Solid State Communications, Vol.63,No.12, pp.1077-1080, P r i n t e d in Great B r i t a i n .
1987.
0 0 3 8 - 1 0 9 8 / 8 7 $3.00 + .00 Pergamon J o u r n a l s L t d .
E L E C T R O N I C S T R U C T U R E O F P U R E A N D D O P E D O R T H O R H O M B I C La2CuO4 Robert V. Kasows~i and William Y. Hsu Central Research and Development Department" E. I. du Pont de Nemours and Company Experimental Station, Wilmington, Delaware 19898 Frank Herman b IBM Almaden Research Center 650 Harry Road, San Jose, California 95120-6099 (Received 25 March 1987 by G. Burns)
The electronic structure of orthorhombic La2CuO( has been investigated by In'st principles pseudofunction band calculations and group theoretical analysis. We find that pure as well as doped compounds remain metallic at all finite temperatures as a conseqence of the Cmca (D~) space group symmetry. The experimentally observed rapid rise in resistiwty below 30*K suggests a structural transition to a lower symmetry space group that could be driven electronically or magnetically. One posslble candidate is monoclinic C2/m (C~0 which is a subgroup of Cmca and can be obtained by distorting the C u - O bonds or rotating the CuO6 octahedra. Imphcations for superconductivity are discussed.
Recently, superconductivity with very high transition temperature T~ has been observed m Ba or Sr doped La2CuO~ "5 and LaCuO3~.6,7as well as in solid solutions of (Y~_xBax)~CuO4.s Undoped La2CuO4 does not become superconducting. According to neutron powder &ffractton s, this compound is tetragonal with space group I4/mmm (DI~) above 533°K and orthorhombic with space group Cmca (D~) below 533°K. The small orthorhomblc distortion increases with decreasing temperature. In the doped, superconducting samples, neutron diffraction experiments suggest that the tetragonal symmetry is maintained down to at least 10°K. However, due to strata-reduced hne broadening, the existence of orthorhombm crystalhtes cannot be ruled out It has recently been suggested that the tetragonal-to-orthorhombic transition ~s driven by a Pelerls or charge density wave (CDW) mstabdity arising from substantml nesting of the half-filled antibonding C u - d - O - p hybrid band. s,9,~° Based on the propos~tlon that the orthorhombic phase has a gap and hence is semlconductmg, it was argued that this phase is detrimental to superconductwlty, and that the role of Ba or Sr doping is to suppress the orthorhombic phase, thus maintaining the metalhc tetragonal phase to much lower temperatures, where it eventually becomes superconducting s,9.10 The reported semlmetalhc behavior of undoped ( o r t h o r h o m b l c ) L a 2 C u O 4 between 30-300 ° K s has been
attributed to imperfect nesting m the real band structure and incomplete band gaps in the Fermi surface. Below 30°K undoped La2CuO4 becomes a semiconductor. In this paper we take issue with earher statements that orthorhombic La:CuO4 is semlconducting due to its lower crystal symmetry and thus incompatible with superconducting behavior. Indeed, we will show that, by virtue of the Cmca space group, the conduction and valence bands of orthorhombic La~CuO4 do not and cannot have a gap and thus La2CuO~ must remain metallic at all temperatures. Doping the orthorhombic phase with Ba, for example, would not change the metallic character of this phase since such doping does not change the overall crystal symmetry. The rapid transition to semlconductmg behavior below 30°K is only consistent with another structural transition to a yet-to-be identified phase. The most likely space group for this phase is C2/m (C]h) whlch is a monochnic subgroup of Cmca and is found m BaBiO3. H The Cu ions in this new phase are inequivalent, allowing a band gap to occur. The gapless nature of the Cmca orthorhombic phase also sets a hm~t 12 to the strength of the electron-phonon interaction responsible for the Peierls mstabihty at 533°K. Contrary to prevaous suggestions,S.9, ~0 we do not believe that the tetragonal phase is essential to superconductivity. Instead, we believe that the orthorhomblc phase, since it is also metallic, could play an ~mportant role in determining the values of T, m th~s class of material, as well as in BaPbl_xBl~O~ 13
,Contributlon No. 4355. bSupported in part by the Office of Naval Research. 1077
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ELECTRONIC STRUCTURE OF PURE AND DOPED ORTHORHOMBIC La2CuO 4 The orthorhomblc structure contains 14 atoms
per primitive unit cell, whereas the tetragonal structure contains only 7 atoms, s For both structures, the Cu atom is octahedrally coordinated to 4 O atoms in the basal plane and 2 O atoms above and below the
1080
8.
9.
10. I1.
method, ~5 using the atomlc coordinates of Jorgensen
et al. 5 Our pseudofunction barns set cons:sts of sp3d s functions on La and Cu atoms and sp 3 functions on the O atoms The pseudofunctions are constructed from overlapping solutions of the muffin-tin potential
ELECTRONIC STRUCTURE OF PURE AND DOPED ORTHORHOMBIC La2CuO 4
M. K. Wu, J. R. Asburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58, 908 (1987); P. H. Hor, L. Gao, R. L. Meng, Z. J. Huang, Y. Q. Wang, K. Forster, J. Vassilious, C. W. Chu, M. K. Wu, J. R. Ashburn and C. J. Torng, tbtd. 58, 911 (1987). L. F. Matthelss, Phys. Rev. Lett. 58, 1028 (1987). J. Yu, A. J. Freeman and 3.-H. Xu, Phys. Rev. Lett. 58, 1035 (1987). D. E. Cox and A. W. Sleight, Acta Crystallogr. B 35, 1 (1979); D. E. Cox and A. W. Sleight, Sohd State Commun. 19, 969 (1976).
Vol. 63, No. 12
12.
Vol. 63, No. 12
C . A . Ba:seiro and L. M. Falicov, Phys. Rev. B
20, 4457 (1979). 13. 14. 15. 16
A . W . Sleight, J. L Gillson and P. E. Bierstedt, Sohd State Commun. 17, 27 (1975). J . C . Slater, Quantum Theory of Molecules and Sohds Vol. 2 (McGraw Hill, New York, 1965), Appendix 3, Section A3-9, pp. 446-451. R . V . Kasowski, M.-H. Tsai, T. N. Rhodin and D. D. Chambliss, Phys. Rev. B 34, 2656 (1986). S. Sugai, S. Uchida, K. Kitazawa, S. Tanaka and A. Katsui, Phys. Rev. Lett. 55, 426 (1985).
Voi, 63, No. 12
ELECTRONIC
STRUCTURE OF PURE At~ DOPED ORTHORHOMBIC La2CuO 4
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Figure 2. Comparison of electronic bands of orthorhombic (LT- ortho) and monoclinically distorted La^CuO, and orthorhombic LaBaCuO 4 z 4 . . . . (Ba doped). The monocltnlc dlstortlon can be by Cu-O bond modulation in the basal plane(breathing mode) or by rotation of CuO 6 octahedra (rotation)
and rotational cases. The energy bands near EF are plotted in Fig. 3 for the breathing-mode case. We found a direct band gap of 0.1 eV which would result in semlconducting behavior at 20°K since most
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Figure 3. Energy bands near the Fermi energy of a breathing-mode monoclinic La2CuO 4,
phonons would be frozen out. The band structure for the rotational case is similar except the band gap is smaller. We have also computed the total energies of
1079
the distorted and undistorted structures. The breathing mode raises the total energy by 0.2 Ry, whereas the rotational mode raises thts by only 0.02 Ry. Thus, rotations of CuO6 oetahedra can be excited more easily than the breathing mode of Cu - O bonds in the basal plane. Similar conclusions have been estabhshed experimentally in BaPb~_xBi=O~ by Raman spectroscopy. ~6 The effect of Ba doping in the orthorhombic Cmca structure was examined by studying the hypothetical compound LaBaCuO4, which ts considerably easier to study theoretically than more reahstic materials with lower levels of doping and La/Ba disorder. Trte energy bands are gwen in Fig. 2 and appear quantitatively very mmdar to the bands of orthorhomblc and the proposed monoclimc La2CuO4. The principal effect of Ba is to move the Fermi level below the double degeneracy at the LZSS' plane, resulting in almost doubhng the DOS at E~ and good metallic behavior. We conclude, therefore, contrary to prewous suggesuons, s,9,~0 that the orthorhombic Cmca structure in itself is not detrimental to superconductwity. We also believe that orthorhombie La2CuO4 might become superconducting if the monoclinic distortion below 30°K could be prevented, for example, by external pressure. In summary, we have shown theoretically that pure and doped orthorhombic La2CuO4 should remain metalhc at any temperature and therefore should not be detrimental to superconductw~ty. Our work suggests that another structural transition, which could be drwen electronically or magnetically, is necessary to account for the semlconducting behavior of La2CuO4 below 30°K. The space group of the suggested distortion could be C2/m, as could be verified by single crystal diffraction or Raman or IR spectroscopy. Raman studies ~6 of BaPb~_xBi=O3 suggest the relevant symmetry xs lower than tetragonal and indicate a correlation between Tc and the rotation of BiO6 octahedraJ 6 Our results strongly suggest a simdar relationship here. Acknowledgements - Helpful dlscusmons with J. C. Calabrese on crystal structure and d~stortions and with C W Chu, R. K Nesbet, S. S. P. Parkin, D. J. S c a l a p m o , . A . W . Sleight, and J. B. Torrance are gratefully acknowledged. We also thank D. J. Scalapino for discussing the results of Ref. 5 before publication.
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3. 4
J . G . Bednorz and K. A. Muller, Z. Phys. B 64, 189 (1986). S. Uchida, H. Takag~, K. Kitazawa and S. Tanaka, Jpn. Appl. Phys. Lett. 26, L1 (1987); H. Takagi, S. Uchlda, K. Kttazawa and S. Tanaka, tbzd., m press C . W . Chu, P. H. Hor, R. L. Meng, L. Gao and Z. J. Huang, Sczence 235, 567 (1987). R . J . Cava, R. B. van Dover, B. Batlogg and E. A. Rietman, Phys. Rev. Lett. 58, 408 (1987).
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1080 8.
9. 10. 1I.
ELECTRONIC STRUCTURE OF PURE AND DOPED ORTHORHOMBIC La2CuO 4
M.K. Wu, J. R. Asburn, C. J. Torng, P. H. Hot, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58, 908 (1987); P. H. Hor, L. Gao, R. L. Meng, Z. J. Huang, Y. Q. Wang, K. Forster, J. Vassihous, C. W. Chu, M. K. Wu, J. R. Ashburn and C. J. Torng, ibid. 58, 911 (1987). L. F. Mattheiss, Phys. Rev. Lett. 58, 1028 (1987). J. Yu, A. J. Freeman and J.-H. Xu, Phys. Rev. Lett. 58, 1035 (1987). D . E . Cox and A. W. Sleight, Acta Crystallogr. B 35, 1 (1979); D. E. Cox and A. W. Sleight, Sohd State Commun. 19, 969 (1976).
12.
Vol. 63, No. 12
C . A . Balseiro and L. M. Falicov, Phys. Rev. B
20, 4457 (1979). 13.
A . W . Sleight, J L. Gillson and P E. Blerstedt, Sohd State Commun. 17, 27 (1975). 14. J . C . Slater, Quantum Theory of Molecules and Solids Vol. 2 (McGraw Hill, New York, 1965), Appendix 3, Section A3-9, pp. 446-451. 15. R . V . Kasowsh, M.-H. Tsai, T. N. Rhodm and D. D. Chambliss, Phys. Rev. B 34, 2656 (1986). 16. S. Sugal, S. Uchida, K. Kltazawa, S. Tanaka and A. Katsui, Phys. Rev. Lett. 55, 426 (1985).