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The formation of negative-U centers in doped HTSC
Physiea C 235-240 (1994) 2313-2314
PHYSlCA
North-Holland
The formation of negative-U centers in doped HTSC. O.M.lvanenko, K.V.Mitsen P.N.Lebedev Physics Institute, 117924 Moscow, Russia A mechanism for the influence of doping on the electron spectrum Nd2_xCexCuO 4 and Bal_xKxBiO3 is proposed. In the framework of simple model taking into account the local nature of HTSC electronic properties the mechanism of formation of "negative-U centers" (with the pair orbital placed at the top of the oxygen subband) is considered• The nature of interelectron attraction on oxygen -U-centers is the dependence of Cul+-state energy on the charge state of nearest O-atom. In tunas out that the energy of Cu 1+-state for copper atom nearest to O i o n is higher than one for copper atom nearest to 02- and O° ions.
Recently [1,2] we supposed that the distinctive feature of H T S C electron spectrum is the pair level with single-electron energy placed on the top of fullfilled oxygen subband. The appearance of pair level was connected with formation o f - U - c e n t e r s [3-6] at oxygen atoms ha certain sites of unit cell. These features result in qualitatively new properties of H T S C normal state• Amo.ag them : - hole carriers appear in oxygen subband by thermal excitation of electrons froni band to pair level, their distribution is nondegenerated - at high hole carrier concentration the non-degenerated distribution tends to predomination o f hole-hole scattering in kinetic processes that results in linear temperature d e p e n d e n c e of dc resistivity and frequency dependencies of relaxation time z-1 occo (to>>T) and ~-locT (to
the cell. The resulting energy of such C u l t ( o 2-) state (with nearest oxygen atom in O2--state) will be equal El: (Fig.la).
cul+(o-) c1+(
cJ+(:-
. I cj+/oo) . b) x=0.15
x.=O
Fig. 1 F r o m the other hand the electrical conductivity in CuO 2 plane would be provided by electron transfer from oxygen to the nearest Cu atom: O2-+Cu 2t---)O-+Cu 1 t However, as evident from experience the expected metal conductivity in undoped copper O ", x"i u' ~~ iS i'iot ,l,Jui~. T~ds is ,-.,v.,,,~,'~'~ by inter-electron correlation on Cu 1¢ that is in excess of band width. That is the final state Cu I ~ with nearest oxygen atom in O- state is separated from El: by charge transfer gap Act (Fig.la). However the energy of Cu 1+ can be lowered by the placement of excess positive charge near the Cu atom that will reduce the
0921-4534:94/$07.00 © 1994 - I-lsevier Science B.V. All rights reserved. SSDi 0921-4534(94)017" • -0
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O.M. lvanenko, K.V. Mitsen/Physica C 235-240 (1994) 2313-2314
repulsion on Cu 1+. In Nd2CuO 4 it can be realized for example by substitution of Nd 3~ by Ce 4~ . At some Ce concentration (x=0.15) the gap Act. is lowered locally [7] to A*ct (Fig. lb) that gives rise of two-electron transitions: 2Cu 2 t +O2---~2Cu I ~+O°. It is possible because in this case the energy of final Cu l~ state is lower than for oneelectron transitions owing to the additional positive charge (hole) on O-atom. This results in formation of the local complexes C u l + - O Cu 1~ and in doing so the energies of Cu 1+(O 2-) and Cu I ~-(O0) states are equal. At the same time the energy of Cul+(O -) state is higher. It means that the oxygen atoms placed between Cu 1+ ions are -U-centers. The pair orbital of arising -U-centers formed from unoccupated O-states will coincide with El:. In fact we have in HTSC a Fermi surface with Coulomb gap for the electron transitions between O- and Cu-atoms. At the same time the transitions between O-atoms become possible as soon as a part of electrons from O-atoms passes to the pair level. Therefore we can consider the HTSC in many cases as a semiconductor with valence band formed by the oxygen orbitals and with acceptor-like pair level placed at the top of fullfilled valence band [1,2]. The occupation of" this orbital results from thermal excitation of electrons from the band. This gives rise of holes which appear on oxygen atoms placed between Cu 2~ ions. The distribution of hole carders is non degenerated. Thus in Nd2_xCexCuO 4 at x>_0.15 the transition from dielectric to metal with hole
conductivity takes place. It is significant that just at this concentration the superconductivity appears. At Tc0 (at x~0.25). This results in the single band spectrum and electron like conductivity (because of doping with 4-valence Ce). In a similar manner the dielectric-metal transition develops in Bal_xKxBiO 3. In this case the empty Bi3~- states resulting from charge disproportion Bi3+-Bi 5+ play the role of Cu 1+. -U-centers form on oxygen atoms placed between Bi3~-ions. The effect of potassium reduces to lowering of energy of Bi3+(O 0) states down to the top of oxygen subband.
1. A.I.Golovashkin, O.M.Ivanenko and K.V. Mitsen, Physiea C, 162-164 (1989) 1661 2. K.V.Mitsen and O.M.Ivanenko. Physiea B 194196 (1994) 1381 3 P.V.Anderson, Phys.Rev.Lett., 34 (1975)953 4. H.-B.Schuttler, M.Jarrell and D.J. Scalapino, Phys. Rev., 39 (1989) 6501 5. C.S.Ting, D.N.Talwar and K.L.Ngai, Phys. Rev. Lett. 45 ( i 980) 1213 6. Y.Bar-Yam, Phys.Rev. B43 (1991) 359 Phys.Rev. B43 (1991) 2601 7. O.M.Ivanenko and K.V.Mitsen, J. of Superconductivity, 7 (1994) 627 8.Z.Z.Wang et al. Phys.Rev. B43 ~1991) 3020
PHYSlCA
North-Holland
The formation of negative-U centers in doped HTSC. O.M.lvanenko, K.V.Mitsen P.N.Lebedev Physics Institute, 117924 Moscow, Russia A mechanism for the influence of doping on the electron spectrum Nd2_xCexCuO 4 and Bal_xKxBiO3 is proposed. In the framework of simple model taking into account the local nature of HTSC electronic properties the mechanism of formation of "negative-U centers" (with the pair orbital placed at the top of the oxygen subband) is considered• The nature of interelectron attraction on oxygen -U-centers is the dependence of Cul+-state energy on the charge state of nearest O-atom. In tunas out that the energy of Cu 1+-state for copper atom nearest to O i o n is higher than one for copper atom nearest to 02- and O° ions.
Recently [1,2] we supposed that the distinctive feature of H T S C electron spectrum is the pair level with single-electron energy placed on the top of fullfilled oxygen subband. The appearance of pair level was connected with formation o f - U - c e n t e r s [3-6] at oxygen atoms ha certain sites of unit cell. These features result in qualitatively new properties of H T S C normal state• Amo.ag them : - hole carriers appear in oxygen subband by thermal excitation of electrons froni band to pair level, their distribution is nondegenerated - at high hole carrier concentration the non-degenerated distribution tends to predomination o f hole-hole scattering in kinetic processes that results in linear temperature d e p e n d e n c e of dc resistivity and frequency dependencies of relaxation time z-1 occo (to>>T) and ~-locT (to
the cell. The resulting energy of such C u l t ( o 2-) state (with nearest oxygen atom in O2--state) will be equal El: (Fig.la).
cul+(o-) c1+(
cJ+(:-
. I cj+/oo) . b) x=0.15
x.=O
Fig. 1 F r o m the other hand the electrical conductivity in CuO 2 plane would be provided by electron transfer from oxygen to the nearest Cu atom: O2-+Cu 2t---)O-+Cu 1 t However, as evident from experience the expected metal conductivity in undoped copper O ", x"i u' ~~ iS i'iot ,l,Jui~. T~ds is ,-.,v.,,,~,'~'~ by inter-electron correlation on Cu 1¢ that is in excess of band width. That is the final state Cu I ~ with nearest oxygen atom in O- state is separated from El: by charge transfer gap Act (Fig.la). However the energy of Cu 1+ can be lowered by the placement of excess positive charge near the Cu atom that will reduce the
0921-4534:94/$07.00 © 1994 - I-lsevier Science B.V. All rights reserved. SSDi 0921-4534(94)017" • -0
....
a
2314
O.M. lvanenko, K.V. Mitsen/Physica C 235-240 (1994) 2313-2314
repulsion on Cu 1+. In Nd2CuO 4 it can be realized for example by substitution of Nd 3~ by Ce 4~ . At some Ce concentration (x=0.15) the gap Act. is lowered locally [7] to A*ct (Fig. lb) that gives rise of two-electron transitions: 2Cu 2 t +O2---~2Cu I ~+O°. It is possible because in this case the energy of final Cu l~ state is lower than for oneelectron transitions owing to the additional positive charge (hole) on O-atom. This results in formation of the local complexes C u l + - O Cu 1~ and in doing so the energies of Cu 1+(O 2-) and Cu I ~-(O0) states are equal. At the same time the energy of Cul+(O -) state is higher. It means that the oxygen atoms placed between Cu 1+ ions are -U-centers. The pair orbital of arising -U-centers formed from unoccupated O-states will coincide with El:. In fact we have in HTSC a Fermi surface with Coulomb gap for the electron transitions between O- and Cu-atoms. At the same time the transitions between O-atoms become possible as soon as a part of electrons from O-atoms passes to the pair level. Therefore we can consider the HTSC in many cases as a semiconductor with valence band formed by the oxygen orbitals and with acceptor-like pair level placed at the top of fullfilled valence band [1,2]. The occupation of" this orbital results from thermal excitation of electrons from the band. This gives rise of holes which appear on oxygen atoms placed between Cu 2~ ions. The distribution of hole carders is non degenerated. Thus in Nd2_xCexCuO 4 at x>_0.15 the transition from dielectric to metal with hole
conductivity takes place. It is significant that just at this concentration the superconductivity appears. At Tc
1. A.I.Golovashkin, O.M.Ivanenko and K.V. Mitsen, Physiea C, 162-164 (1989) 1661 2. K.V.Mitsen and O.M.Ivanenko. Physiea B 194196 (1994) 1381 3 P.V.Anderson, Phys.Rev.Lett., 34 (1975)953 4. H.-B.Schuttler, M.Jarrell and D.J. Scalapino, Phys. Rev., 39 (1989) 6501 5. C.S.Ting, D.N.Talwar and K.L.Ngai, Phys. Rev. Lett. 45 ( i 980) 1213 6. Y.Bar-Yam, Phys.Rev. B43 (1991) 359 Phys.Rev. B43 (1991) 2601 7. O.M.Ivanenko and K.V.Mitsen, J. of Superconductivity, 7 (1994) 627 8.Z.Z.Wang et al. Phys.Rev. B43 ~1991) 3020