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Author: Brian Patterson Neal
Publisher:
ISBN: 9781303792397
Category :
Languages : en
Pages :
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Book Description
The transition metal intermetallic compound NbFe2 displays a magnetic quantum critical point very near stoichiometry, unlike other Fe-based intermetallics, and no field or pressure tuning is required. In this compound we obtain an obvious candidate for the origin of quantum criticality: an accidental Fermi surface "hot stripe" centered on a point of vanishing quasiparticle velocity on the Fermi surface at an unconventional band critical point (uBCP) of NbFe2. Around this uBCP the dispersion is cubic ([epsilon](k) - [epsilon](f) [proportional to] k(superscript 3 over subscript x)) in one direction in the hexagonal basal plane and has a saddle point character in the orthogonal k(y), k(z) plane; both aspects have significant consequences. At such a uBCP Moriya's theory of weak magnetism breaks down due to divergent contributions to the dynamic bare susceptibility from the uBCP, both at Q (rigth arrow) 0 and at momenta spanning the uBCPs. These results are reminiscent of an earlier suggestion that anomalously low Fermi velocities are an essential aspect of the incipient or weak ferromagnetism of TiBe2, and strongly support the viewpoint that, for some quantum critical points, the mechanism may be identifiable in the underlying (mean field) electronic structure. Next, we study YMn2 in the cubic Laves phase (C15). With its highly frustrated pyrochlore type sublattice of Mn sites, it is one of a small but growing class of ordered magnets that lie close to a quantum critical point at stoichiometry. Its ground state displays long-spiral helical magnetic order that is highly sensitive to volume, disappearing due to the substitution of 3% Sc for the larger Y atom (chemical pressure), or by application of just 0.4 GPa pressure. The large change of volume (5%) upon ordering (T(N) = 100 K) argues for itinerant magnetism. In recentyears there have been developments in modeling magnetic fluctuations in itinerant magnets near the ordering point. We extend earlier results of Terao and Yamada on the first principles based energetics versus volume, and quantify the sensitivity of the magnetic state to pressure.Finally, we examine a tertiary nitride compound, Fe3Mo3N, which demonstrates quantum critical behavior and weak magnetism, with a structure consisting of a geometrically frustrated stella quadrangula lattice. Neutron scattering reveals antiferromagnetic ordering, but a 14 T magnetic field induces a ferromagnetic state, as does substitution of 5% Co on the Fe site. We present the energetics of a transition between these states with density functional based fixed spin moment studies. As the fixed spin moment in calculation is reduced, the ferromagnetic state switches to a ferrimagnetic state, approximately 0.2 eV higher in energy per formula unit.
Author: Brian Patterson Neal
Publisher:
ISBN: 9781303792397
Category :
Languages : en
Pages :
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Book Description
The transition metal intermetallic compound NbFe2 displays a magnetic quantum critical point very near stoichiometry, unlike other Fe-based intermetallics, and no field or pressure tuning is required. In this compound we obtain an obvious candidate for the origin of quantum criticality: an accidental Fermi surface "hot stripe" centered on a point of vanishing quasiparticle velocity on the Fermi surface at an unconventional band critical point (uBCP) of NbFe2. Around this uBCP the dispersion is cubic ([epsilon](k) - [epsilon](f) [proportional to] k(superscript 3 over subscript x)) in one direction in the hexagonal basal plane and has a saddle point character in the orthogonal k(y), k(z) plane; both aspects have significant consequences. At such a uBCP Moriya's theory of weak magnetism breaks down due to divergent contributions to the dynamic bare susceptibility from the uBCP, both at Q (rigth arrow) 0 and at momenta spanning the uBCPs. These results are reminiscent of an earlier suggestion that anomalously low Fermi velocities are an essential aspect of the incipient or weak ferromagnetism of TiBe2, and strongly support the viewpoint that, for some quantum critical points, the mechanism may be identifiable in the underlying (mean field) electronic structure. Next, we study YMn2 in the cubic Laves phase (C15). With its highly frustrated pyrochlore type sublattice of Mn sites, it is one of a small but growing class of ordered magnets that lie close to a quantum critical point at stoichiometry. Its ground state displays long-spiral helical magnetic order that is highly sensitive to volume, disappearing due to the substitution of 3% Sc for the larger Y atom (chemical pressure), or by application of just 0.4 GPa pressure. The large change of volume (5%) upon ordering (T(N) = 100 K) argues for itinerant magnetism. In recentyears there have been developments in modeling magnetic fluctuations in itinerant magnets near the ordering point. We extend earlier results of Terao and Yamada on the first principles based energetics versus volume, and quantify the sensitivity of the magnetic state to pressure.Finally, we examine a tertiary nitride compound, Fe3Mo3N, which demonstrates quantum critical behavior and weak magnetism, with a structure consisting of a geometrically frustrated stella quadrangula lattice. Neutron scattering reveals antiferromagnetic ordering, but a 14 T magnetic field induces a ferromagnetic state, as does substitution of 5% Co on the Fe site. We present the energetics of a transition between these states with density functional based fixed spin moment studies. As the fixed spin moment in calculation is reduced, the ferromagnetic state switches to a ferrimagnetic state, approximately 0.2 eV higher in energy per formula unit.
Author: Andreas W Rost
Publisher: Springer Science & Business Media
ISBN: 3642145248
Category : Science
Languages : en
Pages : 154
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Book Description
Our department nominated this thesis for a Springer award because we regard it as an outstanding piece of work, carried out with a remarkable level of independence. Andreas Rost joined us in 2005, as one of the inaugural Prize Students of the Scottish Universities Physics Alliance. Our research group has been working on Sr Ru O , in collaboration with our colleagues in the group of Professor Y. Maeno 3 2 7 at Kyoto, since 1998. By early 2005 we had tantalising evidence that a novel phase was forming at very low temperatures, in an overall phase diagram dominated by quantum ?uctuations. We knew that comprehensive thermodynamic information would be needed in order to understand how this was happening, and that the demanding constraints of low temperature and high magnetic ?eld meant that bespoke apparatus would need to be constructed. Andreas had studied the speci?c heat of glasses below 50 mK during his diploma thesis work at Heidelberg, and was brimming with ideas about how to proceed. We gave him advice, and constantly discussed the physics with him, but quickly realised that the best way to proceed practically was to give him a budget, and let him take the main design decisions, double-checking with us from time to time.
Author: Eberhard Engel
Publisher: Springer Science & Business Media
ISBN: 3642140904
Category : Science
Languages : en
Pages : 543
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Book Description
Density Functional Theory (DFT) has firmly established itself as the workhorse for atomic-level simulations of condensed phases, pure or composite materials and quantum chemical systems. This work offers a rigorous and detailed introduction to the foundations of this theory, up to and including such advanced topics as orbital-dependent functionals as well as both time-dependent and relativistic DFT. Given the many ramifications of contemporary DFT, the text concentrates on the self-contained presentation of the basics of the most widely used DFT variants: this implies a thorough discussion of the corresponding existence theorems and effective single particle equations, as well as of key approximations utilized in implementations. The formal results are complemented by selected quantitative results, which primarily aim at illustrating the strengths and weaknesses of particular approaches or functionals. The structure and content of this book allow a tutorial and modular self-study approach: the reader will find that all concepts of many-body theory which are indispensable for the discussion of DFT - such as the single-particle Green's function or response functions - are introduced step by step, along with the actual DFT material. The same applies to basic notions of solid state theory, such as the Fermi surface of inhomogeneous, interacting systems. In fact, even the language of second quantization is introduced systematically in an Appendix for readers without formal training in many-body theory.
Author:
Publisher: Academic Press
ISBN: 0080582583
Category : Science
Languages : en
Pages : 417
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Book Description
Quantum mechanics can describe the detailed structure and behavior of matter, from electrons, atoms, and molecules, to the whole universe. It is one of the fields of knowledge that yield extraordinary precessions, limited only by the computational resources available. Among these methods is density functional theory (DFT), which permits one to solve the equations of quantum mechanics more efficiently than with any related method.The present volume represents the most comprehensive summary currently available in density functional theory and its applications in chemistry from atomic physics to molecular dynamics. DFT is currently being used by more than fifty percent of computational chemists.
Author: John F. Dobson
Publisher: Springer Science & Business Media
ISBN: 148990316X
Category : Science
Languages : en
Pages : 384
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Book Description
This book is an outcome of the International Workshop on Electronic Density Functional Theory, held at Griffith University in Brisbane, Australia, in July 1996. Density functional theory, standing as it does at the boundary between the disciplines of physics, chemistry, and materials science, is a great mixer. Invited experts from North America, Europe, and Australia mingled with students from several disciplines, rapidly taking up the informal style for which Australia is famous. A list of participants is given at the end of the book. Density functional theory (DFT) is a subtle approach to the very difficult problem of predicting the behavior of many interacting particles. A major application is the study of many-electron systems. This was the workshop theme, embracing inter alia computational chemistry and condensed matter physics. DFT circumvents the more conceptually straightforward (but more computationally intensive) approach in which one solves the many-body Schrodinger equation. It relies instead on rather delicate considerations involving the electron number density. For many years the pioneering work of Kohn and Sham (the Local Density Ap proximation of 1965 and immediate extensions) represented the state of the art in DFT. This approach was widely used for its appealing simplicity and computability, but gave rather modest accuracy. In the last few years there has been a renaissance of interest, quite largely due to the remarkable success of the new generation of gradient functionals whose initiators include invitees to the workshop (Perdew, Parr, Yang).
Author: Daniel Glossman-Mitnik
Publisher: BoD – Books on Demand
ISBN: 1839698454
Category : Science
Languages : en
Pages : 332
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Book Description
Density Functional Theory (DFT) is a powerful technique for calculating and comprehending the molecular and electrical structure of atoms, molecules, clusters, and solids. Its use is based not only on the capacity to calculate the molecular characteristics of the species of interest but also on the provision of interesting concepts that aid in a better understanding of the chemical reactivity of the systems under study. This book presents examples of recent advances, new perspectives, and applications of DFT for the understanding of chemical reactivity through descriptors forming the basis of Conceptual DFT as well as the application of the theory and its related computational procedures in the determination of the molecular properties of different systems of academic, social, and industrial interest.
Author: Reiner M. Dreizler
Publisher: Springer Science & Business Media
ISBN: 3642861059
Category : Science
Languages : en
Pages : 312
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Book Description
Density Functional Theory is a rapidly developing branch of many-particle physics that has found applications in atomic, molecular, solid-state and nuclear physics. This book describes the conceptual framework of density functional theory and discusses in detail the derivation of explicit functionals from first principles as well as their application to Coulomb systems. Both non-relativistic and relativistic systems are treated. The connection of density functional theory with other many-body methods is highlighted. The presentation is self-contained; the book is, thus, well suited for a graduate course on density functional theory.
Author: Maurizio Ferconi
Publisher:
ISBN:
Category : Electronic structure
Languages : en
Pages : 280
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Book Description
Most of the work done in the physics of two-dimensional structures is either limited by the size of the systems that are studied due to the use of exact diagonalization techniques which do not allow to go beyond structures with only few electrons, or by other methods which do not include exchange-correlation effects, or correlation effects only. There are instances in which these effects cannot be neglected. In order to fill this gap we decided to use a density functional approach, which proved to be reliable in other problems. In Chapter 3 I shall present original results on the study of the ground-state properties of quantum dots in the presence of a magnetic field of arbitrary strength obtained within a current-density functional approach. The results obtained agree very well with those obtained by means of exact diagonalization approaches. In the regime not accessible by the latter methods we show how important are exchange-correlation effects, and we also make an analysis of correlation effects, by comparison with Hartree-Fock results. In Chapter 4 we address and study the density profile at the edge of a confined two-dimensional electron gas in the fractional quantum Hall regime. The strong correlation effects are accounted for in a density functional approach. I show that there exists a transition between two regimes as a function of the smoothness of the confining potential. In Chapter 5, we study the magnetophonon dispersion relations of a two-dimensional Wigner crystal in the presence of a strong magnetic field, and of its shear modulus, that we derived within a density functional scheme, that therefore includes correlation effects, and has the advantage of being analytical. In Chapter 6 I present a theory for the calculation of the pinning gap and magnetophonon spectrum of a disordered Wigner crystal. Its main feature resides in the inclusion in the pinning energy of the contribution arising from distortions of the crystal on all length scales.
Author: David S. Sholl
Publisher: John Wiley & Sons
ISBN: 1119840864
Category : Science
Languages : en
Pages : 228
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Book Description
Density Functional Theory A concise and rigorous introduction to the applications of DFT calculations In the newly revised second edition of Density Functional Theory: A Practical Introduction, the authors deliver a concise and easy-to-follow introduction to the key concepts and practical applications of density functional theory (DFT) with an emphasis on plane-wave DFT. The authors draw on decades of experience in the field, offering students from a variety of backgrounds a balanced approach between accessibility and rigor, creating a text that is highly digestible in its entirety. This new edition: Discusses in more detail the accuracy of DFT calculations and the choice of functionals Adds an overview of the wide range of available DFT codes Contains more examples on the use of DFT for high throughput materials calculations Puts more emphasis on computing phase diagrams and on open ensemble methods widely used in electrochemistry Is significantly extended to cover calculation beyond standard DFT, e.g., dispersion-corrected DFT, DFT+U, time-dependent DFT Perfect for graduate students and postdoctoral candidates in physics and engineering, Density Functional Theory: A Practical Introduction will also earn a place in the libraries of researchers and practitioners in chemistry, materials science, and mechanical engineering.
Author: Gang Yang
Publisher: BoD – Books on Demand
ISBN: 1789231329
Category : Science
Languages : en
Pages : 274
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Book Description
Density functional theory (DFT) ranks as the most widely used quantum mechanical method and plays an increasingly larger role in a number of disciplines such as chemistry, physics, material, biology, and pharmacy. DFT has long been used to complement experimental investigations, while now it is also regarded as an indispensable and powerful tool for researchers of different fields. This book is divided into five sections that include original chapters written by experts in their fields: "Method Development and Validation," "Spectra and Thermodynamics," "Catalysis and Mechanism," "Material and Molecular Design," and "Multidisciplinary Integration." I would like to express my sincere gratitude to all contributors and recommend this book to both beginners and experienced researchers.
