Spatially resolved tunneling spectroscopy on Bi2Sr2CaCu2Ox

Physica C 235-240 (1994) 1879-1880 No. :h-Holland

PHYSlCA

Spatially resolved tunneling spectroscopy on Bi2Sr2CaCu20 E.J,G. Boon, A.J.A. van Roy, H. van Kempen University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands We present STM measurements on in-situ cleaved Bi2SraCaCu20~ crystals at 4 2K. By cleaving at low temperature we obtained a clean surface allowing us to image with aiomic resolution. We observed a transltton from clear superconductor - normal metal tun., ling charactenslics Io charging-hke characteristics within a few nanometers.

1. I n t r o d u c t i o n

3, T h e o r y

Electron tunneling spectroscopy has proven to be a good spectroscopic tool to investigate the superco,,ducting state of conventional superconductors. First m planar tunnel junctions [1], later also by use of a scanning tunneling nucroscope (STM). By using an STM one can extract local information about tl e superconducting state In this way it was possible to image directly the Abrlkosov fluxlattice [2] It is therefore no surprise t h a t tunneling experiments were setup on high T~ superconductors after their discovery In 1986 Dependent on surface quality the resulting spectra show insulating, semi-conducting, superconducting and charging phenomena. In this paper we will report about STM measurements on the high T¢ superconductor Bi2Sr2CaCu2Oz. We will show that the tunneling spectra can change from superconducting to charging-like within a few nanometers Since the surface was cleaved at low temperature and showed atomic resolution, we could exclude surface contamination effects and surface damage,

According to the BCS theory the I-V curve of a NlS-junction has overlapping upper- and lowerasymptotes Charging of a single junction on the other hand does have shifted tipper- and lowerasymptotes The latter occurs when a single junction with a capacitance C and a resistance /~T IS connected t o a p e r f e c t current source T h e t u n n o h n g w l t h m tile junclion reduces the charge on the electrodes and can only occur If iln~ does not rose |he total energy E = QU/2(' of lhe junction Tt'..,ehng can therefore only occur for voltages IV[ >_ e/2C

2. S e t u p The experiments on the BI~.Sr~CaCu-.,O~ crystal grown by Dr D Mitzl were perforlned in a home-built low temperature STM [3] The STM was adapted to allow the necessary cleaving of the crystal at LHe temperature The scanning tips were made by cutting Pt(90%)/Ir(10~,) wires

t]ll~ i~ called the ("()li]()llll)t)lo(kade T h e asynlptotes arc shifted o~c'r a (!l,i ,illCl' :]::(12(' (Set" figllrf' 1)

4. E x p e r i m e n t s In figure 1 we see a gradual change from BCShke to charging-like [-V curves when moving the tip over a distance of 40 /~ We stress the point that on the same position, we conld also image the surface with atomic resolution The different kind of curves can be discriminated by looking at tile current above the gap, sa2~ V > 50 mV (see hgure 1) Ill figure 2 the current at selected ~oltage,is measured at every ponlt el1 tile surface along with the height According to figure 1 charging occurs whenever the current at 80 mV ~s lower than expected from ohmm behavior, .o the dark spots m figure 2 indicate the po~ltion~ where I-V curve~ are chargmg-hke, the hght ~pot, indlcai,~ po.~ltions where I-V curveq are BCg-hke

0921-4534/94/S07.00 © 1994 - ElsevmrScience B V All nght~ reserved SSDI 0921-4534(94)01506-6

1880

E.J G Boon et al /Physwa C 235-240 (1994) 1879-1880

5. D i s c u s s i o n •

v

•

/

250 200 <

150

,

.c,.,,0//

I00 L)

50 ':_~~h ar gi n_g-l_ikc_

0 -50 -50

~, 0

e/2C

,

50 100 Voltage (mV)

, 150

200

Figure 1. I-V curves on Bt2Sr2CaCu20, taken at x = 0, 10,20,40 ]~ showing the change from BCS-like (x = 0 ,&) with overlapping asymptotes to charging-like (x = 40 &, e / 2 C ~ 45 mV). Setpoint 200 mV, 240 pA

a)topography, setpoint 100 mV, 200 pA. ~ ~ . .% ~-^;¢= ~.~-_,-,-4~.:~,,,,,,,,,~.-~.~t~.~ ~ , .. ~.~-,~"~'~~',~,' ,~'~,~.,,,~ ,,~,, ~..~-~.~,,','¢5~'~,.~ "" ~~'\,~, ",~ "~ "&'~¢~'~ "~~,~, ,,",,2,/, b)current at 80 mV

Although charging is common m spectroscopy on high-T¢ superconductors [4], the origin is still not clear. For charging to occur in a S T M single junctmn, the junction should be connected to a current source wluch is not the case m STM. Any stray capacitance should be added to the junction capacitance, hereby effectively converting a current source into a voltage source and thereby destroying the Coulomb blockade. To have a Coulomb blockade in a S T M tunnel junction we therefore need a high resmtance near the actual tunnel junction to decouple any stray capacitance. We think that m a y b e a region in the superconductor with a reduced oxygen contents or some other Imperfection could act as such a resistance. In our experiment the oxygen contents is a good candidate because positions where charging occurs could not be correlated with surface topology. This work was supported by the Dutch National Research Program on High Tc Superconductivity I1 and the Dutch Foundation for Fundamental Research of Matter FOM, which in turn is supported by tile Dutch Organization for Scientific Research NWO. REFERENCES 1 2

3 c)current at 60 mV 4

Figure 2. ~:\,t)ography and spectroscopy over an a r e a o f 7 5 × 11 nm The hght part~ (lugh current) m b) and c) indicate places where the I-V curve is BCS-hke and d(, dark parts (Dower curlent) indicate places where (he I-V cur~e I, char~lng-hke

I Glaever, Phys Rev L e t t , 5 (1966)147 ti F lless, R B.Robmson, R C Dynes, J.M. Valles, J r , and J V Waszczak Phys Rev Lett, 62 (1989) 214 JWG Wild6er, A J A van Roy, H van Kempen and C . J . P M. Harmans, submitted to Rev. Scl. Instr P J M . van Bentum, H vail Kempen, L E C van de Leemput and P A A. Teumssen Phys Rev Lett 60 (1988) 369, and the references therem