Observation of Three-Dimensional Fermi Surfaces in a Single-Component Molecular Metal, [Ni(tmdt) 2 ] Hisashi Tanaka, † Madoka Tokumoto, †,¶ Shoji Ishibashi, ‡ David Graf, § Eun Sang Choi, § James S. Brooks, § Syuma Yasuzuka, $ Yoshinori Okano, # Hayao Kobayashi,* ,#,¶ and Akiko Kobayashi | Nanotechnology Research Institute, AIST, Tsukuba, Ibaraki 305-8568, Japan, Research Institute for Computational Sciences, AIST, Tsukuba, Ibaraki 305-8568, Japan, National High Magnetic Field Laboratory and Physics Department, Florida State UniVersity, Tallahassee, Florida 32310, National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan, Institute for Molecular Science, Okazaki 444-8585, Japan, Research Centre for Spectrochemistry, Graduate School of Science, The UniVersity of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan, and CREST, Japan Science and Technology Corporation, Saitama 332-0012, Japan Received May 26, 2004; E-mail: hisashi.tanaka@aist.go.jp Recently considerable attention has been focused on unconven- tional conducting molecular systems such as nanowires, 1 molecular wire junctions, 2 and even DNA. 3 Though the number of intriguing reports on new types of molecular systems are rapidly increasing, there seem to be many systems where the origins of the charge carriers and/or transport mechanisms still remain unclear. We have reported that the single-component molecular crystal consisting of neutral metal complex molecules [Ni(tmdt) 2 ] exhibits metallic behavior down to 0.6 K. 4,5 The crystal has a very simple structure with only one [Ni(tmdt) 2 ] molecule in the unit cell, where all the centers of molecules are on the lattice points. In addition, the existence of many intermolecular contacts much shorter than the van der Waals contacts 5 suggests that in the crystal, the [Ni(tmdt) 2 ] molecules assemble by interactions stronger than the van der Waals interactions, that is, metallic bonds. Needless to say, neutral molecular crystals and metallic crystals have been regarded as two typical but contrasting types of crystals. To prove the existence of a new class of single-component molecular crystals which possess simultaneously the character of molecular crystals and metallic crystals, experimental evidence for Fermi surfaces is essential. To obtain direct evidence for the existence of Fermi surfaces in [Ni(tmdt) 2 ], we have carried out experiments to measure magnetic quantum oscillations, namely de Haas-van Alphen (dHvA) oscil- lations, in high magnetic fields at low temperatures. 6,7 Single crystals of [Ni(tmdt) 2 ] were prepared electrochemically from acetonitrile solution containing ((CH 3 ) 4 N) 2 [Ni(tmdt) 2 ]. Black tiny platelike single crystals with maximum dimension of about 100 μm were grown on a platinum electrode. Methods of torque magnetometry using sensitive piezoresistive cantilevers have been previously reported. 8 To measure tiny crystals (of order 130 × 100 × 20 μm 3 , and 0.5 μg in mass), we employed a commercially available microcantilever for atomic force micro- scope (AFM) 9-11 (see Figure 1a). A simple resistance bridge circuit was used to cancel the background resistance of the two piezo- resistive sensing cantilevers on the AFM assembly. Measurements were carried out at temperatures down to 0.5 K in dc magnetic fields up to 45 T using the hybrid magnet at the National High Magnetic Field Laboratory at Florida. A total of four samples have been studied using a sample rotator, and all have given similar results. Representative experimental torque signals are shown in Figure 1b. Clear, angular-dependent dHvA oscillations were observed, which provided the first unambiguous evidence for the existence of Fermi surfaces in the single-component molecular crystal. Analysis of the temperature dependence of the amplitude of the dHvA oscillations through the Lifshitz-Kosevich formula led to the carrier effective masses, 12,13 which ranged between 1.0 and 1.6 free electron masses depending on field direction. We also found their impurity limited mean free paths through the Dingle temperature, T D ) 4.7 K. In certain directions, as shown in Figure 1b, the signal intensities of dHvA oscillations do not increase monotonically with increasing magnetic field, and the Fourier transform reveals the presence of more than two frequencies. This systematic angular-dependent Fermiological study allowed the mapping of the extremal areas A k of the Fermi surface with respect to the three-dimensional Brillouin zone unit cell. 14 The dHvA signal is clearly seen in all directions, showing that [Ni(tmdt) 2 ] is the 3D metal, as suggested from the tight-binding band calculations. 4,5 To compare the experimental results with electronic structure calculations, we have carried out local density approximation (LDA) calculations based on the ab initio plane-wave norm-conserved pseudopotential method with Troullier-Martins potentials and a cutoff energy at 110 Ry, which gave a band structure consistent with the previous calculations by Rovira et al. 15 We have recently found that the revised extended-Hu ¨ckel tight-binding band calcula- tions gave essentially the same topology for the Fermi surfaces. † Nanotechnology Research Institute, AIST. ‡ Research Institute for Computational Sciences, AIST. § NHMFL/Florida State University. $ National Institute for Materials Science. # Institute for Molecular Science. | The University of Tokyo. ¶ CREST-JST. Figure 1. Torque magnetometry of [Ni(tmdt)2]. (a) Microcrystal on AFM cantilever, (b) An example of raw torque magnetometer signals versus the applied magnetic field at 1.44 K. θ is the angle between the -a* direction and external magnetic field. Published on Web 08/07/2004 10518 9 J. AM. CHEM. SOC. 2004, 126, 10518-10519 10.1021/ja046895n CCC: $27.50 © 2004 American Chemical Society