AgRuO3 , a Strongly Exchange-Coupled Honeycomb Compound Lacking Long-Range Magnetic Order.

Quasi two-dimensional (2D) oxide-based honeycomb lattices have attracted great attention for displaying specific electronic instabilities, which give rise to unconventional bonding patterns and unexpected magnetic exchange couplings. The synthesis of AgRuO3 , another representative exhibiting unique structural properties, is reported here. The stacking sequence of the honeycomb layers (Ru2 O6 ) differs from analogous precedents; in particular, the intercalating silver atoms are shifted from the middle of the interspaces and cap the void octahedral sites of the (□Ru2 O6 ) slabs from both sides. This way, charge neutral, giant 2D "molecules" of Ag/Ru2 O6 /Ag result; a feature that significantly enhances the overall 2D character of AgRuO3 . Measurements of magnetization have revealed extremely strong magnetic exchange coupling to be present, surviving to a temperature as high as 673 K, which is the temperature of thermal decomposition. No indication for long-range magnetic order has, however, been observed. Theoretical analyses confirm the pronounced 2D character of the electronic system, and in particular reveal the inter-honeycomb layer coupling Jc to be distinctly weak.

Na4IrO4 : square-planar coordination of a transition metal in d(5) configuration due to weak on-site coulomb interactions.

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition-metal complexes. For example, square-planar coordination geometries found in transition-metal oxometalates such as cuprates are usually associated with the d(8)  or d(9)  electron configuration. In this work, we address an unusual square-planar single oxoanionic [IrO4 ](4-)  species, as observed in Na4 IrO4 in which Ir(IV) has a d(5)  configuration, and characterize the chemical bonding through experiments and by ab initio calculations. We find that the Ir(IV)  center in ground-state Na4 IrO4 has square-planar coordination geometry because of the weak Coulomb repulsion of the Ir-5d electrons. In contrast, in its 3d counterpart Na4 CoO4 , the Co(IV) center is tetrahedrally coordinated because of strong electron correlation. Na4 IrO4 may thus serve as a simple yet important example to study the ramifications of Hubbard-type Coulomb interactions on local geometries.

Lattice-site-specific spin dynamics in double perovskite Sr2CoOsO6.

Magnetic properties and spin dynamics have been studied for the structurally ordered double perovskite Sr2CoOsO6. Neutron diffraction, muon-spin relaxation, and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature TN1=108 K, cobalt (3d7, S=3/2) and osmium (5d2, S=1) moments fluctuate dynamically, while their average effective moments undergo long-range order. In the second AFM phase below TN2=67 K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at T≈5 K. Ab initio calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin dynamics, making Sr2CoOsO6 distinct from previously reported double-perovskite compounds.

Atomistic designing of 2D quantum materials heterostructures CdF/CrI3for Berry curvature driven tunable intrinsic anomalous Hall state.

The coupling between topology and magnetism can explore rich physics with fundamental interest. Passing through the phase of Bismuth-based topological insulators magnetized by the 3d/4ftransition metal doping, currently the fabrication of quantum heterostructures by suitable new-generation 2D materials, has emerged as a prospective alternative. Following the current trends, the present investigation deals with the atomistic designing and investigation of the quantum heterostructures of the newly predicted massive Dirac semimetal CdF and well-known layered ferromagnetic insulator CrI3using the first-principles density functional theory calculations supplemented by the low energy tight-binding model Hamiltonian. The designed strategy ensures the lattice mismatch should be within the permissible range. We have addressed the physical characteristics of heterostructures in terms of the non-trivial topological band inversion between Cd-5sand I-2porbitals. Proximity effect induces magnetic interactions, breaks the time-reversal symmetry at the interface, and leads to Berry curvature-driven tunable intrinsic anomalous Hall conductance (AHC) at the Fermi energy. Our analysis reveals the electrons with high Fermi velocity (≈106 m s-1) in the heterostructures and the band topology at the Fermi level can be tuned effectively using very small external gate voltage or homogeneous electric field. Our investigation can open up new avenues for designing new topological phases in the heterostructure community and possible tailoring routes of the intrinsic AHC in moderate temperature.

Electronic structure evolution of the transition metals substituted tetragonal graphene: a first-principles investigations.

Motivated by the possibilities of tuning the Fermi level of the metallic band structure of the planar tetragonal graphene (T-graphene), by using the transition metals (TMs) substitution (3d, 4dand 5dseries), the electronic structure investigation has been carried out at low concentration level (≈2.7%) throughab initiodensity functional theory method. We have investigated the influence of the valence electrons of the TM on the evolution of the electronic structure and magnetization and the induced magnetic moments at the carbon atoms in the T-graphene network. The investigations also explored the possibilities of inducing long-range magnetic ordering. In the case of multi TMs substitutions we found the dominance signature of the antiferromagnetic correlations for most of the TM substituted cases. The critical analysis of the magnetization densities indicated the important role of the hybridization between the carbonπandσorbitals with the TM-dstates. We explored that the observed non-monotonic nature of the magnetization and evolution of electronic structure was due to the competing energy scales of electronic correlation, hybridization and crystal field splitting. This study opens up the route for further investigations towards the possibilities of using T-graphene as a potential polymorph of graphene for device applications.

Microscopic investigation of low dimensional magnet Sc2Cu2O5: combined experimental and ab initio approach.

Sc2Cu2O5 is a non centro-symmetric oxide comprising of zig-zag chains made up of Cu2+ ions in a distorted square planer coordination. We present here a combined experimental and theoretical investigation on this compound, which is based on magnetization, electron spin resonance (ESR), heat capacity as well as density functional theory (DFT) based calculations. Short range magnetic correlation prior to the long range order at [Formula: see text] K is evidenced by a broad hump like feature ([Formula: see text]43 K) found in the magnetic contribution of the heat capacity as well as by deviations from a regular Curie-Weiss behavior observed in the bulk magnetization and the Cu2+ ESR intensity. The DFT results indicate the existence of ferro-orbital ordering at the Cu-sites, which gives rise to chain like arrangements of Cu ions along the crystallographic b axis. It also signifies complex nature of the spin structure with nonuniform magnetic interactions along the zig-zag chains. The ground state energy is found to be minimum for ferromagnetically coupled spin-dimers along the chains, whereas the adjacent chains are themselves antiferromagnetically coupled. The experimentally observed short range magnetic correlations possibly arise due to this chain like structure.

Evolution of electronic and magnetic properties in four polytypes of BaRuO3: a first-principles study.

Using density functional theory, we explore the evolution of the electronic and magnetic properties of BaRuO3 in four different phases, 9R, 4H, 6H and 3C, obtained by synthesizing under different pressure conditions. The four different phases differ in the differential proportion of hexagonal versus cubic close stacking of the BaO3 layers, leading to important changes in the structure. By computing the electronic and magnetic properties of the four different phases, and the optical properties of 4H and 9R phases, we find that density functional based calculations are to a large extent able to explain the change in properties of the four different polytypes.