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S. Mazumdar

Publications and source records attributed to S. Mazumdar.

At least 55 records · Page 3Linked to original sources
arXiv2005

Magnetism in BEDT-TTF materials

Strong commensurate antiferromagnetism proximate to superconductivity is found in some members of the kappa-(ET) family, while a spin gap (SG) is found in the theta-(ET). Both kappa- and theta-(ET) materials have frustrated triangular lattice structures. We show from calculations of spin-spin correlations within the effective half-filled band triangular lattice proposed for the kappa-ET, as well as for the real lattice, that long range AFM order is not obtained as a consequence of this frustration. We argue that some other mechanism reduces the magnetic frustration in these systems. We show that the low temperature magnetic states in these materials can only be understood if the effects of the {\it cooperative} charge and bond ordering transitions occurring at higher temperatures in these systems are taken into account. In the kappa-ET, this co-operative transition leads to unequal hole populations on the ET dimers that form the triangular lattice.

cond-mat.str-el↗
arXiv2005

Co-operative density wave and giant spin gap in the quarter-filled zigzag ladder

Strong co-operative interactions occur between four different broken symmetries involving charge-ordering and bond distortions in the quarter-filled correlated zigzag electron ladder. The ground state is singlet, with spin gap several times larger than in the spin-Peierls state of the one-dimensional quarter-filled chain with the same parameters. We propose the quarter-filled zigzag electron ladder model for several different organic charge-transfer solids with coupled pairs of quasi-one-dimensional stacks, the spin-gap transition temperatures in which are unusually high.

cond-mat.str-el↗
arXiv2005

The Quantum Interference Effect Transistor

We propose a new type of molecular transistor, the Quantum Interference Effect Transistor (QuIET), based on tunable current suppression due to quantum interference. We show that any aromatic hydrocarbon ring has two-lead configurations for which current at small voltages is suppressed by destructive interference. A transistor can be created by providing phase relaxation or decoherence at a site on the ring. We propose several molecules which could tunably introduce the necessary dephasing or decoherence, as well as a proof of principle using a scanning tunneling microscope tip. Within the self-consistent Hartree-Fock approximation, the QuIET is shown to have characteristics strikingly similar to those of conventional field effect and bipolar junction transistors.

cond-mat.mes-hall↗
arXiv2004

Role of Electron--Electron Interactions on Spin Effects in Electron--Hole Recombination in Organic Light Emitting Diodes

We extend our theory of electron--hole recombination in organic light emitting diodes to investigate the possibility that high energy singlet and triplet excited states with large electron--hole separations are generated in such processes, over and above the lowest singlet and triplet excitons. Our approach involves a time-dependent calculation of the interchain / intermolecular charge--transfer within model Hamiltonians that explicitly include electron-electron interactions between the $π$-electrons. We show that the electron--hole recombination reaction can be viewed as a tunneling process whose cross section depends on both the matrix element of the interchain part of Hamiltonian and the energy difference between the initial polaron--pair state and the final neutral states. There occurs a bifurcation of the electron--hole recombination path in each of the two spin channels that leads to the generation of both the lowest energy exciton and a specific high energy charge-transfer state, with the matrix elements favoring the lowest energy exciton and the energy difference factor favoring the higher energy state. The overall effect of the electron--electron interactions is to enhance the singlet:triplet yield ratio over the value of 0.25 predicted from statistical considerations that are valid only within noninteracting models.

cond-mat.str-el↗
arXiv2003

Rung-singlet charge-ordering in alpha'-NaV2O5

We show that the proposed zig--zag charge--ordering model for alpha'--NaV2O5 is incompatible with a single transition that induces both charge--ordering and spin gap. We introduce a two--band model for alpha'--NaV2O5 within which the simultaneous charge--spin order transition is driven principally by electron-phonon interactions. The spin gap is due to the formation of local dimers, thus explaining the weak magnetic effect on this transition.

cond-mat.str-el↗
arXiv2003

Designing pi-conjugated polymers with light emission in the infrared

There is currently a great need for solid state lasers that emit in the infrared. Whether or not conjugated polymers that emit in the IR can be synthesized is an interesting theoretical challenge. We show that emission in the IR can be achieved in designer polymers in which the effective Coulomb correlation is smaller than that in existing systems. We also show that the structural requirement for having small effective Coulomb correlations is that there exist transverse $π$--conjugation over a few bonds in addition to longitudinal conjugation with large conjugation lengths.

cond-mat.mtrl-sci↗
arXiv2003

Theory of excited state absorptions in phenylene-based $π$-conjugated polymers

Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of two-photon A$_g$ states in these systems to which photoinduced absorption (PA) can occur. At relatively lower energies there occur A$_g$ states which are superpositions of one electron - one hole (1e--1h) and two electron -- two hole (2e--2h) excitations, that are both comprised of the highest delocalized valence band and the lowest delocalized conduction band states only. The dominant PA is to one specific member of this class of states (the mA$_g$). In addition to the above class of A$_g$ states, PA can also occur to a higher energy kA$_g$ state whose 2e--2h component is {\em different} and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mA$_g$ and the kA$_g$ agree reasonably well to the experimentally observed low and high energy PAs in PPV. The calculated relative intensities of the two PAs are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work a new intense PA at an energy considerably lower than the region where PA have been observed currently. Based on earlier work that showed that efficient charge--carrier generation occurs upon excitation to odd--parity states that involve both delocalized and localized bands, we speculate that it is the characteristic electronic nature of the kA$_g$ that leads to charge generation subsequent to excitation to this state, as found experimentally.

cond-mat↗
arXiv2002

The pattern of the charge ordering in quasi-one- and two-dimensional organic charge transfer solids

We examine critically two different recently proposed models of charge ordering in the nominally 1/4-filled organic charge transfer solids (CTS). In one dimension, the two models are characterized by site charge densities of the form ...1010... and ...1100..., respectively. We establish the following theoretical results: 1) there exists a critical nearest-neighbor Coulomb interaction (V_c > 2|t|) only above which does the ...1010... state become the ground state; 2) Hartree-Fock mean field theory predicts V_c incorrectly; 3) accurate quantum Monte Carlo calculations indicate that V_c increases with decreasing U; 4) for sufficiently strong e-ph interactions, the ...1010... state can undergo a spin-Peierls (SP) transition; 5) for V < V_c, in the presence of e-ph interactions, the ...1100... CO state is the ground state and also undergoes a SP transition. We show that experimental observations clearly indicate the ...1100... CO in the 1:2 anionic CTS and the (TMTSF)_2X class of materials, while the results for (TMTTF)_2X with narrower one-electron bandwidths are more ambiguous. In two dimensions, we focus on the theta-(BEDT-TTF)_2X materials, and establish that the CO pattern there corresponds to the so-called horizontal stripe structure, with ...1100... CO along the two directions with larger electron hopping. We give precise explanations of the observed spontaneous bond distortions in the c-direction of theta-(BEDT-TTF)_2X at the metal-insulator transition and also show that the appearance of the spin gap at lower temperatures is a true two-dimensional effect different from the usual SP transition. Superconductivity in the charge transfer solids appears to be limited to the class of materials which exhibits the ...1100... CO in the insulating phase.

cond-mat.str-el↗
arXiv2002

Prediction of infrared light emission from pi-conjugated polymers: a diagrammatic exciton basis valence bond theory

There is currently a great need for solid state lasers that emit in the infrared, as this is the operating wavelength regime for applications in telecommunications. Existing $π$--conjugated polymers all emit in the visible or ultraviolet, and whether or not $π$--conjugated polymers that emit in the infrared can be designed is an interesting challenge. On the one hand, the excited state ordering in trans-polyacetylene, the $π$--conjugated polymer with relatively small optical gap, is not conducive to light emission because of electron-electron interaction effects. On the other hand, excited state ordering opposite to that in trans-polyacetylene is usually obtained by chemical modification that increases the effective bond-alternation, which in turn increases the optical gap. We develop a theory of electron correlation effects in a model $π$-conjugated polymer that is obtained by replacing the hydrogen atoms of trans-polyacetylene with transverse conjugated groups, and show that the effective on-site correlation in this system is smaller than the bare correlation in the unsubstituted system. An optical gap in the infrared as well as excited state ordering conducive to light emission is thereby predicted upon similar structural modifications.

cond-mat.mtrl-sci↗
arXiv2002

The ubiquitous 1100 charge ordering in organic charge-transfer solids

Charge and spin-orderings in the 1/4-filled organic CT solids are of strong interest, especially in view of their possible relations to organic superconductivity. We show that the charge order (CO) in both 1D and 2D CT solids is of the ...1100... type, in contradiction to mean field prediction of >...1010... CO. We present detailed computations for metal-insulator and magnetic insulator-insulator transitions in the theta-ET materials. Complete agreement with experiments in several theta systems is found. Similar comparisons between theory and experiments in TCNQ, TMTTF, TMTSF, and ET materials prove the ubiquity of this phenomenon.

cond-mat.str-el↗
arXiv2002

Sub-Poissonian Shot Noise in Molecular Wires

We investigate the transport behavior of polyene molecules sandwiched between two metallic contacts using the non-equilibrium Green's function formalism. We calculate both current and noise power as a function of applied voltage and show that they decrease with increasing size of the polyene molecules. We find that even with symmetric connection to metallic contacts, current verus voltage curves can be asymmetric for asymmetrically substituted polyenes. Most importantly, we demonstrate a cross-over from Poissonian to sub-Poissonian behavior in the shot noise as a function of applied voltage. The algorithm for noise power calculation can be used for designing molecules with low noise.

cond-mat.mes-hall↗
arXiv2002

Electron correlation effects in electron-hole recombination in organic light-emitting diodes

We develop a general theory of electron--hole recombination in organic light emitting diodes that leads to formation of emissive singlet excitons and nonemissive triplet excitons. We briefly review other existing theories and show how our approach is substantively different from these theories. Using an exact time-dependent approach to the interchain/intermolecular charge-transfer within a long-range interacting model we find that, (i) the relative yield of the singlet exciton in polymers is considerably larger than the 25% predicted from statistical considerations, (ii) the singlet exciton yield increases with chain length in oligomers, and, (iii) in small molecules containing nitrogen heteroatoms, the relative yield of the singlet exciton is considerably smaller and may be even close to 25%. The above results are independent of whether or not the bond-charge repulsion, X_perp, is included in the interchain part of the Hamiltonian for the two-chain system. The larger (smaller) yield of the singlet (triplet) exciton in carbon-based long-chain polymers is a consequence of both its ionic (covalent) nature and smaller (larger) binding energy. In nitrogen containing monomers, wavefunctions are closer to the noninteracting limit, and this decreases (increases) the relative yield of the singlet (triplet) exciton. Our results are in qualitative agreement with electroluminescence experiments involving both molecular and polymeric light emitters. The time-dependent approach developed here for describing intermolecular charge-transfer processes is completely general and may be applied to many other such processes.

cond-mat.mtrl-sci↗
arXiv2002

Electron-hole versus exciton delocalization in conjugated polymers: the role of topology

There is currently a great need for solid state lasers that emit in the infrared. Whether or not conjugated polymers that emit in the IR can be synthesized is an interesting theoretical challenge. We show that the requirement for such a material is that the exciton delocalization in the system be large, such that the optical gap is small. We develop a theory of exciton delocalization in conjugated polymers, and show that the extent of this can be predicted from the topology of the conjugated polymer in question. We determine the precise structural characteristics that would be necessary for light emission in the IR.

cond-mat.mtrl-sci↗
arXiv2002

Charge ordering in theta-(BEDT-TTF)_2 X materials

We investigate theoretically charge ordered states on the anisotropic triangular lattice characteristic of the theta-(BEDT-TTF)_2 X materials. Using exact diagonalization studies, we establish that the charge order (CO) pattern corresponds to a ``horizontal'' stripe structure, with ...1100... CO along the two directions with larger electron hopping (p-directions), and ...1010... 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 occurence of a charge-induced high temperature transition as well as a spin gap transition at lower temperature.

cond-mat.str-el↗
arXiv2001

Dimensionality dependence of optical nonlinearity and relaxation dynamics in cuprates

Femtosecond pump-probe measurements find pronounced dimensionality dependence of the optical nonlinearity in cuprates. Although the coherent two-photon absorption (TPA) and linear absorption bands nearly overlap in both quasi-one and two-dimensional (1D and 2D) cuprates, the TPA coefficient is one order of magnitude smaller in 2D than in 1D. Furthermore, picosecond recovery of optical transparency is observed in 1D cuprates, while the recovery in 2D involves relaxation channels with a time scales of tens of picoseconds. The experimental results are interpreted within the two-band extended Hubbard model.

cond-mat.str-el↗
arXiv2000

Bond and charge density waves in the isotropic interacting two-dimensional quarter-filled band and the insulating state proximate to organic superconductivity

We report two surprising results regarding the nature of the spatial broken symmetries in the two-dimensional (2D), quarter-filled band with strong electron-electron interactions. First, in direct contradiction to the predictions of one-electron theory, we find a coexisting ``bond-order and charge density wave'' (BCDW) insulating ground state in the 2D rectangular lattice for all anisotropies, including the isotropic limit. Second, we find that the BCDW further coexists with a spin-density wave (SDW) in the range of large anisotropy. Further, in contrast to the interacting half-filled band, in the interacting quarter-filled band there are two transitions: first, a similar singlet-to-AFM/SDW transition for large anisotropy and second, an AFM/SDW-to-singlet transition at smaller anisotropy. We discuss how these theoretical results apply to the insulating states that are proximate to the superconducting states of 2:1 cationic charge-transfer solids (CTS). An important consequence of this work is the suggestion that organic superconductivity is related to the proximate Coulomb-induced BCDW, with the SDW that coexists for large anisotropies being also a consequence of the BCDW, rather than the driver of superconductivity.

cond-mat.str-el↗