- Email: [email protected]
Scanning tunneling spectroscopic studies of quench and melt growth (QMG) YBaCuO crystals at 4.2K
m
_
Physica C 185-189 (1991) 1909-1910 North-Holland
Scanning Tunneling Spectroscopic Studies of Quench and Melt Growth (QMG) YBaCuO crystals at 4.2K. Masamoto Tanaka, Seiki Takebayashi, Kiyoshi Sawano, Satoshi Kashiwaya*, Fuminori Hirayama* and Masao Koyanagi*.
R&D Labs-I, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki, 211 Japan. *Eleetrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki, 305 Japan. Microscopic properties of high temperature superconductors (HTS) were investigated by a low temperature scanning tunneling microscope at 4.2K. We have obtained a topographic image of YBa2Cu30 7 at 4.2K and were able to observe twin boundaries on the cleaved surface for a large area scan (153x153 nm2). Spectroscopic studies were carried out on the same cleaved surface. A superconducting energy-gap (2A) of 20-35 meV at 4.2K was measured in the present study.
1. Introduction Tunneling measurements are among important probes of
cleaved inside a glove box which was filled with nitrogen gas. The cleaved sample was set on an LT-STM probe in
high-temperature superconductors (HTS), providing a
the same glove box. Furthermore, to minimize the surface
direct measure of the superconducting energy-gap parameter, A. Despite their wide use, usual spectroscopic
contamination of the sample, the cryostat of the STM unit
methods such as point-contact or thin film junctions only
was evacuated to 5 x 10-5 Ton" prior to cooling. When the STM unit cooled to 4.2 K, a mechanically ground platinum
give us information about average superconductivity. The
tip was moved to the sample by a stepper motor. The demi~
scanning tunneling microscope (STM) has been a powerful tool for surface and low temperature physics, and its spectroscopic abilities are expected to provide much new
of LT-STM were described e l s e w h ~ 2.
Ln_formations cr superconductors. Namely, only STM enables us to observe the local electronic density of states (LDOS) with atomic scale resolution. Recently, excellent low temperature scanning tunneling microscope (LT-STM) studies were carried out on the traditional superconductors NbSe21 and NbN 2. On the other hand, there have been many contradicting results reported for HTS3"5. In this paper, to study the superconducting energy-gap, we have performed spectroscopic measurements on a bulk YBa2Cu307 surface, and compared the results with the BCS theory. 2. Experiment The bulk specimens were prepared by the Quench and Melt Growth (QMG) process6. It is known that the QMG process can produce a bulk YBa2Cu307 superconductor (Tc=92K) with high critical current density even at 77K and at high magnetic field. The grain size of the QMG processed material was enlarged to 50 mm in diameter and 20 mm in thickness by modifying the process7. The samples used in this study had a typical size of 5 x 5 x 10 mm 3. To avoid the surface contamination, the sample was
Figure 1. The topographic image of YBa2Cu307 at 4.2K. Vb=-I.5V and It=ll30pA. The scan area is about 150 nm square. The height of corrugation is about 3 nm from top to bottom. Figure 1 shows an STM topographic image of the cleaved YBa2Cu307 surface at 4.2K. The tip was bi~ed -1.5 V and tunneling current was 109 pA. The scan area was about 150 nm square. As shown in Fig. 1, many streaks were observed. The intervals of observed streaks were about 20 nm. The streaks are suspected to be twin boundaries. Such twin boundaries may be introduced at the tetragonal-orthorhombic phase transition. Atomic images
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.
M. Tanaka et aL / Scanning tunnelingspectroscopic studies of Quench and Melt Growth
1910
could not be obtained in the present samples. The reason
parameter F (meV) to obtain the best fit to the experimental
may be that an oxygen poor surface layer was formed during the cleavage and after evacuation in the glove box,
data. The experimental curve agreed reasonably with the theoretical curve of 2A=22.6 meV and F=l.4 meV, as it can
or that the surface was not sufficiently smooth for atomic
be seen in Fig.3.
observation. 3. Spectroscopy Results and Discussion
o_
"=
We carried out a spectroscopic study on the cleaved
|
2 YBa Cu'O 2
3
.
•
7
t'U
surface and obtained several conductance spectra at 4.2K as shown in Fig. 2.
i
i
8 I--
i
8 -£
0
0
-20 0 20 Bias Voltage (mV)
40
Figure 3. Conductance spectrum obtained between Pt tip and YBa2Cu30 7 at 4.2K. The calculated line represents a
t'-
I~
1
o
,
-I.511
I -
1011
-
I
I
I
I
.511
U
5U
1O0
BCS theoretical curve of 2A=22.6 meV and F=l.4 meV. 150
Bias Voltage (mV) Figure 2. Conductance spectra obtained between Pt tip and YBa2Cu30 7 at 4.2K. A BCS like spectrum (a) and a semieonducting spectrum (b) were obtained.
In this theoretical fitting, the experimental curve shows good agreement around the both peaks. Despite relatively good agreement, there is a small difference between the theory and the experiment near the zero voltage. This may
There were typically two kinds of conductance spectra. One was a semiconductor-like
spectrum which was
be due to the semiconducting nature of the surface layers.
observed over 80% of the scanned area and the other was a
This assumption is somewhat oversimplified, since we have no information about the surface nature of YBa2Cu30 7
superconducting conductance curve which was observed at
superconductors. More detailed studies of superconducting
the bright line of the central area (see Fig. 1). The
energy-gap for HTS by STM are being carried out.
semiconducting nature may be
due to the oxygen-poor
surface layer and/or contamination by absorbed gas that formed a barrier. The superconducting energy-gaps (2A) estimated from peak-to-peak separation were in the range of 20-35 meV at 4.2K. The representative BCS like conductance spectrum at 4.2K is shown in Fig. 3. The bias voltage of the tip was 100 mV and the tunneling resistance was maintained at 10g ~,~. This curve is fitted to the conductance curve of an N-I-S junction based on the smeared BCS DOS theory which was first proposed by Dynes et ai. 8 The electronic density of states of a superconductor is written in the following form; D(E)=Re {(E-iI")/t(E-iF)2-A2] 1/2 }. We adjusted the energy-gap A (meV) and smearing
Acknowledgments The authors would like to thank Messrs. M. Morita, K. Miyamoto, K. Kimura and Dr. L. Trouilleux for their support and worth-while discussion. References 1. H. F. Hess et al., Phys.Rev. Lett. 64 (1990) 2711-2714. 2. S. Kashiwaya, M. Koyanagi, A. Shoji and H. Shibata, Physica B 169 (1991) 465-466. 3. M. Lee et ai., Phys. Rev. B 39 (i989) 80i. 4. Q. Huanget al., Phys. Rev. B 40 (1989) 9366. 5. M. Gurvitch et al., Phys. Rev. Lett. 63 (1989) 1008. 6. M. Morita et al., l:'hysicaC 172 (1990) 383-387. 7. M. Morita et al., Advances in Superconductivity Ill (Springer Verlag. Tokyo, 1991) P733. 8. R. C. Dynes et al., Phys. Rev. Lett. 41 (1978) 1509.
_
Physica C 185-189 (1991) 1909-1910 North-Holland
Scanning Tunneling Spectroscopic Studies of Quench and Melt Growth (QMG) YBaCuO crystals at 4.2K. Masamoto Tanaka, Seiki Takebayashi, Kiyoshi Sawano, Satoshi Kashiwaya*, Fuminori Hirayama* and Masao Koyanagi*.
R&D Labs-I, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki, 211 Japan. *Eleetrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki, 305 Japan. Microscopic properties of high temperature superconductors (HTS) were investigated by a low temperature scanning tunneling microscope at 4.2K. We have obtained a topographic image of YBa2Cu30 7 at 4.2K and were able to observe twin boundaries on the cleaved surface for a large area scan (153x153 nm2). Spectroscopic studies were carried out on the same cleaved surface. A superconducting energy-gap (2A) of 20-35 meV at 4.2K was measured in the present study.
1. Introduction Tunneling measurements are among important probes of
cleaved inside a glove box which was filled with nitrogen gas. The cleaved sample was set on an LT-STM probe in
high-temperature superconductors (HTS), providing a
the same glove box. Furthermore, to minimize the surface
direct measure of the superconducting energy-gap parameter, A. Despite their wide use, usual spectroscopic
contamination of the sample, the cryostat of the STM unit
methods such as point-contact or thin film junctions only
was evacuated to 5 x 10-5 Ton" prior to cooling. When the STM unit cooled to 4.2 K, a mechanically ground platinum
give us information about average superconductivity. The
tip was moved to the sample by a stepper motor. The demi~
scanning tunneling microscope (STM) has been a powerful tool for surface and low temperature physics, and its spectroscopic abilities are expected to provide much new
of LT-STM were described e l s e w h ~ 2.
Ln_formations cr superconductors. Namely, only STM enables us to observe the local electronic density of states (LDOS) with atomic scale resolution. Recently, excellent low temperature scanning tunneling microscope (LT-STM) studies were carried out on the traditional superconductors NbSe21 and NbN 2. On the other hand, there have been many contradicting results reported for HTS3"5. In this paper, to study the superconducting energy-gap, we have performed spectroscopic measurements on a bulk YBa2Cu307 surface, and compared the results with the BCS theory. 2. Experiment The bulk specimens were prepared by the Quench and Melt Growth (QMG) process6. It is known that the QMG process can produce a bulk YBa2Cu307 superconductor (Tc=92K) with high critical current density even at 77K and at high magnetic field. The grain size of the QMG processed material was enlarged to 50 mm in diameter and 20 mm in thickness by modifying the process7. The samples used in this study had a typical size of 5 x 5 x 10 mm 3. To avoid the surface contamination, the sample was
Figure 1. The topographic image of YBa2Cu307 at 4.2K. Vb=-I.5V and It=ll30pA. The scan area is about 150 nm square. The height of corrugation is about 3 nm from top to bottom. Figure 1 shows an STM topographic image of the cleaved YBa2Cu307 surface at 4.2K. The tip was bi~ed -1.5 V and tunneling current was 109 pA. The scan area was about 150 nm square. As shown in Fig. 1, many streaks were observed. The intervals of observed streaks were about 20 nm. The streaks are suspected to be twin boundaries. Such twin boundaries may be introduced at the tetragonal-orthorhombic phase transition. Atomic images
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.
M. Tanaka et aL / Scanning tunnelingspectroscopic studies of Quench and Melt Growth
1910
could not be obtained in the present samples. The reason
parameter F (meV) to obtain the best fit to the experimental
may be that an oxygen poor surface layer was formed during the cleavage and after evacuation in the glove box,
data. The experimental curve agreed reasonably with the theoretical curve of 2A=22.6 meV and F=l.4 meV, as it can
or that the surface was not sufficiently smooth for atomic
be seen in Fig.3.
observation. 3. Spectroscopy Results and Discussion
o_
"=
We carried out a spectroscopic study on the cleaved
|
2 YBa Cu'O 2
3
.
•
7
t'U
surface and obtained several conductance spectra at 4.2K as shown in Fig. 2.
i
i
8 I--
i
8 -£
0
0
-20 0 20 Bias Voltage (mV)
40
Figure 3. Conductance spectrum obtained between Pt tip and YBa2Cu30 7 at 4.2K. The calculated line represents a
t'-
I~
1
o
,
-I.511
I -
1011
-
I
I
I
I
.511
U
5U
1O0
BCS theoretical curve of 2A=22.6 meV and F=l.4 meV. 150
Bias Voltage (mV) Figure 2. Conductance spectra obtained between Pt tip and YBa2Cu30 7 at 4.2K. A BCS like spectrum (a) and a semieonducting spectrum (b) were obtained.
In this theoretical fitting, the experimental curve shows good agreement around the both peaks. Despite relatively good agreement, there is a small difference between the theory and the experiment near the zero voltage. This may
There were typically two kinds of conductance spectra. One was a semiconductor-like
spectrum which was
be due to the semiconducting nature of the surface layers.
observed over 80% of the scanned area and the other was a
This assumption is somewhat oversimplified, since we have no information about the surface nature of YBa2Cu30 7
superconducting conductance curve which was observed at
superconductors. More detailed studies of superconducting
the bright line of the central area (see Fig. 1). The
energy-gap for HTS by STM are being carried out.
semiconducting nature may be
due to the oxygen-poor
surface layer and/or contamination by absorbed gas that formed a barrier. The superconducting energy-gaps (2A) estimated from peak-to-peak separation were in the range of 20-35 meV at 4.2K. The representative BCS like conductance spectrum at 4.2K is shown in Fig. 3. The bias voltage of the tip was 100 mV and the tunneling resistance was maintained at 10g ~,~. This curve is fitted to the conductance curve of an N-I-S junction based on the smeared BCS DOS theory which was first proposed by Dynes et ai. 8 The electronic density of states of a superconductor is written in the following form; D(E)=Re {(E-iI")/t(E-iF)2-A2] 1/2 }. We adjusted the energy-gap A (meV) and smearing
Acknowledgments The authors would like to thank Messrs. M. Morita, K. Miyamoto, K. Kimura and Dr. L. Trouilleux for their support and worth-while discussion. References 1. H. F. Hess et al., Phys.Rev. Lett. 64 (1990) 2711-2714. 2. S. Kashiwaya, M. Koyanagi, A. Shoji and H. Shibata, Physica B 169 (1991) 465-466. 3. M. Lee et ai., Phys. Rev. B 39 (i989) 80i. 4. Q. Huanget al., Phys. Rev. B 40 (1989) 9366. 5. M. Gurvitch et al., Phys. Rev. Lett. 63 (1989) 1008. 6. M. Morita et al., l:'hysicaC 172 (1990) 383-387. 7. M. Morita et al., Advances in Superconductivity Ill (Springer Verlag. Tokyo, 1991) P733. 8. R. C. Dynes et al., Phys. Rev. Lett. 41 (1978) 1509.