1. Raman spectroscopy of multiphonon emission process in Nidoped PbTeNebojša Romčević, J. Trajić, B. Hadžić, M. Romčević, D. Stojanović, Z. Lazarević, T. A. Kuznetsova, D. Khokhlov, Rebeka Rudolf, Ivan Anžel, 2009, objavljeni znanstveni prispevek na konferenci Opis: In this paper, we present room temperature unpolarized Raman scattering spectra of Ni doped PbTe single crystal sample. Crystal of PbTe(Ni) was grown by the Bridgman method. The Ni concentration in the sample used here was 1* 1019 at./cm3. Well resolved peaks appear at about 126, 143, 181, 362 and 724 cm1. The modes at 126 and 143 cm1, which are also observed in other telluride compounds, originate from vibrations in TeO2. We assume that the mode at about 181 cm1 is connected to excitations of a local phonon mode in the vicinity of an impurity atom (donor Ni3+ state). Modes at about 362 cm1 and 724 cm1 are the second and fourth harmonic of a local phonon mode, registered here due to multiphonon emission. Ključne besede: crystals, Raman scattering Objavljeno: 31.05.2012; Ogledov: 955; Prenosov: 67 Povezava na celotno besedilo 
2. Porosity sensor by using quartz crystals and two excitation signalsVojko Matko, 2003, objavljeni znanstveni prispevek na konferenci Opis: In response to a need for a more accurate porosity measuring method for small solid samples (app. 1 g in mass) the porosity measurement sensor using a sensitive capacitivedependent crystal was developed. This paper presents the new sensor and the probe sensitivity, frequency dependence on the volume. In addition, the new idea of excitation of the entire sensor with stochastic test signals is described, and the porosity measuring method is provided. The latter includes the influence of test signals on the weighting function uncertainty. The experimental results of the porosity determination in volcanic rock samples are presented. The uncertainty of porosity measurements is less than 0.1% in the temperature range 10  30°C. Ključne besede: electrical measurements, porosity, soil, capacitive dependent crystals, sensor, glass test tube, direct digital method Objavljeno: 01.06.2012; Ogledov: 1199; Prenosov: 62 Povezava na celotno besedilo 
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4. Competition between local disordering and global ordering fields in nematic liquid crystalsMatej Cvetko, Milan Ambrožič, Samo Kralj, 2010, izvirni znanstveni članek Opis: We study the influence of external electric or magnetic field B on orientational ordering of nematic liquid crystals or of other rodlike objects (e.g. nanotubes immersed in a liquid) in the presence of random anisotropy field type of disorder. The LebwohlLasher lattice type of semimicroscopic approach is used at zero temperature. Therefore, results are valid well below the transition into the isotropic phase. We calculate the correlation function of systems as a function of B, concentration p of impurities imposing random anisotropy field disorder, the disorder strength W and system dimensionality (2D and 3D systems). In order to probe memory effects we calculate correlation length ▫$xi$▫ for random and homogeneous initial configurations. We determine the crossover fields Bc(p) separating roughly the ordered and disordered regime. Memory effects are apparent only in the latter case, i.e. for B < Bc. Ključne besede: disorder, ImryMa theorem, liquid crystals, memory effect, orientational order Objavljeno: 07.06.2012; Ogledov: 741; Prenosov: 147 Celotno besedilo (747,18 KB) Gradivo ima več datotek! Več...

5. Finitesize effects on order reconstruction around nematic defectsSamo Kralj, Riccardo Rosso, Epifanio G. Virga, 2010, izvirni znanstveni članek Opis: By use of the Landaude Gennes phenomenological theory, we study the texture of a nematic liquid crystal confined within a hybrid cell. Precisely, we consider cylindrically symmetric solutions containing topological defects dictated by appropriate boundary conditions. We focus our attention on cells whose dimensions are comparable with the biaxial correlation length ▫$xi_b$▫. For such severe confinements the order reconstruction (OR) configuration could be stable. Its structural details reflect the balance among boundaryenforced frustration, elastic penalties, and finitesize effects. In particular, we analyze the interplay between finitesize effects and topological defects. We show that defects are always pinned to the negatively (planar) uniaxial sheet of the OR structure. The presence of a ring defect can dramatically increase the critical threshold below which the OR structure is stable. Ključne besede: physics, liquid crystals, nematic crystals, nematic defects, structural transitions Objavljeno: 07.06.2012; Ogledov: 919; Prenosov: 49 Povezava na celotno besedilo 
6. Memory effects in randomly perturbed systems exhibiting continuous symmetry breakingMatej Cvetko, Milan Ambrožič, Samo Kralj, 2009, izvirni znanstveni članek Opis: We studied pattern characteristics in randomly perturbed structures exhibitingcontinuous symmetry breaking. A LebwohlLashertype lattice model was used which described well the onset of orientational ordering of a system of rodlike objects. For example, such systems mimic the orientational ordering tendency in liquid crystals or in an ensemble of nanotubes. We set impurities to impose a random anisotropy type of disorder on the objects. Structural characteristics were studied as a function of concentration of impurities, interaction strength w between impurities and rodlike objects, external ordering strength and history of samples. We showed that the characteristic linear size of patterns and range of ordering strongly depend on the history of samples for weak enough strength w. The twodimensional and threedimensional simulations yielded qualitatively similar results. Ključne besede: liquid crystals, memory effect, nanotubes, structural order, percolation Objavljeno: 07.06.2012; Ogledov: 746; Prenosov: 67 Povezava na celotno besedilo 
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8. Alignment of carbon nanotubes in nematic liquid crystalsPaul van der Schoot, Vlad PopaNita, Samo Kralj, 2008, izvirni znanstveni članek Opis: The selforganizing properties of nematic liquid crystals can be used to aligncarbon nanotubes dispersed in them. Because the nanotubes are so much thinner than the elastic penetration length, the alignment is caused by the coupling of the unperturbed director field to the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In order to relate the degree of alignment of the nanotubes to the properties of the nematic liquid crystal, we treat the two components on the same footing and combine LandaudeGennes free energies for the thermotropic ordering of the liquid crystal and for the lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volumes. The phase ordering of the binary mixture is analyzed as a function of the volume fraction of the carbon nanotubes, the strength of the coupling and the temperature. We find that the degree of ordering of the nanorods is enslaved by the properties of the host liquid and that it can be tuned by raising or lowering the temperature or by increasing or decreasing their concentration. By comparing the theory to recent experiments, we find the anchoring energy of multiwalled carbon nanotubes to be in the range from 1010 to 107 N m1. Ključne besede: liquid crystals, nematic crystals, molecular dynamics, stability, elasticity, carbon nanotubes Objavljeno: 07.06.2012; Ogledov: 955; Prenosov: 35 Povezava na celotno besedilo 
9. Fingered core structure of nematic boojumsSamo Kralj, Riccardo Rosso, Epifanio G. Virga, 2008, izvirni znanstveni članek Opis: Using the Landaude Gennes phenomenological approach, we study the fine biaxial core structure of a boojum residing on the surface of a nematic liquid crystal phase. The core is formed by a negatively uniaxial finger, surrounded by a shell with maximal biaxiality. The characteristic finger's length and the shell's width are comparable to the biaxial correlation length. The finger tip is melted for topological reasons. Upon decreasing the surface anchoring strength below a critical value, the finger gradually leaves the bulk and it is expelled through the surface. Ključne besede: physics, liquid crystals, nematic crystals, line defects, surface phenomena Objavljeno: 07.06.2012; Ogledov: 761; Prenosov: 50 Povezava na celotno besedilo 
10. Annihilation of defects in liquid crystalsMilan Svetec, Milan Ambrožič, Samo Kralj, 2007, objavljeni znanstveni prispevek na konferenci Opis: The annihilation of defect is studied theoretically in liquid crystals (LCs). We consider the annihilation of point disclinations in nematic and line edge dislocations in smectic A LC phase, respectively. We stres s qualitative similarities in these processes. The whole annihilation regime is taken into account, consisting of the precollision, collision, and postcollision stage. Ključne besede: physics, liquid crystals, annihilation, defects, point defects, point disclinations, edge dislocations Objavljeno: 07.06.2012; Ogledov: 698; Prenosov: 48 Povezava na celotno besedilo 