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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Evaluation of the spectral function for the chain nanostructures TTF-TCNQ

Edris Faizabadi, Maryam Soleimani

Published Sep 1, 2007• 0 citations
Publisher

Abstract

The photoemission spectral function for organic charge transfer salt TTF-TCNQ is calculated with a new, simple and efficient method. This method is presented for calculation of the ground and a few excited states of Hubbard chain nanostructures.
DOI
10.1109/icimw.2007.4516617

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A new, simple and efficient method is presented for calculation of the ground and a few excited states of Hubbard chain nanostructures. By using this method, the photoemission spectral function for organic charge transfer salt TTF-TCNQ, is calculated. For a chain with maximum 70 sites the result is in good agreement with the previous works but there is a difference for further number of sites in the chain, which is discussed in the text with all the specifics. We also show that a source of errors in density matrix renormalization group method for a one dimensional chain is the deficiency of the matrix product scheme for generating the desired states of the linear chain.
We present a method for the calculation of few-electron spectral functions of the one-dimensional Hubbard model which relies on a pseudofermion description introduced recently in Ref. [6]. The spectral functions are expressed as a convolution of pseudofermion dynamical correlation functions. Our general method involves the direct evaluation of the matrix elements of pseudofermion operators between the ground state and the excited states. We briefly discuss the application of our general method to the study of the unusual finite-energy spectral properties observed in the quasi-one-dimensional organic conductor TTF-TCNQ.
We calculate the photoemission spectral function of the one-dimensional Hubbard model away from half filling using the dynamical density-matrix renormalization group method. An approach for calculating momentum-dependent quantities in finite open chains is presented. Comparison with exact Bethe ansatz results demonstrates the unprecedented accuracy of our method. Our results show that the photoemission spectrum of the quasi-one-dimensional conductor TTF-TCNQ provides evidence for spin-charge separation on the scale of the conduction bandwidth.
The optical absorption of the one-dimensional Hubbard model is calculated by three different methods. Linear chains of five atoms and rings of five and seven atoms containing four electrons are treated numerically. The infinite-chain problem is solved first in the ttt-matrix approximation of Lyo and Holstein. It is shown that in this approximation, most of the high-frequency absorption is due to a bound state which lies above the band continuum. Finally, the absorption is evaluated in the memory-function formalism of G\"otze and W\"olfle, which reduces to ordinary perturbation theory at high frequency. The three approaches are in qualitative agreement, and the differences between them can be explained by the nature of the approximations involved. Applications to tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and the platinum salt K2{\mathrm{K}}_{2}K2​Pt(CN)4{(\mathrm{CN})}_{4}(CN)4​Br0.3{\mathrm{Br}}_{0.3}Br0.3​ \ifmmode\cdot\else\textperiodcentered\fi{} 3H2{\mathrm{H}}_{2}H2​O (KCP) are discussed.
We present a simple and efficient wave function ansatz for the treatment of excited charge-transfer states in real-space quantum Monte Carlo methods. Using the recently introduced variation-after-response method [E. Neuscamman, J. Chem. Phys. 145, 081103 (2016)], this ansatz allows a crucial orbital optimization step to be performed beyond a configuration interaction singles expansion, while only requiring calculation of two Slater determinant objects. We demonstrate this ansatz for the illustrative example of the stretched LiF molecule, for a range of excited states of formaldehyde, and finally for the more challenging ethylene-tetrafluoroethylene molecule.