Sumit Mazumdar

Controlling quantum transport through a single molecule.

We investigate multiterminal quantum transport through single monocyclic aromatic annulene molecules, and their derivatives, using the nonequilibrium Green function approach within the self-consistent Hartree-Fock approximation. We propose a new device concept, the quantum interference effect transistor, that exploits perfect destructive interference stemming from molecular symmetry and controls current flow by introducing decoherence and/or elastic scattering that break the symmetry. This approach overcomes the fundamental problems of power dissipation and environmental sensitivity that beset nanoscale device proposals.

Ultrafast optical nonlinearity in the quasi-one-dimensional mott insulator Sr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching.

The quantum interference effect transistor

We give a detailed discussion of the quantum interference effect transistor (QuIET), a proposed device which exploits the interference between electron paths through aromatic molecules to modulate the current flow. In the off state, perfect destructive interference stemming from the molecular symmetry blocks the current, while in the on state, the current is allowed to flow by locally introducing either decoherence or elastic scattering. Details of a model calculation demonstrating the efficacy of the QuIET are presented, and various fabrication scenarios are proposed, including the possibility of using conducting polymers to connect the QuIET with multiple leads.

Charge Ordering in θ-(BEDT-TTF)2X Materials

We theoretically investigate charge ordered states on the anisotropic triangular lattice characteristic of the θ-(BEDT-TTF) 2 X materials. Using exact diagonalization studies, we establish that the charge order (CO) pattern corresponds to a “horizontal” stripe structure, with \(\dots\)1100\(\dots\) CO along the two directions with larger electron hopping ( p -directions), and \(\dots\)1010\(\dots\) CO along the third direction ( c -direction). The CO is accompanied by co-operative bond dimerizations along all three directions in the highest spin state. In the lowest spin state bonds along the p -directions are tetramerized. Our theory explains the occurrence of a charge-induced high temperature transition as well as a spin gap transition at lower temperature.

Metal-insulator and insulator-insulator transitions in the quarter-filled band organic conductors

The theory of the 2[ital k][sub [ital F]] and 4[ital k][sub [ital F]] instabilities in quarter-filled band organic conductors is revisited. The phase angles of the 2[ital k][sub [ital F]] bond and charge density waves change as electron correlation is turned on, and this switching of the phase angle is critical for understanding the bond distortion patterns in the real materials. Intersite Coulomb interactions in the real materials must be nonzero but less than a critical value. Both intersite and intrasite charge density waves are de- stabilized in the quasi-two-dimensional regime for realistic parameters, explaining the weakening of these phases in the superconducting materials.

Electron-correlation-induced transverse delocalization and longitudinal confinement in excited states of phenyl-substituted polyacetylenes

Electron-electron interactions in general lead to both ground-state and excited-state confinement. We show, however, that in phenyl-substituted polyacetylenes electron-electron interactions cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain into the phenyl groups, which in turn leads to enhanced confinement in the chain direction. This cooperative delocalization confinement lowers the energy of the one-photon state and raises the relative energy of the lowest two-photon state. The two-photon state is slightly below the optical state in monophenyl-substituted polyacetylenes, but above the optical state in diphenyl-substituted polyacetylenes, thereby explaining the strong photoluminescence of the latter class of materials. We present a detailed mechanism of the crossover in the energies of the one- and two-photon states in these systems. In addition, we calculate the optical-absorption spectra over a wide wavelength region, and make specific predictions for the polarizations of low- and high-energy transitions that can be tested on oriented samples. Within existing theories of light emission from

Theory of Singlet Fission in Polyenes, Acene Crystals and Covalently Linked Acene Dimers

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DESIGNING EMISSIVE CONJUGATED POLYMERS WITH SMALL OPTICAL GAPS : A STEP TOWARDS ORGANIC POLYMERIC INFRARED LASERS

-conjugated polymers, strong photoluminescence should be restricted to materials whose optical gaps are larger than that of trans-polyacetylene. The present work shows that, conceptually at least, it is possible to have light emission from systems with smaller optical gaps.

Quantitative calculations of the excitonic energy spectra of semiconducting single-walled carbon nanotubes within a π -electron model

We report quadruple configuration interaction calculations within the extended Pariser

Photophysics of phenyl-substituted polyacetylenes, theory

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