Ž . Physica C 291 1997 249–256 Nonhomogeneous pressure effects on T in HgBa Ca Cu O c 2 2 3 8qd Xiaojia Chen ) , Zhengkuan Jiao Department of Physics, Zhejiang UniÕersity, Hangzhou 310027, China Received 13 June 1997; revised 4 July 1997; accepted 11 August 1997 Abstract The sheet-charge model is applied to study charge distribution and intrinsic T among the CuO planes in c 2 HgBa Ca Cu O . This model enables us to achieve nonhomogeneous dependence of T on both hole concentration and 2 2 3 8qd c Ž . pressure. Calculations show that there exist two different parabolas of T P for the type-I and type-II CuO planes, which c 2 have a kink at high pressure. Based on the recent experiment of Jover et al., we suggest that at low pressure the T is the c intrinsic T of the type-I CuO planes, and at the highest pressures the effect of the type-II CuO planes dominates. q 1997 c 2 2 Elsevier Science B.V. PACS: 74.72.Gr; 74.80.Dm; 74.62.Fj 1. Introduction The class of mercury-based copper oxide super- w x conductors 1,2 HgBa Ca Cu O has at- 2 ny1 n 2 nq2qd tracted considerable interest in their superconducting properties. The superconducting transition tempera- ture T varies with the number n of CuO planes c 2 wx and maximizes when n s 3 3 . There exist two types of CuO planes which are crystallographically 2 inequivalent when the number of CuO layers is 2 over two. The one in which Cu atom has five-fold pyramidal coordination of oxygen is defined as type-I CuO planes. The other that has four-fold square 2 coordinated CuO planes is called type-II ones. Di 2 wx Stasio et al. 4 used the sheet-charge model to investigate the charge distribution among the in- ) Corresponding author. Fax: q86 571 595 1358; e-mail: phy_zhangxj@zunet.ihep.ac.cn. equivalent CuO planes, they found that the charge 2 distribution is highly nonhomogeneous with a frac- tion of ; 0.1 of holes in the type-II CuO plane and 2 thus a fraction of ; 0.9 of holes in the type-I CuO 2 planes in a triple layer compound. Haines and Tallon wx 5 modified the sheet-charge model to a point-charge model and concluded that the hole distribution is nearly uniform among the CuO planes. The 17 O 2 Ž . wx NMR measurements on Bi,Pb Sr Ca Cu O 6 2 2 2 3 y and 63 Cu NMR measurements on wx HgBa Ca Cu O 7 suggested that the hole con- 2 2 3 8qd Ž . centration n is slightly higher in the type-I CuO H 2 planes than that in the type-II plane. However, 63 Cu 17 w x and O NMR measurements 8,9 for Tl Ba Ca Cu O revealed that the charge distri- 2 2 2 3 10qd bution is almost homogeneous. Recently, we calcu- lated the effective Cu valence in CuO planes for 2 Ž . w x HgBa Ca Cu O n s 1 ; 5 10 , and 2 ny1 n 2 nq2qd found that the valences around Cu atoms in the type-I CuO planes are always higher than that 2 around Cu in the type-II planes, which agrees well 0921-4534r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. Ž . PII S0921-4534 97 01655-9