POINT-CONTACT-SPECTROSCOPY EVIDENCE OF QUASI-PARTICLE INTERACTIONS INRNI2B2C (R=HO, Y)

The point-contact (PC) d(2)V/dI(2)-spectra of HoNi2B2C and YNI2B2C rev eal structure at applied voltages corresponding to the phonon frequenc ies. At about 4 meV a maximum is observed in the phonon density of sta tes by analogy to the soft-phonon structure in neutron scattering expe riments for LuNi2B2C [P. Dervenagas et al., Phys. Rev. B52, R9839 (199 5)] and YNI2B2C [H. Kawano et al., Czech. J. Phys. 46, S2-825 (1996), Phys. Rev. Lett. 77, 4628 (1996)]. In the Ho compound the low-energy p honon peak is suppressed by an applied magnetic field in an anisotropi c way, pointing to an interaction between the phonons and the magnetic systems. Surprisingly, in the nonmagnetic Y compound the 4-meV peak i s also suppressed by a magnetic field. In the Ho-compound contacts whi ch show the [quasi-thermal] behavior, the detailed magnetic-field and temperature dependences of PC spectra suggest that the magnetic order is destroyed due to the coupled phonon-magnon subsystem which is drive n out of equilibrium by electrons that pass through the contact, by an alogy with the nonequilibrium phonon-induced destruction of the superc onducting state in point contacts [I. K. Yanson et al., JETP Lett. 45, 543 (1987)]. The PC electron-phonon interaction (EPI) spectral functi ons are reconstructed and the estimates for the lambda-parameter yield the values of the order of 0.1. Comparison with PC EPI spectra of non superconducting and nonmagnetic LaNi2B2C. [I. K. Yanson et al., Phys. Rev. Lett. 78, 935 (1997)], as well as the comparative study of PC EPI and Andreev-reflection spectra for various contacts with superconduct ing Ho and Y compounds suggest that the low-energy part of the electro n-quasi-particle interaction spectral function is responsible for the Cooper pairing in these materials. (C) 1997 American Institute of Phys ics.