Point-Contact-Spectroscopy of VSi 2 , NbSi 2 and TaSi 2
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OF VSi 2 , NbSi 2 and TaSi 2
U. GOTTLIEBab, 0. P. BALKASHINc, A. G. M. JANSENc, 0. LABORDEac, and R. MADARb, aCentre de Recherches sur les Trbs Basses Temp6ratures, CNRS, BP 166, 38042 Grenoble C6dex 9, France, bLaboratoire des Mat6riaux et du G6nie Physique, ENSPG, INPG, BP 46 38402 St. Martin d'Hbres, France CLaboratoire des Champs Magn~tiques Intenses, CNRS-MPI, BP 166, 38042 Grenoble C6dex 9, France,
ABSTRACT We present point-contact-spectroscopy measurements on VSi 2 , NbSi2 and TaSi 2 high quality single crystals. These three compounds crystallise in the same hexagonal structure (structure C40, space group P6222) and they are isoelectronic. Phonon spectra deduced by this technique are in good agreement with phonon parameters previously obtained from resistivity and low temperature specific heat measured on the same samples.
INTRODUCTION In the last years transition metal silicides have spurred much interest due to their possible applications in Si-based microelectronics. Nevertheless a lot of their fundamental properties are still unknown. We present here point-contactspectroscopy measurements on VSi 2 , NbSi 2 and TaSi 2 in order to proceed our study on fundamental properties of silicides [1,2]. The disilicides of the VB group transition metals, VSi 2 , NbSi 2 and TaSi2 crystallise in the same hexagonal structure (structure type C40, space group P6222). This structure is characterised by hexagonal MSi2-type layers, which are stacked by a ABCA... sequence along the hexagonal c direction. The lattice parameters increase by about 4% from VSi 2 to NbSi2 and decrease by about 0.3% from NbSi 2 to TaSi2 . Further details about crystal structure can be found in reference [2]. The three disilicides are isoelectronic and show a metallic behaviour. The excellent quality of the single crystalline samples was revealed by the residual resistance ratios RRR = p(300K)/p(4.2K) which attain values greater than 1000 for TaSi 2 and NbSi2 and about 20 for VSi 2 [3]. The electronic properties of these silicides are anisotropic as we have shown by resistivity [3] and magnetic susceptibility measurements [4]. From low temperature specific heat and resistivity results we have deduced the Debye temperatures OD of these materials [2,3]. Both determinations by these two methods are in good agreement. 485 Mat. Res. Soc. Symp. Proc. Vol. 320. ©1994 Materials Research Society
The current-voltage (I-V) characteristics of point-contacts between metals can provide informations about the electron-phonon interaction function, O2 F [5]. The spectral density function ca2 F is the convoluted product of the phonon density of states F and the squared matrix element for the electron-phonon interaction a 2. If the dimension of the point-contact is smaller than the electron mean free path, electron transport through the contact is no longer diffuse but ballistic. In this case, for an applied voltage over the metallic contact, the electrons are out of equilibrium close to the contact with an excess energy eV. Relaxation of the electrons occurs via
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