Physica C 162-164 (1989) 1029-1030 North-Holland

CHARACTERISATION AND TRANSPORT MEASUREMENTS ON SINGLE CRYSTALS IN THE BI-SR-CU-O SYSTEM

Andrew MACKENZIE, Elisabeth MARSEGLIA, Ian MARSDEN and Gilbert LONZARICH

Cavendish Laboratory and Interdisciplinary Research Centre in Superconductivity, Cambridge, U.K.

Changkang CHEN and Barbara WANKLYN

Clarendon Laboratory, University of Oxford, Oxford, U.K.

A series of single crystals of Bi2+vSr,~ _,CuO~.~, have been prepared and characterised by A l . , . ' A U~'~

means of high precision electron probe microanalysis and by transport measurements. The superconducting transition temperatures of our samples, for which 0.1 < x-x < 0.4, are found to oe very sensitwe to x and y but insensitive to pressure.

1. INTRODUCTION AND EXPERIMENTAL METHODS The copper oxide superconductor with the

nominal composition Bi2Sr2CuO61, 2 is of special interest as a parent of the high T c Bi-Sr-Ca-Cu-O system having a sufficiently low T c to permit in principle the observation of magneto-oscillatory phenomena at relatively low temperatures and magnetic fields. This study was undertaken to clarify the structure, the precise composition and the nature of the current carriers in single crystals of this material, about which relatively little has so far been reported.

The crystals were grown from a Cu rich melt employing a KF-KCI flux in a Pt crucible and characterised by means of x-ray diffraction and wavelength dispersive electron microprobe analysis (using a Cameca SX-50) at 15 KeV using Bi, SrTiO 3, Cu and MgO standards. Microprobe measurements were made of the primary elements including oxygen (from the oxygen x-ray intensity directly) and of traces of K, F, C1 and Pt, all found to be below detection limits of 500 ppm. The electronic properties were characterised by means of conventional measure-

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ments of the magnetic susceptibility and electrical resistivity as a function of temperature and (for the resistivity) pressure.

2. RESULTS AND CONCLUSIONS Measurements of the composition of ten as-

grown crystals of Bi2+xSr2_x,CuO6+y summar- ised in table 1 and figure 1 indicate that in our samples excess Bi substitutes for Sr, i.e. 0.1 < x-x ' < 0.4, and y is typically less than 0.06. X-ray measurements are consistent with the primary structure for the nominal composition Bi2Sr2CuO 6 reported previously 1 but reveal the existence of a superstructure which is sensitive to X.

Element Mean no. of atoms (Bi+Sr) 4.02 + 0.02 Cu 1.00 + 0.02 O 5.98 + 0.06

TABLE 1 Average compositions of ten as-grown crystals

in the Bi2+xSr2_ x CuO6+y system.

1030 A. Mackenzie et al. / Characterisation and transport measurements on single crystals

2.5 ¸

,~" 2.4' E o

2.3'

~ 2 . 2 '

2.1'

2.0

I'~ [ • Sll~ducling at 4.2K I ~ ~ o Non superr.onduc~g I

Bi+Sr=4 _ ~

1.6 1.8 2.0 Sr (no. of atoms)

2.2

FIGURE 1 The Bi and Sr compositions of ten as-grown

crystals of the Bi2+xSr2_x,CuO6+y system. The solid line represents the case x--x'.

It is interesting to note that, as the oxygen content is close to six, simple valency arguments, assuming a Bi valence of 3+ or slightly higher 3, suggest that the crystals described here may be electron doped in the manner recently reported in

Nd2_xCexCuO4.y 4. Transitions to superconducting states have

been detected by resistivity and susceptibility measurements on samples with x towards the bottom of our range, and T c has been found to be very sensitive to x and to annealing conditions. In marked contrast to the typical behavior of known hole superconductors in the Y-Ba-Cu-O and La-Sr-Cu-O systems, we observe in particular an increase in T c with decrease in oxygen concentration upon vacuum annealing. (For one as-grown sample the initial T c decreased from 5.5K to below 4.2K upon annealing in air at 400 °C for 15 hours and then increased to 12.5K when annealed in vacuum for 8 hours at a similar temperature.)

Measurements of the temperature and pressure dependence of the electrical resistivity in the a-b plane have also been performed and typical results are shown in figure 2. Though very sensitive to x and y as discussed above, T c is insensitive to pressures of up to 5 kbars. The

4000"

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0 " 0

0 | | | i |~11| ||~0 | | | | j ~ | | | | | | 1 0 1 | 1 | | | | | | | | |

a • P=5kbar

100 150 Temperature (Kelvin)

FIGURE 2 The electrical resistivity in the a-b plane vs.

temperature at 0 and 5 kbars of an as-grown crystal of Bi2+xSr2_x,CuO6+y with x--x --0.15.

resistivity in the normal state is not simply linear and in some cases shows a pronounced variation from linearity, with the ratio of the room temperature value to the extrapolated zero tem- perature value being as high as six in some cases.

Our overall results show that small alterations to the stoichiometry can cause major changes in the low temperature electronic properties of Bi2+xSr2_xCuO6+y. The form of the cation doping suggests that material with an oxygen content of 6 or less may be electron doped, and measurements of the Hall and Seebeck coefficients are in progress to investigate this possibility.

REFERENCES

1. C.C.Torardi, M.A.Submmanian~J.C.Cala- brese~ J.Gopalakrishnan, E.M.MCCarron, K.J. Momssey, T.R. Askew, R.B. Flipp~_n, U.Cho- dhry and A.W. Sleight, Phys. Rev. B 38(1988) 225, and references contained therein.

2. P.Strobel, K.Kelleher, F.Holtzberg and T. Worthington, Physica C 156 (1988) 434.

3. Z.-X. Shen, P.A.P.Lindberg, P.Soukiassian, C.B. Eom, I.Lindau, W.E.S~icer and T.H.Geballe, Phys. Rev. B 39 (1988) 823.

4. H.Takagi, S.Uchida and Y.Tokura, Phys. Rev. Letts. 62 (1989) 1197.