arXiv subjects

Tapash Chakraborty

Publications and source records attributed to Tapash Chakraborty.

At least 91 records · Page 5Linked to original sources

arXiv2008

Signature of valley polarization in fractional flux periodicity of a graphene ring

We have studied the interplay of valley polarization and the Coulomb interaction on the energy spectrum, persistent current, and optical absorption of a graphene quantum ring. We show that the interaction has a dramatic effect on the nature of the ground state as a function of the magnetic flux, and that the absence of the exchange interaction in opposite valleys means that the singlet-triplet degeneracy is not lifted for certain states. The additional level crossings (fractional flux periodicity) due to the interaction directly leads to extra steps in the persistent current and intricate structures in the absorption spectrum that should be experimentally observable. By varying the width of the ring, the nature of the ground state at zero field can be varied as well and this is manifest in the measurable properties we discuss.

cond-mat.mes-hall↗

arXiv2008

Electronic Correlations in Double Quantum Dots

We present a study of the electronic structure of two laterally coupled gaussian quantum dots filled with two particles. The exact diagonalization method has been used in order to inspect the spatial correlations and examine the particular spin singlet-triplet configurations for different coupling degrees between quantum dots. The outcome of our research shows this structure to have highly modifiable properties promoting it as an interesting quantum device, showing the possible use of this states as a quantum bit gate.

cond-mat.mes-hall↗

arXiv2007

The influence of polaron size on the conductivity of poly-DNA

The velocity of polaron migration in the long poly-DNA chain (~40 base pairs) in an applied electric field has been studied within a polaron model. We found that the polaron velocity strongly depends on the polaron size. A small polaron shows a slow propagation and strong tolerance to the electric field, while a large polaron is much faster and less stable with increasing electric field. Moreover, the conductance of the DNA molecule within the polaron model is found to be sensitive to structural disorders in the DNA geometry, but that dependence diminishes with increasing temperature.

cond-mat.soft↗

arXiv2007

Polaron tunneling dynamics in the DNA molecule

The formation of polaron and its migration in a DNA chain are studied within a semiclassical Peyrard-Biship-Holstein polaron model. Comparing the energetics of the polaron system found from the quantum chemical and semiclassical calculations, we extract the charge-phonon coupling constant for poly DNA sequences. The coupling constant is found to be larger for the G-C than for the A-T pairs. With this coupling constant we study tunneling in the DNA molecule. The rates and the nature of tunneling have strong dependence on the DNA sequence. By changing the trap positions in the molecular bridge the tunneling rate can by varied up to seven orders of magnitude.

cond-mat.soft↗

arXiv2007

Electronic parameters for the hole transfer in DNA duplex oligomers

We report on our calculations of the inner-sphere reorganization energy and the interaction of the pi orbitals within DNA oligomers. The exponential decrease of the electronic coupling between the highest and second highest occupied base orbitals of the intrastrand nucleobases in the (A-T)n and (G-C)n oligomers have been found with an increase of the sequence number n in the DNA structure. We conclude that for realistic estimation of the electronic coupling values between the nucleobases within the DNA molecule, a DNA chain containing at least four base pairs is required. We estimate the geometry relaxation of the base pairs within the (A-T)n and (G-C)n oligomers (n=1-6) due to their oxidation. The decrease of the inner-sphere reorganization energy with elongation of the oligomer structure participating in the oxidation process have been observed. The maximum degree of geometry relaxation of the nucleobase structures and correspondingly the higher charge density in the oxidized state are found to be located close to the oligomer center.

cond-mat.soft↗

arXiv2007

Spin Configurations and Activation Gaps of the Quantum Hall States in Graphene

We report on our accurate evaluation of spin polarizations of the ground state and particle-hole gaps for partially-filled lowest Landau level, observed in recent experiments on graphene subjected to ultra-high magnetic fields. We find that inter-electron interactions are important at these filling factors, characterized by the non-degenerate ground states and large particle-hole gaps at infinitely large wave vectors. The gaps are the largest for the quantum Hall states in the second Landau level. The weak appearance of quantum Hall plateaus in the experiments for certain filling factors in the second Landau level however indicates that at these filling factors, the system has a soft mode at a finite wave vector due presumably to the presence of disorder.

cond-mat.mes-hall↗

arXiv2006

Coulomb screening and collective excitations in a graphene bilayer

We have investigated the Coulomb screening properties and collective excitations in a graphene bilayer. The static screening effect is anisotropic and is much stronger in the undoped graphene bilayer than in a monolayer graphene [1]. The dynamic screening shows the properties of a Dirac gas in the low frequency and that of a Fermi gas in the high frequency. The transition from the Dirac to the Fermi gas is also observed in the plasmon spectrum. Finally, we find that an electron gas in a doped graphene bilayer has quite similar properties as those of a Fermi gas in materials containing two energy valleys.

cond-mat.mes-hall↗

arXiv2006

The Fock-Darwin States of Dirac Electrons in Graphene-based Artificial Atoms

We have investigated the Fock-Darwin states of the massless chiral fermions confined in a graphitic parabolic quantum dot. In the light of the Klein tunneling, we have analyzed the condition for confinement of the Dirac fermions in a cylindrically-symmetric potential. New features of the energy levels of the Dirac electrons as compared to the conventional electronic systems are dicussed. We have also evaluated the dipole-allowed transitions in the energy levels of the dots. We propose that in the high magnetic field limit, the band parameters can be accurately determined from the dipole-allowed transitions.

cond-mat.mes-hall↗

arXiv2006

Collective Excitations of Dirac Electrons in Graphene

Two-dimensional electrons in graphene are known to behave as massless fermions with Dirac-Weyl type linear dispersion near the Dirac crossing points. We have investigated the collective excitations of this system in the presence or absence of an external magnetic field. Unlike in the conventional two-dimensional electron system, the $ν=\frac1m$ fractional quantum Hall state in graphene was found to be most stable in the $n=1$ Landau level. In the zero field case, but in the presence of the spin-orbit interaction, an undamped plasmon mode was found to exist in the gap of the single-particle continuum.

cond-mat.mes-hall↗

arXiv2006

Charge transfer via a two-strand superexchange bridge in DNA

Charge transfer in a DNA duplex chain is studied by constructing a system with virtual electrodes connected at the ends of each DNA strand. The systeym is described by the tight-binding model and its transport is analyzed by the transfer matrix method. The very weak distance dependence in long (G:C)(T:A)_M(G:C)_3 DNA chain observed in experiment [B. Giese, et al., Nature 412, 318 (2001)] is explained by a unistep two-strand superexchange bridge without the need for the multi-step thermally-induced hopping mechanism or the dephasing effect. The crossover number M_c of (T:A) base pairs, where crossover between strong and weak distance dependence occurs, reflects the ratio of intra- and inter-strand neighboring base-base couplings.

cond-mat.mes-hall↗

arXiv2006

The Fractional Quantum Hall States of Dirac Electrons in Graphene

We have investigated the fractional quantum Hall states for the Dirac electrons in a graphene layer in different Landau levels. The relativistic nature of the energy dispersion relation of the electrons in the graphene significantly modifies the inter-electron interactions. This results in a specific dependence of the ground state energy and the energy gaps for electrons on the Landau level index. For the valley-polarized states, i.e. at ν=1/m, m being an odd integer, the energy gaps have the largest values in the n=1 Landau level. For the valley-unpolarized states, e.g., for the 2/3 state, the energy gaps are suppressed for the n=1 Landau level as compared to the n=0 level. For both the n=1 and n=0 Landau levels the ground state of the 2/3 system is fully valley-unpolarized.

cond-mat.mes-hall↗

arXiv2006

Collective excitations of Dirac electrons in a graphene layer with spin-orbit interaction

Coulomb screening and excitation spectra of electrons in a graphene layer with spin-orbit interaction (SOI) is studied in the random phase approximation. The SOI opens a gap between the valence and conduction bands of the semi-metal Dirac system and reshapes the bottom and top of the bands. As a result, we have observed a dramatic change in the long-wavelength dielectric function of the system. An undamped plasmon mode emerges from the inter-band electron-hole excitation continuum edge and vanishes or merges with a Landau damped mode on the edge of the intra-band electron-hole continuum. The characteristic collective excitation of the Dirac gas is recovered in a system with high carrier density or for a large wave vector.

cond-mat.mes-hall↗

arXiv2006

Spin-Orbit Coupling and Tunneling Current in a Parabolic Quantum Dot

We propose a novel approach to explore the properties of a quantum dot in the presence of the spin-orbit interaction and in a tilted magnetic field. The spin-orbit coupling within the quantum dot manifest itself as anti-crossing of the energy levels when the tilt angle is varied. The anti-crossing gap has a non-monotonic dependence on the magnitude of the magnetic field and exhibits a peak at some finite values of the magnetic field. From the dependence of the tunneling current through the quantum dot on the bias voltage and the tilt angle, the anti-crossing gap and most importantly the spin-orbit strength can be uniquely determined.

cond-mat.mes-hall↗

arXiv2006

Spin Hot Spots in vertically-coupled Few-electron Quantum Dots

The effects of spin-orbit (SO) coupling arising from the confinement potential in single and two vertically-coupled quantum dots have been investigated. Our work indicates that a dot containing a single electron shows the lifting of the degeneracy of dipole-allowed transitions at B=0 due to the SO coupling which disappears for a dot containing two electrons. For coupled dots with one electron in each dot, the optical spectra is not affected by the coupling and is the same as the dot containing one electron. However, for the case of two coupled dots where one partner dot has two interacting electrons while the other dot has only one electron, a remarkable effect is observed where the oscillator strength of two out of four dipole-allowed transition lines disappears as the distance between the dots is decreased.

cond-mat.mes-hall↗

arXiv2005

Spin injection into a short DNA chain

Quantun spin transport through a short DNA chain connected to ferromagnetic electrodes has been investigated by the transfer matrix method. We describe the system by a tight-binding model where the parameters are extracted from the experimental data and realistic metal energy bands. For ferromagnetic iron electrodes, the magnetoresistance of a 30-basepair Poly(G)-Poly(C) DNA is found to be lower than 10% at a bias of < 4 V, but can rach up to 20% at a bias of 5 V. In the presence of the spin-flip mechanism, the magnetoresistance is significantly enhanced when the spin-flip coupling is weak but as the coupling becomes stronger the decreasing magnetoresistance develops an oscillatory behavior.

cond-mat.soft↗

arXiv2005

Energy levels and magneto-optical transitions in parabolic quantum dots with spin-orbit coupling

We report on the electronic properties of few interacting electrons confined in a parabolic quantum dot based on a theoretical approach developed to investigate the influence of Bychkov-Rashba spin-orbit (SO) interaction on such a system. We note that the spin-orbit coupling profoundly influences the energy spectrum of interacting electrons in a quantum dot. Here we present accurate results for the energy levels and optical-absorption spectra for parabolic quantum dots containing upto four interacting electrons, in the presence of spin-orbit coupling and under the influence of an externally applied, perpendicular magnetic field. We have described in detail about a very accurate numerical scheme to evaluate these quantities. We have evaluated the effects of SO coupling on the Fock-Darwin spectra for quantum dots made out of three different semiconductor systems, InAs, InSb, and GaAs.

cond-mat.mes-hall↗

arXiv2005

Energetics of holes trapped in DNA

We report on our study of the electronic properties of guanine traps in the DNA surrounded by adenines. We have shown that for a typical range of DNA parameters, formation of the bound state of two holes at the same guanine trap is possible for the GGG and GGGG traps if the hole-hole interaction is weak, which can be achieved for the DNA in solutions. The origin of the two-hole bound state is the competition between the Coulomb repulsion and the phonon mediated attraction between the holes. For the hole-phonon coupling constant $\approx 1$ two holes will be at the same trap if the on-site hole-hole repulsion energy is $\lesssim 0.9 $ eV.

cond-mat.mes-hall↗

arXiv2005

Spin transitions in an incompressible liquid Coulomb coupled to a quantum dot

We report on our investigation of the low-lying energy spectra and charge density of a two-dimensional quantum Hall liquid at $ν=\frac25$ that is Coulomb coupled to a quantum dot. The dot contains a hole and two/three electrons. We found that any external perturbation (caused by the close proximity of the quantum dot) locally changes the spin polarization of the incompressible liquid. The effect depends crucially on the separation distance of the quantum dot from the electron plane. Electron density distribution in the quantum Hall layer indicates creation of a quasihole that is localized by the close proximity of the quantum dot. Manifestation of this effect in the photoluminescence spectroscopy is also discussed.

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