Spin-gapped Mott insulator with the dimeric arrangement of twisted molecules Zn(tmdt) 2

Abstract

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)...

NMR verification of Dirac nodal lines in a single-component molecular conductor

“…3(b)). Similar behavior is observed in [Zn(tmdt) 2 ], in which χ s increases with T due to triplet excitations from a singlet ground state [25], and attributed to different T -dependences of the principal values of the NMR shift tensor comprised of chemical shift and T -dependent spin shift. This is likely the case in the present system as well.…”

“…The ratio v F /v F = 0.58 is nearly equal to 4t a /∆E = 0.59 when 4t a = 17 meV, consistently indicating a reduction in the band width. The C iso value is reasonably close to C iso of 126 and 114 ppm, respectively, in [Zn(tmdt) 2 ] and [Au(tmdt) 2 ] with tmdt ligand of common molecular frame to dmdt [25,27]. 1/T 1 T in the DF regimes above T cr is nearly solely determined by v F .…”