Return to Rok Žitko's homepage
- 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. (2023)
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
- Topological quantum phase transition of nickelocene on Cu(100)
G. G. Blesio, R. Žitko, L. O. Manuel, A. A. Aligia
SciPost Phys. 14, 042
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.
- Quantum dot Josephson junctions in the strong-coupling limit
Luka Pavešič, Ramón Aguado, Rok Žitko
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
and the origin of the doublet chimney at φ=π.
- Entanglement between quasiparticles in superconducting islands
mediated by a single spin
Juan Carlos Estrada Saldaña, Alexandros Vekris, Luka Pavešič, Rok Žitko,
Kasper Grove-Rasmussen, Jesper Nygård
We discuss in what sense 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
- Incommensurable matter-wave jets in quasi-1D geometry
Tadej Mežnaršič, Rok Žitko, Katja Gosar, Katja Arh, Matevž Jug, Erik
Zupanič, Peter Jeglič
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.
- Impurity Knight shift in quantum dot Josephson junctions
Luka Pavešić, Marta Pita-Vidal, Arno Bargerbos, Rok Žitko
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.
- 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
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.
- Spectroscopy of spin-split Andreev levels in a quantum dot with
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
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.
- 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)
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
- 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)
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
Rok Žitko and Luka Pavešić
Phys. Rev. B 106, 024513 (2022)
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
- 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)
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)
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
(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
(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.
- Subgap states in superconducting islands
Luka Pavešić, Daniel Bauernfeind, and Rok Žitko
Phys. Rev. B 104, L241409 (2021)
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)
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
Jakša Vučičević, Rok Žitko
Phys. Rev. B 104, 205101 (2021)
Featured in Physics
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)
Featured in Editor's Highlights
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,
Nano Lett. 21, 6748 (2021)
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,
Phys. Rev. A 103, 022611 (2021)
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)
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
- 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)
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
E. Vernek, G. B. Martins, and R. Žitko
Phys. Rev. B 102, 155114 (2020)
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)
Vertex corrections to optical conductvity, which are significant on the square
lattice even at high temperatures, are found to contribute less on the
- Erasing odd-parity states in semiconductor quantum dots coupled to
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)
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
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
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)
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
- 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
Shawulienu Kezilebieke, Rok Žitko, Marc Dvorak, Teemu Ojanen, Peter Liljeroth
Nano Lett. 19, 4614 (2019)
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)
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
- 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)
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)
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
Sharareh Sayyad, Rok Žitko, Hugo U. R. Strand, Philipp Werner, and Denis
Phys. Rev. B 99, 045118 (2019)
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
NRG UFU/2019 Advanced
A brief overview of the numerical renormalization group and its "NRG
Ljubljana" implementation. Prepared for the NRG UFU/2019 Advanced
Studies School, Uberlandia, Brazil.
- 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)
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)
Cooper pair splitting can be made perfectly efficient by the addition of quantum dots
in a nanostructure where two parallel nanowires bridge two superconductors.
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)
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)
(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)
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
- Spin-1 two-impurity Kondo problem on a lattice
A.Allerdt, R. Žitko, A. E. Feiguin
Phys. Rev. B 97, 045103 (2018)
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
- Quantum impurity models for magnetic adsorbates on superconductor surfaces
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
- Superconducting quantum dot and the sub-gap states
Proc. SPIE 10732, Spintronics XI, 107321N (20 September 2018)
Brief review of the physics of quantum impurities coupled to superconducting
- Na poti do kvantne prevlade
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)
- 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)
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)
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
- 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)
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)
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)
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)
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)
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
- Magnetic impurities in spin-split superconductors
W.-V. van Gerven Oei, D. Tanasković, R. Žitko
Phys. Rev. B 95, 085115 (2017)
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)
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
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.
- 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)
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)
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)
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
Phys. Rev. B 93, 195125 (2016)
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)
- Z2 gauge theory description of the Mott transition in infinite dimensions
Rok Žitko, Michele Fabrizio
Phys. Rev. B 91, 245130 (2015)
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
Phys. Rev. B 91, 165116 (2015)
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)
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)
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)
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.
- Entanglement switching via the Kondo effect in triple quantum
S. B. Tooski, B. R. Bulka, R. Zitko, A. Ramsak
Eur. Phys. J. B 87, 145 (2014)
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
S. B. Tooski, A. Ramsak, B. R. Bulka, R. Zitko
New J. Phys. 16, 055001 (2014)
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.
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.
Phys. Rev. B 88, 235132 (2013)
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)
NRG scripts used in this paper
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
Denis Golež, Rok Žitko
Phys. Rev. B 88, 054431 (2013)
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)
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)
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,
Phys. Rev. Lett 110, 146406 (2013)
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)
Optical response of a Fermi liquid is computed, assuming a local
self-energy with FL scaling form. The optical conductivity shows a
- How Bad Metals Turn Good: Spectroscopic Signatures of Resilient
X. Deng, J. Mravlje, R. Žitko, M. Ferrero, G. Kotliar, A. Georges
Phys. Rev. Lett 110, 086401 (2013)
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)
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
- Vibrational Andreev bound states in magnetic molecules
D. Golez, J. Bonca, R. Žitko
Phys. Rev. B 86, 085142 (2012)
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
Rok Žitko, Jong Soo Lim, Rosa Lopez, Jan Martinek, Pascal Simon
Phys. Rev. Lett 108, 166605 (2012)
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)
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)
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.
- Kondo effect in the presence of Rashba spin-orbit interaction
R. Žitko, J. Bonca
Phys. Rev. B 84, 193411 (2011)
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
- Kondo resonance lineshape of magnetic adatoms on decoupling
Phys. Rev. B 84, 195116 (2011)
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)
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
Phys. Rev. B 84, 085142 (2011)
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
Comp. Phys. Comm. 182, 2259 (2011)
[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
Phys. Rev. B 83, 195137 (2011)
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)
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
Rok Žitko, Oliver Bodensiek, Thomas Pruschke
Phys. Rev. B 83, 054512 (2011)
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.
- 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)
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
Phys. Rev. Lett. 104, 196102 (2010)
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
- Charge density wave ordering in NbSe3: possible models and the
A Prodan, H J. P. van Midden, R Žitko, E Zupanic, J C Bennett, H Boehm
Solid state Comm. 150, 2134 (2010)
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
Phys. Rev. B 81, 241414(R) (2010)
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)
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
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
- Fano-Kondo effect in side-coupled double quantum dots at finite
temperatures and the importance of the two-stage Kondo screening
Phys. Rev. B 81, 115316 (2010)
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
- Kondo screening in high-spin side-coupled two-impurity
J. Phys.: Condens. Matter 22, 026002 (2010)
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)
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)
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
Physical properties of nanosystes, pp 247-257
NRG study of magnetization curves Sz(B,T) for magnetic impurities. Deviations
from ideal paramagnetic behavior is discussed.
- 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)
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
E. Zupanic, R. Žitko, H. J. P. van Midden, I. Musevic, A. Prodan
Croat. Chem. Acta 82, 485 (2009)
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
Phys. Rev. B 80, 125125 (2009)
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
Phys. Rev. B 79, 233105 (2009)
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 TK. 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
Comm. 180, 1271 (2009)
NRG discretization scheme with adaptable discretization-mesh points reduces
artefacts which result from very low density of states over extended energy
- Splitting of the Kondo resonance in anisotropic magnetic impurities on
R. Žitko, R. Peters, T. Pruschke
New J. Phys. 11, 053003 (2009)
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/TK 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
R. Žitko, T. Pruschke
Phys. Rev. B 79, 085106 (2009)
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)
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
TK is similar to the critical temperature TC.
- Quantum Impurity Physics in Coupled Quantum Dots
R. Žitko, J. Bonča
Book chapter, Encyclopedia
of Complexity and System Science, Springer,
Introduction to the quantum impurity physics in coupled quantum dots
described by the multi-impurity Anderson models.
- Properties of anisotropic magnetic impurities on surfaces
R. Žitko, R. Peters, T. Pruschke
Phys. Rev. B 78, 224404 (2008).
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, TK would be unmeasurably low.
- Numerical renormalization group study of two-channel three-impurity
R. Žitko, J. Bonča
Phys. Rev. B 77, 245112 (2008).
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
J. Mravlje, A. Ramsak, R. Žitko
Physica B 403, 1484 (2008).
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.
- Quantum phase transitions in systems of parallel quantum dots
R. Žitko, J. Bonča
Phys. Rev. B 76, 241305(R) (2007).
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
R. Žitko, J. Bonča
Phys. Rev. Lett. 98, 047203 (2007).
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
- Spin-charge separation and simultaneous spin and charge Kondo effect
R. Žitko, J. Bonča
Phys. Rev. B 74, 224411 (2006).
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
A. Ramšak, J. Mravlje, R. Žitko, J. Bonča
Phys. Rev. B 74, 241305(R) (2006).
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).
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
- Kondo effect in triple quantum dot
R. Žitko, J. Bonča, A. Ramšak, T. Rejec
Phys. Rev. B
73, 153307 (2006).
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
R. Žitko, J. Bonča
Phys. Rev. B
73, 035332 (2006).
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
R. Žitko, I. Muševič
Obz. mat. fiz., 24, 65 (2005).
- Microscopic mechanisms of dephasing due to
R. Žitko, J. Bonča
Phys. Rev. B
68, 085313 (2003).
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.
last updated: Nov 2022