arXiv subjects

Sumit Mazumdar

Publications and source records attributed to Sumit Mazumdar.

At least 19 recordsLinked to original sources

arXiv2026

Correlated-Electron Theory of Triplet-Triplet Multiexciton States in Polypentacene

We present correlated-electron calculations of optical spin-singlet and triplet-triplet multiexciton states in three- and four-unit pentacene oligomers as microscopic models for polypentacene. The calculations use the Pariser-Parr-Pople Hamiltonian, multiple-reference singles and doubles configuration interaction, and a molecular exciton basis that resolves Frenkel, charge-transfer, and triplet-pair (T1T1) configurations in real space. We find that the complete set of 1(T1T1) eigenstates lies in a narrow, nearly degenerate energy window near the lowest optical exciton and that no eigenstate can be identified with a single localized triplet-pair configuration. Instead, each triplet-pair eigenstate is a quantum superposition of configurations containing all accessible intertriplet separations. This electronic structure explains the perceived absence of intramolecular triplet diffusion in pentacene oligomers, polypentacene, and polytetracene solutions, while leaving open the possibility of intermolecular singlet fission in films with appreciable interchain interactions.

cond-mat.mtrl-sci↗

arXiv2025

Comment on "Superconductivity and Mott Physics in Organic Charge Transfer Materials"

Menke et al. recently claimed that superconductivity (SC) in the $\kappa$-phase organic charge-transfer solids (CTS) can be understood within the two-dimensional half-filled anisotropic triangular-lattice Hubbard model. Experimentally, $\kappa$-CTS are mostly but not always antiferromagnetic (AFM) at ambient pressure and SC appears under pressure. In apparent agreement with this observation, Menke et al. found AFM ground states for small $t/U$ and SC over a small region at the interface of AFM and Fermi liquid ground states with increasing $t/U$ at fixed $t'/t$, where $U$ is the Hubbard repulsion. Menke et al's computational results directly contradict those obtained using exact diagonalization and Path Integral Renormalization Group approaches. It is clearly of interest to determine the origin of this discrepancy, especially in view of the facts that (a) related arguments continue to persist in the context of cuprate SC superconductivity (which however involves doping), and (b) there exist CTS in which SC is not proximate to AFM, but is separated by an intermediate charge-disproportionated phase. Here we show that Menke et al's conclusion regarding SC is incorrect and originates from a flawed assumption.

cond-mat.supr-con↗

arXiv2025

Exciton Basis Description of Ultrafast Triplet Separation in Pentacene-(Tetracene)2-Pentacene Intramolecular Singlet Fission Chromophore

Precise understanding of the electronic structures of optically dark triplet-triplet multiexcitons that are the intermediate states in singlet fission (SF) continues to be a challenge. This is particularly true for intramolecular singlet fission (iSF) chromophores, that are oligomers of large monomer molecules. We have performed quantum many-body calculations of the complete set of excited states relevant to iSF in Pentacene-(Tetracene)2-Pentacene oligomers, consisting of two terminal pentacene monomers linked by two tetracene monomers. Our computations use an exciton basis that gives physical pictorial descriptions of all eigenstates, and are performed over an active space of twenty-eight monomer molecular orbitals, including configuration interaction with all relevant quadruple excitations within the active space, thereby ensuring very high precision. We discuss the many-electron structures of the optical predominantly intramonomer spin-singlets, intermonomer charge-transfer excitations, and most importantly, the complete set of low energy covalent triplet-triplet multiexcitons. We are able to explain the weak binding energy of the pentacene-tetracene triplet-triplet eigenstate that is generated following photoexcitation. We explain the increase in lifetime with increasing numbers of tetracene monomers of the transient absorption associated with contiguous pentacene-tetracene triplet-triplet in this family of oligomers. We are consequently able to give a pictorial description of the triplet separation following generation of the initial triplet-triplet, leading to a state with individual triplets occupying only the two pentacene monomers. We expect many applications of our theoretical approach to triplet separation.

cond-mat.mtrl-sci↗

arXiv2024

Computational Demonstrations of Density Wave of Cooper Pairs and Paired-Electron Liquid in the Quarter-Filled Band -- a Brief Review

There has been strong interest recently in the so-called Cooper pair density wave, subsequent to the proposition that such a state occurs in the hole-doped cuprate superconductors. As of now there is no convincing demonstration of such a state in the cuprate theoretical literature. We present here a brief but complete review of our theoretical and computational work on the paired-electron crystal (PEC), which has been also experimentally seen in the insulating phase proximate to superconductivity (SC) in organic charge-transfer solid (CTS) superconductors. Within our theory, SC in the CTS does indeed evolve from the PEC. A crucial requirement for the finding of the PEC is that the proper carrier density of one charge carrier per two sites is taken into consideration at the outset. Following the discussion of CTS superconductors, we briefly discuss how the theory can be extended to understand the phase diagram of the cuprate superconductors that has remained mysterious after nearly four decades of the discovery of SC in this family.

cond-mat.str-el↗

arXiv2023

Valence Transition Theory of the Pressure-Induced Dimensionality Crossover in Superconducting Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$

More than three decades after the discovery of superconductivity (SC) in the cuprates, the nature of the "normal" state and the mechanism of SC remain mysterious. One popular theoretical approach has been to treat the CuO$_2$ layer as coupled two-leg one-band Hubbard ladders. In the undoped two-leg ladder spin-singlets occupy ladder rungs, and doped ladders are characterized by superconducting correlations with quasi-long range order (quasi-LRO). Pressure-induced SC in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ (SCCO) has long been explained within one-band ladder theories. The dramatic pressure-driven crossover from quasi one-to-two dimensional (1D-to-2D) transport and the simultaneous vanishing of the spin gap due to ladder singlets in the metallic state preceding SC however lie outside the scope of ladder-based theories. Recent demonstration of rapid decay of superconducting correlations with distance within a realistic multiband model of the cuprate ladder gives additional credence to this viewpoint. Here we show that the assumption of pressure-driven increase in hole concentration in the ladders cannot explain the dimensionality crossover. The dimensionality crossover is due to discrete change in Cu-ion ionicity accompanied by transfer of holes from the Cu to O-ions, leading to "negative charge-transfer gap". Our theory of valence transition driven dimensionality crossover in SCCO provides a generic explanation of the very large increase in charge carrier density at critical doping concentrations in both hole and electron doped layered cuprates We propose a falsifiable experimental test of our theory.

cond-mat.str-el↗

arXiv2023

Doping asymmetry in the three-band Hamiltonian for cuprate ladders: failure of the standard model of superconductivity in cuprates

The relevance of the single-band two-dimensional Hubbard model to superconductivity in the doped cuprates has recently been questioned, based on Density matrix Renormalization Group (DMRG) computations on extended t-J models that found superconductivity over unrealistically broad doping region upon electron-doping, yet complete absence of superconductivity for hole-doping. We report very similar results from DMRG calculations on Cu$_2$O$_3$ two-leg ladder within the parent three-band correlated-electron Hamiltonian. The strong asymmetry found in our calculations are in contradiction to the deep and profound symmetry between electron- and hole-doped cuprate superconductors, apart from their critical temperatures, that has been found from recent experiments.

cond-mat.str-el↗

arXiv2021

Absence of Luther-Emery Superconducting Phase in the Three-Band Model for Cuprate Ladders

Correlated-electron theories of superconductivity in layered cuprates often start from the premise of a gapped spin-liquid phase proximate to the superconducting state. This assumption is justified based on analytical and numerical demonstrations of a superconducting Luther-Emery phase in the doped 2-leg one-band Hubbard ladder, and the perceived analogy between coupled ladders and the two dimensional CuO2 layer. We demonstrate from accurate density matrix renormalization group studies the absence of the superconducting Luther-Emery phase in the doped 2-leg three-band ladder consisting of both copper and oxygen, even as the spin gap is large in the undoped three-band ladder. For realistic oxygen-oxygen hopping and Hubbard repulsion on the oxygen atoms, density-density rather than pairing correlations are dominant at long range. This result is equally valid whether or not the oxygens outside the ladder proper, over and above the rung and leg oxygens, are included in the computation. These results demonstrate the critical importance of oxygen orbitals, and raise disturbing questions about the applicability of many of the existing correlated-electron theories of superconductivity.

cond-mat.str-el↗

arXiv2020

Wavefunction-Based Analysis of Dynamics Versus Yield of Free Triplets in Intramolecular Singlet Fission

Experiments in several intramolecular singlet fission materials have indicated that the triplet-triplet spin biexciton has a much longer lifetime than believed until recently, opening up loss mechanisms that can annihilate the biexciton prior to its dissociation to free triplets. We have performed many-body calculations of excited state wavefunctions of hypothetical phenylene-linked anthracene molecules to better understand linker-dependent behavior of dimers of larger acenes being investigated as potential singlet fission candidates. The calculations reveal unanticipated features that we show carry over to the real covalently-linked pentacene dimers. Dissociation of the correlated triplet-triplet spin biexciton and free triplet generation may be difficult in acene dimers where the formation of the triplet-triplet spin biexciton is truly ultrafast. Conversely, relatively slower biexciton formation may indicate smaller spin biexciton binding energy and greater yield of free triplets. Currently available experimental results appear to support this conclusion. Whether or not the two distinct behaviors are consequences of distinct mechanisms of triplet-triplet generation from the optical singlet is an interesting theoretical question.

cond-mat.mtrl-sci↗

arXiv2020

Sr$_2$RuO$_4$, like doped cuprates and barium bismuthate, is a negative charge-transfer gap even parity superconductor with $\frac{3}{4}$-filled oxygen band

A comprehensive theory of superconductivity in Sr$_2$RuO$_4$ must explain experiments that suggest even parity superconducting order and others that suggest broken time reversal symmetry. Completeness further requires that the theory applies to Ca$_2$RuO$_4$, a Mott-Hubbard semiconductor that exhibits an unprecedented insulator-to-metal transition driven by very small electric field, and also by doping with very small concentration of electrons, leading to a metallic state proximate to ferromagnetism. A valence transition model, previously proposed for superconducting cuprates [Phys. Rev. B {\bf 98}, 205153] is extended to Sr$_2$RuO$_4$ and Ca$_2$RuO$_4$. The insulator to metal transition is distinct from that expected from the simple melting of the Mott-Hubbard semiconductor. Rather, the Ru ions occur as low spin Ru$^{4+}$ in the semiconductor, and as high spin Ru$^{3+}$ in the metal, the driving force behind the valence transition being the strong spin-charge coupling and consequent large ionizaton energy in the low charge state. Metallic and superconducting ruthenates are two-component systems in which the half-filled high spin Ru$^{3+}$ ions determine the magnetic behavior but not transport, while the charge carriers are entirely on on the layer oxygen ions, which have an average charge -1.5. Spin singlet superconductivity evolves from the correlated lattice frustrated 3/4 filled band of layer oxygen ions alone, in agreement with quantum many body calculations that have demonstrated enhancement by electron-electron interactions of superconducting pair-pair correlations tions uniquely at or very close to this filling [Phys. Rev. B {\bf 93}, 165110 and {\bf 93}, 205111]. Several model specific experimental predictions are made, including that spin susceptibility due to Ru ions will remain unchanged as Sr$_2$RuO$_4$ is taken through superconducting Tc.

cond-mat.supr-con↗

arXiv2020

Free Triplets versus Bound Triplet-Triplet Biexciton in Intramolecular Singlet Fission Materials: Structure-Property Correlations

Recent advances in singlet-fission research make it imperative that structure-property correlations that determine optical signatures of the triplet-triplet spin biexciton as well as its binding energy be understood precisely. We report many-body calculations of excited state absorptions from the triplet exciton and the triplet-triplet biexciton from two transversally linked dimers of pentacene derivatives. Comparison of experiment against theory leads to new interpretations of experiments performed earlier. We show that in the para-linked isomer the triplet-triplet does not dissociate to free triplets through the duration of the measurements. In contrast, even as calculated and experimental transient absorptions agree in the meta-isomer, the experimental observations here are more difficult to interpret, indicating the strong role structural variations can play in determining the rate and yield of free triplets. We also report many-body calculations of the spin gap, the energy difference between the spin quintet versus spin singlet triplet-triplet, as well as the binding energy of the spin singlet triplet-triplet, defined as the energy difference between two free triplets and the bound biexciton. The spin gap and the binding energy of the spin singlet triplet-triplet are different quantities in all but coupled two-level systems. The experimental behavior in the transversally linked dimers as well as previously studied longitudinally linked dimers agree with the trends that would be predicted from the computed biexciton binding energies.

cond-mat.str-el↗

arXiv2019

Correlated Electronic Properties of a Graphene Nanoflake: Coronene

We report studies of the correlated excited states of coronene and substituted coronene within the Pariser-Parr-Pople (PPP) correlated $π$-electron model employing symmetry adapted density matrix renormalization group technique. These polynuclear aromatic hydrocarbons can be considered as graphene nanoflakes. We review their electronic structures utilizing a new symmetry adaptation scheme that exploits electron-hole symmetry, spin-inversion symmetry and end-to-end interchange symmetry. Study of the electronic structures sheds light on the electron correlation effects in these finite-size graphene analogues, which diminishes on going from one-dimensional to higher-dimensional systems, yet is significant within these finite graphene derivatives.

physics.chem-ph↗

arXiv2018

Optical Probes of the Quantum-Entangled Triplet-Triplet State in a Heteroacene Dimer

The nature and extent of the spin-entanglement in the triplet-triplet biexciton with total spin zero in correlated-electron $π$-conjugated systems continues to be an enigma. Differences in the ultrafast transient absorption spectra of free triplets versus the triplet-triplet can give a measure of the entanglement. This, however, requires theoretical understandings of transient absorptions from the optical spin-singlet, the lowest spin-triplet exciton as well as from the triplet-triplet state, whose spectra are often overlapping and hence difficult to distinguish. We present a many-electron theory of the electronic structure of the triplet-triplet, and of complete wavelength-dependent excited state absorptions (ESAs) from all three states in a heteroacene dimer of interest in the field of intramolecular singlet fission. The theory allows direct comparisons of ESAs with existing experiments as well as experimental predictions, and gives physical understandings of transient absorptions within a pictorial exciton basis that can be carried over to other experimental systems.

cond-mat.str-el↗

arXiv2018

The Valence Transition Model of Pseudogap, Charge-Order and Superconductivity in Electron- and Hole-Doped Copper Oxides

We present a valence transition model for electron- and hole-doped cuprates, within which there occurs a discrete jump in ionicity Cu$^{2+} \to$ Cu$^{1+}$ upon doping, at or near optimal doping in the electron-doped compounds and at the pseudogap phase transition in the hole-doped materials. Doped cuprates have negative charge-transfer gaps, just as rare earth nickelates and BaBiO$_3$. Because of strong correlations and small $d-p$ electron hoppings the systems behave as effective $\frac{1}{2}$-filled Cu-band in the undoped state, and as correlated two-dimensional geometrically frustrated nearly $\frac{1}{4}$-filled O-band in the doped state. The theory gives the simplest yet most comprehensive understanding of experiments in the normal states. The robust antiferromagnetism in the conventional T$^\prime$ crystals, the strong role of oxygen deficiency in driving superconductivity and charge carrier sign corresponding to holes at optimal doping are all manifestations of the same quantum state. In the hole-doped pseudogapped state, a biaxial commensurate period 4 charge density wave state of O$^{1-}$-Cu$^{1+}$-O$^{1-}$ spin-singlets coexists with broken rotational C$_4$ symmetry. Finite domains of this broken symmetry state will exhibit the polar Kerr effect. Superconductivity within the model results from a destabilization of the $\frac{1}{4}$-filled band paired Wigner crystal [Phys. Rev. B {\bf 93}, 165110 and {\bf 93}, 205111]. We posit that a similar valence transition, Ir$^{4+} \to$ Ir$^{3+}$, occurs in electron-doped SrIr$_2$O$_4$. We make testable theoretical predictions on cuprates and iridates. Finally, we note that there exist an unusually large number of unconventional superconductors that exhibit superconductivity proximate to exotic charge ordered states, whose bandfillings are also $\frac{1}{4}$, exactly where the paired Wigner crystal is most stable.

cond-mat.str-el↗

arXiv2017

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wavefunctions of the optical singlets, the lowest triplet exciton and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish between the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against published and unpublished experimental transient absorptions in the near and mid infrared we conclude that the end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet and not free triplets. We predict additional transient absorptions at even longer wavelengths, beyond 1500 nm, to the equivalent of the classic 2$^1$A$_g^-$ in linear polyenes.

cond-mat.str-el↗

arXiv2017

Theory of Transient Excited State Absorptions in Solid Pentacene with Implications for Singlet Fission

We report the first theoretical calculations of excited state absorptions (ESAs) from the singlet and triplet excitons, as well as the key intermediate in the singlet fission (SF) process, the spin singlet multiexciton triplet-triplet state, for solid pentacene with herringbone crystal structure. Our goal is to compare theoretical results against ultrafast transient photoinduced absorption (PA) measurements and their interpretations, which have remained controversial. We show that the elusive triplet-triplet state absorbs both in the visible and near infrared (NIR), at or close to the PA energies assigned to the free triplet exciton. In contrast, the triplet PA has nearly vanishing oscillator strength in the NIR within the rigid herringbone structure. Observable oscillator strength for NIR triplet PA requires photoinduced enhancement of coupling between a pair of neighboring pentacene molecules that confers significant charge-transfer (CT) character to the triplet exciton. We discuss the implication of our results for efficient SF in pentacene and related materials.

cond-mat.mes-hall↗

arXiv2016

Correlated Electronic Properties of Some Graphene Nanoribbons: A DMRG Study

The significant electron-electron interactions that characterize the π-electrons of graphene nanoribbons (GNRs) necessitate going beyond one-electron tight-binding description. Existing theories of electron-electron interactions in GNRs take into account one electron-one hole interactions accurately but miss higher order effects. We report highly accurate density matrix renormalization group (DMRG) calculations of the ground state electronic structure, the relative energies of the lowest one-photon versus two-photon excitations and the charge gaps in three narrow graphene nanoribbons (GNRs) within the correlated Pariser-Parr-Pople model for π-conjugated systems. We have employed the symmetrized DMRG method to investigate the zigzag nanoribbon 3-ZGNR and two armchair nanoribbons 6-AGNR and 5-AGNR, respectively. We predict bulk magnetization of the ground state of 3-ZGNR, and a large spin gap in 6-AGNR in their respective thermodynamic limits. Nonzero charge gaps and semiconducting behavior, with moderate to large exciting binding energies are found for all three nanoribbons, in contradiction to the prediction of tight-binding theory. The lowest two-photon gap in 3-ZGNR vanishes in the thermodynamic limit, while this gap is smaller than the one-photon gap in 5-AGNR. However, in 6-AGNR the one-photon gap is smaller than the two-photon gap and it is predicted to be fluorescent.

cond-mat.str-el↗

arXiv2016

Normal state of metal-intercalated phenacene crystals: Role of electron correlations

In this work we study the effect of long range electron-electron correlations on the behavior of the normal state of metal-intercalated phenacene crystals. While the individual phenacene molecules are modeled by the Pariser-Parr-Pople Hamiltonian with long range Coulomb interactions, we derive a correlated minimal model for describing the phenacene ionic crystals. We find that long range electron correlations do not change the behavior of the phenacene ions with molecular valence $-1$ (monoanion) and $-2$ (dianion), compared to that observed for short range electron interactions. The monoanion crystal is a single-band $\frac{1}{2}$-filled antiferromagnetic Mott-Hubbard semiconductor while the dianion crystal is a two-band semiconductor with inter-molecular antiferromagnetic and intra-molecular ferromagnetic spin orderings. However, the trianion crystal is no longer a nearly degenerate $\frac{3}{4}$-filled two-band system. We show that this occurs because the kinetic stability of the $\frac{3}{4}$-filled two-band system with long range correlations is smaller compared to that with short range correlations, for the lattice sizes considered in this study. We argue that with large finite lattices, behavior of the trianion crystal with long range correlations will be same as that with short range correlations. We thus conclude that long range correlations fail to alter the normal states of metal-intercalated phenacene crystals.

cond-mat.str-el↗

arXiv2015

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.

cond-mat.mes-hall↗