P. Vasek

Institute of Chemical Technology Prague, Praha, Praha, Czech Republic

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Publications (43)67.14 Total impact

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    ABSTRACT: We have studied the transport properties of graphene grown on silicon side of SiC. Samples under study have been prepared by two different growth methods in two different laboratories. Magnetoresistance and Hall resistance have been measured at temperatures between 4 and 100 K in resistive magnet in magnetic fields up to 22 T. In spite of differences in sample preparation, the field dependence of resistances measured on both sets of samples exhibits two periods of magneto-oscillations indicating two different parallel conducting channels with different concentrations of carriers. The semi-quantitative agreement with the model calculation allows for conclusion that channels are formed by high-density and low-density Dirac carriers. The coexistence of two different groups of carriers on the silicon side of SiC was not reported before.
    Journal of Physics Conference Series 10/2012; DOI:10.1088/1742-6596/456/1/012038
  • ChemInform 10/2010; 24(40):no-no. DOI:10.1002/chin.199340021
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 09/2010; 23(35):304-304. DOI:10.1002/chin.199235304
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    ABSTRACT: We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional Mn_Ga dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mn-doping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band. Comment: 22 pages, 14 figures
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    ABSTRACT: The design and synthesis of glycol-functionalized porphyrins that contain one to four low molecular weight glycol chains that are linked via ether bonds to the meta-phenyl positions of meso-tetraphenylporphyrin and the comparison of fluorinated and nonfluorinated para derivatives are reported. The cellular uptake and photodynamic activity significantly depend on terminal groups of the glycol substituent. Hydroxy glycol porphyrins, in contrast with methoxy glycol porphyrins, show efficient intracellular transport and a high induction of apoptosis in tumor cell lines in vitro . Furthermore, the ethylene glycol chain at the meta position exhibits a superior efficacy that leads to the permanent ablation of human breast carcinoma (MDA-MB-231) in nude mice. In addition, fluorination enhanced the photosensitizing potential of para-phenyl derivatives. The analysis of the cell-death mechanism revealed that glycol-functionalized porphyrins represent novel nonmitochondrially localized photosensitizers that have a profound ability to induce apoptosis in tumor cells that act upstream of caspase activation. The strong interaction with a tumor marker (sialic acid) indicates the preferential association of these compounds with tumor cells.
    Journal of Medicinal Chemistry 10/2008; 51(19):5964-73. DOI:10.1021/jm8002119 · 5.48 Impact Factor
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    ABSTRACT: We explore the basic physical origins of the noncrystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. The sign of the noncrystalline AMR is found to be determined by the form of spin-orbit coupling in the host band and by the relative strengths of the nonmagnetic and magnetic contributions to the Mn impurity potential. We develop experimental methods yielding directly the noncrystalline and crystalline AMR components which are then analyzed independently. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. Generic implications of our findings for other dilute moment systems are discussed.
    Physical Review Letters 11/2007; 99(14):147207. DOI:10.1103/PhysRevLett.99.147207 · 7.73 Impact Factor
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    ABSTRACT: Series of the samples with nominal compositions Bi3.2Pb0.8Sr4Ca5Cu7Ox and Bi3.2Pb0.8Sr4Ca5Cu7Ox/Ag (10 wt%) were prepared by the standard solid state reaction at the same calcination and sintering temperature and time. Three starting mixtures were used, carbonates, oxides and silver i) with nanoparticles, ii) with micrometer size of particles and iii) powder obtained by modified sol-gel process. The main phase in all samples was Bi-2223 and silver formed the separate phase. The sol-gel process was found to be effective way in term of porosity, bulk density and connectivity of the grains. The effect of the microstructure on the transport properties were described by the temperature dependence of resistivity and critical current densities.
    International Journal of Modern Physics B 07/2007; 21:3246-3249. DOI:10.1142/S0217979207044299 · 0.46 Impact Factor
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    ABSTRACT: We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs superlattices formed by wide and narrow quantum wells and thin Si-doped barriers subject to tilted magnetic fields. It has been shown that illumination of the strongly coupled superlattice with narrow wells leads to reduction of its dimensionality from the 3D to 2D. The illumination-induced transition is revealed by remarkable change of magnetoresistance curves as compared to those measured before illumination. The experimental data along with tight-binding model calculations indicate that the illumination not only enhances the electron concentration but also suppresses the electron tunneling through the barriers.
    Microelectronics Journal 07/2007; DOI:10.1016/j.mejo.2007.07.013 · 0.92 Impact Factor
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    ABSTRACT: The magnetoresistance of the MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been measured in tilted magnetic fields in the as-grown state, and after brief illumination by a red-light diode at low temperature, T is approximately 0.3 K. A remarkable illumination-induced modification of magnetoresistance curves has been observed, which indicates a significant change of the superlattice Fermi surface topology. Analysis of magnetoresistance data in terms of the tight-binding model reveals that not only electron concentration and mobility have been increased by illumination, but also the coupling among 2D electron layers in neighboring quantum wells has been reduced.
    Physical review. B, Condensed matter 07/2007; 75(24). DOI:10.1103/PhysRevB.75.245322 · 3.66 Impact Factor
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    ABSTRACT: In the first part of the work two routes of preparation of undoped superconductor Bi-2223 are compared. The influence of addition of extra PbO is studied as well. The route that leads to higher content of the phase 2223 is then chosen as the appropriate way for preparation of fluorine doped samples of general stoichiometry Bi1.6Pb0.4Sr2Ca2.5Cu3.5Ox-z Fz (z = 0, 0.3, 0.6 and 0.9). These samples are characterized by measuring of their temperature dependence of electrical resistance and critical current density. Their phase composition is determined by XRD. While greater amount of fluorine widenes the superconducting transition and worsenes transport properties, the less doped sample shows increasement of critical current density.
    physica status solidi (c) 09/2006; 3(9):3077-3080. DOI:10.1002/pssc.200567042
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    ABSTRACT: A short synthetic route is outlined, starting from bromo-BN derivatives, via halogen lithium exchange, subsequent Michael reaction with dimethylaminoacrolein, hydrolysis to the corresponding aldehyde, and final condensation with a benzothiazolium unit to produce a BN-pentamethinium system, which absorbs in the visible range around 450 nm. Enantiopure ligands show a decent Cotton effect in the CD spectrum. Preliminary data show potential of these compounds in the area of supramolecular chemistry (enantioselective recognition) and also for medicinal application (induction of apoptosis). [reaction: see text]
    Organic Letters 09/2005; 7(17):3661-4. DOI:10.1021/ol051139n · 6.32 Impact Factor
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    ABSTRACT: We present an experimental study of electron transport in inversion layers of high-mobility Si(001) samples with occupied excited subbands. The second series of oscillations, observed in addition to the main series of Shubnikov-de Hass oscillations, is tentatively attributed to the occupation of a subband associated with the $E_{0'}$ level. Besides, a strong negative magnetoresistance and nonlinear field dependence of the Hall resistance accompany the novel oscillations at high carrier concentrations. The heating of the 2D electron layers leads to suppression of the observed anomalies. Comment: 4 pages, 6 figures, elsart/PHYEAUTH macros; to be presented on the EP2DS-15 Conference in Nara, Japan
    Physica E Low-dimensional Systems and Nanostructures 06/2003; DOI:10.1016/j.physe.2003.12.011 · 1.86 Impact Factor
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    ABSTRACT: We have investigated experimentally the magnetoresistance of strongly asymmetric double-wells. The structures were prepared by inserting a thin Al$_{0.3}$Ga$_{0.7}$As barrier into the GaAs buffer layer of a standard modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure. The resulting double-well system consists of a nearly rectangular well and of a triangular well coupled by tunneling through the thin barrier. With a proper choice of the barrier parameters one can control the occupancy of the two wells and of the two lowest (bonding and antibonding) subbands. The electron properties can be further influenced by applying front- or back-gate voltage. Comment: 4 pages, 5 figures, elsart/PHYEAUTH macros; to be presented on the EP2DS-15 Conference in Nara, Japan. Revised version. To appear in Physica E
    Physica E Low-dimensional Systems and Nanostructures 06/2003; DOI:10.1016/j.physe.2003.11.212 · 1.86 Impact Factor
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    ABSTRACT: Novel analogues of Troger's base were prepared regioselectively from 4-amino-N-methylpyrrole carboxylates in good yield. Catalytic hydrogenation of dibenzyl-4,9-methano-1,6-dimethyl-4,5,9,10-tetrahydro-1H,6H-dipyrrolo-[3,2-b:3',2'-f][1,5]-diazocin-2,7-dicarboxylate 2b led to 4,9-methano-1,6-dimethyl-4,5,9,10-tetrahydro-1H,6H-dipyrrolo-[3,2-b:3',2'-f][1,5]diazocin-2,7-dicarboxylic acid 3 which was used for the preparation of Troger's base derivatives of natural antibiotics via an amide protocol. The novel heterocyclic Troger's bases were characterized by a variety of spectroscopic techniques and compound 2b by X-ray crystallography. Incorporation of guanidine as the terminal group in the N-methylpyrrole Troger's base skeleton opens the possibility for preparation of water soluble derivatives.
    Tetrahedron Letters 06/2003; 44(10). DOI:10.1016/S0040-4039(03)00177-1 · 2.39 Impact Factor
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    ABSTRACT: Samples with the nominal composition Bi1.8−xAxPb0.26Sr2Ca2Cu3Oy (x=0–0.5; A=B, Al, Ga, In) were prepared. Measurements of resistivity, AC susceptibility, critical current density and magnetization have been used for characterization of the samples. All boron-substituted samples had a 2223 phase and narrow transition with the critical temperature Tc≈106–108K and the critical current density Jc (77K)≈105A/m2. The presence of aluminium caused decrease of Tc. The 2212 phase became evident and Jc (77K) sharply decreased at x⩾0.3. The resistivity curves of all the samples with Ga were steeper. The Tc values ranging from 103 to 60K were considerably influenced by the amount of dopant. The volume of 2212 phase increased rapidly at x=0.2 and Jc decreased to zero below 77K. Introducing indium into the samples had a rather positive effect on Tc since the resistivity–temperature curves were always shifted towards higher temperatures with the narrow transition at ∼107K although the XRD patterns showed that the samples also contained the 2212 phase. The samples with x=0.05–0.4 had Jc (77K)≈105A/m2. The highest content of In worsened the quality of the samples and decreased the Jc (77K) to zero.
    Physica B Condensed Matter 08/2002; 321(1):292-294. DOI:10.1016/S0921-4526(02)00863-3 · 1.28 Impact Factor
  • P Svoboda, P Vasek
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    ABSTRACT: The absolute thermoelectric power of the Pd82Si18 and Pd80Si20 metallic glasses has been measured at 1.8-8.5K using a SQUID. The thermopower of Pd82Si18 becomes negative below about 7.7K. This anomaly has been attributed to scattering at magnetic impurities present in the sample. Their negative contribution to the thermopower prevails at sufficiently low temperatures over a positive one from the PdSi matrix. The results for two glasses Pd80Si20 of different purity support this conclusion.
    Journal of Physics F Metal Physics 11/2000; 15(12):2489. DOI:10.1088/0305-4608/15/12/011
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    ABSTRACT: Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent-field calculations of the electronic structure. Comment: 6 pages, 5 figures
    Physical review. B, Condensed matter 04/2000; 62(16). DOI:10.1103/PhysRevB.62.10908 · 3.66 Impact Factor
  • P. Svoboda, P. Chrobok, P. Vasek
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    ABSTRACT: We have developed a simple, computer controlled DC measuring system for calibration of wire-wound 10 kΩ resistors against a QHE structure. The system employs a high-accuracy DMM in both voltage and four-terminal resistance measurement modes. In both cases, resolution down to 10-8 and combined standard uncertainty at the level of 10-7 could be achieved. The system proved to be also useful for testing new QHE samples fabricated for quantum resistance metrology
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    ABSTRACT: Formation of the high temperature Bi-2223 phase has been studied for samples with nominal composition Bi3,2-xPb0.8MxSr4Ca5Cu7Oy. M represented V, Nb, Ta, their contents was altered from 0 to 0.6 mol %. The effect of the additives on the growth of the phase 2223, lattice parameters and the superconducting properties of the samples was detected. It was found that the additives enhanced the volume of the 2223 phase, improved the intergrain connection resp. the critical current density. The additives did not affect the lattice parameters and Tc significantly.
    International Journal of Modern Physics B 12/1999; 13:3738-3740. DOI:10.1142/S0217979299003817 · 0.46 Impact Factor
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    ABSTRACT: The effect of the sintering temperature, the cooling rate and the additive on the critical parameters was studied on samples with nominal composition Bi1.8(Pb0.26)Sr2Ca2Cu3O8+x both undoped and with addition of 5, 9, 12.7 mol % of boron (as B2O3), respectively. The sample with 5 mol % of boron cooled slowly had the critical current density five times higher than undoped sample. The rapid cooling of the samples caused enhancement of the volume of the 2223 phase.
    International Journal of Modern Physics B 12/1999; 13:3729-3731. DOI:10.1142/S0217979299003787 · 0.46 Impact Factor

Publication Stats

290 Citations
67.14 Total Impact Points

Institutions

  • 1999–2010
    • Institute of Chemical Technology Prague
      • Department of Inorganic Chemistry
      Praha, Praha, Czech Republic
  • 1995–2007
    • Academy of Sciences of the Czech Republic
      • Institute of Physics
      Praha, Praha, Czech Republic
  • 1995–2003
    • The Police Academy of the Czech Republic in Prague
      Praha, Praha, Czech Republic
  • 2000
    • Indiana University Bloomington
      • Department of Physics
      Bloomington, Indiana, United States
  • 1997
    • Czech Technical University in Prague
      • Chair of Electrical Engineering (FS)
      Praha, Hlavni mesto Praha, Czech Republic
  • 1992
    • National Center for Scientific Research Demokritos
      Athínai, Attica, Greece
  • 1990
    • Humboldt-Universität zu Berlin
      • Department of Physics
      Berlín, Berlin, Germany