arXiv subjects

Martin Žonda

Publications and source records attributed to Martin Žonda.

28 records · Page 2Linked to original sources

arXiv2019

Second Order Perturbation Theory for a Superconducting Double Quantum Dot

We extend our approach based on the second order perturbation theory in the Coulomb interaction recently developed for quantum dots coupled to superconducting leads to the superconducting double quantum dot setups. Using our perturbative method we evaluate several single-particle quantities such as on-dot induced gap and generalized occupations together with the Andreev in-gap spectra and compare them with numerically exact results from the Numerical Renormalization Group and Quantum Monte Carlo finding a very good correspondence for not too strongly correlated regimes. Thus we can offer in a wide parameter range this method as an efficient and reliable alternative to the heavy numerical tools exclusively used so far for the description of such experimentally relevant systems.

cond-mat.mes-hall↗

arXiv2019

Gapless regime in the charge density wave phase of the finite dimensional Falicov-Kimball model

The ground-state density of states of the half-filled Falicov-Kimball model contains a charge-density-wave gap. At finite temperature, this gap is not immediately closed, but is rather filled in by subgap states. For a specific combination of parameters, this leads to a stable phase where the system is in an ordered charge-density-wave phase, but there is high density of states at the Fermi level. We show that this property can be, in finite dimensions, traced to a crossing of sharp states resulting from the single particle excitations of the localized subsystem. The analysis of the inverse participation ratio points to a strong localization in the discussed regime. However, the pronounced subgap density of states can still lead to a notable increase of charge transport through a finite size system. We show this by focusing on the transmission in heterostructures where a Falicov-Kimball system is sandwiched between two metallic leads.

cond-mat.str-el↗

arXiv2019

Nonequilibrium charge transport through Falicov-Kimball structures connected to metallic leads

Employing a combination of a sign-free Monte Carlo approach and nonequilibrium Green's function techniques, we study nonequilibrium charge transport in a model heterostructure, where a two-dimensional spin-less Falicov-Kimball system is coupled to two noninteracting leads. We show that the transport characteristic depends sensitively on the electrostatic potential in the system and exhibits different properties for different phases of the Falicov-Kimball model. In particular, pronounced step-like changes of the current and transmission are observed at the phase boundaries, evident even on a logarithmic scale. Analyzing finite size effects, we find that with the method used a relatively small system can be utilized to address specific thermodynamic limits.

cond-mat.str-el↗

arXiv2018

Transport theory for femtosecond laser-induced spin-transfer torques

Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser pulse is accompanied by a nonthermal spin-polarized current of hot electrons. These spin currents are studied here theoretically in a spin valve with noncollinear magnetizations. To this end, we introduce an extended model of superdiffusive spin transport that enables to treat noncollinear magnetic configurations, and apply it to the perpendicular spin valve geometry. We show how spin-transfer torques arise due to this mechanism and calculate their action on the magnetization present, as well as how the latter depends on the thicknesses of the layers and other transport parameters. We demonstrate that there exists a certain optimum thickness of the out-of-plane magnetized spin-current polarizer such that the torque acting on the second magnetic layer is maximal. Moreover, we study the magnetization dynamics excited by the superdiffusive spin-transfer torque due to the flow of hot electrons employing the Landau-Lifshitz-Gilbert equation. Thereby we show that a femtosecond laser pulse applied to one magnetic layer can excite small-angle precessions of the magnetization in the second magnetic layer. We compare our calculations with recent experimental results.

cond-mat.mes-hall↗

arXiv2017

Correlation effects in superconducting quantum dot systems

We study the effect of electron correlations on a system consisting of a single-level quantum dot with local Coulomb interaction attached to two superconducting leads. We use the single-impurity Anderson model with BCS superconducting baths to study the interplay between the proximity induced electron pairing and the local Coulomb interaction. We show how to solve the model using the continuous-time hybridization-expansion quantum Monte Carlo method. The results obtained for experimentally relevant parameters are compared with results of self-consistent second order perturbation theory as well as with the numerical renormalization group method.

cond-mat.mes-hall↗

arXiv2017

Quantum dot attached to superconducting leads: Relation between symmetric and asymmetric coupling

We study the Anderson single-level quantum dot attached to two BCS superconducting leads with the same gap size. We reveal that a system with asymmetric tunnel coupling to the leads ($Γ_{L}\neqΓ_{R}$) can be related to the symmetric system with the same net coupling strength $Γ=Γ_{L}+Γ_{R}$. Surprisingly, it is the symmetric case which is the most general, meaning that all physical quantities in case of asymmetric coupling are fully determined by the symmetric ones. We give ready-to-use conversion formulas for the $0-π$ phase transition boundary, on-dot quantities, and the Josephson current, and illustrate them on the NRG results of Oguri, Tanaka and Bauer [Phys. Rev. B 87, 075432 (2013)] for the three-terminal setup. We apply our theory to the recent $0-π$ transition measurement of Delagrange et al. [Phys. Rev. B 93, 196437 (2016)] and determine the asymmetry of the experimental setup from the measured transition width. Finally, we establish that the widely assumed Kondo "universality" of physical quantities depending only on the ratio of the Kondo temperature and the superconducting gap $T_{K}/Δ$ cannot hold for asymmetric junctions.

cond-mat.mes-hall↗

arXiv2017

Josephson-phase-controlled interplay between correlation effects and electron pairing in a three-terminal nanostructure

We study the subgap spectrum of the interacting single-level quantum dot coupled between two superconducting reservoirs, forming the Josephson-type circuit, and additionally hybridized with a metallic normal lead. This system allows for the phase-tunable interplay between the correlation effects and the proximity-induced electron pairing resulting in the singlet-doublet (0-$π$) crossover and the phase-dependent Kondo effect. We investigate the spectral function, induced local pairing, Josephson supercurrent, and Andreev conductance in a wide range of system parameters by the numerically exact Numerical Renormalization Group and Quantum Monte Carlo calculations along with perturbative treatments in terms of the Coulomb repulsion and the hybridization term. Our results address especially the correlation effects reflected in dependencies of various quantities on the local Coulomb interaction strength as well as on the coupling to the normal lead. We quantitatively establish the phase-dependent Kondo temperature $\log T_{K}(ϕ)\propto\cos^{2}(ϕ/2)$ and show that it can be read off from the half-width of the zero-bias enhancement in the Andreev conductance in the doublet phase, which can be experimentally measured by the tunneling spectroscopy.

cond-mat.mes-hall↗

arXiv2016

Perturbation theory for an Anderson quantum dot asymmetrically attached to two superconducting leads

Self-consistent perturbation expansion up to the second order in the interaction strength is used to study a single-level quantum dot with local Coulomb repulsion attached asymmetrically to two generally different superconducting leads. At zero temperature and wide range of other parameters the spin-symmetric version of the expansion yields excellent results for the position of the $0-π$ impurity quantum phase transition boundary and Josephson current together with the energy of Andreev bound states in the $0$-phase as confirmed by numerical calculations using the Numerical Renormalisation Group method. We analytically prove that the method is charge-conserving as well as thermodynamically consistent. Explicit formulas for the position of the $0-π$ phase boundary are presented for the Hartree-Fock approximation as well as for its variant called Generalized Atomic Limit. It is shown that the Generalized Atomic Limit can be used as a quick estimate for the position of the phase boundary at half-filling in a broad range of parameters. We apply our second order perturbation method to the interpretation of the existing experimental data on the phase boundary with very satisfactory outcome suggesting that the so far employed heavy numerical tools such as Numerical Renormalization Group and/or Quantum Monte Carlo are not necessary in a class of generic situations and can be safely replaced by a perturbative approach.

cond-mat.mes-hall↗

arXiv2013

Voltage noise, switching rates, and multiple phase-slips in moderately damped Josephson junctions

We study the voltage noise properties including the switching rates and statistics of phase-slips in moderately damped Josephson junctions using a novel efficient numerical approach combining the matrix continued-fraction method with the full counting statistics. By analyzing the noise results obtained for the RCSJ model we identify different dominating components, namely the thermal noise close to equilibrium (small current-bias regime), the shot noise of (multiple) phase-slips in the intermediate range of biases and the switching noise for yet higher bias currents. We extract thus far inaccessible characteristic rates of phase-slips in the shot noise regime as well as the escape and retrapping rates in the switching regime as functions of various junction's parameters. The method can be extended and applied to other experimentally relevant Josephson junction circuits.

cond-mat.supr-con↗

arXiv2012

Asymmetric voltage noise in superconducting tunnel junctions with electromagnetic environment

We investigate theoretically the V-I characteristics and voltage noise of superconducting tunnel junctions with small critical current via the matrix-continued-fraction method. Special attention is paid to the large hysteresis in the V-I characteristics and to the voltage-noise anomaly observed in preliminary experiments. The current dependence of the voltage noise shows a strong asymmetry between the forward and backward current ramping and a discontinuous change of the noise close to its measured maximum occurring at the switching current. We show that both the large hysteresis and the voltage noise anomaly in this current-biased setup are a consequence of the influence of junction's electromagnetic environment. Typically, the voltage-noise dependence on the junction current parametrized by the external drive contains a loop which is, however, not observed experimentally because of the implementation of the junction-current ramp. Skipping a part of the loop is responsible for the observed hysteresis as well as the noise anomaly.

cond-mat.mes-hall↗