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1
Content available remote

Temperature Dependence of the Zero-Bias Conductance in the Graphene NIS Junction

100%
Wysokiński M.
Acta Physica Polonica A
|
2014
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vol. 126
|
issue 4a
A-36-A-39
EN
The temperature dependence of the zero-bias conductance of the graphene-based, ballistic junction composed of the three consecutive regions: normal, with potential barrier ("insulating") and superconducting (NIS), is analyzed within the extended Blonder-Tinkham-Klapwijk approach. Within this approach we have found that oscillatory behavior of the conductance as a function of barrier strength is suppressed by the temperature - the amplitude diminishes with heating up the junction. Moreover, the subtle, although nontrivial feature of the system is reported: the average over the period of the oscillations of the zero-bias conductance for relatively small Fermi level mismatch behaves non-monotonically with the increase of the temperature with the maximum roughly at T/T_{c} ≈ 0.5.
2
Content available remote

Andreev Transport in Double Quantum Dot Cooper Pair Splitters in the Presence of External Magnetic Field

80%
Trocha P., Weymann I.
Acta Physica Polonica A
|
2015
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vol. 127
|
issue 2
502-504
EN
The effect of external magnetic field on the transport properties of double quantum dots coupled to normal and superconducting leads is studied by means of the real-time diagrammatic technique in the sequential tunneling regime. This device works as a gate-controlled Cooper pair splitter. We focus on the transport regime where the current is blocked due to the spin triplet blockade. It is shown that external magnetic field can modify the Andreev current and differential conductance. In particular, magnetic field can suppress the negative differential conductance associated with the triplet blockade.
3
Content available remote

Tunneling Charge Transport in Graphene-Based Superconductor Junctions

80%
Wojciechowski R., Kowalewski L., Nogala M.
Acta Physica Polonica A
|
2012
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vol. 121
|
issue 5-6
1092-1094
EN
We study the spin polarized electron and hole tunneling transport through a graphene-based ferromagnet(GF_1)-insulator(GI_1)-superconductor(G_{S})-insulator(GI_2)-ferromagnet(GF_2) junction. Proximity induced spin polarization and superconductivity in a graphene sheet are assumed to be created by superconducting and ferromagnetic electrodes placed on the top of the graphene. Using a four-dimensional version of the Dirac-Bogoliubov-de Gennes equation with appropriate boundary conditions we investigate the tunneling processes through the junctions. In particular, we present calculations of the amplitudes of normal and Andreev reflections as a function of the energy of the incident electron for a wide range of the model parameters, such as the strength and orientation of the exchange field, the barrier strength, and the distance between the two ferromagnetic layers. The tunneling transport processes in the graphene-based double junction GF/GI/G_{S}/GI/GF are compared with those in non-graphene-based junctions.
4
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Andreev Spectroscopy in Three-Terminal Hybrid Nanostructure

80%
Michałek G., Bułka B., Urbaniak M., Domański T., Wysokiński K.
Acta Physica Polonica A
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2015
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vol. 127
|
issue 2
293-295
EN
We consider a hybrid three-terminal structure consisting of a quantum dot coupled to three leads, two normal and one superconducting. The current flowing between one of the normal and the superconducting electrodes induces voltage in the other normal (floating) electrode. The value of the induced voltage depends on the position of the Andreev levels in the quantum dot and is a measure of the interplay between the electron transfer and the subgap anomalous reflection processes. When the crossed (i.e. non-local) Andreev reflections dominate the induced potential in the normal electrode becomes negative. This situation occurs for the relatively strong coupling to the superconducting electrode and only outside the Coulomb blockade region. Taking the superconducting electrode as the voltage probe one gets additional information on competition of injected carriers.
5
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Spin Dependent Conductance of a Quantum Dot Side attached to Topological Superconductors as a Probe of Majorana Fermion States

80%
Krychowski D., Lipiński S., Cuniberti G.
Acta Physica Polonica A
|
2018
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vol. 133
|
issue 3
552-554
EN
Spin-polarized transport through a quantum dot side attached to a topological superconductor and coupled to a pair of normal leads is discussed in Coulomb and Kondo regimes. For discussion of Coulomb range equation of motion method with extended Hubbard I approximation is used and Kondo regime is analyzed by Kotliar-Ruckenstein slave boson approach. Apart from the occurrence of zero bias anomaly the presence of Majorana states reflects also in splitting of Coulomb lines. In the region of Coulomb borders the spin dependent negative differential conductance is observed. Due to the low energy scale of Kondo effect this probe allows for detection of Majorana states even for extremely weak coupling with topological wire. In this range no signatures of Majorana states appear in Coulomb blockade dominated transport.
6
Content available remote

Threshold Effects in Three-Terminal Hybrid Systems

80%
Michałek G.
Acta Physica Polonica A
|
2014
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vol. 126
|
issue 1
224-225
EN
In the paper we study transport in the three terminal hybrid devices with quantum dot. Our attention is focused on the conductance threshold effects (so called Wigner singularities) caused by the back action from the side-wire. In particular we study influence of the direct electron transfer (ET) and crossed Andreev reflection (CAR) processes on the conductance caused by the direct Andreev reflection (DAR) processes.
7
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Thermoelectric Effect in the Normal Conductor-Superconductor Junction: A BTK Approach

80%
Wysokiński M.
Acta Physica Polonica A
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2012
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vol. 122
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issue 4
758-764
EN
The thermopower of the junction between normal conductor and s-wave superconductor has been investigated. For this purpose we have analyzed in detail a simple generalization of the Blonder-Tinkham-Klapwijk theory by taking into account explicitly an energy dependence of the density of states near the Fermi level. Both linear and nonlinear thermopowers have been calculated for 3D free electron gas, 3D Fermi liquid, and the case with Van Hove singularity in the vicinity of the Fermi level. In the linear regime, for all models, the thermopower as function of temperature has a clear maximum with its position and the value depending strongly on the junction barrier strength. In the nonlinear regime, we have found very large values of the thermopower (up to 8 k_{B}/e) and strongly asymmetric behavior with respect to the change of the temperature gradient sign.
8
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Spin Dependent Electron Tunneling in a Ferromagnet Superconductor Ferromagnet Junction

80%
Kowalewski L., Wojciechowski R.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
257-259
EN
The coherent spin polarized transport in ferromagnet/superconductor/ferromagnet double barrier junctions is analyzed. Using the Bogolubov-de Gennes equation with appropriate boundary conditions, we calculate probabilities of spin dependent transport processes in the ferromagnet/superconductor/ferromagnet junction. In particular, we discuss the nonlocal processes such as the crossed Andreev reflection and elastic co-tunneling. These processes contribute to tunneling a current when the distance between the two magnetic electrodes is comparable to the superconducting coherence length. The dependences of the tunneling transport processes on the strength of the exchange field in the ferromagnetic electrodes, and on the height of the tunnel barriers are presented.
9
Content available remote

Andreev Conductance through a Quantum Dot Strongly Coupled to Ferromagnetic and Superconducting Leads

80%
Wójcik K. P., Weymann I.
Acta Physica Polonica A
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2017
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vol. 132
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issue 1
143-145
EN
We analyze the Andreev conductance through a quantum dot strongly coupled to one ferromagnetic and one superconducting lead. The transmission due to the Andreev reflection is obtained from the numerical renormalization group method. We show that at low temperatures, depending on the dot level position, the Andreev conductance exhibits a peak at zero bias due to the Kondo effect, which can be split by the exchange field due to spin-dependent coupling to ferromagnetic lead.
10
Content available remote

Andreev Transport through a Magnetic Molecule Weakly Coupled to Ferromagnetic Leads

80%
Pawlicki F., Weymann I.
Acta Physica Polonica A
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2018
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vol. 133
|
issue 3
594-596
EN
The Andreev transport through a single molecular magnet coupled to two external ferromagnetic leads and one superconducting electrode is studied theoretically by means of the real-time diagrammatic technique. The calculations are performed by including the sequential tunneling processes between the molecule and ferromagnetic leads, while the coupling to superconductor can be arbitrary. We analyze the dependence of the Andreev current and tunnel magnetoresistance on various intrinsic parameters of the molecule. The superconducting proximity effect results in the formation of molecular Andreev bound states. We show that the transport behavior depends greatly on the type of internal exchange interaction of the molecule, which can lead to corresponding sign changes of the tunnel magnetoresistance of the device.
11
Content available remote

Josephson Effect in Graphene-Based Junctions

80%
Wojciechowski R., Kowalewski L.
Acta Physica Polonica A
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2018
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vol. 133
|
issue 3
632-634
EN
We study the Josephson effect in graphene based junctions where superconductivity in graphene is induced by the proximity effect from external substrate materials. The electronic properties of the junction is described by the Dirac-Bogoliubov-de-Gennes equations. We consider the junction consisting of two superconductors with different pairing potentials. Using appropriate boundary conditions imposed on the normal region-superconductors interfaces, we calculated the Andreev bound state energy, in the ballistic limit, taking into account two types of reflections namely the retro and specular Andreev reflections.
12
Content available remote

Dynamics of Hotspot Formation in Nanostructured Superconducting Stripes Excited with Single Photons

80%
Jukna A., Kitaygorsky J., Pan D., Cross A., Perlman A., Komissarov I., Sobolewski R., Okunev O., Smirnov K., Korneev A., Chulkova G., Milostnaya I., Voronov B., Gol'tsman G.
Acta Physica Polonica A
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2008
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vol. 113
|
issue 3
955-958
EN
Dynamics of a resistive hotspot formation by near-infrared-wavelength single photons in nanowire-type superconducting NbN stripes was investigated. Numerical simulations of ultrafast thermalization of photon-excited nonequilibrium quasiparticles, their multiplication and out-diffusion from a site of the photon absorption demonstrate that 1.55 μm wavelength photons create in an ultrathin, two-dimensional superconducting film a resistive hotspot with the diameter which depends on the photon energy, and the nanowire temperature and biasing conditions. Our hotspot model indicates that under the subcritical current bias of the 2D stripe, the electric field penetrates the superconductor at the hotspot boundary, leading to suppression of the stripe superconducting properties and accelerated development of a voltage transient across the stripe.
13
Content available remote

The In-Gap Charge Current through the Correlated Quantum Dot Hybridized with Superconductor

80%
Donabidowicz A., Domański T.
Acta Physica Polonica A
|
2008
|
vol. 114
|
issue 1
75-82
EN
We explore the Andreev tunnelling through the strongly correlated quantum dot embedded between the normal and superconducting electrodes. For a small external voltage |eV| <∆_s the electron arriving from the normal lead can be converted into a pair on the quantum dot and further propagates in the superconducting lead while simultaneously the hole is reflected back to the normal electrode. Conductance of such anomalous current is very sensitive to the particle-hole mixing of the quantum dot spectrum. We analyze the influence of the proximity effect and the Coulomb interactions on the differential Andreev conductance focusing on the extreme limit ∆_s → ∞.
14
Content available remote

Single Particle Excitation Spectrum of a Proximized Quantum Dot: The Flow Equation Study

80%
Zapalska M., Domański T.
Acta Physica Polonica A
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2014
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vol. 126
|
issue 4a
A-137-A-140
EN
We study the in-gap states of a quantum dot hybridized with the metallic and superconducting reservoirs applying the continuous unitary transformation to the Anderson-type Hamiltonian. We derive the set of flow equations and analyse the effective single particle excitation spectrum of the correlated quantum dot in presence of the induced electron pairing.
15
Content available remote

Superconductivity of Niobium Thin Films in the BiOCl/Nb Heterostructures

80%
Lotnyk D., Komanický V., Bunda V., Feher A.
Acta Physica Polonica A
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2017
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vol. 131
|
issue 4
1030-1032
EN
In the current paper, electrical transport properties of 25 nm thick Nb films sputtered on the photosensitive semiconductor BiOCl were investigated in the temperature range 7.5 K ≤ T ≤8.5 K. The influence of green (532 nm) and red (640 nm) laser excitations on resistive superconducting transitions of the niobium thin films on a silicon glass and BiOCl single crystal substrates were studied. The temperature dependences of the resistivity for Nb are in good agreement with the McMillan model which indicates the strong influence of the inverse proximity effect induced by the interface. The increased influence of the BiOCl/Nb interface under laser excitation corresponds to the raising of the ratio of the density of normal to superconductivity carriers in the Tıghtarrow0 limit and this observation is in agreement with the photoconductivity study of BiOCl single crystals.
16
Content available remote

Statistics of Tunneling Events in Three-Terminal Hybrid Devices with Quantum Dot

80%
Michałek G., Bułka B., Domański T., Wysokiński K.
Acta Physica Polonica A
|
2018
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vol. 133
|
issue 3
391-393
EN
We investigate statistics of the tunneling events in the short time limit in terms of the waiting time distribution (WTD), defined as the probability for a delay time between two subsequent transitions of particles, and consider it for a quantum dot (QD) strongly coupled to a superconducting and weakly coupled to two normal electrodes. Our study focuses on the WTD in the subgap transport, when coherent exchange of the Cooper pairs occurs between the QD and the superconductor. The dynamics can be described in terms of a Markovian generalized master equation for the reduced density matrix. We observe coherent oscillations between the Andreev bound states in the correlated jumps, both for the local and non-local WTDs. In addition the analysis of the transient currents give us some insight into dominant relaxation processes in short time scales.
17
Content available remote

Interference Effects in Three-Terminal Hybrid Systems with Quantum Dots

80%
Michałek G., Bułka B.
Acta Physica Polonica A
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2012
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vol. 122
|
issue 6
981-983
EN
Conductance in a three-terminal hybrid system with two quantum dots is analyzed. Our attention is focused on an influence of decoherence on interference effects in the Andreev transport. In particular, we have found that a change of coupling to the third electrode can strongly modify a shape of the Fano-type resonances. This effect is due to activation of nonlocal Andreev reflection (CAR) processes.
18
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Content available

Resonance Tunneling in Ferromagnet-Triplet Superconductors Junctions

80%
Kowalewski L., Wojciechowski R., Wojtuś P.
Acta Physica Polonica A
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2007
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vol. 111
|
issue 5
683-691
EN
We discuss the tunneling conductance in a ferromagnet-insulator-triplet superconductor junction. We consider the superconducting order parameters with spin triplet pairing having nodes. The nodal structure of the order parameter has been recently confirmed experimentally in Sr_2RuO_4. In particular, we study how a mid-gap structure of the tunnelling conductance depends on the phase difference of the pairing potential as well as on the orientation of the interface.
19
Content available remote

Microwave Absorption Study on (Bi,Pb)-Sr-Ca-Cu-O Granular Superconductors

80%
Jurga W., Piekara-Sady L., Gazda M.
Acta Physica Polonica A
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2008
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vol. 114
|
issue 1
253-256
EN
(Bi,Pb)-Sr-Ca-Cu-O is considered as a system of 2201, 2212 and 2223 superconductors embedded in the insulating matrix. The size of the grains depends on the time of recrystallization. These types of ceramics exhibit a two-step transition to superconducting state. Because electrical properties depend among other on the Josephson coupling between grains, the magnetically modulated microwave absorption study was undertaken. Magnetically modulated microwave absorption signal was observed to arise just as temperature had been lowered below T₁. The shape of this signal was studied to recognize the second temperature T₂. Some strong oscillations appear on magnetically modulated microwave absorption at lower temperatures, which might be related to local percolation breakdown in superconducting network.
20
Content available remote

Interplay between the Correlations and Superconductivity in Electron Transport through the Double Quantum Dots

80%
Barański J., Domański T.
Acta Physica Polonica A
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2012
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vol. 121
|
issue 5-6
1213-1215
EN
We study the influence of electron correlations on nonequilibrium transport through the quantum dots coupled between one metallic and one superconducting electrode. Such type of nanodevices are characterized by the induced on-dot pairing spread from the superconducting lead (proximity effect) and effectively responsible for mixing the particle with hole excitations. On the other hand, strong Coulomb repulsion between the opposite spin electrons tends to suppress a double occupancy of the quantum dots competing with the on-dot superconducting order. The Coulomb interactions can also cause a screening of the quantum dot spin by itinerant electrons of the metallic lead giving rise to formation of the Kondo resonance. We analyze interplay of such phenomena for the setup of double quantum dots where the quantum interference (such as Fano) effects influence the subgap electron transport through the Andreev scattering.
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