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Vacancy-ordered halide double perovskites hosting 4d/5d transition metals have emerged as a distinct platform for investigating unconventional magnetism arising out of the interplay of strong atomic spin-orbit coupling (SOC) and Coulomb interactions. Focusing on the $d^2$ system Cs$_2$WCl$_6$, our ab initio electronic structure calculation reveals very narrow electronic bands, fulfilling the necessary condition to realize exotic orders. Using this input, we solve the many-body spin-orbit coupled single-site problem by exact diagonalization and show that the multiplet structure of Cs$_2$WCl$_6$ hosts ground non-Kramers doublets on W, with vanishing dipole moment and a small gap to an excited magnetic triplet. Our work provides the rationale for the observed strong deviation from the classic Kotani behaviour in Cs$_2$WCl$_6$ for the measured temperature dependence of the magnetic moment. The non-Kramers doublets on W exhibit non-zero quadrupolar and octupolar moments, and our calculated two-site exchange supports the dominance of inter-site octupolar exchange over quadrupolar interactions. We predict ferro-octupolar order with a transition temperature $T_c \sim 5$K which may get somewhat suppressed by quantum fluctuations and disorder; this could be tested in future low-temperature experiments., Comment: 8 pages, 5 figs, submitted to PRB

Transition metal-based quaternary chalcogenides have gathered immense attention for various renewable energy applications including thermoelectrics (TE). While low-symmetry and complex structure help to achieve low thermal conductivity, the TE power factor and hence the figure of merit (ZT) remains low which hinders to promote these class of materials for future TE applications. Here, we investigated the TE properties of a new system, Cu$_2$ZnSiTe$_4$, with improved electronic transport using first-principles calculation. The presence of heavy chalcogen like Te, helps to achieve a relatively low bandgap (0.58 eV). This, together with unique electronic band topology, leads to a promising value of power-factor of 3.95(n-type) and 3.06(p-type) mWm$^{-1}$K$^{-2}$ at 900 K. Te atoms also play a crucial role in mixing the optical and acoustic phonon branches which, in turn, are responsible for reduced lattice thermal conductivity ($\sim$0.7 Wm$^{-1}$K$^{-1}$ at high temperature). Though the thermal conductivity is not appreciably low, the electronic transport properties (power factor) are quite favorable to yield promising TE figure of merit (ZT $\sim$2.67 (n-type) and $\sim$2.11 (p-type) at 900 K). We propose Cu$_2$ZnSiTe$_4$ to be a potential candidate for TE applications, and believe to attract future experimental/theoretical studies.

Despite a clear demonstration of bistability in spin-crossover (SCO) materials, the absence of long-range magnetic order and poor electrical conductivity limit their prospect in spintronic and nanoelectronic applications. Intending to create hybrid devices made of spin-crossover (SCO)-2D architecture, here, we report an easily processable Fe-based SCO nanostructures grown on 2D reduced graphene oxide (rGO). The heterostructure shows enhanced cooperativity due to formation of interfacial charge transfer induced inter-molecular interaction. The spin transition temperature is controlled by tuning the coverage area of SCO nanostructured networks over the 2D surfaces, thus manipulating hysteresis (aka memory) of the heterostructure. The enhanced magnetic coupling of the heterostructure leads to the spontaneous magnetization states with a large coercive field of $\sim$ 3000 Oe. Additionally, the low conductivity of the pristine SCO nanostructures is addressed by encapsulating them on suitable 2D rGO template, enabling detection of magnetic bistable spin states during high-spin/low-spin conductance change. This adds spin functionality in conductance switching for realizing hybrid 2D spintronic devices. Ab-inito calculations, on the experimentally proposed nanostructures, corroborate the enhanced magnetic interaction in the proposed architecture facilitated by interfacial charge transfer and provide insights on the microscopic mechanism., Comment: 23 pages, 6 figures and supplementary information included

Bi-layer Kagome compounds provide an exciting playground where the interplay of topology and strong correlations can give rise to exotic phases of matter. Motivated by recent first principles calculation on such systems (Phys. Rev. Lett 125, 026401), reporting stabilization of a Chern metal with topological nearly-flat band close to Fermi level, we build minimal models to study the effect of strong electron-electron interactions on such a Chern metal. Using approriate numerical and analytical techniques, we show that the topologically non-trivial bands present in this system at the Fermi energy can realize fractional Chern insulator states. We further show that if the time-reversal symmetry is restored due to destruction of magnetism by low dimensionality and fluctuation, the system can realize a superconducting phase in the presence of strong local repulsive interactions. Furthermore, we identify an interesting phase transition from the superconducting phase to a correlated metal by tuning nearest-neighbor repulsion. Our study uncovers a rich set of non-trivial phases realizable in this system, and contextualizes the physically meaningful regimes where such phases can be further explored., Comment: 16 pages, 14 figures

The study of topological quantum materials for enhanced thermoelectric energy conversion has received significant attention recently. Topological materials (including topological insulators and Dirac/Weyl/nodal-line semi-metals) with unique nature of band structure involving linear and regular parabolic bands near Fermi level (E$_F$) have the potential to show promising TE properties. In this article, we report the promising TE performance of a quaternary chalcogenide (Cu$_2$ZnGeTe$_4$) having non-trivial topological phase. At ambient condition, the compound is a narrow band gap (0.067 eV) semiconductor, with a TE figure of merit (ZT) 1.2. Application of 5% strain drives the system to a topologically non-trivial Weyl semi-metal with the right combination of linear and parabolic bands near E$_F$, giving rise to a reasonable ZT of 0.36. Apart from strain, alloy engineering (Sn substituted at Ge) is also shown to induce topological non-triviality. The present work demonstrates the potential of such unique semimetals for exceptional electronic transport properties and hence excellent thermoelectric performance., Comment: 8 pages, 7 figures

(TaSe$_4$)$_3$I, a compound belonging to the family of quasi-one-dimensional transition-metal tetrachalcogenides, has drawn significant attention due to a recent report on possible coexistence of two antagonistic phenomena, superconductivity and magnetism below 2.5~K (Bera et. al, arXiv:2111.14525). Here, we report a structural phase transition of the trimerized phase at temperature, $T~\simeq$~145~K using Raman scattering, specific heat, and electrical transport measurements. The temperature-dependent single-crystal X-ray diffraction experiments establish the phase transition from a high-temperature centrosymmetric to a low-temperature non-centrosymmetric structure, belonging to the same tetragonal crystal family. The first-principle calculation finds the aforementioned inversion symmetry-breaking structural transition to be driven by the hybridization energy gain due to the off-centric movement of the Ta atoms, which wins over the elastic energy loss., Comment: 11 pages, 5 figures, Under review as a regular article

The gender imbalance in physics higher education and advanced professions is a global problem, and India is not an exception. Although the issue has been acknowledged widely, discrimination needs to be recognized as the driving force. The past three years have witnessed initiatives by different gender groups as well as the Government of India in addressing these lacunae. We report various activities, describe interventions, and present statistics indicating improvements achieved. The Gender in Physics Working Group has brought about significant gender reforms in the Indian Physics Association. The working group organized an open discussion on the issue of sexual harassment in physics professions for the first time in 2018. Subsequently, in 2019, GIPWG organized the first-ever national conference on gender issues, Pressing for Progress. The deliberations of the conference culminated in the Hyderabad Charter, a roadmap towards gender equity in India. The Working Group for Gender Equity constituted under the Astronomical Society of India, also played an impactful role. At the government level, notable new initiatives include Gender Advancement through Transforming Institutions and the proposed Science and Technology Innovation Policy for mainstreaming equity and inclusion., Comment: 3 pages, 1 figure

Recent theoretical and experimental work suggest that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and density-matrix renormalization group (DMRG) calculations, we show that this model exhibits FM order at small $J_3/J_1$ and zig-zag (ZZ) order at large $J_3/J_1$, separated by an intermediate phase, which we label as $\widetilde{\mathrm{SL}}$. This $\widetilde{\mathrm{SL}}$ phase is shown to exhibit spin-liquid-like correlations in DMRG, although we cannot preclude weak broken symmetries, e.g. weak Ising type N\'eel order, given the limits on our explored system sizes. Using a modified parton mean field theory and variational Monte Carlo on Gutzwiller projected wavefunctions, we show that the optimal FM and ZZ orders as well as the intermediate $\widetilde{\mathrm{SL}}$ state are proximate to a `parent' Dirac spin liquid (SL). This Dirac SL is shown to capture the broad continuum in the temperature and magnetic field dependent terahertz spectroscopy of BaCo$_2$(AsO$_4$)$_2$, and the reported low temperature metallic thermal conductivity in Na$_2$Co$_2$TeO$_6$ and BaCo$_2$(AsO$_4$)$_2$ upon incorporating disorder induced broadening., Comment: Final PRB version - 13 pages, 11 figures. Special thanks to Federico Becca, Sasha Chernyshev, Martin Klanjsek

Quantum materials with non-Kramers doublets are a fascinating venue to realize multipolar hidden orders. Impurity probes which break point group symmetries, such as implanted muons or substitutional impurities, split the non-Kramers degeneracy and exhibit a Janus-faced influence in such systems: they can destroy the very order they seek to probe. Here, we explore this duality in cubic osmate double perovskites which are candidates for exotic $d$-orbital octupolar order competing with quadrupolar states. Using {\it ab initio} computations, Landau theory, and Monte Carlo simulations, we show that Janus impurities induce local strain fields, nucleating quadrupolar puddles and suppressing the octupolar $T_c$. At the same time, strains mix the non-Kramers doublet with an excited magnetic triplet, creating parasitic dipole moments which directly expose the hidden octupolar order parameter. Our work unravels this Janus duality in recent impurity nuclear magnetic resonance (NMR) experiments, with important implications for uncovering hidden order in diverse multipolar materials., Comment: Main Text: 14 pages, 7 figures, 1 Table (published - open access)

The spin-orbit coupled double perovskite iridate Ba$_2$YIrO$_6$ with $d^4$ occupancy of Ir is considered as a candidate material for a non-magnetic $J=0$ ground state. The issue of existence of such a state in Ba$_2$YIrO$_6$ however has opened up intense debates both in experimental and theoretical studies. In this study, we revisit the issue using \textit{ab-initio} density functional combined with dynamical mean-field theory to investigate the magnetic properties of Ba$_2$YIrO$_6$ down to zero temperature. To reach the ground state, a recently developed impurity solver based on tensor-product states working directly at zero temperature is employed. We find that Ba$_2$YIrO$_6$ has a small instantaneous non-zero magnetic moment, both at $T=0$ K as well as at room temperature. We did not observe any evidence of magnetic ordering, not even at $T=0$ K. From the calculated local magnetic susceptibility we see that the quantum fluctuations are very strong and effective in screening the instantaneous moments. This dynamical screening, together with frustration effects in the fcc lattice that can lead to almost degenerate magnetic ground states, prevents any long-range ordering., Comment: 11 pages, 8 figures

In order to explore the effects of structural geometry on properties of correlated metals we investigate the magnetic properties of cubic (3C) and hexagonal (4H) BaRuO$_3$. While the 3C variant of BaRuO$_3$ is ferromagnetic below 60K, the 4H phase does not show any long-range magnetic order, however, there is experimental evidence of short-range antiferromagnetic correlations. Employing a combination of computational tools, namely density-functional theory and dynamical mean-field theory calculations, we probe the origin of contrasting magnetic properties of BaRuO$_3$ in the 3C and 4H structures. Our study reveals that the difference in connectivity of RuO$_6$ octahedra in the two phases results in different Ru-O covalency, which in turn influences substantially the strengths of screened interaction values for Hubbard $U$ and Hund's rule $J$. With estimated $U$ and $J$ values, the 3C phase turns out to be a ferromagnetic metal, while the 4H phase shows paramagnetic behavior with vanishing ordered moments. However, this paramagnetic phase bears signatures of antiferromagnetic correlations, as confirmed by a calculation of the magnetic susceptibility. We find that the 4H phase is found to be at the verge of antiferromagnetic long-range order, which can be stabilized upon slight changes of screened Coulomb parameters $U$ and $J$, opening up the possibility of achieving a rare example of an antiferromagnetic metal.

Spin reorientation transitions and other related magnetic phenomena, which owe their origin to the complex interplay between multiple magnetic sublattices, have long attracted scientific attention both from the perspective of fundamental curiosity and technological applications. In this study, combining first principles calculations together with finite temperature Monte Carlo simulations, we explore the possible origins of reorientation transition of Cr spins in NdCrO$_3$. We construct a NdCrO$_3$ specific magnetic model, consisting of symmetric superexchange interactions between magnetic ions, as well as their magnetic anisotropy. We show that the observed spin reorientation in NdCrO$_3$, arises out of a delicate balance between Nd$-$Cr magnetic exchange interactions, single ion anisotropy of Nd spins, and single ion anisotropy of Cr spins. Moreover, though our model does not take into consideration the effect of anti-symmetric and anisotropic-symmetric magnetic exchanges, the qualitative as well as quantitative agreement of the theoretically derived and the experimentally observed spin-reorientation transition in NdCrO$_3$, confirms the merit of our proposed microscopic model. Our results also propose a hitherto unobserved collective magnetic ordering in Nd sublattice, which is challenging to detect as it is an extreme low temperature phenomena, therefore calls for further investigations., Comment: 9 pages, 5 figures and 1 table

The quest for Kitaev quantum spin liquids has led to great interest in honeycomb quantum magnets with strong spin-orbit coupling. It has been recently proposed that even Mott insulators with $3d$ transition metal ions, having nominally weak spin-orbit coupling, can realize such exotic physics. Motivated by this, we study the rhombohedral honeycomb cobaltates CoTiO$_3$, BaCo$_2$(PO$_4$)$_2$, and BaCo$_2$(AsO$_4$)$_2$, using $\textit{ab initio}$ density functional theory, which takes into account realistic crystal field distortions and chemical information, in conjunction with exact diagonalization numerics. We show that these Co$^{2+}$ magnets host $j_\text{eff}=1/2$ local moments with highly anisotropic $g$-factors, and we extract their full spin Hamiltonians including longer-range and anisotropic exchange couplings. For CoTiO$_3$, we find a nearest-neighbor easy-plane ferromagnetic $XXZ$ model with additional bond-dependent anisotropies and interlayer exchange, which supports three-dimensional (3D) Dirac nodal line magnons. In contrast, for BaCo$_2$(PO$_4$)$_2$ and BaCo$_2$(AsO$_4$)$_2$, we find a strongly suppressed interlayer coupling, and significant frustration from additional third-neighbor antiferromagnetic exchange mediated by P/As. Such bond-anisotropic $J_1$-$J_3$ spin models can support collinear zig-zag or coplanar spiral ground states; we discuss their dynamical spin correlations which reveal a gapped Goldstone mode, and argue that the effective parameters of these pseudospin-$1/2$ models may be strongly renormalized by coupling to a low energy spin-exciton. Our results call for re-examining proposals for realizing Kitaev spin liquids in the honeycomb cobaltates., Comment: Main Text: 14 pages, 9 figures ; Supplementary Material: 9 pages, 6 figures v3: added final version, journal ref

In this paper a comparative structural, dielectric and magnetic study of two langasite compounds Ba$_3$TeCo$_3$P$_2$O$_{14}$ (absence of lone pair) and Pb$_3$TeCo$_3$P$_2$O$_{14}$ (Pb$^{2+}$ 6$s^2$ lone pair) have been carried out to precisely explore the development of room temperature spontaneous polarization in presence of stereochemically active lone pair. In case of Pb$_3$TeCo$_3$P$_2$O$_{14}$, mixing of both Pb 6$s$ with Pb 6$p$ and O 2$p$ help the lone pair to be stereochemically active. This stereochemically active lone pair brings a large structural distortion within the unit cell and creates a polar geometry, while Ba$_3$TeCo$_3$P$_2$O$_{14}$ compound remains in a nonpolar structure due to the absence of any such effect. Consequently, polarization measurement under varying electric field confirms room temperature ferroelectricity for Pb$_3$TeCo$_3$P$_2$O$_{14}$, which was not the case of Ba$_3$TeCo$_3$P$_2$O$_{14}$. Detailed study was carried out to understand the microscopic mechanism of ferroelectricity which revealed the exciting underlying activity of poler TeO$_6$ octahedral unit as well as Pb-hexagon.

We have investigated the structural, magnetic and dielectric properties of Pb-based langasite compound Pb$_3$TeMn$_3$P$_2$O$_{14}$ both experimentally and theoretically in the light of metal-oxygen covalency, and the consequent generation of multiferroicity. It is known that large covalency between Pb 6$p$ and O 2$p$ plays instrumental role behind stereochemical lone pair activity of Pb. The same happens here but a subtle structural phase transition above room temperature changes the degree of such lone pair activity and the system becomes ferroelectric below 310 K. Interestingly, this structural change also modulates the charge densities on different constituent atoms and consequently the overall magnetic response of the system while maintaining global paramagnetism behavior of the compound intact. This single origin of modulation in polarity and paramagnetism inherently connects both the functionalities and the system exhibits mutiferroicity at room temperature.

Understanding magnetism in multiple magnetic sublattice system, driven by the interplay of varied nature of magnetic exchanges, is on one hand challenging and on other hand intriguing. Motivated by the recent synthesis of AA'BB'O_6 double double perovskites with multiple magnetic ions both at A- and B-sites, we investigate the mechanism of magnetic behavior in these interesting class of compounds. We find that the magnetism in such multiple sublattice compounds is governed by the interplay and delicate balance between two distinct mechanisms, a) kinetic energy-driven multiple sublattice double exchange mechanism and b) the conventional super-exchange mechanism. The derived spin Hamiltonian based on first-principles calculations is solved by the classical Monte Carlo technique which reproduces the observed magnetic properties. Finally, the influence of off-stoichiometry, as in experimental samples, is discussed. Some of these double double perovskite compounds are found to possess large total magnetic moment and also are found to be half-metallic, which raises the hope of future applications of these large magnetic moment half-metallic oxides in spintronics and memory devices.

Employing first-principles density functional theory calculations and Wannierization of the low energy band structure, we analyze the electronic structure of undoped, infinite-layer nickelate compounds, NdNiO$_2$, PrNiO$_2$ and LaNiO$_2$. Our study reveals important role of non-zero $f$-ness of Nd and Pr atoms, as opposed to $f^{0}$ occupancy of La. The non-zero $f$-ness becomes effective in lowering the energy of the rare-earth 5$d$ hybridized axial orbital, thereby enhancing the electron pockets and influencing the Fermi surface topology. The Fermi surface topology of NdNiO$_2$ and PrNiO$_2$ is strikingly similar, while differences are observed for LaNiO$_2$. This difference shows up in computed doping dependent superconducting properties of the three compounds within a weak coupling theory. We find two gap superconductivity for NdNiO$_2$ and PrNiO$_2$, and possibility of a single gap superconductivity for LaNiO$_2$ with the strength of superconductivity suppressed by almost a factor of two, compared to Nd or Pr compound., Comment: 5+ pages, 3 figures. Submitted

We employ a combination of machine learning and first-principles calculations to predict magnetic properties of rare-earth lean magnets. For this purpose, based on training set constructed out of experimental data, the machine is trained to make predictions on magnetic transition temperature (Tc), largeness of saturation magnetization ({\mu}0Ms), and nature of the magnetocrystalline anisotropy (Ku). Subsequently, the quantitative values of {\mu}0Ms and Ku of the yet-to-be synthesized compounds, screened by machine learning, are calculated by first-principles density functional theory. The applicability of the proposed technique of combined machine learning and first-principles calculations is demonstrated on 2-17-X magnets, Ce2Fe17-xCoxCN. Further to this study, we explore stability of the proposed compounds by calculating vacancy formation energy of small atom interstitials (N/C). Our study indicates a number of compounds in the proposed family, offers the possibility to become solution of cheap, and efficient permanent magnet., Comment: Accepted in Phys. Rev. Applied

We study the deformation of a fluctuating crystalline sheet confined between two flat rigid walls as a simple model for layered solids where bonds among atoms {\it within} the same layer are much stronger than those {\it between} layers. When subjected to sufficiently high loads in an appropriate geometry, these solids deform and fail in unconventional ways. Recent experiments suggest that configurations named {\it ripplocations}, where a layer folds backwards over itself, are involved. These structures are distinct and separated by large free energy barriers from smooth {\it ripples} of the atomic layers that are always present at any non-zero temperature. We use Monte Carlo simulation in combination with an umbrella sampling technique to obtain conditions under which such structures form and study their specific experimental signatures., Comment: 8 pages, 7 figures

Motivated by the recent report on high T$_c$ ferromagnetic insulating state of single transition metal containing double perovskite compound, Sr$_3$OsO$_6$ (Wakabayashi et. al., Nature Commun {\bf 10} 535, 2019), we study this curious behavior by employing first-principles calculations in conjunction with exact diagonalization of full $t_{2g}$ multiplet problem of two Os sites. Our analysis highlights the fact that stabilization of Sr$_3$OsO$_6$ in the cubic phase in epitaxially grown thin film is the key to both ferromagnetic correlation and high temperature scale associated to it. This also provides a natural explanation for the sister compound, Ca$_3$OsO$_6$ to exhibit low T$_N$ antiferromagnetism in its monoclinic structure. Further the insulating property is found to be driven by opening of Mott gap in the half filled spin-orbit coupled $j=3/2$ manifold of $d^{2}$ Os. We point out that Sr$_2$CaOsO$_6$ which naturally forms in the cubic phase would be worthwhile to explore as a future candidate to exhibit high T$_c$ ferromagnetic insulating state in bulk form., Comment: 8 pages, 4 figures, to appear in Phys. Rev. B

In the present study, we explore superconductivity in NdNiO$_2$ and LaNiO$_2$ employing a first-principles derived low-energy model Hamiltonian, consisting of two orbitals: Ni $x^{2}$-$y^{2}$, and an {\it axial} orbital. The {\it axial} orbital is constructed out of Nd/La $d$, Ni 3$z^{2}$-$r^{2}$ and Ni $s$ characters. Calculation of the superconducting pairing symmetry and pairing eigenvalue of the spin-fluctuation mediated pairing interaction underlines the crucial role of inter-orbital Hubbard interaction in superconductivity, which turns out to be orbital-selective. The axial orbital brings in materials dependence in the problem, making NdNiO$_2$ different from LaNiO$_2$, thereby controlling the inter-orbital Hubbard interaction assisted superconductivity., Comment: 6 pages, 4 figures

The binary intermetallic materials, $M_3$Sn$_2$ ($M$ = 3d transition metal) present a new class of strongly correlated systems that naturally allows for the interplay of magnetism and metallicity. Using first principles calculations we confirm that bulk Fe$_3$Sn$_2$ is a ferromagnetic metal, and show that $M$ = Ni and Cu are paramagnetic metals with non-trivial band structures. Focusing on Fe$_3$Sn$_2$ to understand the effect of enhanced correlations in an experimentally relevant atomistically thin single kagome-bilayer, our ab-initio results show that dimensional confinement naturally exposes the flatness of band structure associated with the bilayer kagome geometry in a resultant ferromagnetic Chern metal. We use a multistage minimal modeling of the magnetic bands progressively closer to the Fermi energy. This effectively captures the physics of the Chern metal with a non-zero anomalous Hall response over a material relevant parameter regime along with a possible superconducting instability of the spin-polarised band resulting in a topological superconductor., Comment: 12 pages, 11 figures, 7 tables

Motivated by the puzzling report of the observation of a ferromagnetic insulating state in LaMnO$_3$/SrTiO$_3$ heterostructures, we calculate the electronic and magnetic state of LaMnO$_3$, coherently matched to a SrTiO$_3$ square substrate within a "strained-bulk" geometry. We employ three different density functional theory based computational approaches: (a) density functional theory (DFT) supplemented with Hubbard U (DFT+U), (b) DFT + dynamical mean field theory (DMFT), and (c) a hybrid functional treatment of the exchange-correlation functional. While the first two approaches include local correlations and exchange at Mn sites, treated in a static and dynamic manner, respectively, the last one takes into account the effect of non-local exchange at all sites. We find in all three approaches that the compressive strain induced by the square substrate of SrTiO$_3$ turns LaMnO$_3$ from an antiferromagnet with sizable orbital polarization to a ferromagnet with suppressed Jahn-Teller distortion in agreement with experiment. However, while both DFT+U and DFT+DMFT provide a metallic solution, only the hybrid calculations result in an insulating solution, as observed in experiment. This insulating behavior is found to originate from an electronic charge disproportionation. Our conclusions remain valid when we investigate LaMnO$_3$/SrTiO$_3$ within the experimental set-up of a superlattice geometry using DFT+U and hybrid calculations., Comment: 10 pages, 7 figures

Motivated by the interest in topological quantum paramagnets in candidate spin-$1$ magnets, we investigate the diamond lattice compound NiRh$_2$O$_4$ using {\it ab initio} theory and model Hamiltonian approaches. Our density functional study, taking into account the unquenched orbital degrees of freedom, shows stabilization of $S=1$ and $L=1$ state. We highlight the importance of spin-orbit coupling, in addition to Coulomb correlations, in driving the insulating gap, and uncover frustrating large second-neighbor exchange mediated by Ni-Rh covalency. A single-site model Hamiltonian incorporating the large tetragonal distortion is shown to give rise to a spin-orbit entangled non-magnetic ground state, largely accounting for the entropy, magnetic susceptibility, and inelastic neutron scattering results. Incorporating inter-site exchange within a slave-boson theory, we show that exchange frustration can suppress exciton condensation. We capture the dispersive gapped magnetic modes, uncover `dark states' invisible to neutrons, and make predictions for future experiments., Comment: 5 pages, 3 figures

Using first-principles density functional theory (DFT), we studied the issue of water incorporation in iron bearing wadsleyite (Mg$_2$SiO$_4$)mineral at transition zone pressures and temperature under varying conditions of vacancy concentration and oxygen fugacity. We have considered the effect of varying vacancy concentration by considering single and double vacancy. The affect of varying oxygen fugacity has been modelled through varying ratio of ferric and ferrous ions in the cell. Our first-principles calculations have confirmed that about 30% of the Fe$^{3+}$ can be found at the Si tetrahedral site, which is in accordance with the previous experimental observation by Bolfan-Casanova et al. [2012]. Using different structures of Fe bearing wadsleyite, representing vacancy concentration and oxidizing condition, we calculated elastic properties, such as bulk and shear moduli, phase and shear wave velocity, as a function of pressure and temperature. By comparison of our theoretical prediction with the Preliminary Reference Earth Model (PREM) data, we conclude that the water content of the transition zone decreases with increasing depth, corroborating the previously proposed theories on mantle convection [Bercovici and Karato, 2003]., Comment: one author not agree with results part

We propose the concept of "hybridization-switching induced Mott transition" which is relevant to a broad class of ABO$_3$ perovskite materials including BiNiO$_3$ and PbCrO$_3$ which feature extended $6s$ orbitals on the A-site cation (Bi or Pb), and A-O covalency induced ligand holes. Using {\it ab initio} electronic structure and slave rotor theory calculations, we show that such systems exhibit a breathing phonon driven A-site to oxygen hybridization-wave instability which conspires with strong correlations on the B-site transition metal ion (Ni or Cr) to induce a Mott insulator. These Mott insulators with active A-site orbitals are shown to undergo a pressure induced insulator to metal transition accompanied by a colossal volume collapse due to ligand hybridization switching., Comment: 6 pgs, 4 figs, published version

We study phase ordering kinetics in symmetric and asymmetric binary mixtures, undergoing an order-disorder transition below the critical temperature. Microscopically, we model the kinetics via antiferromagnetic Ising model with Kawasaki spin-exchange kinetics. This conserves the composition while the order-parameter (staggered magnetization) is not conserved. The order-parameter correlation function and structure factor show dynamical scaling, and the scaling functions are independent of the mixture composition. The average domain size shows a power-law growth: $L_\sigma(t)\sim t^\alpha$. The asymptotic growth regime has $\alpha=1/2$, though there can be prolonged transients with $\alpha<1/2$ for asymmetric mixtures. Our unambiguous observation of the asymptotic universal regime is facilitated by using an accelerated Monte Carlo technique. We also obtain the coarse-grained free energy from the Hamiltonian, as a function of two order-parameters. The evolution of these order-parameters is modeled by using \textit{Model C} kinetics. Similar to the microscopic dynamics, the average domain size of the nonconserved order-parameter (staggered magnetization) field exhibits a power-law growth: $L_m(t)\sim t^{1/2}$ at later times, irrespective of the mean value of the conserved order-parameter (composition) field., Comment: 21 Pages, 7 Figures, Accepted in EPJE

Manipulation of intrinsic magnetic and electronic structures of graphene nanoflakes is of technological importance. Here we carry out systematic study of the magnetic and electronic phases, and its manipulation in graphene nanoflakes employing first-principles calculation. We illustrate the intricate shape and size dependence on the magnetic and electronic properties, and further investigate the effects of carrier doping, which could be tuned by gate voltage. A transition from nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance, which we identify to be originated from the armchair defect at the junctions of two sublattices on the edge. Electron, or hole doping simultaneously influences the magnetic and electronic structures, and triggers phase changes. Beyond a critical doping, crossover from antiferromagnetic to ferromagnetic phase is observed for the flakes without sublattice imbalance, while suppression of magnetism, and a possible crossover from magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous to magnetic phase changes, a semiconductor to (half) metal transition is observed, upon carrier doping., Comment: 8 pages, 9 figures

Using combination of Density Functional Theory and Monte Carlo simulation, we study the phase stability and electronic properties of two dimensional hexagonal composites of boron nitride and graphene, with a goal to uncover the role of the interface geometry formed between the two. Our study highlights that preferential creation of extended armchair interfaces may facilitate formation of solid solution of boron nitride and graphene within a certain temperature range. We further find that for band-gap engineering, armchair interfaces or patchy interfaces with mixed geometry are most suitable. Extending the study to nanoribbon geometry shows that reduction of dimensionality makes the tendency to phase segregation of the two phases even stronger. Our thorough study should form an useful database in designing boron nitride-graphene composites with desired properties., Comment: 8 pages, 6 figures

Using spin polarized density functional theory (DFT) based calculations, combined with ab-initio molecular dynamics simulation, we carry out a systematic investigation of the bimetallic Ni$_{13-n}$Ag$_n$ nano clusters, for all compositions. This includes prediction of the geometry, mixing behavior, and electronic properties. Our study reveals a tendency towards formation of a core-shell like structures, following the rule of putting Ni in high coordination site and Ag in low coordination site. Our calculations predict negative mixing energies for the entire composition range, indicating mixing to be favored for the bimetallic small sized Ni-Ag clusters, irrespective of the compositions. The magic composition with the highest stability is found for the NiAg$_{12}$ alloy cluster. We investigate the microscopic origin of core-shell like structure with negative mixing energy, in which the Ni-Ag inter-facial interaction is found to play role. We also study the magnetic properties of the Ni-Ag alloy clusters. The Ni dominated magnetism, consists of parallel alignment of Ni moments while the tiny moments on Ag align in anti-parallel to Ni moments. The hybridization with Ag environment causes reduction of Ni moment.

Hybrid or metal organic framework (MOF) perovskites of general composition, ABX$_3$, are known to show interesting properties which can lead to variety of technological applications. Our first principles study shows they are also potential candidates for exhibiting cooperative spin-state transitions upon application of external stimuli. We demonstrate this by considering two specific Fe-based MOF perovskites, namely Dimethylammonium Iron Formate, [CH$_{3}$NH$_{2}$CH$_{3}$][Fe(HCOO)$_3$] and Hydroxylammonium Iron Formate, [NH$_{3}$OH][Fe(HCOO)$_3$]. Both the compounds are found to undergo high-spin (S=2) to low-spin (S=0) transition at Fe(II) site upon application of moderate strength of hydrostatic pressure, along with large hysteresis. This spin-state transition is signaled by the changes in electronic, magnetic and optical properties. We find both the transition pressure and the width of the hysteresis to be strongly dependent of the choice of A-site cation, Dimethylammonium or Hydroxylammonium, implying tuning of spin-switching properties achievable by chemical variation of the amine cation in the structure. Our finding opens up novel functionalities in this family of materials of recent interest, which can have important usage in sensors and memory devices., Comment: 16 pages

Motivated by the goal of realizing topological phases in thin films and heterostructures of correlated oxides, we propose here a quantum anomalous Hall insulator (QAHI) in ultrathin films of double perovskites based on mixed 3d-5d or 3d-4d transition metal ions, grown along the [111] direction. Considering the specific case of ultrathin Ba2FeReO6, we present a theoretical analysis of an effective Hamiltonian derived from first-principles. We establish that a strong spin-orbit coupling at Re site, t2g symmetry of the low-energy d-bands, polarity of its [111] orientation of perovskite structure, and mixed 3d-5d chemistry results in room temperature magnetism with a robust QAHI state of Chern number C=1 and a large band-gap. We uncover and highlight a non-relativistic orbital-Rashba-type effect in addition to the spin-orbit coupling, that governs this QAHI state. Our prediction of a large topological band-gap of ~100 meV in electronic structure, and a magnetic transition temperature Tc~300K estimated by Monte Carlo simulations, is expected to stimulate experimental efforts at synthesis of such films and enable possible practical applications of its dissipationless edge currents., Comment: 9 pages, 6 figs, submitted to PRB

Using a combination of first-principles density functional theory (DFT) calculations and exact diagonalization studies of a first-principles derived model, we carry out a microscopic analysis of the magnetic properties of the half-metallic double perovskite compound, Sr$_2$CrMoO$_6$, a sister compound of the much discussed material Sr$_2$FeMoO$_6$. The electronic structure of Sr$_2$CrMoO$_6$, though appears similar to Sr$_2$FeMoO$_6$ at first glance, shows non trivial differences with that of Sr$_2$FeMoO$_6$ on closer examination. In this context, our study highlights the importance of charge transfer energy between the two transition metal sites. The change in charge transfer energy due to shift of Cr $d$ states in Sr$_2$CrMoO$_6$ compared to Fe $d$ in Sr$_2$FeMoO$_6$ suppresses the hybridization between Cr $t_{2g}$ and Mo $t_{2g}$. This strongly weakens the hybridization-driven mechanism of magnetism discussed for Sr$_2$FeMoO$_6$. Our study reveals that, nonetheless, the magnetic transition temperature of Sr$_2$CrMoO$_6$ remains high since additional superexchange contribution to magnetism arises with a finite intrinsic moment developed at the Mo site. We further discuss the situation in comparison to another related double perovskite compound, Sr$_2$CrWO$_6$. We also examine the effect of correlation beyond DFT, using dynamical mean field theory (DMFT)., Comment: 8 pages, 3 figures

Using density functional theory calculations, we establish that the half-metallicity of bulk Ba2FeReO6 survives down to 1 nm thickness in BaTiO3/Ba2FeReO6/BaTiO3 heterostructures grown along the (001) and (111) directions. The confinement of the two-dimensional (2D) electron gas in this quantum well structure arises from the suppressed hybridization between Re/Fe d states and unoccupied Ti d states, and it is further strengthened by polar fields for the (111) direction. This mechanism, distinct from the polar catastrophe, leads to an order of magnitude stronger confinement of the 2D electron gas than that at the LaAlO3/SrTiO3 interface. We further show low-energy bands of (111) heterostructure display nontrivial topological character. Our work opens up the possibility of realizing ultrathin spintronic devices.

Emergent phases in the two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides have attracted great attention in the past decade. We present ab-initio electronic structure calculations for the interface between a Mott insulator GdTiO$_3$ (GTO) and a band insulator SrTiO$_3$ (STO) and compare our results with those for the widely studied LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface between two band insulators. Our GTO/STO results are in excellent agreement with experiments, but qualitatively different from LAO/STO. We find an interface carrier density of 0.5$e^{-}$/Ti, independent of GTO thickness in both superlattice and thin film geometries, in contrast to LAO/STO. The superlattice geometry in LAO/STO offers qualitatively the same result as in GTO/STO. On the other hand, for a thin film geometry, the interface carrier density builds up only beyond a threshold thickness of LAO. The positive charge at the vacuum surface that compensates the 2DEG at the interface also exhibits distinct behaviors in the two systems. The top GTO layer is found to be insulating due to correlation-driven charge disproportionation, while the top LAO layer is metallic within band theory and may become insulating due to surface disorder or surface reconstruction., Comment: 7 figures

We reinvestigate the pressure dependence of the crystal structure and antiferromagnetic phase transition in MnTe$_2$ by the rigorous and reliable tool of high pressure neutron powder diffraction. First-principles density functional theory calculations are carried out in order to gain microscopic insight. The measured N\'eel temperature of MnTe$_2$ is found to show unusually large pressure dependence of $12$ K GPa$^{-1}$. This gives rise to large violation of Bloch's rule given by $\alpha=\frac{d\log T_N}{d\log V}=-\frac{10}{3} \approx -3.3$, to a $\alpha$ value of -6.0 $\pm$ 0.1 for MnTe$_2$. The ab-initio calculation of the electronic structure and the magnetic exchange interactions in MnTe$_2$, for the measured crystal structures at different pressures, gives the pressure dependence of the Ne\'el temperature, $\alpha$ to be -5.61, in close agreement with experimental finding. The microscopic origin of this behavior turns to be dictated by the distance dependence of the cation-anion hopping interaction strength.

The main aim of this work is to present two different methodologies for configuration averaging in disordered systems. The Recursion method is suitable for the calculation of spatial or self-averaging, while the Augmented space formalism averages over different possible configurations of the system. We have applied these techniques to a simple example and compared their results. Based on these, we have reexamined the concept of spatial ergodicity in disordered systems. The specific aspect, we have focused on, is the question "Why does an experimentalist often obtain the averaged result on a single sample ?" We have found that in our example of disordered graphene, the two lead to the same result within the error limits of the two methods., Comment: 21 pages, 4 figures in Indian J Phys (2015)

With a view to gain an understanding about the alloying tendency of bimetallic nano alloy clusters of isoelectronics constituents, we studied the structural and mixing behaviors of Mn$_m$Tc$_n$ alloy clusters with $m+n =$13 for all possible compositions, using first principles electronic structure calculations. Our study reports a favorable mixing tendency for the alloy clusters. The average bond lengths of the minimum energy structures show an overall linear variation with concentrations, indicating a Vegard's law like variation for the nano alloy clusters, though the optimized structures undergo a structural transition from a closed and compact structure for the Mn-rich alloy clusters to an open layered like structure for the Tc-rich alloy clusters. We figure out a continuous and smooth interplay between hybridization and magnetization properties of the alloy clusters, which plays a vital role for the Vegard's law like variation in their average bond lengths., Comment: 13 pages and 4 figures