arXiv subjects

Krzysztof P. Wójcik

Publications and source records attributed to Krzysztof P. Wójcik.

13 recordsLinked to original sources

arXiv2026

Kondo singlet from ferromagnetic coupling: an analog of Anderson-Morel superconductivity in the magnetic channel

We consider magnetic impurities coupled to a conduction sea via a fully isotropic ferromagnetic spin-exchange term, the strength of which depends on the conduction-electron modes involved in the scattering. In the single-impurity case we show both analytically and numerically that there exists a parameter regime in which the conventional Kondo effect develops at low temperatures, leading to a singlet ground state. In the case of a lattice of impurities, we show that this leads to a heavy Fermi liquid state that is energetically favored over magnetic ordering in a broad parameter range. We argue that these effects are analogs of Anderson-Morel superconductivity, and discuss routes to their experimental realization.

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arXiv2026

Inverted anisotropy of the partially screened magnetic impurity

We investigate a single magnetic impurity in the presence of strong spin-orbit coupling and single-ion anisotropy. We show that at sufficiently strong coupling there exists a finite temperature window, before the moment is completely screened, where the magnetic anisotropy of the system flips: the hard-axis becomes the easy-axis or vice versa. We derive this rigorously for a single impurity using numerical renormalization group calculations as well as Nozieres' strong-coupling limit and discuss its relevance to heavy-fermion compounds which order magnetically along the hard-direction. We show that the coexistence of Curie-like response and Kondo fluctuations is stabilized along the initially hard direction leading to the anisotropy switch.

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arXiv2024

Signatures of Kondo-Majorana interplay in ac response

We analyze dynamical transport properties of a hybrid nanostructure, comprising a correlated quantum dot embedded between the source and drain electrodes, which are subject to an ac voltage, focusing on signatures imprinted on the charge transport by the side-attached Majorana zero-energy mode. The considerations are based on the Kubo formula, for which the relevant correlation functions are determined by using the numerical renormalization group approach, which allows us to consider the correlation effects due to the Coulomb repulsion and their interplay with the Majorana mode in a non-perturbative manner. We point out universal features of the dynamical conductance, showing up in the Kondo-Majorana regime, and differentiate them against the conventional Kondo and Majorana systems. In particular, we predict that the Majorana quasiparticles give rise to universal fractional values of the ac conductance in the well-defined frequency range below the peak at the Kondo scale. We also show this Kondo scale to actually increase with strengthening the coupling to the topological superconducting wire.

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arXiv2023

Majorana coupling and Kondo screening of localized spins

We perform a theoretical analysis of the fate of local magnetic moment of a quantum dot coupled to a normal metallic lead and a topological superconducting wire hosting Majorana modes at the ends. By means of simple analytical tools and numerical renormalization group calculations we show that the proximity of Majorana mode reduces the magnetic moment from $1/4$, characteristic of a free spin $1/2$, to $1/16$. The coupling to the normal lead then causes the Kondo effect, such that the magnetic moment is fully screened below the Kondo temperature. The latter is vastly increased for strong coupling to Majorana mode.

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arXiv2022

Quantum spin liquid in an RKKY-coupled two-impurity Kondo system

We consider a 2-impurity Kondo system with spin-exchange coupling within the conduction band. Our numerical renormalization group calculations show that for strong intraband spin correlations the competition of these correlations with Kondo spin screening stabilizes a metallic spin-liquid phase of the localized spins without geometric frustration. For weak Kondo coupling the spin liquid and the Kondo singlet phase are separated by two quantum phase transitions and an intermediate RKKY spin-dimer phase, while beyond a critical coupling they are connected by a crossover. The results suggest how a quantum spin liquid may be realized in heavy-fermion systems near a spin-density wave instability.

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arXiv2022

Asymmetry effects on the phases of RKKY-coupled two-impurity Kondo systems

In a related work [arXiv:2106.07519] we have shown that in the two-impurity Anderson (2iA) model with two hosts coupled by spin exchange in the most symmetric case there are either two phase transitions or none. The phases comprise the conventional Kondo and RKKY regimes and a novel one, interpreted as a Kondo-stabilized, metallic quantum spin liquid (QSL). Here we analyze how various types of asymmetry affect this picture. We demonstrate that the transitions are robust against the coupling and particle-hole asymmetries, provided charge transfer is forbidden. This holds true despite the scattering phase shift at each impurity taking non-universal values. Finally, for an extended model including charge transfer between the hosts and a small Coulomb interaction at the host sites directly coupled to impurities, we show that the presence of charge transfer changes the phase transitions into crossovers. Provided the inter-host hopping is sufficiently small, this leads to qualitatively the same physics at non-zero temperature. The relevance of this model for rare-earth atoms in a metallic host is discussed and potential experimental setups for observing our findings are proposed.

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arXiv2022

Spin-resolved thermal signatures of Majorana-Kondo interplay in double quantum dots

We investigate theoretically the thermoelectric transport properties of a T-shaped double quantum dot side-coupled to a topological superconducting nanowire hosting Majorana zero-energy modes. The calculations are performed using the numerical renormalization group method focusing on the transport regime, where the system exhibits the two-stage Kondo effect. It is shown that the leakage of Majorana quasiparticles into the double dot system results in a half-suppression of the second stage of the Kondo effect, which is revealed through fractional values of the charge and heat conductances and gives rise to new resonances in the Seebeck coefficient. The heat conductance is found to satisfy a modified Wiedemann-Franz law. Finally, the interplay of Majorana-induced interference with strong electron correlations is discussed in the behavior of the spin Seebeck effect, a unique feature of the considered setup.

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arXiv2020

Magnetic Kondo regimes in a frustrated half-filled trimer

We analyze theoretically the phase diagram of a triangular triple quantum dot with strong onsite repulsion coupled to ferromagnetic leads. This model includes the competition of magnetic ordering of local or itinerant magnetic moments, geometric frustration and Kondo screening. We identify all the phases resulting from this competition. We find that three Kondo phases -- the conventional one, the two-stage underscreened one, and the one resulting from the ferromagnetic Kondo effect -- can be realized at zero temperature, and all are very susceptible to the proximity of ferromagnetic leads. In particular, we find that the quantum dots are spin-polarized in each of these phases. Further, we discuss the fate of the phases at non-zero temperatures, where a plethora of competing energy scales gives rise to complex landscape of crossovers. Each Kondo regime splits into a pair of phases, one not magnetized and one comprising magnetically polarized quantum dots. We discuss our results in the context of heavy-fermion physics in frustrated Kondo lattices.

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arXiv2020

Majorana-Kondo interplay in T-shaped double quantum dots

The transport behavior of a double quantum dot side-attached to a topological superconducting wire hosting Majorana zero-energy modes is studied theoretically in the strong correlation regime. It is shown that Majorana modes can leak to the whole nanostructure, giving rise to a subtle interplay between the two-stage Kondo screening and the half-fermionic nature of Majorana quasiparticles. In particular, the coupling to the topological wire is found to reduce the effective exchange interaction between the two quantum dots in the absence of normal leads. Interestingly, it also results in an enhancement of the second-stage Kondo temperature when the normal leads are attached. Moreover, it is shown that the second stage of the Kondo effect can become significantly modified in one of the spin channels due to the interference with the Majorana zero-energy mode, yielding the low-temperature conductance equal to $G=G_0/4$, where $G_0 = 2e^2/h$, instead of $G=0$ in the absence of the topological superconducting wire. We also identify a nontrivial spin-charge ${\rm SU}(2)$ symmetry present in the system at a particular point in parameter space, despite lack of the spin nor charge conservation. Finally, we discuss the consequences of a finite overlap between two Majorana modes, as relevant for short Majorana wires.

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arXiv2019

Giant superconducting proximity effect on spintronic anisotropy

We investigate theoretically the interplay of proximity effects due to the presence of a superconductor and normal ferromagnetic leads on the formation of the spintronic quadrupolar exchange field in a large-spin molecule. We show that the spintronic anisotropy can be enhanced by a few orders of magnitude, and tuned by changing the strength of coupling to the superconductor. Especially large anisotropy is generated in the vicinity of the charge parity changing transition of the molecule. We also provide predictions of measurable spectral properties being the hallmarks of these phenomena.

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arXiv2019

Nonlocal Andreev Reflections as a Source of Spin Exchange and Kondo Screening

We report on a novel exchange mechanism, mediating the Kondo screening, in a correlated double quantum dot structure in the proximity of superconductor, stemming from nonlocal Andreev reflection processes. We estimate its strength perturbatively and corroborate analytical predictions with accurate numerical renormalization group calculations using an effective model for the superconductor-proximized nanostructure. We demonstrate that superconductor-induced exchange leads to a two-stage Kondo screening. We determine the dependence of the Kondo temperature on the coupling to superconductor and predict a characteristic modification of conventional low-temperature transport behavior, which can be used to experimentally distinguish this phenomenon from other Kondo effects.

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arXiv2018

Interplay of the Kondo effect with the induced pairing in electronic and caloric properties of T-shaped double quantum dots

We examine the influence of the superconducting proximity effect on the transport properties of a T-shaped double quantum dot strongly coupled to two normal, nonmagnetic or ferromagnetic leads. We show that the two-stage Kondo screening may be suppressed or enhanced by the presence of pairing correlations, depending on the specific geometric arrangement of the device. We explain our results by invoking effective decrease of Coulomb interactions by proximity effect and find qualitatively correct description in many cases, although spin-filtering effect stemming from spin-dependent Fano-Kondo interference occurs to be surprisingly fragile to the presence of induced pairing. The results are obtained within the numerical renormalization group framework in the limit of large superconducting gap, which allows for a reliable examination of the low-temperature sub-gap properties of the considered system. Nevertheless, finite temperature effects are also taken into account.

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arXiv2016

Strong spin Seebeck effect in Kondo T-shaped double quantum dots

We theoretically investigate the thermoelectric and spin thermoelectric properties of a T-shaped double quantum dot strongly coupled to two ferromagnetic leads, focusing on transport regime where the system exhibits the two-stage Kondo effect. We study the dependence of the (spin) Seebeck coefficient, the corresponding power factor and the figure of merit on temperature, leads' spin polarization and dot level position. We show that the thermal conductance fulfills a modified Wiedemann-Franz law. We also demonstrate that the spin thermopower is enhanced at temperatures corresponding to the second stage of Kondo screening. Very interestingly, the spin-thermoelectric response of the system is found to be highly sensitive to the spin polarization of the leads. In some cases spin polarization of the order of 1% is sufficient for a strong spin Seebeck effect to occur. This is explained as a consequence of the interplay between the two-stage Kondo effect and the exchange field induced in the double quantum dot. All calculations are performed with the aid of numerical renormalization group technique.

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