Physica C 153-155 (1988) 1004-1005 North-Holland, Amsterdam

LOW TEMPERATURE STM STUDY ON YBa2Cu307

Sebastian VIEIRA, Miguel A. RAMOS, Andr6s BUENDIA and Arturo M. BARO.

Dpto. FCsica Materia Condensada, C - I l l , Universidad Aut6noma de Madrid, 28049-Madrid, Spain.

We report Scanning Tunneling Microscopy and Spectroscopy on YBa2Cu307_ 6 high-T c superconducting ceramics, at di f ferent temperatures. From our lowest temperature I-V characterist ics, we obtain for the superconducting energy gap 11 < A < 15 meV.

1. INTRODUCTION AND EXPERIMENTAL Since the discovery of high-T c

superconductivity in La-Ba-Cu-O samples (1), a great e f f o r t has been done with the aim of understanding the underlying mechanisms. Several techniques have been used (2-4) to measure the superconducting energy gap A • Local tunneling spectroscopy has shown the e l e c t r i c a l l y inhomogeneous character of the surface in these new materials. Semiconductinq, n o r m a l metall ic and superconducting zones have been reported (2).

We have used a single piezotube Scanning Tunneling Microscope (STM) in order to study the topography and the current-voltage characteristics of the YBa2Cu307_ ~ superconductor. The STM unit is r i g id l y anchored in the inner chamber of a 4He cryostat. The temperature of this chamber can be e lec t r ica l ly controlled. In some measurements helium exchange gas is introduced to achieve thermal equilibrium. The temperature of the sample is measured with a Si diode calibrated between 1.4 K and 300 K. The coarse tip-sample approach is done with a micrometer screw attached to the STM unit. which is driven from the outside of the cryostat. After tunnel current is established, the micrometer is mechanically decoupled. STM electronics is the conventional one. Current-voltage curves are made with a Keithley Electrometer connected to a HP Diqital Voltmeter which transfers data to a computer. Sample voltage is also controlled by the computer. A typical I-V curve of 50 points is taken in 0.5 seconds.

Our system has been tested with Pt and Pb samples using mechanically prepared tungsten and Pt-Rh t ips. We have been able to obtain easily high resolution topography at low temperatures and vacuum tunneling I-V curves.

The samples of YBa2Cu307_ ~ were prepared with the usual recipe to get 6 < 0.1 and showed zero-res is t iv i ty at 91K. The sintering process was done just before the assembly of the sample to the STM unit. After that, samples were always cooled i n 02 atmosphere,

X

FIGURE 1. Topographic images of YBa2Cu3OI_d at 4.2 K (up) and 77 K (down) with scanned x-y areas of 45A x 27A and 685A x 120A, respect ively.

which was removed at ~lOO K and substituted For a small pressure of helium exchange gas.

2. RESULTS AND DISCUSSION We have been able to obtain with the STM

topographic images of YBa2Cu30/_ ~ (F~g. 1) at l i qu id N 2 and l i qu id He temperatures. Two d i f f e ren t kinds of surface topography have been observed throughout the samples, one with tunneling current stable even at high resolution and another one which is unstable and characteristic of non-metallic behaviour, in a similar way as has been reported previously at room temperature (5). In areas of the former kind a structure of h i l l s separated by sharp depressions with typical dimensions of 50-200A has been repeatedly observed {see F ig . l ) .

We have also obtained I-V characteristics of this material at temperatures above and below

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S. Vieira et a L / Low temperature STM study on YBa2Cu307 1005

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FIGURE 2 Typical I-V c h a r a c t e r i s t i c s of YBa2Cu307_ d i s p l a y i n g semiconduct ing behav iour ob ta ined a t : (a) T = 225K: (b) T = 77K.

the superconduct ing t r a n s i t i o n . Fig. 2 shows r e p r e s e n t a t i v e data ob ta ined at 77K (b ) , below T c ~91K, and at 225K ( a l , we l l above T c.

As repor ted by o the r authors ( 2 -4 ) , we obta ined a n o n l i n e a r dependence of the c u r r e n t on v o l t a g e . The Z e l l e r - G i a v e r model f o r t unne l i ng i n t o g ranu la r superconductors could f i t the 77 K curves. Our data show however the same k ind of n o n l i n e a r behav iour below and above T c, demonst ra t ing t ha t t h i s e f f e c t is not connected w i t h s u p e r c o n d u c t i v i t y but r a t he r i n d i c a t e s t h a t l a rge po r t i ons of the sur face sample are semiconduct ing.

Since the conductance of a semiconductor Fa l l s w i th decreasing tempera ture , l i q u i d hel ium temperatures are b e t t e r su i t ed to observe the superconduct ing gap.

Two d i f f e r e n t and r e p r e s e n t a t i v e k inds of I-V curves at 4.2K are p l o t t e d in Fig. 3. For low i n t e n s i t i e s , curves l i k e (a) are observed. When we approach the t i p f u r t h e r and i n t roduce i t i n t o the sample a few 100 A curves l i k e (b) can be ob ta ined . At the same t ime a subs tan t i a l and sudden increase of c u r r e n t takes p lace. Curve (a l is s i m i l a r to those ob ta ined a t h igher tempera tures , but (b) is

V O L T F I G E ( r n V )

FIGURE 3 Two d i f ferent I-V curves of YBa2Cu307_ 6 taken at 4,2 K (See text for deta i ls ) .

quite d i f ferent . The most st r ik ing feature is that the l a t t e r curve is almost l inear and the superconducting gap is much more evident. Only from th is type of I-V characterist ics meaning- ful data about this gap can be obtained. We estimate for A a value of 11-15 meV which gives 2/I/KBT c = 3.3 ± 0.5, not in disagreement with the BCS ra t io in the weak coupling l im i t .

We have also observed that I-V curves taken in locations with more unstable tunnel current as seen in topographic imaging tend to display a more marked semiconducting behaviour but no conclusive results can be drawn vet. Such study is in course in our laboratory.

ACKNOWLEDGEMENTS We thank N. Garcla for helpful discussions.

REFERENCES 1) J.G.

2) 31

(4)

(5)

Bednorz and K.A. MUller~ Z. Phys. B 64 (1986) 189. M.D. K i r k e t a l . Phys. Rev. B35 (1987) 8850 J.R. K i r t l e y e t a l . Phys. Rev. B 35 (1987) 8846. J. Moreland e t a l . Phys. Rev. B35 (1987) 8856, N, Garcia et al. Z. Phys. B 70 (1988) 9.