1,721,039 research outputs found
Low-temperature magnetic susceptibility of Yb(Rh1-xMx)2Si2(M = Ir, Co) single crystals
Full text linkWe report low-temperature measurements of the magnetic susceptibility of Yb(Rh1-xMx)(2)Si-(2)(M=Ir,Co) single crystals for 0 = 0.07 two subsequent antiferromagnetic transitions are observed. (C) 2007 Elsevier B.V. All rights reserved
Unusual weak magnetic exchange in two different structure types: YbPt<sub>2</sub>Sn and YbPt<sub>2</sub>In
No full textWe present the structural, magnetic, thermodynamic and transport properties of the two new compounds YbPt2Sn and YbPt2In. X-ray powder diffraction shows that they crystallize in different structure types, the hexagonal ZrPt2Al and the cubic Heusler type, respectively. Despite quite different lattice types, both compounds present very similar magnetic properties: a stable trivalent Yb3+, no evidence for a sizeable Kondo interaction and very weak exchange interactions with a strength below 1 K as deduced from specific heat C(T). Broad anomalies in C(T) suggest short range magnetic ordering at about 250 mK and 180 mK for YbPt2Sn and YbPt2Sn, respectively. The weak exchange and the low ordering temperature result in a large magnetocaloric effect as deduced from the magnetic field dependence of C(T), making these compounds interesting candidates for magnetic cooling. In addition we found in YbPt2Sn evidences for a charge density wave transition at about 290 K. The occurrence of such transitions within several RET2X compound series (RE = rare earth, T = noble metal, X = In, Sn) is analyzed
Unconventional quantum criticality in YbRh2Si2
Full text linkYbRh2Si2 is a clean heavy fermion system which displays a magnetic field tuned quantum critical point. We present low-temperature electrical resistivity, magnetostriction and magnetization measurements on high-quality single crystals with a residual resistivity ratio of 150. The data provide evidence for a low-energy scale T (H) which vanishes in addition to the boundaries of the magnetic ordering and Landau Fermi liquid regime at the quantum critical point, indicating unconventional quantum criticality. (C) 2007 Elsevier B.V. All rights reserved
Magnetization study of the energy scales in YbRh<sub>2</sub>Si<sub>2</sub> under chemical pressure
Full text linkWe present a systematic study of the magnetization in YbRh2Si2 under slightly negative (6% Ir substitution) and positive (7% Co substitution) chemical pressure. We show how the critical field H0, associated with the high-field Lifshitz transitions, is shifted to lower (higher) values with Co (Ir) substitution. The critical field HN, which identifies the boundary line of the antiferromagnetic (AFM) phase TN(H) increases with positive pressure and it approaches zero with 6% Ir substitution. On the other side, the crossover field H , associated with the energy scale T (H) where a reconstruction of the Fermi surface has been observed, is not much influenced by the chemical substitution. Following the analysis proposed in Refs. 14 we have fitted the quantity with a crossover function to indentify H . The T (H) line follows an almost linear H-dependence at sufficiently high fields outside the AFM phase, but it deviates from linearity at TTN(0) and in Yb(Rh0.93Co0.07)2Si2 it changes slope clearly inside the AFM phase. Moreover, the full width at half maximum (FWHM) of the fit function depends linearly on temperature outside the phase, but remains constant inside, suggesting either that such an analysis is valid only for TTN(0) or that the Fermi surface changes continuously at T=0 inside the AFM phase. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimDFG Research Unit 960 "Quantum Phase Transitions
Das System LaFeAsO in Poly- und Einkristallen
Full text linkIn dieser Arbeit wurde die Ausgangsverbindung der eisenbasierten Supraleiter, LaFeAsO, durch die Synthese und Charakterisierung von poly- und einkristallinen Proben untersucht. Supraleitung kann in den eisenbasierten Supraleitern durch Elektronen- oder Lochdotierung hervorgerufen werden. Die Substitution von Eisen durch Mangan, formal eine Lochdotierung, hat hingegen einen destruktiven Effekt auf die Supraleitung. Dieser ist bei optimal fluordotiertem LaFeAs(O,F) um Größenordnungen stärker ausgeprägt als bei Nd- oder Sm-FeAs(O,F). Indem Lanthan partiell durch das kleinere Yttrium substituiert wurde, konnte gezeigt werden, dass diese unterschiedlich starke Mangantoleranz durch die Unterschiede im Seltenerdmetall-Ionenradius bedingt ist. Weiterhin finden sich Anzeichen, dass die Unterdrückung der Supraleitung durch Mangan mit Elektronenlokalisierung korreliert ist.
Das Fehlen von großen dreidimensionalen Einkristallen der SEFeAsO-Verbindungsklasse stellt ein großes Hindernis in der Erforschung der elektronischen Eigenschaften der eisenbasierten Supraleiter dar. Im Rahmen dieser Arbeit konnte gezeigt werden, dass das Verfahren der Festkörper-Einkristallzüchtung ein geeignetes Mittel darstellt, um große, facettierte SEFeAsO-Einkristalle mit ausgeprägtem Wachstum in c-Richtung zu erhalten.
Mit diesem neu entwickelten Einkristallzüchtungsverfahren konnte ein aktualisiertes Phasendiagramm von La(Fe,Co)AsO erstellt werden. Die Substitution von Eisen durch Cobalt entspricht einer Elektronendotierung und führt zu Supraleitung mit einer maximalen Sprungtemperatur von 12 K. Die Ausgangsverbindung LaFeAsO zeigt bei etwa 156 K einen strukturellen Phasenübergang von einer tetragonalen zu einer orthorhombischen Struktur, weiterhin tritt unterhalb von etwa 138 K eine Spindichtewelle auf. In Einklang mit dem bekannten Phasendiagramm werden mit Cobaltdotierung die beiden Übergänge unterdrückt, mit höheren Cobaltkonzentrationen kommt es zu Supraleitung.
Anders als beim bekannten Phasendiagramm kann eine deutliche Aufspaltung zwischen magnetischem und strukturellen Übergang bei kleinen Cobaltkonzentrationen beobachtet werden. Außerdem findet sich eine Region der Koexistenz zwischen Supraleitung und Spindichtewelle. Bisher konnte ein solcher Zustand im SE(Fe,Co)AsO-System nicht beobachtet werden
Multiple Energy Scales at a Quantum Critical Point
Full text linkWe report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh2Si2. The data provide evidence for an energy scale in the equilibrium excitation spectrum that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ferromagnetische Korrelationen in Kondo-Gittern: YbT2Si2 und CeTPO (T = Übergangsmetall)
Full text linkIm Rahmen dieser Arbeit wurden die Kondo-Gitter YbT2Si2 (T = Rh, Ir, Co) und CeTPO (T = Ru, Os, Fe, Co) untersucht. In diesen Systemen treten starke ferromagnetische Korrelationen der 4f-Momente zusammen mit ausgeprägter Kondo-Wechselwirkung auf, deren theoretische Beschreibung bislang sehr kontrovers diskutiert wird. Diese Arbeit liefert damit einen essentiellen experimentellen Beitrag zur Physik von ferromagnetischen Kondo-Gittern. So konnten qualitativ hochwertige Einkristalle von YbRh2Si2 hergestellt und erstmalig an einem Schwere-Fermion-System deren kritische Fluktuationen um den magnetischen Phasenübergang analysiert werden. Weiterhin konnte das bis dahin unverstandene Auftreten einer Elektron-Spin-Resonanz (ESR)-Linie in YbT2Si2 auf ferromagnetische Korrelationen zurückgeführt werden. Außerdem wurde mit CeFePO ein neues Schwere-Fermion-System mit starken ferromagnetischen Korrelationen entdeckt sowie mit dem isoelektronischen CeRuPO der seltene Fall eines ferromagnetisch geordneten Kondo-Gitters realisiert.Within the context of this thesis the Kondo lattices YbT2Si2 (T = Rh, Ir, Co) and CeTPO (T = Ru, Os, Fe, Co) were investigated. In these systems strong ferromagnetic correlations of the 4f-moments together with pronounced Kondo interactions are present, whose theoretical description are pres-ently controversial discussed. Therefore, this work gives an essential experimental contribution to the physics of ferromagnetic Kondo lattices. The main results include the growth of high-quality single crystals of YbRh2Si2 and the first analysis of the critical fluctuations around the magnetic phase transition in a heavy fermion system. Furthermore, the unexpected observation of an electron spin resonance in YbT2Si2 could be ascribed to ferromagnetic correlations. Moreover, a new heavy fermion system CeFePO with strong ferromagnetic correlations was found and with the isoelec-tronic CeRuPO the rare case of a ferromagnetic Kondo-lattice discovered
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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