Thin superconducting YBaCuO film in strong microwave field

Physma C 235-240 (1994) 2068-2069 North-Holland

PHYSICA

Thin S u p e r c o n d u c t i n g Y B a C u O Film in Strong M i c r o w a v e Field

Yu. V Art, emov", E. S Borovitskayab, V. M. Genkin b, G. l. Levieva, L V Ovchinulkova" alustitute of Solid State Physics, Russian Academy of Sciences Chernogolovka, Moscow (hstr, 142432, RUSSIA blnstitute for Physics of Microstructures, Russian Academy of Sciences Nizhny Novgorod, 46 Uljanov St,, 603600, RUSSIA Nonlinear microwave absorption of the thin superconducting fihll Is .~lndied 1! IS .,,hewn that nonhnearlty could aot be explaned by heat, ulg of the sample. The comeption of the ~ortece~ generated by tile mwrowave field gwes results m agreement with expL lhn"ntal data

The microwave response of thin superconducilug films of conventmnal and high- Tc superconductors is noulinear. This is a reason of harmonic generation, dependence of the surface impedance on the wave amphtude, the mflueuce of the nucrowave power on the critical current [1-3] We used two-frequency method for mvestagation the nonhnear response of a superconductor Weak wa~e was a probulg one and tile reflection at the frequency of ttu~ wave was studied. The amphtude of tile second wa'~e respou,,ib]e for tile change of the electrodynanuc lmrameter~ ol the fihn can be varied to 20Oe. The Y B a C u O film I rain ul dlalneter and 1000A thick was placed on the bottom o f a cyhndrlcal ca,,ily tuned sunultaneously at two frequencies, 03i/2,'r = 9 4GHz and w/~,'r = 18GH'_. The magnetic fields of both modes were liormal to the surface of the fihn At. the lower of the frequencies the power was Inodulated by rectangular pulses and could vary m wide range. "File radiation at the high freguency 03 was contluuou.,, Tile expernnental setup is shown in Fig 1 The setup could record changes of the wal and iniagulary part of the magnetic nloment of tile ~ainple The ~oltage of detectors 7 an(I 8 ~a.> originated b) the interference of the ~ ayes reflected from the ca~ ity and from lhe piston, with alnphtudes I'E,,,, and E,
~ = f(I I'£,,. + tg,f I) 'lhe co,'[ficlent 1' of refleCllon from the ca'~lt3

takes here near the resonanl frequmicy the forln: F = F ' + l r ''

Q-~t _ Q S ~ - i ' x ~ / w o = QT: +-67: +

(1)

where Q< and Qo are the exlernal and nltrinslc Q of cavity,-~o is the CdvIty frequency, and Aw = 03--~o 1~ lhe deviation fioni te.,,onauce Choosing a leference .,,Igilal aniphlude ,,uch that EreS >> I'E,,,,, we get U -.- F'&,,,. E; ~jco.~O+ I'"E,,,,.E,~ rs~n¢, (2)

~a__l~-~[

to computer

Ill; 1 BIo,k di.lgi,lm <,t -etup for tile observation of nonhlwa~ ab',oipllon I o,¢ fllator of frequency ,,,,'l/2rr = 9 t(;fi", 2 ~¢dl,ltol of frequency ,.,.,'12r = 18 t(;ltz, {
7 s illlt~l()V~,tv( d ( ' l e c l o r %

[)O-(Opl< iltlegral~lr Ill pOlVt'l

0921-4534/94/507 00 © 1994 - l-lscvmrScience B V All nghts reserved STDI 0921-4534(94)01600-3

lid ltq

tll

%{Jl ~ Illtq~ I

9-stro-

1]-microwave

Yu V Artemov et al /Physlca C 235-240 (1994) 2068-2069

where ~b is the phase difference between the wave incident on the cavity and the reference wave. Using piston 6 to set a phase shift 4' = 0 or ~b = 7r/2, we can measure the real part F ~ or the imaginary part F" of the reflectmn coefficient. Generator pulse 1 changed the reflection at. the frequency w. At resonance the reflection coefficient is real and the expression for F' takes the simpler form F' --

q~ -- q~

(qe + qo) ~"

(3)

Here qe = Q~-I and qo ": Q o 1 are the energy losses inside the cavity and the coupling elements. The internal losses are determined by dissipation in the walls and in the specimen: qo = qwau + qsp = qwatt + ~ I m M

(4)

Here M Is the magnetic moment of the specmaen, /3 is a geometric factor that takes into account the structure of the mode and the shape of the specimen. If the phase is chosen equal to zero,

0.8

2069

the signal on detectoi 7 t,~ proportional to A I m M during the pulse of generator 1 AU -,, AF' ,--, A I m M

(5)

The ctrcufl, of the detector S (Io,'.- not resl)ond to short pulsed signals, but records slow changes A h n M due to change of the temperature. The temperature dependence of signal for 5 magnitudes of pump power ts shown in Fig.2. The curves demonstrate that T¢ = 85.3/f doesn't shift as function of the pump power. This fact allows us to say that. the film ts heated by microwave power weakly, while the explanation of observed signal by thermal effect reqmres heating on a few kelvins The power of the beginning of nonlinear signal on Fig 2 depends on the temperature as ( T - T o ) 2 Such dependence on temperature points out the vortex character of nonhneanty

[1,41 The change of the Q-factor is defined by the mmgmary part of the magnetic nioment of superconducting film. If we take rote account that the film is placed on the substrate, we found the following expression for the ntagnettc moment ,)

03 -

M - (~fic2(E - 1) + 1)hv~ M,,,

0.5

(6)

where rl >> R , h ale l[le "~tl})'~il'aio's radius and tlnctcn ss, is the dwlectttc ('on..tant of the substrate, M o ts the COml)lex magnetic moment calculated by illean'.:, o f t t l t l a t l O l l fol l h l I i fillo. IFI normal field [5] The good agreem(,nt between lhe tlwor~ an(I tho eXl~'llm,'nl'li data llaa~ be altaiued for fully reasonable values fol London penetration depth and skin length

0.4 #a

REFERENCES

~<1 0.2

0.1

0.0

T eiX]t p e r a t -d.~es FIG 2 Temperature dependence of the imaginary part of magnetic nloment for different magnitudes of incident power Figures near the curves correspond to the power In dB. Maxmnlm power is 50 W

1 S A Govorkov, S K Tolpygc~, V A fuhn, ZETP, 89 (1985) 1704 2 P P Nguyen, D E Oate.~, G Dresselhaus, M S Dresselhaus, Phys Rev B, 48 (1993-[) 6401 3 A M.Porti~, D W Cooke et al App Phys Lett , 58 (1991) 307 4 V I k_t . l_l. r ) d. II .h, O.V ~. I A H I I d I~'X* KI .,,,,l~ '" .N ~',Plg'ma ~ ZETP. 11 (1970) 2,1f~ 5 .1 Pearl, AI>p Phs~ I,ott , 3 (19f;1) 65