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4$d$ transition metal oxide (TMO) offers an intriguing puzzle for their electronic and magnetic ground state. They are in the cross-over regime of strong spin orbit interaction (SOI) and electron-electron correlation ($U$) with quenched orbital angular momentum. Our work unravels the electronic and magnetic ground state of the less investigated 4$d^{3}$ double perovskite ruthenates A$_{2}$LaRuO$_6$ (A = Ca, Ba). The negligible effect of SOI is evident from the bulk magnetic, specific heat measurements and density functional theory (DFT) calculations, indicating a classical spin-only magnetic ground state (${S}$ = 3/2) for the materials. Magnetization measurements show that both materials have long range antiferromagnetic order with high degree of magnetic frustration ($f$ $\approx$13 -15). Interestingly, a near $T^2$- behavior is observed in low-$T$ magnetic heat capacity measurement, indicating the presence of low-dimensional spin-wave exciation and magnetic frustration in both materials. The temperature dependent resistivity measurements and electronic band structure calculations confirm a conventional Mott insulating ground state in these two systems. Moreover, our experimental investigation and DFT calculations highlight the reason for the nonexistence of Sr$_2$LaRuO$_6$., Comment: 11 pages, 12 figures, 5 tables

We employ first-principles density-functional theory, to perform a comparative investigation of the effect of the spin-orbit coupling (SOC) on the electronic and magnetic properties of three experimentally synthesized and characterized hexagonal perovskites Sr$_3$MIrO$_6$(M=Mg, Zn, Cd). The electronic structure calculations show that in all the compounds, Ir is the only magnetically active site in +4[5$d^5$] configuration, whereas M$^{+2}$ (M=Cd, Zn, Mg), remains in nonmagnetic states with Cd/Zn and Mg featuring $d^{10}$ and $d^{0}$ electronic configurations, respectively. The insulating gap could be opened by switching on the correlation parameter $U$ for Sr$_3$CdrO$_6$ and Sr$_3$ZnIrO$_6$ which qualifies it to be a correlated Mott insulator. However, in the case of Sr$_3$MgIrO$_6$ both $U$ and antiferromagnetic ordering is not enough and the gap could only be opened by including the SOC which classifies it to fall under the category of a typical SOC Mott insulator. The $j_{eff}$ states are visualized from the orbital projected band structure. The magnetism is studied from the point of view of exchange interactions and magnetocrystalline anisotropy in the presence of the SOC. We also present the comparative analysis of the renormalized impact of SOC on the three compounds, which shows that all the three compounds fall under the $intermediate$ coupling regime, where Sr$_3$MgIrO$_6$ is comparatively closer to the atomic $j_{eff}=\frac{1}{2}$ picture from the others.

The coupling between electron orbital momentum and spin momentum, known as spin-orbit coupling (SOC), is a fundamental origin of a multitude of fascinating physical phenomena, especially it holds paramount significance in the realm of topological materials. In our work, we have predicted the topological phase in Hg-based chalcopyrite compounds using the first principles density functional theory. The initial focus was on HgSnN 2 , revealing it to be a nonmagnetic Weyl semimetal, while HgSnP 2 displayed characteristics of a strong topological insulator. What makes our work truly unique is that despite both compounds having the same SOC strength, arises from Hg, they exhibit distinct topological phases due to the distinct hybridization effect of the Hg-5d and X-p bands. This finding can address a significant factor, i.e., the effect of the band hybridization in deriving distinct topological phases, keeping the symmetry aspect intact. Our results indicate that due to the presence of band hybridization between the dominant X-np orbitals n=2 and 3 for X=N and P respectively and a minor contribution from Hg-5d, we can tune the topological phase by manipulating SOC strength, which equivalently achievable by chemical substitutions. This investigation stands as a remarkable illustration of the unique roles that hybridization plays in sculpting the topological properties of these compounds while simultaneously preserving their underlying symmetries.

Recent findings of large anomalous Hall signal in nonferromagnetic and nonferrimagnetic materials suggest that the magnetization of the system is not a critical component for the realization of the anomalous Hall effect (AHE). Here, we present a combined theoretical and experimental study demonstrating the evolution of different mechanisms of AHE in a cubic Heusler system MnPt$_{1-x}$Ir$_x$Sn. With the help of magnetization and neutron diffraction studies, we show that the substitution of nonmagnetic Ir in place of Pt significantly reduces the net magnetic moment from 4.17 $ \mu _B$/f.u. in MnPtSn to 2.78 $ \mu _B$/f.u. for MnPt$_{0.5}$Ir$_{0.5}$Sn. In contrast, the anomalous Hall resistivity is enhanced by nearly three times from 1.6 $ \mu \Omega $ cm in MnPtSn to about 5 $ \mu \Omega $ cm for MnPt$_{0.5}$Ir$_{0.5}$Sn. The power law analysis of the Hall resistivity data suggests that the extrinsic contribution of AHE that dominates in the case of the parent MnPtSn almost vanishes for MnPt$_{0.5}$Ir$_{0.5}$Sn, where the intrinsic mechanism plays the major role. The experimental results are well supported by our theoretical study, which shows a considerable enhancement of the spin-orbit coupling when Ir is introduced into the system. Our finding of a crossover of the anomalous Hall effect with chemical engineering is a major contribution toward the recent interest in controlling the band topology of topological materials, both in bulk and thin-film forms., Comment: Published in Physical Review B

The present work aims to address the electronic and magnetic properties of the intermetallic compound GdPd$_2$Bi through a comprehensive study of the structural, magnetic, electrical and thermal transport on a polycrystalline sample, followed by theoretical calculations. Our findings indicate that the magnetic ground state is antiferromagnetic in nature. Magnetotransport data present prominent hysteresis loop hinting a structural transition with further support from specific heat and thermopower measurements, but no such signature is observed in the magnetization study. Temperature dependent powder x-ray diffraction measurements confirm martensitic transition from the high-temperature (HT) cubic Heusler $L2_1$ structure to the low-temperature (LT) orthorhombic $Pmma$ structure similar to many previously reported shape memory alloys. The HT to LT phase transition is characterized by a sharp increase in resistivity associated with prominent thermal hysteresis. Further, we observe robust Bain distortion between cubic and orthorhombic lattice parameters related by $a_{orth} = \sqrt{2}a_{cub}$, $b_{orth} = a_{cub}$ and $c_{orth} = a_{cub}/\sqrt{2}$, that occurs by contraction along $c$-axis and elongation along $a$-axis respectively. The sample shows an unusual `non-saturating' $H^2$-dependent negative magnetoresistance for magnetic field as high as 150 kOe. In addition, non-linear field dependence of Hall resistivity is observed below about 30 K, which coincides with the sign change of the Seebeck coefficient. The electronic structure calculations confirm robust metallic states both in the LT and HT phases. It indicates complex nature of the Fermi surface along with the existence of both electron and hole charge carriers. The anomalous transport behaviors can be related to the presence of both electron and hole pockets., Comment: 13 pages, 12 figures

Recently topologically non-trivial phases have been identified in few time-reversal invariant systems that lack of inversion symmetry. Using density functional theory based first-principles calculations, we report a strong topologically non-trivial phase in chalchopyrite ZnGeSb$_2$, which can act as a model system of strained HgTe. The calculations reveal the non-zero topological invariant ($Z_2$), the presence of Dirac cone crossing in the surface spectral functions with spin-momentum locking. We also show that the application of moderate hydrostatic pressure ($\sim$7 GPa) induces topological phase transition from topological non-trivial phase to a topologically trivial phase. A discontinuity in the tetragonal distortion of non-centrosymmetric ZnGeSb$_2$ plays a crucial role in driving this topological phase transition.

The lack of time-reversal symmetry and Weyl fermions give exotic transport properties to Co-based Heusler alloys. In the present study, we have investigated the role of chemical disorder on the variation of Weyl points in Co\textsubscript{2}Ti\textsubscript{1-x}V\textsubscript{x}Sn magnetic Weyl semimetal candidate. We employ the first principle approach to track the evolution of the nodal lines responsible for the appearance of Weyl node in Co$_2$TiSn as a function of V substitution in place of Ti. By increasing the V concentration in place of Ti, the nodal line moves toward Fermi level and remains at Fermi level around the middle composition. Further increase of the V content, leads shifting of nodal line away from Fermi level. Density of state calculation shows half-metallic behavior for the entire range of composition. The magnetic moment on each Co atom as a function of V concentration increases linearly up to x=0.4, and after that, it starts decreasing. We also investigated the evolution of the Weyl nodes and Fermi arcs with chemical doping. The first-principles calculations reveal that via replacing almost half of the Ti with V, the intrinsic anomalous Hall conductivity increased twice as compared to the undoped composition. Our results indicate that the composition close to the 50\% V doped Co$_2$TiSn, will be an ideal composition for the experimental investigation of Weyl physics., Comment: 14 pages, 12 figures

The 3$d$-5$d$ based double perovskites offer an ideal playground to study the interplay between electron correlation ($U$) and spin-orbit coupling (SOC) effect, showing exotic physics. The Sr$_2$FeIrO$_6$ is an interesting member in this family with ionic distribution of Fe$^{3+}$ (3$d^5$) and Ir$^{5+}$ (5$d^4$) where the later is believed to be nonmagnetic under the picture of strong SOC. Here, we report detailed investigation of structural, magnetic and electronic transport properties along with electronic structure calculations in (Sr$_{1-x}$Ca$_x$)$_2$FeIrO$_6$ series with $x$ from 0 to 1. While the basic interactions such as, $U$ and SOC are unlikely to be modified but a structural modification is expected due to ionic size difference between Sr$^{2+}$ and Ca$^{2+}$ which would influence other properties such as crystal field effect and band widths. While a nonmonotonic changes in lattice parameters are observed across the series, the spectroscopic investigations reveal that 3+/5+ charge state of Fe/Ir continue till end of the series. An analysis of magnetic data suggests similar nonmonotonic evolution of magnetic parameters with doping. Temperature dependent crystal structure as well as low temperature (5 K) magnetic structure have been determined from neutron powder diffraction measurements. The whole series shows insulating behavior. The electronic structure calculations show, SOC enhanced, a noncollinear antiferromagnetic and Mott-type insulating state is the stable ground state for present series with a substantial amount of orbital moment, but less than the spin magnetic moment, at the Ir site and the magnetocrystalline anisotropy. The obtained results imply that the Ca$^{2+}$ has large influence on the magnetic and transport properties, further showing a large agreement between experimental results as well as theoretical calculations., Comment: Accepted in Phys. Rev. B, 19 pages, 18 figures

In this work, we studied time-reversal-breaking topological phases as a result of the interplay between antiferromagnetism and inverted band structures in thin films of antiferromagnetic double perovskite transition-metal Sr$_2$FeOsO$_6$. By combining the first-principles calculations and analytical models, we demonstrate that the quantum anomalous Hall phase and chiral topological superconducting phase can be realized in this system. We find that to achieve time-reversal-breaking topological phases in antiferromagnetic materials, it is essential to break the combined symmetry of time reversal and inversion, which generally exists in antiferromagnetic structures. As a result, we can utilize an external electric gate voltage to induce the phase transition between topological phases and trivial phases, thus providing an electrically controllable topological platform for the future transport experiments., Comment: 5 pages, 4 figures

Using first-principles density functional theory, we have investigated the electronic and magnetic properties of recently synthesized and characterized 5d double-perovskites Sr2BOsO6 (B=Y, In, Sc). The electronic structure calculations show that in all compounds, the Os5+ (5d3) site is the only magnetically active one, while Y3+, In3+ and Sc3+ remain in nonmagnetic states, with Sc/Y and In featuring d0 and d10 electronic configurations, respectively. Our studies reveal the important role of closed-shell (d10) versus open-shell (d0) electronic configurations of the nonmagnetic sites in determining the overall magnetic exchange interactions. Although the magnetic Os5+ (5d3) site is the same in all compounds, the magnetic super-exchange interactions mediated by non-magnetic Y/In/Sc species are strongest for Sr2ScOsO6, weakest for Sr2InOsO6, and intermediate in case of the Y (d0), due to different energy overlaps between Os-5d and Y/In/Sc-d states. This explains the experimentally observed substantial differences in the magnetic transition temperatures of these materials, despite of an identical magnetic site and underlying magnetic ground state. Furthermore, short range Os-Os exchange-interactions are more prominent than long range Os-Os interactions in these compounds, which contrasts with the behavior of other 3d-5d double-perovskites., Comment: 9 pages, 3 figures

The electronic and magnetic properties of monoclinic double perovskite Sr$_2$CeIrO$_6$ were examined based on both experiments and first-principles density functional theory calculations. From the calculations we conclude that low-spin-state Ir$^{4+}$ (5$\textit{d}^5$, S=$\frac{1}{2}$) shows t$_{2g}$ band derived anti-ferro type orbital ordering implying alternating occupations of $\textit{d}_{yz}$ and $\textit{d}_{xz}$ orbitals at the two symmetrically independent Ir sites. The experimentally determined Jahn-Teller type distorted monoclinic structure is consistent with the proposed orbital ordering picture. Surprisingly, the Ir-5$\textit{d}$ orbital magnetic moment was found to be $\approx$ 1.3 times larger than the spin magnetic moment. The experimentally observed AFM-insulating states are consistent with the calculations. Both electron-electron correlation and spin-orbit coupling (SOC) are required to drive the experimentally observed AFM-insulating ground state. This single active site double perovskite provides a rare platform with a prototype geometrically frustrated fcc lattice where among the different degrees of freedom (i.e spin, orbital, and lattice), spin-orbit interaction and Coulomb correlation energy scales compete and interact with each other., Comment: 5 pages, 3 figures

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition metal complexes. For example, square-planar surroundings found in transition oxometalates such as curprates are usually associated with the d8 or d9 electron configuration. In this work, we address an exotic square-planar mono-oxoanion [IrO4]^{4-} as observed in Na4IrO4 with Ir(IV) in d^5 configuration, and characterize the chemical bonding by experiment and ab initio calculations. We find that Na4IrO4 in its ground state evolves a square-planar coordination for Ir(IV) because of the weak Coulomb repulsion of Ir-5d electrons. In contrast, in its 3d counterpart, Na4CoO4, Co(IV) is in tetrahedral coordination, due to strong electron correlation. Na4IrO4 thus may serve as a simple paradigmatic platform for studying the ramifications of Hubbard type Coulomb interactions on local geometries., Comment: 4 pages, 4 figures, accepted by Ang. Chem. Int. Ed. (2015)

Using density-functional theory calculations, we investigated the electronic structure and magnetic exchange interactions of the ordered 3d-5d double perovskite Sr2FeOsO6, which has recently drawn attention for interesting antiferromagnetic transitions. Our study reveals the vital role played by long-range magnetic exchange interactions in this compound. The competition between the ferromagnetic nearest neighbor Os-O-Fe interaction and antiferromagnetic next nearest neighbor Os-O-Fe-O-Os interaction induces strong frustration in this system, which explains the lattice distortion and magnetic phase transitions observed in experiments., Comment: 5 Figures, 2 Tables

We have studied the magnetic properties and spin-dynamics of the structurally-ordered double perovskite Sr$_2$CoOsO$_6$. Our 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 $T_{\mathrm{N}}=108$ K, cobalt (3$d^7$, $S=3/2$) and osmium (5$d^2$, $S=1$) moments fluctuate dynamically, while their \textit{average} effective moments undergo long-range order. In the second AFM phase with $T_{\mathrm{N}}=67$ 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 $T\approx 5$ K. Our \textit{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 Sr$_2$CoOsO$_6$ distinct from previously reported double-perovskite compounds., Comment: Accepted by Phys. Rev. Lett. 12 pages, 3 figures and 1 table

By means of a model Hamiltonian approach we study the role of volume expansion, Hund's coupling and electron correlation in the standard hybridization mechanism for ferroelectricity in cubic CaMnO$_3$, a prototypical non-$d^0$ perovskite. Our results establish that the ferroelectric instability arises from a subtle balance between different energy contributions, explaining the origin of its enhancement under negative pressure. Expansion of volume is found to cause a strong reduction of the elastic energy, while leaving almost unchanged the tendency of Mn states to form covalent bonds with the surrounding oxygens. Hund's coupling with local spins of magnetic cations can reduce and even suppress the instability towards the ferroelectric state., Comment: 7 pages, 5 figures