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Determination of transition curves of high-Tc superconductors by phase contrast microscopy.

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Ann. Physik 4 (1995) 136- 143
Annalen
der Physik
@ Johann Ambrosius Barth 1995
Determination of transition curves of high-T, superconductors
by phase contrast microscopy
Cl. Kriebel, 0. Hoffels, R. Borowski, H. Gottschalk, H. Alexander,
and D. Wohlleben'
11. Phys. Institut, Universiat zu Koln, Ziilpicher Str. 77. D-50937 K6ln. Germany
Received 7 June 1994, 1st revision 22 July, 2nd 31 August, 3rd 13 October, accepted 13 October
1994
Abstract. A new method which allows the detection of the superconducting phase transition of highT, superconductors (HTSC) on a microscopic scale is reported. Micro-size holes in thin foils of superconducting material are examined in a transmission electron microscope at varying temperatures. The
superconducting transition induces small changes in the image intensity within the holes, which can
be detected by using electronic image analysis. Superconducting transition curves are then obtained for
various types of high-T, superconductors and for given values of the applied magnetic field.
Keywords: HTSC; Local detection of superconductivity; Lorentz-microscopy; Phase contrast
microscopy; Meissner-effect; Charge effects; Electron microscopy.
1 Introduction
A technique, which allows for the study of the structure, the stoichiometry, and of the
superconducting properties of a superconductor on a microscopic scale would certainly
be of much value, eg. for the characterization and improvement of materials and to investigate the impact of local structural properties for T,. This is in particular true for
short coherence length superconductors like the high-T, cuprates, where superconductivity is intrinsically sensitive to the local structure and stoichiometry [l -31.
Structure and stoichiometry on a local scale can of course be studied in an electron
microscope. On the other hand, it is also possible to gain information on the superconducting properties by the detection and observation of the magnetic properties in the
superconducting state. The change of the magnetization below T, caused by the
Meissner-Ochsenfeld-effect (Meissner-effect) or by the presence of vortices leads to a
phase shift of the electron wave, which one may hope to detect in the electron
microscope by various techniques. In the past, various electron-optical methods were
used to study the flux expulsion of superconductors such as electron shadow
microscopy, out of focus electron microscopy [4, 51, electron wave interferometry vortex microscopy - [6],and electron holography [7]. The goal was to visualize single
flux quanta and to study their correlation with the material structure. G. Pozzi and U.
Valdrt investigated the electron shadow patterns at the edge of superconducting lead
' Deceased.
Cl. Kriebel, Determination of transition curves of high-T, superconductors
137
in the intermediate state and showed that in the defocused image distorted cycloid patterns appeared on a scale of a few 10 pm as a consequence of the Meissner-effect [4].
With the same resolution Y. Cui-Ying and J.W. Steeds observed at the edge of
YBa2Cu307-6 such caustic shapes as well [8]. Recently K. Harada et al. observed
vortex lattices in Nb-superconductors in real-time with TEM [9] I .
In this paper we report on a particular effect we have found when studying high-T,
materials in an electron microscope: Microsize holes in thin foils of superconducting
material were examined as a function of temperature in magnetic fields H < H c l . We
identify a small change of intensity at T, by comparing images recorded at different
temperatures with a fiber-optically coupled TV and image processing system.
2 Experimental
2. I Sample preparation
Polycrystalline samples of the HTSC YBa2C~307-6
(YBCO) ( T f =' = T,, = 90K)
and Bi,,Pbo~4Sr2Ca2Cu3010-6(Bi-2223) ( Tc0= 110 K) were prepared by a standard
solid state reaction technique using analytically pure powders of carbonates and oxides
1111. All pellets were single phase according to powder X-ray diffractometry. T, was
determined by measurements of the electric resistivity.
Disks with a diameter of 3 mm were cut out of the pellets by sandblasting and then
ground to a thickness of about 30 pm. For further thinning we used the technique of
ion-milling because a chemical method, for example with 1% bromine in methanol
solution [12], is selective; in addition it does not dissolve all the components in the case
of a multiphase preparation. For YBCO and the Bi-system it is known that ion-thinning may cause damage [13, 141. However, severe damage can be avoided by cooling
the HTSC-samples with liquid-nitrogen during the bombardment with 5 kV argon ions.
Holes with a diameter of a few pm required for the observation appear during the thinning process as a result of the falling out of single crystallites. Most of the area of the
specimen, even the edges of the holes, have a thickness of about 1 to 5 pm and are not
electron transparent.
2.2 Experimental setup
The electron microscope used in our study is a Philips EM 400T. The thin foils were
mounted into one of our double-tilting cooling specimen holders (Tmin= 16 K and
Td,, = 89 K, respectively) between two copper grids to improve thermal contact to the
holder. The sample holders allow for rotations around two axes by about 50".
In our experiment we aimed at a detection of the change of the magnetization when
the sample becomes superconducting. In the HTSC the lower critical field H,,is of
order lOmT, whereas the upper critical field HC2is of order 100-200T. The
magnetic field at the position of the sample is determined by the field produced by the
objective lens (including its remanence when working without excitation of the objective), and the fields of the second condensor lens and the diffraction lens. Using a hall
' While writing this paper K. Harada et al. were successful in observing the vortices in a special
prepared Bi-High-T, superconductor [lo].
138
Ann. Physik 4 (1995)
probe we found that a voltage of 120 kV and a magnification factor of 2000 the objective lens of the Philips EM400T electron microscope produces a magnetic field of
about 0.1 Tesla at the position of the specimen. Because of the low values of H,, of
the HTSC and the broadening of the magnetization-curve in an external field H
(>H,,), we used in general fields as small as possible by reducing the excitation of the
objective lens (for H = 10mT the transition width becomes nearly 10K [la]). In this
case the image is formed by the subsequent lenses of the system with a resolution of
about 100 nm. The magnetic field at the sample position with Iobj = OA was measured
to be at least 2 mT parallel and 0.2 mT perpendicular to the electron beam.
After warming up the LaB6-CathOde over a period of at least 2 h the sample is cooled to about 20K in the magnetic field of the lenses. Then constant illumination is
verified over a period of at least 1 h. Images of the hole arc then taken at various fixed
temperatures. Thermal equilibrium at any given temperature turned out to be established within 5 min. With a fiber-optically coupled T V and an image processing system
we can store at each temperature a 512 x 512 pixel, 256 grey level image of the region
of the hole in the computer, whereby all images stored are obtained by an averaging
process over up to 256 images taken in real time.
We note that prolonged irradiation of the specimen by an intense electron beam
tends to induce a loss of oxygen and to create clusters of point defects and should
therefore be avoided [151.
2.3 Data analysis
In all our experiments the change of intensity at T, turned out to be extremely small
and it could not be detected by eye. However, by comparison of the images recorded
at different temperatures, we were able to establish a small change of the intensity
(= 1 - 2% or 3 - 4 of the 256 grey levels) at T,.
Due to the noise of the LaB6-CathOde images always show a scatter in the intensity
value for each pixel (= 1 of the 256 grey levels). Therefore, for image comparison we
introduce a threshold value t, so that a pixel is counted as changed only, if the detected
intensity change is above the given value for t. This threshold value is choosen adequately from the scatter of the intensity.
In order to avoid complications of the data analysis caused by thermal expansion of
the sample holder as well as of the sample, it turned out to be best to compare only
parts of an image (hereinafter referred as windows) and thus parts of the hole. On the
other hand, these windows count less pixels than the whole image, but to guarantee
(see
satisfactory statistics they should contain at least a few thousand pixels (Nror)
Fig. 1). Practically, the spatial resolution, with which changes of the image intensity
due to the superconducting phase transition can be detected, is about (1 pm)2. All
given values for intensity change and scatter are average values in such a window.
Since the contrast is very small, the data analysis is done in the following way:
As a reference image we take the image measured at the lowest temperature, which
is usually 20 K. Each image measured at higher temperatures is then compared with this
reference image by counting the number N of pixels in a window, which have changed
their intensity. We call this method the reference analysis (e.g. Fig. 2).
A different way to analyse the data is to compare in the same way images at two
neighboring temperatures T, and T2 and to plot the resulting N as a function of
T*= (T,
+ T2)/2.We call this a differential analysk
CI. Kriebel, Decermization of transition curves of high-T, supercondxctors
Fig. 1 Elec:ionic k a g e of
Bi, ,,PS3,cS:2Ca2Cu;0,0 - 6.
2
139
:?ole in
I
I
,
I
50 -1
I
i
A
!
-
I
20
I
i
~
04
,=---A-A
-.
c& =-zc&
t
A?=
*
I
I
0
40
:2s
80
:60
I
220
T :!q
snpe:conducto:s
Fig. 2 Sunbe: of c5angel pixek of two YBa,Cu,O-i. T,, = 54 K (threshoid t = 7); 2. T,, = 91 K (threshold t = 4).
(referexe anziysis):
Note that this analysis does not compare the real intensities per pixel, but compares
only whether the intensity at a given pixel has changed (by more than the threshold)
or not. In particu!ar, this znalysis is highly non-linear in the sense that a drastic change
of a pixel-intensity is counted with the s a n e weight as a subtle change only slightly
above the thresnoid.
We p o h t out thar by chis method one obviously does no: get an image of the changed
magnetic structure below Tc or whatever causes the change of intensity. Howeve;, ir
turned out that the superconducti3g traiisition mrve ccm be detected.
140
Ann. Physik 4 (1995)
3 Results
In Fig. 1we show a micrograph of a hole in a thinned Bi-2223 sample with an electronic
window for the data analysis. The edges were not electron transparent. An analysis by
EDX did not reveal any detectable stoichiometrk variations.
Fig. 2 shows the number N of changed pixels versus temperature for two YBCO
samples, extracted with the reference analysis from images recorded at various temperatures. The samples have zero resistance at Tc0= 84 K and Tc0= 91 K, respectively. In
Fig. 3 we show similar data for a Bi-2223 sample with T,o = 110 K. The solid line corresponds to the resistance measured on the same sample. It is apparent that around T,
a change of intensity occurs and the agreement between R(T) and N is obvious. We
note that the curves are reversible, i.e. the same curve is obtained on cooling again
below T, There is only a small influence of the threshold values on the results. The
values used for these analyses are given in the figure captions.
I
1
I
*'O
14
10
ac
-c
2
i
6
2
I
90
I
100
110
120
T M
Fig.3 Number of changed pixels of a Bi-2223 sample with appertaining resistance curve
Cr,, = 110 K, reference analysis, threshold t = 4).
The scatter of the data for T < T, and for T> T, visible in Figs. 2 and 3 has to be
attributed to the noise of the LaB6-cathode. It is much smaller than the change of intensity at T,. We note that N is nearly constant below T,, although these data have
been recorded over a period of more than 1 h. We have in addition stored images every
5 min at a fmed temperature of T = 122.5 K. The scatter in the number of changed pixels gives the error limit of our method.
A differential analysis on a Bi-2223 sample is shown in Fig. 4,where we started with
cooling down the sample (A). In this analysis the change of intensity at T, leads to a
peak at T,, i.e. one measures the first derivative of the curves obtained with the reference analysis.
The dependence of the transition curves on the magnetic field can be studied by varying the excitation of the objective lens. We show data obtained for various magnetic
C1. Kriebel. Determination of transition curves of highT, superconductors
80
-
I
141
I
I
I
A A A ~
A
60-
A
.. o...2 mT
mT
-0-10
A
AA
A
ae
-c 40
z
20
-
O w * ' 90
.
I
I
I
100
110
120
T Il<l
Fig. 4 Number of changed pixels of a Bi-2223 sample-temperature cycle (T, = 110 K, differential
analysis, threshold t = 4).
Fig. 5 Field dependence of the transition curve of a Bi-2223 sample (Tco= 110 K, reference analysis, threshold t = 4).
fields in Fig. 5 . One notes that the transition curves become broader with increasing
magnetic field in agreement with what is expected for the HTSC [16]. The linear increase of the number of changed pixels with increasing magnetic field indicated by the
data in Fig. 5 should not be taken seriously, since our data analysing procedure is
highly non-linear. In particular, the absolute values when comparing different measurements are arbitrary.
We finally point out two further results of experiments: (1) The image is generally
darker for T< T,.(2) The results do not depend on the setting of the defocus. When
142
Ann. Physik 4 (1995)
working without the objective lens it is impossible to gain control over the actual defocus. However, the intensity change at T,occurs when focussing and defocussing.
4 Discussion
From the data presented above one concludes that the change of intensity, which we
observe, is directly correlated with the superconducting transition. We mention that extrinsic effects like the condensation of gases [17] etc. are unlikely to produce the ‘contrast’, since the change of intensity occurs always at Tc. Taken the observed effects as
related to superconductivity one has to explain the origin of the ‘contrast’.
Our aim was of course to study the phase shift caused by the change of magnetization due to the Meissner-effect. However, we have difficulties to explain a magnetic
origin of the intensity change at T, First of all, from the theory of the magnetic phase
contrast one expects a dependence of the contrast on the defocusing distance [18].
Moreover, when assuming a magnetic origin of the contrast it is difficult to explain the
magnitude of the effect because the magnetic field in the hole and the changes due to
the Meissner-effect in our experiment are predominantly parallel to the electron trajectories. In this case a calculation of the contrast has to include several corrections terms
~91.
In order to estimate the reduction of the intensity of the image when entering the
superconducting state we have accomplished several calculations based on different
assumptions. We did this, for instance, for magnetic dipole fields with various tilting
angles and different amounts of appertaining dipole moment near the edge of the hole,
but the contrast did not come out larger than lod4 [20]. For large dipoles (with the
same size as the TEM-disks, where the arising‘effect becomes integral with no local information about superconductivity) the induced phase shift of the electrons wave functions may be large enough, but an exact calculation is not possible because the field
distribution is complicated due to the polycrystalline material.
An electrostatic lens caused by charging effects of the regions around the hole is a
second possibility to explain the origin of the ‘contrast’. Above T, the electron beam
leads to an electric charge in the non-electron transparent material around the hole and
may induce a slight focussing effect. Below T, the voltage and thus this focussing effect should be absent since the resistivity drops to zero. Therefore a slight decrease of
the intensity is expected. In this picture the broadening of the transition curves in a
magnetic field as shown in Fig. 5 may be attributed to the broadening of the resistive
transition [21]. However, one has to notice that the beam current is very low to avoid
sample damaging.
5 Summary
An indirect observation of the superconducting phase transition on a microscopic scale
in an electron microscope has been reported. We have shown that the superconducting
phase transition induces very small changes in the intensity of an image of a hole in
superconducting material. Significant intensity changes were found at the different
critical temperatures of various high-T, superconductors.
An explanation of the ‘contrast’ turned out to be difficult and has not yet been successful, so that several further experiments are necessary to understand the relationship
CI. Kriebel, Determination of transition curves of high-T, superconductors
143
between superconductivity and the ‘contrast’ obtained. However, we believe that the
described technique can help to obtain useful informations about local superconducting properties.
We would like to thank A. Freimuth. B. Roden and W. Schnelle for discussions and acknowledge the
assistance of J. Harnischmacher. This work is supported by the Deutsche Forschungsgemeinschaft
through SFB 341 C 1 and C2.
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