V. Holý

Charles University in Prague, Praha, Praha, Czech Republic

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Publications (358)877.85 Total impact

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    ABSTRACT: Homojunctions between Bi2Se3 and its Mn-doped phase are investigated as a sample geometry to study the influence of spin degrees of freedom on topological insulator properties. n quintuple layers (QLs) of Bi2Se3 are grown ontop of Mn-doped Bi2Se3 by molecular beam epitaxy for 0≤n≤30QLs, allowing to unhamperedly monitor the development of electronic and topological properties by surface sensitive techniques like angle resolved photoemission spectroscopy. With increasing n, a Mn-induced gap at the Dirac point is gradually filled in an "hourglass" fashion to reestablish a topological surface state at n∼9QLs. Our results suggest a competition of upwards and downwards band bending effects due to the presence of an n-p type interface, which can be used to tailor topological and quantum well states independently.
    Full-text · Article · Feb 2016
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    ABSTRACT: The fabrication of regularly ordered Ge quantum dot arrays on Si surfaces usually requires extensive preparation processing, ensuring clean and atomically ordered substrates, while the ordering parameters are quite limited by the surface properties of the substrate. Here, we demonstrate a simple method for fabrication of ordered Ge quantum dots with highly tunable ordering parameters on rippled Si surfaces. The ordering is achieved by magnetron sputter deposition, followed by an annealing in high vacuum. We show that the type of ordering and lattice vector parameters of the formed Ge quantum dot lattice are determined by the crystallographic properties of the ripples, i.e., by their shape and orientation. Moreover, the ordering is achieved regardless the initial amorphisation of the ripples surface and the presence of a thin oxide layer.
    No preview · Article · Nov 2015 · Applied Physics Letters
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    ABSTRACT: A thorough investigation is presented of the magnetic and structural properties of Mn-doped Bi2Se3 topological insulators grown by molecular beam epitaxy on top of insulating BaF2 (111) substrates. The magnetic properties have been studied in the temperature range from 2 K to 300 K in magnetic fields up to 7 T. The systems were further characterized by means of high-resolution X-ray diffraction, electron-microprobe analysis, and X-ray photoemission spectroscopy. Samples with the atomic concentration of Mn up to about 0.06 exhibit an almost perfect crystalline structure while, for higher Mn concentrations, diffuse scattering from defects is observed. Photoemission results suggest a localized non-metallic Mn 3d5 ground state which is weakly or intermediately coupled to the Bi2Se3 environment. The exchange interaction between the Mn moments leads to a ferromagnetic phase at low temperatures with a roughly linear relation between the Curie temperature and the atomic concentration of Mn.
    No preview · Article · Nov 2015 · Physica B Condensed Matter
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    ABSTRACT: Nucleation and growth kinetics of nanoparticles of hexagonal omega phase in a body-centered cubic beta titanium matrix in single crystals of beta-Ti alloys were investigated by small-angle X-ray scattering measured in situ during ageing at various temperatures up to 450 °C. The experimental data were compared with numerical simulations based on a three-dimensional short-range order model of nanoparticle self-ordering. The X-ray contrast of the particles is caused by an inhomogeneous distribution of impurity atoms (Mo, Fe and Al), whose density profile around growing nanoparticles was simulated by solving the corresponding diffusion equation with moving boundary conditions. From the analysis of the experimental data we determined the mean distance and size of the nanoparticles and confirmed the validity of the t.^(1/3) growth law following from the Lifshitz–Slyozov–Wagner theory. From a detailed comparison of the experimental data with simulations we also assessed the diffusion coefficient of the impurity atoms and its activation energy.
    Full-text · Article · Aug 2015 · Acta Materialia
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    Full-text · Article · Aug 2015 · Acta Crystallographica Section A: Foundations and Advances
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    Petr Cejpek · Václav Holý · Oleg Heczko

    Preview · Article · Aug 2015 · Acta Crystallographica Section A: Foundations and Advances
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    ABSTRACT: A common perception assumes that magnetic memories require ferromagnetic materials with a non-zero net magnetic moment. However, it has been recently proposed that compensated antiferromagnets with a zero net moment may represent a viable alternative to ferromagnets. So far, experimental research has focused on bistable memories in antiferromagnetic metals. In the present work we demonstrate a multiple-stable memory device in epitaxial manganese telluride (MnTe) which is an antiferromagnetic counterpart of common II-VI semiconductors. Favorable micromagnetic characteristics of MnTe allow us to demonstrate a smoothly varying antiferromagnetic anisotropic magnetoresistance (AMR) with a harmonic angular dependence on the applied magnetic field, analogous to ferromagnets. The continuously varying AMR provides means for the electrical read-out of multiple-stable antiferromagnetic memory states which we set by heat-assisted magneto-recording and by changing the angle of the writing field. We explore the dependence of the magnitude of the zero-field read-out signal on the strength of the writing field and demonstrate the robustness of the antiferromagnetic memory states against strong magnetic field perturbations. We ascribe the multiple-stability in our antiferromagnetic memory to different distributions of domains with the N\'eel vector aligned along one of the three $c$-plane magnetic easy axes in the hexagonal MnTe film. The domain redistribution is controlled during the heat-assisted recording by the strength and angle of the writing field and freezes when sufficiently below the N\'eel temperature.
    No preview · Article · Aug 2015
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    ABSTRACT: In combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction and first-principle calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite SrIrO3 can be lifted simply by applying epitaxial constraint. In particular, the Dirac gap opens without breaking the Pbnm mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the n-glide operation, which can be selectively broken by different heteroepitaxial structures. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures.
    No preview · Article · Jun 2015
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    ABSTRACT: Optical and magneto-optical properties of single crystal of Ni 50.1Mn28.4Ga21.5 magnetic shape memory alloy during its transformation from martensite to austenite phase were systematically studied. Crystal orientation was approximately along {100} planes of parent cubic austenite. X-ray reciprocal mapping confirmed modulated 10 M martensite phase. Temperature depended measurements of saturation magnetization revealed the martensitic transformation at 335 K during heating. Magneto-optical spectroscopy and spectroscopic ellipsometry were measured in the sample temperature range from 297 to 373 K and photon energy range from 1.2 to 6.5 eV. Magneto-optical spectra of polar Kerr rotation as well as the spectra of ellipsometric parameter Ψ exhibited significant changes when crossing the transformation temperature. These changes were assigned to different optical properties of Ni-Mn-Ga in martensite and austenite phases due to modification of electronic structure near the Fermi energy during martensitic transformation.
    No preview · Article · May 2015 · Journal of Applied Physics
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    ABSTRACT: In systems with broken inversion symmetry spin-orbit coupling (SOC) yields a Rashba-type spin splitting of electronic states, manifested in a k-dependent splitting of the bands. While most research had previously focused on 2D electron systems, recently a three-dimensional (3D) form of such Rashba-effect was found in a series of bismuth tellurohalides. Whereas these materials exhibit a very large spin-splitting, they lack an important property concerning functionalization, namely the possibility to switch or tune the spin texture. This limitation can be overcome in a new class of functional materials displaying Rashba-splitting coupled to ferroelectricity: the ferroelectric Rashba semiconductors (FERS). Using spin- and angle-resolved photoemission spectroscopy (SARPES) we show that GeTe(111) forms a prime member of this class, displaying a complex spin-texture for the Rashba-split surface and bulk bands arising from the intrinsic inversion symmetry breaking caused by the ferroelectric polarization of the bulk (FE). Apart from pure surface and bulk states we find surface-bulk resonant states (SBR) whose wavefunctions entangle the spinors from the bulk and surface contributions. At the Fermi level their hybridization results in unconventional spin topologies with cochiral helicities and concomitant gap opening. The GeTe(111) surface and SBR states make the semiconductor surface conducting. At the same time our SARPES data confirm that GeTe is a narrow-gap semiconductor, suggesting that GeTe(111) electronic states are endowed with spin properties that are theoretically challenging to anticipate. As the helicity of the spins in Rashba bands is connected to the direction of the FE polarization, this work paves the way to all-electric non-volatile control of spin-transport properties in semiconductors.
    Full-text · Article · Mar 2015
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    ABSTRACT: Relativistic current induced torques and devices utilizing antiferromagnets have been independently considered as two promising new directions in spintronics research. Here we report electrical measurements of the torques in structures comprising a $\sim1$~nm thick layer of an antiferromagnet IrMn. The reduced N\'eel temperature and the thickness comparable to the spin-diffusion length allow us to investigate the role of the antiferromagnetic order in the ultra-thin IrMn films in the observed torques. In a Ta/IrMn/CoFeB structure, IrMn in the high-temperature phase diminishes the torque in the CoFeB ferromagnet. At low temperatures, the antidamping torque in CoFeB flips sign as compared to the reference Ta/CoFeB structure, suggesting that IrMn in the antiferromagnetic phase governs the net torque acting on the ferromagnet. At low temperatures, current induced torque signatures are observed also in a Ta/IrMn structure comprising no ferromagnetic layer.
    Full-text · Article · Mar 2015 · Physical Review B
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    ABSTRACT: We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2O3/Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.
    Full-text · Article · Feb 2015 · Nanotechnology
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    Dataset: c4nr06954d1

    Full-text · Dataset · Feb 2015
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    ABSTRACT: This article1 reports the X-ray diffraction-based structural characterization of the α12 multilayer structure SiGe2Si2Ge2SiGe12 [d’Avezac, Luo, Chanier & Zunger (2012 ▶). Phys. Rev. Lett. 108, 027401], which is predicted to form a direct bandgap material. In particular, structural parameters of the superlattice such as thickness and composition as well as interface properties, are obtained. Moreover, it is found that Ge subsequently segregates into layers. These findings are used as input parameters for band structure calculations. It is shown that the direct bandgap properties depend very sensitively on deviations from the nominal structure, and only almost perfect structures can actually yield a direct bandgap. Photoluminescence emission possibly stemming from the superlattice structure is observed.
    Full-text · Article · Feb 2015 · Journal of Applied Crystallography
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    ABSTRACT: A common way of speeding up powder diffraction measurements is the use of one or two dimensional detectors. This usually goes along with worse resolution and asymmetric peak profiles. In this work the influence of a straight linear detector on the resolution function in the Bragg-Brentano focusing geometry is discussed. Due to the straight nature of most modern detectors geometrical defocusing occurs which heavily influences the line shape of diffraction lines at low angles. An easy approach to limit the resolution degrading effects is presented. The presented algorithm selects an adaptive range of channels of the linear detector at low angles, resulting in increased resolution. At higher angles still the whole linear detector is used and the data collection remains fast. Using this algorithm a well-behaved resolution function is obtained in the full angular range, whereas using the full linear detector the resolution function varies within one pattern which hinders line shape and Rietveld analysis.
    Preview · Article · Jan 2015 · Journal of Applied Crystallography
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    ABSTRACT: Weshow that in manganese-doped topological insulator bismuth telluride layers,Mnatoms are incorporated predominantly as interstitials in the van der Waals gaps between the quintuple layers and not substitutionally on Bi sites within the quintuple layers. The structural properties of epitaxial layers withMnconcentration of up to 13% are studied by high-resolution x-ray diffraction, evidencing a shrinking of both the in-plane and out-of plane lattice parameters with increasingMncontent. Ferromagnetism sets in forMncontents around3%and the Curie temperatures rises up to 15K for a Mn concentration of 9%. The easy magnetization axis is along the c-axis perpendicular to the (0001) epilayer plane. Angle-resolved photoemission spectroscopy reveals that the Fermi level is situated in the conduction band and no evidence for a gap opening at the topological surface state with the Dirac cone dispersion is found within the experimental resolution at temperatures close to the Curie temperature. From the detailed analysis of the extended x-ray absorption fine-structure experiments (EXAFS) performed at theMnK-edge, we demonstrate that theMnatoms occupy interstitial positions within the van der Waals gap and are surrounded octahedrally by Te atoms of the adjacent quintuple layers.
    Full-text · Article · Jan 2015 · New Journal of Physics
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    ABSTRACT: Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow gap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, and the giant and fully reversible perpendicular piezoresistance makes iridates a promising material for room temperature piezotronic devices.
    Full-text · Article · Jan 2015 · Nanoscale
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    ABSTRACT: The spatial strain distribution in and around a single axial InAs 1– x P x hetero-segment in an InAs nanowire was analyzed using nano-focused X-ray diffraction. In connection with finite-element-method simulations a detailed quantitative picture of the nanowire's inhomogeneous strain state was achieved. This allows for a detailed understanding of how the variation of the nanowire's and hetero-segment's dimensions affect the strain in its core region and in the region close to the nanowire's side facets. Moreover, ensemble-averaging high-resolution diffraction experiments were used to determine statistical information on the distribution of wurtzite and zinc-blende crystal polytypes in the nanowires.
    Preview · Article · Jan 2015 · Journal of Synchrotron Radiation
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    ABSTRACT: We report on arrangement of iron oxide nanoparticles deposited on flat substrate, on and below graphene, respectively. We combined grazing incidence small angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) to obtain the mean size of the particles and the mean inter-particle distance. While GISAXS provides statistically relevant information averaged over large area, AFM serves to support and clarify the results of GISAXS observations by inspection of the representative area of the sample.
    No preview · Article · Dec 2014 · physica status solidi (b)
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    ABSTRACT: Nanosized particles of ω phase in a β-Ti alloy were investigated by small-angle X-ray scattering using synchrotron radiation. We demonstrated that the particles are spontaneously weakly ordered in a three-dimensional cubic array along the 〈100〉-directions in the β-Ti matrix. The small-angle scattering data fit well to a three-dimensional short-range-order model; from the fit we determined the evolution of the mean particle size and mean distance between particles during ageing. The self-ordering of the particles is explained by elastic interaction between the particles, since the relative positions of the particles coincide with local minima of the interaction energy. We performed numerical Monte Carlo simulation of the particle ordering and we obtained a good agreement with the experimental data.
    Full-text · Article · Dec 2014 · Acta Materialia

Publication Stats

4k Citations
877.85 Total Impact Points

Institutions

  • 2004-2015
    • Charles University in Prague
      • • Faculty of Mathematics and Physics
      • • Department of Condensed Matter Physics
      Praha, Praha, Czech Republic
  • 2013
    • Universität Ulm
      • Institute of Optoelectronics
      Ulm, Baden-Wuerttemberg, Germany
  • 1994-2007
    • Johannes Kepler University Linz
      • Institut für Halbleiter- und Festkörperphysik
      Linz, Upper Austria, Austria
  • 1990-2007
    • Masaryk University
      • • Department of Condensed Matter Physics
      • • Fakulta Přírodovědecká
      Brünn, South Moravian, Czech Republic
  • 1998
    • Slovak Academy of Sciences
      • Institute of Physics
      Bratislava, Bratislavsky Kraj, Slovakia
  • 1997
    • SAS Institute
      North Carolina, United States
  • 1995
    • Universität Regensburg
      Ratisbon, Bavaria, Germany
    • Paul Drude Institute for Solid State Electronics
      Berlín, Berlin, Germany
  • 1985
    • Mendel University in Brno
      Brünn, South Moravian, Czech Republic
  • 1979-1981
    • Jan Evangelista Purkyně University
      • Faculty of Science
      Aussig, Ústecký, Czech Republic