Density Wave State in New α-Type Organic Conductor, α-(BEDT-TTF)₂MHg(XCN)₄ (M = NH₄, X = Se): A Key Material for Universal Phase Diagram of X = S and X = Se Systems

Abstract

A family of organic conductors, α-(BEDT-TTF) 2 MHg(SCN) 4 , is known to show the density wave (DW) state for M = Tl, K, and Rb salts, or the superconducting (SC) state for M = NH 4 salt at low temperatures. In contrast, α-(BEDT-TTF) 2 MHg(SeCN) 4 shows no phase transition and retains its metallic characteristics down to low temperatures. Since no α-(BEDT-TTF) 2 MHg(SeCN) 4 salt shows the DW or SC states, it was unclear whether the system of α-(BEDT-TTF) 2 MHg(SeCN) 4 could be understood by the same phase diagram as that of α-(BEDT-TTF) 2 MHg(SCN) 4 . Here, we succeeded in synthesizing a key material, α-(BEDT-TTF) 2 NH 4 Hg(SeCN) 4 , and determined its crystal structure. The temperature dependence of its electric conductivity and spin susceptibility showed its DW state at low temperatures, indicating that the α-(BEDT-TTF) 2 MHg(SeCN) 4 system is linked to the α-(BEDT-TTF) 2 MHg(SCN) 4 system. From the established phase diagram, we found that the dihedral angle between crystallographically independent A and B molecules, Θ B , and that between A and C molecules, Θ C , are good tuning parameters as in θ-type BEDT-TTF salts. In the Θ B −Θ C plot, the SC salt is located in the large Θ B and small Θ C regions, whereas the metallic salts are located in the small Θ B and large Θ C regions. The DW salts are located in the intermediate Θ B and Θ C regions. The relationship between their location in the Θ B −Θ C plot and the ground states supports the prediction that the local density of state between A and B molecules determines the ground states.

C13–NMR study of charge fluctuations in α–(BEDT–TTF)MXOhnuma1, Kawamoto2 2019Phys. Rev. BSelf Cite1000View full textAdd to dashboardCite

“…2 D-NMR measurements suggested a change in motion of NH + 4 at 130 K and an orderdisorder like transition in the anion layer at 25-40 K [33]. However, the spin susceptibility of NH 4 (SeCN) indicated that the motion of NH + 4 had no effect on its electronic properties [25]. A detailed study of NH + 4 in NH 4 (SCN) and NH 4 (SeCN) is required to reveal its effect on electronic properties.…”

“…Chemical pressure may be more suitable to systematic NMR analyses. Recently, NH 4 (SeCN) was found to be in the DW state and to link the X = S system to the X = Se system [25]. In comparison, Tl(SeCN) was found to be in a simple metallic phase without a DW anomaly [26].…”

“…could be expressed as χ A (250 K):χ B (250 K):χ C (250 K) = 1:0.66:0.46, with the amplitude at the A site being the mean of the amplitudes of the A a and A b peaks, and with the amplitudes of the B and C peaks. From total spin susceptibilities measured using a SQUID magnetometer [χ SQUID = 1 All = 1 2 (2χ A + χ B + χ C )][25], we estimated the local spin susceptibilities to be χ A (250 K) = 6.7 × 10 −5 emu/mol, χ B (250 K) = 4.4 × 10 −5 emu/mol, and χ C (250 K) = 3.1 × 10 −5 emu/mol. Using the chemical shift of 59.8 ppm for A site at 54 • and those of 60.2 ppm for the B site and 58.3 ppm for the C site at 127 • , the hyperfine coupling constants of Tl(SeCN) were determined to be A Aa = 15.2 kOe/μ B and A Ab = 19.4 kOe/μ B at θ = 54 • , A B = 18.4 kOe/μ B , and A C = 17.7 kOe/μ B at θ = 127 • .…”