arXiv subjects

Tapash Chakraborty

Publications and source records attributed to Tapash Chakraborty.

105 records · Page 6Linked to original sources

arXiv2005

Spin precession in a fractional quantum Hall state with spin-orbit coupling

Experimental attempts to realize spin-devices based on concepts derived from single-particle theoretical approaches have not been very successful yet. This raises the fundamental question of whether inter-electron interactions can be neglected in planar electron-based spintronics devices. We report on our results of a many-body approach to the spin configuration in a quantum Hall state in the presence of Bychkov-Rashba type spin-orbit interaction. While some properties of this system are found to be ideally suited for exploitation in spin devices, others might seem to limit its applicability. The latter can however be optimized for device performance.

cond-mat.mes-hall↗

arXiv2005

Optical Signatures of Spin-Orbit Interaction Effects in a Parabolic Quantum Dot

We demonstrate here that the dipole-allowed optical absorption spectrum of a parabolic quantum dot subjected to an external magnetic field reflects the inter-electron interaction effects when the spin-orbit interaction is also taken into account. We have investigated the energy spectra and the dipole-allowed transition energies for up to four interacting electrons parabolically confined, and have uncovered several novel features in those spectra that are solely due to the SO interaction.

cond-mat.mes-hall↗

arXiv2004

Tuning of the Gap in a Laughlin-Bychkov-Rashba Incompressible Liquid

We report on our investigation of the influence of Bychkov-Rashba spin-orbit interaction (SOI) on the incompressible Laughlin state. We find that experimentally obtainable values of the spin-orbit coupling strength can induce as much as a 25% increase in the quasiparticle-quasihole gap Eg at low magnetic fields in InAs, thereby increasing the stability of the liquid state. The SOI-modulated enhancement of Eg is also significant for filling factors 1/5 and 1/7, where the FQH state is usually weak. This raises the intriguing possibility of tuning, via the SO coupling strength, the liquid to solid transition to much lower densities.

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arXiv2004

Electron Dynamics in a DNA Molecule

We report on our theoretical investigations of the electronic states in a DNA molecule. We have used a two-leg charge ladder model where electron-electron interactions and the electron spin have been taken into account. The energy spectra for G-C and A-T base pairs obtained by numerically diagonalizing the Hamiltonian reveal a gap structure and the interaction is found to enhance the energy gaps. We also present the charge distribution in the ground state and low-lying excited states for the A-T and G-C base pairs.

cond-mat.dis-nn↗

arXiv2004

Electron Correlations in a Quantum Dot with Bychkov-Rashba Coupling

We report on a theoretical approach developed to investigate the influence of Bychkov-Rashba interaction on a few interacting electrons confined in a quantum dot. We note that the spin-orbit coupling profoundly influences the energy spectrum of interacting electrons in a quantum dot. Inter-electron interaction causes level crossings in the ground state and a jump in magnetization. As the coupling strength is increased, that jump is shifted to lower magnetic fields. Low-field magnetization will therefore provide a direct probe of the spin-orbit coupling strength in a quantum dot.

cond-mat.dis-nn↗

arXiv2001

Interaction of a quantum dot with an incompressible two-dimensional electron gas

We consider a system of a incompressible quantum Hall liquid in close proximity to a parabolic quantum dot containing a few electrons. We observe a significant influence of the interacting electrons in the dot on the excitation spectrum of the incompressible state in the electron plane. Our calculated charge density indicates that unlike in the case of an impurity, interacting electrons in the dot seem to confine the fractionally charged excitations in the incompressible liquid.

cond-mat.mes-hall↗

arXiv2001

Optical properties of a Quantum-Dot Cascade Structure

We report on our theoretical studies of the luminescence spectra of a quantum cascade laser where the quantum wells in the active regions are replaced by parabolic quantum dots. We analyze the influence of shape and size of the dots on the luminescence spectra. The emission spectra have interaction induced blueshift which increases almost linearly with increasing electron number. The blueshift is smaller for larger and non-circular dots. For large dots, shape of the emission line has weak dependence on the shape of quantum dots.

cond-mat.str-el↗

arXiv2000

Influence of disorder and a parallel magnetic field on a Quantum Cascade Laser

The luminescence spectra of a quantum cascade laser in a strong magnetic field is influenced significantly by the presence of disorder (charged or neutral) in the system. An externally applied magnetic field parallel to the electron plane causes a red shift of the luminescence peak in the absence of any disorder potential. Our results indicate that the disorder potential tends to cancel that red shift and causes a rapid decrease of the luminescence peak. A similar behavior was observed in a recent experiment on QCL in a parallel magnetic field.

cond-mat.mes-hall↗

arXiv2000

Luminescence Spectra of a Quantum-Dot Cascade Laser

A quantum cascade laser where the quantum wells in the active regions are replaced by quantum dots with their atom-like discrete energy levels is an interesting system to study novel features in optical spectroscopy. We study structures suitable for diagonal lasing transitions in coupled dots, and vertical lasing transitions in a single dot, in the active regions of the laser device. The luminescence spectra as a function of electron number and dot size show that for diagonal transitions, a significant amount of blue-shift in the emission spectra can be achieved by increasing electron population in the quantum dots as well as by decreasing the size of the dots.

cond-mat.mes-hall↗

arXiv1999

Temperature dependence of spin polarizations at higher Landau Levels

We report our results on temperature dependence of spin polarizations at $ν=1$ in the lowest as well as in the next higher Landau level that compare well with recent experimental results. At $ν=3$, except having a much smaller magnitude the behavior of spin polarization is not much influenced by higher Landau levels. In sharp contrast, for filling factor $ν=\frac83$ we predict that unlike the case of $ν=\frac23$ the system remains fully spin polarized even at vanishingly small Zeeman energies.

cond-mat.mes-hall↗

arXiv1998

Dynamical Correlations in a Half-Filled Landau Level

We formulate a self-consistent field theory for the Chern-Simons fermions to study the dynamical response function of the quantum Hall system at $ν=1/2$. Our scheme includes the effect of correlations beyond the random-phase approximation (RPA) employed to this date for this system. The resulting zero-frequency density response function vanishes as the square of the wave vector in the long-wavelength limit. The longitudinal conductivity calculated in this scheme shows linear dependence on the wave vector, like the experimentals results and the RPA, but the absolute values are higher than the experimental results.

cond-mat.mes-hall↗

arXiv1997

A Fermi Fluid Description of the Half-Filled Landau Level

We present a many-body approach to calculate the ground state properties of a system of electrons in a half-filled Landau level. Our starting point is a simplified version of the recently proposed trial wave function where one includes the antisymmetrization operator to the bosonic Laughlin state. Using the classical plasma analogy, we calculate the pair-correlation function, the static structure function and the ground state energy in the thermodynamic limit. These results are in good agreement with the expected behavior at $ν=\frac12$.

cond-mat.mes-hall↗

arXiv1997

Impurity Effects and Spin Polarizations in a Narrow Quantum Hall System

The temperature dependence of electron spin polarization for a narrow quantum Hall system shows behavior analogous to that of a two-dimensional system at major filling factors. At the lowest half-filled quantum Hall state for which no two-dimensional analog exists, we find a stable spin partially-polarized system. Periodic Gaussian repulsive impurities (antidots) in such a system leads to novel spin transitions at $ν=\frac13$ and $ν=\frac12$ and the pair-correlation functions provide clues about nature of different ground states in the system. These results can be explored in optical spectroscopy and optically pumped NMR Knight shift measurements.

cond-mat.mes-hall↗

arXiv1997

Theory of Incompressible States in a Narrow Channel

We report on the properties of a system of interacting electrons in a narrow channel in the quantum Hall effect regime. It is shown that an increase in the strength of the Coulomb interaction causes abrupt changes in the width of the charge-density profile of translationally invariant states. We derive a phase diagram which includes many of the stable odd-denominator states as well as a novel fractional quantum Hall state at lowest half-filled Landau level. The collective mode evaluated at the half-filled case is strikingly similar to that for an odd-denominator fractional quantum Hall state.

cond-mat.mes-hall↗

arXiv1997

Spin Polarizations at and about the Lowest Filled Landau Level

The spin polarization versus temperature at or near a fully filled lowest Landau level is explored for finite-size systems in a periodic rectangular geometry. Our results at $ν=1$ which also include the finite-thickness correction are in good agreement with the experimental results. We also find that the interacting electron system results are in complete agreement with the results of the sigma model, i.e., skyrmions on a torus have a topological charge of $Q \ge 2$ and the Q=1 solution is like a single spin-flip excitation. Our results therefore provide direct evidence for the skyrmionic nature of the excitations at this filling factor.

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