Tamal Goswami

Ligand Effects toward the Modulation of Magnetic Anisotropy and Design of Magnetic Systems with Desired Anisotropy Characteristics

Magnetic anisotropy of a set of octahedral Cr(III) complexes is studied theoretically. The magnetic anisotropy is quantified in terms of zero-field splitting (ZFS) parameter D, which appeared sensitive toward ligand substitution. The increased π-donation capacity of the ligand enhances the magnetic anisotropy of the complexes. The axial π-donor ligand of a complex is found to produce an easy-plane type (D > 0) magnetic anisotropy, while the replacement of the axial ligands with π-acceptors entails the inversion of magnetic anisotropy into the easy-axis type (D < 0). This observation enables one to fabricate a single molecule magnet for which easy-axis type magnetic anisotropy is an indispensable criterion. The equatorial ligands are also found to play a role in tuning the magnetic anisotropy. The magnetic anisotropy property is also correlated with the nonlinear optical (NLO) response. The value of the first hyperpolarizability varies proportionately with the magnitude of the ZFS parameter. Finally, it has also been shown that a rational design of simple octahedral complexes with desired anisotropy characteristics is possible through the proper ligand selection.

The impact of surface structure and band gap on the optoelectronic properties of Cu2O nanoclusters of varying size and symmetry

A systematic characterization of Cu2O nanoclusters using classical electrodynamics and time-dependent density functional theory (TDDFT) is performed to investigate their response to light with the alteration of size and symmetry. Absorption and scattering play a crucial role in tuning the surface plasmon resonance (SPR), which is the focal feature of optoelectronic properties. In larger dimensions the SPR is found to be strongly influenced by scattering and in smaller NPs it is dominated by absorption. A blue shift of the SPR peak is observed with decreasing cluster size. The optical properties of Cu2O nanoclusters are also affected by the symmetry aspect. With the variation of size and symmetry the associated surface structure and band gap are also varied. The TDDFT calculation is performed to explore the impact of these two fundamental factors on the optoelectronic nature of (Cu2O)n clusters. The TDDFT study on Cu2O nanoclusters reveals the nature of electronic excitations in photoirradiated (Cu2O)n clusters for n = 1, 2, and 3. The transitions involved in (Cu2O)n are basically categorized as ligand to metal charge transfer (LMCT) and metal to metal charge transfer (MMCT) processes. The change in absorption with varying cluster dimension and symmetry is found to be critically controlled by the relative probabilities of LMCT and MMCT processes. A competing surface reconstruction effect and occupied–virtual energy gap are also found to govern the SPR pattern of the Cu2O nanoclusters. All of these observations provide an appropriate guideline to tune SPR of Cu2O NPs for specific applications.

Probing the Effects of Ligand Field and Coordination Geometry on Magnetic Anisotropy of Pentacoordinate Cobalt(II) Single-Ion Magnets

In this work, the effects of ligand field strength as well as the metal coordination geometry on magnetic anisotropy of pentacoordinated CoII complexes have been investigated using a combined experimental and theoretical approach. For that, a strategic design and synthesis of three pentacoordinate CoII complexes [Co(bbp)Cl2]·(MeOH) (1), [Co(bbp)Br2]·(MeOH) (2), and [Co(bbp)(NCS)2] (3) has been achieved by using the tridentate coordination environment of the ligand in conjunction with the accommodating terminal ligands (i.e., chloride, bromide, and thiocyanate). Detailed magnetic studies disclose the occurrence of slow magnetic relaxation behavior of CoII centers with an easy-plane magnetic anisotropy. A quantitative estimation of ZFS parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the α → β (SOMO-SOMO) excitation contributes almost entirely to the total ZFS values for all complexes. It is worth noting that the excitation pertaining to the most positive contribution to the ZFS parameter is the dxy → dx2-y2 excitation for complexes 1 and 2, whereas for complex 3 it is the dz2 → dx2-y2 excitation.

Concurrent loss of aromaticity and onset of superexchange in Mg3Na2 with an increasing Na–Mg3 distance

Gradual migration of Na+ from Mg32− brings about fascinating change in aromatic and magnetic behavior of inorganic Mg3Na2 cluster, which is addressed at the B3LYP and QCISD levels. During this process, Na+ takes away the electron density from Mg32− causing a net decrease in aromaticity. A tug-of-war between the Pauli repulsion and the aromaticity is shown to be responsible for the observed stability and aromaticity trends in singlet and triplet states. Implications of a spin crossover vis-à-vis a possible superexchange are also explored.

Noncomparative scaling of aromaticity through electron itinerancy

Aromaticity is a multidimensional concept and not a directly observable. These facts have always stood in the way of developing an appropriate theoretical framework for scaling of aromaticity. In the present work, a quantitative account of aromaticity is developed on the basis of cyclic delocalization of π-electrons, which is the phenomenon leading to unique features of aromatic molecules. The stabilization in molecular energy, caused by delocalization of π-electrons is obtained as a second order perturbation energy for archetypal aromatic systems. The final expression parameterizes the aromatic stabilization energy in terms of atom to atom charge transfer integral, onsite repulsion energy and the population of spin orbitals at each site in the delocalized π-electrons. An appropriate computational platform is framed to compute each and individual parameter in the derived equation. The numerical values of aromatic stabilization energies obtained for various aromatic molecules are found to be in close agreement with available theoretical and experimental reports. Thus the reliable estimate of aromaticity through the proposed formalism renders it as a useful tool for the direct assessment of aromaticity, which has been a long standing problem in chemistry.

On magnon mediated Cooper pair formation in ferromagnetic superconductors

Identification of pairing mechanism leading to ferromagnetic superconductivity is one of the most challenging issues in condensed matter physics. Although different models have been proposed to explain this phenomenon, a quantitative understanding about this pairing is yet to be achieved. Using the localized-itinerant model, we find that in ferromagnetic superconducting materials both triplet pairing and singlet pairing of electrons are possible through magnon exchange depending upon whether the Debye cut off frequency of magnons is greater or lesser than the Hunds coupling (J) multiplied by average spin (S) per site. Taking into account the repulsive interaction due to the existence of paramagnons, we also find an expression for effective interaction potential between a pair of electrons with opposite spins. We apply the developed formalism in case of UGe2 and URhGe. The condition of singlet pairing is found to be fulfilled in these cases, as was previously envisaged by Suhl [Suhl, Phys. Rev. Lett. 87, ...

Effect of charge transfer and periodicity on the magnetism of [Cr(Cp*)2][ETCE]

Magnetism in metallocene based donor–acceptor complexes stems from the donor to acceptor charge transfer. Thus, to correlate the exchange coupling constant J and the charge transfer integral, a formalism is developed which enables one to obtain the coupling constant from the value of the charge transfer integral and the spin topology of the system. The variance in the magnetic interaction between donor and acceptor is also investigated along two perpendicular directions in the three dimensional crystal structure of the reference system, decamethylchromocenium ethyl tricyanoethylenecarboxylate [Cr(Cp*)2][ETCE]. These donor–acceptor pairs (V-pair and H-pair), oriented along vertical and horizontal directions respectively, are found to have different extents of J, which is attributed to the difference in exchange coupling mechanisms, viz., direct exchange and superexchange. Next, V-pair and H-pair are taken together to treat both the intrachain and interchain magnetic interactions, since this competition is necessary to decipher the overall magnetic ordering in the bulk phase. In fact, this truncated model produces a small positive value of J supporting the weak ferromagnetic nature of the complex. Lastly, a periodic condition is imposed on the system to comprehend the nature of magnetism in the extended system. Interestingly, the ferromagnetism, prevailing in the aperiodic system, turns into weak antiferromagnetism in the periodic environment. This is explained through the comparison of density of states (DOS) plots in aperiodic and periodic systems. This DOS analysis reveals proximity of the donor and acceptor orbitals, facilitating their mixing in periodic conditions. This mixing causes the antiferromagnetic interaction to prevail over the ferromagnetic one, and imparts an overall antiferromagnetic nature in periodic conditions. This change over in magnetic nature with the imposition of periodicity may be useful to understand the dependence of magnetic behavior with dimensionality in extended systems.