Condensed Matter Physics Group

Bhubaneshwar, India

Condensed Matter Physics Group

Bhubaneshwar, India
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Sahu S.,KIIT University | Panda S.K.,K D Science College | Rout G.C.,Condensed matter Physics Group
Journal of Superconductivity and Novel Magnetism | Year: 2017

Graphene as a possible new magnetic material has stimulated research interest for application in spintronic devices. Pristine graphene does not show magnetism due to equivalence of the two sub-lattices of carbon atoms. It is predicted that impurity effects, substrate-induced gap, and defects, are expected to produce in-equivalence in two carbon sub-lattices leading to magnetism. The spins on the same sub-lattices are found to exhibit ferromagnetic order and spins of different sub-lattices are found to exhibit anti-ferromagnetic order. We consider here impurity and substrate effects as the cause of generation of magnetism in graphene. We report here a microscopic tight-binding study of frequency-dependent neutron scattering spectra for ferromagnetic ordering in the graphene systems. The tight-binding Hamiltonian consists of electron hoppings upto third nearest neighbors and substrate and impurity effects in the presence of Coulomb interaction of electrons separately at two in-equivalent A and B sub-lattices of graphene. We calculate the two-particle electron Green’s functions by using Zubarev’s Green’s function technique. The frequency-dependent scattering intensity of the system is computed numerically. The spectra displays a sharp peak at the neutron momentum transfer energy at low energies and another higher-energy peak appearing at substrate-induced gap. © 2017 Springer Science+Business Media New York


Kar J.K.,Orissa Engineering College, Bhubaneswar | Panda S.,Trident Academy of Technology | Rout G.C.,Condensed Matter Physics Group
AIP Conference Proceedings | Year: 2017

We propose here a tight binding model study of the interplay between charge and spin orderings in the CMR manganites taking anisotropic effect due to electron hoppings and spin exchanges. The Hamiltonian consists of the kinetic energies of eg and t2g electrons of manganese ion. It further includes double exchange and Heisenberg interactions. The charge density wave interaction (CDW) describes an extra mechanism for the insulating character of the system. The CDW gap and spin parameters are calculated using Zubarev's Green's function technique and computed self-consistently. The results are reported in this communication. © 2017 Author(s).


Swain R.,KIIT University | Sahu S.,KIIT University | Rout G.C.,Condensed Matter Physics Group
AIP Conference Proceedings | Year: 2017

We report here a microscopic tight binding theoretical model study of ferromagnetism in graphene taking into account of substrate effect and Coulomb interaction in both the sub-lattices of the honeycomb lattice. The Coulomb interaction is treated here within mean-field approximation giving rise to the ferromagnetic magnetization under different electron occupancies of graphene. The temperature dependent ferromagnetic magnetization is calculated from the electron co-relations obtained from the electron Green's functions. It is observed that ferromagnetic gap displaces sudden change for different electron occupancies separating paramagnetic from the ferromagnetic phase. The effect of on-site Coulomb interaction energy, electron occupancy and temperature on ferromagnetic gap is investigated and is reported in this communication. © 2017 Author(s).


Panda R.,KIIT University | Sahu S.,KIIT University | Rout G.C.,Condensed Matter Physics Group
AIP Conference Proceedings | Year: 2017

We communicate here a tight binding theoretical model study of the band filling effect on the charge gap in graphene-on-substrate. The Hamiltonian consists of nearest neighbor electron hopping and substrate induced gap. Besides this the Coulomb interaction is considered here within mean-field approximation in the paramagnetic limit. The electron occupancies at two sublattices are calculated by Green's function technique and are solved self consistently. Finally the charge gap i.e. Δ=U [〈 na 〉-〈 nb 〉] is calculated and computed numerically. The results are reported. © 2017 Author(s).


Sahu S.,Indian Institute of Technology Bhubaneswar | Rout G.C.,Condensed Matter Physics Group
Journal of Superconductivity and Novel Magnetism | Year: 2017

We address here the anti-ferromagnetic order present in AA-stacked bi-layer graphene in a transversely applied electric field. The system is described by kinetic energy with nearest-neighbor electron hopping with same hopping integral t1 for both the layers. Besides this, Coulomb interaction exists at A and B sub-lattices with same Coulomb correlation energy. The electron Green’s functions are calculated by Zubarev’s Green’s technique. The temperature-dependent anti-ferromagnetic magnetization is calculated from the Green’s function and is computed numerically and self-consistently. The strong on-site Coulomb interaction stabilizes the anti-ferromagnetic order in graphene. We assume that the electron spin at A site in the first layer is directed in the opposite direction to that of A site electron in the second layer. Similar spin order is observed for electrons in B site atom in reversed order. It is observed that anti-ferromagnetic (AFM) magnetization in the first layer nearly remains constant up to certain temperature and then increases with temperature, while the AFM magnetization in the second layer remains nearly constant and then rapidly decreases with temperature. The net AFM magnetization in bi-layer graphene remains constant and then rapidly increases with temperature. The evolution of the AFM magnetization is studied by varying transverse electric field, Coulomb energy, and temperature. © 2017 Springer Science+Business Media New York


Panda S.,Trident Academy of Technology | Kar J.K.,Orissa Engineering College, Bhubaneswar | Rout G.C.,Condensed Matter Physics Group
Journal of Magnetism and Magnetic Materials | Year: 2017

We report here the interplay of ferromagnetism (FM) and charge density wave (CDW) in manganese oxide systems through the study of tunneling conductance spectra. The model Hamiltonian consists of strong Heisenberg coupling in core t2g band electrons within mean-field approximation giving rise to ferromagnetism. Ferromagnetism is induced in the itinerant eg electrons due to Kubo-Ohata type double exchange (DE) interaction among the t2g and eg electrons. The charge ordering (CO) present in the eg band giving rise to CDW interaction is considered as the extra-mechanism to explain the colossal magnetoresistance (CMR) property of manganites. The magnetic and CDW order parameters are calculated using Zubarev's Green's function technique and solved self-consistently and numerically. The eg electron density of states (DOS) calculated from the imaginary part of the Green's function explains the experimentally observed tunneling conductance spectra. The DOS graph exhibits a parabolic gap near the Fermi energy as observed in tunneling conductance spectra experiments. © 2017 Elsevier B.V.


Sahu S.,KIIT University | Panda S.K.,Kd Science College | Rout G.C.,Condensed Matter Physics Group
AIP Conference Proceedings | Year: 2017

We propose here a tight-binding model with nearest-neighbor hopping integral in presence of one site doping. Phonon is coupled to the hopping integral with dimensionless coupling g in presence of bare phonon vibrational energy in graphene plane. Phonon Green's function Dq,q(ω) is calculated from the total Hamiltonian by using Zubarev's Green's function technique and the phonon self-energy is evaluated from the pure electronic Hamiltonian. The Raman spectral density function (SDF) is calculated from the formula SDF=-2π Im[Dq,q(ω+iη)], where η is the small spectral width. The calculated Raman spectra exhibits high energy and highly intense G peak, phonon momentum transfer energy peak and a lower energy anomalous peak. The evolution of these peaks is investigated by varying the model parameters of the graphene system. © 2017 Author(s).


Sahu S.,KIIT University | Parashar S.K.S.,KIIT University | Rout G.C.,Condensed Matter Physics Group
Pramana - Journal of Physics | Year: 2017

We report here a microscopic tight-binding theoretical study of the dynamic dielectric response of graphene-on-polarizable substrate with impurity. The Hamiltonian consists of first, second and third nearestneighbour electron hopping interactions besides doping and substrate-induced effects on graphene. We have introduced electron-electron correlation effect at A and B sublattices of graphene which is considered within Hartree-Fock mean-field approximation. The electron occupancies at both sublattices are calculated and solved self-consistently and numerically for both up-and down-spin orientations. The polarization function appearing in the dielectric function is a two-particle Green's function which is calculated by using Zubarev's Green's function technique. The temperature and optical frequency-dependent dielectric function is evaluated and compared with experimental data by varying Coulomb correlation energy, substrate-induced gap and impurity concentrations. © Indian Academy of Sciences.


Sahu S.,KIIT University | Panda S.K.,Kd Science College | Rout G.C.,Condensed Matter Physics Group
Pramana - Journal of Physics | Year: 2017

We address here a tight-binding model study of frequency-dependent real part of antiferromagnetic susceptibility for the graphene systems. The Hamiltonian consists of electron hopping upto third nearest-neighbours, substrate and impurity effects in the presence of electron-electron interactions at A and B sublattices. To calculate susceptibility, we evaluate the two-particle electron Green's function by using Zubarev's Green's function technique. The frequency-dependent real part of antiferromagnetic susceptibility of the system is computed numerically by taking 1000 × 1000 grid points of the electron momentum. The susceptibility displays a sharp peak at the neutron momentum transfer energy at low energies and another higher energy peak appearing at substrate-induced gap. The evolution of these two peaks is investigated by varying neutron wave vector, Coulomb correlation energy, substrate-induced gap, electron hopping integrals and A-and B-site electron doping concentrations. © Indian Academy of Sciences.


Sahu S.,KIIT University | Rout G.C.,Condensed Matter Physics Group
Journal of Magnetism and Magnetic Materials | Year: 2016

We present here a tight-binding model study of generation of magnetism and pseudo-spin polarization in monolayer graphene arising due to substrate, impurity and Coulomb correlation effects. The model Hamiltonian contains the first-, second- and third-nearest-neighbor hopping integrals for π electrons of graphene besides substrate induced gap, impurity interactions and Coulomb correlation of electrons. The Hubbard type Coulomb interactions present in both the sub-lattices A and B are treated within the mean-field approximation. The electronic Green's functions are calculated by using Zubarev's technique and hence the electron occupancies of both sub-lattices are calculated for up and down spins separately. These four temperature dependent occupancies are calculated numerically and self-consistently. Then we have calculated the temperature dependent pseudo-spin polarization, ferromagnetic and anti-ferromagnetic magnetizations. We observe that there exists pseudo-spin polarization for lower Coulomb energy, u<2.2t1 and pseudo-spin polarization is enhanced with substrate induced gap and impurity effect. For larger Coulomb energy u>2.5t1, there exists pseudo-spin polarization (p); while ferromagnetic (m) and antiferromagnetic (p m) magnetizations exhibit oscillatory behavior. With increase of the substrate induced gap, the ferromagnetic and antiferromagnetic transition temperatures are enhanced with increase of the substrate induced gap; while polarization (p) is enhanced in magnitude only. © 2016 Elsevier B.V.

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