63 research outputs found
Theory of many-body effects in the Kondo-lattice model
Das magnetische Verhalten zahlreicher Materialien lässt sich auf eine indirekte Wechselwirkung lokalisierter magnetischer Momente, vermittelt durch die Elektronen eines Leitungsbandes, zurückführen. Das Kondo-Gitter-Modell hat sich als elegante Möglichkeit bewährt, diesen Prozess quantenmechanisch zu beschreiben. Es reduziert die Physik auf eine intraatomare Wechselwirkung der Spins von lokalisierten und itineranten Elektronen. Die vorliegende Arbeit ist den analytischen Eigenschaften dieses Modells gewidmet. Die besondere Herausforderung des Kondo-Gitter-Modells besteht dabei im Zusammenwirken zweier verschiedener Teilchensorten, beschrieben durch Fermi-Operatoren sowie quantenmechanische Spins. Bisherige Untersuchungen haben sich in der Regel nur auf eine der beiden Teilchensorten konzentriert. Mit der Projektions-Operator-Methode stellen wir eine Möglichkeit vor, beide Teilsysteme in gleicher Qualität zu behandeln. Die Auswertung des Teilsystems der itineranten Elektronen führt auf einen Ausdruck für die Selbstenergie, der lineare und quadratische Effekte in der Wechselwirkung exakt beschreibt. Die resultierenden Zustandsdichten weisen starke Korrelationseffekte auf. Deren Untersuchung dient sowohl der Bestätigung von Ergebnissen weniger systematischer Zugänge als auch dem Aufzeigen neuer Vielteilchen-Phänomene. Die Anwendung der Projektions-Operator-Methode auf das System der lokalisierten Momente führt zu einer Analyse der bereits bekannten RPA (random phase approximation). Zu diesem Zweck werden die Magnonenspektren und die Curie-Temperaturen systematisch untersucht. Dabei treten bisher unbekannte Schwachpunkte der RPA zu Tage, die auch die Kombination mit Theorien für das itinerante Teilsystem verhindern. Verbesserungen und Alternativen zur RPA werden diskutiert.The magnetic behaviour of various materials is due to an indirect interaction of localized magnetic moments, which is based on itinerant electrons in a conduction band. The Kondo-lattice model is an elegant approach for a quantum-mechanical description of this process. It reduces the relevant physics to an intra-atomic exchange interaction of the localized and the itinerant electrons. The aim of the present work is a detailed investigation of analytic properties of this model. Here, the interplay of two distinct types of particles, described by Fermi operators and quantum-mechanical spin operators respectively, is a major challenge of the considered model. Previous studies have focused on one of these subsystems only. Using the projection-operator method, we suggest an efficient way to describe both subsystems on the same level of approximation. An evaluation of the subsystem of itinerant electrons yields an expression for the self-energy, which describes linear and quadratic interaction effects exactly. The densities of states derived with this theory show strong correlation effects. We were able to assess results obtained with less systematic approaches and to predict new many-particle effects. The application of the projection-operator method to the subsystem of localized magnetic moments results in a detailed analysis of the RPA (random phase approximation). The dependence of magnon spectra and Curie temperatures on model parameters are investigated systematically. Previously unknown drawbacks of the RPA are revealed, which prevent the combination of these results with theories for the itinerant subsystem. Improvements beyond RPA and alternative approximations are discussed
Many-body effects in the persistent current problem
In this work some many-body properties of isolated mesoscopic rings are investigated.
Second quantization and tight-binding models for systems of spinless
fermions and fermions with spin are used to derive an expression for the persistent
current. The results obtained for non-interacting systems are in satisfactory
agreement with both experimental measurements and other theoretical results.
Then a Coulomb repulsion is considered for a system of interacting fermions and
a variational approach is adopted. We attempt to improve the description of
the system by introducing rotations of the spin-quantization axis on each site.
Then we go on to show how the emergent Hartree–Fock equations may be treated,
what kind of effects have to be considered and how the trial wave functions can
be chosen accordingly
Electronic properties, low-energy Hamiltonian and superconducting instabilities in CaKFeAs
We analyze the electronic properties of the recently discovered
stoichiometric superconductor CaKFeAs by combining an ab initio
approach and a projection of the band structure to a lowenergy tight-binding
Hamiltonian, based on the maximally localized Wannier orbitals of the 3d Fe
states. We identify the key symmetries as well as differences and similarities
in the electronic structure between CaKFeAs and the parent systems
CaFeAs and KFeAs. In particular, we find CaKFe4As4 to have a
significantly more quasi-two-dimensional electronic structure than the latter
systems. Finally, we study the superconducting instabilities in CaKFeAs
by employing the leading angular harmonics approximation (LAHA) and find two
potential A-symmetry representation of the superconducting gap to be the
dominant instabilities in this system.Comment: 17 pages, 10 figure
A comparison of atomistic and continuum theoretical approaches to determine electronic properties of GaN/AlN quantum dots
In this work we present a comparison of multiband k.p-models, the effective
bond-orbital approach, and an empirical tight-binding model to calculate the
electronic structure for the example of a truncated pyramidal GaN/AlN
self-assembled quantum dot with a zincblende structure. For the system under
consideration, we find a very good agreement between the results of the
microscopic models and the 8-band k.p-formalism, in contrast to a 6+2-band
k.p-model, where conduction band and valence band are assumed to be decoupled.
This indicates a surprisingly strong coupling between conduction and valence
band states for the wide band gap materials GaN and AlN. Special attention is
paid to the possible influence of the weak spin-orbit coupling on the localized
single-particle wave functions of the investigated structure
Ab-initio investigation of lattice distortions in response to van der Waals interactions in FeSe
The electronic structure in unconventional superconductors holds a key to
understand the momentum-dependent pairing interactions and the resulting
superconducting gap function. In superconducting Fe-based chalcogenides, there
have been controversial results regarding the importance of the
dependence of the electronic dispersion, the gap structure and the pairing
mechanisms of iron-based superconductivity. Here, we present a detailed
investigation of the van der Waals interaction in FeSe and its interplay with
magnetic disorder and real space structural properties. Using density
functional theory we show that they need to be taken into account upon
investigation of the 3-dimensional effects, including non-trivial topology, of
FeSeTe and FeSeS systems. In addition, the impact of
paramagnetic (PM) disorder is considered within the spin-space average
approach. Our calculations show that the PM relaxed structure supports the
picture of different competing ordered magnetic states in the nematic regime,
yielding magnetic frustration.Comment: 10 pages, 8 figure
Growth process, characterization, and modeling of electronic properties of coupled InAsSbP nanostructures
Quaternary III-V InAsSbP quantum dots (QDs) have been grown in the form of cooperative InAsSb/InAsP structures using a modified version of the liquid phase epitaxy. High resolution scanning electron microscopy, atomic force microscopy, and Fourier-transform infrared spectrometry were used to investigate these so-called nano-camomiles, mainly consisting of a central InAsSb QD surrounded by six InAsP-QDs, that shall be referred to as leaves in the following. The observed QDs average density ranges from 0.8 to 2 x 10(9) cm(-2), with heights and widths dimensions from 2 to 20 nm and 5 to 45 nm, respectively. The average density of the leaves is equal to (6-10) x 10(9) cm(-2) with dimensions of approx. 5 to 40 nm in width and depth. To achieve a first basic understanding of the electronic properties, we have modeled these novel nanostructures using second-order continuum elasticity theory and an eight-band k . p model to calculate the electronic structure. Our calculations found a clear localization of hole states in the central InAsSb dot. The localization of electron states, however, was found to be weak and might thus be easily influenced by external electric fields or strain. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3624621
Grain boundary diffusion in severely deformed Al-based alloy
Grain boundary diffusion in severely deformed Al-based AA5024 alloy is
investigated. Different states are prepared by combination of equal channel
angular processing and heat treatments, with the radioisotope Co being
employed as a sensitive probe of a given grain boundary state. Its diffusion
rates near room temperature (320~K) are utilized to quantify the effects of
severe plastic deformation and a presumed formation of a previously reported
deformation-modified state of grain boundaries, solute segregation at the
interfaces, increased dislocation content after deformation and of the
precipitation behavior on the transport phenomena along grain boundaries. The
dominant effect of nano-sized AlSc-based precipitates is evaluated using
density functional theory and the Eshelby model for the determination of
elastic stresses around the precipitates.Comment: 13 pages, 7 figure
Low temperature features in the heat capacity of unary metals and intermetallics for the example of bulk aluminum and AlSc
We explore the competition and coupling of vibrational and electronic
contributions to the heat capacity of Al and AlSc at temperatures below 50
K combining experimental calorimetry with highly converged finite temperature
density functional theory calculations. We find that semilocal exchange
correlation functionals accurately describe the rich feature set observed for
these temperatures, including electron-phonon coupling. Using different
representations of the heat capacity, we are therefore able to identify and
explain deviations from the Debye behaviour in the low-temperature limit and in
the temperature regime 30 - 50 K as well as the reduction of these features due
to the addition of Sc.Comment: 10 pages, 6 figures in total, paper submitted to Physical Review
Mechanism of collective interstitial ordering in Fe–C alloys
Collective interstitial ordering is at the core of martensite formation in Fe–C-based alloys, laying the foundation for high-strength steels. Even though this ordering has been studied extensively for more than a century, some fundamental mechanisms remain elusive. Here, we show the unexpected effects of two correlated phenomena on the ordering mechanism: anharmonicity and segregation. The local anharmonicity in the strain fields induced by interstitials substantially reduces the critical concentration for interstitial ordering, up to a factor of three. Further, the competition between interstitial ordering and segregation results in an effective decrease of interstitial segregation into extended defects for high interstitial concentrations. The mechanism and corresponding impact on interstitial ordering identified here enrich the theory of phase transitions in materials and constitute a crucial step in the design of ultra-high-performance alloys
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