arXiv subjects

K. Iwano

Publications and source records attributed to K. Iwano.

4 recordsLinked to original sources

arXiv2023

New approach to extract important degrees of freedom in quantum dynamics using singular value decomposition: Application to linear optical spectrum in two-dimensional Mott insulators

We propose a new approach to extract the important degrees of freedom in quantum dynamics induced by an external stimulus. We calculate the coefficient matrix numerically, where the $i-l$ element of the matrix is the coefficient of the lth basis state at the ith discretized time in the solution of the time-dependent Schrödinger equation induced by the external stimulus. By performing a randomized singular value decomposition of the coefficient matrix, a practically exact solution is obtained using a linear combination of the important modes, where the number of modes is much smaller than the dimensions of the Hilbert space in many cases. We apply this method to analysis of the light absorption spectrum in two-dimensional (2D) Mott insulators using an effective model of the 2D Hubbard model in the strong interaction case. From the dynamics induced by an ultrashort weak light pulse, we find that the practically exact light absorption spectrum can be reproduced by as few as 1000 energy eigenstates in the $1.7 \times 10^{7}$-dimension Hilbert space of a 26-site cluster. These one-photon active energy eigenstates are classified into free holon and doublon (H-D) and localized H-D states. In the free H-D states, the main effect of the spin degrees of freedom on the transfer of a holon (H) and a doublon (D) is the phase shift, and the H and the D move freely. In the localized H-D states, an H and a D are localized with relative distances of $\sqrt{5}$ or $\sqrt{13}$. The antiferromagnetic (AF) spin orders in the localized H-D states are much stronger than those in the free H-D states, and the charge localization is of magnetic origin. There are sharp peaks caused by excitations to the localized H-D states below the broad band caused by excitations to the free H-D states in the light absorption spectrum.

cond-mat.str-el↗

arXiv2020

Excitonic optical spectra and energy structures in a one-dimensional Mott insulator demonstrated by applying a many-body Wannier functions method to a charge model

We have applied a many-body Wannier functions method to theoretically calculate an excitonic optical conductivity spectrum and energy structure in a one-dimensional (1D) Mott insulator at absolute zero temperature with large system size. Focusing on full charge fluctuations associated with pairs of a holon and doublon, we employ a charge model, which is interpreted as a good effective model to investigate photoexcitations of a 1D extended Hubbard model at half-filling in the spin-charge separation picture. As a result, the theoretical spectra with appropriate broadenings qualitatively reproduce the recent experimental data of ET-F$_{2}$TCNQ at 294 K with and without a modulated electric field. Regarding the excitonic energy structure, we have found that the excitons, especially for even-parity, are weakly bound by many-body effects. This is also consistent with the fitting parameters reported in the recent experiment. Thus, our theoretical method presented in this paper is practically useful to understand physical roles of charge fluctuations in many-body excited states of a 1D Mott insulator.

cond-mat.str-el↗

arXiv2019

Photoexcitation of a polarization-inverted domain from the charge-ordered ferroelectric ground state of (TMTTF)$_{2}$PF$_{6}$

We theoretically revealed that a weak photoexcitation achieves the electric polarization-inversion with approximately $18\%$ of all the charges, which was interpreted as a superimposition of multi-exciton states, from the charge-ordered ferroelectric ground state of (TMTTF)$_{2}$PF$_{6}$ at absolute zero temperature. Regarding a relative change of electric polarization ($ΔP/P$), the photoexcitation corresponds to $36\%$, which is much larger than $ΔP/P$ of other typical organic materials. The value of $ΔP/P\sim 36\%$ can be enlarged by a strong photoexcitation. This fact is useful not only for applications of this material and other analogous materials in optical devices but also for researches toward controlling electric polarizations by light, which is one of the recent attracting issues on photoinduced phase transition phenomena. The photoexcitation of $ΔP/P\sim 36\%$ corresponds to the single peak of the optical conductivity in the low-energy region, which was also observed at 10 K. Theoretical calculations are based on a quarter-filled one-dimensional effective model with appropriate parameters and 50 unit cells.

cond-mat.str-el↗

arXiv2018

The optical conductivity for a spin-Peierls ground state of (TMTTF)$_{2}$PF$_{6}$ with tetramer formation

We theoretically investigate the optical conductivity of (TMTTF)$_{2}$PF$_{6}$ in the spin-Peierls ground state within the framework of the exact diagonalization method at absolute zero temperature ($T=0$). As an effective model, a 1/4-filled 1D (one-dimensional) extended Hubbard model with tetramerization is employed. Using appropriate parameters of the model which have already been reported, we clarify the electronic photoexcitation energies from the spin-Peierls ground state. Since some experiments indicate the formation of a tetramer in the spin-Peierls ground state of (TMTTF)$_{2}$PF$_{6}$, our results are useful to understand the effects of tetramerization on the optical properties of (TMTTF)$_{2}$PF$_{6}$.

cond-mat.str-el↗