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NON-RESONANT MICROWAVE ABSORPTION : A
MICROPROBE TO SUPERCONDUCTIVITY IN
Y1Ba2Cu3O7-δ
S. Tyagi, M. Barsoum, K. Rao, N. Karpe
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, decembre 1988
NON-RESONANT MICROWAVE ABSORPTION: A MICROPROBE TO
SUPERCONDUCTIVITY IN Y1Ba2Cu307-S
S. Tyagi (I), M. Barsoum ( I ) , K. V.
F h
(') and N. Karpe (2)( I ) Drexel University, Philadelphia, Pennsylvania 19104, USA (2) Royal Institute of Technology, S-10044 Stockholm, Sweden
Abstract. - The superconducting transition onset temperature in YBaCuO monitored using the microwave absorption
technique shows little change as a function of lattice distortion due to oxygen depletion indicating the presence of the orthorhombic superconducting phase even in highly oxygen-depleted samples.
1. Introduction study was t o gain further insight into the microstruc-
ture of the YBaCuO superconductor as a function of The non-resonant microwave absorption in the high- oxygen removal.
Tc superconductors a t low magnetic fields has been
shown t o result from the damped motion of free flux- ons, which contribute t o the sample surface impedance 2' [I] and is found t o have its onset near the critical tran- sition temperature, Tc [2-71. This absorption thus pro-
vides a sensitive non-contact technique t o study very small (less than a microgram) single phase samples or very small fractions of the superconducting samples is a non-superconducting matrix. This probe using ab- sorption has several advantages over the usual resis- tivity measurements. In the resistivity measurements the observation of zero resistance indicates the exis- tence of a single superconducting path between the sensing leads, whereas in the microwave experiment one can sense even the existence of local supercon- ducting regions. This becomes especially important when the samples are inhomogeneous, multiphased or especially consist of small islands of superconducting phase that do not percolate through the channels. Such multiphasic samples can result, for example, in off-stoichiometeric oxide superconductors according t o a recent model proposed by Khachaturyan and Mor- ris [8].
It is now well established that changing the stoi- chiometry by removal of oxygen from the YBaCuO system plays a crucial role in determining the struc- tural properties and the critical transition tempera- ture Tc [9, 101. The depression of T, as well as the
resulting structure depend on the details of oxygen re- moval. However, independent of the annealing details, the prominent feature of the Tc dependence on the
oxygen content is an initial slow decrease in Tc as 6 is increased, followed by a plateau region in the 50- 60 K range [9, 101. The measurement of Tc in such oxygen removal experiments have been carried out us- ing the usual resistivity and ac-susceptibility measure- ments. However such measurements alone can not pro- vide enough insight into the nature of decomposition proposed in [8]. Our aim in carrying out the present
The starting Y1Ba2CusOY samples were prepared using the standard solid state reaction method. The samples were single phase and showed a sharp resis- tive and ac-susceptibility transition a t about 93 K. The four samples on which the data are reported here were cut from a single disc of diameter 2.5 cm. The microwave absorption experiments were carried out us- ing an X-band Varian E-12 ESR spectrometer at about 9.23 GHz. The details of the experiment are similar to those of a standard ESR experiment except that a pair of Helmholtz coils was used to generate the ex- ternally applied magnetic fields of up to 60 gauss.' A CTI Cryogenics closed cycle helium refrigerator with the associated variable temperature accessory was em- ployed for the low temperature work. The lattice parameters were determined from X-ray diffraction, and oxygen content was analysed thermogravimetri- cally using a Perkin-Elmer TGA-2 system. The oxy- gen was removed by heating a sample t o 600-700 O C
at 5-10 OC/min in argon and monitoring the weight loss of the sample and quenching it t o room temper- ature after the desired weight loss had been achieved. It was assumed that the weight loss was totally due t o removal of oxygen.
3. Results and discussion
The peak to peak amplitude of the modulated mi- crowave absorption signal as a function of temperature for samples S1, S2 and S3 is shown in figure 1 with the inset showing a typical modulated absorption signal a t 40 K. As shown by us elsewhere [ll] the fluctuations in the signal do not represent noise but are actually reproducible signal. The lattice distortion defined as
[(a - b) / b ]
x 100 for the four samples S1, S2, S3, and
C8 - 2230 JOURNAL DE PHYSIQUE
S4 was, respectively, 1.6,0.9,0.5, and zero. Here a and b are the usual orthorhombic lattice parameters. As the oxygen is removed, the onset temperature as deter- mined by the usual four-probe resistivity method and indicated by vertical arrows in figure 1 agrees well with the reported results [9,
lo].
However, the onset T, as determined by the microwave absorption method does not change upon oxygen removal by more than a few degrees, although the width of the transition progres- sively increases. The S4 sample showed no microwave absorption or resistive transition down t o 10 K, the lowest temperature available in our system.Fig. 1. - Plots of t h e modulated microwave absorption signal (a typicaI one a t 40 K is shown in the inset) as a function of temperature for samples S1, S2 and S3. The transition onset temperatures determined by the usual re- sistivity technique is indicated by the vertical arrows. The
dashed lines are drawn as an aid t o the eye.
The apparent independence of the onset T, on the
oxygen content of the YBaCuO system in our ex- periment can be explained by the model proposed in [8]. According t o this model, a near stoichiome- teric orthorhombic ( 0 ) phase is always present in off-stoichiometeric (5
#
0) oxide superconductors, and furthermore the 0 phase in the off-stoichiometeric ox- ide may be intimately embedded in a matrix of the te- tragonal (T) phase. Thus the resistivity onset in such samples may be delayed until a low resistivity path is completed. However, the microwave absorption, as discussed in introduction, will be non-zero near the T, of the embedded 0 phase, although the overall sample composition is not that of the 0 phase.Finally, a comment must be made about the relative magnitude of the microwave absorption in the various samples. At low temperatures the signal amplitude in S3, for example, is nearly half that of in S1. This is not an indication that fifty percent of S3 is super- conducting. Due t o the shielding of the microwaves the absorption in S1 arises mainly from a surface layer and not from its interior [I]. However, upon oxygen removal and a consequent decrease in the shielding, a larger volume of S3 is probed, thus giving rise to an appreciable absorption, although the overall supercon- ducting fraction in S3 compared with S1 is much lower than indicated by the ratios of the signal amplitudes. Acknowledgments
One of us (ST) wishes t o thank the Solid State Divi- sion of the Royal Institute of Technology for their hos- pitality. Part of this research in Sweden is supported by STU.
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