arXiv subjects

Swapan K. Pati

Publications and source records attributed to Swapan K. Pati.

At least 37 records · Page 2Linked to original sources

arXiv2009

Organometallic Vanadium-Borazine Systems: Efficient One- Dimensional Half-Metallic Spin Filters

Using density functional theory, we have investigated the electronic and magnetic properties of finite-size as well as infinitely periodic organometallic vanadium-borazine systems for their possible applications in spintronics devices. From our calculations, we find the finite-size vanadium-borazine systems to be structurally more stable in comparison to their isoelectronic benzene counterparts. All the finite-size vanadium-borazine systems are found to be ferromagnetically stabilized, with the infinite one-dimensional wire exhibiting robust half-metallic behaviour. The finite-size clusters are also found to exhibit efficient spin filter properties when coupled to graphene electrodes.

cond-mat.mes-hall↗

arXiv2009

Intrinsic Half-Metallicity in Modified Graphene Nanoribbons

We perform first-principles calculations based on density functional theory to study quasi one-dimensional edge-passivated (with hydrogen) zigzag graphene nanoribbons (ZGNRs) of various widths with chemical dopants, boron and nitrogen, keeping the whole system isoelectronic. Gradual increase in doping concentration takes the system finally to zigzag boron nitride nanoribbons (ZBNNRs). Our study reveals that, for all doping concentrations the systems stabilize in anti-ferromagnetic ground states. Doping concentrations and dopant positions regulate the electronic structure of the nanoribbons, exhibiting both semiconducting and half-metallic behaviors as a response to the external electric field. Interestingly, our results show that ZBNNRs with terminating polyacene unit exhibit half-metallicity irrespective of the ribbon width as well as applied electric field, opening a huge possibility in spintronics device applications.

cond-mat.mtrl-sci↗

arXiv2008

Molecular Electronics: Effect of external electric field

The effect of electric field, applied on systems in the nanoscale regime has attracted a lot of research in recent times. We highlight some of the recent results in the field of single molecule electronics and then move on to focus on some of our own results in this area. We have first shown how important it is to obtain the spatial profile of the external bias potential across the system, and how this would change in the presence of electron-electron interactions. We have also studied different kinds of insulators in the presence of the spatially varying external bias, and have explicitly shown that a two sublattice structure, caused either by a lattice distortion, or by the presence of substituents with strong dipolar nature, can result in negative differential resistance (NDR) in the transport characteristics. We also find this to be true in case of correlated insulators. Additionally, we have shown clear NDR behavior in a correlated double quantum dot by tuning the electron-electron interaction strength in the system.

cond-mat.mtrl-sci↗

arXiv2008

Comparative study of the electron conduction in azulene and naphthalene

We have studied the feasibility of electron conduction in azulene molecule and compared with that in its isomer naphthalene. We have used non-equilibrium Green's function formalism to measure the current in our systems as a response of the external electric field. Parallely we have performed the Gaussian calculations with electric field in the same bias window to observe the impact of external bias on the wave functions of the systems. We have found that, the conduction of azulene is higher than that of naphthalene inspite of its intrinsic donor-acceptor property, which leads a system to more insulating state. Due to stabilization through charge transfer the azulene system can be fabricated as a very effective molecular wire. Our calculations shows the possibility of huge device application of azulene in nano-scale instruments.

cond-mat.mtrl-sci↗

arXiv2008

Electron-Electron Interactions on the Edge States of Graphene: A Many Body Configuration Interaction Study

We have studied zigzag and armchair graphene nano ribbons (GNRs), described by the Hubbard Hamiltonian using quantum many body configuration interaction methods. Due to finite termination, we find that the bipartite nature of the graphene lattice gets destroyed at the edges making the ground state of the zigzag GNRs a high spin state, whereas the ground state of the armchair GNRs remains a singlet. Our calculations of charge and spin densities suggest that, although the electron density prefers to accumulate on the edges, instead of spin polarization, the up and down spins prefer to mix throughout the GNR lattice. While the many body charge gap results in insulating behavior for both kinds of GNRs, the conduction upon application of electric field is still possible through the edge channels because of their high electron density. Analysis of optical states suggest differences in quantum efficiency of luminescence for zigzag and armchair GNRs, which can be probed by simple experiments.

cond-mat.mtrl-sci↗

arXiv2008

Semiconductor to metal transition in SWNTs caused by interaction with gold and platinum nanoparticles

Single-walled carbon nanotubes (SWNTs) have been coated with gold and platinum nanoparticles either by microwave treatment or by the click reaction and the Raman spectra of these SWNT-metal nanoparticle composites have been investigated. Analysis of the G bands in the Raman spectra shows an increase in the proportion of metallic SWNTs on attachment with metal nanoparticles. This conclusion is also supported by the changes observed in the RBM bands. Ab-initio calculations reveal that semiconductor-metal transition occurs in SWNTs due to Columbic charge transfer between the metal nanoparticles and the semiconducting SWNTs.

cond-mat.mtrl-sci↗

arXiv2008

External Electric Field Mediated Quantum Phase Transitions in One-Dimensional Charge Ordered Insulators: A DMRG Study

We perform density matrix renormalization group (DMRG) calculations extensively on one-dimensional chains with on site (U) as well as nearest neighbour (V) Coulomb repulsions. The calculations are carried out in full parameter space with explicit inclusion of the static bias and we compare the nature of spin density wave (SDW) and charge density wave (CDW) insulators under the influence of external electric field. We find that, although the SDW (U>2V) and CDW (U<2V) insulators enter into a conducting state after a certain threshold bias, CDW insulators require much higher bias than the SDW insulators for insulator-metal transition at zero temperature. We also find the CDW-SDW phase transition on application of external electric field. The bias required for the transitions in both cases decreases with increase in system size.

cond-mat.str-el↗

arXiv2007

Half-Metallicity in Undoped and Boron Doped Graphene Nanoribbons in Presence of Semi-local Exchange-Correlation Interactions

We perform density functional calculations on one-dimensional zigzag edge graphene nano-ribbons (ZGNRs) of different widths, with and without edge doping including semi-local exchange-correlations. Our study reveals that, although the ground state of edge passivated (with hydrogen) ZGNRs prefers to be anti-ferromagnetic, the doping of both the edges with Boron atoms stabilizes the system in a ferromagnetic ground state. Both the local and semi-local exchange-correlations result in half-metallicity in edge passivated ZGNRs at a finite cross-ribbon electric field. However, the ZGNR with Boron edges shows half-metallic behavior irrespective of the ribbon-width even in absence of electric field and this property sustains for any field strength, opening a huge possibility of applications in spintronics.

cond-mat.mtrl-sci↗

arXiv2005

Current-voltage characteristics in donor-acceptor systems: Implications of a spatially varying electric field

We have studied the transport properties of a molecular device composed of donor and acceptor moieties between two electrodes on either side. The device is considered to be one-dimensional with different on-site energies and the non-equilibrium properties are calculated using Landauer's formalism. The current-voltage characteristics is found to be asymmetric with a sharp Negative Differential Resistance at a critical bias on one side and very small current on the other side. The NDR arises primarily due to the bias driven electronic structure change from one kind of insulating phase to another through a highly delocalized conducting phase. Our model can be considered to be the simplest to explain the experimental current-voltage characteristics observed in many molecular devices.

cond-mat.mes-hall↗

arXiv2005

New examples of metalloaromatic Al-clusters: (Al4M4)Fe(CO)3 (M=Li, Na and K) and (Al4M4)2Ni: Rationalization for possible synthesis

Ab-initio calculations reveal that all-metal antiaromatic molecules like Al4M4 (M=Li, Na and K) can be stabilized in half-sandwich complex: (Al4M4)Fe(CO)3 and full-sandwich complexes of the type: (Al4M4)2Ni. The formation of the full-sandwich complex [(Al4M4)2Ni] from its organometallic precursor depends on the stability of the organic-inorganic hybrid (C4H4) Ni (Al4Li4).

physics.chem-ph↗

arXiv2005

Rationalization of pi(sigma) anti(aromaticity) in all metal molecular clusters

A sigma-pi separation analysis of the energies in Al4Li4 reveals that the system is more pi-antiaromatic than the sigma-aromaticity in it. This is true also for C4H4 and Ga4Li4. Unlike C4H4 that has a very large component of pi-antiaromaticity, for these all-metal clusters, these energy scales are comparable though pi-antiaromaticity is the major driving force for the distortion of the these molecules from the square (sigma-aromatic) structure to the rectangular (pi-antiaromatic) architecture. For the dianion Al4Li42-, the sigma-equalization prevails over the pi-distortion in Al4Li4 and for the dication Al4Li42+, pi-equalization is the driving force for the square symmetric structure.

physics.chem-ph↗

arXiv2005

Proton-Pump Mechanism in Retinal Schiff Base: On the molecular structure of the M-state

Theoretical characterizations of the various intermediates in the proton pump cycle of the retinal Schiff base in the Halobacterium salinarium have been performed. Contrary to the general belief over the years that the most stable intermediate, the M-state, is a non-protonated cis-isomer, we find that the M-state is a polarized cis-isomer stabilized due to interactions of the dissociating proton with the pi-electrons. The role of proton in the pump cycle is found to be profound leading to the stabilization or in certain cases destabilization of the intermediates. We propose the chemical structure of the M-state for the first time.

physics.chem-ph↗

arXiv2005

Long range electron transfer across $π$-conjugated systems: role of electron correlations

We consider a prototype polyene chain: donor-$π$(bridge)-acceptor. The distance between the donor and the acceptor is varied by increasing the number of bridged atoms and rate of electron transfer, k$_{et}$ is studied for a series of different donors, D=NH$_2$, SH, OH, and a fixed acceptor, A=NO$_2$. We observe a large k$_{et}$ even at a very large D-A separation of $\sim$ 45 $Å$, unexpected from the well-known and standard theories like the Forster theory. Such a long range electron transfer is due to the strong electron-electron interactions in the bridged orbitals that result in deconfinment of electrons in donor orbitals. Calculations at various levels: semi-empirical and many-body exact, have been performed to accurately account for such correlations.

cond-mat.mes-hall↗

arXiv2004

Charge-transfer induced large nonlinear optical properties of small Al Clusters: Al4M4

We investigate the linear and nonlinear electric polarizabilities of small Al4M4 (M=Li, Na and K) clusters. Quantum chemical calculations reveal that these compounds exhibit an exceptionally high magnitude of linear and nonlinear optical (NLO) coefficients which are orders of magnitude higher than the conventional pi-conjugated systems of similar sizes. We attribute such phenomenal increase to non-centrosymmetricity incorporated in the systems by the alkali atoms surrounding the ring leading to charge transfer with small optical gap and low bond length alternation (BLA). Such a low magnitude of the BLA from a different origin, suggests the possibility that these clusters are aromatic in character and along with the large NLO coefficients they appear to be better candidates for next generation NLO fabrication devices.

cond-mat.mtrl-sci↗

arXiv2001

Exact and Approximate Theoretical Techniques for Quantum Magnetism in Low Dimensions

Quantum magnetism in low dimensions has been one of the central areas of theoretical research for many decades now. One of the key reasons for the long standing interest in this field has been the existence of simplified models, which serve as paradigms for understanding the role of strong interactions in many-electron systems. Although simple, these models quite often can not be solved exactly. In this review, we discuss a variety of analytical and numerical methods, which treat the system in a systematic and controlled manner. The central method employed in all the studies is the density matrix renormalization group (DMRG) method. This is supported by small scale numerical exact calculations, and by analytical methods using field theoretic techniques. We have considered a number of magnetic systems, from magnetic clusters to extended lattices, and have found some novel quantum ground states and low-energy elementary excitations. In some cases, we have also employed the finite-temperature DMRG method to accurately compute the low-temperature thermodynamic properties such as specific heat and magnetic susceptibility.

cond-mat.str-el↗

arXiv1999

Gapless Singlet modes in the Kagome strips: A study through DMRG and strong coupling analysis

Recently Azaria et al have studied strips of the Kagome-lattice in the weak-coupling limit, where they consist of two spin-half chains on the outside weakly coupled to an array of half-integer spins in the middle. Using a number of mappings they have arrived at the interesting result that in this system all spin excitations are gapped but there are gapless spinless modes. Here we study these Kagome strips in the limit where the interchain couplings are comparable to the coupling to the middle spins by density matrix renormalization group and by a strong coupling analysis. In the limit when the coupling to the middle-spin dominates, the 5-spins of the unit-cell reduce to a single S=3/2 spin, and the overall system has well known gapless spin excitations. We study the phase transition from this phase to the weak-coupling phase. We also carry out a strong coupling analysis away from the S=3/2 limit, where the five-spin blocks have four degenerate ground states with S=1/2, which can be thought of as two spin and two pseudospin degrees of freedom. A numerical study of this strong coupling model also suggests a finite spin-gap.

cond-mat↗

arXiv1999

Magnetization Properties of Some Quantum Spin Ladders

The experimental realization of various spin ladder systems has prompted their detailed theoretical investigations. Here we study the evolution of ground state magnetization with an external magnetic field for two different antiferromagnetic systems: a three-legged spin-1/2 ladder, and a two-legged spin-1/2 ladder with an additional diagonal interaction. The finite system density-matrix renormalization group method is employed for numerical studies of the three-chain system, and an effective low-energy Hamiltonian is used in the limit of strong interchain coupling to study the two- and three-chain systems. The three-chain system has a magnetization plateau at one-third of the saturation magnetization. The two-chain system has a plateau at zero magnetization due to a gap above the singlet ground state. It also has a plateau at half of the saturation magnetization for a certain range of values of the couplings. We study the regions of transitions between plateaus numerically and analytically, and find that they are described, at first order in a strong-coupling expansion, by an XXZ spin-1/2 chain in a magnetic field; the second order terms give corrections to the XXZ model. We also study numerically some low-temperature properties of the three-chain system, such as the magnetization, magnetic susceptibility and specific heat.

cond-mat.str-el↗

arXiv1998

Alternating Spin and Orbital Dimerization in Strong-coupling Two-band Models

We study a one-dimensional Hamiltonian consisting of coupled SU(2) spin and orbital degrees of freedom. Using the density matrix renormalization group, we calculate the phase-diagram and the ground state correlation functions for this model. We find that, in addition to the ferromagnetic and power-law antiferromagnetic phases for spin and orbital degrees of freedom, this model has a gapless line extending from the ferromagnetic phase to the Bethe ansatz solvable SU(4) critical point, and a gapped phase with doubly degenerate ground states which form alternating spin and orbital singlets. The spin-gap and the order parameters are evaluated and the relevance to several recently discovered spin-gap materials is discussed.

cond-mat↗