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It has been more than 40 years since superconductivity was discovered in organic conductors, and the way scientists view organic superconductors has changed over time. At first, the fact that organic conductors exhibit superconductivity was a novelty in itself, and subsequently it was shown that behind the superconductivity is the physics of electron correlation, which has been a focus in condensed matter physics at large. Amid the remarkable development of correlation physics, the unique characteristics of organic conductors, e.g., a variety of lattice geometries and the highly compressible feature, led to the elucidation of fundamental principles and the finding of new phenomena, such as bandwidth-controlled Mott transitions and possible quantum spin liquids. However, most organic superconductors have commensurate band fillings, such as a half or a quarter, whereas inorganic superconductors, such as high-$T_{\rm c}$ cuprates and iron-based superconductors, have often been investigated under the variation of their band fillings. Thus, the physical linkage between organic and inorganic superconductors has remained unresolved. In this review article, we focus on the layered nonstoichiometric superconductor, $κ$-(BEDT-TTF)$_4$Hg$_{2.89}$Br$_8$, which is exceptional among organic conductors in that the nonstoichiometry serves as doping to a half-filled band. Moreover, the strong correlation of electrons and a geometrically frustrated triangular lattice make this system exhibit the unique phenomena involved in Mottness, spin liquidity, and superconductivity, which are key concepts of correlated electron physics. This review will summarize what we learned from the pressure study of $κ$-(BEDT-TTF)$_4$Hg$_{2.89}$Br$_8$ and how they relate to the extensively studied issues in inorganic materials.

Glass, a ubiquitous state of matter like a frozen liquid, is a seminal issue across fundamental and applied sciences and has long been investigated in the framework of classical mechanics. A challenge in glass physics is the exploration of the quantum-mechanical behaviour of glass. Experimentally, however, the real quantum manifestation of glass and the relationship between classical and quantum glass are totally unknown and remain to be observed in real systems. Here, we report the direct observation of classical-to-quantum evolution in the frustration-induced charge glass state exhibited by interacting electrons in organic materials. We employ Raman spectroscopy to capture a snapshot of the charge density distribution of each molecule in a series of charge glasses formed on triangular lattices with different geometrical frustrations. In less frustrated glass, the charge density profile exhibits a particle-like two-valued distribution; however, it becomes continuous and narrowed with increasing frustration, demonstrating the classical-to-quantum crossover. Moreover, the charge density distribution shows contrasting temperature evolution in classical and quantum glasses, enabling us to delineate energy landscapes with distinct features. The present result is the first to experimentally identify the quantum charge glass and show how it emerges from classical glass., Comment: 33 pages including 13 figures

The Dirac nodal line (DNL) is a novel form of massless Dirac fermions that reside along lines in momentum space. Here, we verify genuine DNLs in the molecular material, [Ni(dmdt)$_2$], with the combined NMR experiments and numerical simulations. The NMR spectral shift and spin-lattice relaxation rate divided by temperature, $1/T_1T$, decrease linearly and quadratically with temperature, respectively, and become constant at low temperatures, consistent with slightly dispersive DNLs with small Fermi pockets. Comparison of these results with model simulations of DNLs reveals the suppression of the Fermi velocity and the enhancement of antiferromagnetic fluctuations due to electron correlation as well as the influence of the Landau quantization. The present study offers a demonstration to identify the DNL and evaluate the correlation effect with NMR., Comment: 6 pages, 3 figures

Identifying the host genetic factors underlying severe COVID-19 is an emerging challenge. Here we conducted a genome-wide association study (GWAS) involving 2, 393 cases of COVID-19 in a cohort of Japanese individuals collected during the initial waves of the pandemic, with 3, 289 unaffected controls. We identified a variant on chromosome 5 at 5q35 (rs60200309-A), close to the dedicator of cytokinesis 2 gene (DOCK2), which was associated with severe COVID-19 in patients less than 65 years of age. This risk allele was prevalent in East Asian individuals but rare in Europeans, highlighting the value of genome-wide association studies in non-European populations. RNA-sequencing analysis of 473 bulk peripheral blood samples identified decreased expression of DOCK2 associated with the risk allele in these younger patients. DOCK2 expression was suppressed in patients with severe cases of COVID-19. Single-cell RNA-sequencing analysis (n = 61 individuals) identified cell-type-specific downregulation of DOCK2 and a COVID-19-specific decreasing effect of the risk allele on DOCK2 expression in non-classical monocytes. Immunohistochemistry of lung specimens from patients with severe COVID-19 pneumonia showed suppressed DOCK2 expression. Moreover, inhibition of DOCK2 function with CPYPP increased the severity of pneumonia in a Syrian hamster model of SARS-CoV-2 infection, characterized by weight loss, lung oedema, enhanced viral loads, impaired macrophage recruitment and dysregulated type I interferon responses. We conclude that DOCK2 has an important role in the host immune response to SARS-CoV-2 infection and the development of severe COVID-19, and could be further explored as a potential biomarker and/or therapeutic target., 「コロナ制圧タスクフォース」COVID-19疾患感受性遺伝子DOCK2の重症化機序を解明 --アジア最大のバイオレポジトリーでCOVID-19の治療標的を発見--. 京都大学プレスリリース. 2022-08-10.

The spin excitations from the nonmagnetic charge-ordered insulating state of $��$-(BEDT-TTF)$_2$I$_3$ at ambient pressure have been investigated by probing the static and low-frequency dynamic spin susceptibilities via site-selective nuclear magnetic resonance at $^{13}$C sites. The site-dependent values of the shift and the spin-lattice relaxation rate $1/T_1$ below the charge-ordering transition temperature ($T_{CO} \approx$ 135 K) demonstrate a spin density imbalance in the unit cell, in accord with the charge-density ratio reported earlier. The shift and $1/T_1$ show activated temperature dependence with a static (shift) gap $��_S \approx$ 47-52 meV and a dynamic ($1/T_1$) gap $��_R \approx$ 40 meV. The sizes of the gaps are well described in terms of a localized spin model, where spin one-half antiferromagnetic dimer chains are weakly coupled with each other., 22 pages, 7 figures, and 1 table

The molecular conductors [M(tmdt)$_2$] (M=Ni, Pt) consisting of single molecular species are investigated with $^{13}$C NMR and $^1$H NMR. The temperature dependences of $^{13}$C NMR shift and relaxation rate provide microscopic evidences for the metallic nature with appreciable electron correlations. Both compounds exhibit an anomalous frequency-dependent enhancement in $^1$H nuclear spin-lattice relaxation rate in a wide temperature range. These observations signify the presence of extraordinary molecular motions with low energy excitations., Comment: 14 pages, 11 figures

We report a $^{13}$C-NMR study on the ambient-pressure metallic phase of the layered organic conductor $��$-(BEDT-TTF)$_{2}$I$_{3}$ [BEDT-TTF: bisethylenedithio-tetrathiafulvalene], which is expected to connect the physics of correlated electrons and Dirac electrons under pressure. The orientation dependence of the NMR spectra shows that all BEDT-TTF molecules in the unit cell are to be seen equivalent from a microscopic point of view. This feature is consistent with the orthorhombic symmetry of the BEDT-TTF sublattice and also indicates that the monoclinic $I_{3}$ sublattice, which should make three molecules in the unit cell nonequivalent, is not practically influential on the electronic state in the conducting BEDT-TTF layers at ambient pressure. There is no signature of charge disproportionation in opposition to most of the $��$-type BEDT-TTF salts. The analyses of NMR Knight shift, $K$, and the nuclear spin-lattice relaxation rate, $1/T_{1}$, revealed that the degree of electron correlation, evaluated by the Korringa ratio [$\varpropto 1/(T_{1}TK^{2}$)], is in an intermediate regime. However, NMR relaxation rate $1/T_{1}$ is enhanced above $\sim$ 200K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically., 19pages, 6figures

The conducting state of the quasi-two-dimensional organic conductor, $��$-(BEDT-TTF)$_2$I$_3$, at ambient pressure is investigated with $^{13}$C NMR measurements, which separate the local electronic states at three nonequivalent molecular sites (A, B, and C). The spin susceptibility and electron correlation effect are revealed in a locally resolved manner. While there is no remarkable site-dependence around room temperature, the local spin susceptibility gradually disproportionates among the nonequivalent sites with decreasing temperature. The disproportionation-ratio yields 5:4:6 for A:B:C molecules at 140 K. Distinct site- and temperature-dependences are also observed in the Korringa ratio, $\mathcal{K}_i \propto (1/T_{1}T)_iK^{-2}_i$ ($i$ = A, B, and C), which is a measure of the strength and the type of electron correlations. The values of $\mathcal{K}_i$ point to sizable antiferromagnetic spin correlation. We argue the present results in terms of the theoretical prediction of the peculiar site-specific reciprocal-space ($\bm{k}$-space) anisotropy on the tilted Dirac cone, and discuss the $\bm{k}$-dependent profiles of the spin susceptibility and electron correlation on the cone., 12 figures

The static and dynamic local spin susceptibility of the organic Mott insulator $��$-(ET)$_2$Cu$_2$(CN)$_3$, a model material of the spin- 1/2 triangular lattice, is studied by $^{13}$C NMR spectroscopy from room temperature down to 20 mK. We observe an anomalous field-dependent spectral broadening with the continuous and bipolar shift distribution, appearing without the critical spin fluctuations. It is attributable to spatially nonuniform magnetizations induced in the spin liquid under magnetic fields. The amplitude of the magnetization levels off below 1 K, while the low-lying spin fluctuations survive toward the ground state, as indicated by the temperature profile of the relaxation rates., 4 pages, 4 figures