Quasiparticle-resolved variational theory of Andreev spin qubits#
Teodor Iličin, Rok Žitko
arXiv:2610.08557(PDF)
· qdjj_solver
(source code)
· QD Josephson Junction Explorer
(web app)
We develop a general variational framework for Andreev spin qubits
based on quantum-dot Josephson junctions described by an extended
single-impurity Anderson model.
The analytical treatment uses an ansatz with up to two quasiparticles in the
bath. The numerical code qdjj_solver extends this to six quasiparticles.
The method is efficient and accurate, with extensive benchmarking against
NRG and DMRG calculations.
Hubbard vs. Emery model: spectra, transport and relevance for cuprates#
Jakša Vučičević, Rok Žitko
arXiv:2604.08085(PDF)
We investigate realistic minimal lattice models and parameter values
(in particular, the interaction strength U) for cuprates, considering both
Hubbard and Emery models. An improved methodology substantially increases
quantitative accuracy, allowing us to test the theory against an unusually
broad range of compounds and physical observables. The consistency across
materials and measurements provides a more stringent test of the underlying
physical picture than was previously available. We also contribute to the
debate on Planckian dissipation by identifying inconsistencies in earlier
analyses.
Strongly correlated Josephson junction: proximity effect in the
single-layer Hubbard model#
Don Rolih, Rok Žitko
Phys. Rev. B 114, 045124 (2026)(PDF)
We study the superconducting proximity effect in Mott insulators. Weak
coupling to BCS baths splits the Mott midgap peak in the self-energy, while
some spectral weight is transferred from the Hubbard peaks to the energy
scale of the BCS gap. This phase (the M phase) is largely insensitive to the
superconducting phase bias. At stronger coupling, the system undergoes a
first-order quantum phase transition to the S phase, which behaves as a
superconducting metal with coherence peaks and 0-junction Josephson behavior.
At φ = π, the S phase becomes a strongly correlated metal.
Effects of electron-electron interaction and spin-orbit coupling on
Andreev pair qubits in quantum dot Josephson junctions #
Teodor Iličin, Rok Žitko
SciPost Phys. 20, 147
(2026)(PDF)
We investigate the mixing between Andreev bound states and Yu-Shiba-Rusinov
states in an Andreev pair-qubit device. The calculations take into account
electron-electron interactions, spin-orbit coupling, higher-lying
orbitals, and superconducting phase bias. We perform zero-bandwidth, NRG, and
variational calculations. We investigate the behaviour of matrix elements
for transitions in charge, spin, and inductive channels, and discuss the
implications for devices. For U ≈ 2Δ, the matrix elements are large (but
potentially tunable) in all channels.
Andreev bound state spectroscopy of a quantum-dot-based Aharonov-Bohm
interferometer with superconducting terminals#
Peter Zalom, Don Rolih, Rok Žitko
Phys. Rev. B 113, 075130 (2026)(PDF)
We show how the problem of a quantum dot coupled to multiple superconducting
leads reduces to an equivalent problem composed of the original quantum dot,
additional non-interacting side-coupled proximitized levels (representing
Andreev bound states) and a single continuum of Bogoliubov states. Using
this representation, we study the case of an Aharonov-Bohm interferometer
with a quantum dot embedded into one arm. We uncover a general condition for
the existence of persistent doublet states (the "doublet chimney").
Systematic Schrieffer-Wolff-transformation approach to Josephson
junctions: quasiparticle effects and Josephson harmonics#
Ádám Bácsi, Teodor Iličin, Rok Žitko
Phys. Rev. B 113, 064501 (2026)(PDF)
The Schrieffer-Wolff transformation provides a systematic way to derive effective
Hamiltonians for Cooper-pair tunneling in a general setting, including in
the presence of (thermal or non-thermal) quasiparticles. The expansion
can be carried to higher orders to obtain higher Josephson harmonics.
Nobelova nagrada za fiziko 2025#
Rok Žitko
Obzornik za matematiko in
fiziko 72, 141–151 (2026)(PDF)
· Spletna aplikacija (Slovenian only) O Nobelovi nagradi Johnu Clarku,
Michelu Devoretu in Johnu Martinisu za odkritje makroskopskega kvantnega
tuneliranja in kvantizacije energijskih nivojev v elektronskem vezju.
Variational solution of the superconducting Anderson impurity model
and the band-edge singularity phenomena#
Teodor Iličin, Rok Žitko
SciPost Phys. 19, 006 (2025)(PDF)
Using a variational ansatz that covers both the Andreev bound-state and
Yu-Shiba-Rusinov regimes, we uncover an intermediate regime linking
the two. In this "critical regime" the physics is governed by band-edge
singularity effects and the state exhibits strong fluctuations between all
charge states.
Incommensurable matter-wave jets in quasi-1D geometry#
Tadej Mežnaršič, Rok Žitko, Katja Gosar, Katja Arh, Matevž Jug, Erik
Zupanič, Peter Jeglič
Commun. Phys. 8, 59 (2025)(PDF)
We present the discovery of matter-wave jets with velocities in the golden
ratio emitted from a Bose-Einstein condensate. Using numerical simulations
we also predict the formation of a wide jet velocity distribution resembling
the generation of supercontinua in nonlinear optical media. The
incommensurable jets are precursors of the supercontinuum formation.
Kvantna elektronska vezja pri absolutni ničli#
Rok Žitko
Proteus 88, št. 1–2,
september–oktober 2025 (Print only)
(Slovenian only) O Nobelovi nagradi za fiziko 2025 za
makroskopske kvantne pojave v elektronskih vezjih.
Stoletna zgodba#
Rok Žitko
Gea, december 2025 (Print only)
(Slovenian only) O Nobelovi nagradi za fiziko 2025 za prikaz makroskopskih
kvantnih pojavov v superprevodnih elektronskih vezjih.
Inelastic tunneling through normal and superconducting junctions in
the presence of a photonic bath within the Lindbladian formalism#
Ádám Bácsi, Rok Žitko
Phys. Rev. A 110, 012224 (2024)(PDF)
We reveal relations between the P(E) theory for inelastic tunneling and the
Lindbladian quantum master equation approach. As a demonstration, the
Lindbladian approach is then applied to calculate the heat dissipation in
the junction. For superconducting contacts with particle-hole asymmetry in
the normal-state DOS, we find that coherence-factor corrections enter the
expression for I(V) in terms of the P(E) function.
Higher order spin interactions mediated by the substrate#
Szczepan Głodzik and Rok Žitko
Phys. Rev. B 110, 045414 (2024)(PDF)
We discuss how fourth-order exchange coupling (ring exchange)
affects spin alignment in four impurities coupled to a bath of
itinerant electrons in a square lattice.
Generalized transmon Hamiltonian for Andreev spin qubits#
Luka Pavešić, Rok Žitko
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 applied directly to model Andreev spin qubits
embedded in transmon circuits in all parameter regimes, for describing
time-dependent processes, and for the calculation of transition matrix
elements for microwave-driven transmon, spin-flip and mixed transitions that
involve coupling to charge or current degrees of freedom.
Correlation between two distant
quasiparticles in separate superconducting
islands mediated by a single spin#
Juan Carlos Estrada Saldaña, Alexandros Vekris, Luka Pavešić, Rok Žitko,
Kasper Grove-Rasmussen, Jesper Nygård
Nat. Commun. 15, 3465
(2024)(PDF)
We discuss how a single quantum dot can be YSR screened by two
quasiparticles, one in each of two superconductors. This enables
ferromagnetic correlations between fermionic excitations over a micrometer
distance. Such alternating chains of quantum dots and superconducting
islands represent a new platform for controllable large-scale spin coupling.
Strong-coupling theory of quantum-dot Josephson junctions: Role of a
residual quasiparticle#
Luka Pavešić, Ramón Aguado, Rok Žitko
Phys. Rev. B 109, 125131 (2024)(PDF)
We present a theory of a quantum-dot Josephson junction (an Anderson impurity
model coupled to two superconducting baths with a phase difference) in the
strong-coupling limit. It explains the changing nature of the doublet states
as a function of coupling strength and phase difference, as well as the origin
of the doublet chimney at φ=π.
True random number generators with flicker noise: stochastic model,
min-entropy calculation and online test#
Rok Žitko
Research
Square preprint(PDF)
It is shown that it is possible to build a random number generator for
cryptographic use in the presence of strong flicker (1/f, pink) noise.
An online test based on measuring Allan variance is proposed.
Auxiliary master equation approach to the Anderson-Holstein impurity
problem out of equilibrium#
Daniel Werner, Rok Žitko, and Enrico Arrigoni
Phys. Rev. B 109, 075156 (2024)(PDF)
An impurity with local electron-electron and electron-phonon interactions,
coupled to two conducting leads under a voltage bias, is studied using
the auxiliary master equation approach. The method is applicable to the
regime of sizeable interactions at high bias voltage.
Mbit/s-range alkali vapour spin noise
quantum random number generators#
Matija Koterle, Samo Beguš, Jure Pirman, Tadej Mežnaršič, Katja Gosar,
Erik Zupanič, Rok Žitko & Peter Jeglič
EPJ Quantum
Technol. 11, 11 (2024)(PDF) [open access]
An alkali-vapour spin-noise quantum random number generator can reach
generation rates of 1 Mbit/s. This is achieved by increasing the scattering
rate through coating removal, higher temperatures, and a magnetic-field
gradient, together with an improved bit-generation protocol.
Richardson model with complex level structure and spin-orbit coupling
for hybrid superconducting islands: Stepwise suppression of pairing and
magnetic pinning#
Juan Carlos Estrada Saldaña, Luka Pavešić, Alexandros Vekris,
Kasper Grove-Rasmussen, Jesper Nygård, Rok Žitko
Phys. Rev. B
108, 224507 (2023)(PDF)
We discuss Coulomb spectroscopy of quantum dots coupled to superconducting
islands in hybrid semiconductor-superconductor nanowires. The results show
equal spacing as a function of the gate voltage applied to the island. We propose a
model involving Andreev levels in the semiconducting core of the hybrid wire
that become spin-polarized step by step with increasing magnetic
field, with the even-odd oscillations being washed out through spin-orbit
coupling.
Exchange interaction between two quantum dots coupled through a
superconducting island#
Ádám Bácsi, Luka Pavešić, Rok Žitko
Phys. Rev. B 108, 115160 (2023)(PDF)
We study the inter-dot exchange coupling in a QD-SI-QD device (QD=quantum dot,
SI=superconducting island). We find a qualitative difference between
simplified flat-band and full-model calculations. This indicates that
zero-bandwidth models can produce misleading results.
Spectroscopy of spin-split Andreev levels in a quantum dot with
superconducting leads#
Arno Bargerbos, Marta Pita-Vidal, Rok Žitko, Lukas J. Splitthoff, Lukas
Grünhaupt, Jaap J. Wesdorp, Yu Liu, Leo P. Kouwenhoven, Ramón Aguado,
Christian Kraglund Andersen, Angela Kou, Bernard van Heck
Phys. Rev. Lett.
131, 097001 (2023)(PDF)(Supplementary information)
Spin-orbit coupling in a hybrid superconductor-semiconductor transmon device leads to
spin splitting of the doublet subgap states. Spin-flip excitations can
be performed using all-electrical control.
Impurity Knight shift in quantum dot Josephson junctions#
Luka Pavešić, Marta Pita-Vidal, Arno Bargerbos, Rok Žitko
SciPost Phys. 15, 070
(2023)(PDF)(Data & notebooks)
We discuss the coupling between the spin degree of freedom of a quasiparticle
trapped in a QD Josephson junction and the supercurrent, which manifests as
a phase-dependent renormalization of the Zeeman splitting. This coupling
could find wide application in superconducting spintronics.
Direct manipulation of a superconducting spin qubit strongly coupled
to a transmon qubit#
Marta Pita-Vidal, Arno Bargerbos, Rok Žitko, Lukas J. Splitthoff, Lukas
Grünhaupt, Jaap J. Wesdorp, Yu Liu, Leo P. Kouwenhoven, Ramón Aguado,
Bernard van Heck, Angela Kou, Christian Kraglund Andersen
Nat. Phys. 19, 1110 (2023)(PDF)(self-archived PDF)
An Andreev spin qubit is implemented using the spin-split doublet ground state
of an electrostatically defined quantum-dot Josephson junction with a large
charging energy. This spin qubit is coherently coupled to 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 STM spectra of nickelocene adsorbed on the Cu(100) surface reveal
the existence of a topological quantum phase transition between two distinct
Fermi-liquid ground states of Landau and non-Landau type, the latter with a
nontrivial value of the Luttinger integral.
Quantum capacitance of a superconducting subgap state in an
electrostatically floating dot-island#
Filip K. Malinowski, R. K. Rupesh, Luka Pavešić, Zoltán Guba, Damaz de Jong,
Lin Han, Christian G. Prosko, Michael Chan, Yu Liu, Peter Krogstrup, András
Pályi, Rok Žitko, Jonne V. Koski
arXiv:2210.01519(PDF)
RF reflectometry is used to characterize the subgap states in a device
consisting of a quantum dot coupled to a superconducting island. The device
as a whole is electrostatically floating, so that the total electron number
remains constant on a long time scale, providing access to ground states that
cannot be reached in electrically grounded devices. Capacitance loading
of the resonator can be strongly reduced by thermal excitations.
Spin-orbital Yu-Shiba-Rusinov states in single Kondo molecular magnet#
Hui-Nan Xia, Emi Minamitani, Rok Žitko, Zhen-Yu Liu, Xin Liao, Min Cai,
Zi-Heng Ling, Wen-Hao Zhang, Svetlana Klyatskaya, Mario Ruben, Ying-Shuang Fu
Nat. Commun. 13, 6388 (2022)(PDF)
[open access]
Tb2Pc3 phthalocyanine-based triple-decker dinuclear
sandwich-type terbium complexes on the superconducting Pb(111) surface
have two pairs of Yu-Shiba-Rusinov states delocalized over the molecule. One
level corresponds to a well-defined local moment, the other to a
valence-fluctuating orbital.
Singlet-Doublet Transitions of a Quantum Dot Josephson Junction
Detected in a Transmon Circuit#
Arno Bargerbos, Marta Pita-Vidal, Rok Žitko, Jesús Ávila, Lukas J.
Splitthoff, Lukas Grünhaupt, Jaap J. Wesdorp, Christian K. Andersen, Yu Liu,
Leo P. Kouwenhoven, Ramón Aguado, Angela Kou, and Bernard van Heck
PRX Quantum 3, 030311 (2022)(PDF)(Supplementary)(Josephson
potentials)
[open access]
Microwave spectroscopy of a hybrid superconductor-semiconductor transmon
device is used to probe the ground-state parity of the Josephson junction quantum dot, in
agreement with predictions of a single-impurity Anderson model with
superconducting leads. Quasiparticle dynamics can be resolved across the
parity phase boundaries.
Yu-Shiba-Rusinov states, BCS-BEC crossover, and exact solution in the
flat-band limit#
Rok Žitko and Luka Pavešić
Phys. Rev. B 106, 024513 (2022)(PDF)
We study how YSR states depend on 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 flat-band 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 for the problem of a magnetic impurity in a
superconducting bath.
Excitations in a superconducting Coulombic energy gap#
Juan Carlos Estrada Saldaña, Alexandros Vekris, Luka Pavešić, Peter
Krogstrup, Rok Žitko, Kasper Grove-Rasmussen & Jesper Nygård
Nat. Commun. 13, 2243 (2022)(PDF)(Supplementary)
[open access]
We show how the Coulomb repulsion on the superconducting island changes the
nature of the singlet sub-gap state and breaks the electron-hole energy
symmetry of the excitations.
Qubit based on spin-singlet Yu-Shiba-Rusinov states#
Luka Pavešić and Rok Žitko
Phys. Rev. B 105, 075129 (2022)(PDF)
We explore the possibility of using a device that consists of a quantum dot
in a junction between two superconducting islands as a qubit based on
spin-singlet Yu-Shiba-Rusinov subgap states. Such a qubit is insensitive to
magnetic-field fluctuations and can be operated at a gate-voltage sweet spot
where it is also resilient to electric-field fluctuations.
Nobelova nagrada za fiziko 2022 za poskuse s kvantno prepletenimi delci#
R. Žitko
Alternator, 31/2022(PDF) (Slovenian only) O Nobelovi nagradi za fiziko 2022 za pionirske poskuse s
kvantno prepletenimi delci (dokaz kršitve Bellove neenačbe, kvantna
teleportacija, izmenjava prepletenosti).
Stanje na področju generatorjev naključnih števil#
Matjaž Depolli, Peter Jeglič, Roman Novak, Erik Zupanič, Rok Žitko
CRP V1-2119-P1(PDF) (Slovenian only) Poročilo o stanju na področju generatorjev naključnih
števil, namenjeno razvijalcem programske in strojne opreme,
administratorjem računalniških sistemov, strokovnjakom za varnost, uporabnikom
naključnih števil v znanstvene namene in zaposlenim v loterijskih organizacijah.
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. Interactions in the
bath make conventional impurity solvers inapplicable. We show that the
Hamiltonian has a matrix-product-operator representation with small matrices,
enabling a solution using the density-matrix renormalization group (DMRG).
We use this method to systematically study 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
interactions. Such states interpolate between Yu-Shiba-Rusinov states
and Coulombic subgap states. Subgap states also exist for odd occupancy
of the superconducting island.
Universal Magnetic Oscillations of dc Conductivity in the
Incoherent Regime of Correlated Systems#
Jakša Vučičević, Rok Žitko
Phys. Rev. Lett. 127, 196601 (2021)(PDF)
Editor's Suggestion
Featured in Physics
Interactions induce quantum oscillations at high temperatures with a
frequency that is independent of the electron density. These oscillations
could provide information about the scattering mechanisms and dominant
interactions in strongly correlated materials. The finding is in excellent
qualitative agreement with the recent experiments on graphene superlattices.
We elucidate the key roles of the off-diagonal elements of the current
vertex and the incoherence of electronic states, and explain the trends with
respect to temperature and doping.
Electrical conductivity in the Hubbard model: Orbital effects of
magnetic field#
Jakša Vučičević, Rok Žitko
Phys. Rev. B 104, 205101 (2021)(PDF)
Editor's Suggestion
Featured in Physics [open access]
We present the formalism for calculating conductivity in the presence of
a perpendicular magnetic field, focusing on orbital effects. We prove that the
Kubo bubble preserves gauge and translational invariance, and that within
DMFT the vertex corrections cancel regardless of the magnetic field. We
present the DMFT results for the spectral function and both the longitudinal
and Hall conductivities in several regimes of parameters. We analyze
thoroughly the quantum oscillations of the longitudinal conductivity and
identify a high-frequency oscillation component, arising as a combined
effect of scattering and temperature.
Iron phthalocyanine on Au(111) is a “non-Landau” Fermi liquid#
R. Žitko, G. G. Blesio, L. O. Manuel, A. A. Aligia
Nat. Commun. 12, 6027 (2021)(PDF)(Supplementary)Featured in Editor's Highlights
[open access]
We show that an iron phthalocyanine molecule on a gold surface has a
ground state that is a non-Landau Fermi liquid in that it is not
adiabatically connected to the noninteracting limit. In this
work we extend the theory of topologically nontrivial local Fermi liquids
to multi-orbital problems with magnetic fields, which allowed us to
make this inference.
Correlation of Yu–Shiba–Rusinov States and Kondo Resonances in
Artificial Spin Arrays on an s-Wave Superconductor#
Anand Kamlapure, Lasse Cornils, Rok Žitko, Maria Valentyuk, Roberto Mozara,
Saurabh Pradhan, Jonas Fransson, Alexander I. Lichtenstein, Jens Wiebe,
Roland Wiesendanger
Nano Lett. 21, 6748 (2021)PDF
An Fe adatom interacting with assemblies of interstitial magnetic atoms
on the superconducting Ta(100) surface shows an evolution of the in-gap
states with the same phenomenology as the two-impurity Kondo problem.
We interpret this to be an effect of the formation of a collective
spin degree of freedom with increasing magnetic alignment of interstitials.
Single-shot Stern-Gerlach magnetic gradiometer with an expanding
cloud of cold cesium atoms#
Katja Gosar, Tina Arh, Tadej Mežnaršič, Ivan Kvasič, Dušan Ponikvar, Tomaž Apih, Rainer Kaltenbaek, Rok Žitko, Erik Zupanič, Samo Beguš,
Peter Jeglič
Phys. Rev. A 103, 022611 (2021)(PDF)
We demonstrate a gradiometer using a single image of an expanding atomic Cs cloud with the phase difference imprinted along the cloud.
The resolution of our single-shot gradiometer is not limited by thermal motion of atoms and has an estimated absolute accuracy below ±0.2 mG/cm (±20 nT/cm).
Coupled Yu-Shiba-Rusinov States Induced by a Many-Body Molecular Spin
on a Superconductor#
Carmen Rubio-Verdú, Javier Zaldívar, Rok Žitko, Jose Ignacio Pascual
Phys. Rev. Lett. 126, 017001 (2021)(PDF)
Iron porphyrin molecules on Pb(111) have an S = 1 ground state
due to Hund's-like exchange coupling between two orbitals. The orbitals have
opposite potential scattering, yielding two sub-gap states with particular
spatial patterns showing particle-hole asymmetry inversion across the
molecule.
Two-impurity Yu-Shiba-Rusinov states in coupled quantum dots#
J. C. Estrada Saldaña, A. Vekris, R. Žitko, G. Steffensen, P. Krogstrup, J.
Paaske, K. Grove-Rasmussen, and J. Nygård
Phys. Rev. B 102, 195143 (2020)(PDF)
We demonstrate the superconducting analog of the two-impurity Kondo problem.
In addition to antiferromagnetic and Kondo-screened spin-singlet regimes,
the phase diagram includes doublet Yu-Shiba-Rusinov regimes where a
single local moment is screened.
Anisotropic Kondo screening induced by spin-orbit coupling in quantum
wires#
E. Vernek, G. B. Martins, and R. Žitko
Phys. Rev. B 102, 155114 (2020)(PDF)
We study an Anderson impurity coupled to a 1D wire with Rashba and Dresselhaus
spin-orbit coupling (SOC) with numerical renormalization group (NRG)
using no approximations to the hybridisation
function. Fingerprints of anisotropy in the wire are revealed through the
anisotropic response to an 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 conductivity, which are significant on the square
lattice even at high temperatures, are found to contribute less on the
triangular lattice.
Erasing odd-parity states in semiconductor quantum dots coupled to
superconductors#
Z. Su, R. Žitko, P. Zhang, H. Wu, D. Car, S. R. Plissard, S. Gazibegovic, G.
Badawy, M. Hocevar, J. Chen, E. P. A. M. Bakkers, S. M. Frolov
Phys. Rev. B 101, 235315 (2020)(PDF)
We assess to what degree the erasure of odd-parity regimes in open quantum
dots coincides at low and high biases.
Emission of correlated jets from a driven matter-wave soliton in a
quasi-one-dimensional geometry#
Tadej Mežnaršič, Rok Žitko, Tina Arh, Katja Gosar, Erik Zupanič, and Peter Jeglič
Phys. Rev. A 101, 031601(R) (2020)(PDF)(GPE solver)
Ejection of correlated atom jets from a soliton can be triggered by
modulating the interaction strength. Number correlations beyond mean-field
are demonstrated.
Conductivity in the Square Lattice Hubbard Model at High Temperatures:
Importance of Vertex Corrections#
J. Vučičević, J. Kokalj, R. Žitko, N. Wentzell, D. Tanasković, and J. Mravlje
Phys. Rev. Lett. 123, 036601 (2019)(PDF)(suppl)
We show that while the self-energy is practically local at high temperatures,
the vertex corrections to conductivity remain rather important. At high
temperatures, finite-size effects are small even for a 4×4 lattice,
and Lanczos diagonalization gives results close to the exact result in the
thermodynamic limit.
Charge localization and reentrant superconductivity in a
quasi-ballistic InAs nanowire coupled to superconductors#
J. C. Estrada Saldaña, R. Žitko, J. P. Cleuziou, E. J. H. Lee, V.
Zannier, D. Ercolani, L. Sorba, R. Aguado and S. De Franceschi
Sci. Adv. 5, eaav1235 (2019)PDF [open access]
We explore a nanowire at low subband filling in the presence
of the superconducting proximity effect and a magnetic field, and show
that the plateau of ballistic conductance actually hides the presence
of Coulomb-blockade and Kondo physics.
Observation of Coexistence of Yu-Shiba-Rusinov States and Spin-Flip
Excitations#
Shawulienu Kezilebieke, Rok Žitko, Marc Dvorak, Teemu Ojanen, Peter Liljeroth
Nano Lett. 19, 4614 (2019)PDF
The crossover from weak impurity-substrate hybridization with inelastic
spin-flip excitations outside the gap to strong hybridization with
Yu-Shiba-Rusinov peaks inside the gap is smooth, with a parameter range
where both spectral features are visible at the same time.
Kondo screening in a charge-insulating spinon metal#
M. Gomilšek, R. Žitko, M. Klanjšek, M. Pregelj, C. Baines, Y. Li, Q. M.
Zhang, A. Zorko
Nat. Phys. 15, 754 (2019)(self-archived PDF)
We report the first experimental observation of Kondo screening
by neutral quasiparticles in a charge-insulating quantum spin liquid,
where spinon excitations forming a Fermi surface take on the role of conduction
electrons.
Cesium bright matter-wave solitons and soliton trains#
Tadej Mežnaršič, Tina Arh, Jure Brence, Jaka Pišljar, Katja Gosar, Žiga
Gosar, Rok Žitko, Erik Zupanič, and Peter Jeglič
Phys. Rev. A 99, 033625 (2019)(PDF)
We demonstrate the production of bright solitons and soliton trains in a
cesium Bose-Einstein condensate (BEC). The condensate also fragments after
release from confinement into free space.
Mottness collapse without metallization in the domain wall of the
triangular-lattice Mott insulator 1T-TaS2#
J. Skolimowski, Ya. Gerasimenko, R. Žitko
Phys. Rev. Lett. 122, 036802 (2019)(PDF)(suppl)
The 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:
full metallization is preempted by the splitting of the quasiparticle
band into bonding and antibonding subbands separated by a band gap. This
picture is consistent with the experimental tunneling spectra.
Comparative study of nonequilibrium insulator-to-metal transitions in
electron-phonon systems#
Sharareh Sayyad, Rok Žitko, Hugo U. R. Strand, Philipp Werner, and Denis
Golež
Phys. Rev. B 99, 045118 (2019)(PDF)
Different impurity solvers are compared for their ability to describe the
equilibrium and nonequilibrium properties of electron-phonon systems
described by the Hubbard-Holstein model using the dynamical mean-field theory.
Numerical renormalization group#
Rok Žitko
NRG UFU/2019 Advanced
Studies School(PDF)
A brief overview of the numerical renormalization group and its "NRG
Ljubljana" implementation. Prepared for the NRG UFU/2019 Advanced
Studies School, Uberlândia, 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)(PDF)(suppl)
We demonstrate the Josephson effect in a serial double quantum dot defined
in a nanowire with epitaxial superconducting leads. The consistency between
experiment and theory indicates that our device is a faithful realization of
the two-impurity Anderson model.
Aharonov-Bohm and Aharonov-Casher effects in a double quantum dot Josephson junction#
Damian Tomaszewski, Piotr Busz, Rosa López, Rok Žitko, Minchul Lee, and Jan Martinek
Phys. Rev. B 98, 174504 (2018)(PDF)
Cooper-pair splitting can be made perfectly efficient by adding quantum dots
in a nanostructure where two parallel nanowires bridge two superconductors.
The Aharonov-Casher
phase is different for non-spin-flip and spin-flip processes.
Reversal of particle-hole scattering-rate asymmetry in the Anderson impurity model#
R. Žitko, H. R. Krishnamurthy, and B. Sriram Shastry
Phys. Rev. B 98, 161121(R) (2018)(PDF)
In the single-impurity Anderson model, we identify a line in the (U, n)
parameter plane where the scattering-rate asymmetry vanishes. This line,
defined through impurity dynamics, traces the crossover between the
weak-coupling and strong-coupling regimes.
Yu–Shiba–Rusinov screening of spins in double quantum dots#
K. Grove-Rasmussen, G. Steffensen, A. Jellinggaard, M. H. Madsen, R. Žitko, J. Paaske, J. Nygård
Nat. Commun. 9, 2376 (2018)(PDF)(Supplementary) [open access]
A 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-Shiba-Rusinov states that delineate the different ground
states of the system.
Aharonov-Bohm and Aharonov-Casher effects for local and nonlocal Cooper pairs#
Damian Tomaszewski, Piotr Busz, Rosa López, Rok Žitko, Minchul Lee, Jan Martinek
Phys. Rev. B 97, 214506 (2018)(PDF)
In geometries allowing for both local and non-local (split)
Cooper-pair Josephson current, both Aharonov-Bohm (AB) and
Aharonov-Casher (AC) effects can occur. For the AC effect, a Cooper
pair must be split and nonlocally preserve its entangled singlet
state, and each electron of the pair has to experience a different
Rashba phase.
Spin-1 two-impurity Kondo problem on a lattice#
A. Allerdt, R. Žitko, A. E. Feiguin
Phys. Rev. B 97, 045103 (2018)(PDF)
A two-impurity spin-1 model is studied on a square lattice for different
inter-impurity separations using the DMRG. When the impurities are
underscreened, the dangling spin-1/2 degrees of freedom generate the
inter-impurity entanglement. Kondo screening and RKKY spin alignment
coexist.
Quantum impurity models for magnetic adsorbates on superconductor surfaces#
R. Žitko
Physica B
536, 230 (2018)
Classical and quantum impurity models for magnetic adsorbates on
superconductors differ in several qualitative respects: the number and
degeneracies of the sub-gap states, presence of symmetry breaking, and
the effect of the external magnetic field. The high-spin adsorbates
with Ising (easy-axis) magnetic anisotropy show features that are
different from both isotropic quantum impurities and classical
impurities.
Na poti do kvantne prevlade#
R. Žitko
Gea, april 2018 (Slovenian only) O kvantnem računalništvu in drugih nastajajočih 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)(PDF)
The quantum spin-liquid state in Zn-brochantite undergoes a transition in
an arbitrarily small applied magnetic field to a different spin-liquid
state with a full or 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 transverse transport and optical response in the infinite-U Hubbard
model at low temperatures and extract longitudinal and transverse relaxation
rates. A single rate controls both types of transport in the lowest-temperature
Fermi-liquid regime. In the strange-metal regime, two distinct 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 Mihailović, 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 μSR measurements. Between 55 K and 200 K, T2 relaxation dynamics
are observed, while below 55 K a new gapless state with a reduced density of spin excitations is
observed.
Spin-orbit coupling in three-orbital Kanamori impurity model and its relevance for transition-metal oxides#
Alen Horvat, Rok Žitko, and Jernej Mravlje
Phys. Rev. B 96, 085122 (2017)(PDF)
The spin-orbit coupling (SOC) in the three-orbital impurity model with Kanamori interaction
becomes effective if it is larger in strength than the orbital Kondo temperature.
Depending on the sign of SOC, the system then behaves as either a nonmagnetic
Van Vleck impurity or a J = 2 impurity. Regardless of the SOC strength, 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 (a Gutzwiller-correlated
Fermi liquid and a Gutzwiller-correlated strange metal) in the U = ∞,
d = ∞ 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
the zero-frequency imaginary part of the self-energy, which starts
to deviate from its low-temperature asymptotic behavior on
the scale of the Fermi-liquid temperature.
Nonperturbative effects and indirect exchange interaction between quantum impurities on metallic (111) surfaces#
A. Allerdt, R. Žitko, and A. E. Feiguin
Phys. Rev. B 95, 235416 (2017)(PDF)
We study the exchange interaction between two impurities on a metallic (111)
surface that hosts surface states. A density-matrix renormalization group
representation allows high-accuracy results in the thermodynamic limit.
Ferromagnetic correlations develop only at short distances.
Scaling of subgap excitations in a superconductor-semiconductor nanowire quantum dot#
Eduardo J. H. Lee, Xiaocheng Jiang, Rok Žitko, Ramón Aguado, Charles M. Lieber, and Silvano De Franceschi
Phys. Rev. B 95, 180502(R) (2017)(PDF)
Editor's Suggestion
We study the energies of subgap Andreev excitations in a hybrid
superconductor-semiconductor device using a simple Anderson impurity model
with superconducting leads. Model parameters are extracted from the
normal-state linear conductance, including temperature and magnetic-field
effects, and used to calculate superconducting subgap spectra. Theory and
experiment agree well on the scaling of the excitation energy with
T_K/Delta, where T_K is the Kondo temperature and Delta is the BCS gap.
Non-Fermi-liquid behavior in quantum impurity models with superconducting channels#
R. Žitko and M. Fabrizio
Phys. Rev. B 95, 085121 (2017)(PDF)
In the limit of a small superconducting gap in the bath, the subgap spectrum of overscreened Kondo models shows universal features
characteristic of the fixed point of the normal-state problem. These effects could be detected in hybrid devices with mesoscopic
superconducting islands.
Magnetic impurities in spin-split superconductors#
W.-V. van Gerven Oei, D. Tanasković, R. Žitko
Phys. Rev. B 95, 085115 (2017)(PDF)
We study the sub-gap (Shiba) state of classical and quantum impurities
in a superconductor with Zeeman splitting of Bogoliubov quasiparticle states.
For classical impurities, this results in a trivial shift of the Shiba state.
For quantum impurities, we find a non-monotonic dependence of the singlet-doublet
quantum phase transition line in the phase diagram in the (gap,field) plane.
Fine structure of the spectra of the Kondo lattice model: Two-site cellular dynamical mean-field theory study#
Žiga Osolin, Rok Žitko
Phys. Rev. B 95, 035107 (2017)(PDF)
A cellular DMFT calculation uses 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 matrix space. We discuss the
limitations of the method due to significant computational demands and
aggressive state truncation.
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 uvedbo topoloških konceptov za razlago
osnovnih lastnosti 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)(PDF)
We derive spectral moments of dynamical response functions of the Hubbard
model using high-temperature series expansions, then reconstruct the response
functions. We find excellent agreement with DMFT results obtained using
state-of-the-art numerical methods over a surprisingly wide temperature range.
At high temperatures, the resistivity is linear and does not saturate,
even though the scattering rate saturates.
Low-energy physics of three-orbital impurity model with Kanamori interaction#
Alen Horvat, Rok Žitko, Jernej Mravlje
Phys. Rev. B 94, 165140 (2016)(PDF)
The three-orbital impurity model with Kanamori interaction,
appropriate for Hund metals, maps to a Kondo model with spin, orbital,
and quadrupole (as well as mixed) coupling terms. We study the degree
of separation between the spin and orbital degrees of freedom. We
find slow screening of the spin degrees of freedom. At low energies,
the SU(3) orbital symmetry is restored.
Kondo effect at low electron density and high particle-hole asymmetry in 1D, 2D, and 3D#
Rok Žitko, Alen Horvat
Phys. Rev. B 94, 125138 (2016)(PDF)
We discuss the relation between the impurity binding energy and the Kondo
temperature, including the effects of band-edge van Hove singularities on
the Kondo scale. We find strong deviations in 1D and 3D due to the
nontrivial renormalization of potential scattering.
Spectral properties of Shiba subgap states at finite temperatures#
Rok Žitko
Phys. Rev. B 93, 195125 (2016)(PDF)
We study the temperature dependence of the sub-gap part of the spectral function
of the Anderson impurity model with a superconducting bath. We find a background continuum
at finite temperatures due to inelastic exchange scattering of thermally excited
Bogoliubov quasiparticles. In the charge-fluctuation regime with a 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 measurements of LSX zeolites reveal a doping-dependent crossover from
an insulating to a metallic state. In the metallic state, the nuclear
spin-lattice relaxation time follows Korringa behavior.
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)(PDF)
The half-filled Hubbard model in infinite dimensions maps exactly onto free
fermions coupled in a Z2 gauge-invariant manner to auxiliary Ising
spins in a transverse field, separating charge and spin degrees of freedom.
The zero-temperature insulator-to-metal transition corresponds to spontaneous
breaking of the local Z2 gauge symmetry. This is not forbidden in
infinite dimensions and is not an artifact of a mean-field decomposition,
giving the Mott transition 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 a 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 splitting of doublet Shiba states and for observing
both singlet and triplet Shiba states. Inter-dot capacitive coupling is
also considered: it may lead to a crossover at half-filling and to phase
transitions close to quarter-filling. Finally, for two impurities coupled
solely by the exchange interaction, we find a quantum phase transition
between two singlet states.
Repulsive versus attractive Hubbard model: Transport properties and spin-lattice relaxation rate#
Rok Žitko, Žiga Osolin, Peter Jeglič
Phys. Rev. B 91, 155111 (2015)(PDF)
We compare the repulsive and attractive Hubbard models in the paramagnetic state at 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 the Seebeck coefficient between the positive-U model at half-filling in a magnetic field and
the negative-U model away from half-filling, arising from the partial particle-hole 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 Néel
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 unequal 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 López, Ramón Aguado
Phys. Rev. B 91, 045441 (2015)(PDF)
We study the sub-gap spectra of a quantum impurity coupled to one superconducting
and one normal-state lead (tunneling probe). We show under which conditions a zero-bias
anomaly appears either due to the presence of the normal-state lead or due to Zeeman
splitting of the doublet Shiba state. The calculations cover arbitrary
ratios Delta/T_K, so Kondo correlations are properly taken into account.
Entanglement switching via the Kondo effect in triple quantum
dots#
S. B. Tooski, B. R. Bułka, R. Žitko, A. Ramšak
Eur. Phys. J. B 87, 145 (2014)(PDF)
Entanglement properties of a triple quantum dot system are studied
with a focus on switching between separable and entangled states
due to Kondo singlet formation.
Effect of assisted hopping on thermopower in an interacting
quantum dot#
S. B. Tooski, A. Ramšak, B. R. Bułka, R. Žitko
New J. Phys. 16, 055001 (2014)(PDF) [open
access]
Assisted hopping (occupancy dependence of the tunneling amplitudes)
can lead to anomalies in the mixed-valence regime which, in turn,
are associated with a 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.
Shastry
Phys. Rev. B 88, 235132 (2013)(PDF)
We analyze the doped Mott insulator using DMFT(NRG) and ECFL.
The ECFL ansatz captures the features of
the self-energy, in particular the asymmetry between particle and hole
excitations, even when symmetric (Fermi-liquid-like) auxiliary
self-energies are used as the input to the ECFL formalism.
Spin thermopower in the overscreened Kondo model#
R. Žitko, J. Mravlje, A. Ramšak, T. Rejec
New J. Phys. 15, 105023 (2013)(PDF)
[open access]
We study the spin thermopower in various members of the family of
Kondo models, focusing on the two-channel spin-1/2 case, which exhibits
a non-Fermi-liquid ground state. We show that the critical exponent
can be deduced from the magnetic-field and temperature dependence
of the spin Seebeck coefficient.
Lifshitz phase transitions in the ferromagnetic regime of the Kondo
lattice model#
Denis Golež, Rok Žitko
Phys. Rev. B 88, 054431 (2013)(PDF)(suppl)
We establish the phase diagram of the topologically different
ferromagnetic phases of spin-1/2 and spin-1 Kondo lattice models
using the DMFT and static mean-field approaches. We determine the
"ferromagnetic Doniach diagram".
Tunneling spectroscopy of a single quantum dot coupled to a
superconductor: From Kondo ridge to Andreev bound states
#
J.-D. Pillet, P. Joyez, Rok Žitko, M. F. Goffman
Phys. Rev. B 88, 045101 (2013)(PDF)
Described in
Physics ViewpointPRB 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 the normal or superconducting state. The results are in
quantitative agreement with a single-level Anderson model.
Padé approximant approach for obtaining finite-temperature spectral
functions of quantum impurity models using the numerical
renormalization group technique#
Žiga Osolin, Rok Žitko
Phys. Rev. B 87, 245135 (2013)(PDF)
We present a new method for obtaining finite-temperature spectral
functions of impurity models using the NRG, based on performing
Padé continuation to the real-frequency axis of raw data accumulated
on the imaginary axis. The method provides improved results for spectral
functions on frequency scales of order T.
Unconventional Superconductivity from Local Spin Fluctuations
in the Kondo Lattice#
Oliver Bodensiek, Rok Žitko, Matthias Vojta, Mark Jarrell,
Thomas Pruschke
Phys. Rev. Lett. 110, 146406 (2013)(PDF)
We find a singlet s-wave superconducting solution for the Kondo
lattice model within single-site DMFT. We argue that the pairing is
due to strong local spin fluctuations.
Non-Drude universal scaling laws for the optical response of
local Fermi liquids#
C. Berthod, J. Mravlje, X. Deng, R. Žitko, D. van der Marel, A. Georges
Phys. Rev. B 87, 115109 (2013)(PDF)
Editor's Suggestion
The optical response of a Fermi liquid is computed assuming a local
self-energy with a Fermi-liquid scaling form. The optical conductivity shows a
non-Drude "foot".
How Bad Metals Turn Good: Spectroscopic Signatures of Resilient
Quasiparticles#
X. Deng, J. Mravlje, R. Žitko, M. Ferrero, G. Kotliar, A. Georges
Phys. Rev. Lett. 110, 086401 (2013)(PDF)(suppl)
In hole-doped Mott insulators, resilient quasiparticles remain
well defined above the Fermi temperature and dominate transport
in the intermediate regime up to the Mott-Ioffe-Regel regime.
SU(3) Kondo effect in spinless triple quantum dots#
R. López, T. Rejec, J. Martinek, R. Žitko
Phys. Rev. B 87, 035135 (2013)(PDF)
We study Kondo physics in a system of three spin-polarized
quantum dots, which provide a realization of the SU(3) Kondo model. We
discuss the stability of the SU(3) Kondo effect with respect to
different perturbations.
Vibrational Andreev bound states in magnetic molecules#
D. Golež, J. Bonča, R. Žitko
Phys. Rev. B 86, 085142 (2012)(PDF)
A deformable magnetic molecule adsorbed on a superconducting surface
is predicted to exhibit vibrational side peaks alongside the main subgap
Andreev bound state (Shiba state). We discuss phonon hardening due to the
anharmonic potential of the vibrational mode, which competes with phonon
softening due to electron-phonon coupling.
Tunable Kondo Effect in a Double Quantum Dot Coupled to
Ferromagnetic Contacts#
Rok Žitko, Jong Soo Lim, Rosa López, Jan Martinek, Pascal Simon
Phys. Rev. Lett. 108, 166605 (2012)(PDF)
The phase diagram for a DQD coupled to spin-polarized contacts is
established. The two-impurity Kondo crossover is generalized to the
(p, t) plane, where p is the polarization and t is 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. Ramšak
Phys. Rev. B 85, 085117 (2012)(PDF)
We study the spin thermopower of a quantum dot in a magnetic field: the
temperature difference applied across the dot drives a pure spin
current without accompanying charge current.
Ground state of the parallel double quantum dot system#
R. Žitko, J. Mravlje, K. Haule
Phys. Rev. Lett. 108, 066602 (2012)(PDF)
We show that CT-QMC and NRG produce consistent results for the
low-temperature behavior of a parallel double quantum dot (a singular
Fermi-liquid state), 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. Bonča
Phys. Rev. B 84, 193411 (2011)(PDF)
We study the Anderson model for an impurity hybridizing with a
2D electron gas with Rashba spin-orbit interaction.
The spectral function exhibits an additional peak
at the bottom of the band which becomes more pronounced due to
a diverging effective density of states. The Kondo temperature changes
only slightly (non-exponentially).
Kondo resonance lineshape of magnetic adatoms on decoupling
layers#
R. Žitko
Phys. Rev. B 84, 195116 (2011)(PDF)
Excellent numerical agreement between the theoretical Kondo line-shape
and the experimental results for the Ti/CuN/Cu(100) zero-bias anomaly
is found, including even the asymptotic logarithmic tails. No
potential scattering needs to be included in the impurity model: the
valence of the impurity atom is well defined. The extraction of the g
factor is also discussed: two interpretations are possible.
Quantum impurity coupled to Majorana edge fermions#
R. Žitko, P. Simon
Phys. Rev. B 84, 195310 (2011)(PDF)
An 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 a weak diamagnetic contribution to the total
magnetic susceptibility of the system.
Hydrogenation properties of the TiBx structures#
R. Žitko, H. J. P. van Midden, E. Zupanič, A. Prodan, S. S. Makridis,
D. Niarchos, A. K. Stubos
Int. J. Hydrogen Energy 36, 12268–12278 (2011)
A random structure search using DFT has been performed for hydrogenated
titanium boride structures: TiB2, TiB, and Ti2B.
Quantitative determination of the discretization and truncation
errors in numerical renormalization-group calculations of spectral
functions#
R. Žitko
Phys. Rev. B 84, 085142 (2011)(PDF)
Errors in the spectral-function calculations using NRG can be
quantitatively estimated. Application to magnetic-field splitting
of the Kondo resonance.
SNEG - Mathematica package for symbolic calculations with
second-quantization-operator expressions#
R. Žitko
Comput. Phys. Commun. 182, 2259 (2011)(PDF)
[SNEG download page]
Description of the philosophy and design choices behind SNEG, a package
for symbolic computation with second-quantization operators.
Detection of Majorana edge states in topological
superconductors through non-Fermi-liquid effects induced in an
interacting quantum dot#
R. Žitko
Phys. Rev. B 83, 195137 (2011)(PDF)
The presence of the continuum of Majorana fermion
edge states along the perimeter of a chiral topological superconductor
can be probed using an interacting quantum dot coupled to three
terminals: the lead supporting the Majorana edge states and two
spin-polarized (ferromagnetic) measurement leads.
Low-energy properties of the Kondo lattice model#
O. Bodensiek, R. Žitko, R. Peters, T. Pruschke
J. Phys.: Condens. Matter 23, 094212 (2011)(PDF)
The heavy-fermion physics in the Kondo lattice model is studied within
DMFT using NRG as the solver. The crossover between large and
small Fermi surfaces is discussed, as well as the effect of phonons.
Effects of magnetic anisotropy on the subgap excitations induced by quantum
impurities in a superconducting host#
Rok Žitko, Oliver Bodensiek, Thomas Pruschke
Phys. Rev. B 83, 054512 (2011)(PDF)
Magnetic anisotropy leads to the splitting of sub-gap states localized
on magnetic impurities coupled to a superconducting host. This
splitting may have already been experimentally resolved in
high-resolution STM studies of Mn and Cr on superconducting Pb.
We also discuss the effect of the external magnetic field on the sub-gap
states, and the sub-gap spectrum of the two-impurity model, where
two different singlet states, a pair of doublet states, and a triplet
state compete to become the ground state of the system.
Josephson current in strongly correlated double quantum dots#
Rok Žitko, Minchul Lee, Rosa López, Ramón Aguado, Mahn-Soo Choi
Phys. Rev. Lett. 105, 116803 (2010)(PDF)
A phase diagram of the 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 Zupanič, Rok Žitko, Herman J. P. van Midden, Igor Muševič, Albert
Prodan
Phys. Rev. Lett. 104, 196102 (2010)(PDF)
Substitutional Ag defects in the topmost atomic layer of Cu(111)
act as pinning centers for Co adatoms without affecting their electronic
or magnetic properties (the Kondo resonance remains essentially
the same). The Ag impurities are hardly detectable in
topographic imaging as they produce no standing-wave pattern and the
dI/dV recorded above them is hardly different from that of the clean
surface.
Charge density waves in NbSe3: The models and the
experimental evidence#
A. Prodan, H. J. P. van Midden, R. Žitko, E. Zupanič, J. C. Bennett, H. Böhm
Solid State Commun. 150, 2134 (2010)(PDF)
Discussion of charge-density-wave (CDW) modes in NbSe3. It is pointed
out that the standard model with two incommensurate modulations
localized on different prismatic columns does not account for the
totality of experimental results.
Quantum impurity on the surface of a topological insulator#
R. Žitko
Phys. Rev. B 81, 241414(R) (2010)(PDF)
The Hamiltonian for a quantum magnetic impurity on
the surface of a topological insulator can be mapped to the
conventional pseudo-gap Anderson impurity model. This implies
that the magnetic moment is fully Kondo screened, thus at low enough
temperature a single magnetic dopant is effectively non-magnetic
and does not break the time-reversal-invariance (TRI) protection
of the surface states.
Many-particle effects in adsorbed magnetic atoms with easy-axis
anisotropy: the case of Fe on the 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 of 8–9 T 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. Zupanič, R. Žitko, S. S. Makridis,
A. K. Stubos
J. Phys. Chem. C 114, 4221–4227 (2010)
Two interstitial H atoms in close proximity in a ZrCr2 host repel each
other.
Fano-Kondo effect in side-coupled double quantum dots at finite
temperatures and the importance of two-stage Kondo screening#
Rok Žitko
Phys. Rev. B 81, 115316 (2010)(PDF)
The antiresonance in transport experiments on a side-coupled double
quantum dot may be explained in terms of two-stage Kondo screening
with the experimental temperature between the two different Kondo
scales.
Kondo screening in high-spin side-coupled two-impurity
clusters#
Rok Žitko
J. Phys.: Condens. Matter 22, 026002 (2010)(PDF)
The two-stage Kondo screening phenomenology is
generalized to a pair of side-coupled Kondo impurities with large
spin. Both antiferromagnetic and ferromagnetic (as in
Hund's rule) exchange coupling between the spins is analysed.
Two-impurity Anderson model revisited: Competition between Kondo
effect and reservoir-mediated superexchange in double quantum dots#
Minchul Lee, Mahn-Soo Choi, Rosa López, Ramón Aguado, Jan Martinek, Rok Žitko
Phys. Rev. B 81, 121311(R) (2010)(PDF)
Editor's Suggestion
A conduction-band-mediated superexchange mechanism leads to an
effective antiferromagnetic interaction between the two impurity spins
in the infinite-U limit, thereby significantly modifying the transport
properties of the system. This work points out the deficiencies of
a static mean-field description of inter-impurity coupling.
Superconductivity in the Kondo lattice model#
O. Bodensiek, R. Žitko, Th. Pruschke
J. Phys.: Conf. Ser. 200, 012162 (2010)(PDF)
DMFT(NRG) study of the Kondo lattice model with attractive interaction
between the conduction-band electrons.
Magnetization Curves for Anisotropic Magnetic Impurities Adsorbed on a Normal Metal Substrate#
R. Žitko
Physical Properties of Nanosystems, pp. 247–257 (2010)(PDF)
NRG study of magnetization curves Sz(B,T) for magnetic impurities. Deviations
from ideal paramagnetic behavior are discussed.
Van Hove singularities in the paramagnetic phase of
the Hubbard model: DMFT study#
R. Žitko, J. Bonča, T. Pruschke
Phys. Rev. B 80, 245112 (2009)(PDF)
Electron-electron interactions smooth out van Hove singularities in
the spectral functions of the Hubbard model. However, the logarithmic
singularity (for 2D square lattices) pinned at the Fermi level
survives. In this latter case, the magnetic-field-induced
metal-insulator transition falls in a different universality class from
the case with a featureless density of states, as there is no
field-induced localization of quasiparticles. The 3D cubic lattice and
a density of states with a power-law divergence are also considered.
Charge-density waves in pure and intercalated Nb3Te4#
A. Prodan, Herman J. P. van Midden, S. Šturm, R. Žitko,
E. Zupanič, J. Craig Bennett, R. MacDougall, P. Starowicz
Solid State Sci. 11, 1556 (2009)
Intercalation of Tl and In in Nb3Te4 flattens
the Fermi surfaces and enhances the formation of charge density waves (CDWs).
Low-temperature Scanning Tunneling Microscopy and Spectroscopy of
Noble-metal Surfaces#
E. Zupanič, R. Žitko, H. J. P. van Midden, I. Muševič, A. Prodan
Croat. Chem. Acta 82, 485 (2009)(PDF)
We describe some results obtained with the low-temperature STM built
at the Jožef Stefan Institute. The instrument has high mechanical
stability, allowing improved spectroscopic measurements and
the manipulation of individual atoms.
Convergence acceleration and stabilization of
dynamical mean-field theory calculations#
R. Žitko
Phys. Rev. B 80, 125125 (2009)(PDF)
Using advanced mixing techniques (the modified Broyden 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 metastable solutions of
DMFT equations can be obtained.
Numerical renormalization group calculations of ground-state energy: Application to
correlation effects in the adsorption of magnetic impurities on metal surfaces#
R. Žitko
Phys. Rev. B 79, 233105 (2009)(PDF)
The binding energy of the single-impurity Anderson model is computed
to high accuracy. The Kondo energy is only a small
contribution to the total correlation energy (total binding energy
minus Hartree-Fock binding energy), which is of order Γ2,
rather than of order TK. This work shows that NRG can be applied to
calculate the ground-state energies of quantum impurity models with
excellent accuracy, sufficient to extract various quantities of
physical interest (susceptibilities, etc.).
Adaptive logarithmic discretization for numerical renormalization group
methods#
R. Žitko
Comput. Phys.
Commun. 180, 1271 (2009)(PDF)
An NRG discretization scheme with adaptable discretization-mesh points reduces
artifacts that result from a 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 that map onto 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 artifacts in the numerical
renormalization group#
R. Žitko, T. Pruschke
Phys. Rev. B 79, 085106 (2009)(PDF)
We show how to compute high-resolution spectral functions of quantum impurity
models using numerical renormalization group techniques. We present results
for the Anderson and Anderson-Holstein impurity models and for the Hubbard
model (within DMFT).
This work also introduces a new discretization technique which largely
eliminates the discretization artifacts 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
make a 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 Systems Science, Springer,
New York (2009) (PDF)
Introduction to quantum impurity physics in coupled quantum dots
described by multi-impurity Anderson models.
Properties of anisotropic magnetic impurities on surfaces#
R. Žitko, R. Peters, T. Pruschke
Phys. Rev. B 78, 224404 (2008)
(PDF)
We study the Kondo model with XXZ anisotropy as a model for
anisotropic magnetic impurities on surfaces. Both easy-axis and
hard-axis cases are considered. For hard-axis half-integer spin cases
(such as S=3/2 for Co adatoms on CuN/Cu), we point out a strong
enhancement of the Kondo temperature due to effective doubling of the
transverse component of the Kondo coupling; without this exponential
enhancement, TK would be unmeasurably low.
Numerical renormalization group study of two-channel three-impurity
triangular clusters
#
R. Žitko, J. Bonča
Phys. Rev. B 77, 245112 (2008)
(PDF)
Editor's Suggestion
The phase diagram of triangular three-impurity clusters is determined.
Two-stage Kondo screening is relevant even for crossovers
between non-Fermi-liquid fixed points. We find such a direct transition
between two-impurity and two-channel Kondo model fixed points. We point
out that in problems with exponentially low energy scales, it is
dangerous to study transport at "zero temperature".
Vibrational effects on low-temperature properties of molecular
conductors#
J. Mravlje, A. Ramšak, R. Žitko
Physica B 403, 1484 (2008)(PDF)
In the Anderson model with additional phonon-assisted coupling to the
odd conduction channel, the zero-temperature conductance is found to
be qualitatively the same as in the model with no coupling to the
vibrational mode. An unstable two-channel Kondo model fixed point is
found at the point where the couplings match.
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 an abrupt change in average impurity occupancy. These effects
arise from the interplay of strong-correlation effects and decoupling
of certain combinations of states from the continuum (the Dicke effect).
Fermi-liquid versus non-Fermi-liquid behavior in triple quantum
dots#
R. Žitko, J. Bonča
Phys. Rev. Lett. 98, 047203 (2007)(PDF)
The conductance through a 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 signatures of this NFL behavior.
Correlation effects in side-coupled quantum dots#
R. Žitko, J. Bonča
J. Phys.: Condens. Matter 19, 255205 (2007)
A review of the two-stage Kondo effect in a side-coupled double quantum dot,
a demonstration of the three-stage Kondo effect in a side-coupled triple
quantum dot, and a generalization to the multistage Kondo effect in more
general systems. We also study simultaneous spin and charge Kondo effects
in a side-coupled configuration.
Spin-charge separation and simultaneous spin and charge Kondo effect#
R. Žitko, J. Bonča
Phys. Rev. B 74, 224411 (2006)(PDF)
Due to the spin-charge separation, a single conduction channel may screen
both spin and charge (isospin) degrees of freedom: spin and charge Kondo
effects may occur simultaneously and independently. Such an effect might
occur in specifically tuned single-molecule transistors.
Spin qubits in double quantum dots: Entanglement versus the Kondo
effect#
A. Ramšak, J. Mravlje, R. Žitko, J. Bonča
Phys. Rev. B 74, 241305(R) (2006)(PDF)
The entanglement (concurrence) between two localized spins in a double
quantum dot has universal behaviour as a function of inter-dot
coupling, magnetic field and temperature for different topologies of
the coupling to the leads.
Multiple-impurity Anderson model for quantum dots coupled in parallel#
R. Žitko, J. Bonča
Phys. Rev. B 74, 045312 (2006)(PDF)
If several Anderson impurities couple to the same orbital of the conduction
band, the inter-impurity RKKY interaction is ferromagnetic. The spins align
at low temperatures and the system undergoes S = N/2 Kondo screening, where N
is the number of impurities. An analytic estimate is provided for the RKKY
exchange constant. Perturbation terms and induced quantum phase transitions
are studied.
Kondo effect in triple quantum dot#
R. Žitko, J. Bonča, A. Ramšak, T. Rejec
Phys. Rev. B
73, 153307 (2006)(PDF)
A phase diagram for the triple quantum dot is established. At strong
inter-dot coupling the electrons consecutively fill molecular orbitals and
the Kondo effect emerges for an odd number of electrons. At weak coupling, the
electrons tend to form rigid antiferromagnetic spin chains or undergo
two-stage Kondo screening.
Enhanced conductance through side-coupled double
quantum dots#
R. Žitko, J. Bonča
Phys. Rev. B
73, 035332 (2006)(PDF)
A 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 temperatures. 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
Obzornik za matematiko in fiziko 52, 65–76 (2005)
Microscopic mechanisms of dephasing due to
electron-electron interactions#
R. Žitko, J. Bonča
Phys. Rev. B
68, 085313 (2003)(PDF)
A method for calculating the scattering of a single electron incident on
a region of strong electron-electron interaction where additional electrons
are bound. We apply it to study dephasing in an Aharonov-Bohm ring.
(PDF)
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