Light-induced metal±insulator transition in Lu 2 V 2 O 7 S. Shamoto a, * , H. Tazawa a , Y. Ono a , T. Nakano b , Y. Nozue b , T. Kajitani a a Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan b Department of Physics, Tohoku University, Sendai 980-8578, Japan Abstract Ferromagnetic Mott±Hubbard insulator Lu 2 V 2 O 7 has been studied under continuous Ar laser irradiation. One of the photon energies, i.e. E  2:41 eV; corresponds to a transition from the lower Hubbard band of the V 3d orbital to the upper Hubbard band. Upon cooling, the activation-type electrical resistivity shows an anomalous drop within the temperature range of 70± 130 K. Below the transition temperature, the resistivity shows no appreciable temperature dependence. The temperature increase due to the laser heating is estimated to be within the range of 15±35 K, which cannot be responsible for the observed anomalous drop of the resistivity. The metal±insulator transition is discussed in connection with the ferromagnetism of the present compound. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; A. Magnetic materials; D. Electrical conductivity; D. EPR Pyrochlore Lu 2 V 2 O 7 has been studied extensively as a ferromagnetic semiconductor since the discovery by Bazuev et al. [1]. Although a metallic state can be expected from the simple band picture because of the 3d 1 con®guration of vanadium ions in Lu 2 V 2 O 7 , the compound behaves as if it were a semiconductor with a 0.2 eV activation gap. The value of U eff /W in Lu 2 V 2 O 7 was estimated to be about 6 [2], where U eff and W are the Coulomb energy and the bandwidth usually used in the Hubbard model, respectively. The energy level of V 3d is expected to be higher than that of O 2p as reported in Ref. [3]. Therefore, the ferromagnetic semiconductor Lu 2 V 2 O 7 should be regarded as a ferromag- netic Mott±Hubbard insulator, although the origin of the ferromagnetism is not clear yet. A systematic distortion of VO 6 octahedra in pyrochlore produces a trigonal ®eld to a vanadium ion, leading to one electron in the doubly degen- erated e g orbitals, as shown in Fig. 1. The bond angle of V± O(1)±V was found to be about 1318 from our powder X-ray diffraction data. It is dif®cult to determine whether the origin of the ferromagnetism is the superexchange interaction via two orthogonal O 2p orbitals based on this angle, or orbital degeneracy [4]. Apart from the origin of the ferromagnet- ism, it would be interesting to study a ferromagnetic insu- lator±ferromagnetic metal transition in the system with S  1=2: The substitution of various ions for Lu 31 has been studied. Although it was possible to substitute 10% Mg 21 for Lu 31 , which led to the decrease of the ferromag- netic transition temperature (T c ) from 73 to 68 K, the metal± insulator (M±I) transition has not been observed so far. The electron excitation from the lower Hubbard band (LHB) to the upper Hubbard band (UHB) by photons is usually forbidden by the selection rule of photon excitation, since both the LHB and UHB have the same d orbital char- acter. However, as Yamanouchi et al. showed in their opti- cal study, the transition is allowed because of the mixing of O 2p orbitals and V 3d orbitals [5]. In their optical conduc- tivity spectrum, two peaks at about 2.0 (1.0±2.6 eV) and 3.5 eV (2.5±4.3 eV) are assigned as one transition from LHB to UHB and another from the O 2p orbital to UHB, respectively. Here, we have studied a continuous laser irra- diation effect on DC resistivity of Lu 2 V 2 O 7 mainly by the photons with E  2:41 eV, which corresponds to the excita- tion from LHB to UHB, in addition to the study of the magnetism and the structure without laser irradiation. The laser heating effect has also been studied by setting a temperature sensor covered with Lu 2 V 2 O 7 powder on the Lu 2 V 2 O 7 pellet sample that was used for the DC resistivity measurement. Appropriate amounts of mixtures of Lu 2 O 3 (3 N), V 2 O 3 (3 N), and V 2 O 5 (3 N), were pelletized and calcined at 14008C for 4 h in a vacuum of about 10 24 Torr. The powder sample was examined by X-ray diffraction and a SQUID magnetometer (Quantum Design). The obtained sample Journal of Physics and Chemistry of Solids 62 (2001) 325±329 0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(00)00157-8 www.elsevier.nl/locate/jpcs * Corresponding author. Tel./Fax: 181-22-217-7970. E-mail address: shamoto@mail.cc.tohoku.ac.jp (S. Shamoto).