PHYSICAL REVIEW B VOLUME 27, NUMBER 5 1 MARCH 1983

Response to "Comment on 'Tunneling a2F(ra) as a functionof composition in A15 NbGe' "

by B. R. Sood

K. E. Kihlstrom' and T. H. GeballeDepartment ofApplied Physics, Stanford University, Stanford, California 94305

(Received 27 July 1982)

We present arguments that mode softening is present in our Nb36e samples as a function ofcomposition [although we agree with Dr. Sood that other factors, such as a softening ofn (cu), could also be present]. Since quantitative results depend on the product 0. F(co) rath-

er than either component, they are unaffected by Dr. Sood's question.

Dr. Sood questions both the quantitative resultsobtained from our analysis and the interpretation thatthe increased strength at low energies in our a'F(a&)curves is due to mode softening. He raises the objec-tion due to the increased weighting of the a F(ru)curves at all energies as stoichiometry is approached,thus they do not obey the sum rule.

In answer to his first question, while F(co) mustobey the sum rule, a'(co) need not, and it is physi-cally reasonable that it would not. Thus it is reason-able for the product n'F(ca) not to obey the sumrule. It is true that tunneling alone cannot separateu'(cu) from F(cu), but since all quantitative resultsdepend on the product a'F(co) rather than eithercomponent, attributing effects to one or the otherdoes in no way affect the quantitative results.

His second question does have some merit. Sinceby tunneling alone we cannot separate a'(cu) fromF(e), attributing the enhanced weighting at low en-ergies to mode softening [implying it is due tochanges in F(co)] is an assumption. Certainly, if

there were enhanced weighting alone, it could be at-tributed to a'(cu). But in addition to enhancedweighting we see a discrete movement of the lowestenergy peak to lower frequencies even when theenhanced weighting [which may be due to u'(ru)] isnormalized out. This is far more likely to be due to amovement in the phonon mode than an artifact of0,'(co). In fact Muller, et al. ' reported a correspond-ing movement observed in F(e) time-of-flight neu-tron spectroscopy. Also despite the increase in thehigh-frequency modes, the weighted frequency mo-ments still show a decrease ( (aP) goes from -240 to-200). Thus we do not need to attribute the entireincrease at high energies to a'(co) for our con-clusions to hold. Therefore we feel there is evidenceto support an interpretation of mode softening. ButDr. Sood's point is well taken that there may also bea significant contribution due to softening of 0,"(cu).Further neutron spectroscopy' on high- T, Nb3Geshould help to resolve the relative contributions.

'Address after September 1, 1982: Naval ResearchLaboratories, Washington, D.C 20375.

~Also at Bell Laboratories, Murray Hill, N.J. 07974.'P. Muller, N. Nucker, W. Reichardt, and A. Muller, in the

Fourth Conference on Superconductivity in d- and f-BandMetals, Kernforschungszentrum, Karlsruhe, West Ger-many, 1982 (unpublished).

2J. Geerk et al. (unpublished).

3082 Qc1983 The American Physical Society