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1
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Conductance of Mesoscopic Magnetic Systems

100%
Krompiewski S.
Acta Physica Polonica A
|
2000
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vol. 97
|
issue 1
15-24
EN
Theoretical studies of electrical conductance of various nanowires are performed within the framework of a semi-realistic s-d tight-binding model. The presented results concern both homogeneous paramagnetic and ferromagnetic systems as well as trilayers composed of 2 magnetic slabs separated by a nonmagnetic spacer. On the one hand, in the case of the homogeneous systems the attention is focused on conductance quantization, which manifests itself when a contact gets open and conduction decays in a spectacular stepwise way. A new approach is developed by assuming that in the last stage of the breaking of the contact between wires there are fewer and fewer, distributed at random, conduction paths passing through the nanowire cross-section. The corresponding conductances are calculated within the quasiballistic regime, using the Kubo formula and a recursion Green function technique. The results for weak ferromagnets (when both majority and minority bands intersect the Fermi surface) are qualitatively different from those for strong ferromagnets (only the minority bands do), which may explain experimental cumulative conductance histograms of Fe and Ni. On the other hand, giant magnetoresistances of magnetic trilayers are studied for both current-perpendicular-to-plane and current-in-plane geometries. The corresponding magnetoresistances are compared with each other and with the interlayer exchange coupling.
2
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Effect of External Contacts on Edge Magnetic Moments in Graphene Nanoribbons

100%
Krompiewski S.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
523-524
EN
The main problem of interest of this study is the influence of external electrodes on the edge magnetic moments in graphene nanoribbons. The studies are carried out within the framework of tight-binding method (for π -state electrons) and the Landauer-Büttiker formalism combined with the Green function technique. It is shown that the edge atom moments get reduced (and eventually disappear) when the graphene nanoribbon/electrode interface becomes more and more transparent for electrons.
3
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Theoretical Studies of Tunnel Magnetoresistance and Shot Noise in a Schottky-Barrier Carbon Nanotube Transistor with Ferromagnetic Contacts

100%
Krompiewski S.
|
EN
A Schottky-barrier carbon nanotube field-effect transistor with ferromagnetic contacts was modelled. The theoretical method combines a tight-binding model and the non-equilibrium Green function technique. Tunnel magnetoresistance as well as current noise of the carbon nanotube field-effect transistor are the main issues addressed in this study. It is shown that the former may exceed 50%, whereas the latter is characterized by the Poissonian Fano factor (F) within the sub-threshold region, and the sub-Poissonian F≈0.5 for elevated gate voltages. Remarkably, reorientation of relative magnetization alignments of the contacts may lead to noticeable changes in the current noise.
4
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Spin-Sensitive Conductance in Magnetically Contacted Graphene Flakes

100%
Krompiewski S.
|
EN
The aim of this study is to gain a deeper insight into the impact of geometrical dimensions (aspect ratio) and current direction (zigzag vs. armchair) on transport characteristics. It is found that there is a pronounced dependence of the giant magnetoresistance (in setups with 2 ferromagnetic electrodes), as well as the spin polarization of current (in the case of one paramagnetic and one ferromagnetic electrode) on the bias and gate voltages, meaning a possible electrical control of magnetic properties of these quantities.
5
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Electronic Transport in Multi-Terminal Graphene Devices with Various Arrangements of Electrodes

100%
Krompiewski S.
Acta Physica Polonica A
|
2014
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vol. 126
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issue 1
194-195
EN
This study is devoted to the problem of electronic transport in graphene nanodevices in 4-terminal systems with various arrangements of electrodes. The electrodes are attached to square and rectangular graphene nanoflakes with armchair (a) and zigzag (z) edges. Apart from the known case of the zzzz-configuration, with all the electrodes coupled to the zigzag fragments of the edges, also the aaaa- and zaza-type cases are considered here. The adopted theoretical approach is based on a tight-binding method combined with the wideband approximation for electrodes, and an effective iterative knitting-type Green's function algorithm.
6
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Graphene Nanoribbons with End- and Side-Contacted Electrodes

100%
Krompiewski S.
Acta Physica Polonica A
|
2012
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vol. 121
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issue 5-6
1216-1218
EN
This contribution reports on theoretical studies of electronic transport through graphene nanoribbons in the two-terminal geometry. The method combines the Landauer-type formalism with Green's function technique within the framework of the standard tight-binding model. The aim of this study is to gain some insight on how fundamental electric current characteristics (conductance and shot noise) depend on interface conditions imposed by graphene nanoribbon/metal-electrode contact details. Calculations have been carried out for both end- and side-contact geometries, and metallic (zigzag-edge) as well as semiconducting (armchair-edge) graphene nanoribbons. It turns out that results for side-contacted systems depend on the ratio between the free-standing graphene nanoribbon length to that covered by the electrode. For sufficiently long nanoribbons the results start converging when this ratio exceeds 0.5. In the case of ferromagnetic contacts, the giant magnetoresistance coefficient is also discussed.
7
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Comparative Studies on Giant Magnetoresistance in Carbon Nanotubes and Graphene Nanoribbons with Ferromagnetic Contacts

100%
Krompiewski S.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
319-321
EN
This contribution reports on comparative studies on giant magnetoresistance in carbon nanotubes and graphene nanoribbons of similar aspect ratios (i.e. perimeter/length and width/length ratios, for the former and the latter, respectively). The problem is solved at zero temperature in the ballistic transport regime, by means of the Green functions technique within the tight-binding model and with the so-called wide band approximation for electrodes. The giant magnetoresistance effect in graphene is comparable to that of carbon nanotubes, it depends strongly on the chirality and only slightly on the aspect ratio. It turns out that graphene, analogously to carbon nanotubes may be quite an interesting material for spintronic applications.
8
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Effect of Magnetic Zigzag Edges in Graphene-like Nanoribbons on the Thermoelectric Power Factor

64%
Krompiewski S., Cuniberti G.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
535-537
EN
This study shows that magnetic edge states of graphene-like nanoribbons enhance effectively the thermoelectric performance. This is due to the antiparallel alignment of magnetic moments on opposite zigzag edges and the confinement effect, which jointly lead to the appearance of a gap in the electronic energy spectrum. Consequently, the Seebeck coefficient as well as the thermoelectric power factor get strongly enhanced (with respect to other alignment cases) at room temperature and energies not far away from the charge neutrality point. Moreover the corresponding figure of merit (ZT) is also improved as a result of the reduced electronic thermal conductance.
9
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Perpendicular and Parallel Transport, and Interlayer Exchange Coupling in Magnetic Trilayers with Extra Cap-Layers

64%
Krompiewski S., Zwierzycki M.
Acta Physica Polonica A
|
2000
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vol. 97
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issue 3
555-558
EN
Oscillations of Magnetoresistance and Interlayer Exchange Coupling Vs. both Non-magnetic Spacer and Cap-layer Thicknesses Are Studied Within the Framework of the S-d Model. the Studies Are Carried Out by Means of the Kubo Formula and a Green Function Recursion Technique. Transport Calculations Concern Multilayers (sandwiched Between Two Semi-infinite Ideal Lead Wires) Oriented Either Parallel Or Perpendicular To the Current Direction. the Considered Structures for the Above Mentioned Two Basic Geometries, Are Cubic Grains Identical In: (i) Size, (ii) Number of Sublayers, and (iii) Thicknesses of Particular Sublayers, Which Makes It Possible To Compare, in a Direct and Reliable Way, the Corresponding Magnetoresistances. the Current-in-plane Magnetoresistance Is Found To Be Usually Lower Than the Current-perpendicular-to-plane One, But Both of Them Show Oscillatory Behaviours Mutually Correlated, and Related To the Fermi Surface Callipers. moreover, the Current-perpendicular-to-plane Giant Magnetoresistance for Structures in a Form of Infinite in Layer-plane Trilayers Capped with An Extra Bilayer (ferromagnet/paramagnet) Is Also Discussed.
10
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Ab Initio Study of the Edge States of Graphene Nanoribbons in the Presence of Electrodes

64%
Zwierzycki M., Krompiewski S.
Acta Physica Polonica A
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2010
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vol. 118
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issue 5
856-858
EN
Thanks to its outstanding electronic properties, like very high mobility of carriers, graphene has emerged in recent years as exciting candidate for use in new electronic devices. When it is patterned in the form of ribbons with widths in the range of nanometers, its transport properties become strongly influenced by the presence of the states localized at the edges of the ribbon. Using first principles calculations we study the properties of these states for both isolated ribbons and in the presence of metallic electrodes. The calculations were performed for end contacted geometry i.e. for graphene sheets at right angle to the electrodes. Both para- and ferromagnetic electrodes were considered.
11
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CPP-Giant Magnetoresistance and Thermoelectric Power of Multilayers

64%
Krompiewski S., Krey U.
Acta Physica Polonica A
|
1997
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vol. 91
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issue 1
221-224
EN
Oscillations of magnetoresistance and thermoelectric power vs. both nonmagnetic spacer and ferromagnetic slab thicknesses are studied in the current-perpendicular-to-plane geometry, in terms of the single-band tight-binding model. The spin-dependent conductance was calculated from the Kubo formula by means of a recursion Green function technique, and the thermoelectric power directly from the well-known Onsager relations. In general, the observed oscillations may have either just one or two periods. In the latter case the long period of oscillations, related to spectacular beats, is apparently of non-RKKY type. The relative thermoelectric power oscillations are strongly enhanced in comparison with those of the giant magnetoresistance, have the same periods, but different phases and a negative bias.
12
Content available remote

In-Plane Edge Magnetism in Graphene-Like Nanoribbons

64%
Krompiewski S., Cuniberti G.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
828-829
EN
This paper is devoted to identification of the most important factors responsible for formation of magnetic moments at edges of graphene-like nanoribbons. The main role is attributed to the Hubbard correlations (within unrestricted Hartree-Fock approximation) and intrinsic spin-orbit interactions, but additionally a perpendicular electric field is also taken into account. Of particular interest is the interplay of the in-plane edge magnetism and the energy band gap. It is shown that, with the increasing electric field, typically the following phases develop: magnetic insulator (with in-plane spins), nonmagnetic narrow-band semiconductor, and nonmagnetic band insulator.
13
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Ab Initio Analysis of a Quantum Dot Induced by a Local External Potential in a Semiconducting Carbon Nanotube

64%
Kostyrko T., Krompiewski S.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
387-389
EN
Using the density functional theory we study the influence of external charge probes on the electronic structure of semiconducting carbon nanotubes in the vicinity of the Fermi level. We show that the spatially limited potential due to the probe can create localized electronic states in the energy gap and at the edges of the conductance band. By filling these localized states with additional electrons one obtains a quantum dot, which can be tuned by modifying the properties of the external charge probe. We analyze dependence of the electronic structure of the dot on the spatial extension of the potential as well as on the nanotube radius.
14
Content available remote

Exchange Coupling and Magnetization Profiles of Binary and Ternary Magnetic Multilayers

51%
Süss F., Krey U., Krompiewski S.
Acta Physica Polonica A
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1997
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vol. 91
|
issue 2
281-284
EN
Within a spin-polarized LMTO approach in the atomic-sphere approximation we calculate ab initio the magnetic properties of various binary and ternary multilayers composed of Fe, Co, Ni, Cr, V and Cu. The emphasis lies on the indirect exchange interaction of the magnetic sandwiches across the antiferromagnetic or non-magnetic spacers, and on the profiles of the intrinsic resp. induced magnetic moments. Among other results we find (i) that Ni is very sensitive on its neighborhood and that (ii) at the interface to Fe, V gets almost as strongly polarized as Cr, whereas in the interior layers, the V sandwich remains non-polarized.
15
Content available remote

Transport through Single-Wall Carbon Nanotubes Weakly Coupled to External Leads

51%
Weymann I., Krompiewski S., Barnaś J.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
296-298
EN
We consider transport properties of single-wall metallic carbon nanotubes weakly coupled to external leads. In particular, we analyze the conductance and shot noise of nanotubes coupled to nonmagnetic leads and show that the shot noise may become super-Poissonian depending on the ground state of the nanotube. In addition, we also show that when the nanotube is coupled to one ferromagnetic and one nonmagnetic lead, it can operate as a gate-controlled spin diode.
16
Content available remote

Calculation of Oscillatory Exchange in bcc-Fe/Cu Multilayers

51%
Krompiewski S., Pirnay J., Krey U., Süss F.
Acta Physica Polonica A
|
1994
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vol. 85
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issue 2
475-478
EN
Within a tight-binding LMTO approach in the atomic sphere approximation we calculate the exchange interaction, J, between magnetic Fe slabs in a periodic multilayer arrangement m Fe/n Cu/ .... We concentrate on cases, where Cu adapts to the bcc structure of Fe. For the "open" (100)-interface our results suggest that a minimal number of m = 13 Fe monolayers per Fe slab is necessary to get antiferromagnetic coupling. For m = 16 we find antiferromagnetic coupling for 8 ≤ n ≤ 14, i.e. a long period of ca. 11 and a short period of 2 Cu monolayers. In contrast, for the (110)-interfaces, only a long period of 14 monolayers is found, i.e. the coupling is antiferromagnetic for 7 ≤ n ≤ 14 already for small values of m. We then model the roughness by mutual interchange of 50% of the Fe and Cu atoms, respectively, in the interface layers and find that, due to the interchange, the coupling constant J, as a function of the Cu-spacer thickness n, roughly shifts by half a mono-layer to the right. In the (100)-case, the short period gets washed out by the roughness.
17
Content available remote

Proceedings of the European Conference Physics of Magnetism 96. Preface.

39%
Krompiewski S., Micnas R., Fechner B., Kowalewski L., Morkowski J., Szajek A., Thomas M.
Acta Physica Polonica A
|
1997
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vol. 91
|
issue 1
3-6
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