Birabar Ranjit Kumar Nanda

The 5d transition metal oxides, in particular iridates, host novel electronic and magnetic phases due to the interplay between onsite Coulomb repulsion (U) and spin-orbit coupling (SOC). The reduced dimensionality brings another degree of freedom to increase the functionality of these systems. By taking the example of ultrathin films of SrIrO3, theoretically we demonstrate that confinement led localization can introduce large magnetic anisotropy energy (MAE) in the range of 2-7 meV/Ir, which is one to two order higher than that of the traditional MAE compounds formed out of transition metals and their multilayers. Furthermore, in the weak correlation limit, tailored terminations can yield multiple Dirac states across a large energy window of 2 eV around the Fermi energy, which is rare phenomena in correlated oxides and upon experimental realization it will give rise to unique transport properties with excitation and doping.

Ferroelectric oxides have gained research attention in the field of ferroelectric photovoltaics (PV) after the discovery of power conversion efficiency exceeding the Shockley-Queisser limit in BaTiO3 (BTO) crystals. However, advancement in this field is hindered by the wide bandgap (>3 eV) nature of ferroelectric oxides. In this work, a novel lead-free ferroelectric (1 − x)BTO − xBi(Ni2/3Nb1/3)O3 system was proposed and demonstrated to show bandgap reduction without compromising the polarization. Notably, the system displayed a bandgap reduction from 3.1 to 2.4 eV upon varying the composition from x = 0.0 to 0.05. Particularly, the optimal composition x = 0.02 showed enhancement in polarization (Pmax = 16 μC/cm2) and anomalous PV response with an open-circuit voltage of 6 V at 300 K. The origin of the bandgap reduction and polarization retention is explored experimentally by Raman spectroscopic measurements and analyzed theoretically by density functional theory. Our results revealed that the oxygen octahedral distortions and Ni2+ doping favor bandgap lowering, and Bi3+ ions stabilize the ferroelectric polarization. This study provides insight into the origin of bandgap tuning and paves the route for exploring new low-bandgap ferroelectric material with room temperature polarization.

The 4d and 5d transition metal oxides have become important members of the emerging quantum materials family due to the competition between on-site Coulomb repulsion (U) and spin-orbit coupling (SOC). Specifically, the systems with d5 electronic configuration in an octahedral environment are found to be capable of possessing invariant semimetallic state and perturbations can lead to diverse magnetic phases. In this work, by formulating a multiband Hubbard model and performing SOC tunable density functional theory+U calculations on prototypes SrIrO3 and CaIrO3 and extending the analysis to other isostructural and isovalent compounds, we present eight quantum phases that can be observed in the family of low-spin d5 perovskites. In the cubic configuration, the U-SOC phase diagram shows stabilization of nonmagnetic metal phase in the weak U regime irrespective of the strength of SOC with the doubly degenerate t2g-J1/2 states occupying the Fermi surface. However, the system become ferromagnetic metal with increasing U while the SOC is maintained low. As the SOC increases, the moderate and higher values of U makes the transition to an antiferromagnetic metal and eventually to an antiferromagnetic insulating state. The GdFeO3-type orthorhombic distortion through tilting and rotation of the octahedra reform the t2g states through orbital intermixing to introduce a noncollinear spin arrangement. In the weak correlation regime, the nonmagnetic metal phase transform to ferromagnetic metal phase for weak SOC and an invariant Dirac semimetal phase for the strong SOC. On increasing the correlation strength, the ferromagnetic metal phase becomes insulating while the Dirac semimetal phase becomes a canted antiferromagnetic metal and finally transform to the canted antiferromagnetic insulating phase. Interestingly, in the higher U and higher SOC regimes the normal-spin (Sz) component vanishes to form a pure coplanar spin arrangement. The presence of several soft phase boundaries makes the family of d5 perovskites an ideal platform to study electronic and magnetic phase transitions under external stimuli.

A-site ordered perovskites, CaCu3B4O12, which are derivatives of conventional ABO3 perovskites, exhibit varying electronic and magnetic properties. With the objective of examining the role of Cu in this work, we have studied CaCu3Ti4O12 and CaCu3Zr4O12 and presented the cause of the crystallization of A-site ordered perovskite from conventional ABO3 perovskite and the underlying mechanism leading to the stabilization of nontrivial and experimentally established G-type antiferromagnetic (G-AFM) ordering in these systems. The first-principles electronic structure calculations supplemented with phonon studies show that the formation of A-site ordered perovskite is driven by Jahn-Teller distortion of the CuO12 icosahedron. The crystal orbital Hamiltonian population analysis and magnetic exchange interactions estimated using spin dimer analysis infer that the nearest and next-nearest-neighbor interactions (J1 and J2) are direct and weakly ferromagnetic, whereas the third-neighbor interaction (J3) is unusually strong and antiferromagnetic driven by an indirect superexchange mechanism. The structural geometry reveals that stabilization of G-AFM requires J1<2J2, J1<2J3. The experimental and theoretical values of Néel temperature agree well for U≈ 7 eV, highlighting the role of strong correlation. The magnetic ordering is found to be robust against pressure and strain.

Double-perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. In this paper, we report the electronic structure and magnetic properties of an ordered double perovskite material, Ho2CoMnO6. In the double perovskites with general class A2BB′O6 (A = rare-earth ions; B, B′ = transition metal ions), the octahedral B and B′ sites have a distinct crystallographic site. The Rietveld refinement of x-ray diffraction data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibits a long-range ferromagnetic ordering at Tc∼76 K owing to superexchange interaction between Co2+ and Mn4+ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms the ferromagnetic behavior of this double perovskite. We observed a reentrant glassy state in the intermediate-temperature regime, which is attributed to inherent antisite disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. Temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above Tc in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infer the compound to be a narrow-band-gap insulator with the gap arising between the lower and upper Hubbard Co d subbands. Our results demonstrate that antisite disorder and complex 3d-4f exchange interactions in the spin lattice account for the observed electronic and magnetic properties in this promising double perovskite material.

In the Ca3Co2O6 (CCO) system, the large contribution of the orbital moment to the magnetization and the strong magnetocrystalline anisotropy (MCA) are considered to give rise to the Ising magnetism. In this study, the dominant role of both spin-orbit-coupling (SOC) and crystal-field (CF) effects behind this Ising character of magnetism is qualitatively elucidated from the temperature and field dependence of magnetization in the presence of hydrostatic pressures up to 1.04 GPa in a CCO single crystal (SC). The local trigonal prismatic environment is compressed with the application of high pressure, resulting in higher trigonal CF as compared to ambient conditions. It reduces the effect of SOC due to the initiation of orbital quenching that finally decreases orbital moment contributions to both the magnetization and the MCA, respectively. This interplay of triagonal CF and SOC effects is further shown from the detailed quantitative analysis of the field-dependent magnetization in different orientations of CCO and Ca3Co1.8Fe0.2O6 (CCFO) SCs at ambient pressure by employing a simple classical model and second-order perturbative analysis of SOC. The complete quenching of the orbital moment of Fe3+ (S=5/2) in CCFO weakens the MCA and also helps in deducing the SOC effect. Furthermore, the estimated anisotropic constants using density functional theory very well capture the Ising magnetism in CCO and deviation from it in CCFO compared to that of classical results.

State-of-the-art defect engineering techniques have paved the way to realize unique quantum phases out of pristine materials. Here, through density-functional calculations and model studies, we show that the chain-doped monolayer transition metal dichalcogenides, where M atoms on a single zigzag chain are replaced by a higher-valence transition-metal element M′ (MX2/M′), exhibit one-dimensional (1D) bands. These 1D bands, occurring in the fundamental gap of the pristine material, are dispersive along the doped chain but are strongly confined along the lateral direction. This confinement occurs as the bare potential of the dopant chain formed by the positively charged M′ ions resembles the potential well of a uniformly charged wire. These bands could show unique 1D physics, including another type of Tomonaga-Luttinger liquid behavior, multiorbital Mott insulator physics, and an unusual optical absorption due to the simultaneous presence of the spin-orbit coupling, strong correlation, multiple orbitals, Rashba spin splitting, and broken symmetry. We find the broadening of the half-filled 1D bands with correlation. It is surprising since correlation reduces the effective hopping interactions and in turn reduces the bandwidth. This is interpreted to be due to multiple orbitals forming the single Hubbard band at different points of the Brillouin zone. Furthermore, due to the presence of an intrinsic electric field along the lateral direction, the 1D bands are Rashba spin-split and provide a mechanism for tuning the valley-dependent optical transitions.

Ruthenium-based pyrochlore oxides exemplify a unique material system in which the competition of Mottness (U), Hundness (JH), and effective p-d hybridization brings exotic physical characteristics. To understand and manipulate these interactions, we have investigated the structural, magnetic, and electrical properties of the pyrochlore ruthenate Y2Ru2O7 and its doped variants (Y1-xCax)2Ru2O7. The results of our magnetic measurements reveal a systematic suppression of antiferromagnetic (AFM) ordering and increased frustration with the progressive substitution of Ca at the Y site leading to a nonequilibrium glassy magnetic state at low temperature above the doping level of 10%. Moreover, the screening of the net effective moment of Ru in the doped samples, as derived from the experimental results, is attributed to the induced itinerant character of Ru5+. Such an effect is also reflected in the substantially reduced electrical resistivity, possibly due to enhanced d-p hybridization. Consistent with the experimental results, density functional theory calculations confirm the bulk AFM ordering and a drop in the band gap of the doped samples. We further claim the low-temperature magnetic state of Y2Ru2O7 to possess a noncollinear all-out-type spin configuration, which transforms into a randomized glassy phase upon hole doping.