COMMENTS

Comments are short papers which criticize or correct papers of other authors previously published inPhysical Review B. EachComment should state clearly to which paper it refers and must be accompanied by a brief abstract. The same publication schedule asfor regular articles is followed, and page proofs are sent to authors.

Comment on ‘‘Mechanism of the electric-field effect in the high-Tc cuprates’’

N. ChandrasekharDepartment of Physics, Indian Institute of Science, Bangalore 560012, India

Oriol T. Valls and A. M. GoldmanSchool of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455

~Received 14 December 1995!

The electric-field effect in the high-Tc cuprates has been claimed to arise from the field-driven mobility offree charges in the superconductor, a conclusion based on the observation of the field effect inBi2Sr2CaCu2O81y at one sign of the bias. We show that this conclusion is unwarranted.

The electric-field effect in high-Tc cuprates is a matter ofconsiderable scientific and technological importance. It hassparked a certain amount of controversy. Two different, al-though not necessarily mutually exclusive, mechanisms havebeen proposed, based on early experiments restricted to in-vestigations of the electric-field effect in1 YBa2Cu3O72x .The first is the conventional mechanism, and has its origin inthe Coulomb interaction of an applied field with freecharges.1 This is fundamentally electronic, characterized by afast time constant, and results in the enhancement or deple-tion of the number of carriers in a surface channel. Thisshould be a symmetric effect: changes in the number of car-riers in the surface channel, and consequently changes in theresistance observed, are equal in magnitude~but not in sign!for a certain value of the bias, whether positive or negative.

The second explanation is based on the basal plane oxy-gen diffusion process,2 driven by the oxygen electric dipolemoments interacting with the applied external field. This pro-cess is characterized by a slower time constant and, at leastin YBa2Cu3O72x , by asymmetry in the magnitude of theeffect: the changes in resistance and carrier concentration areunequal in magnitude at positive and negative bias of iden-tical absolute value. There are experimental data supportingthis mechanism in studies performed in geometries identicalto that discussed above.3 The predicted asymmetry2 has beenobserved,1,3 and is also found to depend on oxygen content.The general picture of the basal plane oxygen diffusion pro-cess is also supported by experimental and theoretical workon persistent photoconductivity.4,5

In their recent paper6 Freyet al. claim to resolve the con-troversy about the mechanism responsible by observing thefield effect in Bi2Sr2CaCu2O81y . They argued that, be-cause they observed a field effect in this compound, whichlacks the CuO chains present in YBa2Cu3O72x , an expla-nation in terms of the oxygen diffusion mechanism2 wasruled out.

This claim is erroneous. The presence of chains inYBa2Cu3O72x makes it very convenient to develop asimple model to describe the oxygen diffusion process in thiscompound, and to make some quantitative predictions. But acareful reading of Ref. 2 shows that the basic physics of theprocess described there does not specifically require the pres-ence of oxygen chains. All that is required is that there bepermanent oxygen dipole moments taking different valuesdepending on the chemical environment of the site. The pro-cess in YBa2Cu3O72x can be qualitatively described with-out explicit reference to chains: in the presence of an electricfield, oxygen migrates in order to lower the energy associ-ated with its dipole moment. The migration of the oxygenchanges the valence state of some of the carrier donor atoms~Cu in YBa2Cu3O72x) and hence changes the carrier con-centration. We will argue below that a very similar situationshould occur in Bi2Sr2CaCu2O81y , and therefore the ob-servation of electric-field effects in this compound in no wayrules out oxygen diffusion as an explanation, even thoughthe quantitative conclusions based on a chain model2 do notnecessarily apply.

Structure and superconducting properties are intimatelyconnected7 in the compounds under study. The crystal struc-ture of Bi2Sr2CaCu2O81y has been widely investigated byseveral complementary techniques.8 The unit cell has twoequivalent but shifted halves, each of which is 16 Å long inthe c direction. The crystal is composed of parallel planes.Bismuth atoms are located at the centers of BiO6 octahedra~sixfold coordinated with oxygen! and Cu atoms are locatedat the centers of square pyramids. Double Bi-O planes areweakly held together by Van der Waals bonds. Both theBi-O and the Cu-O planes contribute to the electronic densityof states at the Fermi level. The Cu and the Bi atoms share acommon oxygen. The Cu-O bond is stronger than the Bi-Obond, and the former determines the crystal lattice parameter.The natural Bi-O bond is shorter than that dictated by the Cu

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plane below, thus causing strain in the crystal. The atoms inthe BiO plane shift to relieve this strain. While the largestdisplacement of atomic positions is in the Bi-O planes, thereis a significant buckling of all planes, including Cu-O planes.Bismuth, like copper, exhibits two valence states 31 and51 (11 and 21 for Cu!. These observations establish that theoxygen in the Bi-O planes sits in a distorted structural envi-ronment. Consequently there will be some charge asymmetryaround the oxygen, and the existence of dipole moments onthese oxygen atoms follows logically. The determination ofthese dipole moments is beyond the scope of the presentwork. It is clear, however, that the specific value of the di-pole moments at each site will vary depending on the localdistorted environment and the presence of any vacanciescaused by the nonstoichiometry.

The Bi-O layers act as a source of holes, with the totalcarrier concentration arising from the Bi-O layers and theoxygen nonstoichiometry that is accommodated in theselayers.9 X-ray absorption near edge spectroscopy studieshave confirmed the role of the Bi-O layers as a source ofholes.10 The role of the Bi-O layers is not independent of theoxygen stoichiometry, and their effect, in the presence ofoxygen defects, is more pronounced, as demonstrated by re-cent experiments.11 This is similar to the role played by theCu-O basal plane layer (CuO chain layer! in theYBa2Cu3O72x compound, where the oxygen nonstoichiom-etry is accommodated in this layer, which also determinesthe carrier concentration in the CuO2 planes. Samples ofBi 2Sr2CaCu2O81y with the same oxygen content, but dif-ferent thermal histories, have very different transitiontemperatures.11 It is clear that the position of the oxygen inthe Bi-O plane controls the hole concentration in the CuO2plane and thereby the transition temperature. This is identicalto the role played by the Cu-O chains in YBa2Cu3O72x .Thus, changes in normal state and superconducting proper-ties are due not only to changes in total hole concentration,but also to changes in structure, caused by adjustment andaccommodation of oxygen in the Bi-O layers.

These experimental observations clearly establish thesimilar roles of the Cu-O chain layers~basal planes! inYBa2Cu3O72x and the Bi-O planes inBi 2Sr2CaCu2O81y . Recent point contact spectroscopy ex-periments by Grajcaret al.12 on Bi2Sr2CaCu2O101y haveshown that electric-field-induced oxygen motion is a realphenomenon in these materials, as evidenced by temporalevolution of point contact resistances. Similar observationswere reported by Rybal’chenkoet al.13 on YBa2Cu3O72x .Estimates of oxygen diffusion coefficients by Grajcaret al.12

yielded a numerical value comparable to that in YBa2Cu3O72x . Since the Bi2Sr2CaCu2O81y and the Bi2Sr2Ca2Cu3O101y compounds form a homologous series, differingonly in the number of Cu-O planes, one may conclude that asimilar mechanism of oxygen disorder in the Bi-O layerscausing changes in doping is operative in Bi2Sr2CaCu2O81y .From these arguments, it is clear that the situations con-

cerning the electric-field effect in YBa2Cu3O72x andBi 2Sr2CaCu2O81y are not at all dissimilar. Further, we canfind even in the data of Ref. 6 support for the above argu-ments. It follows from our reasoning that the observed ef-fects ~changes in the normal-state resistance and the super-

conducting transition temperature! should then scale withNBi , the areal density of Bi atoms. This is because the fieldeffect is caused by changing the oxidation state of the cationBi. That this is indeed the case may easily be confirmed bychanging the abscissa of Fig. 4 in Freyet al.6 to NBi . Theobservations of Fig. 3 in Freyet al.6 relating the field in-duced shifts ofTc0 and RDS to the thicknesses of theBi 2Sr2CaCu2O81y and the YBa2Cu3O72x films used, canalso be explained. The YBa2Cu3O72x unit cell has oneCu-O chain layer for every 11.7 Å thickness. TheBi 2Sr2CaCu2O81y unit cell has 2 Bi-O planes for every 32Å thickness. The YBa2Cu3O72x unit cell is smaller in thea and b crystallographic directions~3.85 Å average! thanthe Bi2Sr2CaCu2O8y

unit cell ~5.4 Å average!. This resultsin a higher areal density of the donor Cu atoms~per Cu-Oplane! than the donor Bi atoms~per Bi-O plane!. Simplegeometry can then be used to show that the scaling relation-ship obtained in Fig. 3 follows, since for a given change indoping caused by oxygen disorder, YBa2Cu3O72x has to bethinner than Bi2Sr2CaCu2O81y by a factor of 1.5.

Further, Freyet al.6 present data at only one sign of bias.There is no demonstration of an ‘‘enhancement’’ of carrierdensity in the channel. Only depletion has been shown. If themechanism were only due to the Coulomb interaction, theeffect should be opposite in sign when the bias is changedfrom positive to negative, i.e., there should be a decrease inthe normal-state resistance, and an increase in the transitiontemperature, while the oxygen diffusion mechanism is likelyto lead to an asymmetry. There are also no data on the dy-namical response of the Bi2Sr2CaCu2O81y superconductingfield effect transistor~SuFET!. If the effects observed weredue to the Coulomb interaction, then the temporal responseshould be governed by theRC time constant of the device. Inthe absence of these missing pieces of evidence, the conclu-sion drawn by Freyet al.6 is premature at the least, since thedata are clearly inadequate. The temporal response of thedevice is crucially important in determining the mechanism.

Frey et al.6 state in the last paragraph of their paper thattheir results ‘‘do not imply that field-driven oxygen diffusionmay not play a role in samples with a metastable oxygendistribution.’’ In films that are at most 250 Å thick, the onlypossible oxygen distribution is a metastable one. Such a dis-tribution is susceptible to relaxation under any applied exter-nal perturbation, such as thermal cycling, external electricfields, etc. Equilibrium distributions may be attained in thecase of thicker films such as those investigated by Kula andSobolewski,3 whose observations are inconsistent with thesimple Coulomb interaction mechanism, and can be ex-plained only in terms of an oxygen diffusion-migrationmechanism.

In conclusion, the mechanism of the electric-field effect inBi 2Sr2CaCu2O81y is amenable to explanation in terms ofoxygen diffusion, with the same physics as the model whichhas been used earlier2 to explain the field effects observed inYBa2Cu3O72x . In thick films of YBa2Cu3O72x where an‘‘equilibrium oxygen distribution’’ may be expected to exist,the experimental observations support this approach.3 Fur-ther evidence in support of the relevance of the oxygen or-dering model picture to the properties of the cuprates comesfrom experiments on persistent photoconductivity4 for whicha similar oxygen diffusion model has been used.5

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1See, e.g., J. Mannhartet al., Phys. Rev. Lett.67, 2099~1991!; X.Xi et al., ibid. 68, 1240~1992!.

2N. Chandrasekhar, Oriol T. Valls, and A. M. Goldman, Phys.Rev. Lett.71, 1079~1993!; Phys. Rev. B49, 6220~1994!.

3W. Kula and R. Sobolewski, Phys. Rev. B49, 6428~1994!.4K. Tanabe, S. Kubo, F. Hosseini Teherani, H. Asano, and M.Suzuki, Phys. Rev. Lett.72, 1537~1994!.

5G. Grigelionis, E. E. Tornau, and A. Rosengren, Phys. Rev. B53,425 ~1996!.

6T. Freyet al., Phys. Rev. B51, 3257~1995!.7B. W. Veal, H. You, A. P. Paulikas, H. Shi, Y. Fang, and J. W.Downey, Phys. Rev. B42, 4770~1990!; G. Ceder, R. P. McCor-mack, and D. de Fontaine,ibid. 44, 2377~1991!.

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ibid. 37, 9856 ~1988!; J. M. Tarasconet al., ibid. 37, 9382~1988!.

9M. S. Hybertsen and L. F. Matheiss, Phys. Rev. Lett.60, 1661~1988!; F. Herman, R. V. Kasowski, and W. Y. Hsu, Phys. Rev.B 38, 204 ~1988!.

10A. Q. Pham, F. Studer, N. Merrien, A. Maignan, C. Michel, andB. Raveau, Phys. Rev. B48, 1249~1993!; H. Krakawer and W.E. Pickett, Phys. Rev. Lett.60, 166 ~1988!.

11P. Krishnaraj, M. Lelovic, N. G. Eror, and U. Balachandran,Physica C246, 271 ~1995!.

12M. Grajcar, A. Plecenik, S. Benacka, Ju Revenko, and V. M.Svistunov, Physica C218, 82 ~1993!.

13L. F. Rybal’chenko, V. V. Fisun, N. L. Bobrov, I. K. Yanson, A.V. Bondarenko, and M. A. Obolenskii, Sov. J. Low Temp. Phys.17, 105 ~1991!.

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