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2009 METALS, SUPERCONDUCTORS...Magnetic torque experiments on the magnetic-field-induced organicsuperconductor λ-(BETS) 2 FeCl 4Layered organic superconductors have upper critical fieldsexceeding the theoretical Pauli limit for superconductivitywhen the magnetic field is applied parallel to the layers.Theory predicts then the possibility of a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state[Fulde and Ferrel, Phys. Rev. 135, A550 (1964);Larkin and Ovchinnikov, Sov. Phys. JETP 20, 762(1965)]. Besides κ-(BEDT-TTF) 2 Cu(NCS) 2 , the two dimensionalfield-induced superconductor λ-(BETS) 2 FeCl 4 ,where BETS is bis(ethylenedithio)tetraselenafulvalene, is acandidate which fulfils all necessary conditions. When themagnetic field is applied parallel to the metallic layers, asuperconducting phase appears above 17 T below 1 K.In our first experiment we focused on the critical field at17 T. Figure 98 shows torque data taken at different temperaturesduring field sweeps. Surprisingly, a broad kinklikeanomaly appears only above 22 T indicating the transitioninto the field-induced superconducting state. Below200 mK a pronounced hysteresis appears. The experimentsshow no evidence for additional phase transitions withinthe superconducting state. The phase diagram clearly differsfrom that reported by Uji et al. A misalignment of thesample can be ruled out. A possible explanation may befound in the rather rapid cooling conditions in our experimentwhich is known to cause anion-disorder effects in 1Dorganic superconductors. The result stimulates further experimentsunder more controlled experimental conditions.This field-induced superconductivity is well understoodin the framework of the Fischer theory, based on theJaccarino-Peter effect. Uji et al. reported possible FFLOstates at the two critical fields based on dip structures inthe resistivity [Uji et al., Phys. Rev. Lett. 97, 157001(2006) and references therein]. This motivated us to lookfor a thermodynamic phase transition between the homogeneoussuperconducting phase and the FFLO states bymeans of magnetic torque experiments using a capacitancecantilever technique. The probe was equipped with a rotator,which allowed us to align the layers parallel to the fielddirection with a precision of 0.001 ◦ . Single crystals of λ-(BETS) 2 FeCl 4 were grown by the standard electrochemicaloxidation technique.Figure 98: Magnet torque of λ-(BETS) 2 FeCl 4 is plotted as afunction of field at different temperatures.I. SheikinR. Lortz (The Hong Kong University of Science and Technology, Kowloon, Hong Kong), Y. Nakazawa ( University ofOsaka, Osaka, Japan), B. Zhou, A. Kobayashi, H. Kobayashi (Nihon University, Tokyo, Japan)71