The recently observed variations of the transition temperature (T sub c) with oxygen content in the Bi based (2212) and (2223) superconductors are analyzed in terms of p+, the hole concentration per CuO2 sheet. This analysis shows that in this system, T sub c increases with p+ initially, reaching maxima at p+ = 0.2 approx. 0.3, followed by monotonic decrease of T sub c with p+. The forms of these variations are similar to those observed in the La(2-x)Sr(x)CuO4 and YBa2Cu3Oy systems, suggesting that p+ may be an important variable governing superconductivity in the cuprate superconductors

Seehra, M. S.

Zhao, J.

English

NASA Technical Reports Server

N92-21612
DEPENDENCE OF TRANSITION TEMPERATURE ON HOLE CONCENTRATION
PER CuO2 SHEET IN THE BI-BASED SUPERCONDUCTORS
J. Zhao and M. S. Seehra
Department of Physics, West Virginia University
Morgantown, West Virginia 26506, USA
Abstract
The recently observed variations of the transition temperature Tc with
oxygen content in the Bi-based (2212) and (2223) superconductors are analyzed
in terms of p+, the hole concentration per CuO2 sheet. This analysis shows that
in this systems Tc increases with p+ initially, reaching maxima at p+= 0.2 N 0.3,
followed by monotonic decrease of Tc with p+. The forms of these variations
are similar to those observed in the La2-xSrxCuO4 and YBa2Cu3Oy systems,
suggesting that p+ may be an important variable governing superconductivity
in the cuprate superconductors.
Among the cuprate high-Tc superconductors, La2-xSrxCuO4-5 is a
prototype since the unit cell of this system contains only a single CuO2 sheet
[1]. The hole concentration or excess valence p+ on the CuO2 sheet in an ionic
3+
r2x+(CuO2)-2+P+O2- 5 is given by p+ x 2_i,charge lattice represented by La2.xS = -
meaning that p+ can be varied by varying either x or 5 [2,3]. With 5 = 0, Tc
increases from 0 to 38 K as x increases from 0.06 to 0.15, Tc starts to decrease for
x > 0.15 and superconductivity disappears for x > 0.27 [2,3]. This result indicates
the existence of two superconducting states: a hole (carrier)-deficient state
and hole (carrier)-excess state. For the hole-deficient state, Tc increases with
holes concentration whereas for the hole-excess state, T c decreases as the hole
concentration increases.
For the (123) family of superconductors represented by YBa2Cu3Oy [4],
superconductivity vanishes for stoichiometric composition y = 6.5 and for y >
6.5, Tc increases with y, reaching 93 K for y ~ 7.0. In this system, each unit
cell contains two CuO2 sheets and a CuO chain. The question of where the
excess holes resides leading to superconductivity has been studied by Tokura et
al [5]. According to their analysis, the excess holes p given by p = (2y - Q - 6)/3
is distributed as Psh = (Y - 6.5)/2, and Pch = (Y - 6.5) with p = (2psh + Pch)/3. In
the above, Q is the total cationic charge (excluding Cu), and Psh and Pc h
respectively represent the excess holes in the CuO2 sheets and CuO chain. They
further suggested that holes associated with chains, Pch, are localized and
merely provide an insulating reservoir of charge whereas Psh is the principal
variable governing superconductivity. If p+ represents hole concentration
per CuO2 sheet, then p+ = Psh/2 for the (123) system. By doping with La and Ca,
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Tokura et al. showedthat Tc initially increaseswith p+ for p+ > 0, reaches
maximumof Tc- 85K at aroundp+ = 0.2 andfor p+ > 0.2, a decreaseof Tc is seen.
The idealizedstructureof the Bi2Sr2CalCu208+8 (2212) superconductoris
quite similar to that of the (123) compoundwith Ca, SrO and double BiO layers
replacingY, BaO and CuO chain layers respectively. Bi2Sr2Ca2Cu3010+8(2223)
phasecan be formed by inserting one more (CuO2 + Ca) slab into the (2212)
phase. From the structure similarities, one may expect that excessoxygen
atoms are residing in the double BiO layer, playing the samerole as the excess
oxygen in the CuO chains for the (123) superconductor. However, the situation
for the Bi (2212) phase is complicated. First of all, the exact location of the
excess oxygen atoms in the compound is still unclear. Secondly, there are
numerous deviations from the idealized structure and composition in the Bi
and T1 superconductor systems, especially in the double Bi(TI)O layers and the
Sr(Ba)O layers. Deviations such as strontium deficiency [6] (Sr:Ca < 2:1 for the
(2212) phase) also creates holes on the CuO2 sheets. Attemp to vary the oxygen
content will inevitably alter the structure to some extent. Thirdly, on
synthesizing the high Tc (2223) phase, intergrowth with a low Tc phase often
occurs in this system which makes quantitative study on a single phase
difficult.
A number of studies [7-11] have shown that Tc in the Bi superconductors is
sensitive to oxygen stoichiometry. For example, Morris et al [9] observed
increase in T c with the decrease of oxygen content for the (2212) phase. They
varied oxygen content by annealing the samples at different oxygen pressures
[9]. We used a TGA (Thermogravimetric Analysis) apparatus to heat a
Bi2Sr2CalCu208+ 8 (2212) sample to < 850°C and subsequently quench the
samples in Ar gas [10]. This method is relatively simple. Since the sample
treatment was carried out at atmosphere pressure, it is less destructive than
other methods and the weight variation can be monitored in-situ with 1 gg
sensitivity. By varying the quenching temperature, we were able to affect the
oxygen content by different levels and we observed an increase of T c of 25 K as
a result of loss of 0.09 oxygens per formula unit. By the same technique, we
studied a Pb-doped Bi superconductor consisting of the (2223) phase with an
admixture of the (2212) phase [11]. Here we observed that with loss of oxygen,
Tc of the (2212) phase increased from 70 K to 90 K, whereas that of the (2223)
phase decreased from 107 to 90 K. These results suggested that the as-prepared
Bi (2212) phase is in the hole-excess state whereas the Bi (2223) phase is in the
hole-deficient state. For an as-prepared TI (2223) phase sample, Tc has been
observed to decrease with decrease of oxygen content [12,13] whereas for an
as-prepared T1 (2201) phase sample [14], T c increases with decrease of oxygen
content.
At first glance, the above findings on the variation of Tc with hole
concentration in the (2212) and (2223) phases of the Bi superconductors do not
fit into a consistent picture. Furthermore, the Tc's of the Bi (2201), (2212),
(2223) and (2234) phases are quite different viz. 10, 85, 110 and 90 K
respectively and these results need to be understood. Here we show that when
the observed findings of the variation of Tc with oxygen content in the Bi
superconductor are analyzed in terms of p+, the hole concentration per CuO2
sheet, a consistent picture emerges. In addition, the variation of Tc with p+ in
56
Bi and possibly in TI superconductorsis quite similar to that shown in the La2-
xSrxCuO4.s and the YBa2Cu3Oy superconductors,providing a consistent picture
for the four families of the cuprate superconductors.
The Bi (2223)+(2212)phasesamplewe preparedis not a simple mixture
of two compounds. As mentionedin the previous paragraph,during the
formation of the (2223) phase, intergrowth of the (2212) phase occurs.
Therefore the double Bi-O layers are often sharedby the (2212) and the (2223)
phases. If we assumethat excessoxygenatoms are residing in the double BiO
layers, the amount of the holes provided by the excessoxygen atoms will be
sharedby two CuO2 sheetsin the (2212) phasecaseand by three CuO2sheetsin
the (2223)phasecase. Accordingto literature[15,16], 8 in Bi2Sr2CaCu208+_
varies from 0.15 to 0.6 for the as-prepared(2212) phasesample. With the
considerationof the holes created by strontium deficency [6], we assumethere
is 0.8 holes per formula unit. Then in average,the hole concentrationper
sheet,p+, is givenby p+ = 0.8/n. This givesp+ = 0.4 and0,27 for the (2212)and
(2223) phasesrespectively. Using these two values for the as-prepared
samples,the variations of Tc with p+ for the (2212) and (2223) phasesbasedon
our data [10,11] are shownin Fig. 1. A different choiceof 8 shifts p+ at which
the maximum Tc is observed. Howevergeneral shapeof the curve remainsthe
same. Fig. 1 shows that the as-preparedBi (2223) is in a carrier-deficientstate
whereasthe as-preparedBi (2212) phaseis in a carrier-excessstate. This
explains the opposite directions of the Tc variation observedin the (2223) and
(2212) phasesas a result of loss of oxygen[7-11]. Accordingto the above
scheme,one may explain that Tc for the (2234) phasewith four CuO2 sheetsis
lower (90 K) than that for the (2223) phasebecauseof the smaller p+ value and
the low Tc (- 10 K) for the (2201)phasewith one Cu02 sheetbecauseof the
larger p+ value. In addition, the as-preparedBi (2234) and (2201) phase
samples are expected to belong to carrier-deficient and carreir-excessstates
respectively if the structure of the double BiO layer and the SrO layers remains
the sameas that for the (2212) and (2223) phases. We note that Urland and
Tietz [15] have reportedthe variation of Tc with p+ for the Bi (2212)phase. Tc
variation was achieved by varying Sr:Ca ratio and oxygen content. They
showed that as p+ increases from 0.1 to 0.6, Tc first increases from 73 to 80 K,
remaining at 80 K for p+ = 0.2 - 0.4, then falling for p+ > 0.4. Their results
suggest that for a single Bi or TI phase, the variation of Tc with p+ may follow
the same shape as that in Fig. 1. Recently, similar results have been reported
by Yoshizaki et al [17] and Torrance et al [18] respectively in Y doped, and Na
and K doped Bi (2212) samples. Maximum Tc's were found at p+ = 0.15 ~ 0.25. In
the case of La doped (2201) Bi2Sr2-xLaxCuO6+8 compound [19], Groen et al
showed that the as-prepared (2201) phase is in a carrier-excess state.
Maximum Tc = 28 K was observed at p+ = 0.3 and for p÷ < 0.21, the sample became
non-superconductor.
In the above, we have shown that the variation of T c with p+ for the Bi
(2212) and (2223) phases may be represented by Fig. 1. Moreover the shape of
the curve in Fig. 1 is similar to that for the (123) [5] and La2_xSrxCuO4+8 [2,3]
superconductors although the magnitude of p+ corresponding to the maximum
Tc is somewhat different for different families of superconductors. According
57
to previous experimentalresults [12,13,14], it is likely that the variation of Tc
with p+ for the Tl-based superconductingphasesmay follow a pattern similar
to that in Fig. 1 with maximumTc = 125 K. Theseresultssuggestthat p+ (excess
valence per CuO2 sheet)may be an important variable for the cuprate
superconductorsand these results should form the major test for any
successful theory of stiperconductivity.
It is interesting to note that without the hole reservoir, Tc for
La2-xSrxCuO4-8is below40 K. With CuOchain,doubleBiO layers anddoubleTIO
layers as hole reservoirs,Tc increasesto 93 K, 110K and 125 K respectively. It
appearsthat the key to the differencesin Tc is the interaction between the
hole reservoir part and the CuO2 sheets. Bishop et al [20] argued that although
superconductivity itself takes place on the CuO2 sheets, the attractive pairing
mechanism is not two-dimensionalbut may require dynamic coupling of these
sheets to the surrounding polarizable environment in order to understand the
difference in the Tc's of the four families of superconductors.
Acknowledgements: This research was supported in part by a grant from
the Energy Research Center of West Virginia University.
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Fig. 1.
120
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0.1 0.2 0.3 0.4 0.5
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The variations of Tc's with p+ for the Bi (2212) and (2223) phases.
59



Dependence of transition temperature on hole concentration per CuO2 sheet in the Bi-based superconductors

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