1,721,013 research outputs found
Origin, symmetry and elastic tuneability of charge density waves in the superconductor BaNi(AsP)
Full text linkDer Ursprung und die Wechselwirkung verschiedener elektronischer Ordnungsphänomene wie Ladungsdichtewellen und Supraleitung sind ein hochinteressantes Forschungsfeld in vielen Quantenmaterialen. Insbesondere die Beeinflussung durch chemische Substituierung oder chemischen „Druck“, sowie hydrostatischen und uniaxialem Druck haben sich als besonders erkenntnisreich erwiesen. BaNiAs ist ein aussichtsreiches Material zur Untersuchung des Ursprunges und der Wechselwirkung zwischen (dynamischer) Nematizität, Ladungsdichtewellen und Supraleitung. Der Ursprung, Ordnungsmechanismus, Symmetrie und Beeinflussbarkeit durch Druck, vor allem für die Ladungsdichtewellen, sind größtenteils unbekannt und unerforscht. In dieser Doktorarbeit werden die zuvor genannten offenen Fragen genauer untersucht. Dafür werden inelastische Röntgenstreuung, thermische diffuse Röntgenstreuung, Dichtefunktionaltheorie, resonante elastische Röntgenstreuung und Röntgendiffraktion an reinem und phosphorsubstituiertem BaNiAs angewendet. Die Röntgendiffraktion wird zusätzlich unter hydrostatischem und uniaxialem Druck durchgeführt.
In dieser Doktorarbeit wird gezeigt, dass die inkommensurable Ladungsdichtewelle in BaNiAs durch ein niederenergetisches Phonon getrieben wird und nur wenig durch Phosphorsubstituierung beeinflusst wird. Die Ab-Initio-Rechnungen finden keine Anzeichen für Fermi-Flächen-Nesting oder -abhängige Elektron-Phonon-Kopplung, was auf eine unkonventionelle Natur der Ladungsdichtewelle hindeutet. Im Gegensatz dazu lässt sich für die kommensurable Ladungsdichtewelle in den Experimenten kein vergleichbares niederenergetisches Phonon ausmachen, das für die Instabilität verantwortlich ist. Die energie- und azimutwinkelabhängigen resonant elastischen Röntgenstreuungsexperimente offenbaren eine größtenteils vergleichbare orbitale Ordnung für beide Ladungsdichtewellen, die wahrscheinlich mit den d Orbitalen zusammenhängt. Dabei zeigt die Untersuchung des tensorialem atomaren Strukturfaktors, dass die lokale Symmetrie an der Nickelposition monoklin oder niedriger ist. Zudem wird für eine vollständige Beschreibung des Experiments neben Dipole-Dipole Übergängen auch Dipole-Quadrupole Übergänge benötigt. In den Diffraktionsexperimente unter hydrostatischem Druck werden zwei neue monokline Strukturen, die eng mit neuen Ladungsdichtewellen zusammenhängen, gefunden. Die ursprüngliche inkommensurable Ladungsdichtewelle, zeigt sich dabei als ungewöhnlich widerstandsfähig gegenüber hydrostatischem Druck. Erst in einer monoklinen Struktur über 11 GPa sind alle Ladungsdichtewellen verschwunden. Die monokline Struktur weist dabei einige Übereinstimmungen mit der kollabierten tetragonalen Struktur, die in anderen Pniktiden gefunden wurde, auf. Innerhalb dieser Struktur sind die Nickel-Arsen Bindungen deutlich verändert. Dies hebt die Bedeutung der hybridisierung der Nickel- und Arsenorbitale für die Formierung der Ladungsdichtewellen hervor. Die Kombination von Phosphorsubstitution und hydrostatischem Druck zeigt weitgehend das gleiche Phasendiagramm. Allerdings werden alle Tieftemperaturstrukturen und die damit verbundenen Ladungsdichtewellen teilweise oder vollständig unterdrückt. Die Messungen unter uniaxialem Druck zeigen eine Verstärkung der inkommensurablen Ladungsdichtewelle entlang der Druckrichtung und eine Unterdrückung der inkommensurablen Ladungsdichtewellen orthogonal dazu. Dies ist jedoch nur in der geordneten Phase und nicht in der fluktuierenden Phase zu beobachten. Zusammengefasst zeigt diese Doktorarbeit die Wichtigkeit der orbitalen Ordnung und Hybridisierung in BaNiAs, insbesondere für die Formierung der Ladungsdichtewellen, auf
Proximity-Induced Magnetization in SrIrO₃ Thin Films
Full text linkTo reduce the increasing power consumption in information technology, energy-efficient devices are the need of the hour. In contrast to conventional electronic devices, which rely on charge transport, spintronic devices offer energy-saving alternatives and could pave the way for next-generation information processing. The magneto-electric coupling, essential for these devices, is usually provided by spin-orbit coupling (SOC). In 3d-transition metal oxides (TMOs), electron-electron correlation is strong; however, SOC, which evolves with increasing atomic numbers, is only small. On the other hand, in 5d-TMOs, which display strong SOC, the electron correlation is rather weak. Therefore, combining both 3d and 5d- TMOs in artificially grown heterostructures (HSs) provides a promising platform to search for new quantum materials showing exotic effects and potential for next-generation spintronic devices.
In this thesis, perovskite SrIrO3 (SIO) HSs, in which 5d-TMO SIO is combined with magnetic 3d-TMO perovskite, are analyzed in detail with respect to proximity induced changes of the magnetization and electronic transport in the presence of a neighboured magnetic layer, e.g., LaXO3 or NdNiO3 (LXO, NNO) with X = Mn, Fe and Co. To this purpose, high-quality LXO(NNO)/SIO HSs were produced by pulsed laser deposition (PLD). PLD is a commonly used thin film deposition technique to produce epitaxial TMO thin films and HSs with high epitaxial quality and thickness resolution of one unit cell.
In contrast to the SIO single layer, the LCO/SIO HSs, where LCO is a ferromagnetic (FM) insulator, display a strong intrinsic anomalous Hall effect (AHE) and anisotropic magnetoresistance indicating in-plane antiferromagnetic (AFM) ordering similar to that of layered Sr2IrO4 with effective moment (meff) along the pseudo-cubic (110) direction. Separating LCO and SIO by a 4 monolayer thick, insulating SrTiO3 layer and increasing the SIO thickness in LCO/SIO HSs suppress the effects strongly and document proximity-induced AHE, magnetic moment and magnetocrystalline anisotropy in SIO.
Electrostatic doping by electric field effect in back gate geometry enhances AHE by a factor of 7 but leaves the magnetic moment of meff ≈0.02 μB/Ir nearly unaffected. Therefore, the origin of the strong AHE is very likely caused by enhanced Berry curvature (BC) due to the topological band structure of SIO (also supported by recently published DFT results).
Substitution of the LCO layer by other magnetic 3d-TMO layers allows for more detailed analysis of the proximity effect, magnetic exchange, and coupling between SIO and the 3d magnetic layer. With increasing 3d-electron number, interfacial charge transfer is observed from 5d-SIO to 3d-TMO which increases systematically from Mn to Co (Ni). However, the induced spin and orbital magnetic moment in SIO is very similar in all HSs. In contrast, the AHE shows significant enhancement when a FM layer (LCO, LMO) is interfaced with SIO, in comparison to SIO/AFM HSs; where the AHE is one order of magnitude smaller, indicating the strong influence of the effective magnetic moment and effective local field associated with the magnetic layer at the interface on the BC and hence AHE.
The obtained results provide important information on the interfacial properties of 3d/5d HSs and demonstrate effective coupling between perovskite-related 3d-TMO and SIO. On the other side, proximity-induced SOC in the 3d-TMO layer is also very likely and could be useful for magnetization manipulation in 3d-TMOs by electric field via the Rashba effect. This issue will be addressed in future works
Strange bedfellows inside a superconductor
Full text linkThe discovery of superconducting cuprates in 1986 is considered a watershed moment in the study of superconductivity—not only because of their high superconducting temperatures (TC’s) but also on account of their highly exotic properties, which are still largely enigmatic (1). On page 1506 of this issue, Wahlberg et al. (2) bring insights into the intriguing physics of cuprates’ nonsuperconducting state by connecting two widely studied phenomena previously believed to be completely independent of each other: the linear resistivity of the strange metallic phase and charge density waves (CDWs)
Strange metal dynamics in SrIrO3
No full textThe interplay of electronic correlations, multi-orbital excitations, and strong spin-orbit coupling is a fertile ground for new states of matter in quantum materials. Here, we report on a confocal Raman scattering study of momentum-resolved charge dynamics from a thin film of semimetallic perovskite . We demonstrate that the charge dynamics, characterized by a broad continuum, is well described in terms of the marginal Fermi liquid phenomenology. In addition, over a wide temperature regime, the inverse scattering time is for all momenta close to the Planckian limit . Thus, is a semimetallic multi-band system that is as correlated as, for example, the cuprate superconductors.
The usual challenge to resolve the charge dynamics in multi-band systems with very different mobilities is circumvented by taking advantage of the momentum space selectivity of polarized electronic Raman scattering.
The Raman responses of both hole- and electron-pockets display an electronic continuum extending far beyond 1000\icm (125 meV), much larger than allowed by the phase space for creating particle-hole pairs in a regular Fermi liquid. Analyzing this response in the framework of a memory function formalism, we are able to extract the frequency dependent scattering rate and mass enhancement factor of both types of charge carriers, which in turn allows us to determine the carrier-dependent mobilities and electrical resistivities. The results are well consistent with transport measurement and demonstrate the potential of this approach to investigate the charge dynamics in multi-band systems
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
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