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PHYSICA ELSEVIER Physica C 282-287 (1997) 162-165 Transport properties of MBE grown Cuprate Spin Ladders M. Lagu~sa, J.Y.Lavalb , C.F.Beurana, C. Deville Cavellin~, B.Eustache c, J.B.Moussyc, C.Partiota and X.Z.Xu ~ = Laboratoire Surfaces et Supraconducteurs - Universit# Paris 12 - CNRS UPR5, b Laboratoire de Physique des Solides - CNRS UPR5, Ecole Sup~rieure de Physique et de Chimie Industrielles - 10 rue Vauquelin, 75005, Paris, France c Wintici SA - 17 rue Jean Moulin, 94300 Vincennes, France Thin films belonging to the family of Spin Ladders (Sr, Ca)h_lCUh.lO2h are grown by MBE. The deposition is performed under atomic oxygen using real time control by the RHEED intensity. The structural parameters measured by four-circle X-ray diffraction agree with the values observed for cuprate Spin Ladders in bulk compounds from various laboratories. HRTEM images shows the double Cu rows within the CuO planes which are characteristic of ladder structures. The room temperature resistivity ranges from 1 nuO.cm for Ca2Cu406 to 1 fLcm for Sr2Cu406. Doping with BiO planes was tested. A superconducting transition at 40K was observed in a BiSrCaCuO compound with a composition compatible with two-leg ladders. 1. INTRODUCTION Cuprate Spin Ladders were In'st synthesized under high pressure by Hiroi and Takano. ~ They describe electrical and magnetic properties of the two gust members Sr2Cu406 and Sr4Cu6Olo , of the Srh_lCU~+lO2hhomologous series. Figure 1 compares their unit cell with the structure of the Infinite Layer Compounds 2 (ILC). These compounds are referred as Spin Ladders because the CuO4 tetragonal units are distributed along stripes looking like ladders. The quasi one-dimensionnal magnetic properties of these compounds 3'4 are interesting. It was suggested by Rice5 that they could lead to superconductivity with high critical temperatures. It was reported 6 that the ladder compound (La, Sr)CuO2.5 shows no superconductivity. A related phase, Sr14Cu24041 , is suggested to be the intergrowth of the Spin Ladder unit cell Sr2Cu406 (figure 1) with a SrTCuloO20 block containing CuO2 chains 7'8. One may also notice the relation between this phase and the superconducting behavior observed but not well described in some Infinite Layer Compounds : the SrI4Cu2404~ phase shows up often as a secondary phase when superconductivity is observed in ILC, both in the case of n-type9 and p-type doping 1°'11. A recent report describes the superconductivity under high pressure for a (Sr, Ca)14Cu2404~ compound~2 with a Tc of 12K. All these ladder compounds are bulk materials prepared under very 0921-4534/97/$17.00 © Elsevier Science B.V. All fights reserved. PII S0921-4534(97)00251-7 SrCu02 (ILC) Sr4CusO~o Sr2Cu406 Fig. 1 Structure of the unit cell for the first two members (h=3 and h=5) of the Spin Ladder family Srh.lCuh+lO2h compared to the ILC structure. high pressure. The compounds presented here are thin films prepared by atomic engineering. Depending on the composition we observe low resistivities and metallic behavior -but no superconducting properties as yet- on thin l-rims belonging to the family (Sr, Ca)h.lCUla+lO2h . Their room temperature resistivity ranges from 1 mgLcm for Ca2Cu406 to

Transport properties of MBE grown cuprate spin ladders

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PHYSICA ELSEVIER Physica C 282-287 (1997) 162-165

Transport properties of MBE grown Cuprate Spin Ladders

M. Lagu~s a, J.Y.Laval b , C.F.Beuran a, C. Deville Cavellin ~, B.Eustache c, J.B.Moussy c, C.Partiot a and X.Z.Xu ~

= Laboratoire Surfaces et Supraconducteurs - Universit# Paris 12 - CNRS UPR5, b Laboratoire de Physique des Solides - CNRS UPR5,

Ecole Sup~rieure de Physique et de Chimie Industrielles - 10 rue Vauquelin, 75005, Paris, France c Wintici SA - 17 rue Jean Moulin, 94300 Vincennes, France

Thin films belonging to the family of Spin Ladders (Sr, Ca)h_lCUh.lO2h are grown by MBE. The deposition is performed under atomic oxygen using real time control by the RHEED intensity. The structural parameters measured by four-circle X-ray diffraction agree with the values observed for cuprate Spin Ladders in bulk compounds from various laboratories. HRTEM images shows the double Cu rows within the CuO planes which are characteristic of ladder structures. The room temperature resistivity ranges from 1 nuO.cm for Ca2Cu406 to 1 fLcm for Sr2Cu406. Doping with BiO planes was tested. A superconducting transition at 40K was observed in a BiSrCaCuO compound with a composition compatible with two-leg ladders.

1. INTRODUCTION

Cuprate Spin Ladders were In'st synthesized under high pressure by Hiroi and Takano. ~ They describe electrical and magnetic properties of the two gust members Sr2Cu406 and Sr4Cu6Olo , of the Srh_lCU~+lO2h homologous series. Figure 1 compares their unit cell with the structure of the Infinite Layer Compounds 2 (ILC). These compounds are referred as Spin Ladders because the CuO4 tetragonal units are distributed along stripes looking like ladders. The quasi one-dimensionnal magnetic properties of these compounds 3'4 are interesting. It was suggested by Rice 5 that they could lead to superconductivity with high critical temperatures. It w a s reported 6 that the ladder compound (La, Sr)CuO2.5 shows no superconductivity. A related phase, Sr14Cu24041 , is suggested to be the intergrowth of the Spin Ladder unit cell Sr2Cu406 (figure 1) with a SrTCuloO20 block containing CuO2 chains 7'8. One may also notice the relation between this phase and the superconducting behavior observed but not well described in some Infinite Layer Compounds : the SrI4Cu2404~ phase shows up often as a secondary phase when superconductivity is observed in ILC, both in the case of n-type 9 and p-type doping 1°'11. A recent report describes the superconductivity under high pressure for a (Sr, Ca)14Cu2404~ compound ~2 with a Tc of 12K. All these ladder compounds are bulk materials prepared under very

0921-4534/97/$17.00 © Elsevier Science B.V. All fights reserved. PII S0921-4534(97)00251-7

SrCu02 (ILC)

Sr4CusO~o

Sr2Cu406 Fig. 1 Structure of the unit cell for the first two members (h=3 and h=5) of the Spin Ladder family Srh.lCuh+lO2h compared to the ILC structure.

high pressure. The compounds presented here are thin films prepared by atomic engineering. Depending on the composition we observe low resistivities and metallic behavior -but no superconducting properties as yet- on thin l-rims belonging to the f a m i l y (Sr, Ca)h.lCUla+lO2h . Their room temperature resistivity ranges from 1 mgLcm for Ca2Cu406 to

M. Lagu~s et al./Physica C 282-287 (1997) 162-165 163

1 fLcm for S r 2 C u 4 0 6. A BiSrCaCuO compound with a composition [4,4,3,10] compatible with a two-leg Spin Ladder shows superconducting properties.

2. EXPERIMENTAL

Films with a typical thickness of 30 nm were grown by the Sequentially Imposed Layer Epitaxy (SILE). Using such SILE deposition with our conditions, we find that the growth of compounds belonging to the system (Sr,Ca,Cu,O) is more stable for Spin Ladder structures than for infinite layer compounds ~315 . The deposition chamber is a Riber Eva32 equipped with an atomic oxygen plasma source. The substrate were MgO or SrTiO3 (100). During the deposition

the local pressure near the sample is kept to 10 -4 Tort of atomic oxygen. The subsWate temperature is in the range 500 to 600°C for SrCaCuO compounds, and 700 to 800°C for BiSrCaCuO. Bismuth, strontium and calcium are deposited from Knudsen cells. Copper is evaporated by an electron gun. The surface structure of the film is monitored in real time using the diffraction pattern of a 35 keV electron beam. The oscillations of the Reflection High Energy Electron Diffraction (RI-IEED) intensity are used to stop the deposition of each atomic layer when an extremum is reached. As the sticking coefficients of the different species change

16 during the deposition of typically the first 5 nm, the deposition sequence is adjusted in real time for these fu'st few atomic layers. A deposition sequence controlled by computer is then used to complete the film thickness up to around 30nm. Rutherford Backscattering allows absolute measurements of the average amount of each element (except for the oxygen) deposited during one layering sequence. The oxygen content used in the following compositions, i.e. C a 2 C u 4 0 6 , is a formal value without quantitative meaning.

3. STRUCTURE

Figure 2 shows a typical High Resolution Trnasmission Electron Microscopy (HRTEM) picture observed in Sr2Cu406 f-dins grown on SrTiO3 . The double Cu row observed here in the Cu203 planes is characteristic of the two-leg cuprate ladders. A detailed study of the structure

:i:i: 3ii

:i:

Fig 2 HRTEM picture of Sr2Cu406 showing the double copper rows. Upper: raw image, lower • after Wiener filtering.

shows a c-axis cell parameter corresponding to four Cu203 planes 17. This corresponds to the 13.43 /~ measured by X-ray diffraction for the (OO1) lattice parameter. In this structure, the copper-copper distance is modulated from one Cu203 plane to the following between 3.25 3.45/~. The measurement of the in-plane structure on a four-circle diffractometer is shows the expected lattice parameters for a two leg ladder, a = 3.95 /~ and b=11.57 /~ . The same structural study is under progress for Ca2Cu406 fdms on MgO, in which a = 3.90/k and b=l 1.40 ~, and c = 12.40 ,~. Copper rich unconventional BiSrCaCuO compounds were sequentially deposed including B i t reservoir block layers. One can use a general ladder scheme to describe their composition • it is assumed that the copper layers exhibit a Cal_l~CUl+lthO2 composition where l/h equals zero for conventional compounds. Compared to the conventional Bi2Sr2Ca~qCunOx we expect to observe a general composition :

[Bi2Sr2Caa_l] 1-1~a[Cun] l+lcaOy [11 We assume here that the reservoir block-layer builds itself by epitaxy onto the copper-oxide

164 M. Lagu#s et aL /Physica C 282-287 (1997.) 162-165

ladders planes, with a reduced surface density as do the Ca layers. The sample for which electric and magnetic properties are described in the following exhibits an absolute composition for each cell of BiL3Srl.4Ca0.9Cua.aOy in units of 0.675 atoms/cm 2. (In these units the cuprate plane composition is simply CuO2). Such a copper rich composition without presence of copper oxide phases in the X- ray pattern, corresponds precisely to the composition of a two-leg ladder structure (h=3) with an intergrowth having equal parts of n=2 and n=3. The composition of such a sample according to [1] should BiL3SrL3CaCu3.3Oy, Bi4Sr4Ca3CuloOy for three cells . HRTEM observations are presently in progress in order to verify the presence of ladders in these compounds.

4. TRANSPORT and MAGNETIC PROPERTIES

Figure 3 shows the temperature dependence of the

S r 2 C u 4 0 6 grown on SrTiO3 and resistivity for 1000

~= 100 0

d ~ 1 0

Z"

(D 0.1 n"

0 0 1

0

4

oE

v

._z- a

rr 2 i i i i ,

100 200 300 T (K)

Fig 3 Resistivity of Sr2Cu406 and CazCu406 films grown on MgO

Ca2Cu406 grown on MgO. The Sr two-leg ladder sample shows an insulating behavior with an activation energy of about 160 meV for T>130 K. For T>150K a metallic behavior is observed for Ca ladders without any special doping process. Figure 4 presents the temperature dependence of the

resisitivity for the unconventional Bi[4,4,3,10] sample. Figure 5 presents the susceptibility of this last sample for which a zero resistance is observed at 40K. These properties were already presented elsewhere. 19 The diamagnetic onset corresponds well with the resistive transition, as shown in figure 5.

0.6

E o

0.4

._z- • > 0.2

n"

0 . 0 '

0 100 200 300 T (K)

Fig 4 Resistivity of the Bi [4,4,3,]0] sample

=. E

0,000

-0.001

- 0 . 0 0 2

f .......

/ x,ooi !

20 4 0 60 80 100 T (K)

Fig 5 Zero Field Cooling magnetisation of the Bi [4,4,3,10] sample mesured under 50 Oe (contiuous line), zoom xl00 (dashed line), versus the log of the resistivity (dotted line)

4. DISCUSSION

A detailed HRTEM structural study was performed

for Sr2Cu406 grown on SrTiO3. The in plane parameters a and b agree with those of bulk compounds 1 but the c-axis parameter of our films corresponds to four Cu203 planes. This compound is thus different from the bulk compound which shows c=3.40 /~. HRTEM structural study is currently under progress for Ca2Cu406 and the unconventional Bi compounds. Polarized X-my

M. Lagu#s et al./Physica C 282-287 (1997) 162-165 165

Absorption Fluorescence studies were also performed and measurements of the Cu L3-edge lead to an average Cu valence of 2.15 for the unconventional Bi compounds and 2.26 for Sr2CunO6. This high oxidation level, corresponding

to a composition Sr2Cu406.5 , could explain the difference of the c-axis structure between this compound and the bulk Sr2Cu406 Spin Ladder 2° • the film shows c = l l.40A leading to an average distance of 3.35]k between the Cu203 planes, while in the bulk compound this distance is 3.43A.

5. CONCLUSION

Thin films belonging to the family of Spin Ladders (Sr, Ca)h_lCuh+lO2h are grown by MBE. In plane lattice parameters measured by X-ray diffraction and HRTEM are in agreement with the values observed for cuprate Spin Ladders in bulk compounds. The room temperature resistivity ranges from 1 mfLcm for Ca2Cu406 -showing a metallic behavior above 150K- to 1 fLcm for Sr2CthO6. A superconducting transition at 40K was observed in an unconventional BiSrCaCuO compound whose composition could correspond to a two-leg spin ladder. The presence of ladders in this sample has to be verified, for instance by HRTEM.

Acknowledgments SQUID measurements were performed by N.Bontemps at ENS (Paris). Cu-edge XAS in collaboration with F.Studer, and four-circle diffraction in collaboration with J.P.Lauriat were performed at LURE (Orsay). RBS measurements were performed in Universit6 Paris 7 with the support of the GRD86 of CNRS. This work was financially supported by Soffmova, Anvar, Conseil R6gional d'Ile de France and CRITI" Chimie Ile de France.

6. REFERENCES

1..Z.Hiroi, and M.Takano, Physica C 235-240 (1994) 29

2. T.Tatsuki, S.Adachi, M.Itoh, T.Tamura, X.J.Wu, C.Q.Jin, H.Yamauchi, N.Koshizuka and S,Tanaka, Jpn. J. Appl. Phys. 34 (1995) L615

3. M.Azuma, Z.Hiroi, M.Takano, K.Ishida and Y.Kitaoka, Phys.Rev.Lett. 73 (1994) 3463

4. T.M.Rice, Europhys. Lett. 23 (1993) 445 5. Z.Hiroi and M.Takano, Nature 377 (1995) 41 6. M.Lagu~, C.F.Beuran, C.Coussot, C.DeviUe

Cavellin, B.Eustache, C.Hatterer, P.Laffez, V.Mairet, X.M. Xie and X.Z.Xu Coherence in superconductors p70 Ed. G.Deutscher & A. Revcoleschi (World Scientific 1996)

7. N.Sugii, M.Ichikawa, K.Kubo, M.Ichlkawa, K.Yamamoto, H.Yamauchi and S.Tanaka, Physica C 196 (1992) 129

8. X.Zhou, J.W.Li, F.Wu, J.Q.Li, B.Yin, S.L.Jia, Y.S.Yao and Z.X.Zhao, Physica C. 223 (1994) 30

9. C.Prouteau, P.Strobel, J.J.Capponi, C.Chaillout and J.L.Tholence, Physica C 228 (1994) 63

10. Mc Carton, M.A Subramanian, J.C.Calabrese and R.L.Harlow, Mat. Res. Bull. 23 (1988) 1355

11. H. Shaked, Y.Shimakawa, B.A. Hunter, R.L.Hitterman, J.D. Jorgensen, P.D.Han and D.A. Payne, Phys. Rev. B 51 (1995) 11784

12. M.Uehara, T.Nagata, J.Akimitsu, H.Takahashi, N.Mori, K.Kinoshita J.Phys.Soc Jpn 65 (1996) 2764

13. V. Mairet PhD thesis Univ. Paris VI P. & M.Curie (21 december 1995)

14. C.Hatterer PhD thesis Univ. Paris VI P. & M.Curie (11 july 1996)

15. B.Eustache, C.F.Beuran, C.Deville Cavelin, Ch. Hatterer, V.Mairet, C.Partiot, X.Z.Xu, P.Germain, M.Lagu~s J. of Alloys and compounds (1997) in press

16. X.Z.Xu, M.Viret, H.Tebbji and M.Lagu~s Applied Superconductivity 1 (1993) 755

17. J.Y.Laval & al. to be published 18. C. Deville Cavellin, B.Eustache, C.J.Hatterer,

J.-P. Lauriat, X.Z. Xu, V. Mairet, F. Beuran and M. Lagu~s (this meeting)

19. M. Lagu~s, N.Bontemps, C.Partiot, B.Eustache, J.B.Moussy, C.F.Beuran, X.Z.Xu and C. Deville Cavellin MRS meeting (Boston, USA, 1-5 december 1996)

20. Z.Hiroi, M.Azuma,M.Takano and Y.Bando J.Sol.Stat. Chem. 95 (1991) 230