rf absorption in HTSC

Physma C 235-240 (1994) 2058-2059 North-Holland

PHYSICA

Role of magnetic field modulation in causing the fine structure of non-resonant microwave/rf absorption in HTSC Y.S. Sudershan ~, Amit Rastogi a, M.S. Hegde b and S.V. Bhat a "Dep~wtment of Physics, Indian Institute of Science, Bangalore 560 012, India bSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India. The fine structure m non-resonant microwave/rf absorption, hitherto observed m HTSC single crystals, are also observed in ~hm films It Is strongly dependent on the amplitude and frequency of magnetic field modulatmn and is absent when frequency modulation is uscd instead Arguments are presented to show that magnetic field modulation also causes fluxon nucleation thai, is rcsponsible for the fine structure.

1. I N T R O D U C T I O N

3. R E S U L T S

An interesting feature of non-resonant microwave absorption (NRMA) [l] in HTSC is the occurrence of closely spaced line spectra [2], often referred to as fine structure, in single crystalline samples. Thesc lines were found to occur at temperature dependent threshold microwave power levels and broaden out into bands at higher powers [2]. We report here the observation of the .~ame in both microwave and rf ranges in c-axis orwnted YBa2CuaO7 ~ thin films. We observe: (1) the fine >trncture only with magnetic field modulation but not with frequency modulation [3], (2) that it is dependent on frequency (f~) of magnetic field modulation and (3) that it is a sensitive function of magnetic field modulation amplitude H~r.

Fig. (1) shows N R M A signals recorded in the rf range. The fine structure is evident in Fig.(la) which is recorded with magnetic field modulation. Fig.(lb), recorded with frequency modulation does not show any fine structure even though this technique was demonstrated [3] to be very useful in recording the electromagnetic response from HTSC. Figs. (lc) and (!d) are recorded with magnetic field modulation at f~/27r = 87 Hz and 1 kHZ respectively. The lines are narrower and spaced relatively closer in the latter. Fig. (2) shows the fine structure recorded [5] at various amplitudes of magnetic field modulation in the microwave range. 4. D I S C U S S I O N

2. E X P E R I M E N T A L

The films (T,. = 90 K) were deposited oh YZrO= substrate b~ pulsed laser ablation [4]. A home built C W NMR spectrometer equipped with a lexel limited Robinson oscillator ~o/2~=20 Milz) with facility for ti'equency modulation [3] and a Bruker Eli 200D X band E P R spectrometer were used Both ':'oe'..troll~et.,r.: ;'.'~re equipped x~lth Oxford t'[:',¢t'l~,,|CD|~ t':)Ill.,l*~(,.', helium tie'c, t r3ostats and tim dc [mid (mtld be swept between + ~alues as desired ghe film was oriented with it.~ ~-axls parallel to the tic - magnetic field.

The origin of fine structure has been attributed to [2] microwave current induced fluxon nucleation in the Josephson junctions. However, the absence of fine structure under frequency modulation, Fig. (lb), strongly suggests that it is also caused b~ fluxon nucleation reduced by magnetic fiehl m,)dulation. This is further supported by Fig. (2) In whmh the fine structure evolves with mag,~"tic field modulation amplitude arid is best resolved at H~I = 0.125 Oe. Fhe dependence on IIat ~tl~o indicates that there exists a threshold value of |I,~t at whwh ttuxon nucleation occurs and broadens at higher HM Another evidence for IIa~ itlduced flu.,,on mmleation can be found

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2059

YS Sude,~han et al /Physica C 235-240 (1994) 2058-2059

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ACKNOWLEDGEMENTS

The authors wi.-,h to thank I;(;C, l)S'l' and (;SIR India for financial support

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M a g n e t i c Field (Oe) Figure 1. (a) Fine structure with field modulation, (b) its absence under frequency modulation, (c L" d) Fine structure at Ft/21r = 87 Hz and 1 kHz respectively. All these were recorded at 77 K and 25 l~W of rf power.

At ~2/27r = 87 Hz, Fig. (lc), the lines are widely spaced implying that the thre~htld vMue is crossed a fewer number of times in a given modulation cycle than in the 1 kHz case , Fig (ld). The separation between the successive hne structure lines for the two frequencies is inversely pro-

REFERENCES

1.

S.V. Bhat, P. Cangulv, l.~ Ramaknshnan and C.N.R. Rao, J. Phys C 20 (1987) L559. 2. K.W. Blazey, A.M. Portis, K.A. Muller and F.H. Holtzberg, Enrophys. Lett. 6 (1988) .157 and K.W. Blazey, A.M. Portis and F tt. Ho[tzberg, Phvsica C 157 (1989) 16 3. S.V. Bhat, Amit Rastogl and Y.S Sudershan. Solid State Comm. 89 (199.1) 633 4. D.M. ttuang, T Venkatesan, C.('. ('han~, L Nazer, X.D. Wu, A. Inaln and M.S ltegde Appl Phys. Lett. 54 (19,~9) 1702. 5. Amlt. Rastogl, M.S.Hegde and S V ]3itat i;r~,ceechnKs of the Sohd Sial(' Phvsl(s S~tnpo smm, 3 5 - C (lq(12) 296. orgam~ed b\ I ) \ t ' India