Small Fermi Pocket in Layered Organic Superconductor -(BDA-TTP) 2 SbF 6 Syuma YASUZUKA  , Hiroaki KOGA, Yasuhisa YAMAMURA, Kazuya SAITO, Shinya UJI 1 , Taichi TERASHIMA 1 , Hirohito AIZAWA 2 , Kazuhiko KUROKI 3 , Masahisa TSUCHIIZU 4 , Hiroki AKUTSU 5 , and Jun-ichi YAMADA 5 Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan 1 National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0003, Japan 2 Institute of Physics, Faculty of Engineering, Kanagawa University, Yokohama 221-8686, Japan 3 Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan 4 Department of Physics, Nagoya University, Nagoya 464-8602, Japan 5 Department of Material Science, Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan (Received December 22, 2011; accepted January 30, 2012; published online February 29, 2012) KEYWORDS: -(BDA-TTP) 2 SbF 6 , AMRO, Fermi surface, ab initio band-structure calculation -(BDA-TTP) 2 SbF 6 is a quasi-two-dimensional (Q2D) organic superconductor with T c ¼ 6:5 K, where BDA-TTP stands for 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapen- talene. 1) The crystal structure has a triclinic symmetry and the arrangement of the BDA-TTP donor molecules is of -type. 1) According to the tight-binding band-structure calculation, a nearly isotropic cylindrical Fermi surface (FS) should exist, which is characteristic of -type organic conductors. 1) However, previous angular-dependent magnetoresistance oscillation (AMRO) measurement at the high field of 33 T 2) showed that the FS is significantly more ellipsoidal than that determined by the tight-binding band calculation. The maximum Fermi wave vector extends over the Brillouin zone; the energy bands cross the zone boundaries. The energy bands are likely gapped at the zone boundaries, forming a small closed pocket. In this case, AMRO measurement at lower fields will enable us to determine the presence of a small closed pocket unambiguously because a possible magnetic breakdown (MB) effect is strongly suppressed. Moreover, ab initio band-structure calculations will provide more fruitful discussion on the FS structure than the tight- binding band calculation. In this paper, we report the AMRO at 14.8 T for -(BDA-TTP) 2 SbF 6 , and compare the results with those of ab initio band-structure calculations. The single crystals were synthesized electrochemically. 1) The crystal orientation was carefully determined by X-ray diffraction. The interlayer resistance was measured by a four-probe ac technique with electric current along the b  -axis, which is normal to the highly conducting a–c plane. The samples were mounted on a two-axis rotator in a 4 He cryostat with a 17 T superconducting magnet. The residual resistivity ratio (300 K)/(10 K) exceeds 300, showing very high quality. Figure 1 shows the typical AMRO results at 1.7 K for H ¼ 14:8 T. The angles  and  are defined in the inset. For  ¼ 0  , sharp dips due to the superconducting transition are evident at  90  . In the wide-field-angle region, the oscillatory structure is clearly seen. In previous studies, AMRO was observed only above 25 T at 50 mK. 2) We successfully observe clear AMRO behavior in fields down to 9 T using a single crystal of very high quality. The AMRO in Q2D conductors is known as a powerful tool to probe their FSs. 3–6) The periodicity  of the AMRO peaks as a function of tan  is given by ðÞ¼ =dk k ðÞ, 3) where d is the interlayer spacing and the projection of the wave vector k k ðÞ is 6) k k ðÞ 2 ¼½k min F cosð  Þ 2 þ½k max F sinð  Þ 2 ; ð1Þ where k min F ðk max F Þ is the minimum (maximum) Fermi wave vector, and  is the inclination of the minor axis from [101]. Figure 2 shows the polar plot of k k determined by ðÞ. The solid line is the result fitted by eq. (1) with k min F ¼ 0:12  A 1 , k max F ¼ 0:51  A 1 , and  ¼ 9  . Assuming an ellipsoidal FS cross section, we obtain the area k min F k max F  0:19  A 2 , corresponding to about 25% of the first Brillouin zone. This cross-sectional area is significantly small, only half the Fig. 1. Angular-dependent magnetoresistance oscillation (AMRO) at 1.7 K for H ¼ 14:8 T. Fig. 2. Polar plot of k k determined by ðÞ. The solid line is the result fitted by eq. (1) with k max F ¼ 0:51  A 1 , k min F ¼ 0:12  A 1 , and  ¼ 9  . The thick solid line represents the corresponding FS.  Present address: Faculty of Engineering, Hiroshima Institute of Technol- ogy, Hiroshima 731-5193, Japan. E-mail: yasuzuka@cc.it-hiroshima.ac.jp Journal of the Physical Society of Japan 81 (2012) 035006 035006-1 SHORT NOTES #2012 The Physical Society of Japan DOI: 10.1143/JPSJ.81.035006