Raman modes ofRNi2B2C (R=Lu, Ho, Y) single crysta

Abstract

Results of Raman measurements on RNi 2 B 2 C ͑RϭLu, Ho, Y͒ single crystals are reported. Raman peaks are observed at 190 cm Ϫ1 and near 830 cm Ϫ1 . From the polarization-dependence measurements, these peaks are assigned to be the Ni-B 1g and the B-A 1g modes, respectively. Raman study on the effects of boron isotope 10 B substitution for 11 B in the LuNi 2 B 2 C crystals confirms that the mode near 830 cm Ϫ1 is due to the vibration of B atoms. However, from these single crystals, the expected Ni-E g and B-E g modes are not observed.

Raman- and infrared-active phonons in superconducting and nonsuperconducting rare-earth transition-metal borocarbides from full-potential calculations

“…In general our calculated equilibrium z B parameters are in good agreement with the experimental values ͑Table I͒. The calculated Raman-active phonon frequencies for the RT 2 B 2 C compounds are given in Table II along with the available experimental values from Raman-scattering measurements 42,43,58,59 and inelastic neutron-scattering experiments with Born-von Karman ͑BvK͒ model evaluation. 44 The experimental results 43 could not identify the Ni-E g and B-E g modes from polarization-dependent Raman measurements for RNi 2 B 2 C single crystals.…”

“…This may be a reason for the problem of observing the E g mode in some of the experimental investigations. 43 The phonon frequency of the heavier Ni atom is expected to be lower than that of B and C. Figure 7 is consistent with this expectation that the Ni-B 1g mode has the lowest frequency.…”

“…In recent years several experimental and theoretical studies have been made on Raman spectra for superconducting RNi 2 B 2 C. From Raman-scattering measurements on polycrystalline YNi 2 B 2 C Hadjiev et al 42 assigned all four Raman-active modes (B-A 1g , Ni-B 1g , Ni-E g , and B-E g ). On the other hand, Park et al 43 did not observe the Ni-E g and B-E g modes in RNi 2 B 2 C (RϭLu, Ho or Y͒ by measurements on single crystals. So, calculations on Raman-active phonons are needed to settle this issue.…”

“…The calculated Raman-active phonon frequencies for the RT 2 B 2 C compounds are given in Table II along with the available experimental values from Raman-scattering measurements 42,43,58,59 and inelastic neutron-scattering experiments with Born-von Karman ͑BvK͒ model evaluation. 44 The experimental results 43 could not identify the Ni-E g and B-E g modes from polarization-dependent Raman measurements for RNi 2 B 2 C single crystals. However, our calculated phonon frequencies of the Ni-E g and B-E g modes for YNi 2 B 2 C are found to be in excellent agreement with the phonon frequencies obtained by Raman-scattering measurements of Hadjiev et al 42 and Hartmann et al 59 which should clarify the controversy between the three reports.…”

Boron isotope effect in single crystals of superconductor

“…The phonons responsible for the superconductivity in these materials are thought to be the high-frequency boron A 1g optical modes [24][25][26][27]. For non-magnetic, superconducting members of the series, the boron isotope effect (IE) of T c has been measured: significant boron IE was reported for polycrystalline YNi 2 B 2 C, partial isotope effect exponent, α B =0.25±0.04, [5]; and for single crystals of YNi 2 B 2 C, α B =0.21±0.07, and LuNi 2 B 2 C, α B =0.11±0.05 [6].…”