Magnetic excitations in a single crystal of the Kondo semiconductor YbB 12 F. Iga a, * , A. Bouvet b,c , L.P. Regnault b,c , T. Takabatake a , A. Hiess c , T. Kasuya d a ADSM, Hiroshima University, Higashi–Hiroshima 739-8526, Japan b SPSMS/MDN, CEA/DRFMC, 38054 Grenoble Cedex 9, France c Institute Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France d Faculty of Science, Tohoku University, Sendai 981-0845, Japan Abstract Inelastic neutron-scattering measurements have been performed on a single crystal of the Kondo semiconductor YbB 12 by using a triple-axis spectrometer. We have found two magnetic excitations; one is dispersive from 14 to 20 meV and the other centered at 37 meV is less dispersive. The excitation energy of the former agrees with the gap energy derived from the specific heat and transport measurements. With increasing temperature, both excitations disappear at 60 K, above which a quasi-elastic peak and another peak centered around 25 meV appear.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic materials; B. Crystal growth; C. Neutron scattering; D. Electronic structure; D. Magnetic properties 1. Introduction YbB 12 is the only Yb-based Kondo semiconductor with a narrow gap at the Fermi level [1,2]. The system changes from a quasi-metallic state with localized magnetic moments at high temperatures to a semiconducting non- magnetic state at low temperatures. However, the mechan- ism of gap formation has not been clarified yet, as in the other Kondo semiconductors, like for e.g. CeNiSn, regard- less of considerable efforts in the last two decades [3]. In order to elaborate this mechanism, information about the k- dependence of mixing interaction between conduction elec- trons and f-electrons is indispensable. For this purpose, an inelastic neutron scattering study was performed on a poly- crystalline-powdered sample of YbB 12 [4]. The observed spectra showed the presence of three characteristic magnetic excitations centered at about 15.5, 20 and 37 meV, respec- tively. The first and the second were assigned, respectively, to an excitonic-like peak and an excitation from the Kondo singlet to a magnetic state. The last was thought to be a crystal field excitation. These features should be carefully compared with the unconventional magnetic excitations reported for SmB 6 [5], which is a Kondo semiconductor with valence fluctuations. For single crystalline SmB 6 ,a sharp excitonic peak was observed around 14.5 meV with a weak dispersion, which was attributed to the formation of an exciton. However, magnetic contributions of YbB 12 at 15.5 and 20 meV, were not seperated so clearly from the phonon parts because of accidental degeneracies. Recently we have succeeded in growing a large single crystal of YbB 12 by a floating-zone method using a newly developed image-furnace with four xenon lamps [6]. In this article, we report the first measurements of inelastic neutron scattering of a single crystal YbB 12 , from which we obtained the q-dependence of the dynamical response function. 2. Experimental method Single crystalline samples of YbB 12 for this study were prepared by using the traveling-solvent floating-zone method as reported in Ref. [6]. We used the isotope 11 B enriched by 99.5% in order to remove the other isotope 10 B with a large absorption crosssection for thermal neutrons. The sample used in the present experiment was a cylinder of 5 mm  × 15 mm. The neutron scattering experiment was carried out on the IN8 three-axis spectrometer at the reactor of ILL. Energy Journal of Physics and Chemistry of Solids 60 (1999) 1193–1196 0022-3697/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(99)00081-5 * Corresponding author. Fax: + 81-824-24-0720. E-mail address: figa@sci.hiroshima-u.ac.jp (F. Iga)