Showing papers by "Tanusri Saha-Dasgupta published in 2017"

Manipulating the mechanical properties of Ti 2 C MXene: Effect of substitutional doping

Abstract: With the aim of manipulating the mechanical properties of the recently discussed two-dimensional material MXene, we investigate the effect of alloying. We consider substitutional doping of B and V at Ti and C sites of ${\mathrm{Ti}}_{2}\mathrm{C}$. Calculations of quantities such as in-plane stiffness, Young's modulus, and critical strain through rigorous first-principles technique establish that B doping is highly effective in improving the elastic properties. Oxygen passivation of B-doped ${\mathrm{Ti}}_{2}\mathrm{C}$ in addition to improved elastic properties also exhibits reasonably high critical strains making them ideally suited for applications in flexible devices. Our study further reveals the presence of strong spin-phonon coupling in unpassivated ${\mathrm{Ti}}_{2}\mathrm{C}$ compounds which influences the mechanical behavior. The damage of ${\mathrm{Ti}}_{2}\mathrm{C}$ in its magnetic ground state of A-type antiferromagnetic structure is found to occur at much higher strain than that of the nonmagnetic ${\mathrm{Ti}}_{2}\mathrm{C}$.

Ferromagnetism at Room Temperature Induced by Spin Structure Change in BiFe1−xCoxO3 Thin Films

Abstract: The coexistence and coupling of ferromagnetic and ferroelectric orders in a single material is crucial for realizing next-generation multifunctional applications. The coexistence of such orders is confirmed at room temperature in epitaxial thin films of BiFe1-x Cox O3 (x ≤ 0.15), which manifests a spin structure change from a low-temperature cycloidal one to a high-temperature collinear one with canted ferromagnetism.

First principles study of bimetallic Ni13−nAgn nano-clusters (n = 0–13): Structural, mixing, electronic, and magnetic properties

Abstract: Using spin polarized density functional theory based calculations, combined with ab initio molecular dynamics simulation, we carry out a systematic investigation of the bimetallic Ni13−nAgn nano-clusters, for all compositions. This includes prediction of the geometry, mixing behavior, and electronic properties. Our study reveals a tendency towards the formation of a core-shell like structure, following the rule of putting Ni in a high coordination site and Ag in a 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 NiAg12 alloy cluster. We investigate the microscopic origin of a core-shell like structure with negative mixing energy, in which the Ni–Ag inter-facial interaction is found to play a role. We also study the magnetic properties of the Ni–Ag alloy clusters. The Ni do...

Design and Control of Cooperativity in Spin-Crossover in Metal–Organic Complexes: A Theoretical Overview

Abstract: Metal organic complexes consisting of transition metal centers linked by organic ligands, may show bistability which enables the system to be observed in two different electronic states depending o ...

Magnetic and electronic crossovers in graphene nanoflakes

Abstract: Manipulation of magnetic and electronic structures of graphene nanoflakes is of great technological importance. Here, we systematically study the magnetic and electronic phases of graphene nanoflakes within first-principles calculations. 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 crossover from the nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance. We identify this as originating from the armchair defects at the junctions of two sublattices on the edge. Electron or hole doping simultaneously influences the magnetic and electronic structures and triggers phase crossover. Beyond a critical doping, antiferromagnetic to ferromagnetic phase crossover is observed for the flakes without sublattice imbalance. In contrast, suppression of magnetism and a possible crossover from the magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous with magnetic phase changes, a semiconductor to (half) metal transition is observed upon carrier doping. Our findings should have important implications in graphene-based spintronics.

Properties at the interface of graphene and Ti 2 C MXene

Abstract: Employing ab initio calculations, we characterize the interfaces formed between graphene, a much discussed two-dimensional material, and MXene, another two-dimensional material of recent interest. Our study considering the specific case of ${\mathrm{Ti}}_{2}\mathrm{C}$, a member of the MXene family, shows the formation of chemical bonds between Ti atoms and C atoms of graphene. This results in reconstruction of the electronic structure at the interface, making the interface metallic, though graphene is a zero-gap semiconductor and ${\mathrm{Ti}}_{2}\mathrm{C}$ is an antiferromagnetic insulator in their respective native form. The optical and phonon properties of the interfaces are found to be strongly dependent on the stacking arrangement, driven by the nature of chemical-bond formation. Consideration of O-passivated ${\mathrm{Ti}}_{2}\mathrm{C}$ is found to weaken the interaction between graphene and ${\mathrm{Ti}}_{2}\mathrm{C}$ substantially, making it a physisorption process rather than chemisorption in the unpassivated situation. Our first-principles study is expected to motivate future experimental investigation.

First principles study of bimetallic Ni$_{13-n}$Ag$_n$ nano-clusters ($n =$ 0$-$13) : Structural, mixing, electronic and magnetic properties

Abstract: 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.

Rocksalt versus layered ordering in double perovskites: A case study with La 2 CuSnO 6 and La 2 CuIrO 6

Abstract: The nature of ordering of B and $\mathrm{B}{}^{\ensuremath{'}}$ transition metal ions in double perovskite compounds of general composition ${\mathrm{A}}_{2}\mathrm{BB}{}^{\ensuremath{'}}{\mathrm{O}}_{6}$ is an important topic, since the physical properties crucially depend on it. In the present study, considering the specific cases of ${\mathrm{La}}_{2}{\mathrm{CuSnO}}_{6}$ and ${\mathrm{La}}_{2}{\mathrm{CuIrO}}_{6}$, we carry out first-principles calculations with an aim to obtain microscopic understanding on this issue. Our study reveals the presence of Jahn-Teller distorted B ion, like ${\mathrm{Cu}}^{2+}$ helps in band energy stabilization of the layered ordering over the rocksalt ordering. However, introduction of magnetism may reverse this trend, especially in the presence of a second magnetic ion at $\mathrm{B}{}^{\ensuremath{'}}$ site, which may introduce a strong superexchange path involving B-O-$\mathrm{B}{}^{\ensuremath{'}}$, as found in the case of ${\mathrm{La}}_{2}{\mathrm{CuIrO}}_{6}$. We further find the spin-orbit coupling at Ir site drives the ${\mathrm{La}}_{2}{\mathrm{CuIrO}}_{6}$ compound to be a spin-orbit assisted Mott insulator.

First‐principles prediction of Si‐doped Fe carbide as one of the possible constituents of Earth's inner core

Abstract: We perform a computational study based on first-principles calculations to investigate the relative stability and elastic properties of the doped and undoped Fe carbide compounds at 200–364 GPa. We find that upon doping a few weight percent of Si impurities at the carbon sites in Fe7C3 carbide phases, the values of Poisson's ratio and density increase while VP, and VS decrease compared to their undoped counterparts. This leads to marked improvement in the agreement of seismic parameters such as P wave and S wave velocity, Poisson's ratio, and density with the Preliminary Reference Earth Model (PREM) data. The agreement with PREM data is found to be better for the orthorhombic phase of iron carbide (o-Fe7C3) compared to hexagonal phase (h-Fe7C3). Our theoretical analysis indicates that Fe carbide containing Si impurities can be a possible constituent of the Earth's inner core. Since the density of undoped Fe7C3 is low compared to that of inner core, as discussed in a recent theoretical study, our proposal of Si-doped Fe7C3 can provide an alternative solution as an important component of the Earth's inner core.

Universality of domain growth in antiferromagnets with spin-exchange kinetics.

Abstract: 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 the 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 Model C kinetics. As for 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.

Spin-order-induced multiferroicity in LiCuFe2(VO4)3 and disorder effects in NaCuFe2(VO4)3

Abstract: Mixed spin chain compounds, ACuFe2(VO4)3 (A= Li,Na), reach magnetically ordered state at TN ~ 11 K (Li) or ~ 9 K (Na) and experience further transformation of magnetic order at T* ~ 7 K (Li) or ~ 5 K (Na), evidenced in magnetic susceptibility chi and specific heat Cp measurements. While no anomaly has been detected in dielectric property of NaCuFe2(VO4)3, the step-like feature precedes a sharp peak in permittivity epsilon at TN in LiCuFe2(VO4)3. These data suggest the spin-order-induced ferroelectricity in Li compound and no such thing in Na compound. On the contrary, the Moessbauer spectroscopy study suggests similarly wide distribution of hyperfine field in between T* and TN for both the compounds. The first principles calculations also provide similar values for magnetic exchange interaction parameters in both compounds. These observations lead us to conclude on the crucial role of alkali metals mobility within the channels of the crystal structure needed to be considered in explaining the improper multiferroicity in one compound and its absence in other.