Steric hindrance

1. Pressure-induced Superconductivity of b-(meso-DMBEDT-TTF)2PF6and Related MaterialsISSP H.Mori, S. Kimura, H. Suzuki, J. Yamaura,H. Tajima, M. Koeda, M. Hedo, Y. UwatokoToho Univ. R. Chiba,Y. Nishio, K. KajitaTIT T. Kawamoto, T. MoriKEK T. Kakiuchi, H. Sawa

2. Introduction Introduction of steric hindrance Chemical negative pressure Chemical negative pressure Steric hindrance ET derivatives C5ET C6ET meso-DMBEDT-TTF

3. ET derivatives with steric hindrance C5ET C6ET C6 part perpendicular to the plane C5 part parallel to the plane →weak dimerization →dimer structure →metallic character →semicon.

4. ET derivatives with steric hindrance --- meso-DMeET Moderate dimerization dimethyl introduced to the ET molecule R axial S equatorial meso-DMeET l-FeCl4, l-GaCl4 b-PF6 b-AsF6, b-SbF6 b-TaF6

5. Crystal and band structures of b-(meso-DMBEDT-TTF)2PF6 0.03 eV (W = 0.9 eV) Overlap integral *dimeric b-type structure * two-dimensional -> similar to b-ET2I3 (W = 1.0 eV) r1 = 8.2 (x10-3) r2 = 22.6 p = -4.8 q1 = 4.4 q2 = 11.5 dimer

6. Resistivity and magnetic susceptibility of b-(meso-DMBEDT-TTF)2PF6 resistivity 16 Scm-1 susceptibility

7. Superlattice and lattice parameter for b-(meso-DMBEDT-TTF)2PF6 at low temperatures Mac Science DIP320V imaging plate, -0.5 K/min Superlattice (a*, b*/2, c*/2) 90 K 90 K

8. Lattice distortion of b-(meso-DMBEDT-TTF)2PF6 at 90 K 90 K P R P R dimer dimer Rigaku AFC7RIP, KEK(PF, BL-1A ), -1 K/min Room temperature 11.5 K (< 90 K) (a, b, c) (2c, -b+c, a) (h, k, l) superstructure(h, k/2, l/2)) V = 906 A3V = 1727A3

9. Crystal structure of b-(meso-DMBEDT-TTF)2PF6 at 11.5 K Checkerboard-type charge ordering (CO) 1. Molecular shape 2. Bond length charge rich d = 0.732 charge poor d = 0.790 3. HOMO level charge rich -9.25 eV charge poor -9.31 eV KEK (PF BL-1A) cf. ET3(MnCl4)2 ET2+ ET+ 0.17 eV

10. Crystal structure of b-(meso-DMBEDT-TTF)2PF6 at 11.5 K CO pattern 1. Charge disproportion within a dimer 2. ○RRPPRRPP → CBCO × RPRPRPRP → stripe CO →* V= (e2/4pe)(r2 + l2) r: intermolecular distance l: effective molecular size * electron-phonon interaction R R P P R R

11. Pressure dependence of resistivity for b-(meso-DMBEDT-TTF)2PF6 16 Scm-1 * Tc (onset) = 4.3 K under 4 kbar S. Kimura et al., Chem. Commun., 2454-2455(2004).

12. Resistivity and magnetic susceptibility of b-(meso-DMBEDT-TTF)2X (X = PF6 , AsF6 , SbF6 , TaF6 ) (Resistivity) (Magnetic susceptibility) TMI 10～ 20 Scm-1 ＴＭＩ＝90 Ｋ No anomaly atＴＭＩ Similar behaviors to PF6 salt

13. Pressure dependence of resistivity for b-(meso-DMBEDT-TTF)2X (X = PF6 , AsF6) X = AsF6 :New SC Tc = 4.3 K (3.8 kbar) X = PF6 Tc = 4.3 K (4 kbar)

14. Pressure dependence of resistivity for b-(meso-DMBEDT-TTF)2X (X = PF6 , AsF6) X = PF6 X = AsF6 Tc Tmin

15. Phase diagram of b-(meso-DMBEDT-TTF)2X (X = PF6 , AsF6) X = PF6 X = AsF6 DTc/DP = -0.6 K/kbar DTc/DP = -0.5 K/kbar cf. ET salts: DTc/DP = -0.8 ~ -1.3 K/kbar

16. Magnetoresistance of b-(meso-DMBEDT-TTF)2X (X = PF6 , AsF6) X = AsF6 X = PF6 P = 4 kbar P = 4 kbar Competition of CO and SC states

17. Phase diagram of b-(meso-DMBEDT-TTF)2X (X = PF6 , AsF6) X = PF6 X = AsF6 H H Competition of SC and CO states

18. Conclusion 1 Charge-ordered state of pressure-induced SC b-(meso-DMBEDT-TTF)2PF6 *Checkerboard-typeCO *CO within a dimer *SC by suppression of CO 2 Related materials *New SCb-(meso-DMBEDT-TTF)2AsF6 Tc = 4.3 K (3.8 kbar) *Magnetoresistance, Resistivity under pressure Competition of SC and CO states