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Electronic structure investigation of the relationship between oxygen ordering and superconductivity in YBa2Cu3O6.5
Physica C 171 (1990) 465-467 North-Holland
Electronic structure investigation of the relationship between oxygen ordering and superconductivity in YBa2Cu306.5 R a j u P. G u p t a Centre d'Etudes Nucl~aires de Saclay, Section de Recherches de Mktallurgie Physique, 91191 Gif sur Yvette Cedex, France
Mich~le G u p t a Universitk de Paris-Sud, Institut des Sciences des Mat~riaux, 91405 Orsay Cedex, France Received 31 August 1990
Results of electronic structure calculations for the compound YBa2Cu306.5 are presented. Three models for the crystal structure have been considered: (a) an alternate chain (AC) model where each alternate chain is fully intact and the adjacent one fully empty, ( b ) an identical chain (IC) model where all chains are identical but are broken, and (c) tetragonal. Our calculations show that there is practically no charge transfer from the CuO2 planes towards the plane of the chains in the latter two cases. However, in the first case ~ 0.18 holes/CuO2 are created. This shows that the ordered chains are absolutely essential for the superconductivity and the disorder in the chains destroys superconductivity.
One of the most interesting aspects of the YBa2CuaOT_~ family of superconductors is the dramatic dependence of the superconducting transition temperature Tc on the oxygen stoichiometry ~. This compound is superconducting with a maximum To~90 K for ~---0 while for ~~ 1 it is nonmetallic and nonsuperconducting [1-6]. Indeed, as ~ increases from 0 to 1, Tc decreases continuously going through a well known two-plateau behaviour. Tc remains constant at ~ 9 0 K for J < 0 . 2 but for ~>0.2 there is a sharp drop until it stabilizes at ~ 60 K for 0.3 < ~< 0.5. For ~> 0.5 there is again a rapid decline until the material becomes nonsuperconducting for ~> 0.65. The transition from the superconducting to the nonsuperconducting state is accompanied by an orthogonal to tetragonal ( O - T ) phase transition. The origin of the existence of this two plateau region is not clear. Electron microscopy investigations [4] show that there is a short range ordering of the oxygen atoms as the oxygen is removed from the fully superconducting compound YBa2Cu3OT. In the YBa2Cu307 compound the O ( 5 ) sites along the aaxis are empty, resulting in the formation of the socalled CuO chains along the b-axis. In the 90 K plateau region (for ~< 0.2) there is a partial oxygen de-
ficiency at random positions in the chain which how'ever does not seriously affect their fully ordered character. The 60 K plateau region is, on the other hand, dominated by a long range ordered 2a superlattice where the alternate chains are fully ordered and completely intact as in the superconducting compound YBa2Cu307 while the adjacent ones fully empty as in the insulating nonsuperconducting compound YBa2Cu306. Such an ordering of the chains is absent when the compound is tetragonal and nonsuperconducting. Although the exact mechanism of superconductivity in the cuprate superconductors is still not understood, it is nonetheless well established that the superconductivity in these compounds arises when the two-dimensional CuO2 planes are conducting and that there is a direct correlation [ 7-10 ] between the hole concentration in these CuO2 planes and To. In the insulating YBa2Cu306 compound the lack of oxygens at the chain sites isolates the CuO2 planes from the chains so that these planes are nonconducting. In the fully stoichiometric superconducting YBa2Cu307 compound the presence of oxygen atoms on the chain sites couples the CuO2 planes to the CuO chains
0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
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R.P. Gupta, M. Gupta / Electronic structure calculations for YBa2Cu306 5
which act as electron reservoirs for electron transfer from the planes to the chains [7]. If the stoichiometry in oxygen is the dominant criterion for electron transfer from the planes to the chains, the compound YBa2Cu306.5 should remain superconducting in both orthorhombic and tetragonal crystal structures, which is evidently not the case since the compound is superconducting only in the orthorhombic phase when the chains are fully intact. It is therefore important to understand how the hole concentration in the CuO2 planes is affected as the chains are ordered or disordered. With this in mind we have performed electronic structure calculations for the compound YBa2Cu306.5 using the tight binding recursion method and determined the hole concentration in three different crystal structures, two of which are orthorhombic and the third tetragonal. In model I the order in the chains is fully preserved so that each alternate chain is fully intact, and the adjacent one fully empty (ACM). The structure remains orthorhombic but there is a doubling of the lattice parameter along the a-axis. This full-empty-full .... configuration has been found experimentally [2,4] for the 60 K plateau region. In model II we consider the case where all of the chains are identical (ICM). In this case all chains are obviously broken since an alternate oxygen atom on each chain is missing. The orthorhombic symmetry is however retained as there are no oxygen atoms on the O (5) sites along the a-axis. In model III the crystal symmetry is tetragonal so that the O (4) and O (5) ~ite occupancies are identical. In table I we have listed the charges on the copper and oxygen sites on a CuO2 plane obtained from our calculation for the three models. For the purpose of comparison the charges for the superconducting Y B a 2 C u 3 0 7 and the nonsuperconducting Y B a 2 C u 3 0 6 are also given from a previous publication [7 ]. It should be noted that in model I there are two different types of Cu sites that exist on a given CuO2 plane, a Cu just above or below a fully chain and a Cu just above or below an empty chain. There is a small difference between the charges at these sites because of differences in local environments. Due to considerations of simplicity this has been ignored and the charge on a Cu shown in table I is the average charge. The same also holds true for model III where three different Cu sites are present on a CuO2 plane.
Table I Number of electrons at Cu and O sites on the CuO2 plane. Also given are the number of holes per CuO2 plane measured from the half-filled band position. Compound
Number of electrons Cu O CuO2
Number of holes per CuO2 unit
YBa2Cu306 YBa2Cu307 YBa2Cu306.5 Model I
9.55 9.36
5.59 5.53
20.73 20.42
0.0 0.31
9.43
5.56
20.55
0.18
9.49
5.60
20.69
0.04
9.49
5.59
20.67
0.06
(ACM) Model II (ICM) Model Ill (Tetragonal)
Further, in the orthorhombic structure there is usually a small difference in the charge of oxygen at the O( 1 ) and 0 ( 2 ) sites. Again this has been ignored in table I and the charge presented is the average of the two. The hole concentration is obtained by taking the charge on a CuO2 plane in the insulating YBa2Cu306 compound as a reference. We thus see from table I that when the chains are broken and disordered or when the crystal symmetry is tetragonal there is practically no charge transfer from the CuO2 planes to the planes of the chains. The hole count < 0.06/CUO2 found in our calculation falls in the limit where electron localization is expected to occur and the material is nonmetallic and not superconducting [ 8-10 ]. However, when the chains are ordered with the full-empty-full sequence we find less charge at the Cu and O sites in the CuO2 plane, and there is a charge transfer from the CuO2 planes to the chains. We find a hole count of 0.18/CUO2 compared to a value of 0.31/CUO2 which was obtained earlier [7], in good agreement with experiment [ 11 ], for the fully superconducting compound YBa2Cu307. This reduction in hole count is in agreement with the reduction in Tc from 90 K to 60 K found experimentally.
References [ 1 ] R.J. Cava et al., Phys. Rev. B36 (1987) 5719. [2] C. Chaillout et al., Solid State Commun. 65 ( 1988 ) 283. [ 3 ] R. Beyers et al., Nature 340 (1989) 619.
R.P. Gupta, M. Gupta / Electronic structure calculations for YBa2Cu306.5 [4] R.J. Cava et al., Physica C 165 (1989) 419. [5] J.D. Jorgensen et al., Phys. Rev. B41 (1990) 1863. [ 6 ] H. Shaked et al., Phys. Rev. B41 (1990) 4173. [ 7 ] R.P. Gupta and M. Gupta, Solid State Commun. 67 ( 1988 ) 129.
[8] J.B. Torrance et al., Phys. Rev. Lett. 61 (1988) 1127. [9] Y. Tokura et al., Phys. Rev. B40 (1989) 8872. [ 10] Y.J. Uemura et al., Phys. Rev. Lett. 62 (1989) 2317. [ 11 ] Z.Z. Wang et al., Phys. Rev. B36 (1987) 7222.
467
Electronic structure investigation of the relationship between oxygen ordering and superconductivity in YBa2Cu306.5 R a j u P. G u p t a Centre d'Etudes Nucl~aires de Saclay, Section de Recherches de Mktallurgie Physique, 91191 Gif sur Yvette Cedex, France
Mich~le G u p t a Universitk de Paris-Sud, Institut des Sciences des Mat~riaux, 91405 Orsay Cedex, France Received 31 August 1990
Results of electronic structure calculations for the compound YBa2Cu306.5 are presented. Three models for the crystal structure have been considered: (a) an alternate chain (AC) model where each alternate chain is fully intact and the adjacent one fully empty, ( b ) an identical chain (IC) model where all chains are identical but are broken, and (c) tetragonal. Our calculations show that there is practically no charge transfer from the CuO2 planes towards the plane of the chains in the latter two cases. However, in the first case ~ 0.18 holes/CuO2 are created. This shows that the ordered chains are absolutely essential for the superconductivity and the disorder in the chains destroys superconductivity.
One of the most interesting aspects of the YBa2CuaOT_~ family of superconductors is the dramatic dependence of the superconducting transition temperature Tc on the oxygen stoichiometry ~. This compound is superconducting with a maximum To~90 K for ~---0 while for ~~ 1 it is nonmetallic and nonsuperconducting [1-6]. Indeed, as ~ increases from 0 to 1, Tc decreases continuously going through a well known two-plateau behaviour. Tc remains constant at ~ 9 0 K for J < 0 . 2 but for ~>0.2 there is a sharp drop until it stabilizes at ~ 60 K for 0.3 < ~< 0.5. For ~> 0.5 there is again a rapid decline until the material becomes nonsuperconducting for ~> 0.65. The transition from the superconducting to the nonsuperconducting state is accompanied by an orthogonal to tetragonal ( O - T ) phase transition. The origin of the existence of this two plateau region is not clear. Electron microscopy investigations [4] show that there is a short range ordering of the oxygen atoms as the oxygen is removed from the fully superconducting compound YBa2Cu3OT. In the YBa2Cu307 compound the O ( 5 ) sites along the aaxis are empty, resulting in the formation of the socalled CuO chains along the b-axis. In the 90 K plateau region (for ~< 0.2) there is a partial oxygen de-
ficiency at random positions in the chain which how'ever does not seriously affect their fully ordered character. The 60 K plateau region is, on the other hand, dominated by a long range ordered 2a superlattice where the alternate chains are fully ordered and completely intact as in the superconducting compound YBa2Cu307 while the adjacent ones fully empty as in the insulating nonsuperconducting compound YBa2Cu306. Such an ordering of the chains is absent when the compound is tetragonal and nonsuperconducting. Although the exact mechanism of superconductivity in the cuprate superconductors is still not understood, it is nonetheless well established that the superconductivity in these compounds arises when the two-dimensional CuO2 planes are conducting and that there is a direct correlation [ 7-10 ] between the hole concentration in these CuO2 planes and To. In the insulating YBa2Cu306 compound the lack of oxygens at the chain sites isolates the CuO2 planes from the chains so that these planes are nonconducting. In the fully stoichiometric superconducting YBa2Cu307 compound the presence of oxygen atoms on the chain sites couples the CuO2 planes to the CuO chains
0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
466
R.P. Gupta, M. Gupta / Electronic structure calculations for YBa2Cu306 5
which act as electron reservoirs for electron transfer from the planes to the chains [7]. If the stoichiometry in oxygen is the dominant criterion for electron transfer from the planes to the chains, the compound YBa2Cu306.5 should remain superconducting in both orthorhombic and tetragonal crystal structures, which is evidently not the case since the compound is superconducting only in the orthorhombic phase when the chains are fully intact. It is therefore important to understand how the hole concentration in the CuO2 planes is affected as the chains are ordered or disordered. With this in mind we have performed electronic structure calculations for the compound YBa2Cu306.5 using the tight binding recursion method and determined the hole concentration in three different crystal structures, two of which are orthorhombic and the third tetragonal. In model I the order in the chains is fully preserved so that each alternate chain is fully intact, and the adjacent one fully empty (ACM). The structure remains orthorhombic but there is a doubling of the lattice parameter along the a-axis. This full-empty-full .... configuration has been found experimentally [2,4] for the 60 K plateau region. In model II we consider the case where all of the chains are identical (ICM). In this case all chains are obviously broken since an alternate oxygen atom on each chain is missing. The orthorhombic symmetry is however retained as there are no oxygen atoms on the O (5) sites along the a-axis. In model III the crystal symmetry is tetragonal so that the O (4) and O (5) ~ite occupancies are identical. In table I we have listed the charges on the copper and oxygen sites on a CuO2 plane obtained from our calculation for the three models. For the purpose of comparison the charges for the superconducting Y B a 2 C u 3 0 7 and the nonsuperconducting Y B a 2 C u 3 0 6 are also given from a previous publication [7 ]. It should be noted that in model I there are two different types of Cu sites that exist on a given CuO2 plane, a Cu just above or below a fully chain and a Cu just above or below an empty chain. There is a small difference between the charges at these sites because of differences in local environments. Due to considerations of simplicity this has been ignored and the charge on a Cu shown in table I is the average charge. The same also holds true for model III where three different Cu sites are present on a CuO2 plane.
Table I Number of electrons at Cu and O sites on the CuO2 plane. Also given are the number of holes per CuO2 plane measured from the half-filled band position. Compound
Number of electrons Cu O CuO2
Number of holes per CuO2 unit
YBa2Cu306 YBa2Cu307 YBa2Cu306.5 Model I
9.55 9.36
5.59 5.53
20.73 20.42
0.0 0.31
9.43
5.56
20.55
0.18
9.49
5.60
20.69
0.04
9.49
5.59
20.67
0.06
(ACM) Model II (ICM) Model Ill (Tetragonal)
Further, in the orthorhombic structure there is usually a small difference in the charge of oxygen at the O( 1 ) and 0 ( 2 ) sites. Again this has been ignored in table I and the charge presented is the average of the two. The hole concentration is obtained by taking the charge on a CuO2 plane in the insulating YBa2Cu306 compound as a reference. We thus see from table I that when the chains are broken and disordered or when the crystal symmetry is tetragonal there is practically no charge transfer from the CuO2 planes to the planes of the chains. The hole count < 0.06/CUO2 found in our calculation falls in the limit where electron localization is expected to occur and the material is nonmetallic and not superconducting [ 8-10 ]. However, when the chains are ordered with the full-empty-full sequence we find less charge at the Cu and O sites in the CuO2 plane, and there is a charge transfer from the CuO2 planes to the chains. We find a hole count of 0.18/CUO2 compared to a value of 0.31/CUO2 which was obtained earlier [7], in good agreement with experiment [ 11 ], for the fully superconducting compound YBa2Cu307. This reduction in hole count is in agreement with the reduction in Tc from 90 K to 60 K found experimentally.
References [ 1 ] R.J. Cava et al., Phys. Rev. B36 (1987) 5719. [2] C. Chaillout et al., Solid State Commun. 65 ( 1988 ) 283. [ 3 ] R. Beyers et al., Nature 340 (1989) 619.
R.P. Gupta, M. Gupta / Electronic structure calculations for YBa2Cu306.5 [4] R.J. Cava et al., Physica C 165 (1989) 419. [5] J.D. Jorgensen et al., Phys. Rev. B41 (1990) 1863. [ 6 ] H. Shaked et al., Phys. Rev. B41 (1990) 4173. [ 7 ] R.P. Gupta and M. Gupta, Solid State Commun. 67 ( 1988 ) 129.
[8] J.B. Torrance et al., Phys. Rev. Lett. 61 (1988) 1127. [9] Y. Tokura et al., Phys. Rev. B40 (1989) 8872. [ 10] Y.J. Uemura et al., Phys. Rev. Lett. 62 (1989) 2317. [ 11 ] Z.Z. Wang et al., Phys. Rev. B36 (1987) 7222.
467