Phys.: Condens. Matter 22, 026002 (2010)
(PDF)
The twostage Kondo screening phenomenology is
generalized to a pair of sidecoupled Kondo impurities with large
spin. Both antiferromagnetic and ferromagnetic (as in
Hund's rule) exchange coupling between the spins is analysed.
 The Twoimpurity Anderson Model Revisited: Competition between Kondo
Effect and Reservoirmediated Superexchange in Double Quantum Dots
Minchul Lee, MahnSoo Choi, Rosa Lopez, Ramon Aguado, Jan Martinek, Rok Žitko
Phys. Rev. B 81, 121311(R) (2010)
(PDF)
Editor's Suggestion
A conductionband mediated superexchange mechanism leads to an
effective antiferromagnetic interaction between the two impurity spins
in the infiniteU limit, thereby significantly modifying the transport
properties of the system. This work points out the deficiencies of
static meanfield description of interimpurity 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 conductionband electrons.
 Magnetization Curves for Anisotropic Magnetic Impurities Adsorbed on a Normal Metal Substrate
R. Žitko
Physical properties of nanosystes, pp 247257
(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)
Electronelectron 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 magneticfieldinduced
metalinsulatortransition falls in a different universality class as
for the case with featureless density of states, as there is no
fieldinduced localization of quasiparticles. 3D cubic and a DOS with
powerlaw 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.
 Lowtemperature Scanning Tunneling Microscopy and Spectroscopy of
Noblemetal 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 lowtemperature 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
dynamicalmeanfieldtheory calculations
R. Žitko
Phys. Rev. B 80, 125125 (2009)
(PDF)
Using advanced mixing techniques (modified Broyden's method) the
convergence to a selfconsistent solution in DMFT calculations can be
significantly accelerated (or even made possible in some extreme
parameter regimes). In addition, unstable and metastable solutions of
DMFT equations can be obtained.
 NRG calculations of the groundstate 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 singleimpurity Anderson model is computed
to high accuracy. The Kondo energy is only a small
contribution to the total correlation energy (total binding energy
minus HartreeFock binding energy), which is order Γ^{2},
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 discretizationmesh 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 spin1/2 Kondo model can be described using effective
gfactors determined by projection of the spinS operators on the
spin1/2 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 highresolution results for the Anderson
and AndersonHolstein 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 multiimpurity 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 easyaxis and
hardaxis cases are considered. For hardaxis halfinteger 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 twochannel threeimpurity
triangular clusters
R. Žitko, J. Bonča
Phys. Rev. B 77, 245112 (2008).
J. Bonča
(PDF)
Editor's Suggestion
Phase diagram of triangular threeimpurity clusters is determined.
Twostage Kondo screening is a relevant concept even for crossovers
between nonFermi liquid fixed point. We find such a direct transition
between twoimpuriy and twochannel 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 lowtemperature properties of molecular
conductors
J. Mravlje, A. Ramsak, R. Žitko
Physica B 403, 1484 (2008).
(PDF)
In the Anderson model with additional phononassisted coupling to the
odd conduction channel, the zerotemperature conductance is found to
be qualitatively the same as in the model with no coupling to the
vibrational mode. An unstable twochannel 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 strongcorrelation effects and decoupling
of certain combinations of states from the continuum (the Dicke effect).
 Fermiliquid versus nonFermiliquid 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
particlehole symmetric case is unitary at zero temperature. For weak
interdot coupling, the Kondo screening occurs in two stages. For a
range of parameters, the twochannel Kondo nonFermi liquid (NFL)
fixed point is approached at finite temperatures; we discuss the
transport of this NFL behaviour.
 Correlation effects in sidecoupled quantum dots
R. Žitko, J. Bonča
J. Phys.: Condens. Matter 19 255205 (2007).
Review of twostage Kondo effect in sidecoupled double quantum dot,
demonstration of threestage Kondo effect in sidecoupled triple
quantum dot and a generalization to multistage Kondo effect in more
general systems. Study of simultaneous spin and charge Kondo effect in
sidecoupled configuration.
2006
 Spincharge separation and simultaneous spin and charge Kondo effect
R. Žitko, J. Bonča
Phys. Rev. B 74, 224411 (2006).
(PDF)
Due to the spincharge 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 interdot
coupling, magnetic field and temperature for different topologies of
the coupling to the leads.
 Multiimpurity 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 interimpurity 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
interdot 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
twostage Kondo screening.
 Enhanced conductance through sidecoupled double
quantum dots
R. Žitko, J. Bonča
Phys. Rev. B
73, 035332 (2006).
(PDF)
Sidecoupled double quantum dot has two Kondo plateaus of high conductance
when the interdot coupling is strong. For weak interdot 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
electronelectron 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 electronelectron interaction where additional electrons
are bound. Application to the AharonovBohm ring where dephasing is studied.
PhD dissertation
Contact: rok.zitko@ijs.si,
last updated: Dec 2023