8 results on '"Kneidinger, Friedrich"'
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2. Non-centrosymmetric superconductivity of intermetallic compounds in absence of strong correlations among electrons
- Author
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Kneidinger, Friedrich
- Subjects
non-centrosymmetric systems ,unconventional superconductivity ,unkonventionelle Supraleitung ,nicht-zentrosymmetrische Systeme - Abstract
Supraleitung beschreibt die Eigenschaft eines Materials, unterhalb einer Sprungtemperatur Tc einen verschwindenden elektrischen Widerstand aufzuweisen, sowie ein kleines ��u��eres magnetisches Feld (��0Hcth oder ��0Hc1) komplett zu verdr��ngen. Die mikroskopische Begr��ndung wird durch die Kondensation von jeweils zwei Elektronen zu einem Cooper-Paar gegeben. Die einzelnen Cooper-Paare weisen eine Korrelation in Form einer einheitlichen Phase auf. Es entsteht somit eine makroskopische Wellenfunktion. Die genauen Paarungsmechanismen bzw. Voraussetzungen der Supraleitung sind bisher nicht vollst��ndig erkl��rt. Spin-Triplet Paarung, sowie verschiedene ungew��hnliche Paarungskan��le haben das Interesse auf dem Weg zur Kl��rung des Ph��nomens Supraleitung weiter geweckt, um letztendlich Materialien und Strukturen zu finden, die Supraleitung bei h��heren Temperaturen erlauben. Diese Dissertation handelt von Supraleitern ohne Inversionszentrum und ohne schwere Fermionen. Polykristalline und einkristalline Materialien wurden daf��r im Lichtbogen geschmolzen. Die Probenmaterialien konnten im Anschluss durch R��ntgendiffraktometrie eindeutig einer Raumgruppe ohne Inversionszentrum zugeordnet werden. Mithilfe von EPMA (electron probe microanalysis) Messungen konnten zudem die stoichiometrischen Zusammensetzungen verifiziert und die Prim��rphasen identifiziert werden. Hierf��r werden die Proben mit Elektronen beschossen und angeregt. Die dabei emittierten R��ntgenstrahlen sind f��r jedes Element charakteristisch. Die Materialen wurden im Anschluss weiter geschnitten und poliert. Im folgenden Schritt wurden die temperatur- und magnetfeldabh��ngigen physikalischen Eigenschaften, insbesondere der elektrische Widerstand, die spezifische W��rmekapazit��t und die magnetische Suszeptibilit��t, bestimmt. Vereinzelt wurde der elektrische Widerstand der Probenmaterialien unter dem Einfluss von ��u��erem hydrostatischen Druck untersucht. Hierbei konnten insgesamt f��nf weitere Supraleiter ohne Inversionszentrum (SrNiSi3, SrPdSi3, SrPtSi3, HfRhGe und LaPtSi) sowie ein weiterer Supraleiter (La3Pd4Si4) mit m��glicherweise zwei Energiel��cken von unterschiedlicher Gr����e identifiziert und charakterisiert werden. Die supraleitenden Sprungtemperaturen befinden sich dabei zwischen 1 K und 3.6 K. Die experimentellen Daten wurden durch theoretische Berechnungen der Bandstrukturen unterst��tzt und zeigen zum Teil sehr starke k abh��ngige Aufspaltungen der Spin-Auf als auch Spin-Ab B��nder, sowie m��gliche Spin Triplet Paarungskan��le. Weitere Besonderheiten der Supraleiter ohne Inversionszentrum, wie die Unterschiede im oberen kritischen Magnetfeld (��0Hc2) bei unterschiedlichen physikalischen Messgr����en, wurden im Zusammenhang mit der Perkolationstheorie diskutiert. Die Kompositionen ohne Inversionszentrum zeigen hierbei vorwiegend eine vollst��ndige, knotenlose Energiel��cke, sowie ein s- symmetrisches, konventionelles BCS Verhalten mit vernachl��ssigbarem Anteil an Triplet Cooper Paaren. Mikroskopische ��berpr��fungen der Singlet Paarungskan��le wurden durch ��SR Messungen erm��glicht. Dar��ber hinaus konnte die Materialklasse der EpTX3 (Ep = Sr; Ba;T = Ni; Pd; Pt;X = Si; Ge) Verbindungen vollst��ndig in ihren physikalischen Charakteristiken analysiert werden. Bei den Seltenen-Erden-Verbindungen wurden weitere Untersuchungen durchgef��hrt. So f��hrt ein Austausch des La durch Ce zum m��glichen Auftreten von magnetischen Ph��nomenen. In dieser Folge konnten interessante Effekte wie magnetische Ordnungen im Einfluss eines Kristallfeldes untersucht werden, sowie m��gliches Auftreten eines Kondo Verhalten diskutiert werden., Superconductivity describes the behavior of a material to show absolute zero electrical resistivity as well as an absolute expulsion of a small external magnetic field (��0Hcth or ��0Hc1) below a transition temperature Tc. This is microscopically based on the condensation of two electrons to one Cooper pair. Each single Cooper pair shows a correlation to each other by a single phase. Therefore, a macroscopic wave function emerges. The exact pairing mechanisms and preconditions for the existence of superconductivity are not fully clarified, yet. Spin triplet as well as other unconventional pairing channels have attracted much interest on the topic of superconductivity to further synthesize new materials and structures, allowing superconductivity at higher temperatures. This doctoral thesis deals with superconductors lacking a center of inversion and strong correlations among Fermions. Polycrystalline and single crystalline materials were prepared by arc-melting. X-ray diffraction was used to precisely assign the samples to a certain point group without inversion center. With the help of EPMA (electron probe microanalysis) measurements, the stochiometric composition and the primary phases could be proven and verified. Samples get bombarded by electrons and become excited. The X-rays subsequently emitted are characteristic for every specific element. Afterward, the specimen were further cut and polished. Furthermore, their physical properties in dependence of temperature and external magnetic fields, especially the electrical resistivity, specific heat and magnetic susceptibility were determined. On certain samples, electrical resistivity measurements were performed while exposed to external hydrostatic pressure. Here, five new superconductors lacking a center of inversion, i.e. SrNiSi3, SrPdSi3, SrPtSi3, HfRhGe and LaPtSi, as well as a new superconductor (La3Pd4Si4) with possible two gaps, were identified and characterized. The superconducting transition temperatures are between 1 K and 3.6 K. The experimental data were supported by theoretical calculations of the band structure and partly show strong k-dependent splitting of the spin up and spin down bands and, therefore, possible spin triplet pairing channels. Further peculiarities of the non-centrosymmetric superconductors, i.e. the differences of the upper critical field (��0Hc2) of different physical properties, were discussed in the sense of the percolation theory. Predominately, the non-centrosymmetric compositions show a nodeless, full energy gap, and an s-wave, conventional BCS behavior with a negligible contribution of triplet Cooper pairing. Microscopical analysis of the singlet paring channels were enabled by means of the ��SR measurement technique. Furthermore, the material class of EpTX3 (Ep = Sr; Ba;T = Ni; Pd; Pt;X = Si; Ge) compounds could be fully examined. The rare earth compounds were further investigated. An exchange of La by Ce can lead to a formation of magnetic phenomena. Therefore, interesting effects like magnetic ordering in the presence of crystalline electric field splitting were investigated and the possible existence of Kondo behavior could be discussed.
- Published
- 2014
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3. BaAl4 derivative phases in the sections {La,Ce}Ni2Si2–{La,Ce}Zn2Si2: phase relations, crystal structures and physical properties
- Author
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Failamani, Fainan, primary, Malik, Zahida, additional, Salamakha, Leonid, additional, Kneidinger, Friedrich, additional, Grytsiv, Andriy, additional, Michor, Herwig, additional, Bauer, Ernst, additional, Giester, Gerald, additional, and Rogl, Peter, additional
- Published
- 2016
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4. Ferromagnetic Transition at 2.5 K in the Hexagonal Kondo-Lattice Compound CeRh6Ge4
- Author
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Matsuoka, Eiichi, primary, Hondo, Chisato, additional, Fujii, Tatsuya, additional, Oshima, Akihiro, additional, Sugawara, Hitoshi, additional, Sakurai, Takahiro, additional, Ohta, Hitoshi, additional, Kneidinger, Friedrich, additional, Salamakha, Leonid, additional, Michor, Herwig, additional, and Bauer, Ernst, additional
- Published
- 2015
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5. BaAl4 derivative phases in the sections {La,Ce}Ni2Si2–{La,Ce}Zn2Si2: phase relations, crystal structures and physical properties.
- Author
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Failamani, Fainan, Malik, Zahida, Salamakha, Leonid, Kneidinger, Friedrich, Grytsiv, Andriy, Michor, Herwig, Bauer, Ernst, Giester, Gerald, and Rogl, Peter
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BARIUM compounds ,NICKEL compounds ,ZINC compounds ,CRYSTAL structure ,PHASE transitions ,SOLUTION (Chemistry) - Abstract
Phase relations and crystal structures have been evaluated within the sections LaNi
2 Si2 –LaZn2 Si2 and CeNi2 Si2 –CeZn2 Si2 at 800 °C using electron microprobe analysis and X-ray powder and single crystal structure analyses. Although the systems La–Zn–Si and Ce–Zn–Si at 800 °C do not reveal compounds such as “LaZn2 Si2 ” or “CeZn2 Si2 ”, solid solutions {La,Ce}(Ni1−x Znx )2 Si2 exist with the Ni/Zn substitution starting from {La,Ce}Ni2 Si2 (ThCr2 Si2 -type; I4/mmm) up to x = 0.18 for Ce(Ni1−x Znx )2 Si2 and x = 0.125 for La(Ni1−x Znx )2 Si2 . For higher Zn-contents 0.25 ≤x≤ 0.55 the solutions adopt the CaBe2 Ge2 -type (P4/nmm). The investigations are backed by single crystal X-ray diffraction data for Ce(Ni0.61 Zn0.39 )2 Si2 (P4/nmm; a = 0.41022(1) nm, c = 0.98146(4) nm; RF = 0.012) and by Rietveld refinement for La(Ni0.56 Zn0.44 )2 Si2 (P4/nmm; a = 0.41680(6) nm, c = 0.99364(4) nm; RF = 0.043). Interestingly, the Ce–Zn–Si system contains a ternary phase CeZn2 (Si1−x Znx )2 of the ThCr2 Si2 structure type (0.25 ≤x≤ 0.30 at 600 °C), which forms peritectically at T = 695 °C but does not include the composition “CeZn2 Si2 ”. The primitive high temperature tetragonal phase with the CaBe2 Ge2 -type has also been observed for the first time in the Ce–Ni–Si system at CeNi2+x Si2−x , x = 0.33 (single crystal data, P4/nmm; a = 0.40150(2) nm, c = 0.95210(2) nm; RF = 0.0163). Physical properties (from 400 mK to 300 K) including specific heat, electrical resistivity and magnetic susceptibility have been elucidated for Ce(Ni0.61 Zn0.39 )2 Si2 and La(Ni0.56 Zn0.44 )2 Si2 . Ce(Ni0.61 Zn0.39 )2 Si2 exhibits a Kondo-type ground state. Low temperature specific heat data of La(Ni0.56 Zn0.44 )2 Si2 suggest a spin fluctuation scenario with an enhanced value of the Sommerfeld constant. [ABSTRACT FROM AUTHOR]- Published
- 2016
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6. Pressure Study of the Noncentrosymmetric 5d-Electron Superconductors CaMSi3(M= Ir, Pt)
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Eguchi, Gaku, primary, Kneidinger, Friedrich, additional, Salamakha, Leonid, additional, Yonezawa, Shingo, additional, Maeno, Yoshiteru, additional, and Bauer, Ernst, additional
- Published
- 2012
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7. Magnetic Properties of Highly Textured Fe85Ga15.
- Author
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Kriegisch, Martin, Groessinger, Roland, Grijalva, Cristina, Muhammad, Atif, Kneidinger, Friedrich, Mehboob, Nasir, Kubel, Frank, and Turtelli, Reiko Sato
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MAGNETIC properties of iron compounds ,METALS ,CRYSTAL texture ,MAGNETOSTRICTION ,X-ray diffraction ,THERMAL properties of metals - Abstract
Hysteresis loops and magnetostriction were measured on ring-shaped Fe85Ga15 samples. The samples were annealed at T=980^\circC for 100 h and cooled from annealing temperature in two different ways: one was slowly cooled in an oven and the other one was quenched in cold water. X-ray diffraction results showed a strong texture in these samples, which is dependent on the cooling process. The anisotropy energy and magnetostriction of the samples were calculated based on these determined textures. These results agree qualitatively with magnetization and magnetostriction experiments and support the thermal history influence on the magnetostriction values. [ABSTRACT FROM AUTHOR]
- Published
- 2014
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8. Pressure Study of the Noncentrosymmetric $5d$-Electron Superconductors Ca$M$Si3($M = \text{Ir}$, Pt)
- Author
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Eguchi, Gaku, Kneidinger, Friedrich, Salamakha, Leonid, Yonezawa, Shingo, Maeno, Yoshiteru, and Bauer, Ernst
- Abstract
We report a hydrostatic pressure study on the Rashba-type noncentrosymmetric superconductors Ca$M$Si3($M=\text{Ir}$, Pt). The temperature dependence of the resistivity of both compounds is well described by the conventional Bloch--Grüneisen formalism at each pressure. This fact suggests that electron--phonon scattering is dominant in these compounds. The superconducting critical temperature $T_{\text{c}}$ decreases with pressure above 0.41 to 2 GPa at a rate of ${\sim}0.2$ K/GPa for both compounds. This $T_{\text{c}}$ behavior can be explained with a modest decrease in the density of states based on the conventional BCS theory.
- Published
- 2012
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