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Inelastic neutron and Raman scattering of high-Tc Nb3(Al, Ge)
Physica 135B (1985) 355-357 North-Holland, Amsterdam
INELASTIC N E U T R O N AND R A M A N SCATTERING ON HIGH-T c Nbs(AI, Ge) P. M I 3 L L E R , R. H A C K L , R. K A I S E R Fakultdt fiir Physik E21, Technische Universitiit Miinchen. D-8046 Garching. Fed. Rep. Germany
N. NI~ICKER Kernforschungszentrum Karlsruhe, lnstitut far Nukleare Festk6rperphysik, D-7500 Karlsruhe, Fed. Rep. Germany
A. M O L L E R Research Laboratories, Siemens AG, D-8520 Erlangen, Fed. Rep. Germany
We report on inelastic neutron and Raman scattering measurements on a N b 3 (AI, Ge) sample with 75.9 at%Nb and 6.1at% Ge, and Tc = 19.4 K. The generalized phonon density of states G(w) was obtained from the inelastic neutron spectra at 300 K and 7 K. Our strongest peak in the phonon density of states at 17.6 meV at 300 K also shows up in the Raman spectrum as impurity induced scattering. In addition, the Raman spectrum reveals the two modes of Eg and F2gtype allowed in A15 compounds due to symmetry. Upon cooling, the F2g phonon softens strongly and the low energy part of G(to) is enhanced.
1. Introduction
2. Experimental
The A15 c o m p o u n d s attracted considerable interest since they have been discovered. E v e n if sensational results such as a new high-T c material are missing, some effort has been put into the investigation of these c o m p o u n d s [1]. Recent theoretical calculations [2] have shed light on the question for the very high values of the superconducting transition t e m p e r a t u r e T c in these materials. In addition, they give an idea for future research. The calculations roughly divide the high-T c Nb-based A15 c o m p o u n d s into two groups with Nb3Sn and N b 3 G e on the one hand, and Nb3AI and N b 3 G a on the other hand. In connection with the rather detailed experiments on Nb3Sn it seems interesting to investigate a c o m p o u n d out of the second group. We decided to take Nb3A10.75Ge0.25 , which is very similar to Nb3A1. M o r e o v e r , out of all the A15 compounds, which are thermodynamically stable at the stoichiometric composition, Nb3(A1 , Ge) has the highest T c [3]. We present inelastic neutron and R a m a n scattering results, which are discussed in connection with theoretical predictions.
The Nb3(AI, Ge) sample was p r e p a r e d by HF-melting and a subsequent annealing procedure as described earlier [3]. The midpoint of the inductively measured transition is 19.4K. The composition of Nb0.76A10.18Ge0.06 guarantees pure A15 phase as also confirmed by neutron diffraction. From the about 40 g p r e p a r e d for the neutron m e a s u r e m e n t s we took a small single crystalline grain with a surface oriented close to (110) for the R a m a n experiments. The inelastic neutron scattering spectra were measured with the Karlsruhe Time of Flight spectrometer located at the Melusine reactor, C E N G , Grenoble. The incident neutron energies were 44.15meV and 1 2 m e V at 3 0 0 K and 44.15 meV at 7 K. The generalized p h o n o n density of states G(~o) was calculated with the usual averaging procedure [4]. The R a m a n data were taken with the experimental set up described earlier [5]. The scattering geometries were Eg, and F2g with a small admixture of Eg. The spectra were measured at a holder t e m p e r a t u r e of 300 K and 1 0 K , respectively, with a laser power of
0378-4363 / 85 / $03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P. Mfiller et al. / Phonons of high-T Nb3(Al, Ge)
356
100 roW. As estimated from thermal conductivity data the temperature raise by the incident laser beam is less than (15 _+ 10) K.
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3. Results and discussion >"
Fig. 1 shows G(o~) at 300 K and 7 K. The gross features look very similar to the G(o~) of Nb3AI. In fact, the spectrum can be synthesized by a 75% to 25% superposition of the spectra of Nb3AI [6] and Nb3Ge [4]. The main peak at about 18 meV is found in all Nb-based A15 compounds. The peak at 36 meV is due to the A1 vibrations. At 7 K, the shoulder at 12 meV is drastically increased. The intensity of the peaks at 18meV and 23 meV decreases. The A1 structure remains unchanged. Fig. 2 contains the Raman data in E g geometry. For reasons of symmetry the peak at 26 meV can be identified as the Raman active E g ( ~ 2 ) vibration. The broad structure between 10 and 20 meV has to be attributed to impurity induced scattering thus revealing the phonon density of states. In contrast to the refractory compounds like NbN the correspondence between the Raman and the neutron data is not very striking for moderately disordered A15 materials, because here the spectrum is dominated by the allowed modes. Nevertheless, in Nb3(A1, Ge) the two neutron peaks at 18meV and 23meV (fig. 1) attributed to the vibrating Nb atoms also show up
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Fig. 2. Raman spectrum in Eg geometry at 300K. The allowed Eg mode is located at 26 meV. The features around 15 meV are due to impurity induced scattering.
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Fig. 1. Fig. 1. G(~o) at 300K (©) and 7 K ( 0 ) . Note the pronounced softening of the low energy part upon cooling.
Fig. 3. (a) Raman spectrum in F2g geometry at 300 K. The peak at 14 meV is the Raman active F2g vibration. There is also a small contribution of the Eg mode at 26 meV. The impurity induced structures at 17 and 23 meV can clearly be resolved in the neutron spectrum (fig. I). (b) F 2 g spectrum at 10 K. The F2~ peak is shifted by about 14% to 12 meV.
P. Miiller et al. / Phonons of high-T,. Nb3(Al, Ge)
in the Raman spectrum as small structures as indicated in fig. 3a. In addition, fig. 3a shows the Raman active F2g (F25,) vibration at 14 meV and a contribution of the Eg mode due to improper orientation of the surface. We like to emphasize that the energy of the F2g phonon is the lowest of all the Nb-based A15 compounds investigated until now. A further difference to the other A15's is the strong depression of the F2g frequency upon cooling (fig. 3b), whereas the Eg mode only softens moderately by about 5%.
4. Conclusions We have performed inelastic neutron and Raman scattering experiments of high-T c Nb 3(AI, Ge) as a function of temperature. The low frequency and the pronounced temperature dependence of the F2g mode in contrast to the Eg mode are predicted by e l e c t r o n - p h o n o n calculations [2]. For Nb3(A1, Ge) the Fermi level E F lies far away from the ~12 electronic state and intersects the bands emanating from the F12 point at large values of q. Thus, small variations of E F with temperature affect the Eg mode frequency only slightly. On the other hand the F2g mode becomes very sensitive to temperature and is renormalized down in frequency. Simple geometric reasons also lead to a lower Fzg frequency.
357
Therefore, the softening of the low energy part of the phonon spectrum as usually observed in all the high-T c A15 compounds is caused here - and probably in Nb3A1 and Nb3Ga as w e l l - b y a different set of phonon modes compared to Nb3Sn.
Acknowledgements We are grateful to Dr. W. Weber, Karlsruhe, for a discussion on his theory. The neutron and the Raman experiments have been supported by the Bundesministerium f/Jr Forschung und Technologie and the Deutsche Forschungsgemeinschaft, respectively.
References [1] For a review see J. Muller, Rep. Prog. Phys. 43 (1980) 641. [2] W. Weber, Physiea 126B (1984) 217. [3] A. Miiller, Z. Naturforschung 25a (1970) 1659. [4] P. Mtiller, U. Buchenau, N. Niicker, B. Renker and A. Miiller, Proc. LT-17, U. Eckern et al., eds. (NorthHolland, Amsterdam, 1984) p. 599; and references therein. [5] R. Hackl, R. Kaiser and S. Schicktanz, J. Phys. C16 (1983) 1729. [6] B.E SchweiB,B. Renker, E. Schneider and W. Reichardt, Superconductivity in d- and f-Band Metals, D.H. Douglas, ed. (Plenum, New York, 1976) p. 189.
INELASTIC N E U T R O N AND R A M A N SCATTERING ON HIGH-T c Nbs(AI, Ge) P. M I 3 L L E R , R. H A C K L , R. K A I S E R Fakultdt fiir Physik E21, Technische Universitiit Miinchen. D-8046 Garching. Fed. Rep. Germany
N. NI~ICKER Kernforschungszentrum Karlsruhe, lnstitut far Nukleare Festk6rperphysik, D-7500 Karlsruhe, Fed. Rep. Germany
A. M O L L E R Research Laboratories, Siemens AG, D-8520 Erlangen, Fed. Rep. Germany
We report on inelastic neutron and Raman scattering measurements on a N b 3 (AI, Ge) sample with 75.9 at%Nb and 6.1at% Ge, and Tc = 19.4 K. The generalized phonon density of states G(w) was obtained from the inelastic neutron spectra at 300 K and 7 K. Our strongest peak in the phonon density of states at 17.6 meV at 300 K also shows up in the Raman spectrum as impurity induced scattering. In addition, the Raman spectrum reveals the two modes of Eg and F2gtype allowed in A15 compounds due to symmetry. Upon cooling, the F2g phonon softens strongly and the low energy part of G(to) is enhanced.
1. Introduction
2. Experimental
The A15 c o m p o u n d s attracted considerable interest since they have been discovered. E v e n if sensational results such as a new high-T c material are missing, some effort has been put into the investigation of these c o m p o u n d s [1]. Recent theoretical calculations [2] have shed light on the question for the very high values of the superconducting transition t e m p e r a t u r e T c in these materials. In addition, they give an idea for future research. The calculations roughly divide the high-T c Nb-based A15 c o m p o u n d s into two groups with Nb3Sn and N b 3 G e on the one hand, and Nb3AI and N b 3 G a on the other hand. In connection with the rather detailed experiments on Nb3Sn it seems interesting to investigate a c o m p o u n d out of the second group. We decided to take Nb3A10.75Ge0.25 , which is very similar to Nb3A1. M o r e o v e r , out of all the A15 compounds, which are thermodynamically stable at the stoichiometric composition, Nb3(A1 , Ge) has the highest T c [3]. We present inelastic neutron and R a m a n scattering results, which are discussed in connection with theoretical predictions.
The Nb3(AI, Ge) sample was p r e p a r e d by HF-melting and a subsequent annealing procedure as described earlier [3]. The midpoint of the inductively measured transition is 19.4K. The composition of Nb0.76A10.18Ge0.06 guarantees pure A15 phase as also confirmed by neutron diffraction. From the about 40 g p r e p a r e d for the neutron m e a s u r e m e n t s we took a small single crystalline grain with a surface oriented close to (110) for the R a m a n experiments. The inelastic neutron scattering spectra were measured with the Karlsruhe Time of Flight spectrometer located at the Melusine reactor, C E N G , Grenoble. The incident neutron energies were 44.15meV and 1 2 m e V at 3 0 0 K and 44.15 meV at 7 K. The generalized p h o n o n density of states G(~o) was calculated with the usual averaging procedure [4]. The R a m a n data were taken with the experimental set up described earlier [5]. The scattering geometries were Eg, and F2g with a small admixture of Eg. The spectra were measured at a holder t e m p e r a t u r e of 300 K and 1 0 K , respectively, with a laser power of
0378-4363 / 85 / $03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P. Mfiller et al. / Phonons of high-T Nb3(Al, Ge)
356
100 roW. As estimated from thermal conductivity data the temperature raise by the incident laser beam is less than (15 _+ 10) K.
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Ln
c D o
I
0,8
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3. Results and discussion >"
Fig. 1 shows G(o~) at 300 K and 7 K. The gross features look very similar to the G(o~) of Nb3AI. In fact, the spectrum can be synthesized by a 75% to 25% superposition of the spectra of Nb3AI [6] and Nb3Ge [4]. The main peak at about 18 meV is found in all Nb-based A15 compounds. The peak at 36 meV is due to the A1 vibrations. At 7 K, the shoulder at 12 meV is drastically increased. The intensity of the peaks at 18meV and 23 meV decreases. The A1 structure remains unchanged. Fig. 2 contains the Raman data in E g geometry. For reasons of symmetry the peak at 26 meV can be identified as the Raman active E g ( ~ 2 ) vibration. The broad structure between 10 and 20 meV has to be attributed to impurity induced scattering thus revealing the phonon density of states. In contrast to the refractory compounds like NbN the correspondence between the Raman and the neutron data is not very striking for moderately disordered A15 materials, because here the spectrum is dominated by the allowed modes. Nevertheless, in Nb3(A1, Ge) the two neutron peaks at 18meV and 23meV (fig. 1) attributed to the vibrating Nb atoms also show up
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Fig. 2. Raman spectrum in Eg geometry at 300K. The allowed Eg mode is located at 26 meV. The features around 15 meV are due to impurity induced scattering.
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Fig. 1. Fig. 1. G(~o) at 300K (©) and 7 K ( 0 ) . Note the pronounced softening of the low energy part upon cooling.
Fig. 3. (a) Raman spectrum in F2g geometry at 300 K. The peak at 14 meV is the Raman active F2g vibration. There is also a small contribution of the Eg mode at 26 meV. The impurity induced structures at 17 and 23 meV can clearly be resolved in the neutron spectrum (fig. I). (b) F 2 g spectrum at 10 K. The F2~ peak is shifted by about 14% to 12 meV.
P. Miiller et al. / Phonons of high-T,. Nb3(Al, Ge)
in the Raman spectrum as small structures as indicated in fig. 3a. In addition, fig. 3a shows the Raman active F2g (F25,) vibration at 14 meV and a contribution of the Eg mode due to improper orientation of the surface. We like to emphasize that the energy of the F2g phonon is the lowest of all the Nb-based A15 compounds investigated until now. A further difference to the other A15's is the strong depression of the F2g frequency upon cooling (fig. 3b), whereas the Eg mode only softens moderately by about 5%.
4. Conclusions We have performed inelastic neutron and Raman scattering experiments of high-T c Nb 3(AI, Ge) as a function of temperature. The low frequency and the pronounced temperature dependence of the F2g mode in contrast to the Eg mode are predicted by e l e c t r o n - p h o n o n calculations [2]. For Nb3(A1, Ge) the Fermi level E F lies far away from the ~12 electronic state and intersects the bands emanating from the F12 point at large values of q. Thus, small variations of E F with temperature affect the Eg mode frequency only slightly. On the other hand the F2g mode becomes very sensitive to temperature and is renormalized down in frequency. Simple geometric reasons also lead to a lower Fzg frequency.
357
Therefore, the softening of the low energy part of the phonon spectrum as usually observed in all the high-T c A15 compounds is caused here - and probably in Nb3A1 and Nb3Ga as w e l l - b y a different set of phonon modes compared to Nb3Sn.
Acknowledgements We are grateful to Dr. W. Weber, Karlsruhe, for a discussion on his theory. The neutron and the Raman experiments have been supported by the Bundesministerium f/Jr Forschung und Technologie and the Deutsche Forschungsgemeinschaft, respectively.
References [1] For a review see J. Muller, Rep. Prog. Phys. 43 (1980) 641. [2] W. Weber, Physiea 126B (1984) 217. [3] A. Miiller, Z. Naturforschung 25a (1970) 1659. [4] P. Mtiller, U. Buchenau, N. Niicker, B. Renker and A. Miiller, Proc. LT-17, U. Eckern et al., eds. (NorthHolland, Amsterdam, 1984) p. 599; and references therein. [5] R. Hackl, R. Kaiser and S. Schicktanz, J. Phys. C16 (1983) 1729. [6] B.E SchweiB,B. Renker, E. Schneider and W. Reichardt, Superconductivity in d- and f-Band Metals, D.H. Douglas, ed. (Plenum, New York, 1976) p. 189.