4,000 research outputs found
Nature of the Mott transition in Ca2RuO4
We study the origin of the temperature-induced Mott transition in Ca2RuO4. As
a method we use the local-density approximation+dynamical mean-field theory. We
show the following. (i) The Mott transition is driven by the change in
structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it
occurs together with orbital order, which follows, rather than produces, the
structural transition. (ii) In the metallic L-Pbca phase the orbital
polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy
orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce
the effective masses in the metallic phase. Our results indicate that a similar
scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5
structures electrons are progressively transferred to the xz/yz bands with
increasing x, however we find no orbital-selective Mott transition down to ~300
K.Comment: 4 pages, 3 figures; published versio
Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS
Electron correlation effects are studied in ZrSiS using a combination of
first-principles and model approaches. We show that basic electronic properties
of ZrSiS can be described within a two-dimensional lattice model of two nested
square lattices. High degree of electron-hole symmetry characteristic for ZrSiS
is one of the key features of this model. Having determined model parameters
from first-principles calculations, we then explicitly take electron-electron
interactions into account and show that at moderately low temperatures ZrSiS
exhibits excitonic instability, leading to the formation of a pseudogap in the
electronic spectrum. The results can be understood in terms of
Coulomb-interaction-assisted pairing of electrons and holes reminiscent to that
of an excitonic insulator. Our finding allows us to provide a physical
interpretation to the unusual mass enhancement of charge carriers in ZrSiS
recently observed experimentally.Comment: 6 pages, 4 figures. Final versio
Effective Heisenberg model and exchange interaction for strongly correlated systems
We consider the extended Hubbard model and introduce a corresponding
Heisenberg-like problem written in terms of spin operators. The derived
formalism is reminiscent of Anderson's idea of the effective exchange
interaction and takes into account nonlocal correlation effects. The results
for the exchange interaction and magnetic susceptibility are expressed in terms
of single-particle quantities, which can be obtained efficiently in realistic
calculations of multiband systems. In the strongly spin-polarized limit, when
the local magnetic moment is well-defined, the exchange interaction reduces to
a standard expression of the density functional theory that has been
successfully used in practical calculations of magnetic properties of real
materials.Comment: Accepted to Physical Review Letter
Half-Metallic Ferromagnetism and the spin polarization in CrO
We present electronic structure calculations in combination with local and
non-local many-body correlation effects for the half-metallic ferromagnet
CrO. Finite-temperature Dynamical Mean Field Theory results show the
existence of non-quasiparticle states, which were recently observed as almost
currentless minority spin states near the Fermi energy in resonant scattering
experients. At zero temperatures, Variational Cluster Approach calculations
support the half-metallic nature of CrO as seen in superconducting point
contact spectroscopy. The combination of these two techniques allowed us to
qualitatively describe the spin-polarization in CrO.Comment: 5 pages, 3 figure
Hydrogen on graphene: Electronic structure, total energy, structural distortions, and magnetism from first-principles calculations
Density functional calculations of electronic structure, total energy,
structural distortions, and magnetism for hydrogenated single-layer, bilayer,
and multi-layer graphene are performed. It is found that hydrogen-induced
magnetism can survives only at very low concentrations of hydrogen (single-atom
regime) whereas hydrogen pairs with optimized structure are usually
nonmagnetic. Chemisorption energy as a function of hydrogen concentration is
calculated, as well as energy barriers for hydrogen binding and release. The
results confirm that graphene can be perspective material for hydrogen storage.
Difference between hydrogenation of graphene, nanotubes, and bulk graphite is
discussed.Comment: 8 pages 8 figures (accepted to Phys. Rev. B
Orbital Kondo effect in Cobalt-Benzene sandwich molecules
We study a Co-benzene sandwich molecule bridging the tips of a Cu nanocontact
as a realistic model of correlated molecular transport. To this end we employ a
recently developed method for calculating the correlated electronic structure
and transport properties of nanoscopic conductors. When the molecule is
slightly compressed by the tips of the nanocontact the dynamic correlations
originating from the strongly interacting Co 3d shell give rise to an orbital
Kondo effect while the usual spin Kondo effect is suppressed due to Hund's rule
coupling. This non-trivial Kondo effect produces a sharp and
temperature-dependent Abrikosov-Suhl resonance in the spectral function at the
Fermi level and a corresponding Fano line shape in the low bias conductance
Non-quasiparticle states in CoMnSi evidenced through magnetic tunnel junction spectroscopy measurements
We investigate the effects of electronic correlations in the full-Heusler
CoMnSi, by combining a theoretical analysis of the spin-resolved density of
states with tunneling-conductance spectroscopy measurements using CoMnSi as
electrode. Both experimental and theoretical results confirm the existence of
so-called non-quasiparticle states and their crucial contribution to the
finite-temperature spin polarisation in this material.Comment: Repalced Fig. 1. of PRL, 100, 086402 (2008), better k-space
resolution for DOS around Fermi energ
Electron energy spectrum of the spin-liquid state in a frustrated Hubbard model
Non-local correlation effects in the half-filled Hubbard model on an
isotropic triangular lattice are studied within a spin polarized extension of
the dual fermion approach. A competition between the antiferromagnetic
non-collinear and the spin liquid states is strongly enhanced by an
incorporation of a k-dependent self-energy beyond the local dynamical
mean-field theory. The dual fermion correc- tions drastically decrease the
energy of a spin liquid state while leaving the non-collinear magnetic states
almost non-affected. This makes the spin liquid to become a preferable state in
a certain interval of interaction strength of an order of the magnitude of a
bandwidth. The spectral function of the spin-liquid Mott insulator is
determined by a formation of local singlets which results in the energy gap of
about twice larger than that of the 120 degrees antiferromagnetic Neel state.Comment: 6 pages, 4 figure
Local magnetic moments in iron and nickel at ambient and Earth's core conditions
Some Bravais lattices have a particular geometry that can slow down the
motion of Bloch electrons by pre-localization due to the band-structure
properties. Another known source of electronic localization in solids is the
Coulomb repulsion in partially filled d- or f-orbitals, which leads to the
formation of local magnetic moments. The combination of these two effects is
usually considered of little relevance to strongly correlated materials. Here
we show that it represents, instead, the underlying physical mechanism in two
of the most important ferromagnets: nickel and iron. In nickel, the van Hove
singularity has an unexpected impact on the magnetism. As a result, the
electron-electron scattering rate is linear in temperature, in violation of the
conventional Landau theory of metals. This is true even at Earth's core
pressures, at which iron is instead a good Fermi liquid. The importance of
nickel in models of geomagnetism may have therefore to be reconsidered.Comment: Supplementary Information available at
https://www.nature.com/articles/ncomms16062#supplementary-informatio
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