arXiv subjects

Swapan K Pati

Publications and source records attributed to Swapan K Pati.

At least 19 recordsLinked to original sources

arXiv2026

Strong Optical-Optical Avoided Crossings Suppress Thermal Conductivity in Ga-Substituted TlInTe$_2$

In crystalline solids, avoided crossing between acoustic and optical phonons is widely recognized as an effective mechanism for suppressing lattice thermal conductivity ($κ_l$). However, the role of avoided crossings among optical phonons remains largely unexplored due to their weak contribution to heat transport. Here, using first-principles calculations combined with the linearized Wigner transport equation (LWTE), we demonstrate that optical-optical avoided crossings can effectively reduce ($κ_l$) in TlIn$_{0.5}$Ga$_{0.5}$Te$_2$. Pristine TlInTe$_2$ exhibits strong optical phonon-dominated heat transport, where optical phonons contribute nearly 63% of $κ_l$. The phonon dispersion of TlInTe$_2$ shows several crossing points in the optical region, which evolve into avoided crossings after 50% Ga substitution. Irreducible representation analysis reveals that the crossing phonon branches in TlInTe$_2$ belong to different symmetry representations, whereas the corresponding branches in TlIn$_{0.5}$Ga$_{0.5}$Te$_2$ possess the same symmetry representation, which enables phonon modes to couple and results in gap opening at the crossing points. These avoided crossings significantly suppress the optical phonon group velocity, thereby reducing the optical phonon contribution from 63% to 44% and lowering $κ_l$ from 0.568 to 0.482 Wm$^{-1}$K$^{-1}$ at 300 K. Mode-averaged transport analysis further confirms that the suppression of $κ_l$ is primarily governed by reduced phonon group velocity ($v_g$), while enhanced anharmonic scattering provides an additional secondary contribution. Our results establish symmetry-modified optical-optical avoided crossing as an effective route to suppress optical phonon transport and reduce $κ_l$ in systems where optical phonons significantly contribute to heat transport.

cond-mat.mtrl-sci↗

arXiv2023

Repesentation of general spin-$S$ systems using a Restricted Boltzmann Machine with Softmax Regression

Here, we propose a novel method for representation of general spin systems using Restricted Boltzmann Machine with Softmax Regression (SRBM) that follows the probability distribution of the training data. SRBM training is performed using stochastic reconfiguration method to find approximate representation of many body wave functions. We have shown that proposed SRBM technique performs very well and achieves the trial wave function, in a numerically more efficient way, which is in good agreement with the theoretical prediction. We demonstrated that the prediction of the trial wave function through SRBM becomes more accurate as one increases the number of hidden units. We evaluated the accuracy of our method by studying the spin-1/2 quantum systems with softmax RBM which shows good accordance with the Exact Diagonalization(ED). We have also compared the energies of spin chains of a few spin multiplicities($1, 3/2$ and $2$) with ED and DMRG results.

cond-mat.dis-nn↗

arXiv2021

Realizing high Near-Room-Temperature Thermoelectric Performance in n-type Ag2Se through Rashba Effect and Entropy Engineering

Although there are enormous numbers of high-temperature thermoelectric materials present, designing a near-room-temperature especially n-type thermoelectric material with high zT is extremely challenging. Generally, pristine Ag2Se exhibits unusual low thermal conductivity along with high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudoternary phase, Ag2Se0.5Te0.25S0.25, which shows improved thermoelectric performance (zT ~ 2.1 at 400 K). Density functional theory reveals that the Rashba type of spin-dependent band spitting originated because of Te-doping, enhancing carrier mobility. Using density functional perturbation theory, we hereby realize that the intrinsic carrier mobility is not only controlled by carrier effective mass, neither deformation potential theory, instead it is substantially limited by longitudinal optical phonon scattering. In fact, locally off-centered S atoms and rising configurational entropy via substitution of Te and S atoms in Ag2Se significantly reduce the lattice thermal conductivity (klat ~ 0.34 at 400 K). In order to accurately obtain electrical as well as thermal transport coefficient, we adopt deformation potential theory based on Boltzmann transport formalism. The combined consequence of the Rashba effect coupled with configurational entropy synergistically results in such high thermoelectric performance with the development of new n-type thermoelectric material working at the near-room-temperature regime.

cond-mat.mtrl-sci↗

arXiv2020

Loss of classicality in alternating spin-$\frac{1}{2}$/spin-$1$ chain, in the presence of next-neighbor couplings and Dzyaloshinskii-Moriya interactions

We have considered and alternating Heisenberg spin chain with nearest-neighbor ($J_1$), next-nearest neighbor ($J_2$) antiferromagnetic couplings along with z-component of the Dzyaloshinskii-Moriya(DM) ($D_z$) interactions. The Hamiltonian has been studied using (a) Linear Spin-Wave Theory(LSWT) and (b) Density Matrix Renormalization Group (DMRG). The system had been reported earlier as a classical ferrimagnet only when nearest neighbor exchange interactions are present. Both the antiferromagnetic next-nearest neighbor interactions and DM interactions introduce strong quantum fluctuations and due to which all the signatures of ferrimagnetism vanishes. We find that the nonzero $J_2$ introduces strong quantum fluctuations in each of the spin sites due to which the z-components of both spin-1 and spin-1/2 sites average out to be zero. The ground state becomes a singlet. The presence of $J_1$ along with $D_z$ introduces a short range order but develops long range order along the XY plane. $J_1$ along with $J_2$ induces competing phases with structure factor showing sharp and wide peaks, at two different angles reflecting the spin spiral structure locally as well as in the underlying lattice. Interestingly, we find that the $D_z$ term removes the local spin spiral structure in z-direction, while developing a spiral order in the XY plane.

cond-mat.str-el↗

arXiv2019

Vibrational Spectra of MO (M=Sn/Pb) in Their Bulk and Single Layer Forms: Role of Avoided Crossing in their Thermodynamic Properties

We report ab-initio calculations of the phonon dispersion relation on the bulk and single layer of SnO and PbO. We identify Raman active modes and infrared active modes at the zone center Γ point. In agreement with experimental observations of Raman spectroscopy measurement, we find that A1g mode is higher in frequency than that of Eg mode. Moreover, the reason behind the shift of A2u mode to higher frequency for monolayer of both SnO and PbO is revealed from our calculations. We also find that long-range Coulomb interaction enhances the dielectric constant and Born effective charges in bulk SnO and bulk PbO, compared to their monolayer. Here, we observe avoided crossing or Landau degeneracy between longitudinal acoustics (LA) and low energetic transverse optical (TO) modes in bulk form of both SnO and PbO. Additionally, monolayer SnO also shows low energetic Raman modes (Eg and A1g) of same frequency as bulk. As a result, we notice avoided crossing between LA and TO modes in monolayer SnO. Interestingly, higher Born effective charge and low dielectric constant enhances self-force constants and the interatomic force constants (IFCs) between the M-O bonds. The enhanced force constants give rise to higher vibrational frequency of phonon modes for monolayer PbO. Our studies reveal that due to avoided crossing between two degenerate bands, the phonon dispersion near high symmetry X point lowers specific heat and vibrational entropy in bulk SnO, bulk PbO and only in monolayer SnO upto temperature 150 K. Moreover, the large mass difference between Pb and Oxygen atoms and absence of interlayer van der Waal interactions give rise to high phonon vibration which reduces the occurrence of band crossing between two degenerate energy levels. The absence of avoided crossing leads higher specific heat and vibrational entropy in monolayer PbO at low temperatures.

cond-mat.mtrl-sci↗

arXiv2016

Engineering the De-localized States of Graphene Quantum Dots

Employing a combination of many-body configuration interaction method described by extended Hubbard model along with first principle calculations we predict the emergence of high oscillator strength at near-IR region which originates from the Davydov type of splitting in doped graphene quantum dots (GQD). Incorporation of strain in GQD promotes closely spaced bright states inciting for coherent excitation. Controlling the destructive interference of the functionalized nano graphene quantum states, the dark states can be tuned towards red end ensuing the system as a good candidate for photocell whereas coherent states can be tailored to concentrate the light at very high intensity resulting an opportunity for photonic device

cond-mat.mtrl-sci↗

arXiv2014

Nitrogen Doped Graphene Quantum Dots as Possible Substrates to Stabilize Planar Conformer of Au 20 Over its Tetrahedral Conformer: A Systematic DFT Study

Utilizing the strengths of nitrogen doped graphene quantum dot (N-GQD) as a substrate, here in, we have shown that one can stabilize the catalytically more active planar Au 20 (P-Au 20 ) compared to the thermodynamically more stable tetrahedral structure (T-Au 20 ) on an N-GQD. Clearly, this simple route avoids the usage of traditional transition metal oxide substrates which have been suggested and used for stabilizing the planar structure for a long time. Considering the experimental success in the synthesis of N-GQDs and in the stabilization of Au nanoparticles on N-doped graphene, we expect our proposed method to stabilize planar structure will be realized experimentally and will be useful for industrial level applications.

cond-mat.mtrl-sci↗

arXiv2014

Effects of Edge Passivations on the Electronic and Magnetic Properties of Zigzag Boron-Nitride Nanoribbons with Even and Odd-Line Stone-Wales (5-7 Pair) Defects

First-principles spin-polarized calculations have been performed on passivated Boron-Nitride Nanoribbons (BNNRs) with pentagon-heptagon line-defects (PHLDs) (also called as Stone-Wales line-defects). Two kinds of PHLDs, namely, even-line and odd-line PHLDs, have been added either at one edge or at both edges of BNNRs. Single-edge (with all its different possibilities, for example, for a BNNR with 2-line PHLD at single-edge there are 8 possibilities) as well as both-edge passivations have been considered for all the ribbons in this study by passivating each edge atom with hydrogen atom. Density of states (DOS) and projected-DOS (pDOS) analysis have been accomplished to understand the underlying reason for various properties. We find that passivation lead to different effects on the electronic and magnetic properties of a system, and the effects are mainly based on the line-defect introduced and/or on the atoms which are present at the passivated edge. In general, we find that, passivation can play a key role in tuning the properties of a system only when it has a zigzag edge.

cond-mat.mtrl-sci↗

arXiv2014

Electronic properties of zigzag, armchair and their hybrid quantum dots of graphene and boron-nitride with and without substitution: A DFT study

Spin-polarized density functional theory calculations have been performed on armchair graphene quantum dots and boron-nitride quantum dots (A-G/BN-QDs) and the effect of carbon/boron-nitride substitution on the electronic properties of these A-G/BN-QDs has been investigated. As a first step to consider more realistic quantum dots, quantum dots which are a combination of zigzag QDs and armchair QDs have been considered. Effect of substitution on these hybrid quantum dots has been explored for both GQDs and BNQDs and such results have been compared and contrasted with the results of substituted A-G/BN-QDs and their zigzag analogues. Our work suggests that the edge substitution can play an important tool while tuning the electronic properties of quantum dots.

cond-mat.mtrl-sci↗

arXiv2013

Computational Studies on Structural and Excited State Properties of Modified Chlorophyll f with Various Axial Ligands

Time Dependent Density Functional Theory (TDDFT) calculations have been used to understand the excited state properties of "modified-chlorophyll" f ('Chlide' f), Chlide a, Chlide b and axial ligated (with Imidazole, H2O, CH3OH, CH3COOH, C6H5OH) Chlide f molecules. The computed differences among the Qx, Qy, Bx and By band absorbance wavelengths of Chlide a, b and f molecules are found to be comparable with the experimentally observed shifts for these bands in 'chlorophyll' a ('chl' a), chl b and chl f molecules. Our computations provide an evidence that the red shift in Qy band of chl f is due to the extended delocaliztion of macrocycle chlorin ring due to the presence of -CHO group. The local contribution from the -CHO substituent to the macrocycle chlorin ring stabilizes the corresponding molecular orbitals (LUMO of the Chlide f and LUMO-1 of the Chlide b). All the absorption bands of Chlide f shift to higher wavelengths on the addition of axial ligands. Computed redox potentials show that, among the axial ligated Chlide f molecules, Chlide f -Imidazole acts as a good electron donor and Chlide f -CH3COOH acts as a good electron acceptor.

cond-mat.mtrl-sci↗

arXiv2013

Electronic and Magnetic Properties of Zigzag Boron-Nitride Nanoribbons with Even and Odd-line Stone-Wales (5-7 pair) Defects

Spin-polarized first-principles calculations have been performed on zigzag Boron-Nitride Nanoribbons (z-BNNRs) with lines of alternating fused pentagon (P) and heptagon (H) rings (Pentagon-Heptagon-line-defect) at single edge as well as at both edges. The number of line (n) of the Pentagon-Heptagon-defect has been varied from 1 to 8 for 10-zBNNRs. Among the different spin-configurations which we have studied, we find that, the spin-configuration with ferromagnetic ordering at each edge and anti-ferromagnetic ordering across the edges is quite interesting. For this spin-configuration, we find that, if the introduced PH-line-defect is odd numbered, the systems behave as spin-polarized semi-conductors, but, for even numbered, all the systems show interesting anti-ferromagnetic half-metallic behavior. Robustness of these results has been cross checked by the variation of the line-defect position and also by the variation of the width [from ~ 1.1 nm (6-zBNNR) to ~ 3.3 nm (16-zBNNR)] of the ribbon. Density of States (DOS), projected-DOS and band-structure analysis have been accomplished to understand the reasons for these differences between even and odd-line-defects. The main reason for many of the observed changes was traced back to the change in edge nature of the BNNR, which indeed dictates the properties of the systems.

cond-mat.mtrl-sci↗

arXiv2013

Structural Stability, Electronic, Magnetic and Optical Properties of Rectangular Graphene and Boron-Nitride Quantum Dots: Effects of Size, Substitution and Electric Field

Using density functional theory calculations, we have examined the structural stability, electronic, magnetic and optical properties of rectangular shaped quantum dots (QDs) of graphene (G), Boron Nitride (BN) and their hybrids. Different hybrid QDs have been considered by substituting a GQD (BNQD) with BN-pairs (carbon atoms) at different positions. Several parameters like size, amount of substitution etc. have been varied for all these QDs (GQDs, BNQDs, hybrid-QDs) to monitor the corresponding changes in their properties. Among the considered parameters, we find that substitution can act as a powerful tool to attain interesting properties with these QDs, for example, broad range of absorption (~2000 nm) in the near infrared (NIR) region, spin-polarized HOMO-LUMO gaps without the application of any external-bias etc., which are highly required in the preparation of opto-electronic, electronic/spintronic devices etc. Explanations have been given in details by varying different factors, like, changing the position and amount of substitution, application of external electric-field etc., to ensure the reliability of our results.

cond-mat.mtrl-sci↗

arXiv2013

Tuning the Electronic and Optical properties of Graphene and Boron-Nitride Quantum Dots through Molecular Charge-transfer Interactions

Spin-polarized first-principles calculations have been performed to tune the electronic and optical properties of graphene (G) and boron-nitride (BN) quantum dots (QDs) through molecular charge-transfer using Tetracyanoquinodimethane (TCNQ) and Tetrathiafulvalene (TTF) as dopants. From our calculations, we find that the nature of interaction between the dopants and QDs is similar to the interaction between the dopants and their two-dimensional counter parts of the QDs, namely, graphene and hexagonal boron-nitride sheets. Based on the values of formation energy and distance between QDs and dopants, we find that both the dopants are physisorbed on the QDs. Also, we find that GQDs interact strongly with the dopants compared to the BNQDS. Interestingly, though the dopants are physisorbed on QDs, their interaction lead to a decrement in the HOMO-LUMO gap of QDs by more than half of their original value. We have also observed a spin-polarized HOMO-LUMO gap in certain QD-dopant complexes. Mulliken population analysis, Density of states (DOS), projected DOS (pDOS) plots and optical conductivity calculations have been performed to support and understand the reasons behind the above mentioned findings.

cond-mat.mtrl-sci↗

arXiv2012

Cyclopentadienyl-Benzene Based Sandwich Molecular Wires Showing Efficient Spin Filtering, Negative Differential Resistance, and Pressure Induced Electronic Transitions

Using density functional theory, we investigate TM-cyclopentadienyl-benzene sandwich molecular wires (SMWs) which are composites of TM-cyclopentadienyl and TM-benzene wires (TM = transition metal (V, Fe)). All the SMWs are found to be highly stable ferromagnetic half-metals, showing spin switching behavior. Transport calculations show that finite size clusters display spin filter property when coupled with Au electrodes on either side. I-V characteristics of all systems confirm the spin filter property, with Au-BzVCpVBz-Au displaying exceptionally high performance. In addition to spin filtering, the Au-BzFeCpFeBz-Au system also shows negative differential resistance (NDR). Compression causes an abrupt reduction in magnetic moment and a transition to a metallic phase, while stretching causes an increase in magnetic moment. Half-metallicity is preserved for modest amounts of stretching and compression.

cond-mat.mes-hall↗

arXiv2009

Negative differential resistance in nanoscale transport in the Coulomb blockade

Motivated by recent experiments, we have studied transport behavior of coupled quantum dot systems in the Coulomb blockade regime using the master (rate) equation approach. We explore how electron-electron interactions in a donor-acceptor system, resembling weakly coupled quantum dots with varying charging energy, can modify the systems response to an external bias, taking it from normal Coulomb blockade behavior to negative differential resistance (NDR) in the curent-voltage characteristics.

cond-mat.mes-hall↗

arXiv2008

Effect of Electric Field on One-Dimensional Insulators: A DMRG study

We perform density matrix renormalization group (DMRG) calculations extensively on one dimensional Mott and Peierls chains with explicit inclusion of the static bias to study the insulator-metal transition in those systems. We find that the electric field induces a number of insulator-metal transitions for finite size systems and at the thermodynamic limit, the insulating system breaks down into a completely conducting state at a critical value of bias which depends strongly on the insulating parameters. Our results indicate that the breakdown, in both the Peierls and Mott insulators, at thermodynamic limit, does not follow the Landau-Zener mechanism. Calculations on various size systems indicate that an increase in the system size decreases the threshold bias as well as the charge gap at that bias, making the insulator-metal transition sharper in both cases.

cond-mat.str-el↗

arXiv2007

Effect of Protonation on the electronic properties of DNA base pairs: Applications for molecular electronics

Protonation of DNA basepairs is a reversible phenomenon which can be controlled by tuning the pH of the system. Under mild acidic conditions, the hydrogen bonding pattern of the DNA basepairs undergoes a change. We study the effect of protonation on the electronic properties of the DNA basepairs to probe for possible molecular electronics applications. We find that, under mild acidic pH conditions, the A:T basepair shows excellent rectification behaviour which is, however, absent in the G:C basepair. The mechanism of rectification has been discussed using a simple chemical potential model. We also consider the non-canonical A:A basepair and find that it can be used as efficient pH dependent molecular switch. The switching action in A:A basepair is explained in the light of pi-pi interactions which lead to efficient delocalization over the entire basepair.

cond-mat.mtrl-sci↗

arXiv2006

Theory of high bias Coulomb Blockade in ultrashort molecules

We point out that single electron charging effects such as Coulomb Blockade (CB) and high-bias staircases play a crucial role in transport through single ultrashort molecules. A treatment of Coulomb Blockade through a prototypical molecule, benzene, is developed using a master-equation in its complete many-electron Fock space, evaluated through exact diagonalization or full Configuration Interaction (CI). This approach can explain a whole class of non-trivial experimental features including vanishing zero bias conductances, sharp current onsets followed by ohmic current rises, and gateable current levels and conductance structures, most of which cannot be captured even qualitatively within the traditional Self Consistent Field (SCF) approach coupled with perturbative transport theories. By comparing the two approaches, namely SCF and CB, in the limit of weak coupling to the electrode, we establish that the inclusion of strong-correlations within the molecule becomes critical in addressing the above experiments. Our approach includes on-bridge-correlations fully, and is therefore well-suited for describing transport through short molecules in the limit of weak coupling to electrodes.

cond-mat.mes-hall↗