arXiv subjects

Tapash Chakraborty

Publications and source records attributed to Tapash Chakraborty.

At least 55 records · Page 3Linked to original sources

arXiv2013

Super-intense highly anisotropic optical transitions in anisotropic quantum dots

Coulomb interaction among electrons is found to have profound effects on the electronic properties of anisotropic quantum dots in a perpendicular external magnetic field, and in the presence of the Rashba spin-orbit interaction. This is more evident in optical transitions, which we find in this system to be highly anisotropic and super-intense, in particular, for large values of the anisotropy parameter.

cond-mat.mes-hall↗

arXiv2013

Incompressible States of Dirac Fermions in Graphene with Anisotropic Interactions

We report on the properties of incompressible states of Dirac fermions in graphene in the presence of anisotropic interactions and a quantizing magnetic field. We introduce the necessary formalism to incorporate the anisotropy in the system. The incopmpressible state in graphene is found to survive the anisotropy upto a critical value of the anisotropy parameter. The anisotropy also introduces two branches in the collective excitations of the corresponding Laughlin state. It strongly influences the short-range behavior of the pair-correlation functions in the incompressible ground state.

cond-mat.mes-hall↗

arXiv2013

Traits and Characteristics of Interacting Dirac fermions in Monolayer and Bilayer Graphene

The relativistic-like behavior of electrons in graphene significantly influences the interaction properties of these electrons in a quantizing magnetic field, resulting in more stable fractional quantum Hall effect states as compared to those in conventional (non-relativistic) semiconductor systems. In bilayer graphene the interaction strength can be controlled by a bias voltage and by the orientation of the magnetic field. The finite bias voltage between the graphene monolayers can in fact, enhance the interaction strength in a given Landau level. As a function of the bias voltage, a graphene bilayer system shows transitions from a state with weak electron-electron interactions to a state with strong interactions. Interestingly, the in-plane component of a tilted magnetic field can also alter the interaction strength in bilayer graphene. We also discuss the nature of the Pfaffian state in bilayer graphene and demonstrate that the stability of this state can be greatly enhanced by applying an in-plane magnetic field.

cond-mat.mes-hall↗

arXiv2012

The Fractional Quantum Hall Effect of Tachyons in a Topological Insulator Junction

We have studied the tachyonic excitations in the junction of two topological insulators in the presence of an external magnetic field. The Landau levels, evaluated from an effective two-dimensional model for tachyons, and from the junction states of two topological insulators, show some unique properties not seen in conventional electrons systems or in graphene. The $ν=1/3$ fractional quantum Hall effect has also a strong presence in the tachyon system.

cond-mat.mes-hall↗

arXiv2012

Superluminal Tachyonlike Excitations of Dirac Fermions in a Topological Insulator Junction

We have considered a system of two topological insulators and have determined the properties of the surface states at the junction. Here we report that these states, under certain conditions exhibit superluminous (tachyonic) dispersion of the Dirac fermions. Although superluminal excitations are known to exist in optical systems, this is the first demonstration of possible tachyonic excitations in a purely electronic system. The first ever signature of tachyons could therefore be found experimentally in a topological insulator junction.

cond-mat.mes-hall↗

arXiv2012

Quantum transport anomalies in DNA containing mispairs

The effect of mispair on charge transport in a DNA of sequence (GC)(TA)_N(GC)_3 connected to platinum electrodes is studied using the tight-binding model. With parameters derived from ab initio density functional result, we calculate the current versus bias voltage for DNA with and without mispair and for different numbers of (TA) basepairs N between the single and triple (GC) basepairs. The current decays exponentially with $N$ under low bias but reaches a minimum under high bias when a multichannel transport mechanism is established. A (GA) mispair substituting a (TA) basepair near the middle of the (TA)_N sequence usually enhances the current by one order due to its low ionization energy but may decrease the current significantly when an established multichannel mechanism is broken.

cond-mat.mes-hall↗

arXiv2012

Strong Enhancement of Rashba spin-orbit coupling with increasing anisotropy in the Fock-Darwin states of a quantum dot

We have investigated the electronic properties of elliptical quantum dots in a perpendicular external magnetic field, and in the presence of the Rashba spin-orbit interaction. Our work indicates that the Fock-Darwin spectra display strong signature of Rashba spin-orbit coupling even for a low magnetic field, as the anisotropy of the quantum dot is increased. An explanation of this pronounced effect with respect to the anisotropy is presented. The strong spin-orbit coupling effect manifests itself prominently in the corresponding dipole-allowed optical transitions, and hence is susceptible to direct experimental observation.

cond-mat.mes-hall↗

arXiv2011

Spin-orbit Interaction induced Singlet-Triplet Resonant Raman Transitions in Quantum-dot Helium

From our theoretical studies of resonant Raman transitions in two-electron quantum dots (artificial helium atoms) we show that in this system, the singlet-triplet Raman transitions are allowed (in polarized configuration) only in the presence of spin-orbit interactions. With an increase of the applied magnetic field this transition dominates over the singlet-singlet and triplet-triplet transitions. This intriguing effect can therefore be utilized to tune Raman transitions as well as the spin-orbit coupling in few-electron quantum dots.

cond-mat.mes-hall↗

arXiv2011

Electrically tunable charge and spin transitions in Landau levels of interacting Dirac fermions in trilayer graphene

Trilayer graphene in the fractional Quantum Hall Effect regime displays a set of unique interaction-induced transitions that can be tuned entirely by the applied bias voltage. These transitions occur near the anti-crossing points of two Landau levels. In a large magnetic field ($> 8$ T) the electron-electron interactions close the anti-crossing gap, resulting in some unusual transitions between different Landau levels. For the filling factor $ν=\frac23$, these transitions are accompanied by a change of spin polarization of the ground state. For a small Zeeman energy, this provides an unique opportunity to control the spin polarization of the ground state by fine tuning the bias voltage.

cond-mat.str-el↗

arXiv2011

Interacting Dirac Fermions on a Topological Insulator in a Magnetic Field

We have studied the fractional quantum Hall states on the surface of a topological insulator thin film in an external magnetic field, where the Dirac fermion nature of the charge carriers have been experimentally established only recently. Our studies indicate that the fractional quantum Hall states should indeed be observable in the surface Landau levels of a topological insulator. The strength of the effect will however be different, compared to that in graphene, due to the finite thickness of the topological insulator film and due to the admixture of Landau levels of the two surfaces of the film. At a small film thickness, that mixture results in a strongly non-monotonic dependence of the excitation gap on the film thickness. At a large enough thickness of the film, the excitation gap in the lowest two Landau levels are comparable in strength.

cond-mat.mes-hall↗

arXiv2011

On the nature of interlayer interactions in a system of two graphene fragments

With the help of the quantum chemistry methods we have investigated the nature of interlayer interactions between graphene fragments in different stacking arrangements (AA and AB). We found that the AB stacking pattern as the ground state of the system, is characterized by the effective inter-band orbital interactions which are barely present in the AA. Their vanishing induces electronic decoupling between the graphene layers, so that the bonding interaction $ΔE_{oi}$ between the flakes is drastically reduced from -0.482 eV to -0.087 eV as the stacking pattern is changed from AB to AA. The effective way to improve the bonding interaction between layers preserving the same AA lattice order is to induce rotation of the layer. As the flake is rotated, the bonding interactions are improved mostly due to suppression of the Pauli repulsion which in turn increases the interlayer orbital interactions, while the inter-band part of those remain negligible on the whole range of the rotation angle. The Pauli repulsion is also found to be the main force moving apart two fragments as the stacking pattern is changed from AA to AB. This enhances the equilibrium interlayer distance, which for the AA staking is larger than the established value for the AB stacking (3.4 A)

cond-mat.mes-hall↗

arXiv2011

Interlayer repulsion and decoupling effects in stacked turbostratic graphene flakes

We have explored the electronic properties of stacked graphene flakes with the help of the quantum chemistry methods. We found that the behavior of a bilayer system is governed by the strength of the repulsive interactions that arise between the layers as a result of the orthogonality of their $π$ orbitals. The decoupling effect, seen experimentally in AA stacked layers is a result of the repulsion being dominant over the orbital interactions and the observed layer misorientation of 2$^{\circ}-5^{\circ}$ is an attempt by the system to suppress that repulsion and stabilize itself. For misorientated graphene, in the regions of superposed lattices in the Moiré pattern, the repulsion between the layers induce lattice distortion in the form of a bump or, in rigid systems local interlayer decoupling.

cond-mat.mes-hall↗

arXiv2011

Zipping and unzipping of nanoscale carbon structures

We demonstrate theoretically that hydrogenation and annealing applied to nanoscale carbon structures play a crucial role in determining the final shape of the system. In particular, graphene flakes characterized by the linear and non-hydrogenated zigzag or armchair edges have high propensity to merge into a bigger flake or a nanotube (the formation of a single carbon-carbon bond lowers the total energy of the system by up to 6.22 eV). Conversely, the line of the $sp^2$ carbon bonds (common for pure carbon structures such as graphene or a carbon nanotube) converted into the $sp^3$ type by hydrogenation shows an ability to disassemble the original structure by cutting it along the line of the modified bonds. These structural transformations provide us with an understanding of the behavior of mobile carbon structures in solution and a distinct scenario of how to preserve the original structure which would be a crucial issue for their application in carbon-based electronics.

cond-mat.mtrl-sci↗

arXiv2011

A Stable Pfaffian State in Bilayer Graphene

Here we show that the Pfaffian state proposed for the $\frac52$ fractional quantum Hall states in conventional two-dimensional electron systems can be readily realized in a bilayer graphene at one of the Landau levels. The properties and stability of the Pfaffian state at this special Landau level strongly depend on the magnetic field strength. The graphene system shows a transition from the incompressible to a compressible state with increasing magnetic field. At a finite magnetic field of ~10 Tesla, the Pfaffian state in bilayer graphene becomes more stable than its counterpart in conventional electron systems.

cond-mat.mes-hall↗

arXiv2011

Misoriented bilayer graphene in a magnetic field: Optical transitions at commensurate angles

Misoriented bilayer graphene with commensurate angles shows unique magneto-optical properties. The optical absorption spectra of such a system strongly depend on the angle of rotation. For a general commensurate twist angle the absorption spectra has a simple single-peak structure. However, our studies indicate that there are special angles at which the absorption spectra of the rotated bilayer exhibit well developed multi-peak structures. These angles correspond to even symmetry of the rotated graphene with respect to the sublattice exchange. Magneto-spectroscopy can therefore be a potentially useful scheme to determine the twist angles.

cond-mat.mes-hall↗

arXiv2011

Taking stock of the quantum Hall effects: Thirty years on

The quantum Hall effects, discovered about thirty years ago have remained one of the most spectacular discoveries in condensed matter physics in the past century. Those discoveries triggered huge expansion in the field of low-dimensional electronic systems, the area grew at an unprecedented rate and continues to expand. Novel and challenging observations, be it theoretical or experimental, have been reported since then on a regular basis. Additionally, the effects have inspired physicists to find analogous situations in far-flung fields as disparate as string theory or black hole physics.

cond-mat.mes-hall↗

arXiv2010

Irradiated bilayer graphene

We describe the gated bilayer graphene system when it is subjected to intense terahertz frequency electromagnetic radiation. We examine the electron band structure and density of states via exact diagonalization methods within Floquet theory. We find that dynamical states are induced which lead to modification of the band structure. We first examine the situation where there is no external magnetic field. In the unbiased case, dynamical gaps appear in the spectrum which manifest as dips in the density of states. For finite interlayer bias (where a static gap is present in the band structure of unirradiated bilayer graphene), dynamical states may be induced in the static gap. These states can show a high degree of valley polarization. When the system is placed in a strong magnetic field, the radiation induces coupling between the Landau levels which allows dynamical levels to exist. For strong fields, this means the Landau levels are smeared to form a near-continuum of states.

cond-mat.mes-hall↗

arXiv2010

Graphene, Nobel Prize and All that Jazz

Graphene, a single atomic layer of graphite, first isolated in 2004, has made a quantum leap in the exploration of the physics of two-dimensional electron systems. Since the initial report of its discovery, many thousands of papers have been published, attempting to explain every aspect of the exotic electronic properties of this system. The graphene euphoria has culminated with the 2010 Nobel Prize in physics being awarded jointly to Andre Geim and Konstantin Novoselov of the University of Manchester, UK, "for groundbreaking experiments regarding the two-dimensional material graphene". But, what are the properties of graphene, and how was it made? Why it is so exciting for so many researchers, and why the Nobel Prize?

cond-mat.mes-hall↗