From charge- and spin-ordering to superconductivity in the organic charge-transfer solids

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

https://www.mdpi.com/2073-4352/11/7/838/pdf

Modern History of Organic Conductors: An Overview

This study concerns development of a non-destructive method to control conduction and magnetism of molecular solids such as single crystals of charge-transfer complexes. The method is named “optical doping”, where appropriate irradiation is utilized under ambient conditions. Owing to this feature, it can be applied to a wide range of substances while measuring the properties during the control. In addition, the method adds unique conduction and magnetic properties to common insulators. Unlike other doping methods, optical doping only affects the properties and/or structures of the irradiated part of a sample while leaving the rest of the sample unchanged. There are two patterns in the optical doping. Irreversible optical doping produces junction-structures on the single molecular crystals, which exhibit characteristic behavior of semiconductor devices such as diodes and varistors. Reversible optical doping produces “giant photoconductors” and “photomagnetic conductors” by realizing unprecedented metallic ...

Development of a Control Method for Conduction and Magnetism in Molecular Crystals

We review recent progress in understanding the different spatial broken symmetries that occur in the normal states of the family of charge-transfer solids (CTS) that exhibit superconductivity (SC), and discuss how this knowledge gives insight to the mechanism of the unconventional SC in these systems We show that a unified theory of the diverse broken symmetry states necessarily requires explicit incorporation of strong electron-electron interactions and lattice discreteness, and most importantly, the correct bandfilling of one-quarter Uniquely in the quarter-filled band, there is a very strong tendency to form nearest neighbor spin-singlets, in both one and two dimensions The tendency to spin-singlets, a quantum effect, drives a commensurate charge-order in the correlated quarter-filled band This charge-ordered spin-singlet, which we label as a paired-electron crystal (PEC), is different from and competes with both the antiferromagnetic state and the Wigner crystal of single electrons Further, unlike these classical broken symmetries, the PEC is characterized by a spin gap The tendency to the PEC in two dimensions is enhanced by lattice frustration Following this characterization of the spatial broken symmetries, we critically reexamine spin-fluctuation and resonating valence bond theories of frustration-driven SC within half-filled band Hubbard and Hubbard-Heisenberg Hamiltonians for the superconducting CTS We develop a valence-bond theory of SC within which the superconducting state is reached by the destabilization of the PEC by additional pressure-induced lattice frustration that makes the spin-singlets mobile Our proposed mechanism for SC is the same for CTS in which the proximate semiconducting state is antiferromagnetic instead of charge-ordered, with the only difference that SC in the former is generated via a fluctuating spin-singlet state as opposed to static PEC

Solid state high-resolution NMR of 1H and 13C along with 15N is analyzed to investigate the electronic states of the charge transfer salts (DMe-DCNQI)2M, (M=Li, Ag, and Cu). We determined the spin/charge distribution in a DMe-DCNQI molecule of the Li-salt from the Knight shifts at each atom on the molecule. It is found that the obtained charge distribution is similar to the theoretical prediction. The charge density on the DCNQI molecules of the Ag-salt is found to be smaller by 20% than the Li-salt, which could be an origin of differences from the Li-salt. This result is consistent with the first principle calculations (Miyazaki and Terakura, Phys. Rev. B 54, 10452, 1996).

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Determination of the π-charge distribution of the DMe-DCNQI molecule in (DMe-DCNQI)2M, M=Li, Ag, and Cu

We experimentally determined the spin density distribution to investigate the electronic states of the ${(\mathrm{DMe}\text{\ensuremath{-}}\mathrm{DCNQI})}_{2}M$ systems, where $M$ is Li, Ag, and Cu. The Knight shift is measured with solid-state high-resolution NMR of $^{1}\mathrm{H}$ and $^{13}\mathrm{C}$ and is analyzed together with the reported $^{15}\mathrm{N}$ data. The Knight shift data at each atom of the DMe-DCNQI molecule give the $\ensuremath{\pi}$-electron spin distribution of the Li salt, which might correspond to the charge distribution in the insulating and localized spin system of the Li salt. The average fraction of the spin susceptibility ${\ensuremath{\chi}}_{\mathrm{DCNQI}}∕{\ensuremath{\chi}}_{0}$ in the DMe-DCNQI molecule of the Ag and Cu salts relative to that of the Li salt is obtained to be 0.82 and 0.71, respectively. Thus, if the valence of the Ag ion is unity, the DMe-DCNQI molecule of the Ag salt has the missing spin fraction by 0.18. One possible origin of this missing spin is a modification of the lowest unoccupied molecular orbital of the DMe-DCNQI molecule due to the $\ensuremath{\pi}\text{\ensuremath{-}}d$ hybridization with the $\mathrm{Ag}\phantom{\rule{0.2em}{0ex}}{d}_{xy}$ orbitals, which could be one of the origins for the difference of the physical properties from the Li salt, as in the enhancement of the charge hopping rate between the neighboring DMe-DCNQI columns in the Ag salt. In the Cu salt case, the missing spin fraction of 0.29 would correspond to the $d$-spin hole number of 0.28 of the $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}{d}_{xy}$ orbital. Finally, it is stressed that all the present results are consistent with both the first-principles calculation and the X-ray photoelectron spectroscopy study.

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Yukie Shinohara

Papers

Determination of the π-charge distribution of the DMe-DCNQI molecule in (DMe-DCNQI)2M, M=Li, Ag, and Cu

Spin density distribution and electronic states in ( DMe − DCNQI ) 2 M ( M = Li , Ag , Cu ) from high-resolution solid-state NMR