Solid State Communications, Vol. 71, No. 1, pp. 33-38, 1989. 0038-1098/89 $3.00 + .00 Printed in Great Britain. Maxwell Pergamon Macmillan plc

TOP OGR APHY AND LOCAL C O N D U C T A N C E IMAGES OF YBa2Cu307 CRYSTALS F R A C T U R E D IN ULTRA H I G H VACUUM

C.J. Chen and C.C. Tsuei

IBM Research Division, Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA

(Received 13 March 1989 by G. Burns)

Atomic-scale topography and local dynamic conductance images of polycrystaUine YBa2Cu307 samples fractured in ultra high vacuum are obtained with a cross-sectional scanning tunneling microscope. Topo- graphic features are found which consist of strips with a 12 A periodicity. Simultaneous measurements of normalized conductance (d In I/dV) on these surfaces exhibit a sharp contrast from strip to strip. The corresponding tunneling spectra at these areas show an alternation of metallic and semiconducting characteristics. Possible implication of the results on the basic understanding of high temperature superconductivity is discussed.

SINCE the discovery of superconducting La-Ba-Cu oxides by Bednorz and Mueller [1], high-temperature superconductivity has been found in a number of cuprates, including the intensively studied ABa2Cu307 (A = Y, Ho, La, Eu . . . e t c ) [2], and the recently dis- covered Bi(TI)-Sr(Ba)-Ca-Cu-O compounds [3, 4] which contain no rare-earth elements. These new superconducting oxides are characterized by low- dimensional structural entities such as the CuO2 square-lattice planes and Bi(T1)-O layers. Further- more, strong anisotropy in normal state and super- conducting state properties of these materials is well- established experimentally [5-8]. The underlying correlation between the low-dimensional structures and their relevance to high temperature supercon- ductivity are of great current interest and, by and large, unknown. It is, therefore, highly desirable to study structural features along with some atomic-scale electronic-state measurements on this new class of materials.

Recent advances in scanning tunneling micro- scopy (STM) [9] and scanning tunneling spectroscopy (STS) using the method of current imaging tunneling spectroscopy [10] have demonstrated that it is feasible to image surface topographic features as well as elec- tronic structures. Thus, STM and STS are ideal for revealing the electronic structure of these oxide super- conductors down to atomic scale. Experimental effort on STM and STS study of the oxide superconductors, to date, is hampered by the existence of a thick (at least 100 A) native insulating film on the surface of the cuprate specimens, which results in a feedback instab-

ility during the STM measurements, and requires a high bias (above 1 volt) to obtain images [11]. In fact, the reported STM works on YBa2Cu307 are either the topography on the as-made (001) surfaces at room temperature in air [11], or the I-V measurements at one fixed point at a cryogenic temperature by plung- ing the tunneling tip through the insulating layer [12]. In this work, we report new measurements on YBa2Cu307 crystals freshly fractured in ultra high vacuum using a cross-sectional STM. Our results show that stable images can be obtained at a low bias (0.1 V) and a relatively high set current (1 nA). We also obtain simultaneous topographic and local conduc- tance images as well as local tunneling spectra on different regions.

The ideal experiment if of course to cleave (or fracture) a high T, single crystal in ultra high vacuum or a clean chamber filled with a certain amount of pure oxygen and carry out the STM and STS measurements immediately. Although high-quality single crystals of YBazCu307 are available recently [13], the crystal size is in general very small. Especially, the thickness in the c direction is usually less than 0.2 mm. It is very dif- ficult to tunneling into the (1 0 0) and (0 1 0) cleavages, which is of primary interest. However, using our cross- sectional STM which is capable of searching over vast areas of the surface without crashing the tip [14], we have found features of interest on fractured polycrys- talline samples. In this investigation, we use dense, polycrystalline ceramic samples of YBa2Cu307 pre- pared by the standard procedures of mixing, grinding and sintering. All the samples used in this study are

33

34 YBa2Cu307 CRYSTALS F R A C T U R E D IN ULTRA H I G H VACUUM Vol. 71, No. 1

Fig. 1. A scanning electron micrograph of fractured YBazCu3Ov polycrystals. The flat surfaces are (00 1) surfaces.

single-phase with the well-known orthorhombic triple- deck perovskite structure [15]. The resistive transition completes at 91.2 K and the 10% to 90% transition width is about 1 K. The density of our samples (4.89 gcm -3) is about 80% of the theoretical value as estimated from the lattice parameters. The relatively high density ensures that the crystal grains in the polycrystalline samples are tightly interlocked with each other and facilitates the fracture process once a crack propagation is initiated at certain spots of the specimen. The samples are cut into rectangular bars of 2 mm × 5 mm × 0.5 mm in size. A notch is scribed with a pin file on the sample before it is transferred into the STM chamber (5 × 10-mTorr) which is equipped with an in-situ cleaving mechanism. The morphology of the fractured samples is studied with scanning electron microscope after the STM experi- ment. The following features are observed: First, samples consist of single crystals of 20 to 50/~m in size with good integrity. Second, about one half of the fractured facets are grain boundaries, and the rest of the facets are transgranular fractured surfaces of single crystals which are either very flat or irregularly curved, which indicates a mixture of crystallographic cleavages and concoidal fractures. A scanning elec- tron microscope image showing different facets of a fractured surface is shown in Fig. 1.

All the STM experiments were done under ultra

high vacuum condition to prevent contamination of the fractured surface. No substantial change of the observed images was found several hours after cleav- ing. The cross-sectional STM residing in the ultra high vacuum chamber is equipped with a two-dimensional precision computer-controlled stepper allowing the tip to traverse along the sample surface with step sizes of 100 ___ 20 A and total travel distance of more than one millimeter for searching areas of interest without crashing the tip. The cleaving mechanism in the main chamber is designed to be able to break small samples and mount it on the STM immediately. Chemically etched tungsten tips were used in all measurements. The tips were heat-treated in the same ultra-high- vacuum chamber at about 800°C to evaporate the surface oxides and then mounted on the STM in situ. The calibration of the spatial dimension on the STM was made with two independent methods: (1) a direct measurement of the mechanical displacement while applying a voltage of a few hundred volts on the piezodrives, and (2) calibration against images of the (1 1 1) surface of silicon crystal freshly cleaved in the ultra high vacuum chamber. The two calibrations were in agreement within 5%. The z-scale was cali- brated with mechanical measurement only. The details of the cross-sectional scanning tunneling microscope will be published elsewhere [14].

To study local electronic properties, along with

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each topographical image, an image of the normalized conductance (d In I/d V) was acquired simultaneously with point-to-point correspondence. It was obtained by modulating the bias with an a.c. sinusoidal signal from a lock-in amplifier. The d.c. component of the tunneling current was picked up through a low-pass filter to activate the feedback circuit. The a.c. com- ponent of the tunneling current, after a logarithmic amplifier, was detected with the same lock-in amplifier. All the experiments reported here were performed with a bias of VB = 0.25 V, and a 5 kHz a.c. modula- tion signal of 0.1 V rms. The time constant of the lock-in amplifier was set to be 3 ms. The scanning rate was set such that the time duration for each pixel was 10ms, i.e. much longer than the time constant of the lock-in amplifier. The output of the lock-in amplifier was recorded simultaneously with the topograph to form a second image. Therefore, a true point-point correspondence of the two images was achieved. The normalized dynamic conductance is a measure of the rate of variation of the local density of state (LDOS) of the sample ~s(e) on the surface [16]. In the present study, the range of bias is small, the tunneling matrix element can be considered as a constant. A simplified expression for normalized conductance can be obtained [16] as follows: ]l

~- #s (eV) Qs (e)de (1) dV

where the Fermi level i s taken as the zero point of energy. Suppose that the density of states varies with energy as Qs(e,) oc e", where n is a constant, it follows from equation (1) that the ratio of dynamic to static conductivity r is

v d l n I r = d1,1 - n + 1, (2)

i.e. it is directly proportional to the exponent n in the expression of LDOS vs energy. In.general, it is a measure of how fast the LDOS changes with energy. For metals, the LDOS near the Fermi level does not change substantially, thus, r - 1; whereas for semi- conductors, the LDOS near the Fermi level is very low but increases rapidly at a certain bias from the Fermi level, thus, r ~ 1. This has been demonstrated experi- mentally, for example, for nickel and silicon [17]. As a function of x, y, it provides a normalized conductance image which is independent of the topographical image.

The tunneling experiments on uncleaved native YBa2Cu307 surfaces exhibits non-vacuum tunneling behavior. By changing the set current by a factor of two or changing the bias by a factor of two, the feedback circuit draws the tip by more than 50 A. However, the tunneling characteristics of the surfaces

YBa2fu307 CRYSTALS F R A C T U R E D IN U L T R A H I G H V A C U U M

13

35

N

b

I_ zeo -[

0

Fig. 2. The "broad band" type images. The width of the bands is about 100 A. The dynamic conductance at the edges is much higher than that on the planes.

of YBa2Cu307 freshly fractured in ultra high vacuum is found to be quite different. Stable images can be obtained at a low bias, e.g. + 0.1 V, and a relatively high set current (1 nA). By changing the distance of the tip to the sample by 1 A, the tunneling current can change more than twice, which indicates that the tunneling is indeed through vacuum.

By moving the tip on the fractured surfaces of the YBa2Cu307 sample with the stepper, several differ- ent kinds of surfaces are found with the scanning tunneling images. Some of the areas exhibit similar tunneling characteristics as the native surface, which are presumably the exposed grain surfaces. On areas where vacuum tunneling characteristics prevail, the observed images can be classified in general into two categories. The first kind of images consist of bands of a few hundred A in width (Fig. 2a). Steps with depth

36

of 12 + 3 A are found at the border of the bands. The simultaneous dynamic conductance image shows that the value of (d In I/d V) at the border of each band is about 3 times larger than that of the band itself. It is an indication that the surface of the band is metallic whereas the border area is much similar to a semicon- ductor. A possible interpretation is that near the steps, oxygen in easier to escape from the surface leaving a local structure with oxygen deficiency. From the height of the steps (about 12A) it is suggestive that the crystallographic c-axis is in the z-direction of tunnel- ing image. The second kind of images consists of long strips of about 12A in width (Fig. 3a). On the nor- malized conductance images, a sharp contrast is found (Fig. 3b). We will discuss these images in detail as follows.

By overlapping the topographic image onto the normalized conductance image we find that there are three types of strips, as indicated schematically in Fig. 3c. Type one is high on the topographical image and low on normalized conductance. Type two is slightly (1 to 2A) lower than the first type, but very high on normalized conductance. Type three, which is about 4 A lower than the first one, has a normalized conductance as low as the first type. The maximum values of the quantity (d In I/d V) (at the darkest areas in Fig. 3b) is about three times greater than that in the lightest areas in Fig. 3b. On both topographic and local conductance images, a clear 12A periodicity is observed. The local dI/dV curves obtained simul- taneously with those images show that on strips with lower values of normalized conductance, the dI/d V curves vary less than a factor of two within a bias range of -0 .5 V, whereas the other one has very low values of normalized conductance at zero bias which increases rapidly at about 0.3V. The first kind of dim V curve is typical for metals whereas the second type is typical for a narrow-gap semiconductor.

The observed images can be understood in terms of possible cleavage surfaces of YBa2Cu307 from cleaving experiments [18] and from structural analysis [15]. As the crystal fractures, the planes consist of weaker interplane bonds have higher probability to break. The cleaving experiments of YBa2Cu307 single crystals [18] show planes of types (100), (010) and (001) are the easy fracture planes in the crystal. Among which the ab plane, i.e. the (001) plane, is the easiest to cleave, as is well known, these crystals behave like a layered material. The next most fragile one is the plane containing Ba and Y atoms with missing oxygen atoms, i.e. (200) plane. The (010) planes, which are perpendicular to the copper-oxygen chains, are probably the most difficult to break because the copper atoms are four-fold coordinated

YBa2Cu307 CRYSTALS FRACTURED IN ULTRA HIGH VACUUM Vol. 71, No. 1

o ,¢

b 0 ,¢

180 .~ =-

Fig. 3. The striplike images. (a), topographic image. (b) simultaneous normalized dynamic conductance image. The normal conductance of the darkest region is about 3 times larger than that of the lightest region. (c) three types of strips: 1, high in topography and low in normalized conductivity, 2, high in topography and high in normalized conductivity, 3, low in topography and low in normalized conductivity.

with a relatively short bond length [15]. The 12A periodicity in our images indicates hat the c-direction is perpendicular to the orientation of the strips. The continuity of the strips indicates that they are parallel to the b orientation, i.e. parallel to the chains. There- fore, the surfaces Fig. 3 are identified to be the (2 0 0)

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4

YBa2fu307 CRYSTALS F R A C T U R E D IN U LTRA H I G H VACUUM 37

B o #

_ o r b o o

o

o

# i o (9

o

i i i t , , , , I , , , , -0.5 0.0 0.5 -0.5 0.0 0.5

BIAS (V) BIAS (V) (o) (b)

Fig. 4. Differentiated tunneling spectra (dI/dV) on two kinds of regions. (a), on regions with low nor- malized conductance, metallic; and (b), on regions with high normalized conductance, semiconducting.

planes. The orientation assignment is tentative in view of the polycrystalline nature of the samples studied.

By comparing the images with the structural information, an interpretation of the dynamic conduc- tivity images is suggested as follows: As shown in Fig. 5, when the crystal is fractured at (2 0 0) planes, the barium atoms have equal probability to remain on the sample or to stay with the broken-away portion of the sample. Therefore, two kinds of surfaces are exposed. One consists of copper and oxygen atoms, and the other consists of barium atoms and oxygen atoms. According to the results of band-structure cal- culations [19, 20], near the Fermi level, the muffin-tin projected DOS on the oxygen sites in the Cu-O3 chains have a very high value of LDOS within an

• Cu O 0

O Y O B o

j / z ." _: - ;

METALLIC SEMICONDUCTING METALLIC

Fig. 5. A schematic diagram showing possible fractured surfaces of YBa2Cu307.

interval of _0 .5eV. Thus, the normalized conduc- tance should be low. On the barium sites, the LDOS is orders of magnitude lower than that of those oxygen sites near the Fermi level. Thus, the surface with bar- ium atoms should be similar to a semiconductor, and the normalized conductance should be much higher. Although our observation of the two types of tunnel- ing spectra is qualitatively consistent with the band- structure calculations, one should keep in mind that such calculations do not take into account the effect of the strong electron-electron correlations which may be very important in these superconducting oxides.

There is another unknown factor in the interpreta- tion of the images, that is, to what extent the surface reconstructions make the exposed surface different from the bulk. Our results and those of others" how- ever provide no evidence of such surface reconstruc- tion effects. At room temperature, the displacements of the surface atoms are not expected to exceed a few /~ngstrrms as an activation energy far more than a small fraction of an eV is required. It is unlikely that the possible surface reconstruction is to alter the nature of the two kinds of surfaces mentioned above.

In the following, we will discuss some possible implications of our STM observations on high tem- perature superconductivity. The role of the Cu-O linear chains in high temperature superconductivity has been controversial. According to the crystal structure (Fig. 5) and the analysis of our observations, the Cu-O chains are part of the 12 A wide metallic "r ibbons" in the (1 00) planes, parallel to the b-direction. The atomic arrangement in the "ribbons" is essentially the same as that in the CuO2 planes if we ignore the buckling of the Cu-O bonds in the Cu-O basal planes. From the viewpoint of electrical conduction, the YBa2fu307 crystals can be considered as an assembly of basic structures consist of two adjacent CuO2 square-lattice sheets coupled by vertical Cu-O ribbons 4 A apart. The ribbons form a parallel array of con- ducting bridges and make the entire structure less two-dimensional. In terms of the formation of super- conducting state, this presumably leads to a reduced phase fluctuations and a higher To. We note that by reducing the oxygen content in the linear chains, the transition temperature decreases significantly. Further- more, the transition temperatures of the purely two- dimensional compounds, e.g. the La-Sr(Ba)-Cu-O systems, are lower than 40 K as they do not have the one-dimensional structure between CuO2 layers. We also note that the (BiO)2 or (TIO)2.1 planes occupy an equivalent position as the CuO ribbons in the layered structure of Bi(T1)Sr(Ba)CaCuO systems. There are band structure calculations suggesting that the (BiO)2 layers are metallic [20]. There are also evidences that

38

the Bi-O layers exhibit a one-dimensional superstruc- ture which forms separate chainlike entities of 27 A periodicity [21]. It is tempting to suggest that this type of one-dimensional substructure may play a similar role as the linear chains in YBa2Cu307 crystals. Thus, the linear chain in the YBa2Cu307 compound is essen- tial in enhancing the transition temperature from the level of La-Sr(Ba)-Cu-O to above liquid nitrogen temperature.

In conclusion, we have observed various topo- graphic and dynamic conductivity images and local tunneling spectra on YBa2Cu307 crystals freshly frac- tured in ultra high vacuum with a cross-sectional STM. Features of 12 A-wide long ribbons are observed in both topographical and local conductance images. Simultaneous measurements of local tunneling spectra show that these ribbons alternate with metallic and semiconducting characteristics.

Acknowledgements - The authors would like to thank J.E. Demuth, R. Hammers, P. Horn, F. Holtz- berg, J. Clabes, N.C. Yeh, and S.I. Park for stimulat- ing discussions, and J. Wolf, R.I. Kaufman for their technical assistance in developing the cross-sectional STM.

YBa2Cu307 CRYSTALS FRACTURED IN

8.

REFERENCES

1. G. Bednorz & K.A. Miiller, Z. Phys. B 64, 189 (1986).

2. M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang & C.W. Chu, Phys. Rev. Lett. 58, 908 (1987).

3. H. Maeda, Y. Tanaka, M. Fukutomi & T. Asano, Jpn. J. Appl. Phys. 27, No. 2 (1988).

4. Z.Z. Sheng & A.M. Hermann, Nature 332, 55 (1988).

5. S.W. Tozer, A.W. Kleinsasser, T. Penney, D. Kaiser, & F. Holtzberg, Phys. Rev. Lett. 59, 1768 (1987).

6. S.J. Hagen, T.W. Jing, Z.Z. Wang, J. Horvath, & N.P. Ong, Phys, Rev. B37, 7928 (1988).

7. T. Penney, S. von Molnar, D. Kaiser, F. Holtz- berg, & A.W. Kleinsasser, Phys. Rev. B38, 2918 (1988).

ULTRA HIGH VACUUM Vol. 71, No. 1

S. Martin, A.T. Fiory, R.M. Fleming, L.F. Scheemeyer & J.V. Waszczak, Phys. Rev. Lett. 60, 2194 (1988).

9. G. Binnig, H. Rohrer, Ch. Gerber & E. Weiber, Phys. Rev. Lett. 49, 57 (1982); P. Hansma & J. Tersoff, J. Appl. Phys. 50, 120 (1983).

10. R.J. Hammers, R.M. Tromp, & J.E. Demuth, Phys. Rev. Lett, 56, 1972 (1986); R.M. Tromp, R.J. Hamers, & J.E. Demuth, Science 234, 304 (1987).

11. N. Garcia, S. Vieira, A.M. Baro, J. Tornero, M. Pazos, L. Vazquez, J. Gomez, A. Aguilo, S. Bourgeal, A. Buendia, M. Hortel, M.A. Lopez de la Torre, M.A. Romos, R. Villar, K.V. Rao, D.-X. Chen, J. Nogues, & N. Karpe, Z. Phys. B 70, 9 (1988); D. Anselmetti, H. Hein- zelmann, R. Wiesendanger, H. Jenny, H.-J. Giintherodt, M. Diiggelin, & R. Guggenheim, Physica C-B, in print.

12. J.R. Kirtley, C.C. Tsuei, S.I. Park, C.C. Chi, J. Rozen & M.W. Shafer, Phys. Rev. B35, 7216 (1987).

13. D.L. Kaiser, F. Holtzberg, M.F. Chisholm & T.K. Worthington, J. Crys. Growth 85, 593 (1987).

14. C.J. Chen, R. Sci. Instrum. (to be published). 15. T. Siegrist, S. Sunshine, D.W. Murphy, R.J.

Cava & S.M. Zahurak, Phys. Rev. B35, 7137 (1987).

16. J. Tersoff & D. Hamann, Phys. Rev. B31, 805 (1985); C.J. Chen, J. Vac. Sci. Technol. A6, 7216 (1988).

17. J.A. Stroscio, R.M. Feenstra, & A.P. Fain, Phys. Rev. Lett. 57, 2579 (1986).

18. R.F. Cook, T.R. Dinger, & D.R. Clarke, Appl. Phys. Lett. 51, 454 (1987).

19. L.F. Mattheiss & D.R. Hamman, Solid State Commun. 63, 395 (1987).

20. M.S. Hybertsen & L.F. Mattheiss, Phys. Rev. Lett. 60, 1661 (1988); P.A. Sterne & C.S. Wang, J. Phys. C 21, 949 (1988).

21. Y. Gao, P. Lee, P. Coppens, M.A. Subra- manian, & A.W. Slight, Science 241, 954 (1988); M.D. Kirk, J. Nogami, A.A. Baski, A. Kapitul- nik, T.H. Geballe, & C.F. Quate, ibid., 242, 1673 (1988).