Birabar Ranjit Kumar Nanda

The practical application of Li–S batteries is severely limited due to low sulfur utilization, sluggish sulfur redox kinetics, intermediate polysulfide dissolution/shuttling, and subsequent anode degradation. A smart cathode with efficient electrocatalyst and a protected anode is necessary. Herein, hollow carbon (HC) spheres are used as a sulfur host to improve the electrical conductivity and buffer the volume expansion of active materials. Considering the weak interaction between carbon and lithium polysulfides (LiPS), tungsten diboride (WB2) nanoparticles are used as a conductive additive. Both experimental and density functional theory (DFT) comprehensively exhibit that metallic WB2 nanoparticles can firmly anchor the LiPS through B–S bond formation, accelerate their electrocatalytic conversion, and immobilize them. DFT also reveals that boron interacts with LiPS either through molecular or dissociative adsorption depending on its boron layer arrangement in WB2. Further, a freestanding lithiated-poly(4-styrene sulfonate) membrane constructed on lithium, offers a homogeneous Li-ion flux, stable interface, and protection from LiPS. Finally, cells with the HC-S+WB2 cathode and protected anode exhibit improved active material utilization, superior rate performance, and impressive cycling stability, even at high sulfur loading and less quantity of the electrolyte. Further, the pouch cells demonstrate high reversible capacity and an excellent capacity retention.

Artificial confinement of electrons by tailoring the layer thickness has turned out to be a powerful tool to harness control over competing phases in nano-layers of complex oxides. We investigate the effect of dimensionality on transport properties of d-electron–based heavy-fermion metal CaCu3Ru4O12. Transport behavior evolves from metallic to localized regime upon reducing thickness and a metal-insulator transition is observed below 3 nm film thickness for which sheet resistance crosses h/e2 ∼ 25 kΩ, the quantum resistance in 2D. Magnetotransport study reveals a strong interplay between inelastic and spin-orbit scattering lengths upon reducing thickness, which results in weak-antilocalization (WAL) to weak-localization (WL) crossover in magnetoconductance.

Anionic alloying (halide mixing) in lead-free halide double perovskites is an effective strategy to tailor the optoelectronic properties including band gap. An important question that needs to be addressed is whether halide ions mix up homogeneously at the atomic scale as has already been inferred in a hybrid halide solid solution. Here, we show from Raman spectral analyses that halide ions (X = Cl-, Br-) preferentially form Br-rich or Cl-rich octahedra in Cs2AgBiBr6-xClx (x = 0 to 6; M = Ag, Bi) double perovskites. Octahedral vibrations show discontinuity in Raman shifts upon alloying, and the observation of octahedral modes from both [MCl6-xBrx]5- and [MBr6-xClx]5- with a shift from the end-member vibrational frequencies confirms the absence of homogeneous mixing (i.e., octahedra [MBr3Cl3]5-) and preferential formation of X-rich octahedra. The lattice parameter and the optical band gap of Cs2AgBiBr6-xClx vary linearly resembling Vegard’s rule, suggesting a macroscopic solid solution behavior while maintaining, at the sublattice level, the preferential X-rich octahedra. This is further corroborated through comprehensive first-principles calculations that the alloyed structure with preferential occupation of halide ions, instead of local phase segregation or homogeneous mixing, tends to be the more stable configuration. An equal number of dissimilar halogen atoms in each octahedron as conventionally assumed is not a stable configuration. The linear variation of the band gap is attributed to the fact that individual Ag-X and Bi-X interactions add up to form the electronic structure, and therefore, the band gap is primarily correlated to the concentration of Cl and Br anions rather than their distribution in the individual octahedra.

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.

The halide perovskites have truly emerged as efficient optoelectronic materials and show the promise of exhibiting nontrivial topological phases. Since the bandgap is the deterministic factor for these quantum phases, here, we present a comprehensive electronic structure study using first-principle methods by considering nine inorganic halide perovskites CsBX3 (B = Ge, Sn, Pb; X = Cl, Br, I) in their three structural polymorphs (cubic, tetragonal, and orthorhombic). A series of exchange-correlation (XC) functionals are examined toward accurate estimation of the bandgap. Furthermore, while 13 orbitals are active in constructing the valence and conduction band spectra, here, we establish that a 4 orbital based minimal basis set is sufficient to build the Slater-Koster tight-binding (SK-TB) model, which is capable of reproducing the bulk and surface electronic structures in the vicinity of the Fermi level. Therefore, like the Wannier based TB model, the presented SK-TB model can also be considered an efficient tool to examine the bulk and surface electronic structures of the halide family of compounds. As estimated by comparing the model study and DFT band structure, the dominant electron coupling strengths are found to be nearly independent of XC functionals, which further establishes the utility of the SK-TB model.

Skyrmions in reduced dimensions such as thin layers and interfaces are of both fundamental and technological importance. In these systems, itinerant electrons are often present together with the Rashba and Dresselhaus spin-orbit coupling (SOC). Here, we show that an itinerant electron in the presence of these interactions can nucleate the skyrmion state, even when the standard Dzyaloshinskii-Moriya interaction (DMI) is absent, and the electron can become self-trapped in the skyrmion core, forming the "skyrmionic polaron"(SkP). The formation of the SkP is investigated from a continuum model of the electron, exchange coupled to the lattice spins, by solving the appropriate Euler-Lagrange equations. The skyrmion (antiskyrmion) texture is favored by the Rashba (Dresselhaus) SOC, with the binding energy increasing quadratically with the strength of the interaction. In contrast, if the skyrmion is already formed due to a nonzero DMI, the electron is delocalized and avoids the skyrmion core until the strength of the Rashba or Dresselhaus SOC exceeds a critical value. Below this critical value, the electron is not bound to the skyrmion core, the polaron does not form, and the electron has little effect on the skyrmion state. Our work envisions the possibility of manipulating the skyrmion state in device applications by altering the strength of the Rashba or Dresselhaus interactions, e.g., by an external electric field.

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.

Density functional theory computations are used to predict the electronic structure of Cd-based hybrid organic-inorganic high-TC ferroelectric perovskites with TMCM-CdCl3 being one representative. We report Rashba-Dresselhaus spin splitting in the valence band of these nonmagnetic compounds. Interestingly, we find in computations that the splitting is not necessarily sensitive to the polarization of the material but to the organic molecule itself which opens a way to its chemical tunability through the choice of the molecule. Further chemical tunability of spin splitting is shown to be possible through a substitution of Cl in the CdCl3 chains as the valence band was found to originate from Cl-Cl weekly bonding orbitals. For example, the substitution of Cl with Br in TMCM-CdCl3 resulted in a ten times increase of spin splitting. Furthermore, the spin polarization in these materials gives origin to persistent spin textures which are coupled to the polarization direction, and, therefore, can be controlled by the electric field. This is promising for spintronics applications.