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1
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Superconductivity of Carbon Materials - Unstable Phases

100%
Kempiński W.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
627-636
EN
Different intercalated carbon systems: K_xC_{60}, Rb_xC_{60} as well as highly oriented pyrolytic graphite + potassium with unstable structural and superconducting behavior were investigated with the EPR and magnetically modulated microwave absorption techniques. Three characteristic, well-separated EPR signals were observed for A_xC_{60} (A = K or Rb) systems at the early stages of doping process. We ascribed these signals to C_{60}^+, C_{60}^{1-}, and C_{60}^{3-} radicals. Evolution of the EPR spectrum characterizes different phases of K_xC_{60} system including superconducting one. Two superconducting phases with T_c^{(1)}=(21±0.5) K (unstable) and T_c^{(2)}=(18.5±0.5) K (final, stable) were distinctly separated for K_xC_{60} system. For C_{60} intercalated by rubidium a similar evolution of EPR spectrum is observed. Prior to well-defined superconducting Rb_3C_{60} phase was created, strong instabilities of the magnetically modulated microwave absorption signal were observed at the narrow part of the doping process. For potassium intercalated highly oriented pyrolytic graphite, EPR oscillations below 100 K were observed as well as Josephson hysteresis loops were registered at 5 K.
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EPR and MMMA Study of C_{60} upon K-Doping

64%
Kempiński W., Stankowski J.
Acta Physica Polonica A
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1995
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vol. 88
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issue 3
549-552
EN
EPR and ΜΜMΑ studies of C_{60} upon K-doping have been performed. Two different and well separated EPR narrow lines were detected for C^{1-}_{60} and C at temperatures below 100 K. Time dependent changes in the intensities of both and C^{3-}_{60} lines were observed when the system undergoes an eutectoid transformation. The evolution of superconductivity with two T_{c} related to different valences (v) of C^{v}_{60} ion radicals have been found.
3
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Free Radicals in K and Rb Admixtured Fullerene C_{60}

51%
Stankowski J., Kempiński W., Byszewski P., Trybuła Z.
Acta Physica Polonica A
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1993
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vol. 84
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issue 6
1117-1123
EN
Molecules of C_{60} form Van der Waals type crystals. The observations reported here concern EPR investigations of such C_{60} samples with only a slight amount of Rb or K. Detected defects are (C_{60})^{+} or (C_{60})^{y-} ion-radicals. A hole (h) or trapped electron (t) are localized on one fullerene ball. In the case of potassium fullerites C_{60}:K the line shape of EPR signal was of the Dysonian form which is characteristic of conducting electrons in metal. The EPR lines of holes (C_{60})^{+} are characterized by g_{h} ≥ g_{0} whereas for electrons g_{t} is below the value g_{0} = 2.0023 characteristic of a free electron (g_{t} ≤ g_{0}). The EPR linewidth 2ΔB^{h}_{1s} of the (C_{60})^{+} weakly increased with decreasing temperature whereas the EPR linewidth attributed to the electron 2ΔB^{t}_{1s} significantly decreases with decreasing temperature. The C_{60}:K sample reached superconducting phase below T_{c} = 11 K which is significantly less than T_{c} = 16.5 K observed for K_{x}C_{60} where 2 ≤ x ≤ 4.
4
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Pseudorotational Averaging of EPR Spectrum of Cu(II)O_{5} Complex in YBa_{2}Cu_{3}O_{7-δ} in Low Temperatures

51%
Stankowski J., Kempiński W., Trybuła Z.
Acta Physica Polonica A
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1991
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vol. 80
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issue 4
571-581
EN
In low temperatures the condensation of oxygen was found to occur at 05 site in an elementary cell of YBa_{2}Cu_{3}O_{7-δ} being the fifth ligand forming the CuO_{5} complex around Cul copper in chain. This change of coordinantion from CuO_{4} to CuO_{5} is the origin of a pseudorotation related to a strong vibronic coupling of two distorted configurations: a tetragonal pyramid C_{4v} and a trigonal bipyramid D_{3h}, which yields a pseudocubic EPR spectrum in low temperatures. The averaged spectroscopic splitting coefficient is related to a superposition of vibronically coupled orbital states |x^{2} - y^{2}〉 and |3z^{2} - r^{2}〉. The averaged spectrum was for the first time observed in low temperatures since the oxygen condensation in YBa_{2}Cu_{3}O_{7-δ} at 05 site of the chain occur only when oxygen undergoing fast diffusion among the chains, gets localized with decreasing temperature. The activation energy of oxygen desorption from the 05 site is 36 K.
5
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Size Modification of Nanographite System of Activated Carbon Fibers Studied by EPR

51%
Kempiński M., Kempiński W., Śliwińska-Bartkowiak M.
Acta Physica Polonica A
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2005
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vol. 108
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issue 2
339-343
EN
We report results of EPR measurements of activated carbon fibers. Experiments made for pristine activated carbon fibers and activated carbon fibers with adsorbed molecules (CCl_4, C_6H_5NO_2, and H_2O) confirmed the localized character of paramagnetic centers observed in the system. Pristine activated carbon fibers are characterized by single Lorentzian line. Broader component of EPR signal appears when guest molecules are adsorbed in nanopores. The strongest localization is observed for water-filled activated carbon fibers nanopores (with hydrophobic pore walls) where changes in distance between nanographite particles were monitored by the g-shift to higher values. This process is related to stronger spin-orbit interaction of electrons trapped at nanographite particles compressed by guest molecules.
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EPR and Impedance Measurements of Graphene Oxide and Reduced Graphene Oxide

45%
Kempiński M., Łoś S., Florczak P., Kempiński W., Jurga S.
Acta Physica Polonica A
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2017
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vol. 132
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issue 1
81-85
EN
We report the observations of electron paramagnetic resonance and impedance measurements of graphene oxide and reduced graphene oxide performed in the wide temperature range in order to get insight into the electronic properties of graphene-based materials and the role of oxygen functionalities in the charge carrier transport phenomena. In such systems the strong spin localization, hopping charge carrier transport as well as the formation of adsorption layers are observed, all the phenomena changing significantly after the heavily oxidized graphene is reduced.
7
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Electric Conductivity of Carbon Nanoparticles Stimulated by Electric Field

39%
Markowski D., Kempiński W., Kempiński M., Trybuła Z., Kaszyńska K., Śliwińska-Bartkowiak M.
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issue 3
457-458
EN
Host-guest interactions can be the unique method of spin manipulation in nanoscale. Strong changes in spin localization are generated when potential barriers between nanographitic units of activated carbon fibers are modified by interaction with adsorbed molecules. Stronger modifications occur when dipolar guest molecules are stimulated with external electric field. We report experimental results which show the influence of electric field on the spin localization in activated carbon fibers.
8
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Oxygen Deficient YBa_2Cu_3O_{7-δ}: Two Superconducting Phases

39%
Stankowski J., Ślązak A., Kempiński W., Andrzejewski B., Reichel B., Plesch G.
Acta Physica Polonica A
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2004
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vol. 105
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issue 3
287-294
EN
A set of oxygen deficient powder and granular samples of YBa_2Cu_3O_{7-δ} ceramics was studied by means of thermogravimetric and magnetically modulated microwave absorption methods. Especially the last method has turned out to be a powerful method, which was used to determine the dependence of critical temperature T_c on the oxygen parameterδ. Using these methods it has been shown that the removing of oxygen evokes the inhomogeneous oxygen distribution in the samples and a change in carrier concentration till to the loss of superconducting properties. A superconducting phase with T_c=60 K, which occurs in oxygen deficient YBa_2Cu_3O_{7-δ} sample, is an unstable phase.
9
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Superconducting Regions and Kondo Effect of MgB_2 Formed by Implantation of Magnesium Ions into Boron Substrate

33%
Trybuła Z., Kempiński W., Andrzejewski B., Piekara-Sady L., Kaszyński J., Trybuła M., Piekoszewski J., Stanisławski J., Barlak M., Richter E.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
657-660
EN
The results of investigation of the polycrystalline boron implanted by magnesium and argon plasma pulse treatment are presented. The four-probe electric conductivity measurements and magnetically modulated microwave absorption showed the presence of superconducting islands below the temperature of 25 K. Below T=23 K we detected the Kondo effect, a logarithmic increase in the resistivity as the temperature is lowered, due to iron impurity.
10
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Superconductivity of MgB_2 Layers Prepared on Silicon Substrate by Implantation of Magnesium Ions into Boron Substrate

33%
Trybuła Z., Kempiński W., Łoś S., Kaszyńska K., Trybuła M., Piekoszewski J., Werner Z., Barlak M.
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issue 2
323-325
EN
The results of investigation of the MgB_2 layers prepared on silicon substrate by implantation of Mg ions into boron substrate are presented. After implantation the annealing processes were carried out at temperatures 673 K, 773 K, and 873 K in a furnace in an atmosphere of flowing Ar-4%H_2 gas mixture. The samples were characterized by: four-probe electric conductivity measurements and magnetically modulated microwave absorption. Our results showed that due to silicon substrate the diffusion of implanted Mg ions into boron materials should be limited, and the superconducting phase forms a continuous MgB_2 layer and the resistivity for all samples fall down to zero below T_{c}. The transition temperature T_{c} becomes higher with increasing annealing temperature: T_{c}=18 K (for annealing at T_{A}=673 K), T_{c}=20 K (for annealing at T_{A}=773 K), and T_{c}=27 K (for annealing at T_{A}=873 K).
11
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Ion Implantation and Transient Melting: A New Approach to Formation of Superconducting MgB_2 Phases

33%
Piekoszewski J., Kempiński W., Stankowski J., Prokert F., Richter E., Stanisławski J., Werner Z., Szymczyk W.
Acta Physica Polonica A
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2004
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vol. 106
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issue 6
861-868
EN
An attempt to synthesize superconducting MgB_2 inter-metallic compound from the liquid state is presented. The process consists of two steps. In the first one, boron ions are implanted into a magnesium substrate. In the second one, the near-surface region of such system is melted by short, intense hydrogen plasma pulses without necessity of annealing in Mg vapor. A magnetically modulated microwave absorption method was used to detect superconducting regions in the obtained MgB_2 layer. Percolation between nano-regions (islands) of MgB_2 has not been observed. However, a superconducting state of the insulated islands has been experimentally proven with transition temperatures T_C as high as 31 K.
12
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The Evolution of Superconducting Phase MgB_x

33%
Łoś S., Kempiński W., Piekara-Sady L., Andrzejewski B., Jurga W., Kaszyńska K., Piekoszewski J., Werner Z., Barlak M.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
179-184
EN
Thin layers of MgB_x were studied in order to define evolution of superconducting phase after Mg ions implantation into boron substrate. Three fluencies of energies 140, 80, and 40 keV were used to establish proper stoichiometry to synthesize homogeneous MgB_2 film. Additionally, the annealing processes were carried out at temperatures 400, 500, and 600°C in a furnace in an atmosphere of flowing Ar-4%H_2 gas mixture. The quality of the superconducting material was examined by magnetically modulated microwave absorption, and magnetic and resistivity measurements. The results showed that T_c becomes higher with increasing annealing temperature. However, the fraction of superconducting phase decreases, due to partial evaporation of Mg ions and their deeper migration into boron substrate.
13
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Formation of Superconducting Regions of MgB_2 by Implantation of Boron Ions into Magnesium Substrate

27%
Trybuła Z., Kempiński W., Andrzejewski B., Piekara-Sady L., Kaszyński J., Piekoszewski J., Werner Z., Richter E., Prokert F., Stanisławski J., Barlak M.
Acta Physica Polonica A
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2005
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vol. 108
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issue 1
165-170
EN
The results of investigation of the MgB_2 inter-metallic compound with the use of boron ions implantation and plasma pulse treatment are presented. The samples were characterized by: four-probe electric conductivity measurements, magnetically modulated microwave absorption, and magnetic measurements. For hydrogen and argon pulsed plasma treatment the samples with T_c ranging from 10 K to 32 K were obtained. The superconducting phase does not form a continuous layer since the resistivity does not fall down to zero. Apparently, separate islands of superconducting phase are connected through metallic Mg paths. All samples are still below the percolation threshold.
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