arXiv subjects

Tapash Chakraborty

Publications and source records attributed to Tapash Chakraborty.

At least 73 records · Page 4Linked to original sources

arXiv2010

Electron Correlations in Bilayer Graphene

The nature of electron correlations in bilayer graphene has been investigated. An analytic expression for the radial distribution function is derived for an ideal electron gas and the corresponding static structure factor is evaluated. We also estimate the interaction energy of this system. In particular, the functional form of the pair-correlation function was found to be almost insensitive to the electron density in the experimentally accessible range. The inter-layer bias potential also has a negligible effect on the pair-correlation function. Our results offer valuable insights into the general behavior of the correlated systems and serve as an essential starting-point for investigation of the fully-interacting system.

cond-mat.mes-hall↗

arXiv2010

Controllable, driven phase transitions in the Fractional quantum Hall states in bilayer graphene

Here we report from our theoretical studies that in biased bilayer graphene, one can induce phase transitions from an incompressible fractional quantum Hall state to a compressible state by tuning the bandgap at a given electron density. The nature of such phase transitions is different for weak and strong inter-layer coupling. Although for strong coupling more levels interact there are lesser number of transitions than for the weak coupling case. The intriguing scenario of tunable phase transitions in the fractional quantum Hall states is unique to bilayer graphene and never before existed in conventional semiconductor systems.

cond-mat.mtrl-sci↗

arXiv2010

Spin interactions in a quantum dot containing a magnetic impurity

The electron and hole states in a CdTe quantum dot containing a single magnetic impurity in an external magnetic field are investigated, using the multiband approximation which includes the heavy hole-light hole coupling effects. The electron-hole spin interactions and s,p-d interactions between the electron, the hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. A novel mechanism is proposed here to manipulate impurity-spin in the quantum dot which allows us to drive selectively the spin of the magnetic atom into each of its six possible orientations.

cond-mat.mes-hall↗

arXiv2010

Sustained ferromagnetism induced by H-vacancies in graphane

The electronic and magnetic properties of graphane with H-vacancies are investigated with the help of quantum-chemistry methods. The hybridization of the edges is found to be absolutely crucial in defining the size of the bandgap, which is increased from 3.04 eV to 7.51 eV when the hybridization is changed from the sp^2 to the sp^3 type. The H-vacancy defects also influence the size of the gap depending on the number of defects and their distribution between the two sides of the graphane plane. Further, the H-vacancy defects induced on one side of the graphane plane and placed on the neighboring carbon atoms are found to be the source of ferromagnetism which is distinguished by the high stability of the state with a large spin number in comparison to that of the singlet state and is expected to persist even at room temperatures. However, the ferromagnetic ordering of the spins is obtained to be limited by the concentration of H-vacancy defects and ordering would be preserved if number of defects do not exceed eight.

cond-mat.mtrl-sci↗

arXiv2010

Graphene and graphane: New stars of nanoscale electronics

Discoveries of graphene and graphane possessing unique electronic and magnetic properties offer a bright future for carbon based electronics, with future prospects of superseding silicon in the semiconductor industry.

cond-mat.mtrl-sci↗

arXiv2010

Reentrant fractional quantum Hall states in bilayer graphene: Controllable, driven phase transitions

Here we report from our theoretical studies that in biased bilayer graphene, one can induce phase transitions from an incompressible state to a compressible state by tuning the bandgap at a given electron density. Likewise, variation of the density with a fixed bandgap results in a transition from the FQHE states at lower Landau levels to compressible states at intermediate Landau levels and finally to FQHE states at higher Landau levels. This intriguing scenario of tunable phase transitions in the fractional quantum Hall states is unique to bilayer graphene and never before existed in conventional semiconductor systems.

cond-mat.mtrl-sci↗

arXiv2010

Properties of Graphene: A Theoretical Perspective

In this review, we provide an in-depth description of the physics of monolayer and bilayer graphene from a theorist's perspective. We discuss the physical properties of graphene in an external magnetic field, reflecting the chiral nature of the quasiparticles near the Dirac point with a Landau level at zero energy. We address the unique integer quantum Hall effects, the role of electron correlations, and the recent observation of the fractional quantum Hall effect in the monolayer graphene. The quantum Hall effect in bilayer graphene is fundamentally different from that of a monolayer, reflecting the unique band structure of this system. The theory of transport in the absence of an external magnetic field is discussed in detail, along with the role of disorder studied in various theoretical models. We highlight the differences and similarities between monolayer and bilayer graphene, and focus on thermodynamic properties such as the compressibility, the plasmon spectra, the weak localization correction, quantum Hall effect, and optical properties. Confinement of electrons in graphene is nontrivial due to Klein tunneling. We review various theoretical and experimental studies of quantum confined structures made from graphene. The band structure of graphene nanoribbons and the role of the sublattice symmetry, edge geometry and the size of the nanoribbon on the electronic and magnetic properties are very active areas of research, and a detailed review of these topics is presented. Also, the effects of substrate interactions, adsorbed atoms, lattice defects and doping on the band structure of finite-sized graphene systems are discussed. We also include a brief description of graphane -- gapped material obtained from graphene by attaching hydrogen atoms to each carbon atom in the lattice.

cond-mat.mtrl-sci↗

arXiv2010

Coulomb screening and collective excitations in biased bilayer graphene

We have investigated the Coulomb screening properties and plasmon spectrum in a bilayer graphene under a perpendicular electric bias. The bias voltage applied between the two graphene layers opens a gap in the single particle energy spectrum and modifies the many-body correlations and collective excitations. The energy gap can soften the plasmon modes and lead to a crossover of the plasmons from a Landau damped mode to being undamped. Plasmon modes of long lifetime may be observable in experiments and may have potentials for device applications.

cond-mat.mes-hall↗

arXiv2009

Doping graphene by adsorption of polar molecules at the oxidized zigzag edges

We have theoretically investigated the electronic and magnetic properties of graphene whose zigzag edges are oxidized. The alteration of these properties by adsorption of $\mathrm{H_{2}O}$ and $\mathrm{NH_3}$ molecules have been considered. It was found that the adsorbed molecules form a cluster along the oxidized zigzag edges of graphene due to interaction with the electro-negative oxygen. Graphene tends to donate a charge to the adsorbates through the oxygen atoms and the efficiency of donation depends on the intermolecular distance and on the location of the adsorbed molecules relative to the plane of graphene. It was found that by appropriate selection of the adsorbates, a controllable and gradual growth of $p$-doping in graphene with a variety of adsorbed molecules can be achieved.

physics.chem-ph↗

arXiv2009

Enhanced Rashba effect for hole states in a quantum dot

The effect of Rashba spin-orbit (SO) interaction on the hole states in a quantum dot is studied in the presence of an external magnetic field. We demonstrate here that the Rashba SO coupling has a profound effect on the energy spectrum of the holes revealing level repulsions between the states with the same total momentum. We also show that the resulting spin-orbit gap is much larger than the corresponding one for the electron energy levels in a quantum dot. Inter-hole interactions only marginally reduce the spin-orbit gap. This enhanced Rashba effect would manifest itself in the tuneling current which depends on the spin-orbit coupling strength.

cond-mat.mes-hall↗

arXiv2009

Generation of valley polarized current in bilayer graphene

We propose a device for the generation of valley polarized electronic current in bilayer graphene. By analyzing the response of this material to intense terahertz frequency light in the presence of a transverse electric field, we demonstrate that dynamical states are induced in the gapped energy region, and if the system parameters are properly tuned, these states exist only in one valley. The valley polarized states can then be used to filter an arbitrary electron current, so generating a valley polarized current.

cond-mat.mes-hall↗

arXiv2009

Electronic compressibility of graphene: The case of vanishing electron correlations and the role of chirality

A recent surprising finding that electronic compressibility measured experimentally in monolayer graphene can be described solely in terms of the kinetic energy [J. Martin, et al., Nat. Phys. 4, 144 (2008)] is explained theoretically as a direct consequence of the linear energy dispersion and the chirality of massless Dirac electrons. For bilayer graphene we show that contributions to the compressibility from the electron correlations are restored. We attribute the difference to the respective momentum dependence of the low energy band structures of the two materials.

cond-mat.mes-hall↗

arXiv2009

Tunable bandgap and magnetic ordering by adsorption of molecules on graphene

We have studied the electronic and magnetic properties of graphene and their modification due to the adsorption of water and other gas molecules. Water and gas molecules adsorbed on nanoscale graphene were found to play the role of defects which facilitate the tunability of the bandgap and allow us to control the magnetic ordering of localized states at the edges. The adsorbed molecules push the wavefunctions corresponding to $α$-spin (up) and $β$-spin (down) states of graphene to the opposite (zigzag) edges. This breaks the sublattice and molecular point group symmetry that results in opening of a large bandgap. The efficiency of the wavefunction displacement depends strongly on the type of molecules adsorbed

cond-mat.mtrl-sci↗

arXiv2009

Impurity induced spin gap asymmetry in nanoscale graphene

We propose a unique way to control both bandgap and the magnetic properties of nanoscale graphene, which might prove highly beneficial for application in nanoelectronic and spintronic devices. We have shown that chemical doping by nitrogen along a single zigzag edge breaks the sublattice symmetry of graphene. This leads to the opening of a gap and a shift of the molecular orbitals localized on the doped edge in such a way that the spin gap asymmetry, which can lead to half-metallicity under certain conditions, is obtained. The spin-selective behavior of graphene and tunable spin gaps help us to obtain semiconductor diode-like current-voltage characteristics, where the current flowing in one direction is preferred over the other. The doping in the middle of the graphene layer results in an impurity level between the HOMO and LUMO orbitals of pure graphene (again, much like in semiconductor systems) localized on the zigzag edges thus decreasing the bandgap and adding unpaired electrons, and this can also be used to control graphene conductivity.

cond-mat.mtrl-sci↗

arXiv2008

Long range Coulomb interaction in bilayer graphene

We report on our studies of interacting electrons in bilayer graphene in a magnetic field. We demonstrate that the long range Coulomb interactions between electrons in this material are highly important and account for the band asymmetry in recent optical magneto-absorption experiments. We show that in the unbiased bilayer (where both layers are at the same electrostatic potential), the interactions can cause mixing of Landau levels in moderate magnetic fields. For the biased bilayer (when the two layers are at different potentials), we demonstrate that the interactions are responsible for a change in the total spin of the ground state for half-filled Landau levels in the valence band.

cond-mat.mes-hall↗

arXiv2008

Long range Coulomb interaction in the ground state of bilayer graphene

We report on our studies of interacting electrons in bilayer graphene in a magnetic field. We demonstrate that the long range Coulomb interactions between electrons in this material are highly important. We show that in the unbiased bilayer (where both layers are at the same electrostatic potential), the interactions can cause mixing of Landau levels in moderate magnetic fields. For the biased bilayer (when the two layers are at different potentials), we demonstrate that the interactions are responsible for a change in the total spin of the ground state for half-filled Landau levels in the valence band.

cond-mat.mes-hall↗

arXiv2008

Thermodynamics of G.A mispairs in DNA: continuum electrostatic model

An analysis of the stability of a duplex containing G.A mispairs or G.A/A.G tandem during DNA melting has revealed that duplex stability depends on both DNA sequences and on the conformations of the G.A mispairs. The thermodynamics of single pair opening for G(anti).A(syn) and G(anti).A(anti) conformations adopted by G.A mispairs is found to strongly correlate with that of the canonical base pairs, while for sheared conformation a significant difference is observed.

physics.bio-ph↗

arXiv2008

How the surrounding water changes the electronic and magnetic properties of DNA

Strong influence of water molecules on the transport and magnetic properties of DNA, observed in this study, opens up real opportunities for application of DNA in molecular electronics. Interaction of the nucleobases with water molecules leads to breaking of some of the $π$ bonds and appearance of unbound pi electrons. These unbound electrons contribute significantly to the charge transfer at room temperature by up to 1000 times, but at low temperature the efficiency of charge transfer is determined by the spin interaction of the two unbound electrons located on the intrastrand nucleobases. The charge exchange between the nucleobases is allowed only when the spins of unbound electrons are antiparallel. Therefore, the conductance of the DNA molecule can be controlled by a magnetic field. That effect has potential applications for developing a nanoscale spintronic device based on the DNA molecule, where efficiency of spin interaction will be determined by the DNA sequence.

physics.chem-ph↗