Research Activities

 

Field of Interest: Theoretical and Computational Condensed Matter Physics

 

Areas Of Interest:

1. First principles electronic structure calculations of complex crystalline solids, novel magnetic systems including multiferroic and spintronic materials.

2. Study of electronic structure of strongly correlated systems and low dimensional quantum spin systems.

3. Extraction of low energy model Hamiltonians using ab-initio approaches (NMTO Downfolding).

4. Multiscale simulations of novel functional materials.

5. Electronic Structure and Magnetism in nano-systems.

6. Study of superconductivity in disordered systems using real-space recursion method.

7. Quantum transmittance in two and three dimensional disordered media and quasi-periodic systems using vector recursion.

 

The research work of the group primarily involve the study of contemporary problems in the electronic structure of materials using numerical and analytic techniques. Some of these work are carried out in close collaboration with experimentalists. Research work is devoted to the study of both weakly and strongly correlated materials where the dominant electronic energy is the kinetic energy and the Coulomb repulsion respectively. The study of weakly correlated systems is based on density functional theory (DFT) implemented in variety of basis sets. For strongly correlated systems, the main objective is to develop methods where chemical realism may be introduced in their description by deriving realistic Hubbard and Heisenberg Hamiltonians and solving them using numerical methods (Exact Diagonalization, QMC, DMFT ). In addition, research work is also carried out to develop efficient real space methods based on the recursion technique to study disordered systems, a recent interest has been on the study of  superconductivity in disordered alloys. Recent research work of the group  primarily focused on the following topics: 

 

(a) Electronic Structure of Strongly Correlated Systems and Low Dimensional Quantum Spin Systems: 

Strongly correlated systems are investigated by deriving material dependent Hubbard Hamiltonians using a basis set of Wannier functions obtained from the N-th order muffin  tin orbital (NMTO) downfolding method. The Hubbard model at half-lling in the limit of strong correlation reduces to the Heisenberg model. The NMTO down-folding method is employed to understand variety of low dimensional quantum spin systems by deriving material specific Heisenberg Hamiltonians. These calculations not only provide accurate estimate of the exchange interactions but also clarify the exchange paths and identify the relevant spin Hamiltonians necessary to study these systems. In some cases the spin Hamiltonians are solved using quantum Monte Carlo (QMC) technique with the stochastic series expansion (SSE) algorithm. The NMTO downfolding coupled with QMC-SSE was applied to study the electronic structure of the spin gap compound Sr2Cu(BO3)2. This calculation illustrated that a careful analysis of the electronic structure plays a key role for the identification of the correct low energy model Hamiltonian for this system. The validity of the model was checked by calculating the magnetic susceptibility as a function of temperature and magnetization both as a function of temperature as well as field using QMC-SSE technique and comparing the calculated results with the available experimental data. This comparison established the suitability of the coupled dimer model for the description of the low energy physics of Sr2Cu(BO3)2 (Phys. Rev. B 86, 054434 2012) .

 

  

 

In addition, NMTO downfolding method was employed to identify the exchange paths and understand the experimental data for several other low dimensional quantum spin systems that included diamond chain antiferromagnets Ba3Cu3X4O12 (X=Sc, In) ( J. Phys: Condens. Matter 24, 236001 2012), the stair case Kagome lattice system PbCu3TeO7 ( J. Phys: Condens. Matter 25, 336003 2013) and a nearly 2D Heisenberg model Na2CuP2O7 ( J. Phys: Condens Matter 21, 025603 2009).

Research work is also devoted to understand the role of spin-orbit coupling in strongly correlated systems. While spin-orbit coupling is expected to be weak for 3d systems, recently it was shown that it may have important consequences even for such systems. This was illustrated by considering the spinel compound FeCr2S4. Ab-initio calculations based on density functional theory as well as model calculations of the electronic structure of FeCr2S4 provided a microscopic understanding of the origin of insulating behavior of this compound which turned out to be Coulomb enhanced spin-orbit coupling (SOC)operative within the Fe-d manifold (Phys. Rev. B 80, Rapid Comm. 201101 2009). In the recent years, 5d based oxides have attracted considerable attention where a combined inuence of band-structure, electron-electron correlation and spin orbit coupling lead to emergent quantum phenomena. Until few years ago the common belief has been that due to the extended nature of the 5d orbitals, the effective electron correlation is quite small in these systems and density functional theory within local density approximation can explain the metallic ground state. Contrary to this expectation, recently there are reports of insulating antiferromagnetic ground state in 5d transition metal oxides, where in addition to the crystal field and Coulomb repulsion, strong spin-orbit coupling plays a key role. In this respect, of particular interest are d5 Ir oxides where due to large crystal field splitting and strong SOC the t2g orbitals are renormalized into doubly degenerate Jeff= 1/2 and quadruply degenerate Jeff=3/2 states leading to narrow half-filled Jeff =1/2 states. Inclusion of moderate Coulomb interaction in the Jeff = 1/2 manifold opens up a gap explaining the insulating property of some of these iridates. We have investigated the realization of the novel Jeff = 1/2 state for the insulating double perovskite Sr2CeIrO2 (Mod. Phys. Lett. 27, 1350041 (2013)) as well as for the the metallic IrO2 (Phys Rev B 89 155102 (2014))  .

 

 

In addition, we have studied the electronic structure of the hexagonal NiS using LDA+U as wll as LDA + Dynamical Mean Field Theory (LDA + DMFT) method and addressed the several decade long controversy whether the low temperature (LT) phase of NiS is a metal or an insulator. Detailed calculations conclusively established that all experimental data for the LT phase of NiS can be understood in terms of a rather unusual ground state of NiS that is best described as a self-doped nearly compensated, antiferromagnetic metal thereby resolving the age old controversy for this system.( Scientific Report 3, 2995 2013, New Journal of Physics 16, 093049 (2014) )

 

 

(b ) Electronic structure of novel magnetic systems:

Research was also devoted to understand variety of novel magnetic systems that included multiferroic materials, double perovskites, rare-earth intermetallics, magnetic shape memory alloys etc. Multiferroic materials with simultaneous presence of ferroelectricity and magnetism are in focus of attention in the recent times. Of particular importance are improper multiferroics where ferroelectricity (FE) is induced by an inversion symmetry breaking magnetic ordering resulting in strong coupling between the two order parameters. Some spiral magnets belong to these class of materials where it is suggested that ferroelectricity can appear in these system if the spin rotation axis is not parallel to the spin propagation vector. This correlation between the FE polarization and the cycloidal-spiral spin structure is suggested to be associated with the antisymmetric part of the exchange coupling, the so called Dzyaloshinskii-Moriya (DM) interaction where the presence of the spin orbit coupling is indispensable in generating dipole moments. In this context, the origin of ferroelectric polarization in the spiral magnetic structure of MnWO4 is intriguing with a nominally d5, L=0 orbitally quenched state. In a recent work (PRB 81, 212406 2010), this puzzle was resolved and a microscopic understanding of the phenomena was obtained with the aid of detailed ab-initio electronic structure calculations supplemented with X-ray absorption spectroscopy.

In another project on multiferroic materials, using first principles density functional calculations, the electronic structure of the low-dimensional multiferroic compound FeTe2O5Br was studied to investigate the origin of the magnetoelectric (ME) effect and the role of Te ions in this system. Calculations reveal without magnetism, even in the presence of Te 5s lone pairs, the system remains centrosymmetric due to the antipolar orientation of the lone pairs. The exchange striction within the Fe tetramers as well as between them is found to be responsible for the ME effect in FeTe2O5Br. Further the Te4+ ions are found to play an important role in the inter tetramer exchange striction as well as contributing to the electric polarization in FeTe2O5Br, once the polarization is triggered by the magnetic ordering (Phys. Rev. B 88, 094409 2013).

 

 

The double perovskite LaSrVMoO6 attracted considerable attention as a possible realization of rare half-metallic antiferromagnet. Very recently careful experiments done at IACS revealed that the ground state of this compound is neighter half-metallic nor antiferromagnetic. Most importantly the study showed that the local chemical order in this material is very different from the usually perceived long range rock-salt type chemical order expected for double perovskites. These experinents reveal a narrow scale chemical fluctuation driven by an unusual anity of the V(Mo) ions towards La(Sr) ions. The origin of this novel cationic order was understood using various indicators of chemical bonding in the framework of first principles electronic structure calculations. The impact of this novel cationic order on the magnetic properties of LaSrVMoO6. was also investigated and compared with available experiments. ( Phys. Rev. B 86, 014203 2012, Phys. Rev. B Rapid Comm. 82, 180407 2010).  

We have investigated the metamagnetic property of the rare-earth intermetallic compound Gd2In. Our calculations conclusively established that Gd2In is a rather unique system where both localized Gd-f electrons as well as itinerant 5-d electrons originating from Gd are responsible for its novel metamagnetic behavior with hardly any role of the In s-p electrons. (J. Appl. Phys. 111, 053709 2012). 

 

(c) Electronic Structure and Magnetism in Nanosystems:

Materials at nano scale offer an unique possibility of tuning properties by tailoring sizes and shapes. Recently coupled quantum dots of ZnSe and CdS were synthesized that provided an alternative route to tune the electronic properties via band offset engineering. Electronic structure calculations clarified the chemical bonding at the interface of these coupled dots and also provided an estimate of the band off
sets that played a key role in understanding and controlling long-range visible fluorescence tunability of these coupled dots. Such novel coupled quantum dots could be potentially useful for quantum information processing and optoelectronic devices. This joint experiment and theoretical work was published in ( Advanced Materials, 23 1998, 2011, Phys Rev B
 89, 245423) and was also highlighted by Nature India. The strain proles of coupled quantum dots are calculated and compared with core-shell nanowires. Calculations reveal that the effect of strain is minimum in the coupled quantum dots thereby providing an opportunity for  designing interface as a novel quantum device.

 

 

The importance of the ligands in controlling the crystal structure of nano-systems was investigated. Recently with the aid of careful experiments it was shown that CdS nanocrystals can be thermodynamically stabilized in both wurtzite and zinc-blende crystallographic phases at will, just by the proper choice of the capping ligand. Ab-initio theoretical calculations on nano crystalline CdS revealed that the binding energy of the capping ligand trioctylphosphine molecules on the (001) facets of zinc-blende CdS is significantly larger than that for any of the wurtzite facets. As a consequence, trioctylphosphine as a capping agent stabilizes the zinc-blende phase via influencing the surface energy that plays an important role in the overall energetics of a nano crystal. (J. Phys. Chem Lett. 2, 706, 2011), ( J. Phys. Chem. C 116 6507 2012)  

The origin of magnetism in transition metal (TM) doped nano-crystalline ZnO was investigated (APL 94, 192503 2009), (J. Appl. Phys. 108, 123911 2010) by calculating the energetics and magnetic interactions in Fe doped ZnO nano-clusters by ab initio density functional theory. The calculations revealed that defects under suitable conditions can induce ferromagnetic interactions between the dopant Fe atoms whereas the anti ferromagnetic coupling dominates in a neutral defect-free cluster. The calculations also suggested that in the presence of charged defects the Fe atoms residing at the surface of the nano cluster may have an unusual oxidation state, that plays an important role to render the cluster ferromagnetic. This possibility was later veried by detailed XMCD measurements. 

The magnetic properties of unique cluster assembled solids namely Mn doped Ge46 and Ba8Ge46 clathrates was investigated. Calculations conrmed that ferromagnetic (FM) ground states may be realized in both the compounds when doped with Mn. In Mn2Ge44, ferromagnetism is driven by hybridization induced negative exchange splitting, a generic mechanism operating in many diluted magnetic semiconductors. However, for Mn-doped Ba8Ge46 clathrates incorporation of conduction electrons via Ba encapsulation results in RKKY-like magnetic interactions between the Mn ions. The RKKY mechanism also provided a natural explanation to the experimental results available for Ba8Mn2Ge44 clathrate (J. Phys: Condens. Matter 24, 505501 2012). 

 

 

(d) Study of Superconductivity in Disordered Systems: 

An efficient real space approach (Phys. Rev. B 79, 224204, 2009) to solve Bogoliubov de-Gennes (BdG) equations in systems modeled by disordered attractive Hubbard model was developed using the recursion method and the augmented space formalism. This method proved to be important to understand the effect of disorder on superconductivity in both single band as well as multi-band systems. In multiband systems in the presence of only intraband pairing in a two-band disordered system with disorder in either or both bands, calculations reveal that the gap survives in the quasiparticle spectrum; similar to single band systems. However, for interband pairing the gap in the quasiparticle spectrum ceases to exist beyond a critical value of the disorder strength. In the presence of both interband and intraband pairing interaction, depending on the relative magnitude of the pairing strength, only a particular kind of pairing is possible for a half filled two-band system (Phys. Rev B 84, 174508, 2011).

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