Project Details
Projekt
Formation of the excitonic insulator state in Ta2NiSe5
Applicant
Dr. Satoshi Ejima
Subject Area
Theoretical Condensed Matter Physics
Term
from 2020 to 2023
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 433553211
More than half a century ago, theory predicts that electrons and holes will pair in proximity to a semiconductor-semimetal transition and even may form a macroscopically quantum coherent state at low temperatures, the so-called excitonic insulator (EI). But only recently there has been strong experimental evidence for such an equilibrium condensation phenomenon in a bulk material, mainly in systems with reduced electronic dimension. The most promising compound in this respect is quasi-one-dimensional (1D) Ta_2NiSe_5 (TNS), where the distinct flattening of the valence-band top observed in the angle-resolved photoemission spectroscopy below the structural transition temperature (TT_c. Therefore the main objective of this project is to construct first an appropriate microscopic model for TNS, which takes into account the correct electronic structure and both electron-electron and electron-phonon interactions, and solve, in a second step, this model using unbiased numerical techniques such as the density-matrix renormalization group (DMRG) techniques and its further developments, which is possible for a quasi-1D setting. By utilizing a time-evolution scheme in matrix-product-state representation, dynamical quantities can be simulated with high accuracy, which allows for a direct, even quantitative comparison between experiment and theory. A key to understand the formation and condensation of excitons in this material is the optical conductivity, showing a peculiar low-energy peak in experiments, which cannot be reproduced by standard density-functional-theory calculations. We are convinced that we can figure out the origin of this low-energy peak within our DMRG-based scheme, clarifying thereby the interplay between electron-electron and electron-phonon interaction, against the background of partly contradictory theoretical and experimental statements. Most notably, we will develop a numerical scheme that will enable us to describe very recent (non-equilibrium) pump-probe experiments on TNS with a view to prove or disprove that TNS develops an EI state for T
DFG Programme
Research Grants
Co-Investigators
Professor Dr. Holger Fehske; Professor Dr. Hidenori Takagi
