Physica C 167 (1990)
North-Holland
520-524
THE EFFECT OF VARIABLE MICROWAVE POWER ON THE LOW-FIELD
HIGH-Z’, SUPERCONDUCTING
CUPRATES
ABSORPTION
IN
,R. JANES, R.S. LIU and P.P. EDWARDS
IRC in Superconductivity, University of Cambridge, West Cambridge Site, Madingley Road, Cambridge, CB3 OHE, UK
J.L. TALLON
Physics and Engineering Laboratory, D.S.I.R.. PO Box 31313. Lower Hutt, New Zealand
Received
26 February
1990
The well established low-field microwave absorption below T, has been investigated using the ESR technique at 9 GHz, as a
function of incident microwave power, in a range of superconducting
cuprates, namely Er2Ba4Cu70,5_-x, T12Ba2CeCu20, and
YBa&&O,_,
In all cases, the signal was “saturated”
above a critical microwave power level; we observed that this effect was
sample-dependent.
1. Introduction
It is now well established that the transition to the
superconducting
state in high-T, copper oxide based
materials is characterized by the observation
of an
intense microwave absorption at low magnetic fields
[ 11. Rettori et al. [ 21 interpreted this effect as arising from changes in the diamagnetic
susceptibility,
on moving from the Meissner state into the mixed
state.
This response has been studied by a large number
of groups, and an extensive literature has accumulated [ 3,4] showing amongst other things that ESR
is a useful diagnostic tool for screening superconducting samples. However, a controversy still exists
as to the precise nature of the signal. For example,
one explanation
invokes the circulation of Josephson currents (induced by the oscillating microwave
field impinging on the sample) in these granular materials [ 51, the shape of the absorption being a reflection of the distribution
of domain sizes. Furthermore, closely spaced fluctuations,
which appear
superimposed
on the broad absorption,
have been
assigned to Josephson oscillations [ 61. A number of
recent
measurements
on
single
crystals
of
YBaZCu307_-x have revealed an anisotropic multiple
line structure which is sensitive to both microwave
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power and sample temperature
[ 7,8] ; this has been
attributed to flux quantisation
effects. Signals of this
nature have also been reported in pelletised samples,
and indeed we have detected reproducible substructure in the ESR response, up to high field values, ca.
6000 G at 9.1 GHz [9].
The non-resonant
nature of the signal was demonstrated by Lin and co workers [ lo] who obtained
essentially identical spectra at S-, X- and Q-band frequencies, both for the microwave field parallel and
perpendicular
to the static magnetic field. In this regard we have also studied the signal using an ESR
spectrometer operating at a frequency of 300 MHz
[ 111 in a range of samples, which again yielded the
same signal as was detected using an X-band spectrometer for the same material. Interestingly, Sastry
and co-workers [ 3,121 have also reported a field independent component, together with an absorptionlike signal close to T,, and in addition proposed that
a resonant absorption also takes place, the latter becoming swamped by the predominant
non-resonant
signal as the sample temperature
is lowered below
T,. Furthermore,
Siu [ 13 ] has recently discussed the
low-field microwave absorption
in high-T, superconductors as being a convolution
of both non-resonant and spin-resonant
components, the latter arising from copper d-electron triplet states.
R. Janes et al. / Low-jieldmicrowaveabsorptionin superconductingcuprates
In this work we report the observation
of a lowfield microwave
absorption
in superconducting
Er2Ba4Cu7015_-x, T12Ba2CeCu20, and YBa2Cu307_x,
and demonstrate
the effect of raising the incident
microwave power thereon. In all cases saturation occurred at a sample dependent threshold power level.
521
(well below the T, of the materials)
employing a
quartz insert Dewar and liquid nitrogen. Measurements in the 85-293 K range were obtained using
liquid nitrogen and a variable temperature accesory,
the temperature being measured most accurately by
placing a thermocouple
inside the ESR tube just
above the sample position.
2. Experimental
3. Results and discussion
ErzBa&u,O,,_,
was synthesised in oxygen in one
atmosphere, as described previously
[ 141. Briefly,
Erz03, Ba(N03)2 and submicron powders of CuO
were reacted in stoichiometric
proportions at 860 OC
in flowing oxygen. The precursors were mixed with
0.2 mol NaN03 per formula unit to enhance the reaction rate. During the first 24 h the reacted material
was ground, milled and die-pressed into pellets, tinally being sintered for 3-4 days. All samples were
rapidly air-quenched, followed by annealing in flowing oxygen at 300-350°C
to produce the maximum
T, for this material [ 15 1.
Samples of TlzBazCeCuzO,
were prepared from
high purity BaC03, CeOz and CuO powders, which
were initially calcined in air at 900-930°C. The precursors were then ground and mixed with the appropriate amount of T1203 to produce a nominal
composition
of T12Ba2CeCu20,. The mixture was
then pressed into a cylindrical pellet and wrapped in
gold foil to prevent loss of thallium during heating.
The samples were then sintered at 880°C for 10 min
in an oxygen atmosphere, followed by quenching in
air.
Samples of YBa2Cu307_-x were prepared by well
documented
routes.
X-ray diffraction (XRD) studies were carried out
with CuKa radiation using a Spectrolab CPS-120
diffractometer.
A standard four-probe method was
used for electrical resistance measurements.
AC susceptibility was determined
using the mutual inductive technique at a frequency of 823 Hz. Field-cooled
magnetization
(at 100 G) was studied using a
SQUID magnetometer
(Quantum
Design).
Microwave measurements
were carried out using
a conventional
X-band ESR spectrometer
(Bruker
ER200D) employing either 12.5 or 100 KHz field
modulation
and a standard rectangular TEio2 mode
cavity. Most measurements
were carried out at 77 K
Most of the ESR/microwave
absorption measurements were carried out at 77 K, at which temperature all the samples were superconducting,
evidenced by the appearance of an intense magnetic field
dependent microwave absorption beginning at lowfield. In all cases, we found that on raising the microwave power level incident on the sample, the lowfield signal initially increased in intensity and then
saturated above a sample dependent threshold level.
Significantly, the latter was always accompanied
by
a sudden degradation
of the cavity Quality factor
(detuning)
as evidenced by a shift in the detector
diode current. We appeared therefore, to be effectively driving the material back into the normal (i.e.
metallic) state by increasing the intensity of the microwave field (Hi) at the sample position. “Lossy”
or conducting samples appreciably perturb the field
patterns in the resonator - resulting in large shifts in
resonant frequency and reduction of Q.
Hi is related to the cavity Q (loaded) by the following equation:
2(H1 ),a, =a
(Qf’o)“’ >
where cr is a constant, and PO is the incident power
level.
For a conventional
ESR transition,
this will determine its intensity (in the absence of saturation).
Certainly, the term saturation in the accepted sense
of a conventional
ESR experiment, is something of
a misnomer here, as we are clearly dealing with what
is essentially a non-resonant
response (i.e. there is
no upper spin level to populate). These cavity perturbations have been studied in detail by Rettori et
al. [ 2 1, who noted a dramatic enhancement
of the
Q on cooling a sample of YBa2Cu30,_,
below T,;
this was accompanied by a large increase in the cavity resonant frequency, an effect which forms the ba-
R. Janes et al. /Low-field microwave absorption in superconducting cuprates
522
sis of the “leakage-current”
measurements
of Sastry
et al. [3,12].
Considering each system individually,
fig. 1 shows
the temperature
dependence
of the normalised
resistance
and
AC
susceptibility
for
the
Er2Ba4Cu7015_-x sample. The superconducting
transition
temperatures
were as follows:
T, (onset)=100
K, T, (mid-point)=98
K and T,
(zero) =92 K. The temperature
dependence of the
AC susceptibility is also shown, the diamagnetic signal indicating T, (onset) = 93 K, consistent with the
resistivity measurements.
The powder XRD pattern
for this sample was indicative of nearly monophasic
ERZBa4Cu,015_-x, this phase being responsible for
the superconductivity
[ 141. Figure 2 illustrates the
low-field microwave absorption for this phase (measured at 77 K) which grew in at T,,and exhibited a
saturation-type
effect on decreasing the attenuation.
The signal loss occurred between 30-40 mW at 77
K, a process which was completely reversible.
We also studied this effect in the recently synthesized superconducting
T1,Ba,CeCuzO,
phase [ 16 1.
Figure 3 shows the normalized
resistance versus
temperature
curve for a sample of this material,
showing T, (onset) = 100 K, T, (mid-point)
= 87 K
and T, (zero) = 82 K. The temperature dependence
of the mass diamagnetic
susceptibility
in the field
cooled ( 100 G) situation
is also included.
The
Meissner signal shows T, (onset) to be around 85 K,
consistent
with the resistance measurement.
The
powder XRD pattern was indicative of a Ce-doped
T1,BazCuOe phase, which is responsible for the su-
0.8 -
5
2
a’
0.6 -
t
0.4 -
2
r
0.2 0
0
IIIIIIII
20
40
60
80
100
120
140
Temperature(K)
Fig. 1. The temperature dependence of the normalised resistance
and the AC susceptibility
for an Er2Ba4Cu7015_-x sample.
1
ZOOOG
SOOG
I
I_
30mW
t
40mW
Fig. 2. Low-field microwave absorption
(derivative display) for
the Er2Ba4Cu7015_-x sample as a function of microwave power,
sample temperature=
77 K, v= 9.1 GHz. The narrow signal at ca.
3200 G, is due to a DPPH (diphenyl-picryl-hydrazyl,
g= 2.0036 )
marker, mixed with the sample.
perconducting
behaviour
[ 161. Figure 4 shows the
saturation of the low-field signal in this sample, where
a much lower power level was required to effect this
response. Furthermore, the same detuning effect was
observed and the signal returned to its original intensity on attenuation. The broad singlet at g= 2.150
300 G) is assigned to Cu2+ in impurity or
(A&,contaminant
phases on the basis of previous studies
[ 17-191. The intensity of this signal increased on
cooling consistent with a Curie-type susceptibility,
this also occurred on cooling through T,,suggesting
the impurity phase is located at the surface or grain
boundaries. The intensity of the localised Cu*+ signal, and indeed the DPPH signal in rig. 2, was approximately the same at both power levels, indicat-
R. Janes et al. / Low-field microwave absorption in superconducting cuprates
\\
\\
‘\\\
\
\
\
‘\
\
\
0.2 -
0
I
0
1
20
1
I
40
I
1
60
1
-1
\
\
I
SO
100
120
140
Temperature(K)
Fig. 3. The temperature
dependence of the normalised resistance
and the magnetisation
for the T1,BazCeCu,O,
sample.
5OOOG
\
523
single crystals. In this study individual components
of the spectrum were observed to increase intensity
at different rates, and high power levels caused linebroadening
and eventual
saturation.
Conversely,
Blazey et al. [ 5 ] have reported a completely new series of absorptions to grow in as the incident power
was increased, the variance being attributed by Dultic et al. [ 7 ] to sample homogeneities. Certainly, the
results of our investigations
point towards the microwave response being sample dependent with the
signal disappearing at different values of microwave
magnetic field strength, for the different materials
examined. As discussed earlier, one explanation for
the origin of the absorption has been proposed to be
microwave induced supercurrents
[ 5 1, and we may
speculate that as the microwave power is increased,
we reach a level sufficient to exceed the critical current (J,) of the weak links in the material concerned.
Indeed Dulcic et al. [ 71 have also discussed this possibility. Therefore, it is possible that the saturation
effect reported here is a function of the ceramic microstructure, rather than being a phase dependent response, and if so, may prove to be a useful probe of
microstructure
and of the effects of microstructural
engineering.
References
Fig. 4. Microwave
absorption
derivative
spectrum
for the
TI,Ba2CeCu20,
sample (T= 77 K, ~~9.1 GHz) showing the effect of varying the incident microwave power level.
ing the lossy nature of the material in the normal state
does not significantly reduce spectrometer sensitivity.
We extended
our studies to the well-known
YBa2Cu307_-x phase, and again the same signal-loss
occurred as the incident microwave power level was
raised. However, the sample studied required significantly higher power levels (circa 50-60 mW) to
facilitate this response.
The actual origin of the saturation effect reported
here is somewhat unclear, and indeed Dulcic and coworkers [ 71 have recently reported an extensive
study of the effect of microwave power on the lowfield multiline spectrum detected in YBa2Cu307_-x
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