Papers by Monodeep Chakraborty
Physical Review B
We use the variational exact diagonalization to investigate the single polaron properties for fou... more We use the variational exact diagonalization to investigate the single polaron properties for four different dual models, combining a short-range off-diagonal (Peierls) plus a longer-range diagonal (Holstein or breathingmode) coupling. This allows us to investigate the sensitivity of various polaron properties both to the range of the diagonal coupling and to the specific diagonal coupling chosen. We find strong sensitivity to the range for all dual models as the adiabatic limit is approached; however, considerable sensitivity is observed for some quantities even in the antiadiabatic limit. Also, strong dependence of the results on the specific form of the diagonal coupling is observed everywhere in the parameter space. Taken together, these results suggest that a careful consideration must be given to the specific coupling and its proper range, when quantitative comparisons with experiments are sought.
Monodeep Chakraborty, Sankeerth S. Narayan, Vigneshwaran R., and Mona Berciu 5 Centre for Quantum... more Monodeep Chakraborty, Sankeerth S. Narayan, Vigneshwaran R., and Mona Berciu 5 Centre for Quantum Science and Technology, Chennai Institute of Technology, Chennai, India-600037 Computer Science Engineering Department, Chennai Institute of Technology, Chennai, India-600037 Mechatronics Engineering Department, Chennai Institute of Technology, Chennai, India-600037 Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z1 Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4 (Dated: April 21, 2022)
Bulletin of the American Physical Society, 2017
We study two particles in an infinite chain and coupled to phonons by interactions that modulate ... more We study two particles in an infinite chain and coupled to phonons by interactions that modulate their hopping as described by the Peierls/Su-Schrieffer-Heeger (SSH) model. In the case of hard-core bare particles, we show that exchange of phonons generates effective nearest-neighbor repulsion between particles and also gives rise to interactions that move the pair as a whole. The two-polaron phase diagram exhibits two sharp transitions, leading to light dimers at strong coupling and the flattening of the dimer dispersion at some critical values of the parameters. This dimer (quasi)self-trapping occurs at coupling strengths where single polarons are mobile. On the other hand, in the case of soft-core particles/ spinfull fermions, we show that phonon-mediated interactions are attractive and result in strongly bound and mobile bipolarons in a wide region of parameter space. This illustrates that, depending on the strength of the phonon-mediated interactions and statistics of bare particles, the coupling to phonons may completely suppress or strongly enhance quantum transport of correlated particles.
Bulletin of the American Physical Society, 2020
Зарубіна Зінаїда Василівна, старший викладач Харківський національний автомобільно-дорожній уніве... more Зарубіна Зінаїда Василівна, старший викладач Харківський національний автомобільно-дорожній університет Герасимчук Тетяна Володимирівна, кандидат педагогічних наук, старший викладач Харківський національний технічний університет сільського господарства імені П. Василенка ТРАДИЦІЙНІ ПРОТИ СУЧАСНИХ МЕТОДІВ ЕФЕКТИВНОГО НАВЧАННЯ ІНОЗЕМНІЙ МОВІ У наші дні он-лайн курси / тренінги стали надзвичайно популярними, так як все більше і більше компаній пропонують курси он-лайн. Проте, незважаючи на популярність онлайн-освіти, деякі люди уникають такі методи, в основному із-за неправильного уявлення. У той же час, незважаючи на зростання популярності он-лайн-курсів, традиційне (класичне) навчання супроводжується і намагається використовувати нові способи збереження інтересів учасників. Для деяких осіб більш досконалим є он-лайнове навчання, для інших-традиційне. Незважаючи на практичну цінність технічної освіти, життя-це більше, ніж робота, і вища освіта є одним з важливих факторів досягнення балансу між роботою та життям. Вважають, що основна мета освіти-це не підготовка до роботи, а передача знань, культури і традицій майбутнього покоління. Мета класичної освіти складалася з того, щоб забезпечити учнів інструментами для критичного і незалежного мислення на протязі всього життя. Ключові слова: класичне освіта, онлайн-освіта, технічне навчання, методики навчання.
Scientific Reports, 2018
We perform ab initio investigations of the bulk and surface band structures of LaSb and LaBi and ... more We perform ab initio investigations of the bulk and surface band structures of LaSb and LaBi and resolve the existing disagreements about the topological property of LaSb, considering LaBi as a reference. We examine the bulk band structure for band inversion, along with the stability of surface Dirac cones (if any) to time-reversal-preserving perturbations, as a strong diagnostic test for determining the topological character of LaSb, LaBi and LaSb-LaBi multilayer. A detailed ab initio investigation of a multilayer consisting of alternating unit cells of LaSb and LaBi shows the presence of band inversion in the bulk and a massless Dirac cone on the (001) surface, which remains stable under the influence of time-reversal-preserving perturbations, thus confirming the topologically non-trivial nature of the multilayer in which the electronic properties can be tailored as per requirement. A detailed 2 invariant calculation is performed to arrive at a holistic conclusion.
Physical Review B, 2019
Using density-functional theory calculations and experimental investigations on structural, magne... more Using density-functional theory calculations and experimental investigations on structural, magnetic and dielectric properties, we have elucidated a unique tetragonal ground state for MnTi2O4, a Ti 3+ (3d 1)-ion containing spinel-oxide. With lowering of temperature around 164 K, cubic MnTi2O4 undergoes a structural transition into a polar P 41 tetragonal structure and at further lower temperatures, around 45 K, the system undergoes a paramagnetic to ferrimagnetic transition. Magnetic superexchange interactions involving Mn and Ti spins and minimization of strain energy associated with cooperative Jahn-Teller distortions plays a critical role in stabilization of the unique tetramer-orbital ordered ground state which further gives rise to lattice chirality through subtle Ti-Ti bond-length modulations.
Effect of Ca doping on the Pr magnetic moment in Pr0.75Ca0.25MnO3
AIP Conference Proceedings, 2017
We investigate the electronic structure of CMR manganite Pr1-xCaxMnO3 at x = 0 and 0.25, within d... more We investigate the electronic structure of CMR manganite Pr1-xCaxMnO3 at x = 0 and 0.25, within density functional theory (DFT). We find that the Pr atoms in PrMnO3 do not posses any magnetic moment whereas in Pr0.75Ca0.25MnO3, the Pr moment changes to the high spin value of −1.95μB per formula unit. The appearance of the moment in Pr site, noticed experimentally long back, is an outstanding problem. Our ab initio electronic structure investigation leads us to the conclusion that the modified crystal ambience resulting from Ca doping holds the key to the origin of large Pr magnetic moment in Pr0.75Ca0.25MnO3.
Physical Review B, 2016
Employing a self-consistent (optimized) variational diagonalization scheme we investigate the for... more Employing a self-consistent (optimized) variational diagonalization scheme we investigate the formation of polaronic quasiparticles in a spinless fermion-boson transport model that couples the movement of charge carriers to fluctuations and correlations of a background medium. The background is parameterized by bosonic degrees of freedom. The variational fermion-boson Hilbert space is constructed to achieve high accuracy in one to three spatial dimensions with modest computational requirements. To characterize the ground-state properties of the Edwards model in the single-particle sector, we present exact numerical results for the polaron band dispersion, quasiparticle weight, Drude weight, mass enhancement, and the particle-boson correlations in a wide parameter regime. In the Edwards model, transport will be quasi-free, diffusive or boson-assisted in the weakly fermion-boson coupled, fluctuation-dominated or strongly correlated regimes, respectively. Thereby correlated transport is not only assisted but also limited by the bosonic excitations. As a result, the Drude weight remains finite even in the limit of very small boson frequencies. For a strongly correlated background, closed loops are important, in any dimension, to generate a finite effective particle mass even when the free fermion has an infinite mass.
Scientific Reports, 2017
We study two identical fermions, or two hard-core bosons, in an infinite chain and coupled to pho... more We study two identical fermions, or two hard-core bosons, in an infinite chain and coupled to phonons by interactions that modulate their hopping as described by the Peierls/Su-Schrieffer-Heeger (SSH) model. We show that exchange of phonons generates effective nearest-neighbor repulsion between particles and also gives rise to interactions that move the pair as a whole. The two-polaron phase diagram exhibits two sharp transitions, leading to light dimers at strong coupling and the flattening of the dimer dispersion at some critical values of the parameters. This dimer (quasi)self-trapping occurs at coupling strengths where single polarons are mobile. This illustrates that, depending on the strength of the phonon-mediated interactions, the coupling to phonons may completely suppress or strongly enhance quantum transport of correlated particles.
Polar octahedral rotations, cation displacement and ferroelectricity in multiferroic SmCrO3
EPL (Europhysics Letters), 2014
ABSTRACT Our thorough synchrotron diffraction studies provide a clue on the origin of ferroelectr... more ABSTRACT Our thorough synchrotron diffraction studies provide a clue on the origin of ferroelectricity in SmCrO3. Careful observation demonstrates that polar order develops in the paramagnetic state. Rietveld refinement of the diffraction data confirms that emergence of polar order is correlated with the structural transformation from centrosymmetric Pbnm to noncentrosymmetric Pna21 space group of the distorted orthorhombic structure. Rotations of polar CrO6 octahedra and Sm displacement are proposed to be correlated with the emergence of polar order, which is extended over a wide temperature range and increases gradually with decreasing temperature. This is consistent with the relaxor behavior as evident from the frequency-dependent dielectric response satisfying the Vogel-Fulcher law. A non-collinear to collinear spin transformation is suggested well below the spin reorientation transition. Appearance of ferroelectricity without any correlation to the antiferromagnetic order in SmCrO3 suggests a new class of ferroelectricity. All-electron full-potential first-principles calculation demonstrates significant Sm-Cr hybridization near the Fermi level, which substantiates the experimental findings
Physical Review B, 2016
We investigate the orbital effects of strong external magnetic field on the ground state properti... more We investigate the orbital effects of strong external magnetic field on the ground state properties of a two dimensional holstein polaron, employing variational approaches based on the exact diagonalization (VAED). From the ground state energy and the wave function we calculate electron-phonon correlation function, the average phonon number and the Drude weight and investigate the evolution of a 2D holstein polaron as a function of the magnetic flux. Although the external magnetic field affects the polaron throughout the parameter regime, we show that the magnetic field has a stronger effect on a loosely bound (spatially extended) polaron. We also find that the magnetic field can be used as a tuning parameter, particularly for weakly coupled polaron, to reduce the spatial extent of a large polaron.
We investigate the orbital effects of strong external magnetic field on the ground state properti... more We investigate the orbital effects of strong external magnetic field on the ground state properties of a two dimensional holstein polaron, employing variational approaches based on the exact diagonalization (VAED). From the ground state energy and the wave function we calculate electron-phonon correlation function, the average phonon number and the Drude weight and investigate the evolution of a 2D holstein polaron as a function of the magnetic flux. Although the external magnetic field affects the polaron throughout the parameter regime, we show that the magnetic field has a stronger effect on a loosely bound (spatially extended) polaron. We also find that the magnetic field can be used as a tuning parameter, particularly for weakly coupled polaron, to reduce the spatial extent of a large polaron.
Electronic Structure and Magnetism of Nickel Thin Films
International Journal of Modern Physics B, 2003
We present calculations of the electronic and magnetic structure of thin films of Ni, using the r... more We present calculations of the electronic and magnetic structure of thin films of Ni, using the recursion technique based on the tight-binding linearized muffin-tin orbitals basis. We find that the surface layers of free standing Ni thin films do carry enhanced magnetic moment.
Solid State Communications, 2008
In this communication we present a first principle study of Pr 1−x Sr x MnO 3 with x = 0.25. Whil... more In this communication we present a first principle study of Pr 1−x Sr x MnO 3 with x = 0.25. While the parent compounds of this system are antiferromagnetic insulators with different structural and magnetic ground states, the x = 0.25 is in the colossal magnetoresistance regime of the Pr 1−x Sr x MnO 3 phase diagram[1]. Our band structure calculations for the end-point compounds matches well with the existing theoretical and experimental results[1,2]. Interestingly, our calculations show that the Pr 0.75 Sr 0.25 MnO 3 has a half-metallic character with a huge band gap of 2.8 eV in the minority band. We believe this result would fuel further interest in some of these special compositions of colossal magnetoresistive manganites as they could be potential candidates for spintronic devices. We discuss the half-metallicity of the Pr 0.75 Sr 0.25 MnO 3 in the light of changes in the orbital hybridization as a result of Sr doping in PrMnO 3. Further, we highlight the importance of half-metallicity for a consolidated understanding of colossal magnetoresistance effect.
Physical Review B, 2012
We have investigated bipolaron formation in the Holstein-Hubbard and the Fröhlich-Hubbard model o... more We have investigated bipolaron formation in the Holstein-Hubbard and the Fröhlich-Hubbard model on a discrete one-dimensional lattice. Phonon enriched basis has been generated by exploiting the idea of the Lang-Firsov transformation, which results in better convergence in the strong coupling regime. The incorporation of the finite-range electron-phonon (e-ph) interaction reduces the bipolaron effective mass. We systematically increase the spatial extent of the e-ph interaction to study the effect of the long-range tail of the Fröhlich model. The electron-electron (e-e) interaction also plays a crucial role in lowering the effective mass for both the Holstein and the Fröhlich bipolarons. In the presence of the extended e-e interaction, the intersite S1 Holstein bipolaron becomes unstable, while the effective mass of the Fröhlich bipolaron becomes significantly reduced for both the spin singlet and triplet cases.
Physical Review B, 2007
In this study, we report the observation of a pseudogap associated with the insulator-metal trans... more In this study, we report the observation of a pseudogap associated with the insulator-metal transition in compositions of Pr1−xSrxMnO3 system with no charge ordering. Our valence band photoemission study shows that the observed shifts in the near Fermi level density of states are abrupt at the Curie transtion and occur over an energy scale of ∼ 1 eV, strongly suggesting that the charge-ordering gap observed earlier in other manganites and the pseudogap observed here may indeed have same origin. These results could be understood within the framework of models based on electronic phase separation where it has been shown that the pseudogap is a generic feature of the mixed-phase compositions. Also, our band structure calculations on Pr0.75Sr0.25MnO3 show the possible existence of half-metallicity in this system.
Physical Review B, 2014
Employing the recently developed self-consistent variational basis generation scheme, we have inv... more Employing the recently developed self-consistent variational basis generation scheme, we have investigated the bipolaron-bipolaron interaction within the purview of Holstein-Hubbard and the extended-Holstein-Hubbard (F2H) model on a discrete one-dimensional lattice. The density-matrix renormalization group (DMRG) method has also been used for the Holstein-Hubbard model. We have shown that there exists no bipolaron-bipolaron attraction in the Holstein-Hubbard model. In contrast, we have obtained clear-cut bipolaron-bipolaron attraction in the F2H model. Composite bipolarons are formed above a critical electron-phonon coupling strength, which can survive the finite Hubbard U effect. We have constructed the phase diagram of F2H polarons and bipolarons, and discussed the phase separation in terms of the formation of composite bipolarons.
Journal of Magnetism and Magnetic Materials, 2005
In this paper we carry out a detailed orbital ('-mm s) resolved study of the way in which the loc... more In this paper we carry out a detailed orbital ('-mm s) resolved study of the way in which the local magnetic moment changes as we go from the bulk to a planar or a rough surface. For a rough surfaces (FCC Ni (1 0 0) and BCC Fe (1 0 0) surfaces) we shall concentrate on specific structures on them and study the way in which local environment affects the local magnetic moment. Results will be shown for Fe, Co and Ni planar surfaces.
ATAT@WIEN2k: An interface for cluster expansion based on the linearized augmented planewave method
Computer Physics Communications, 2010
ABSTRACT We have developed an interface between the all-electron density functional theory code W... more ABSTRACT We have developed an interface between the all-electron density functional theory code WIEN2k, and the MIT Ab-initio Phase Stability (MAPS) code of the Alloy-Theoretic Automated Toolkit (ATAT). WIEN2k is an implementation of the full-potential linearized augmented planewave method which yields highly accurate total energies and optimized geometries for any given structure. The ATAT package consists of two parts. The first one is the MAPS code, which constructs a cluster expansion (CE) in conjunction with a first-principles code. These results form the basis for the second part, which computes the thermodynamic properties of the alloy. The main task of the CE is to calculate the many-body potentials or effective cluster interactions (ECIs) from the first-principles total energies of different structures or supercells using the structure-inversion technique. By linking MAPS seamlessly with WIEN2k we have created a tool to obtain the ECIs for any lattice type of an alloy. We have chosen fcc Al–Ti and bcc W–Re to evaluate our implementation. Our calculated ECIs exhibit all features of a converged CE and compare well with literature results.
AIP Conference Proceedings, 2017
We study the ground state of 1D Holstein single polaron with next nearest neighbour electron hopp... more We study the ground state of 1D Holstein single polaron with next nearest neighbour electron hopping (NNN), employing a variational approximation based on exact diagonalization. Our investigation reveals that, depending upon the sign and magnitude of the NNN hopping integral with respect to nearest neighbour hopping, the polaron band minima may occur at non-zero k GS. We compare the present scenario with the SSH polarons, where a similar feature is also observed, albeit, due to very different mechanism. Our initial investigation of triplet bipolarons, in presence of an attractive extended Hubbard interactions, further substantiates the differences between the present model and the SSH model.
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Papers by Monodeep Chakraborty