41 research outputs found
Orbital-dependent singlet dimers and orbital-selective Peierls transitions in transition metal compounds
We show that in transition metal compounds containing structural metal dimers
there may exist in the presence of different orbitals a special state with
partial formation of singlets by electrons on one orbital, while others are
effectively decoupled and may give e.g. long-range magnetic order or stay
paramagnetic. Similar situation can be realized in dimers spontaneously formed
at structural phase transitions, which can be called orbital-selective Peierls
transition. This can occur in case of strongly nonuniform hopping integrals for
different orbitals and small intra-atomic Hund's rule coupling JH. Yet another
consequence of this picture is that for odd number of electrons per dimer there
exist competition between double exchange mechanism of ferromagnetism, and the
formation of singlet dimer by electron on one orbital, with remaining electrons
giving a net spin of a dimer. The first case is realized for strong Hund's rule
coupling, typical for 3d compounds, whereas the second is more plausible for
4d-5d compounds. We discuss some implications of these phenomena, and consider
examples of real systems, in which orbital-selective phase seems to be
realized.Comment: to be published in PRB Rapid Co
Electronic correlations and competing orders in multiorbital dimers: a cluster DMFT study
We investigate the violation of the first Hund's rule in 4 and 5
transition metal oxides that form solids of dimers. Bonding states within these
dimers reduce the magnetization of such materials. We parametrize the dimer
formation with realistic hopping parameters and find not only regimes, where
the system behaves as a Fermi liquid or as a Peierls insulator, but also
strongly correlated regions due to Hund's coupling and its competition with the
dimer formation. The electronic structure is investigated using the cluster
dynamical mean-field theory for a dimer in the two-plane Bethe lattice with two
orbitals per site and -filling, that is three electrons per dimer. It
reveals dimer-antiferromagnetic order of a high-spin (double exchange) state
and a low-spin (molecular orbital) state. At the crossover region we observe
the suppression of long-range magnetic order, fluctuation enhancement and
renormalization of electron masses. At certain interaction strengths the system
becomes an incoherent antiferromagnetic metal with well defined local moments.Comment: 11 pages, 10 figure
Suppression of Magnetism in Ba\u3csub\u3e5\u3c/sub\u3eAlIr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e11\u3c/sub\u3e: Interplay of Hund\u27s Coupling, Molecular Orbitals, and Spin-Orbit Interaction
The electronic and magnetic properties of Ba5AlIr2O11 containing Ir-Ir dimers are investigated using the generalized gradient approximation (GGA) and GGA + spin-orbit coupling (SOC) calculations. We found that the strong suppression of the magnetic moment in this compound recently found by Terzic et al. [Phys. Rev. B 91, 235147 (2015)] is not due to charge ordering but is related to the joint effect of the spin-orbit interaction and strong covalency, resulting in the formation of metal-metal bonds. They conspire and act against the intraatomic Hund\u27s rule exchange interaction to reduce total magnetic moment of the dimer. We argue that the same mechanism could be relevant for other 4d and 5d dimerized transition metal compounds
Ab initio guided minimal model for the "Kitaev" material BaCo(AsO): Importance of direct hopping, third-neighbor exchange and quantum fluctuations
We present a simple three-parameter exchange model to describe the
interactions of the lowest doublet of the honeycomb cobaltate
BaCo(AsO), which has been proposed as a possible candidate for
Kitaev physics. Remarkably, it is the third-neighbor interactions, both
isotropic and anisotropic, that are responsible for the unique ground state of
BaCo(AsO), stabilized by quantum fluctuations. By considering two
{\it ab initio}-based complementary approaches, we analyze the electronic
structure of BaCo(AsO) and extract effective spin models that
justify the minimal model. Both methods show that the dominant direct hopping
makes the bond-dependent Kitaev term negligible moving the material away from
the sought-after spin-liquid regime. Moreover, a significantly large
third-nearest neighbor hopping supports the observed importance of the
third-neighbor interactions in the stabilization of the standout double-zigzag
ground state of BaCo(AsO).Comment: 8 pages, 4 figure
Pressure-induced magnetic transitions with change of the orbital configuration in dimerised systems
We suggest a possible scenario for magnetic transition under pressure in dimerised systems where electrons are localised on molecular orbitals. The mechanism of transition is not related with competition between kinetic energy and on-site Coulomb repulsion as in Mott-Hubbard systems, or between crystal-field splitting and intra-atomic exchange as in classical atomic spin-state transitions. Instead, it is driven by the change of bonding-antibonding splitting on part of the molecular orbitals. In the magnetic systems with few half-filled molecular orbitals external pressure may result in increase of the bonding-antibonding splitting and localise all electrons on low-lying molecular orbitals suppressing net magnetic moment of the system. We give examples of the systems, where this or inverse transition may occur and by means of ab initio band structure calculations predict that it can be observed in α−MoCl(4) at pressure P ~ 11 GPa
A valence bond liquid on the honeycomb lattice
The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+
cations on cooling below 270C, where the magnetic susceptibility vanishes. We
use density functional theory calculations to show that this reflects the
formation of a 'valence bond crystal', with a strong bond disproportionation.
On warming, x-ray diffraction shows that discrete three-fold symmetry is
regained on average, and the dimerization apparently disappears. In contrast,
local structural measurements using high-energy x-rays, show that disordered
dimers survive at the nanoscale up to at least 650C. The high temperature phase
of Li2RuO3 is thus an example of a valence bond liquid, where thermal
fluctuations drive resonance between different dimer coverages, a classic
analogue of the resonating valence bond state often discussed in connection
with high T cuprates.Comment: 5 pages, 4 figures, References correcte
Two-step antiferromagnetic transition and moderate triangular frustration in Li2Co(WO4)2
We present a detailed investigation of the magnetic properties of the
spin- system LiCo(WO) by means of magnetic
susceptibility and specific heat. Our experimental results show that in
LiCo(WO) a short-range antiferromagnetic (AFM) correlations appear
near 11 K and two successive long-range AFM phase
transitions are observed at T 9 K and T 7 K. The
frustration factor, /T3, indicates that the
system is moderately frustrated, which is identifiable by the broken triangular
symmetry within both - and -planes for the triclinic crystal structure.
The magnetic isotherm at temperatures below T shows a field-induced
spin-flop transition, and a complete H-T phase diagram for the two-step AFM
system is mapped. ~ band structure calculations suggest that the
strongest exchange coupling does not correspond to the shortest Co-Co distance
along the -axis, but rather along the diagonal direction through a
Co-O-W-O-Co super-superexchange path within the -planeComment: 7 Pages, 10 Figure
Electric field control of the magnetic chiralities in ferroaxial multiferroic RbFe(MoO4)2
The coupling of magnetic chiralities to the ferroelectric polarisation in
multiferroic RbFe(MoO) is investigated by neutron spherical
polarimetry. Because of the axiality of the crystal structure below
= 190 K, helicity and triangular chirality are
symmetric-exchange coupled, explaining the onset of the ferroelectricity in
this proper-screw magnetic structure - a mechanism that can be generalised to
other systems with "ferroaxial" distortions in the crystal structure. With an
applied electric field we demonstrate control of the chiralities in both
structural domains simultaneously.Comment: 5 pages, 4 figure