218 research outputs found
Group theory of Wannier functions providing the basis for a deeper understanding of magnetism and superconductivity
The paper presents the group theory of best localized and symmetry-adapted
Wannier functions in a crystal of any given space group G or magnetic group M.
Provided that the calculated band structure of the considered material is given
and that the symmetry of the Bloch functions at all the points of symmetry in
the Brillouin zone is known, the paper details whether or not the Bloch
functions of particular energy bands can be unitarily transformed into best
localized Wannier functions symmetry-adapted to the space group G, to the
magnetic group M, or to a subgroup of G or M. In this context, the paper
considers usual as well as spin-dependent Wannier functions, the latter
representing the most general definition of Wannier functions. The presented
group theory is a review of the theory published by one of the authors in
several former papers and is independent of any physical model of magnetism or
superconductivity. However, it is suggested to interpret the special symmetry
of the best localized Wannier functions in the framework of a nonadiabatic
extension of the Heisenberg model, the nonadiabatic Heisenberg model. On the
basis of the symmetry of the Wannier functions, this model of strongly
correlated localized electrons makes clear predictions whether or not the
system can possess superconducting or magnetic eigenstates
K-space magnetism as the origin of superconductivity
The nonadiabatic Heisenberg model presents a nonadiabatic mechanism
generating Cooper pairs in narrow, roughly half-filled "superconducting bands"
of special symmetry. Here we show that this mechanism may be understood as the
outcome of a special spin structure in the reciprocal space, hereinafter
referred to as k-space magnetism. The presented picture permits a vivid
depiction of this new mechanism highlighting the height similarity as well as
the essential difference between the new nonadiabatic and the familiar
Bardeen-Cooper-Schrieffer mechanism
Constraining forces causing the Meissner effect
As shown in former papers, the nonadiabatic Heisenberg model presents a novel
mechanism of Cooper pair formation which is not the result of an attractive
electron-electron interaction but can be described in terms of quantum
mechanical constraining forces. This mechanism operates in narrow, roughly
half-filled superconducting bands of special symmetry and is evidently
responsible for the formation of Cooper pairs in all superconductors. Here we
consider this new mechanism within an outer magnetic field. We show that in the
magnetic field the constraining forces produce Cooper pairs of non-vanishing
total momentum with the consequence that an electric current flows within the
superconductor. This current satisfies the London equations and, consequently,
leads to the Meissner effect. This theoretical result is confirmed by the
experimental observation that all superconductors, whether conventional or
unconventional, exhibit the Meissner effect
Nonadiabatic Atomic-like State Stabilizing Antiferromagnetism and Mott Insulation in MnO
In this paper I report evidence that the antiferromagnetic and insulating
ground state of MnO is caused by a nonadiabatic atomic-like motion as it is
evidently the case in NiO. In addition, I show that the experimental findings
of Goodwin et al. [Phys. Rev. Lett. (2006), 96,~047209] corroborate my
suggestion that the rhombohedral-like distortion in antiferromagnetic MnO as
well as in antiferromagnetic NiO is an inner distortion of the monoclinic
base-centered Bravais lattice of the antiferromagnetic phases.Comment: arXiv admin note: text overlap with arXiv:1911.0819
Variational Principle of Bogoliubov and Generalized Mean Fields in Many-Particle Interacting Systems
The approach to the theory of many-particle interacting systems from a
unified standpoint, based on the variational principle for free energy is
reviewed. A systematic discussion is given of the approximate free energies of
complex statistical systems. The analysis is centered around the variational
principle of N. N. Bogoliubov for free energy in the context of its
applications to various problems of statistical mechanics and condensed matter
physics. The review presents a terse discussion of selected works carried out
over the past few decades on the theory of many-particle interacting systems in
terms of the variational inequalities. It is the purpose of this paper to
discuss some of the general principles which form the mathematical background
to this approach, and to establish a connection of the variational technique
with other methods, such as the method of the mean (or self-consistent) field
in the many-body problem, in which the effect of all the other particles on any
given particle is approximated by a single averaged effect, thus reducing a
many-body problem to a single-body problem. The method is illustrated by
applying it to various systems of many-particle interacting systems, such as
Ising and Heisenberg models, superconducting and superfluid systems, strongly
correlated systems, etc. It seems likely that these technical advances in the
many-body problem will be useful in suggesting new methods for treating and
understanding many-particle interacting systems. This work proposes a new,
general and pedagogical presentation, intended both for those who are
interested in basic aspects, and for those who are interested in concrete
applications.Comment: 60 pages, Refs.25
Spectral properties of transition metal pnictides and chalcogenides: angle-resolved photoemission spectroscopy and dynamical mean field theory
Electronic Coulomb correlations lead to characteristic signatures in the
spectroscopy of transition metal pnictides and chalcogenides: quasi-particle
renormalizations, lifetime effects or incoherent badly metallic behavior above
relatively low coherence temperatures are measures of many-body effects due to
local Hubbard and Hund's couplings. We review and compare the results of
angle-resolved photoemission spectroscopy experiments (ARPES) and of combined
density functional dynamical mean field theory (DFT+DMFT) calculations. We
emphasize the doping-dependence of the quasi-particle mass renormalization and
coherence properties
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