Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems

Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems

We adopted the x-ray diffraction, oxygen contents, positron annihilation technology and simulation methods to investigate systematically YBa₂Cu₃–x(Al,Zn,Co)xO₇–δ (x = 0–0.5) cuprates. The experimental results and simulated calculations support the existence of cluster effect. Moreover, it is conclud...

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Дата:2010
Автори: Yufeng, Z., Dandan, W., Pinglin, L.
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Опубліковано: Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2010
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Свеpхпpоводимость, в том числе высокотемпеpатуpная
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Цитувати:Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems / Z. Yufeng, W. Dandan, L. Pinglin // Физика низких температур. — 2010. — Т. 36, № 2. — С. 206-211. — Бібліогр.: 47 назв. — англ.

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spelling irk-123456789-1168962017-05-19T03:02:56Z Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems Yufeng, Z. Dandan, W. Pinglin, L. Свеpхпpоводимость, в том числе высокотемпеpатуpная We adopted the x-ray diffraction, oxygen contents, positron annihilation technology and simulation methods to investigate systematically YBa₂Cu₃–x(Al,Zn,Co)xO₇–δ (x = 0–0.5) cuprates. The experimental results and simulated calculations support the existence of cluster effect. Moreover, it is concluded that the cluster effect is an important factor on suppression of the superconductivity and the Tc does not depend directly on the density of valence electron in the samples. 2010 Article Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems / Z. Yufeng, W. Dandan, L. Pinglin // Физика низких температур. — 2010. — Т. 36, № 2. — С. 206-211. — Бібліогр.: 47 назв. — англ. 0132-6414 PACS: 68.65.–k, 47.54.De, 47.54.Bd. http://dspace.nbuv.gov.ua/handle/123456789/116896 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
topic Свеpхпpоводимость, в том числе высокотемпеpатуpная
Свеpхпpоводимость, в том числе высокотемпеpатуpная
spellingShingle Свеpхпpоводимость, в том числе высокотемпеpатуpная
Свеpхпpоводимость, в том числе высокотемпеpатуpная
Yufeng, Z.
Dandan, W.
Pinglin, L.
Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems
Физика низких температур
description We adopted the x-ray diffraction, oxygen contents, positron annihilation technology and simulation methods to investigate systematically YBa₂Cu₃–x(Al,Zn,Co)xO₇–δ (x = 0–0.5) cuprates. The experimental results and simulated calculations support the existence of cluster effect. Moreover, it is concluded that the cluster effect is an important factor on suppression of the superconductivity and the Tc does not depend directly on the density of valence electron in the samples.
format Article
author Yufeng, Z.
Dandan, W.
Pinglin, L.
author_facet Yufeng, Z.
Dandan, W.
Pinglin, L.
author_sort Yufeng, Z.
title Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems
title_short Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems
title_full Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems
title_fullStr Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems
title_full_unstemmed Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems
title_sort superconductivity without dependence on valence electron density in (al, zn, co) doped ybco systems
publisher Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate 2010
topic_facet Свеpхпpоводимость, в том числе высокотемпеpатуpная
url http://dspace.nbuv.gov.ua/handle/123456789/116896
citation_txt Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems / Z. Yufeng, W. Dandan, L. Pinglin // Физика низких температур. — 2010. — Т. 36, № 2. — С. 206-211. — Бібліогр.: 47 назв. — англ.
series Физика низких температур
work_keys_str_mv AT yufengz superconductivitywithoutdependenceonvalenceelectrondensityinalzncodopedybcosystems
AT dandanw superconductivitywithoutdependenceonvalenceelectrondensityinalzncodopedybcosystems
AT pinglinl superconductivitywithoutdependenceonvalenceelectrondensityinalzncodopedybcosystems
first_indexed 2025-07-08T11:16:49Z
last_indexed 2025-07-08T11:16:49Z
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fulltext © Zhang Yufeng, Wang Dandan, and Li Pinglin, 2010 Fizika Nizkikh Temperatur, 2010, v. 36, No. 2, p. 206–211 Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems Zhang Yufeng Department of Mathematics and Physics, Shanghai University of Electric Power 28 Xuehai Road, Nanhui District, Shanghai 201300, PRC E-mail: zyfeng1011@yahoo.com.cn Wang Dandan and Li Pinglin School of Physics and Engineering, Zhengzhou University, Zhengzhou 450052, PRC Received July 30, 2009, revised September 1, 2009 We adopted the x-ray diffraction, oxygen contents, positron annihilation technology and simulation methods to investigate systematically YBa2Cu3–x(Al, Zn, Co)xO7–δ (x = 0–0.5) cuprates. The experimental results and simulated calculations support the existence of cluster effect. Moreover, it is concluded that the cluster effect is an important factor on suppression of the superconductivity and the Tc does not depend directly on the density of valence electron in the samples. PACS: 68.65.–k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties; 47.54.De Experimental aspects; 47.54.Bd Theoretical aspects. Keywords: YBCO, simulated calculations, positron annihilation, cluster effect, valence electron. 1. Introduction The high-Tc cuprates, especially YBCO systems, are still the highlight in the superconducting researches [1–5], however, the microscopic mechanism is challenging harshly to all theories, even as Zaanens statements: «The high-Tc superconductivity is on the list of the most pro- found physics problems…» [6]. In order to search and comprehend such a problem, scientists suggested many new theories and experimental technologies [7–9], for ex- ample, elemental substitution that play an important role in the research of high-Tc cuprates until today [10–13], namely, copper is replaced by magnetic or nonmagnetic elements. According to the traditional theory of magnetic pair-breaking, the nonmagnetic ions doped into supercon- ductors will suppress less the superconductivity than mag- netic ions. However, Zn2+ doped in YBCO, a kind of non- magnetic ion, suppresses more forcefully the superconduc- tivity than other nonmagnetic and magnetic ions, like Al and Co. At present, most researchers believe that the su- perconducting phenomenon occurs on CuO2 planes [14– 16], where Zn2+ ions substitute for Cu(2) sites. Hence, Zn doping will suppress noticeably the superconductivity [17,18]. Some scientists suggested Zn doping destroys the background of antiferromagnetism on CuO2 planes [19]; however, this proposal is still one of viewpoints of mag- netic pair-breaking. Besides, what a reason causes the tran- sition temperature Tc to fall faster and faster with the in- crease of doped concentration [20–25] and so on? Many queries are now still unclear; therefore, further investiga- tions of theories and experiments will be necessary in Al, Zn and Co doped YBCO cuprates. In order to realize the distribution of doped ions, here we will calculate the total defect energy and average bind- ing energy inside a cluster based on Islam’s method [26]. Simulated results reveal the possibility of cluster effect. When the doped concentration increases, the doped ions combine into different clusters on CuO2 planes and in CuO chains. Afterwards, we use positron annihilation technique (PAT) to probe the change of electron structure as Al, Zn and Co substitute for Cu in YBCO. PAT plays an impor- tant role in the investigation of such condensed matters as semiconductors, metal materials and high-Tc cuprates [27– 30]. The positron studies examine the properties of normal and superconducting states, such as Fermi surface, O–T transition and the carrier concentration [31–34]. In recent years, Jean and Li et al. reported many important results from positron experiments and theories concerning high-Tc superconducting mechanism [20–24,33–35]. In the present article, we report the systematical investigation concerning Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems Fizika Nizkikh Temperatur, 2010, v. 36, No. 2 207 YBa2Cu3–x(Al, Zn, Co)xO7–δ (x = 0–0.5) samples. The theo- retic calculations and experimental results of PAT and oxygen content support the existence of cluster effect. Moreover, it is reasoned that cluster effects, as an impor- tant factor, suppress the cuprate superconductivity and the Tc variations do not directly depend on the density of va- lence electron in the samples. 2. Experiments Samples of YBa2Cu3-x(Al, Zn, Co)xO7-δ (x = 0–0.5) were prepared by the same method as in Refs. 20 and 22–24. Due to the sensitivity of the PAT experiments, all samples were sintered in the same conditions in order to reduce the dispersion of the experimental results. The superconduc- ting transition temperature Tc was measured by the stan- dard dc four-probe method with a voltage resolution of 10–7 V (HP3457A). The crystal structures of samples were analyzed by powder x-ray diffraction (XRD) using the D/max-BX-ray diffractometer. The positron lifetime spec- tra were measured by the ORTEC-100U fast-fast coin- cidence lifetime spectrometer. Two pieces of identical samples (∅13×3 mm) were sandwiched together with a 10-µC22 Na positron source deposited on a thin Mylar foil (about 1.2 mg/cm2 thickness). We apply Pilot-U plastic flicker sensor, which was tested by 60Co and showed the excellent time resolution over 220 ps. Each spectrum con- tains more than 1·106 counts to guarantee the sufficient statistic precision. After subtracting background and source contributions, the lifetime spectra were fitted with two- lifetime components by POSITRON-FIT-EXTENDED program with the best fit (χ2 ＝1.0–1.1). The positron life- time spectra of all samples were measured at the identical environment temperature, (283 ± 1) K, and the results were reproducible. 3. Results and discussions 3.1. Simulated calculations In order to understand the characteristics of cluster structure, here we perform simulated calculations on the energy minimization principle and framework of Born model, in which the effective pairwise potentials represent the interatomic forces in the following form [26]: 2 6 0 exp , 4 i j ij ij ij ij ij ij ij Z Z e r C A r rπε ρ ⎛ ⎞ Φ = + − −⎜ ⎟⎜ ⎟ ⎝ ⎠ (1) where Φij is the effective potentials between ions i and j; Zi and Zj are ion valences; rij is the distance between ions i and j; Aij , Cij and ρij are the relative characteristic con- stants. The first term is the long-range Coulomb interac- tion; the remaining terms represent the short-range interac- tion and the shielded revision. Because charge equilibrium determines the ion coordination characteristics in clusters, the clusters should be electrically neutral. The neutral clus- ters may show several structures, two of which are illus- trated as (a) and (b) on CuO2 planes in Refs. 20–24. Here (a) as Hexamer denotes six doped ions with two common- lateral squares, and (b) as Double Square does seven doped ions with two common-apical squares. On CuO2 planes Cu ions are ＋2.25 valence, and doped Zn ions are +2, more- over holes exist on oxygen ions [25,36]. These two factors together make electrons lose; consequently, the number of the oxygen ions has to decrease in order to keep the charge equilibrium. For example, in four doped ions cluster, every four Zn2+ ions may exclude an oxygen ion, which can be described as {4Zn2+ → 4Cu2.25+ － O2–}. Nevertheless, one Zn ion alone cannot exclude an oxygen ion. Hence, the binding energy of Zn2+ cluster is: 2 2.25 2 bind 1 2 2.25 2 2 3 (4Zn 4Cu O ) 4 (Zn Cu ) (O ) . b b b E E E E + + − + + − = → − − − → + (2) where Eb1, Eb2 and Eb3 are all represented as E ＝ ∑Φij . The first term Eb1 is the cluster total energy, Eb2 is the binding energy that a Zn2+ substitutes for a Cu2.25+ ion, and Eb3 is the binding energy that an O2－ ion is lost. For both Hexamer and Double Square, Ebind have similar formula- tion. The average binding energy of every doped ion is Emean = Ebind/N where N is the number of doped ions in a cluster. Other clusters may likewise be described and cal- culated. The calculation results are listed in Table 1, in which the negative value of the binding energy indicates the system is bound. As well known, the larger average binding energy is，the more stable a cluster combines. Consequently, the doped ions should prefer to form the cluster with largest average binding energy. It can be con- cluded from the results listed in Table 1 that doped Al and Co ions prefer to form Hexamer cluster of six ions, while for Zn doping Double Square (a common point, seven ions) clusters have the largest probability. As for the larger cluster, it can be composed by some small clusters, for example, the cluster of eight ions are composed by double four ions and so on. Table 1. The mean binding energy of per doped ion in the doped YBCO Cluster Al3+ – Em Co3+ – Em Cluster Zn2+ – Em D + O –2.73 –2.80 D –1.67 TL + 2O –2.25 –2.44 TL–O –1.49 TS + 2O –2.71 –2.82 TS–O –1.64 TZ + 2O –2.10 –2.15 TZ–O –1.36 H + 3O –2.83 H–2O –1.68 H + 4O –2.85 –3.03 H–O –1.75 DS + 4O –2.81 –2.91 DS–2O –1.78 N o t e s: Here the negative value indicates the binding energy. Em (eV/ion) is the mean binding energy per doped ion. D + O = Dimmer {2M→2Cu2++O2–} (M = Al3+, Co3+), «+O» represents an O2– ion is attracted into the cluster; TL + 2O = Tetramer {4M → 4Cu2+ + 2O2–} Linear; TS + 2O = Tetramer {4M → 4Cu2+ + 2O2–} Square; TZ + 2O = Tetramer {4M → 4Cu2+ + 2O2–} Zigzag; H + 4O = Hexamer {6M → 6Cu2+ + 4O2－}; DS + 4O = Double Square {7M → 7Cu2+ + 4O2–}. Besides, Zn2+ clusters will squeeze out oxygen ion, which is marked as TL－O and so on. «–O» represents an O2– ion is squeeze out the cluster. Zhang Yufeng, Wang Dandan, and Li Pinglin 208 Fizika Nizkikh Temperatur, 2010, v. 36, No. 2 3.2. X-ray diffraction results and cuprate superconductivity XRD results show that undoped and low-doped Y-123 samples have the well single phase. As doped concentra- tion x increase, samples show slight impure phases at x = = 0.12, x = 0.20 and at x = 0.25 for Zn, Al, and Co doping, respectively. Here in-order three x values correspond with the average binding energy Em of Zn, Al, and Co. Every cluster in Table 1, namely, x (Zn) < x (Al) < x (Co) has the same relation to Em(Zn) < Em(Al) < Em(Co). Such a result reveals that the small theoretic Em samples form easily the impure phases in the experiments, so our theoretic calcula- tions are able to reflect the experimental data. All the lat- tice parameters were calculated by the least square method with powder XRD data, and the results of Zn and Al doped samples are shown in Fig. 1 and Fig. 2. Due to Co doped samples have the similar results to Al doped, the former lattice parameters are not illustrated. For Zn doped sam- ples, the lattice parameter a and b increase slightly with x increasing, because the Zn2+ radius (0.74 Å) is larger than the Cu2+ radius (0.72 Å), but the O–T transition does not appear in x = 0–0.4. While for Al and Co doping, O–T transition occurs near x = 0.15 and 0.12, respectively. Generally, O–T transition results from the change of oxygen content in Cu–O chains [20–23]. Thus the variation of lattice parameters a and b indicates that Al and Co ions enter mainly Cu(1) sites, which is consistent with Hoff- mann et al. experiments [37,38]. It should be noticed that O–T transition occurs before the appearance of impure phases, which reflects the intrinsic properties of Al and Co doped Y123 systems. For the same reason, the variation of lattice parameters shows that Zn ions enter mainly Cu(2) sites on CuO2 planes [39,41], where the superconductivity occurs. It implies that the superconductivity suppressed by Zn doping should be stronger than by Al and Co doping. In our experiments [20,22–24] , the Tc is 92 K for Y123 com- pounds without elemental substitution, which call undoped samples in the text. For Al doping, during x = 0–0.10, the Tc falls slightly to 91 K; at x = 0.20, the Tc is 83.2 K; and at x = 0.40, Tc = 42.5 K. And the Tc falls to about 72.3 K for Zn concentration x = 0.10; when x = 0.20, the Tc is 42.6 K and for x = 0.40, Tc = 15.7 K. While for Co doped samples, the Tc decreases stepwise with x increasing, when x = 0.05, the Tc is about 80.0 K; at x = 0.10, the Tc is 67.5 K; at x = 0.20, the Tc is 44.5 K; and if x = 0.30, one has Tc = 21.0 K. Obviously, when x < 0.20, Zn doping suppresses the superconductivity much more severely than Al and Co doping, however, when x > 0.20, Co doping suppresses most strongly the superconductivity in the three doped ions. It suggests that the cluster effect is still an im- portant factor in suppression of the superconductivity be- cause Co ions show the strongest cluster effect as above calculation results. 3.3. Oxygen content variations The insert picture in Fig. 1 and Fig. 2 show the experi- mental results of oxygen content concerning Al and Zn doped samples [17]. Here the oxygen content of Co doping is omitted as being similar to Al doping. For M3+ (Al3+, Co3+) doping, M3+ valence is higher than Cu2+ in Cu–O chains, the dispersed ions cannot capture one oxygen ion due to charge equilibrium. As x increases the ions form clusters, which are able to capture oxygen ions, so the oxygen content should rise in Al and Co doped samples. However, for Zn doping, the oxygen content falls obvi- ously with x increase, such variation characteristics are consistent with the simulated calculations, because Zn2+ valence is lower than Cu2.25+ and the dispersed ions cannot extrude oxygen ions to achieve electric neutrality. While the cluster has not formed yet and Zn ions are distributed randomly, there are hardly oxygen vacancies around Zn ions and the oxygen content should not decrease. With x increasing, Zn ions form more and more clusters, because Zn ions require less oxygen ions for coordination than Cu, Fig. 1. Lattice parameter a and b variation with Al doped concentration x in YBCO systems. The insert picture indicates the oxygen content variation with x (Ref. 17). 00 0,10,1 0,20,2 0,30,3 0,40,4 3,813,81 3,823,82 3,833,83 3,843,84 3,853,85 3,863,86 3,873,87 00 0,10,1 0,20,2 0,30,3 0,40,4 bb aa L at ti ce p ar am et er s, L at ti ce p ar am et er s, ÅÅ Al doped concentrationAl doped concentration xx Al doped concentrationAl doped concentration õõ 6,806,80 6,856,85 6,906,90 6,956,95 7,007,00 7,057,05 O x y g en co n te n t O x y g en co n te n t Fig. 2. Lattice parameter a and b variation with Zn doped concentration x in YBCO systems. The insert picture indicates the oxygen content variation with x (Ref. 17). 00 0,10,1 0,20,2 0,30,3 0,40,4 3,803,80 3,813,81 3,823,82 3,833,83 3,843,84 3,853,85 3,863,86 3,873,87 3,883,88 3,893,89 3,903,90 00 0,10,1 0,20,2 0,30,3 bb aa Zn doped concentrationZn doped concentration xx Zn doped concentrationZn doped concentration xx 6,756,75 6,806,80 6,856,85 L at ti ce p ar am et er s, L at ti ce p ar am et er s, ÅÅ O x y g en co n te n t O x y g en co n te n t Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems Fizika Nizkikh Temperatur, 2010, v. 36, No. 2 209 the extra oxygen ions will be extruded from the clusters. Therefore, oxygen vacancies would appear on CuO2 planes, and then the oxygen content should descend no- ticeably (Fig. 2, insert). We can conclude that cluster effect is indeed a factor that causes oxygen content to reduce in Zn doped YBCO. From the above analysis, oxygen content variation characteristics in those doped samples can be explained convincingly by the cluster effect. In fact, the PAT experiments have the same conclusions. 3.4. Positron experiments PAT experiments are described in detail in our other papers [20–24]. The positron lifetime is defined as the in- verse of annihilation rate. According to the model of two- state capture in condensed matter [42] with the short life- time τ1 and the long lifetime τ2 , the processes of positron annihilation are attributed to free-state annihilation and trapping-state annihilation, respectively. The former inten- sity I1 denotes the proportion of free-state annihilation to total annihilation events, the latter intensity I2 = 1 – I1. τ1 reflects mainly the annihilation process in the perfect crys- tal lattices and can be used to detect the electron distribu- tion of the inner microstructure. τ2 reflects mainly the process of positron captured in the imperfect areas inside the materials; it can characterize the intrinsic structure and preparation quality of samples. If positrons annihilate in the imperfections, such as oxygen vacancy, twin boundary, dislocation, and cation vacancies, τ2 will be larger, some- times it is noted as τ3, generally τ3 > 800 ps [43–45]. Our samples do not contain τ3 component, this indicates the credibility of the sample quality. According to the characteristics of positron annihila- tion, local electron density 2 0 bulk1 ( )en r cπ τ= ／ , where 0 r is the classical electron radius, c is the velocity of light, and bulkτ is the systematical lifetime parameter, it is de- fined as: 1 1 2bulk 21 .I Iτ τ τ= + (3) Some relational calculation results [46,47] show that about 90% positrons annihilate with the valence electrons, and only about 10% positrons annihilate with the electrons within the atomic kernel. These results indicate that the ne amount is determined mainly by the valence electrons, so we may regard the density of valence electron as the ne without influence on the general conclusions. When M+3 (Al+3, Co+3) enter Cu–O chains as above, the dispersive M+3 contains one valence electron less than Cu2+, so the ne should drop. However, the doped ions form clusters as x increases, oxygen ions will be attracted into the crystal lattice. Once introduced by the clusters, every oxygen ion will attract two valence electrons in terms of charge equilibrium. Therefore the valence electron density rises, namely, the ne shows a rising trend. When the ne dropping and rising reach a balance, the low saturation emerges as shown in the experiments [24]. Obviously, such a result originates from the cluster effect. In contrast, while +2 valence ions enter CuO2 planes, Zn2+ loses fewer electrons than Cu2.25+, so the density should rise on the consideration of valence state. However, as x increase, the Zn ions start to form clusters. As mentioned before, every four Zn2+ will extrude an O– (including a hole) which car- ries away two valance electrons, so the density begin to fall. When the rising and falling of the density reach a bal- ance, the density ne tends to high saturation [20], which is influenced evidently by the cluster effect. As a result, the ne saturation also can be elucidated satisfactorily through the cluster effect as the oxygen content variations above. 3.5. Valence electron density and the Tc of cuprates Figure 3 shows the relationship between the Tc and the reduced ne in Al, Zn and Co doped samples. For Al and Co doped samples, both reduced ne have the great decrease in the little doping, the Tc descends only slightly, especially in the Al doped samples, but the reduced ne of Co doped samples descends faster than that of Al doped in the same concentration x, as shown in the figure, so the ne–Tc curve turning point in Co doped samples is further from ne = 1.00 than that in Al doped, such a result still gives evidence that the Em is larger in Co doped samples. Below the curve turning point in both samples, the Tc seems to lose further the association with the reduced density ne, in particular, when both Tc drop sharply, the reduced ne varies only slightly. In fact, when x is low, both ions enter the crystal lattice in individual, M3+ loses more valence electrons than Cu2+, so the density ne will decrease. However, the Tc de- scends slightly with the density change because the dis- persed ions cannot destroy the crystal lattice. Below the curve turning point the Tc starts to drop sharply with clus- ters appearing. On our idea, the stronger cluster effect is, the stronger it will distort the structure of crystal lattice. So the superconductivity is seriously suppressed in both sam- ples. In contrast with both above, the ne–Tc curve of Zn doping has the opposite variation trend in the beginning stage, though the reduced density raises greatly, the Tc de- scends still slightly, below the curve turning point the Tc Fig. 3. The variation of superconducting transition temperature Tc with reduced valence electron density ne in Al, Zn and Co doped YBCO systems. 1010 2020 3030 4040 5050 6060 7070 8080 9090 100100 0,920,92 0,940,94 0,960,96 0,980,98 1,001,00 1,021,02 1,041,04 1,061,06 CoCo ZnZnAlAl Local electronic density nLocal electronic density n , arb. units, arb. unitsee TT , K , K cc Zhang Yufeng, Wang Dandan, and Li Pinglin 210 Fizika Nizkikh Temperatur, 2010, v. 36, No. 2 starts to drop sharply from 72.3 K to 15.7 K, whereas the reduced ne increases only slightly. As above statements, similarly, when x is small, Zn ions enter the crystal lattice in individual; Zn2+ loses fewer electrons than Cu2.25+, so the density will increase. However, the Tc descends slightly with the density increasing, because the dispersed ions do not destroy the crystal lattice. While the Zn clusters appear in the doped samples, the structures of crystal lat- tice will be distorted by the cluster effect, which will influ- ence directly the pairing and transportation of carriers, and thus the superconductivity is suppressed markedly, then the Tc starts to drop sharply. Anyway, when the reduced density increases or decreases essentially, the Tc alters only slightly; on a contrary, while the Tc falls dramatically, the reduced density changes hardly. As a result, we can con- clude that the Tc has no direct connection with the valence electron density. However, when the cluster effect strengthens in the region of turning point, the Tc falls no- ticeably. Therefore, the cluster effect is an important factor that suppresses the cuprate superconductivity. 4. Conclusions In conclusion, the high-Tc cuprates YBa2Cu3–x(Al,Zn,Co)xO7–δ (x = 0–0.5) have been analyzed and studied systematically by XRD, positron annihilation technique, oxygen content, and calculations of binding energy. The simulated calcula- tions, the variations of oxygen content and positron ex- periments support congruously the existence of cluster effect. Especially, it is reasoned that the cluster effect is an important factor in suppression of high-Tc cuprate super- conductivity and the Tc has no direct connection with the density of valence electron. This work is supported by The Natural Science Founda- tion of China (No.10647145). 1. U. Schwingenschlogl and C. Schuster, Phys. Rev. B79, 092505 (2009). 2. H. Yamase, Phys. Rev. B79, 052501 (2009). 3. X. Wang, A. Dibos, and J.Z. Wu, Phys. Rev. B77, 144525 (2008). 4. H.H. Song, M.W. Davidson, and J. Schwartz, Supercond. Sci. Tech. 22, 062001 (2009). 5. W. Markowitsch, W. Lang, J.D. Pedarnig, and D. Bauerle, Supercond. Sci. 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