26 research outputs found
Mechanism of basal-plane antiferromagnetism in the spin-orbit driven iridate Ba2IrO4
By ab initio many-body quantum chemistry calculations, we determine the
strength of the symmetric anisotropy in the 5 j 1/2 layered
material BaIrO. While the calculated anisotropic couplings come out in
the range of a few meV, orders of magnitude stronger than in analogous 3d
transition-metal compounds, the Heisenberg superexchange still defines the
largest energy scale. The ab initio results reveal that individual layers of
BaIrO provide a close realization of the quantum spin-1/2
Heisenberg-compass model on the square lattice. We show that the experimentally
observed basal-plane antiferromagnetism can be accounted for by including
additional interlayer interactions and the associated order-by-disorder
quantum-mechanical effects, in analogy to undoped layered cuprates.Comment: 8 pages, 2 figure
Strong magnetic frustration and anti-site disorder causing spin-glass behavior in honeycomb Li2RhO3
With large spin-orbit coupling, the electron configuration in
-metal oxides is prone to highly anisotropic exchange interactions and
exotic magnetic properties. In iridates, given the existing variety of
crystal structures, the magnetic anisotropy can be tuned from antisymmetric to
symmetric Kitaev-type, with interaction strengths that outsize the isotropic
terms. By many-body electronic-structure calculations we here address the
nature of the magnetic exchange and the intriguing spin-glass behavior of
LiRhO, a honeycomb oxide. For pristine crystals without Rh-Li
site inversion, we predict a dimerized ground state as in the isostructural
iridate LiIrO, with triplet spin dimers effectively placed on a
frustrated triangular lattice. With Rh-Li anti-site disorder, we explain the
observed spin-glass phase as a superposition of different, nearly degenerate
symmetry-broken configurations.Comment: 10 page
CaIrO3 post-perovskite, a j = 1/2 quasi-one-dimensional antiferromagnet
The 5d5 iridate CaIrO3 is isostructural with the post-perovskite phase of
MgSiO3, recently shown to occur under extreme pressure in the lower Earth's
mantle. It therefore serves as an analogue of post-perovskite MgSiO3 for a wide
variety of measurements at ambient conditions or achievable with conventional
multianvile pressure modules. By multireference configuration-interaction
calculations we here provide essential information on the chemical bonding and
magnetic interactions in CaIrO3. We predict a large antiferromagnetic
superexchange of 120 meV along the c axis, the same size with the interactions
in the cuprate superconductors, and ferromagnetic couplings smaller by an order
of magnitude along a. CaIrO3 can thus be regarded as a j = 1/2
quasi-one-dimensional antiferromagnet. While this qualitatively agrees with the
stripy magnetic structure proposed by resonant x-ray diffraction, the detailed
microscopic picture emerging from our study, in particular, the highly uneven
admixture of t2g components, provides a clear prediction for resonant inelastic
x-ray scattering experiments
Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers
A promising route to tailoring the electronic properties of quantum materials
and devices rests on the idea of orbital engineering in multilayered oxide
heterostructures. Here we show that the interplay of interlayer charge
imbalance and ligand distortions provides a knob for tuning the sequence of
electronic levels even in intrinsically stacked oxides. We resolve in this
regard the -level structure of layered SrIrO by electron spin
resonance. While canonical ligand-field theory predicts -factors
for positive tetragonal distortions as present in SrIrO, the
experiment indicates . This implies that the iridium
levels are inverted with respect to their normal ordering. State-of-the-art
electronic-structure calculations confirm the level switching in SrIrO,
whereas we find them in BaIrO to be instead normally ordered. Given the
nonpolar character of the metal-oxygen layers, our findings highlight the
tetravalent transition-metal 214 oxides as ideal platforms to explore
-orbital reconstruction in the context of oxide electronics
Strongly frustrated triangular spin lattice emerging from triplet dimer formation in honeycomb Li2IrO3
In quantum magnetism spin dimers are typically associated with spin-singlet
states. To date, has not been observed in any 3D or
quasi-2D material. With this in mind we here discuss the electronic structure
of the spin-orbit driven Mott insulator LiIrO, a
honeycomb-lattice system with two crystallographically inequivalent Ir-Ir
bonds. From many-body calculations we find that, while both
Heisenberg and Kitaev couplings are present, the magnetic interactions are
dominated by a strong isotropic ferromagnetic exchange on only one set of
bonds. This causes the formation of triplet spin dimers effectively placed on a
strongly frustrated triangular lattice. The triplet dimers remain protected in
a large region of the phase diagram, suggesting that LiIrO has a
long-range incommensurate magnetic ground state that is pushed by the Kitaev
exchange interactions beyond a standard planar helix configuration.Comment: 5 pages, 3 figures, revised from the previous versio
Combined unitary and symmetric group approach applied to low-dimensional Heisenberg spin systems
A novel combined unitary and symmetric group approach is used to study the spin-1/2 Heisenberg model and related Fermionic systems in a total spin-adapted representation, using a linearly-parameterised Ansatz for the many-body wave function. We show that a more compact ground-state wave function representation-indicated by a larger leading ground-state coefficient-is obtained when combining the symmetric group S-n, in the form of permutations of the underlying lattice site ordering, with the cumulative spin coupling based on the unitary group U(n). In one-dimensional systems the observed compression of the wave function is reminiscent of block-spin renormalization group approaches, and allows us to study larger lattices (here taken up to 80 sites) with the spin-adapted full configuration interaction quantum Monte Carlo method, which benefits from the sparsity of the Hamiltonian matrix and the corresponding sampled eigenstates that emerge from the reordering. We find that in an optimal lattice ordering the configuration state function with highest weight already captures with high accuracy the spin-spin correlation function of the exact ground-state wave function. This feature is found for more general lattice models, such as the Hubbard model, and ab initio quantum chemical models, exemplified by one-dimensional hydrogen chains. We also provide numerical evidence that the optimal lattice ordering for the unitary group approach is not generally equivalent to the optimal ordering obtained for methods based on matrix-product states, such as the density-matrix renormalization group approach
Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations
NaIrO, a honeycomb 5 oxide, has been recently identified as a
potential realization of the Kitaev spin lattice. The basic feature of this
spin model is that for each of the three metal-metal links emerging out of a
metal site, the Kitaev interaction connects only spin components perpendicular
to the plaquette defined by the magnetic ions and two bridging ligands. The
fact that reciprocally orthogonal spin components are coupled along the three
different links leads to strong frustration effects and nontrivial physics.
While the experiments indicate zigzag antiferromagnetic order in NaIrO,
the signs and relative strengths of the Kitaev and Heisenberg interactions are
still under debate. Herein we report results of ab initio many-body electronic
structure calculations and establish that the nearest-neighbor exchange is
strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the
Heisenberg contribution is significantly weaker and antiferromagnetic. The
calculations further reveal a strong sensitivity to tiny structural details
such as the bond angles. In addition to the large spin-orbit interactions, this
strong dependence on distortions of the IrO plaquettes singles out the
honeycomb 5 oxides as a new playground for the realization of
unconventional magnetic ground states and excitations in extended systems.Comment: 13 pages, 2 tables, 3 figures, accepted in NJ