Two Pressure-Induced Superconducting Anion Radical Salts Exhibiting Different Spin States at Ambient Pressure

Abstract

Pressure-induced superconducting behavior was found in two anion radical salts, EtMe3Z[Pd(dmit)2]2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate, Z = P, As), that are Mott insulators and exhibit different magnetic and structural transitions at ambient pressure.

Mott Transition in a Valence-Bond Solid Insulator with a Triangular Lattice

“…1 inset [8,10], while most of the Pd(dmit) 2 salts having anisotropic triangular lattice (0.55 < t ′ /t < 0.85) show antiferromagnetic order [9]. Application of hydrostatic pressure, which increases the bandwidth, induces a superconducting transition at T c = 5 K [8]. However, the nature of the insulator and metal phases under pressure and the Mott transition between them are less understood.…”

“…An anion radical salt EtMe 3 P[Pd(dmit) 2 ] 2 is the 2D VBS Mott insulator synthesized recently [8], where Et and Me denote C 2 H 5 and CH 3 respectively, and Pd(dmit) 2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate, C 3 S 2− 5 ) is an electron-acceptor molecule. Its crystal structure consists of two layers: the Pd(dmit) 2 layer that involves in conduction and magnetism, and the insulating closed-shell EtMe 3 P + layer.…”

“…The ground state is, however, settled into a spin-gapped phase by the lattice distortion via the spin-phonon coupling at 25 K, as shown in Fig. 1 inset [8,10], while most of the Pd(dmit) 2 salts having anisotropic triangular lattice (0.55 < t ′ /t < 0.85) show antiferromagnetic order [9]. Application of hydrostatic pressure, which increases the bandwidth, induces a superconducting transition at T c = 5 K [8].…”

Variety of valence bond states formed of frustrated spins on triangular lattices based on a two-level system Pd(dmit)2

“…The structures similar to, but not identical to, the β structure are found in the EtMe 3 P salt [16,17] and in the Me 4 N salts [36,37]. At least two polymorphs are found in the EtMe 3 P salt.…”

“…For example, the χ behaviour showing strong frustration effect is commonly observed in many Z [Pd(dmit) 2 ] 2 salts [2], whereas, among the κ-(BEDT-TTF) 2 X salts, only the Cu 2 (CN) 3 salt exhibits such behaviour [4], the other κ-salts being metals or paramagnetic insulators with significant antiferromagnetic correlation [5,6]. In the Z [Pd(dmit) 2 ] 2 series, in contrast, a variety of unconventional valence bond states [13][14][15][16][17] appear around the spin-frustrated state depending on the cation and pressure. For this reason, the Z [Pd(dmit) 2 ] 2 series occupy a unique position in molecular materials.…”