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.
We investigated a system based solely on a single molecular species, Cu(tmdt) 2 , accommodating d and π orbitals within the molecule.
The orbital of electrons in a solid affects their mutual interaction, which is a key to emergent phenomena. Therefore, control of the interplay between distinct orbitals makes the behavior of solids fertile. Such a case is substantiated by the aggregation of the molecule, M(tmdt) 2 , which contains π orbitals extended over the organic ligand, tmdt, and a level-tunable d orbital centered at the metal ion M. Among them, Au(tmdt) 2 exemplifies the critical π-d mixing, which arguably causes an over-100 K antiferromagnetic metal whose nature remains elusive. Here, we track the π-d interplay in Au(tmdt) 2 with dual orbital-resolved probes, 13 C NMR and synchrotron x-ray diffraction. We find substantial intramolecular (interorbital) redistribution of electrons on cooling, triggering a commensurate π-d antiferromagnetic metal to emerge, which invokes an orbital-selective doped Mott insulator caused by intramolecular self-doping. This demonstrates that vital traffic of electrons between distinct orbitals brings about an unique correlated phase.
Abstract13 C nuclear magnetic resonance measurements were performed for a single-component molecular material Zn(tmdt) 2 , in which tmdt's form an arrangement similar to the so-called κ-type molecular packing in quasi-two-dimensional Mott insulators and superconductors. Detailed analysis of the powder spectra uncovered local spin susceptibility in the tmdt π orbitals. The obtained shift and relaxation rate revealed the singlet-triplet excitations of the π spins, indicating that Zn(tmdt) 2 is a spin-gapped Mott insulator with exceptionally large electron correlations compared to conventional molecular Mott systems. 1A new type of molecular conductors M(tmdt) 2 (M = Ni, Au, Pt, Cu, and Pd) are of current interest not only because it is the first molecular metal composed of a single molecular species 1 but also affords novel electronic states owing to the multi-orbital nature, which conventional charge-transfer salts do not possess. Furthermore, the systems of M(tmdt) 2 are expected to host highly correlated electrons compared with conventional molecular materials with dimeric molecular structures called 2 : 1 compounds, where a dimer playing as one lattice site accommodates one carrier and thus the on-site Coulomb repulsion U is reduced by the spatial extension of the dimer orbital. In case that one molecule accommodates one carrier as in some of the systems M(tmdt) 2 , the U is not reduced by the dimerization.The M(tmdt) 2 consists of a single molecular species in which a transition-metal ion, M, is coordinated by molecular ligands, tmdt, from both sides 2,3 . The pπ molecular orbitals extended over the tmdt ligand lie near the Fermi level, whereas the energy level of the dpσ molecular orbital located around M strongly depends on M. 4-7 For M = Cu, the dpσ orbital lies close to the pπ orbitals, leading to the formation of a dpσ-pπ multi-orbital system 5,7 ;indeed, NMR studies of Cu(tmdt) 2 found that the dpσ and pπ orbitals host antiferromagnetic and spin-gapped Mott subsystems, respectively 8,9 . For M = Ni, Pt and Zn, the dpσ orbitals lie away (upward in the former two and downward in the latter) from the two pπ orbitals located near the Fermi level 4,10 . In these systems, two pπ orbitals in M(tmdt) 2 accommodate two electrons, likely giving pπ-band semimetals; the ab-initio band structure calculation predicts small Fermi pockets 4,10,11 . As expected, Ni(tmdt) 2 and Pt(tmdt) 2 are metals 1,10,12 and appreciable electron correlation was revealed by 13 C nuclear magnetic resonance (NMR) studies 13 .It is to be noticed, however, that Zn(tmdt) 2 is an insulator 14 ; the typical resistivity data are shown in Fig. 1 Fig. 1(a). In the 2 solid, two tmdt ligands are arranged face-to-face and they form a checker-board pattern in two-dimensional layers (Figs. 1(b) and (c)). This type of molecular arrangement is similar to the "κ-type configuration", which is familiar in the 2 : 1 charge-transfer salts represented by κ-(ET) 2 X, including dimer-Mott insulators and over-10-K superconductors 15 . Most remarkably, Zn(tmdt)...
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