arXiv subjects

Tanusri Saha-Dasgupta

Publications and source records attributed to Tanusri Saha-Dasgupta.

66 records · Page 4Linked to original sources

arXiv2008

A local-density approximation for the exchange energy functional for excited states : the band gap problem

We present excited states density functional theory (DFT) to calculate band gap for semiconductors and insulators. For the excited states exchange-correlation functional, we use a simple local density approximation (LDA) like functional and it gives the result which is very closed to experimental results. The linear muffin-tin potential is used to solve the self consistent Kohn-Sham equation

cond-mat.mtrl-sci↗

arXiv2008

First principles study of structural stability and electronic structure of CdS nanoclusters

Using first-principles density functional calculations, we have studied the structural stability of stoichiometric as well as non-stoichiometric CdS nanoclusters at ambient pressure with diameters ranging up to about 2.5 nm. Our study reveals that the relative stability of the two available structures for CdS, namely zinc blende and wurtzite, depends sensitively on the details like surface geometry and/or surface chemistry. The associated band gap also exhibits non-monotonic behavior as a function of cluster size. Our findings may shed light on reports of experimentally observed structures and associated electronic structures of CdS nanoclusters found in the literature.

physics.atm-clus↗

arXiv2008

Microscopic modeling of a spin crossover transition

In spin crossover materials, an abrupt phase transition between a low spin state and a high spin state can be driven by temperature, pressure or by light irradiation. Of a special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition between a spin S=0 and a spin S=2 state shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of X-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within an ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between high spin Fe(II) centers is as important as elastic couplings for understanding the phase transition. We discuss the relevance of these interactions for the cooperative behavior in these materials.

cond-mat.mtrl-sci↗

arXiv2007

Cu-based metalorganic systems: an ab initio study of the electronic structure

Within a first principles framework, we study the electronic structure of the recently synthesized polymeric coordination compound Cu(II)-2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP), which has been suggested to be a good realization of a Heisenberg spin-1/2 chain with antiferromagnetic coupling. By using a combination of classical with ab initio quantum mechanical methods, we design on the computer reliable modified structures of CuCCP aimed at studying effects of Cu-Cu coupling strength variations on this spin-1/2 system. For this purpose, we performed two types of modifications on CuCCP. In one case, we replaced H in the linker by i) an electron donating group (NH2) and ii) an electron withdrawing group (CN), while the other modification consisted in adding H2O and NH3 molecules in the structure which change the local coordination of the Cu(II) ions. With the NMTO-downfolding method we provide a quantitative analysis of the modified electronic structure and the nature of the Cu-Cu interaction paths in these new structures and discuss its implications for the underlying microscopic model.

cond-mat.mtrl-sci↗

arXiv2005

Electrical transport in deformed nanostrips: electrical signature of reversible mechanical failure

We calculate the electrical conductivity of a thin crystalline strip of atoms confined within a quasi one dimensional channel of fixed width. The conductivity shows anomalous behavior as the strip is deformed under tensile loading. Beyond a critical strain, the solid fails by the nucleation of alternating bands of solid and {\em smectic} like phases accompanied by a jump in the conductivity. Since the failure of the strip in this system is known to be reversible, the onductivity anomaly may have practical use as a sensitive strain transducer.

cond-mat.mes-hall↗

arXiv2005

Electronic structure of and Quantum size effect in III-V and II-VI semiconducting nanocrystals using a realistic tight binding approach

We analyze the electronic structure of group III-V semiconductors obtained within full potential linearized augmented plane wave (FP-LAPW) method and arrive at a realistic and minimal tight-binding model, parameterized to provide an accurate description of both valence and conduction bands. It is shown that cation sp3 - anion sp3d5 basis along with the next nearest neighbor model for hopping interactions is sufficient to describe the electronic structure of these systems over a wide energy range, obviating the use of any fictitious s* orbital, employed previously. Similar analyses were also performed for the II-VI semiconductors, using the more accurate FP-LAPW method compared to previous approaches, in order to enhance reliability of the parameter values. Using these parameters, we calculate the electronic structure of III-V and II-VI nanocrystals in real space with sizes ranging upto about 7 nm in diameter, establishing a quantitatively accurate description of the band-gap variation with sizes for the various nanocrystals by comparing with available experimental results from the literature.

cond-mat.mtrl-sci↗

arXiv2004

Phase stability analysis in Fe-Pt and Co-Pt alloy systems: An augmented space study

We have studied the problem of phase stability in Fe-Pt and Co-Pt alloy systems. We have used the orbital peeling technique in the conjunction of augmented space recursion based on the tight binding linear orbital method as the method for the calculation of pair interaction energies. In particular, we have generalized our earlier technique to take into account of magnetic effects for the cases where the magnetic transition is higher than the order disorder chemical transition temperature as in the case of Co$_3$Pt. Our theoretical results obtained within this framework successfully reproduce the experimentally observed trends.

cond-mat.mtrl-sci↗

arXiv2004

Magnetic properties of X-Pt (X=Fe,Co,Ni) alloy systems

We have studied the electronic and magnetic properties of Fe-Pt, Co-Pt and Ni-Pt alloy systems in ordered and disordered phases. The influence of various exchange-correlation functionals on values of equilibrium lattice parameters and magnetic moments in ordered Fe-Pt, Co-Pt and Ni-Pt alloys have been studied using linearized muffin-tin orbital method. The electronic structure calculations for the disordered alloys have been carried out using augmented space recursion technique in the framework of tight binding linearized muffin-tin orbital method. The effect of short range order has also been studied in the disordered phase of these systems. The results show good agreements with available experimental values.

cond-mat.mtrl-sci↗

arXiv2003

Symmetry reduction in the augmented space recursion formalism for random binary alloys

We present here an efficient method which systematically reduces the rank of the augmented space and thereby helps to implement augmented space recursion for any real calculation. Our method is based on the symmetry of the Hamiltonian in the augmented space and keeping recursion basis vectors in the irreducible subspace of the Hilbert space.

cond-mat.mtrl-sci↗

arXiv2003

Study of Phase Stability in NiPt Systems

We have studied the problem of phase stability in NiPt alloy system. We have used the augmented space recursion based on the TB-LMTO as the method for studying the electronic structure of the alloys. In particular, we have used the relativistic generalization of our earlier technique. We note that, in order to predict the proper ground state structures and energetics, in addition to relativistic effects, we have to take into account charge transfer effects with precision.

cond-mat.mtrl-sci↗

arXiv2002

Electronic and Optical Properties of ZnIn$_2$Te$_4$

Band structure and optical properties of defect- Chalcopyrite type semiconductor ZnIn$_2$Te$_4$ have been studied by TB-LMTO first principle technique. The optical absorption calculation suggest that ZnIn$_2$Te$_4$ is a direct-gap semiconductor having a band gap of 1.40 eV., which confirms the experimentally measured value. The calculated complex dielectric-function $ε(E) = ε_1(E) + iε_2(E)$ reveal distinct structures at energies of the critical points in the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2002

Optical properties of perovskite alkaline earth titanates : a formulation

In this communication we suggest a formulation of the optical conductivity as a convolution of an energy resolved joint density of states and an energy-frequency labelled transition rate. Our final aim is to develop a scheme based on the augmented space recursion for random systems. In order to gain confidence in our formulation, we apply the formulation to three alkaline earth titanates CaTiO_3, SrTiO_3 and BaTiO_3 and compare our results with available data on optical properties of these systems.

cond-mat.mtrl-sci↗