d. l. rousseau et al- comment on "two-magnon resonant raman scattering in mnf"

8/3/2019 D. L. Rousseau et al- Comment on "Two-Magnon Resonant Raman Scattering in MnF"

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Electrical Engineering, Department of

P. F. (Paul Frazer) Williams Publications

University of Nebraska - Lincoln Year

Comment on Two-Magnon Resonant

Raman Scattering in MnF2

D. L. Rousseau R. E. Dietz

P. F. Williams H. J. Guggenheim

Bell Laboratories, Murray Hill, New Jersey

Bell Laboratories, Murray Hill, New JerseyBell Laboratories, Murray Hill, New Jersey

Bell Laboratories, Murray Hill, New Jersey

This paper is p osted at DigitalCommons@University of Nebraska - Lincoln.

http://digitalcommons.unl.edu/elecengwilliams/36


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VOLUME 6, NUMBER8 P H Y S I C A L R E V I E W L E T T E R S 3 MAY1976(1973). ''E. U . Condon and G. H. Shortleg, The Theory of

4 ~ .. Eastman and J . L. Freeouf, Phys. Rev. Lett . Atomic Spectra (Cambridge Univ . P r e s s , London,34 , 1624 (1975).- 1963), p. 269.

5 ~ . . Eastman and J . L. Freeouf, Phys. Rev. Lett. ' 'condon and Shor tley , Ref. 10 , p. 299, and disc uss ion33, 1601 (1974).- on p. 271.

6 ~ .. Freeouf and D . E . Eastrnan, Crit . Rev. Solid '2~ ill oug hby . Cady, Phys. Rev.o,22 (1933).State Sci. 5 , 245 (1975). l 3 ~ h i s ay be due to an experimental aspect of the ap-7 ~ .. Lapeyre and J . Anderson, Phys. Rev. Lett. E , para tus of Ref. 1, viz., their energy resolution AE117 (1975). - 0.007Ep ; ther efore , as the Born approximation i s ap-8 ~ r a n k ermann and Sherwood Skillman, Atomic proached the ir energy resolution approaches o r exceedsStructure Calculations (Pre ntic e-Ha ll, Englewood the spin-orbit splitting of the 3d cor e levels being dis-Cliffs , N. J . , 19631, pp. 2-7 an d 3-9. cussed.

' ~ o s i t a k aOnodera and Yutaka Toyozawa, J . Phys . ' ' ~ ~ ~Ref. 1) sees large differences between Ge(ll1 )Soc. Jpn. 22, 833 (1975). 8 x 8 and Ge(100) 2x 2.

Com ment o n "Two-Magnon Resonant Ram an Scattering in MnF, "D. L. Rousseau, R. E. Dietz, P. F. Will iams, * and H. J. GuggenheimBe11 Labovatories, Muway Hill,New Jersey 07974

(Received 4 August 1975; revised manuscript received 22 December 1975)We have not been able lo reproduce the recently reported resonance enhancement of

Raman scat terin g from magnons in MnF, by Amer, Chiang, and Shen. Fifteen cr ys ta lsranging from extremely pur e sam ples to ones with high concentrations of impurities, inaddition t o one supplied by Amer, Chiang, and Shen, we re investiga ted. By observing theoff-resonanc e sca tter ing, we conclude that if an enhancement of gre at er than a factor of2 were present , it would have been seen in our resonance sp ectr a.

Recently Amer , Chiang, and Shen (ACS)' re -ported very interes ting two-magnon resonance-Raman-scatter ing spe ctr a fr om MnF,. They ex-cited MnF, with a dye la se r in the re gion of themagnon sidebands of the E, and E, excitons andobserved the th re e two-magnon Raman modes,corresponding to the o,, a,, and U, sidebands,to be resonantly enhanced. We found this exp eri-ment particularly intriguing because it may leadboth to better understanding of the basic reso -nance-Raman processes in crystals and to betterunderstanding of the very complex exciton-mag-non dynamics in MnF,. On the othe r hand, it isquite sur prisi ng that this ver y intense Ramanscat terin g was not observed by Dietz et al.' whenthey pre ss ur e tuned the a, sideband through the5419-A output of a xenon laser, nor was it re-ported by M acfarlane and Luntz3 in the ir lifetimeexperiments in which a dye la se r was tunedthrough the u, sideband.

We undertook to study resonant-Raman scat ter-ing fr om MnF, in the region of the magnon sid e-bands by illuminating a MnF, cr ys ta l of ext rem e-ly h igh puri ty ~ 4 t hhe output of a cw dye la se r(Rhodamine 110). The la se r output power wasabout 100 mW throughout the region of interestaround the magnon sidebands. Severa1 experi-

ments were performed. In some, the beam wasfocused into crystals which were immersed insuperfluid helium a t < 2K. In othe rs, c rys tal swer e placed on the cold fing er of an Air ProductsHeli-Tran cryostat. In this c ase we estimate thecry sta l tempera ture to have been about 10K.The sca ttere d light was gathered at 90" and ana-lyzed with a Spex 1401 double monochromator.

In al1 of o ur expe riments the utmost ca re wasused to ass ure that no experimental artifactswer e presen t in our data. Specifically we tookthe following mea sur es: (1) Caref ul alignmentand ap eraturing was done to minimize the amountof dye-laser fluorescence reaching the spectrom-eter. (2) To be su re that no exciton migrationoccu rred , the diame ter of the intrinsic lumines-cence emanating fro m the waist of the focusedla se r beam was measured. It was found to bele ss than 20 y m, roughly what we would expectfo r the width of the dye-las er waist itself. Thisguaranteed that misalignment effects in whichwe could have observed intrins ic luminescencebut not Raman sca tte rin g would be impos sible inour experiments. (3 ) Excitation spe ctra wererun af ter any change in optical alignment to de-ter min e if any thermal-heating effe cts wer e oc-curr ing at resonance. When such effects were


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P H Y S I C A L R E V I E W L E T T E R S

EXClTATlON FREQUENCY ( c m - ' ) SCATTERED FREQUENCY ( c m - )FIG. 1 . (a) Excitatio n spe ctr um of MnF2 showing the

a l and u2 sidebands. The arrow indicates the la se r fre-quency used to obtain the scattering spectrum in (b).(b) Scatterin g spectr um of MnFz excited a t " 8 480cm-'. The strong feature at 18367 cmDi s the intrin-s ic alh luminescence. The M 1 and 1M2 ar ro ws desi gna tethe expected frequenc ies of the two-magnon resona nceRaman scatteri ng associated with the al and a 2 side-bands, respectively.

se en the la se r power was accordingly reduced.Many cry sta l orientations were trie d in our ex-

peri ment s although emph asis was placed on look-ing at tens or components expected to exhibit thestro nges t nonresonant, two-magnon, Raman sca t-ter ing 4 (o,, and a,,). In addition, emphas is wasplaced on orienting the cry stal so a s to observethe s catte ring along the c axis. In this axial ori -entation2 we were ab le to eli minate n early com-ple te ly the Zn++ and ~ g " mpurity exciton lines5at 18 383 and 18 372 cm'', respectively, fr omour spectra .

In Fig. l (a ) we show the fluo resce nce excitation

spec trum of one of our mos t pure cr ys ta ls (No, 1in the Table 1). As expected, in the orientationshown he re (incident polarization perpendicularto the c axis) we were able to s ee the E, and E,excitons and the a, and o, sidebands.' By ro -tating the incident polarization we were able toobs erv e the n1 sideband. In Fig. 1 b) we pres entthe X(YX)Z Raman scattering spectrum at 2OKwith the dye-laser frequency set in the valley be-tween the a, and @, sidebands (-18 480 cm").This position is designated by the arrow in theexcitation spect rum . Note that the Zntt and Mg+*impurity lines a r e absent from our luminescencespectrum and the only feature is alL at 18 367cm''. The long ta il fr om 18 370 o 18 400 cm''does not result from impurities but is the expect-ed intrinsic ~lL- lum ine sce nce hape.' Fo r thisincident frequency f rom the data presented inFig. 3 of Ref. 1, two two-magnon Raman bandsar e located at the M, and M, positions designatedby the arro ws . It should be noted that at approx-imately this s ame excitation frequency in the r e-port of ACS1 one of the two-magnon resonanceRaman lines had a peak intensity of - 5@6 of thepeak intensity of the olL int rin sic luminescence.We investigated many excitation frequen cies cov-eri ng the en tir e range which theyl studied. In nocas e were we able to obse rve the two-magnonRaman scattering in the resonance region. Fur -therm ore, because of our cr yst al orientation andhigh purity, in our spe ctr a we wer e able to com-pletely eliminate the Mgtt impurity luminescenceat 18 372 cm" which was the strong est fea tur e inthe sp ec tr a of ACSL and part ially masked th e two-

TABLE 1. Source, growth methcd, and total inlpurity content of cry sta ls studied fro m eniission spectro graph icanalysis.

- --

Cry stal Source Growth methodTotal impurities

( P P ~ )1 BTL Horizontal zone


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VOLUME6,NUMBER8 P H Y S I C A L R E V I E W L E T T E R S j MAY1976magnon Raman lines. We would there fore expectto have gr eat er sensitivity than the irs fo r detect-ing these Raman lines.

To determine whether o r not this fir st c rys talwas unique in its failure to exhibit the reportedresonance spect ra, we investigated severa1 othe rcry sta ls. Al1 of our cr yst als with th ei r methodof growth and impurity content are listed in Ta-ble 1. The impurity contents listed in Table 1 a r eupper limits of the total cation impurities basedon emission spectrographic analysis and con-firmed by secondary ion mass sp ectrometry andspark-source mas s spectroscopy. Fo r the pur-es t cryst als we believe a real istic estimate ofthe total cation impurity leve1 to be in the rangeof 100 ppm. In the excitation s pec trum of someof the crys ta ls with high impuri ty concent rationswe observed eith er a splitting or a substantialbroadening of the intrinsic exciton and magnonsideband lines. However in none of the crys ta lswer e we able to s ee the two-magnon resonance-Raman sca tte ring. Recently we have exchangedcry st al s with ACS. We found that the ir cr ys ta lexhibited seve ra1 str ong impurity lines. As inour cryst als we were unable to observe any reso-nance enhancement of the two-magnon Ramanscattering.

To deter mine the expected intensity of the two-magnon Raman scattering in the resonance regionwe investigated an additional crystal at " 10Kcut f rom the sam e boule a s cr ystal No. 1. In thisse ri es of experim ents we repeated the off-reso-nance Raman measu remen ts of Fleury , Porto,and Loudon4 with 4880-A exci tation and then u s-ing the discrete lines of argon and krypton laserswe followed the Raman scattering to the 5308-Akrypton la se r excitation which overlapped thewavelength range of the dye la se r. The Ramanscattering was then followed as the dye laser wastuned into resonance. In Fig. 2 we show two ofthese spectra when the excitation is near reso-nance with o,. M i s the two-magnon Raman sca t-tering. Fo r 18 524 cm'' excitation the two-mag-non sca tte ring li es to the high-energy side of theE l exciton, while at 18 506 cm'' it has moved in-to the wing of the magnon sideband. As the ex-citation frequency was moved further into reso-nance with o, the two-magnon Raman scatteringbecame lost in luminescence from the sharplysloping magnon sideband.

ACS' repor ted a fact or of 7 enhancement of thetwo-magnon Raman scatt ering at resonance withthe o, sideband. In addition they reported thatin the resonance region the two-magnon line-

1 1 I I I l l I18,360 18,440

SCATTERED FREPUENCY ( CM - I IFIG. 2. Re-emission spec trum fro m MnFz slightly

away fro m resonance with the u, sideband. The l o 3and l o 5 refer to counts per second full scale. E l , uiL,and M refer to emission from the E l exciton, i ts mag-non sideband, and the two-magnon Raman sca tte rin g,respectivel y. The top spectr um was obtained with18 524-cm-' excitation and the bottom with 18 506-cm-'excitation.

width was instrumentally limited. To dete rminethe limit s of our se nsitivity fo r detecting the two-magnon resonance Raman scattering we measuredthe re- emi ssion with excitation of about 18 530cm'' so that both the E l em ission with an instru-mentally limited width and the off-resonance Ra-man emission were reso lved and observed in thesame spectrum. These data were stored on onequadrant of a multichannel analyzer. The inte-grated intensity of the exciton was adjusted toequal that of the Raman scattering. The excita-tion frequency was then tuned to the peak of o,and a spectrum was obtained [it appeared qualita-tively si mil ar to that shown in Fig. l (a )] andsto red in a different quadrant of the analyzer.The positions of the two spe ct ra were adjustedsuch that the E l exciton line in the fir st spectrumcoincided with the expected position of the two-magnon resonance Raman scattering in the sec-


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V O L U M E6,N U M B E R8 P H Y S I C A L R E V E W L E T T E R S 3 MAY1976ond spec trum . By adding the two sp ec tra andva ry ing the i r r e la t ive in ten s i t i e s we then wereable to de term ine th e level of enhancement ob-ser va ble on the a l, -1uminescence background ino u r ex p e r im e n t s. F r o m t h e s e m e a s u r e m e n t s afac tor of 7 enhancement , a s repor ted by ACS,'would be very readi ly observab le in ou r da ta . In-deed if the enhancement we r e a s sm a l l a s a f ac -t o r of 2 we would have been able to o bserv e i t .The con t ra s t be tween ou r r e s u l t s and those ofACS1 is s t r ik ing . They repo r t an enhancementof a n o rd er of m agnitude and a peak inten sity ofthe r e sonance Raman sca t t e r i ng comparable tothat of al,. Any poss ible enhancement in ourc r y s t a l s is l e s s than a f ac tor of 2 and the totalRam an two-magnon intensity is down from U,,by mo re than two ord er s of magnitude. We havee x am i n ed s e v e r a 1 c r y s t a l s r an g in g f r o m s o m ewhich we be l ieve to be among the p ures t ava i l -able to ot he rs which have high concen tra t ions ofimpu r i t i e s , hoping to de te rmine if a c e r t a i n i m -pur i ty l eve l was r e spons ib le for the sca t t e r ingdata rep or ted by ACS,' but in no case s were weable to obse rve any enhancement.As ide f rom our inabi l ity to r eproduce the r e so -nantly enhanced Raman s pe ctr a of ACS,' we be-l i ev e t h e r e a r e o t h e r s t r o n g g r o u nd s f o r r e j e c t-ing the va l idi ty of the i r c la ims. Most impo rtant-ly, they neglec t the s t r ong exci ton-magnon in-te rac t io n which was shown by Die tz et ale2 o besolely r e spons ib le for the d i f f e rence in shapesbetween a , and a, . Th is was demo nstra ted byshowing tha t th e magnon s idebands of E l an d E2have d i f f eren t shap es only in an absorp t ion pro-ce ss when the exc i ton and magnon a re c rea teds imultaneously. In f luorescence , the exci ton isannihilated with the sub sequen t creat ion of a mag-

non, with the resul t tha t th ere is no s t r ong in te r -ac t ion, and consequent ly both s ideband s have thesa m e shape , indica t ing tha t E l an d E2 have iden-t ica l disp ers io ns . A ca lcula t ion of the emiss io ns ideband shape6 us ing neut ron-sca t te r ing pa ra m-e te r s7 for the magnon d i spe r s ion d emons t ra tedtha t both E l and E2 had le s s than 0.5 cm" dis-pe r s ion , cont ra ry to the d i spe r s ion of 7 cm'' as-s u m ed f o r E2 in the ana lys is by ACS.' The effectof a disp ers io nless E2 exci ton would be to shif tthe position of the a, r e sonance we l l ou t s ide therange of th e ir exper imen ta l uncer ta inty. Appro-pr i a te t rea tm ent of the exciton-magnon and mag-non-magnon in teract ion would significantly broad-en (5-10 cm") the resonan ce Raman sca t te r ing .In view of the ser iou s i r repro duc ibi l i ty of theresults of ACS coupled with the theoretical ob-jec t ions ra ised above , we be l ieve the dif ferencesbe tween our r e su l t s and the i r s mus t l i e in someexpe r imenta l a r t i f ac t in the i r da ta.

*Prese nt addre ss: Department of Physics, Universi-ty of Puerto Rico, Rio Pie dra s, Puerto Rico 00931.

'N. M. Amer, T.-c. Chiang, and Y. R. Shen, Phys.Rev. Lett.a, 454 (1975).'R . E . Dietz, A. E. Meizner, H. J . Guggenheim, andA . Misetich, J . Lumin.u,79 (1970).

3 ~ . . Macfarl ane and A. C. Luntz, Phys. Rev. Let t.31, 832 (1973).-

~ . . Fleury, S. P. S. Po rto, and R. Loudon, Phys.Rev. Lett.E, 58 (1967).

5 ~ . isetich, R. E. Dietz, and H. J . Guggenheim,Localized Excitations En Solids (Plenum, New York,1968), p. 379.

6 ~ . . Dietz, A. E. Meizner, H . J . Guggenheim, andA. Misetich, Phys. Rev. Lett. a, 067 (1968).

'G. G. Low, A . Okazakl, K . C. Turberfield, andR. W . H. Stevenson, Phys. Lett. 8, 9 (1964).

d. l. rousseau et al- comment on "two-magnon resonant raman scattering in mnf"

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