arXiv subjects

Piotr Trocha

Publications and source records attributed to Piotr Trocha.

At least 19 recordsLinked to original sources

arXiv2024

Spin-polarized transport in quadruple quantum dots attached to ferromagnetic leads

Motivated by the experimental evidence of the Nagaoka ferromagnetism in quantum dot systems by Dehollain et al [1], we search for possible confirmation of such kind of ferromagnetism by analyzing the spin-resolved transport properties of a quadruple quantum dot system focusing on the linear response regime. In particular, we consider four quantum dots arranged in a two-by-two square lattice, coupled to external ferromagnetic source and drain electrodes. Turning on and off the specific conditions for the Nagaoka ferromagnetism to occur by changing the value of the intra-dot Coulomb interactions, we determine the transport coefficients, including the linear conductance, tunnel magnetoresistance and current spin polarization. We show that a sign change of the current spin polarization may be an indication of a ferromagnetic order of Nagaoka type which develops in the system.

cond-mat.mes-hall↗

arXiv2024

Thermal Generation of Spin Current in a Quantum Dot Coupled to Magnetic Insulators

In this work, we study thermally-generated spin current in the system consisting of a quantum dot connected to two magnetic insulators. The external leads are kept at different temperatures which leads to an imbalance of magnon populations in two magnetic insulators resulting in the flow of the magnon (spin) current. We take into account many-body magnon interactions and incorporate energy-dependent density of states of the magnetic insulators. Both features can strongly affect magnon distribution in the magnetic insulators and the coupling strengths between the leads and the dot, and thus, the thermally generated spin current. All the calculations are carried out in the weak coupling regime. We show, that results obtained with a density of states being a function of energy differ significantly from the ones obtained with a density of states taken as a constant. In turn, magnon interactions in the leads proved to be important at high temperatures and large values of energy of transported spin waves.

cond-mat.mes-hall↗

arXiv2018

Cross-correlations in a quantum dot Cooper pair splitter with ferromagnetic leads

We investigate Andreev transport through a quantum dot attached to two external ferromagnetic leads and one superconducting electrode. The transport properties of the system are studied by means of the real-time diagrammatic technique in the sequential tunneling regime. To distinguish various contributions to Andreev current we calculate the current cross-correlations, i.e. correlations between currents flowing through two junctions with normal leads. We analyze dependence of current cross-correlations on various parameters of the considered model, both in linear and nonlinear transport regimes. The processes and mechanisms leading to enhancement, suppression or sign change of current cross-correlations are examined and discussed. Interestingly, our results show that for specific transport regimes splitted Cooper pair results in two uncorrelated electrons. However, utilizing ferromagnetic leads instead of non-magnetic electrodes can result in positive current cross-correlations.

cond-mat.mes-hall↗

arXiv2017

The SU(4) Kondo effect in double quantum dots with ferromagnetic leads

We investigate the spin-resolved transport properties, such as the linear conductance and the tunnel magnetoresistance, of a double quantum dot device attached to ferromagnetic leads and look for signatures of SU(4) symmetry in the Kondo regime. We show that the transport behavior greatly depends on the magnetic configuration of the device, and the spin-SU(2) as well as the orbital and spin-SU(4) Kondo effects become generally suppressed when the magnetic configuration of the leads varies from the antiparallel to the parallel one. Furthermore, a finite spin polarization of the leads lifts the spin degeneracy and drives the system from the SU(4) to an orbital-SU(2) Kondo state. We analyze in detail the crossover and show that the Kondo temperature between the two fixed points has a non-monotonic dependence on the degree of spin polarization of the leads. In terms of methods used, we characterize transport by using a combination of analytical and numerical renormalization group approaches.

cond-mat.mes-hall↗

arXiv2017

Magnon transport through a quantum dot: Conversion to electronic spin and charge currents

We consider a single-level quantum dot coupled to magnetic insulators (magnonic reservoirs) and magnetic metals (electronic reservoirs). The whole system is in an external magnetic field. In a general case, the system includes two magnonic and two electronic reservoirs, but we also present results for some specific situations, where only two or three reservoirs are effectively connected to the dot. The main objective is the analysis of the conversion of magnon current to electronic spin and charge currents, and {\it vice versa}. We consider the limiting case of large Coulomb energy in the dot (Coulomb blockade), as well as the case when the Coulomb energy is finite and double occupancy is allowed.

cond-mat.mes-hall↗

arXiv2017

Spin-dependent thermoelectric phenomena in a quantum dot attached to ferromagnetic and superconducting electrodes

We investigate transport and thermoelectric properties of hybrid systems based on a single-level quantum dot and one superconducting lead. The other lead is generally normal-metallic ferromagnet. In the latter case single-particle transport is spin-polarized. Our main interest is in the interplay of Andreev tunneling of Cooper pairs and single-particle tunneling. The latter is responsible for relatively large thermopower and figure of merit due to a diverging density of single-particle states at the superconducting gap edges. System with ferromagnetic and superconducting leads can also reveal spin thermoelectric phenomena. Finite superconducting gap is considered within the BCS theory, and the thermoelectric coefficients are calculated by means of nonequilibrium Green's function technique within Hartree-Fock like approximation with respect to the intradot Coulomb interaction.

cond-mat.mes-hall↗

arXiv2017

Current cross-correlations in double quantum dot based Cooper pair splitters with ferromagnetic leads

We investigate the current cross-correlations in a double quantum dot based Cooper pair splitter coupled to one superconducting and two ferromagnetic electrodes. The analysis is performed by assuming a weak coupling between the double dot and ferromagnetic leads, while the coupling to the superconductor is arbitrary. Employing the perturbative real-time diagrammatic technique, we study the Andreev transport properties of the device, focusing on the Andreev current cross-correlations, for various parameters of the model, both in the linear and nonlinear response regimes. Depending on parameters and transport regime, we find both positive and negative current cross-correlations. Enhancement of the former type of cross-correlations indicates transport regimes, in which the device works with high Cooper pair splitting efficiency, contrary to the latter type of correlations, which imply negative influence on the splitting. The processes and mechanisms leading to both types of current cross-correlations are thoroughly examined and discussed, giving a detailed insight into the Andreev transport properties of the considered device.

cond-mat.mes-hall↗

arXiv2017

Spin-dependent thermoelectric effects in a strongly correlated double quantum dot

We investigate spin-dependent thermoelectric transport through a system of two coupled quantum dots attached to reservoirs of spin-polarized electrons. Generally, we focus on the strongly correlated regime of transport. To this end, a slave-boson method for finite $U$ is employed. Our main goal is to show that, apart from complex low-temperature physics, such a basic multilevel system provides a possibility to examine various quantum interference effects, with particular emphasis put on the influence of such phenomena on thermoelectric transport. Apart from the influence of interference effects on spin-degenerate charge transport, we show how spin-dependent transport, induced by ferromagnetic leads, can be modified as well. Finally, we also consider the case where the spin-relaxation time in the ferromagnetic leads is relatively long, which leads to the so-called spin thermoelectric effects.

cond-mat.mes-hall↗

arXiv2015

Spin-resolved Andreev transport through double-quantum-dot Cooper pair splitters

We investigate the Andreev transport through double-quantum-dot Cooper pair splitters with ferromagnetic leads. The analysis is performed with the aid of the real-time diagrammatic technique in the sequential tunneling regime. We study the dependence of the Andreev current, the differential conductance, and the tunnel magnetoresistance on various parameters of the model in both the linear and nonlinear response regimes. In particular, we analyze the spin-resolved transport in the crossed Andreev reflection regime, where a blockade of the current occurs due to enhanced occupation of the triplet state. We show that in the triplet blockade, finite intradot correlations can lead to considerable leakage current due to direct Andreev reflection processes. Furthermore, we find additional regimes of current suppression resulting from enhanced occupation of singlet states, which decreases the rate of crossed Andreev reflection.We also study how the splitting of Andreev bound states, triggered by either dot level detuning, finite hopping between the dots, or finite magnetic field, affects the Andreev current. While in the first two cases the number of Andreev bound states is doubled, whereas transport properties are qualitatively similar, in the case of finite magnetic field further level splitting occurs, leading to a nontrivial behavior of spin-resolved transport characteristics, and especially that of tunneling magnetoresistance. Finally, we discuss the entanglement fidelity between split Cooper pair electrons and show that by tuning the device parameters, fidelity can reach unity.

cond-mat.mes-hall↗

arXiv2014

Spin-dependent thermoelectric properties of a Kondo-correlated quantum dot with Rashba spin-orbit coupling

Thermoelectric transport phenomena in a single-level quantum dot coupled to ferromagnetic leads are considered theoretically in the Kondo regime. The dot is described by the Anderson model with Rashba type spin-orbit interaction. The finite-U mean field slave boson technique is used to describe the transport characteristics, like heat conductance, thermopower, thermoelectric efficiency (figure of merit). The role of quantum interference effects in thermoelectric parameters is also analyzed.

cond-mat.mes-hall↗

arXiv2014

Spin-polarized Andreev transport influenced by Coulomb repulsion through two quantum dot system

Spin-polarized transport through a double quantum dot system attached to a common superconducting lead and two ferromagnetic electrodes (fork geometry) is investigated theoretically. The key objective of the analysis is to describe the influence of electrodes' ferromagnetism on the Andreev tunneling. Both direct and crossed Andreev tunneling processes are considered, in general. The other objective is a detailed analysis of the role of Coulomb interaction and its impact on the Andreev tunneling processes.

cond-mat.mes-hall↗

arXiv2012

Large enhancement of thermoelectric effects in a double quantum dot system due to interference and Coulomb correlation phenomena

Thermoelectric effects in a double quantum dot system coupled to external magnetic/nonmagnetic leads are investigated theoretically. The basic thermoelectric transport characteristics, like thermopower, electronic contribution to heat conductance, and the corresponding figure of merit, have been calculated in terms of the linear response theory and Green function formalism in the Hartree-Fock approximation for Coulomb interactions. An enhancement of the thermal efficiency (figure of merit ZT) due to Coulomb blockade has been found. The magnitude of ZT is further considerably enhanced by quantum interference effects. Both the Coulomb correlations and interference effects lead to strong violation of the Wiedemann-Franz law. The influence of spin-dependent transport and spin bias on the thermoelectric effects (especially on Seebeck and spin Seebeck effects) is also analyzed.

cond-mat.mes-hall↗

arXiv2011

The role of the indirect tunneling processes and asymmetry in couplings in orbital Kondo transport through double quantum dots

System of two quantum dots attached to external electrodes is considered theoretically in orbital Kondo regime. In general, the double dot system is coupled via both Coulomb interaction and direct hoping. Moreover, the indirect hopping processes between the dots (through the leads) are also taken into account. To investigate system's electronic properties we apply slave-boson mean field (SBMF) technique. With help of the SBMF approach the local density of states for both dots and the transmission (as well as linear and differencial conductance) is calculated. We show that Dicke- and Fano-like line shape may emerge in transport characteristics of the double dot system. Moreover, we observed that these modified Kondo resonances are very susceptible to the change of the indirect coupling's strength. We have also shown that the Kondo temperature become suppressed with increasing asymmetry in the dot-lead couplings when there is no indirect coupling. Moreover, when the indirect coupling is turned on the Kondo temperature becomes suppressed. By allowing a relative sign of the nondiagonal elements of the coupling matrix with left and right electrode, we extend our investigations become more generic. Finally, we have also included the level renormalization effects due to indirect tunneling, which in most papers is not taken into account.

cond-mat.mes-hall↗

arXiv2011

The influence of spin-flip scattering on the preparation and detection of a single spin state in a quantum dot attached to a spin battery

Recently, the possibility of an all electrical scheme of preparation and readout for a single spin state in a single quantum dot attached to spin biased leads has been shown [F. Chi et al., Phys. Rev. B 81, 075310 (2010)]. However, spin scattering mechanisms have been omitted. To remedy this lack we consider the influence of the spin-flip scattering process on the proposed preparation and readout scheme.

cond-mat.mes-hall↗

arXiv2011

Beating in electronic transport through quantum dot based devices

Electronic transport through a two-level system driven by external electric field and coupled to (magnetic or non-magnetic) electron reservoirs is considered theoretically. The basic transport characteristics such as current and tunnel magnetoresistance (TMR) are calculated in the weak coupling approximation by the use of rate equation connected with Green function formalism and slave-boson approach. The time dependent phenomenon is considered in the gradient expansion approximation. The results show that coherent beats pattern can be observed both in current and TMR. The proposed system consisting of two quantum dots attached to external leads, in which the dots' levels can be tuned independently, can be realized experimentally to test this well known physical phenomenon. Finally, we also indicate possible practical applications of such device.

cond-mat.mes-hall↗

arXiv2010

Orbital Kondo effect in double quantum dots

Orbital Kondo effect in a system of two single-level quantum dots attached to external electron reservoirs is considered theoretically. The dots are coupled via direct hoping term and Coulomb interaction. The Kondo temperature is evaluated from the scaling approach and slave boson technique. The later method is also used to calculate linear conductance of the system. Nonlinear conductance, in turn, is calculated in terms of the nonequilibrium Green function formalism.

cond-mat.mes-hall↗

arXiv2009

Negative tunnel magnetoresistance and differential conductance in transport through double quantum dots

Spin-dependent transport through two coupled single-level quantum dots weakly connected to ferromagnetic leads with collinear magnetizations is considered theoretically. Transport characteristics, including the current, linear and nonlinear conductance, and tunnel magnetoresistance are calculated using the real-time diagrammatic technique in the parallel, serial, and intermediate geometries. The effects due to virtual tunneling processes between the two dots via the leads, associated with off-diagonal coupling matrix elements, are also considered. Negative differential conductance and negative tunnel magnetoresistance have been found in the case of serial and intermediate geometries, while no such behavior has been observed for double quantum dots coupled in parallel. It is also shown that transport characteristics strongly depend on the magnitude of the off-diagonal coupling matrix elements.

cond-mat.mes-hall↗

arXiv2008

Kondo-Dicke resonances in electronic transport through triple quantum dots

Electronic transport through a triple quantum dot system, with only a single dot coupled directly to external leads, is considered theoretically. The model includes Coulomb correlations in the central dot, while such correlations in the two side-coupled dots are omitted. The infinite-U mean-field slave-boson approach is used to obtain basic transport characteristics in the Kondo regime. When tuning position of the side-coupled dots' levels, transition from subradiant to superradiant like mode (and vice versa) has been found in the spectral function, in analogy to the Dicke effect in atomic physics. Bias dependence of the differential conductance and zero frequency shot noise is also analysed.

cond-mat.mes-hall↗