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Resistivity anisotropy and Josephson coupling in lead-substituted bismuth cuprates
F.-X. Régi, J. Schneck, H. Savary, R. Mellet, P. Müller, R. Kleiner
To cite this version:
F.-X. Régi, J. Schneck, H. Savary, R. Mellet, P. Müller, et al.. Resistivity anisotropy and Josephson coupling in lead-substituted bismuth cuprates. Journal de Physique III, EDP Sciences, 1994, 4 (11), pp.2249-2257. �10.1051/jp3:1994270�. �jpa-00249259�
Classification Physic-s Abstracts
74.70J 74.50
Resistivity anisotropy and Josephson coupling in lead- substituted bismuth cuprates
F.-X. Rdgi ('), J. Schneck ('), H. Savary ('), R. Mellet ('), P. Miiller (2) and R. Kleiner (2)
(') France Telecom, CNET Bagneux, 92220 Bagneux, France
(2) Walther-Meissner Institut, 8046 Garching, Germany
(Received 3 Februaiy 1994, revised and accepted 14 April 1994)
Rdsumd. Nous prdsentons des rdsultats expdrimentaux, qui montrent que l'augmentation de la substitution partielle du bismuth par le plomb dans des monocristaux de Bi-2 :2 :1: 2, peut permettre de rdduire l'anisotropie. Le renforcement des forces de liaison entre plans Bi-O
adjacents, induit par cette substitution, est confirmd par de nouveaux rdsultats sur l'intercalation d'iode entre ces plans. Dans la phase normale, la substitution par le plomb a pour effet de diminuer la rdsistivitd dans la direction c d'un facteur 60. Dans la phase supraconductrice, elle augmente le
couplage Josephson entre les feuillets. Il en rdsulte des densitds de courant critique plus importantes dans la direction c, jusqu'h 5000A/cm~, ainsi que des caractdristiques I-V
hystdrdtiques, avec deux dtats, sans branches intermddiaires.
Abstract. We report experimental results showing that the increase of the lead-substitution in Bi-2 : 2: :2 single crystals can offer the possibility to reduce the anisotropy. The strengthening
of the binding forces between adjacent Bi-O planes, induced by lead-substitution, is confirmed by
new results about iodine intercalation. In the normal phase, the lead-substitution has the effect to decrease the resistivity along the c-direction by a factor as high as 60. In the superconducting phase, it increases the Josephson coupling between the layers, resulting in high densities of critical
current along the c-axis, up to 5 000 A/cm~ and two-state hysteretic I-V characteritics without
intermediate branches.
1. Introduction.
Owing to their layered structure and their high degree of anisotropy it has been argued that the
interlayer coupling in bismuth cuprates Bi~sr~cacu~os
~,
superconductors can be described in term of a Josephson coupling of the superconducting CUO~ bilayers with the BiO and SrO
planes acting as insulating or metallic barriers. This description, corresponding to the Lawrence-Doniach model [11, is consistent with the fact that in this material, the c-acis coherence length f~ is small as compared to the 15 I spacing between the CUO~ bilayers. This
picture has been first indirectly supported by several experiments devoted to investigation of
2250 JOURNAL DE PHYSIQUE III N° II
the temperature and angular dependence of the upper critical fields [21. More recently the a-c- and d-c- manifestations of the intrinsic Josephson effect have been observed in thin single crystals of Bi~Sr~CaCU~O~
_,,
showing directly that a crystal behaves like a series array of tunnel Josephson junctions [3].
Finding a way to tune the anisotropy of the material and possibly change the characteristics of the Josephson coupling between the superconducting CUO~ bilayers should allow to
experimentally investigate the important issue concerning the influence of the interlayer coupling on the superconducting properties.
In this paper, we show that increasing the lead-substitution in single crystals of
Bizsr~cacu~og_~ can offer the possibility to progressively reduce the anisotropy. This conclusion relies on a consistent set of experimental observations, concerning the evolution of
some structural properties, as well as the modifications of the perpendicular transport
properties, relatively to the amount of lead. We particularly emphasize this latter aspect by showing a progressive decrease of the resistivity anisotropy, in the normal phase and a
progressive increase of the perpendicular critical current density, in the superconducting phase. We discuss the effect of the lead-substitution on the Josephson interlayer coupling, through the observation of striking modifications in the I (V) characteristics induced by the
presence of lead. The possible physical implications of these results and relevance for
applications are also discussed.
2. Experimental.
Our studies have been carried out on single-crystals of the Bi-2
: 2 : 2 phase elaborated by a
method we have previously described [41. Crystals with differente lead concentrations have
been obtained by changing the PbO amount in the melt. The cations proportions have been
checked by means of an electron microprobe of I ~Lm diameter. We found for each crystal a
uniform composition over the surface, and for each melt a good reproductibility in the average
composition from one crystal to another. Changing the lead amount in the melt mainly affects the respective fraction of bismuth and lead cations in the average formulas, which correspond approximately to Bi~ _,Pb,Sr~cacuzo~, with x varying from 0 to 0.4. We have consistently
assumed that lead is mainly substituted at the bismuth site, a type of substitution which is also consistent with previous structural considerations [41 and with recent investigations of the
~°~Pb nuclear magnetic resonance of lead-substituted BiSrCaCUO compounds [5]. The good crystalline quality of our as grown crystals is attested by the observation, on X-ray precession photographs of the (hk0) and (0k/ reciprocal planes, of very well defined Bragg reflections, and in particular with no diffused strikes along the I * reciprocal direction which would result
from a disordered stacking of the layers constituting the crystals. Iodine intercalation has been
achieved by annealing crystals and iodine in evacuated Pyrex glass ampoules, at 150 °C,
during ten days. We have measured the c-parameter variations owing to this intercalation, both on X-ray precession photographs and with a two circles X-ray diffractometer. We have
performed the resitivity anisotropy measurements with the standard Montgomery method [6],
on platelets of typical dimension 000 x 500 ~Lm~ and of thickness between 9 ~Lm and 3 ~Lm
depending on the sample, by using an a-c- technique with currents between 0.I and lo mA at I kHz. The d-c- and a-c- Josephson characteristics with the current flowing along the c-axis
have been performed on crystals which have been prepared with two different methods. The first one consists to cut and cleave single-crystals perpendicularly to the c-axis, to obtain thin-
rectangular platelets of typical lateral dimensions 50 x 50 ~Lm~ and of thickness of approxi- mately 1-3 ~Lm. The other one uses a technological method. By a combination of photolitog-
~aphy and of Ar-ion milling we have patterned sets of cylindrical mesas on single crystal piatejets j7j the diameter of the mesas varying between 30 ~Lm and 60 ~Lm and their thickness
varying between 2 0001 and 7 000 I. In that
case the Josephson transport measurements
across the mesas were performed by using the etched plane of the single crystal platelet as the
lower electrode, and the top of the investigated mesa as the upper electrode. The microwave emission measuring technique is described in details in reference [3].
3. Experimental results.
3,I STRUCTURAL ANISOTROPY. In order to specify the consequence of lead-substitution at
the structural level, we have compared the ability of iodine to intercalate in lead-free and lead- substituted crystals. We have performed iodine intercalation in Bi~_,Pb,Sr~cacu,O,, com- Pounds with.r ranging from 0 to 0.4. We have found an increase of the c-lattice parameter from
30.8 to 38.0 and 37.61, for,;
= 0 and 0.I respectively. This is the result of a stage-I
intercalation [8]. But as schematically described in figure I, iodine no longer intercalates in the
crystals with x ~ 0.36. As iodine intercalation in lead-free Bi~sr~cacu~og
~,,
is permitted by the very weak inter-plane binding forces between adjacent BiO planes, we have concluded that
a sufficient amount of lead substitution should increase this binding force to such a point that it prevents iodine intercalation. We can say that lead-substitution reduces the «structural anisotropy » of the BiO planes which is related to the difference between the intra-plane and the inter-plane binding forces. This conclusion is also consistent with that we have previously
drawn from X-ray diffraction studies [41, which have pointed-out that lead-substitution induces the following structural modifications a (.-parameter decrease, an increase of the incommen-
surate modulation wavelength and the in-phase stacking of the blocks relatively to the
incommensurate modulation. Indeed, these results have been previously discussed consistently
with the occurrence of a strengthening of the inter-plane binding forces between adjacent Bi-O
planes, which could result from the establishment of a covalent type of bonding [41.
A
B14J B14J
Bi-O
~
@
~c.
.~
Bi-O
O~
B
Fig. I. Schematic representation of iodine intercalation in Bi~_,Pb,Sr~CaCU~O,. A) For x
=
0 and
; 0. I iodine intercalates. The bloks are out-of-phase stacked and weakly bonded along the c-direction, relatively to the incommensurate modulation and cm 37.61. B) For ; m 0.35, the bloks are in-phase
stacked and are strongly bonded. c. 30.7 I. Iodine
no longer intercalates.
2252 JOURNAL DE PHYSIQUE III N° II
3.2 RESISTIVITY ANISOTROPY. The temperature dependence of the in-plane p~~ and out-of-
plane p~ resistivities for lead-free and lead-substituted crystals, with x ranging from 0. I to 0.4,
are represented in figures 2 and 3. Comparison of the transition temperatures for the different
pc (Ohms,cm)
50
o.75 ~
40 f.
. pb/B%25%
~'
~ %
30
"..~ '
50
. 150
~
..
~ 20
.
~
10
'..
.
o
50
T@Q
.
Fig. 2. - Temperature of
ead-substitution.
(.j : T~ = 86.6 K. (v) Bij ~~Pbo : T~ = 84
Bii : ~ = K, the inset
shows p, vs. this
cristal with a
dilated
P
~
/P ~b lE7
~...
.. .. . ..
. . .
'..
lE4
~....-...
50
100 150
200 250 300
T@Q
.
Fig. 3. Temperature dependence of the resistivity anisotropy as a function of the lead-substitution.
(.) Bi~Sr~CaCU~O~ T~ 86.6 K. (v) Bij »Pbo 2~Sr2CaCu20, : T~ 84 K. (m) Bij ~Pbo ~Sr~CaCU~O,.
T~ 79.2 K. Note that each curve differs from each other by one order of magnitude.
samples yields a T~ reduction from approximately 88 K to 80 K, as measure as the lead amount increases in the crystals. It is shown in figure 2, that increasing the amount of lead in the
crystals induces a progressive decrease of the room temperature magnitude of the out-of-plane resistivity p,, of approximately a factor 25, from 20 n-cm to 0.8 Q-cm- Near T~, the decrease of p~ corresponds to approximately one order of magnitude for intermediate lead concentration
x= 0.25 and reaches a factor 60 at the optimum of lead-substitution xi 0.4. This is
accompanied by a progressive modification of the semiconducting type of temperature dependence (dp,/dT
~ 0), which has been previously reported in lead-free crystals [91, toward
a quasi-metallic type of behaviour. Actually, for the optimum of lead-substitution, p, decreases linearly with decreasing temperature down to around 130 K where it presents a
shallow minimum and then increases slightly until T~, where it drops abruptly. The
temperature dependences of the anisotropy p~/p~~, for crystals with different amounts of lead substitution, are represented in figure 3. It clearly appears, in the whole temperature range between room temperature and the superconducting transition, that lead-substituted crystals have a weaker resistivity anisotropy than the lead-free ones. This decrease of anisotropy mainly results from the progressive increase of the out-of-plane conductivity, as measure as the
amount of lead-substitution increases in the samples. For intermediate lead-concentrations, the
resistivity anisotropy is one order of magnitude lower than that of lead-free samples. And for the optimum of lead-substitution, this decrease of the anisotropy is up to two orders of
magnitude weaker. Consistently, we have not observed significant modifications resulting
from the lead addition in the magnitude and in the temperature behaviour of the in-plane
resistivity p~~.
3.3 JOSEPHSON couPLiNG. The intrinsic Josephson effects, I-e- with each CUO~ bilayers
acting as the superconducting electrode and the BiO and SrO layers acting as a the barrier, have been observed and described in the lead-free Bi-2 : 2 : 2 single crystals [31. We show that the Josephson effects along the c-direction, still exist in the lead-substituted crystals but with characteristics consistent with a strengthening of the interlayer coupling. We present here
some typical manifestations of the intrinsic Josephson effect, obtained for the most heavily
substituted crystals (~
=
0.4 the existence of hysteretic I-V characteristics, when the current flows along the c-axis (see Fig. 4) and microwave emission (see Fig. 5). The I(V) curves exhibit two branches, a superconducting one and a resistive one. We have checked that the
voltage jump, corresponding to the switching from the superconducting branch to the resistive one, is proportional to the thickness of the crystals. This result is consistent with an increase of
the number of junctions with the thickness of the crystals, if we suppose the crystals
constituted with a series array of identical junctions. The following results of microwave emission confirms this assumption. The emission detector set-up we have used has its range of
operation around lo GHz. For a Single junction and according to the frequency-voltage Josephson relation, Josephson emission in this frequency range should correspond to a voltage of only few tens of microvolts across the junction. In consequence to observe emission coming
from a crystal, we have performed the measurement at 72K, a temperature near
T~ (T~ = 74 K) where the characteristic voltage of the I(V) curve is strongly decreased.
Figure 5 shows the microwave emission detected at f
=
I1.05 GHz. We observe a rather
broad emission because at such a high temperature all junctions within a crystal radiate
incoherently. The peak maximum arises at V =16.8 mV. Hence, the Josephson relation
f
=
2 eV/hN allows us to calculate the number N of radiating junctions in a series array. For
the case under consideration, this gives N
=
740 junctions with a voltage of 22 ~LV per
junction. Multiplying the number N by lsi, I-e- the spacing between consecutive CUO~ bilayers, we obtain I. ~Lm, which is roughly the thickness of the sample measured with
a scanning electron microscope.
2254 JOURNAL DE PHYSIQUE III N°
o.i
o.05
'i'f
0
< =70
.o.05 '~
-o.i
.i -o.5 o o.5
v (volts)
Fig. 4. I-V characteristics for a crystal with a maximum lead-substitution. Bij ~Pbo~Sr~CaCU~O,. for two different temperatures. Note that there is only two branche~. Inside characteristic at T
=
4 K, the hysteresis is maximum. Outside characteristic at 70 K, near T~ (T~ 74 K), the value of R~I~ is of 80 mV, which remains relatively high.
0.0045
I
~ ~- 0.0035
E G~
f
< 5
~ 0.0025
f -0.025
n
o.ooi 5 -o.05
-o.i -o.05 o o.05 o.i
v (vi
Fig. 5. I-V characteristic and microwave emission for a crystal with a maximum lead-substitution, Bii~Pbo4Sr~CaCu~O~ (contact resistances have not been substracted), at T
= 74 K (T~ = 76 K ). The peak
maximum arises at V
=
16.8 mV when the voltage values are corrected from the contact resistances.
Comparing the I(V) characteristics at 4.2K of as grown lead-free samples and lead substituted samples we have observed two important consequences of the lead-substitution. In the first place, we have found out-of-plane critical current densities as high as 5 000 A/cm~, corresponding to an increase by nearly two orders of magnitude with respect to reported values in as grown lead-free crystals [3, 10].
In the second place, the I (V ) curves, with a two branches characteristic, similar to a « single
effective » junction (see Fig. 4), seem specific of the lead-substituted samples. Indeed, these
I(V curves do not show the large number of branches whose existence has been reported in
the lead-free single crystals. Such branches has been interpreted [3] as the result of a
succession of switching from the superconducting to the resistive state, of the slightly different
junctions stacked in the crystals. We have carefully checked that the distinct behaviour does not result from an unexpected effect of our technological process of preparation of the samples [7], by verifying the persistence off (V curves with multiple branches, for samples prepared
in the same manner with lead-free crystals.
4. Discussion.
The peculiar characteristics of the intrinsic Josephson effect in lead-substituted samples can be
related to the decrease of anisotropy described above. In particular the decrease of
p, with lead-substitution is fully consistent with the observation of a higher critical current
density in the arrays of junctions, since it is known that in Josephson junction (of the SIS or SNS types), the critical current density is proportional to the inverse of the normal state
resistance [10]. Hence, this decrease of p~ implies a strengthening of the inter-layer Josephson coupling and an enhancement of the Josephson tunneling.
We believe the absence of multiple branching in the I(V) curves of the lead-substituted samples to be due to a less sensitivity of this system, with respect to the lead-free one, to the
spreading of the parameters of the junctions, which results from the structural disorder of the
crystals. Indeed the occurence of the multiple branching in lead-free crystals has been
interpreted [3] as the result of a succession of switching from the superconducting state, of the
slightly different junctions staked in the crystals. In the lead-substituted case the greater
« tolerance » to these slight differences is possibly related to the important problem of the interaction between junctions [[[J. In lead-substituted samples, a more effective interaction
may occur, which has the effect that the switching of one junction to the resistive state, drive
the global switching of the entire array. This triggering of the global switching of the array by a
single junction, could be assigned to a modification of the conductance in the barrier, leading
to a more effective dynamic coupling of the junctions. Indeed, the higher conductivity of the barrier could play the role of a matched resistive load, similar to the extemal resistive load of
electronic circuits, which is known to stabilize the in-phase locking of the Josephson
oscillation in artificial arrays of slightly non identical junctions [12].
The origin of the more efficient coupling along c in the lead-substituted compound is not clear at present. This origin has to be found in the changes in the electronics properties of the material which certainly result from the two following factors the valence difference between lead and bismuth, and the structural modifications induced by lead-substitution. The valence difference between lead and bismuth might be compensated by several types of mechanisms including hole doping of the CUO~ planes, oxygen departure from the BiO planes or
coexistence of Pb~ + with Pb~+ Indeed, some recent therrnoelectric power measurements and
'~O NMR investigations seem to support the occurrence of hole doping in lead-substituted materials [13]. This would give consistency to the existence of some qualitative similar features, to those reported here, in oxygen annealed lead-free crystals [14], a treatment which is known to increase the hole concentration in the CUO~ planes. This similarities include the T~ reduction [15], the shrinking of the c-parameter [16], the increase of the out-of-plane
conductance [3, 15] and the increase of the c-axis critical current density [3]. However it is
important to note that, while the magnitude of the T~ reduction is the same in lead-doped crystals than in lead-free oxygen annealed crystals, the amplitude of the decrease of anisotropy
in the transport properties is significantly more important in lead-substituted samples.
