PHYSICAL REVIEW B VOLUME 36, NUMBER 13 1 NOVEMBER 1987

Superconductivity in La-deficient and stoichiometric La2cu04

S. A. Shaheen, N. Jisrawi, Y. H. Lee, Y. Z. Zhang, M. Croft, and W. L. McLeanDepartment of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08855-0849

H. Zhen, L. Rebelsky, and S. HornDepartment of Physics, New York University, 4 Washington Place, New YorkN, ew York l0003

(Received 20 July 1987)

The effects of La deficiency and of impurities on the occurrence of filamentary superconductivi-

ty in La2Cu04 have been investigated by magnetic-susceptibility and electrical-resistivity mea-surements. Reducing the purity of the starting material degrades the superconductivity. Ladeficiency causes a fully reentrant superconducting-to-normal-state transition with decreasingtemperature at temperatures below T, . This behavior clarifies the incipient reentrance which isobserved in nominally stoichiometric LazCu04. We associate the reentrant transition with the re-cently observed structural transformation from the orthorhombic to a semiconducting monoclinicphase.

The well-confirmed observation of superconductivity inLa2 „B„Cu04—y type compounds, ' where 8 is Ba, Sr,or some other divalent ion, has stimulated considerable in-terest in the ground-state properties of the parent com-pound La2Cu04. ' Although it had been believed thatstoichiometric La2Cu04 is a Mott-Hubbard insulator, '

experiments on nominally pure La2Cu04 have revealedthat it is superconducting with an onset temperature closeto 40 K. A frequently suggested origin of the supercon-ductivity has been deficiency of lanthanum or excess ofcopper or oxygen, thus providing an eff'ect similar to Sr orBa doping. Since Ba- and Sr-doped La2 B„Cu04type compounds are superconducting in the 40-K range,an obvious concern is whether superconductivity in un-doped La2Cu04 is intrinsic or an artifact of an impurityphase. This second possibility is strongly suggested by thevery small Meissner signal observed in superconductingLa2Cu04 samples (1 part in 6000 of ideal Meissnereffect). "

We report results on La2Cu04 in which both the purityof starting materials and the La content have been varied.We confirm a filamentary type of superconductivity whoseoccurrence we find to be insensitive to material purity butwhose percolative paths through the sample can be dis-rupted by purity eA'ects. La-deficient material is shown tohelp clarify the origin of superconductivity in a novel way.The La deficiency is found to destabilize the superconduc-tivity by promoting a low-temperature reentrant transi-tion to the normal state. In the context of recent litera-ture we propose that the superconductivity occurs in anorthorhombic metallic phase of La2Cu04 which under-goes a structural transformation to a monoclinic phase atlow temperatures. The monoclinic phase is semiconduct-ing and cannot support superconductivity.

Samples with nominal compositions Lai 90Cu04, La& 95-Cu04, and La2Cu04 were prepared by solid-state reactionusing proportionate amounts of powders of high purityLa203 (99.99%) and CuO (99.999%), supplied byJohnson Mattey Company (AESAR). The nominallystoichiometric La2Cu04 samples 2 and 3 were prepared

using CuO of, respectively, 99.9% and 96% purity sup-plied by Morton Thiokol, Inc. (Alfa Products). Accord-ing to optical emission spectrography of the supplies, thecontent of any single metallic impurity is less than threepart per million in 99.99% and higher purity materials.The 96% pure CuO contains 0.22% insolubles, 0.02%moisture, and 98.8% CuO. Thoroughly mixed powders ofthe starting materials were calcinated in air at980-1050 C for 24 h. Pellets were then pressed at —4kbar. The compressed pellets were annealed in a flowingoxygen atmosphere for 16 h at 950 C then slowly cooledto 520'C and kept at this temperature for 5 h, and finallycooled to 200 C in 2 h. X-ray spectra using Cu Ka& radi-ation in a Scintag diAractometer showed the sharp peakscharacteristic only of the orthorhombically distortedK2NiF4 structure (space group, Cmca). The lattice pa-rameters were the same for both La-deficient andstoichiometric samples and agree well with published re-sults.

Four-probe resistivity measurements were made on rec-tangular bars of typical dimensions, 2x2x9 mm . Thecontacts were made by indium soldering. Temperaturemeasurements were made using calibrated Pt, Ge, Si-diode, and carbon-glass thermometers in diferent cryo-stats. The standard Faraday method was used for mag-netic susceptibility measurements.

The temperature dependence of the resistivities of thethree nominally stoichiometric La2Cu04 samples isdisplayed in Fig. 1. The resistivity increases with decreas-ing temperature from its room temperature value, risingsharply below 100 K like a semiconductor. The activationenergy in the 50-100 K range is about 14 meV. Theresistivity anomaly at the anticipated antiferromagneticordering temperature [in the 210-240 K range] is barelyvisible in the resistivity data. There is a sudden drop inresistivity at —45 K for sample 1 and at 40 K for sample2. The resistivity of sample 3 flattens between 45 and 39K before dropping sharply. The current and magneticfield dependencies of the electrical resistivity displayed inthe inset in Fig. 1 suggest that the drops in resistivity are

7214 1987 The American Physical Society

SUPERCONDUCTIVITY IN La-DEFICIENT AND. . . 7215

20

La&Cu0&

15—V

010

CA

M

5

'gg

/0

p ~

sarnpte1 ( ~ )5QlTI pie 2 ( x )sample 3 (~ )

g. L.~ ~ ~ ~gl I

I I

O0

~ ~

100 200Temperature (K)

T(K)~ ~ ~ ~

I

300

FIG. 1. The resistivity for three LazCu04 samples. Samples1, 2, and 3 are prepared from Cu oxide of purity 99.999%,99.99%, and 96%, respectively. The inset shows current andfield dependence of the resistivity of sample 1.

due to superconductivity. The resistivities of samples 1

and 2, which are the relatively high purity samples, dropbelow the instrumental noise at —18 and 16 K, respec-tively, at current densities below 2 mA/cm. For thelowest purity sample 3 the resistivity remains nonzerodown to 4.2 K. If the superconductivity were a conse-quence of impurities in the starting materials, the lowerpurity sample 3 would have been a better superconductor.Thus our experimental observations do not support the hy-pothesis that the superconductivity arises from an impuri-

ty phase produced by impurities in the starting materials.A comparison of the temperature dependencies of the

resistivities of La-deficient and stoichiometric samples ismade in Fig. 2. The La-deficient samples behave verymuch like the stoichiometric one, except that instead of

attaining a zero value below T, their resistivities show up-turns at lower temperatures. The origin of these upturnswill become clear when we discuss complementary mag-netic susceptibility results.

Figure 3 shows the temperature dependence of the stat-ic magnetic susceptibility, Z(T), for La-deficient andstoichiometric LaqCu04 samples. The behavior above 50K is nearly the same for all three samples. The magneticsusceptibility increases slowly from its room-temperaturevalue, flattens in the range of 200-240 K, and remainsnearly constant down to 50 K.

In order to prevent rounding of anticipated features inthe magnetic susceptibility related to the reported antifer-romagnetism and superconductivity we used very lowfields (75-100 Oe) for the susceptibility measurements.Even then the only features possibly related to antiferro-magnetic ordering in the range 22O-290 K were the veryshallow maxima near 210 K in the stoichiometricLapCu04 sample. In contrast, maxima observed by oth-ers "are much more pronounced, even in higher fields.Johnston, Stokers, Goshorn, and Lewandowski haveshown that the susceptibility maxima are very sensitive tooxygen content, shift towards higher temperature with ox-ygen vacancies, and disappear when the oxygen contentapproaches exact stoichiometry. Thus it appears that theoxygen content in our samples is very close to, or exactlyon, stoichiometry.

La& 9OCUO~aa ~ ~ oooo ooo oooo ~ oo ooo ~ ~ ~ oooo oooo ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~

30l 1

La& gpCUO&+

La~ OOCu0~

I a) 9~CgQ4~ ~ ~ ~ ~I ~ ~ ~ o ~

~2 OOCu 04~ ~ 0 ~ ~ ~ ~ o ~ ~ ~

~ ~

t

0 —M. 0 0+ + +

+

80

0~ ~ o ~

~~ ~y ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ gyes

2 L

~ ~

~' 0I

100 200

gO

300I

100I

200 300 T(K)

T(K)FIG. 2. Logarithm of the resistivity (0cm) of La&&OCu04

and LaqooCu04 (sample I) as a function of temperature. Insetshows lnp vs T for La& 95Cu04.

FIG. 3. The static magnetic-susceptibility data forLa~.9OCu04, La]95Cu04, and Lap.ppCu04 (sample 2) as a func-tion of temperature. Inset shows enlarged view of the suscepti-bility data on La& 95Cu04 below 50 K.

7216 S. A. SHAHEEN et aI.

Another interesting aspect of the data is the dramaticincrease in the absolute value of L with La deficiencies.The values between 50-300 K for La) 95Cu04 are of 2-3times larger than those for LaqCu04. The correspondingenhancement factor for Lai 9OCu04 is 10-12. We attri-bute these increases to the change in the valence of Cucaused, as expected, by the La deficiencies.

As is apparent in Fig. 3, both La-deficient andstoichiometric samples show fairly sharp drops in themagnetic susceptibility starting around 40 K. The magni-tude of the magnetic susceptibility drop is nearly the samein all three samples in spite of the varying magnitudes ofthe drops in their resistivities in the 40-K range (see Fig.2). The ratio of the observed Meissner signal to the idealMeissner signal reveals that only about -0.04% of eachsample is superconducting. Together with the currentdependence of the resistivity, the susceptibility data implya filamentary rather than a bulk nature of the supercon-ductivity in both La-deficient and stoichiometricLaqCu04.

Perhaps the most interesting features of our X(T) mea-surements are the observed upturns below the supercon-ducting transition. Paramagnetic impurities are a verycommon origin of such low-temperature upturns in di-amagnetic materials. However, the varying magnitude ofthe upturn with La deficiency and its dramatic shape forLai 95Cu04 suggest that this feature is not due to magnet-ic impurities. In fact, the upturn provides interesting in-formation related to the structural transformation men-tioned earlier.

The inset in Fig. 3 shows an expanded view of themagnetic-susceptibility data below 50 K for the La~ 95Cu-04 sample. The noteworthy feature is the reentrant-likesharp rise in the magnetic susceptibility starting at -22K. The magnetic susceptibility rises very sharply until 19K and then Aattens at lower temperatures at the samevalue as above 50 K. This behavior together with the cor-responding upturns in the resistivity data suggests that thesuperconductivity in the La-deficient material disappearsagain below —19 K and the system reenters the normalstate. The rapidity with which the Meissner signal isquenched with decreasing temperature suggests that thereentrance is being driven by a phase transition into anonsuperconducting phase. Such a phase transition couldbe structural or magnetic. The close correspondence ofthe susceptibility above and below the reentrant supercon-ducting phase, as shown by the La) 95Cu04 curve, arguesagainst a magnetic transition.

Our observations are particularly interesting in view ofthe recent first-principles pseudopotential band calcula-tions and group theoretical analysis of Kasowski, Hsu,and Herman, ' who claim that on the basis of the space-group symmetry Cmca the orthorhombic La&Cu04 shouldremain metallic at all temperatures. They have suggestedthat the experimentally observed semiconducting behaviorof La~Cu04 at low temperatures is probably a conse-quence of a structural transformation to monoclinic sym-metry. The very recent X-ray studies of Skelton et al. '

indeed provide experimental evidence for a structuraltransformation to monoclinic symmetry in the 15-30range. With this background in mind, we believe that the

observed reentrant superconducting behavior is associatedwith a transformation from the orthorhombic to a mono-clinic phase. The reentrant features in the magnetic sus-ceptibility and electrical resistivity seem very sensitive toLa deficiencies. These observations together with (1) theearlier mentioned equality of the lattice parameters forthe La-deficient and nominally stoichiometric samples,and (2) the absence of a structural transformation in someof the previous low-temperature x-ray studies ' make itan open question whether the structural transformation innominally stoichiometric samples is La-deficiency assistedor not. As far as the La-deficiency assisted origin of su-perconductivity is concerned, the present experimentaldata do not bring any support for this possibility. On theother hand, it cannot be completely ruled out, first, be-cause the La concentrations in our experiments are knownonly with limited accuracy, and second, because theMeissner effect is underestimated in the La-deficient sam-ples owing to the reentrance to the normal state below T, .Nevertheless, the present experimental data and the ob-servatons of Tarascon et al. ,

' who have been successfulin producing superconductivity by introducing excess oxy-gen in a previously nonsuperconducting, nominallystoichiometric, LaqCu04 sample, provide ample reason toconsider that superconducting in LaqCu04 is not La-deficiency assisted.

The very small values of the Meissner signal observed inthe present and previous studies can be understood in ei-ther of two ways: (1) by assuming that superconductivityis a bulk phenomenon whose bulk nature is masked by theproximity of a structural transformation close to T„assuggested by Skelton et al. ,

' or (2) by considering thatsuperconductivity emerges from regions of the samplescontaining oxygen in excess of stoichiometry, i.e.,LaqCu04+y.

The assumption of the occurrence of a structural trans-formation close to T, on which the first interpretation isbased might be realistic for the more strongly reentrantLa-deficient materials. On the other hand, the structural-transformation-related reentrant features in ourmagnetic-susceptibility data on a nominally stoichiomet-ric sample appear at 6 K, which is far below T, (—40 K).The lack of a bulk Meissner effect on this sample cannotbe explained within this interpretation. There is also noobvious explanation for the observed activated electricalconduction above T, and well above the structural-transformation temperature. However, by invoking thesluggish nature of the structural transformation, as re-vealed by x-ray studies, ' one can possibly reconcile theabove observations within this interpretation.

The second explanation of the small size of the Meiss-ner signal is strongly suggested by recent observations ofTarascon et al. ,

' mentioned earlier. This interpretationis convincing (1) because the presence of oxygen in excessof stoichiometry will provide additional charge carriers forconduction, and (2) because the charge-balancing re-quirement would tend to drive Cu + into Cu +, a situa-tion which is believed to be very favorable for supercon-ductivity in this class of materials. ' The same considera-tions suggest a reduced conductivity if there are oxygenvacancies. It is not unrealistic in this class of materials to

SUPERCONDUCTIVITY IN La-DEFICIENT AND. . . 7217

assume that there is a local variation of oxygen content, sothat both weakly activated conduction and filamentary su-perconductivity may co-exist in the same sample.

In summary, we have shown that superconductivity in

LaqCu04 is not an artifact of any phase produced fromimpurities in the starting materials nor does it appear tobe due to La deficiences. We propose that, whether asso-ciated with regions containing oxygen in excess ofstoichiometry or not, superconductivity in LaqCu04 is as-sociated with the orthorhombic phase. La deficiencies ap-pear to destabilize superconductivity by driving a reen-

trant superconducting-to-normal transition at low temper-atures. These observations are understood to arise from astructural transformation of the orthorhombic phase to areportedly' monoclinic semiconducting phase at T & T,.

S. A. Shaheen acknowledges useful discussions withD. C. Johnston. This work was supported by the State ofNew Jersey Commission on Science and Technology un-

der Grants No. 86-240040-13, and No. 87-240090-13,and National Science Foundation Grant No. DMR 85-11982.

~ J. G. Bednorz and K. A. Miiller, Z. Phys. B 64, 189 (1986).S. Uchida, H. Takagi, K. Kitazawa, and S. Tanaka, Jpn. J.

Appl. Phys. Lett. 26, Ll (1987); H. Takagi, S. Uchida, K. Ki-tazawa, and S. Tanaka, Jpn. J. Appl. Phys. 26, L123 (1987).

C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. H. Huang, andY. Q. Wang, Phys. Rev. Lett. 5$, 405 (1987).

4R. J. Cava, R. B. van Dover, B. Batlogg, and P. A. Rietman,Phys. Rev. Lett. 5$, 408 (1987).

5J. D. Jorgensen, H. B. Schutler, D. G. Hinks, D. W. Capone II,K. Zhang, M. B. Brodsky, and D. J. Scalapino, Phys. Rev.Lett. 5$, 1024 (1987).

S. M. Fine, M. Greenblatt, S. Simizu, and S. A. Friedberg,Phys. Rev. B 36, 5716 (1987).

7D. C. Johnston, J. P. Stokers, D. P. Goshorn, and J. T. Lewan-dowski, Phys. Rev. B 36, 4007 (1987).

D. Vaknin, S. K. Sinha, D. E. Moncton, D. C. Johnston,J. Newsam, C. R. Safinya, and H. E. King, Jr., Phys. Rev.Lett. 5$, 2802 (1987).

S. Usida, H. Tagaki, Y. Yangisawa, K. Kishio, K. Titagawa,K. Fueki, and S. Tanaka, Jpn. J. Appl. Phys. 26, L445(1987).

' T. Fujita, Y. Aoki, Y. Maeno, J. Sakurai, H. Fukuba, and

H. Fugii, Jpn. J. Appl. Phys. 26, L368 (1987)."P. M. Grant, S. S. P. Parkin, V. Y. Lee, E. M. Engler, M. L.

Ramirez, J. E. Vaquez, C. Liu, R. D. Jacowitz, and R. L.Greene, Phys. Rev. Lett. 5$, 2482 (1987).

' S. A. Shaheen, N. Jisrawi, Y. H. Min, H. Zhen, L. Rebelsky,M. Croft, W. L. McLean, and S. Horn, in Proceedings of theInternational Workshop on Novel Mechanisms of Superconductivity, Berkeley, June 22-26, 1987, edited by S. A. Wolfand V. Z. Kresin (Plenum, New York, 1987).

'3P. W. Anderson, Science 235, 1196 (1987).R. V. Kasowski, W. Y. Hsu, and F. Herman, Solid State Com-mun. (to be published).

~5E. F. Skelton, W. T. Elam, D. U. Gubser, V. L. Letourneau,M. S. Osofsky, S. B. Qadri, L. E. Toth, and S. A. Wolf, Phys.Rev. B 36, 5713 (1987).

' S. Mitsuda, G. Shirane, S. K. Sinha, D. C. Johnston, M. S. Al-

varez, D. Valein, and D. E. Moncton, Phys. Rev. B 36, 822(1987).

' J. M. Tarascon, L. H. Greene, B. G. Bagley, W. R. McKin-non, P. Barboux, and G. W. Hull (unpublished).