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Electromagnetic and microstructural properties of bulk bicrystal grain boundaries in Bi2Sr2CaCu2O8+δ superconductors
PHYSlCA@ ELSEVIER
Physica C 341 348 (2000) 1407-1410 www.elsevier.nl/locate/physc
Electromagnetic and microstructural properties o f bulk bicrystal grain boundaries in BizSrzCaCu2Os+8 superconductors Qiang Lia, Y. N. Tsaya, Y. Zhua, M. Suenagaa, G. D. Gub, and N. Koshizukb aDepartment of Applied Sciences, Brookhaven National Laboratory, Upton, New York 11973, USA bsuperconductivity Research Laboratory, ISTEC, Tokyo 135, Japan We present systematic studies of electromagnetic and microstructurai properties of bulk Bi2Sr2CaCu2Os+8 (Bi2212) bicrystals having synthetic [001] twist or naturally-grown [100] tilt boundaries. We find that [001] twist boundaries in as-prepared bicrystals, regardless of twist angles, conduct virtually the same critical current L as the constituent single crystals. We also irradiated some of the [001] twist bicrystals with 2.2 GeV Auions, and found that the [001] twist grain boundaries carry significantly less critical current than the irradiated single crystals in magnetic field. For naturally grown [100] tilt boundaries, we fred a general T¢depression by several degrees at the grain boundary. Strong coupling behavior was observed in several high angle tilt grain boundaries below the grain boundary Tc°b.
1. INTRODUCTION Bi-based high-T¢ superconducting tapes are widely regarded as potential candidates for largescale applications of high-T¢ superconductors, primarily due to the fact that the individual Bi2212 or Bi2223 grains in a silver-sheathed tape can be highly textured through thermo-mechartical processing. Due to their extremely anisotropic nature, the textured Bi-tapes contain about 80% of [001] or c-axis twist grain boundaries, forming socalled colonies [1]. The colonies are connected through tilt grain boundaries. Depending on the rotation axis, these tilt grain boundaries can be grouped into two types: in-plane tilt and off-plane tilt. The in-plane tilt has the c-axis as rotation axis; hence the basal ab-planes of both grains are parallel. At the off-plane tilt grain boundaries the basal planes are not parallel. In textured tapes, the colonies are usually connected through these offplane tilt grain boundaries, often with a certain degree of twist component. Despite much success in achieving a high degree of grain alignment, the overall value of J¢ is still far from that needed for practical applications at high field and elevated temperatures. The two possible causes for limiting J¢ in these Bi-based superconducting tapes are low magnetic flux pinning inherent within superconductor grains and additional weak-link type grain boundaries in the textured tapes. In grain boundary
studies, the focus is on identifying the type of strongcoupled grain-boundaries for carrying high critical current density J¢ in a textured tape. We performed electromagnetic and structural characterization of bulk Bi2212-bicrystais having synthetic [001] twist boundaries or naturally grown off-plane [100] tilt boundaries. This is followed by extensive studies on the correlation between the transport properties and microstructures of the grain boundaries. In addition, we explore the relative strength of the grain boundary coupling and the flux pinning in some of the twist bicrystals by changing the intrinsic pinning strength of the bicrystals through heavy iron irradiation. 2. E ~ E R I M E ~ S
The bicrystal grain boundaries used in this work were prepared from high quality Bi-2212 single crystals grown by the traveling solvent floating zone technique. The synthetic [001] twist bierystals were made by first cleaving a single crystal in the ab plane. One cleave was rotated a desired angle about the c-axis with respect to the other, and placed atop it. The twist grain boundary was formed in a controlled sintering process for 30 hours in air or controlled 02 pressure, just below the melting point
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PI1 S0921-4534(00)00959-X
(~ Li et aL/Physica C 341-348 (2000) 1407-1410
1408
of Bi2212 [2]. Tilt grain boundaries are naturallygrown and mechanically isolated from original crystal bar. For a direct comparison, six-probe electrical configurations were used to measure the resistance and the voltage-current (V-/) characteristics across the grain boundary and within their constituent single crystals simultaneously. The transport measurements were performed in an exchange-gas cryostat equipped with a 9 T superconducting magnet, where temperature can be controlled within an accuracy of 0.01 K between 2 and 350 K. Magnetic fields were applied along the chosen direction by appropriate alignment of the bicrystal with respect to the magnetic field, such as along the c-axis, parallel to the ab plane of one of the constituent single crystals, and parallel or perpendicular to the grain boundary plane.
transmission electron microscopy (TEM), including 0.17 nm high-resolution imaging (HRTEM), electron energy-loss spectroscopy (EELS), and energy dispersive x-ray spectroscopy (EDS) with a 2 nm field-emission probe. Fig. la shows a HRTEM cross-section view of a 37.45 ° [001] twist grain boundary. Details on the structural characterization of the twist boundaries can be found elsewhere [2]. Briefly, we observed that the boundary interface plane is at the double Bi-O layer, and this double Bi-O layer at the boundary appears to be the same as that in the constituent single crystals. No significant strucRkral disorder along the basal ab-planes was observed across the interface. Fig. lb shows a HRTEM cross-section image taken on a 30° [100] non-basal-plane-faced tilt grain boundary. The boundaries are all microscopically flat and clean without any impurities. The boundary planes appeared straight on a scale of 100 A. However, at atomic scale, facets at the grain boundary planes are always observed.
4. TRANSPORT MEASUREMENTS
(a)
)?:?!
Figure 1. Cross-section HRTEM micrographs of (a) a 37.45 ° [001] twist grain boundary, The top crystal is viewed along the a-axis, and the common c-axis is perpendicular to the grain boundary. (b) A 30° [100] non-basai-plane-faced tilt grain-boundary with facets along the boundary plane. The saw tooth line is a guide to indicate the facets. The double Bi-O layer spacing is 15.3 A. 3. GRAIN-BOUNDARY MCROSTRUCTURES The microstructure of these grain-boundaries was extensively characterized using advanced
4.1 [001] twist grain boundaries characterized in brick-wall configuration
In order to provide a close comparison to the grain boundaries in the textured Bi-tapes, measurements were performed on the bicrystals having approximately the same aspect ratio as the platelet-like grains in the tapes. Furthermore, we used a six-probe electric configuration, shown in Fig. 2a, to mimic the current transport pattern (brick-wall configuration) comparable to that in a practical textured Bi-tape. We measured the resistance and the V-I characteristics across the grain-boundary and within their constituent single crystals simultaneously at various magnetic fields and temperatures. The magnetic irreversibility line Hire(T) was determined by the onset of the resistive transition. We observed a similar dissipation behavior in R(T, H) and Hirr(T) across the [001] twist boundaries and within the single crystals in as-prepared bicrystals, regardless of twist angles. Furthermore, we found that the [001] twist boundaries conduct virtually the same Ic as constitute single crystals in a broad H-T plane. In these twist bicrystals, the dissipation at grain boundary seems to be dominated
Q Li et al./Pl~vsica C 341 348 (2000) 1407-1410
1409
critical currents, as well as the Josephson critical currents of [001] twist grain boundary when the current is driven across the layers.
Figure 2. Crystallographic orientation of a c-axis twist bicrystal and the six-probe brick-wall configurations for transport measurements.
by the extremely weak pinning of the single crystal grains. To increase the pinning of the bicrystals, we irradiated some of the [001] twist bicrystals with 2.2 GeV Au-ions [3]. At temperatures far below the irreversibility line, a large increase of 1¢ was observed only in the single crystals when current flows in the same brick-wall configuration (Fig. 2) as that does between the grains in a practical textured Bi-tape. This is due to enhanced vortex pinning in single crystal, at the same time no change is expected in the Josephsoncoupling strength along the c-axis. It is clear that in this low temperature region, Ic of the irradiated bicrystals is limited by the grain boundary, in contrast to the case before irradiation, where the critical current is limited by the lack of strong pinning centers in unirradiated single crystals.
4.2 [001] twist grain boundaries characterized in Josephson junction configuration In Bi2212, the superconducting coherence length along the c-axis is of the order of 0.1 nm. This is much less than the interlayer spacing between the CuO double layers (1.5 nm). Therefore, Bi2212 can be described as a stack of discrete superconducting layers whose order parameters are coupled by Josephson interaction. This long series of Josephson junctions consists of superconducting CuO double layers separated by the BiO and SrO layers acting as weak links. This layered structure provides great opportunities to make direct measurements of intrinsic single crystal Josephson
Figure 3. (a) Top view of a 45 ° c-axis twist bicrystal junction with six attached electrical leads. (b) Sketch of a side view of the bicrystal, with six attached leads in Josephson junction configuration.
A dozen [001] twist Bi2212 bicrystal grain boundaries were prepared using identical single crystal cleaves stacked and twisted an arbitrary angle 4o about the c-axis. An optical micrograph of a 45° caxis twist bicrystal characterized in Josephson junction configuration is pictured in Fig. 3a, while a schematic side view of the bicrystal is sketched in Fig. 3b. Note that the overlap area of the twist junction is about half the cross-sectional area of the identical single crystal cleaved pieces. The current I was fed along the common c-axis, and the voltages Vsl and Vs2 were measured across each single crystal cleave. The voltage Vj across the twist junction was measured from the voltage lead on the bottom surface of the top crystal to the top lead on the bottom crystal. In this configuration, it is possible to measure simultaneously the c-axis V-I characteristics across either of the single crystal parts of the bicrystal and across the twist junction. We found that for each bicrystal, regardless
Q. Li et al./Physica C 341-348 (2000) 1407-1410
1410
of twist angle, the ratio of the measured c-axis Josephson critical current density across the twist junction to that across either single crystal part is close to unity for T < T¢ [4]. Hence, the Josephsoncoupling strength across the twist boundary is almost identical to the intrinsic coupling strength within the single crystals along the c-axis.
by the change of the local oxygen content and strain field [5]. At T < T0°b, we observed a similar superconducting and dissipation behavior for both the grain boundaries and the single crystals in a broad range of magnetic field. Surprisingly, some high angle grain boundaries carry lc as high as their constituent single crystals. Most importantly, these features seem to be independent of the misorientation angle, based on the bicrystals we have studied so far. The orientation of the grain boundary planes and the facets at the grain boundary appear to play an important role in the coupling strength of the tilt grain boundaries, as well as in pinning of the vortices.
5. SUMMARY
C I~ . . . . V
/
C2
/
/- vs(rvo;Vvs -\ Figure 4 (a) An optical micrograph of a 20° [100] non-basal-planed-faced tilt grain boundary and (b) a sketch of crystal orientation with a six-probe in-line configuration for transport measurement.
In summary, we prepared Bi2212 bicrystal c-axis twist grain boundaries of extremely high quality, and found that Josephson-coupling strength across the twist boundary is identical to the intrinsic coupling strength within the single crystals along the c-axis, independent of the twist angle. For naturally grown [100] tilt bicrystal grain boundaries, our studies indicate that the misorientation angle in this type of Bi2212 tilt grain boundary is less likely to be a dominant factor in determining the grain boundary critical current, as compared to the case of YBCO. This work was supported by the U. S. DOE, Office of BES, under contracts No. DE-AC02-98CH10886.
4.3 1100] twist grain boundaries REFERENCES
Bicrystals with a single naturally-grown [100] tilt grain boundaries were isolated from crystal bar with further cleavage along ab-plane and cutting by a precision low-speed wire-saw to reduce the cross section of the bicrystals. Smaller cross sections of the bicrystals require less current, allowing an extended V-I measurement without over-heating the specimen. Fig. 4a shows an optical micrograph of a bicrystal containing a20 ° [100] non-basal-planefaced tilt grain-boundary. Fig. 4b shows a sketch of crystal orientation with a six-probe in-line configuration for transport characterization of the grain boundary and its constituent single crystals simultaneously. For naturally-grown [100] tilt boundaries, we found a general T¢ depression of 610 K at the grain boundary, which is likely induced
1. Y. Zhu, M. Suenaga, and R. L. Sabatini, Appl. Phys. Lett. 65, (1994) 1832. 2. Qiang Li, Y. N. Tsay, Y. Zhu, M. Suenaga, G. D. Gu, and N. Koshizuka, IEEE transaction on Appl. Supercond. 7, (1997) 1584; Y. Zhu, Qiang Li, Y. N. Tsay, M. Suenaga, G. D. Gu, and N. Koshizuka, Phys. Rev. B 57, (1998) 8601. 3. Qiang Y. N. Tsay, G. Wirth, M. Suenaga, G. D. Gu, and N. Koshizuka, Appl. Phys. Lett. 74, (1999) 1323. 4. Qiang Li, Y. N. Tsay, M. Suenaga, R. A. Klemm, G. D. Gu, and N. Koshizuka, Phys. Rev. Lett. 83, (1999) 4160. 5. Qiang Li, Y. N. Tsay, Y. Zhu, M. Suenaga, G. D. Gu, and N. Koshizuka, Appl. Phys. Lett. 70, (1997) 1164.
Physica C 341 348 (2000) 1407-1410 www.elsevier.nl/locate/physc
Electromagnetic and microstructural properties o f bulk bicrystal grain boundaries in BizSrzCaCu2Os+8 superconductors Qiang Lia, Y. N. Tsaya, Y. Zhua, M. Suenagaa, G. D. Gub, and N. Koshizukb aDepartment of Applied Sciences, Brookhaven National Laboratory, Upton, New York 11973, USA bsuperconductivity Research Laboratory, ISTEC, Tokyo 135, Japan We present systematic studies of electromagnetic and microstructurai properties of bulk Bi2Sr2CaCu2Os+8 (Bi2212) bicrystals having synthetic [001] twist or naturally-grown [100] tilt boundaries. We find that [001] twist boundaries in as-prepared bicrystals, regardless of twist angles, conduct virtually the same critical current L as the constituent single crystals. We also irradiated some of the [001] twist bicrystals with 2.2 GeV Auions, and found that the [001] twist grain boundaries carry significantly less critical current than the irradiated single crystals in magnetic field. For naturally grown [100] tilt boundaries, we fred a general T¢depression by several degrees at the grain boundary. Strong coupling behavior was observed in several high angle tilt grain boundaries below the grain boundary Tc°b.
1. INTRODUCTION Bi-based high-T¢ superconducting tapes are widely regarded as potential candidates for largescale applications of high-T¢ superconductors, primarily due to the fact that the individual Bi2212 or Bi2223 grains in a silver-sheathed tape can be highly textured through thermo-mechartical processing. Due to their extremely anisotropic nature, the textured Bi-tapes contain about 80% of [001] or c-axis twist grain boundaries, forming socalled colonies [1]. The colonies are connected through tilt grain boundaries. Depending on the rotation axis, these tilt grain boundaries can be grouped into two types: in-plane tilt and off-plane tilt. The in-plane tilt has the c-axis as rotation axis; hence the basal ab-planes of both grains are parallel. At the off-plane tilt grain boundaries the basal planes are not parallel. In textured tapes, the colonies are usually connected through these offplane tilt grain boundaries, often with a certain degree of twist component. Despite much success in achieving a high degree of grain alignment, the overall value of J¢ is still far from that needed for practical applications at high field and elevated temperatures. The two possible causes for limiting J¢ in these Bi-based superconducting tapes are low magnetic flux pinning inherent within superconductor grains and additional weak-link type grain boundaries in the textured tapes. In grain boundary
studies, the focus is on identifying the type of strongcoupled grain-boundaries for carrying high critical current density J¢ in a textured tape. We performed electromagnetic and structural characterization of bulk Bi2212-bicrystais having synthetic [001] twist boundaries or naturally grown off-plane [100] tilt boundaries. This is followed by extensive studies on the correlation between the transport properties and microstructures of the grain boundaries. In addition, we explore the relative strength of the grain boundary coupling and the flux pinning in some of the twist bicrystals by changing the intrinsic pinning strength of the bicrystals through heavy iron irradiation. 2. E ~ E R I M E ~ S
The bicrystal grain boundaries used in this work were prepared from high quality Bi-2212 single crystals grown by the traveling solvent floating zone technique. The synthetic [001] twist bierystals were made by first cleaving a single crystal in the ab plane. One cleave was rotated a desired angle about the c-axis with respect to the other, and placed atop it. The twist grain boundary was formed in a controlled sintering process for 30 hours in air or controlled 02 pressure, just below the melting point
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PI1 S0921-4534(00)00959-X
(~ Li et aL/Physica C 341-348 (2000) 1407-1410
1408
of Bi2212 [2]. Tilt grain boundaries are naturallygrown and mechanically isolated from original crystal bar. For a direct comparison, six-probe electrical configurations were used to measure the resistance and the voltage-current (V-/) characteristics across the grain boundary and within their constituent single crystals simultaneously. The transport measurements were performed in an exchange-gas cryostat equipped with a 9 T superconducting magnet, where temperature can be controlled within an accuracy of 0.01 K between 2 and 350 K. Magnetic fields were applied along the chosen direction by appropriate alignment of the bicrystal with respect to the magnetic field, such as along the c-axis, parallel to the ab plane of one of the constituent single crystals, and parallel or perpendicular to the grain boundary plane.
transmission electron microscopy (TEM), including 0.17 nm high-resolution imaging (HRTEM), electron energy-loss spectroscopy (EELS), and energy dispersive x-ray spectroscopy (EDS) with a 2 nm field-emission probe. Fig. la shows a HRTEM cross-section view of a 37.45 ° [001] twist grain boundary. Details on the structural characterization of the twist boundaries can be found elsewhere [2]. Briefly, we observed that the boundary interface plane is at the double Bi-O layer, and this double Bi-O layer at the boundary appears to be the same as that in the constituent single crystals. No significant strucRkral disorder along the basal ab-planes was observed across the interface. Fig. lb shows a HRTEM cross-section image taken on a 30° [100] non-basal-plane-faced tilt grain boundary. The boundaries are all microscopically flat and clean without any impurities. The boundary planes appeared straight on a scale of 100 A. However, at atomic scale, facets at the grain boundary planes are always observed.
4. TRANSPORT MEASUREMENTS
(a)
)?:?!
Figure 1. Cross-section HRTEM micrographs of (a) a 37.45 ° [001] twist grain boundary, The top crystal is viewed along the a-axis, and the common c-axis is perpendicular to the grain boundary. (b) A 30° [100] non-basai-plane-faced tilt grain-boundary with facets along the boundary plane. The saw tooth line is a guide to indicate the facets. The double Bi-O layer spacing is 15.3 A. 3. GRAIN-BOUNDARY MCROSTRUCTURES The microstructure of these grain-boundaries was extensively characterized using advanced
4.1 [001] twist grain boundaries characterized in brick-wall configuration
In order to provide a close comparison to the grain boundaries in the textured Bi-tapes, measurements were performed on the bicrystals having approximately the same aspect ratio as the platelet-like grains in the tapes. Furthermore, we used a six-probe electric configuration, shown in Fig. 2a, to mimic the current transport pattern (brick-wall configuration) comparable to that in a practical textured Bi-tape. We measured the resistance and the V-I characteristics across the grain-boundary and within their constituent single crystals simultaneously at various magnetic fields and temperatures. The magnetic irreversibility line Hire(T) was determined by the onset of the resistive transition. We observed a similar dissipation behavior in R(T, H) and Hirr(T) across the [001] twist boundaries and within the single crystals in as-prepared bicrystals, regardless of twist angles. Furthermore, we found that the [001] twist boundaries conduct virtually the same Ic as constitute single crystals in a broad H-T plane. In these twist bicrystals, the dissipation at grain boundary seems to be dominated
Q Li et al./Pl~vsica C 341 348 (2000) 1407-1410
1409
critical currents, as well as the Josephson critical currents of [001] twist grain boundary when the current is driven across the layers.
Figure 2. Crystallographic orientation of a c-axis twist bicrystal and the six-probe brick-wall configurations for transport measurements.
by the extremely weak pinning of the single crystal grains. To increase the pinning of the bicrystals, we irradiated some of the [001] twist bicrystals with 2.2 GeV Au-ions [3]. At temperatures far below the irreversibility line, a large increase of 1¢ was observed only in the single crystals when current flows in the same brick-wall configuration (Fig. 2) as that does between the grains in a practical textured Bi-tape. This is due to enhanced vortex pinning in single crystal, at the same time no change is expected in the Josephsoncoupling strength along the c-axis. It is clear that in this low temperature region, Ic of the irradiated bicrystals is limited by the grain boundary, in contrast to the case before irradiation, where the critical current is limited by the lack of strong pinning centers in unirradiated single crystals.
4.2 [001] twist grain boundaries characterized in Josephson junction configuration In Bi2212, the superconducting coherence length along the c-axis is of the order of 0.1 nm. This is much less than the interlayer spacing between the CuO double layers (1.5 nm). Therefore, Bi2212 can be described as a stack of discrete superconducting layers whose order parameters are coupled by Josephson interaction. This long series of Josephson junctions consists of superconducting CuO double layers separated by the BiO and SrO layers acting as weak links. This layered structure provides great opportunities to make direct measurements of intrinsic single crystal Josephson
Figure 3. (a) Top view of a 45 ° c-axis twist bicrystal junction with six attached electrical leads. (b) Sketch of a side view of the bicrystal, with six attached leads in Josephson junction configuration.
A dozen [001] twist Bi2212 bicrystal grain boundaries were prepared using identical single crystal cleaves stacked and twisted an arbitrary angle 4o about the c-axis. An optical micrograph of a 45° caxis twist bicrystal characterized in Josephson junction configuration is pictured in Fig. 3a, while a schematic side view of the bicrystal is sketched in Fig. 3b. Note that the overlap area of the twist junction is about half the cross-sectional area of the identical single crystal cleaved pieces. The current I was fed along the common c-axis, and the voltages Vsl and Vs2 were measured across each single crystal cleave. The voltage Vj across the twist junction was measured from the voltage lead on the bottom surface of the top crystal to the top lead on the bottom crystal. In this configuration, it is possible to measure simultaneously the c-axis V-I characteristics across either of the single crystal parts of the bicrystal and across the twist junction. We found that for each bicrystal, regardless
Q. Li et al./Physica C 341-348 (2000) 1407-1410
1410
of twist angle, the ratio of the measured c-axis Josephson critical current density across the twist junction to that across either single crystal part is close to unity for T < T¢ [4]. Hence, the Josephsoncoupling strength across the twist boundary is almost identical to the intrinsic coupling strength within the single crystals along the c-axis.
by the change of the local oxygen content and strain field [5]. At T < T0°b, we observed a similar superconducting and dissipation behavior for both the grain boundaries and the single crystals in a broad range of magnetic field. Surprisingly, some high angle grain boundaries carry lc as high as their constituent single crystals. Most importantly, these features seem to be independent of the misorientation angle, based on the bicrystals we have studied so far. The orientation of the grain boundary planes and the facets at the grain boundary appear to play an important role in the coupling strength of the tilt grain boundaries, as well as in pinning of the vortices.
5. SUMMARY
C I~ . . . . V
/
C2
/
/- vs(rvo;Vvs -\ Figure 4 (a) An optical micrograph of a 20° [100] non-basal-planed-faced tilt grain boundary and (b) a sketch of crystal orientation with a six-probe in-line configuration for transport measurement.
In summary, we prepared Bi2212 bicrystal c-axis twist grain boundaries of extremely high quality, and found that Josephson-coupling strength across the twist boundary is identical to the intrinsic coupling strength within the single crystals along the c-axis, independent of the twist angle. For naturally grown [100] tilt bicrystal grain boundaries, our studies indicate that the misorientation angle in this type of Bi2212 tilt grain boundary is less likely to be a dominant factor in determining the grain boundary critical current, as compared to the case of YBCO. This work was supported by the U. S. DOE, Office of BES, under contracts No. DE-AC02-98CH10886.
4.3 1100] twist grain boundaries REFERENCES
Bicrystals with a single naturally-grown [100] tilt grain boundaries were isolated from crystal bar with further cleavage along ab-plane and cutting by a precision low-speed wire-saw to reduce the cross section of the bicrystals. Smaller cross sections of the bicrystals require less current, allowing an extended V-I measurement without over-heating the specimen. Fig. 4a shows an optical micrograph of a bicrystal containing a20 ° [100] non-basal-planefaced tilt grain-boundary. Fig. 4b shows a sketch of crystal orientation with a six-probe in-line configuration for transport characterization of the grain boundary and its constituent single crystals simultaneously. For naturally-grown [100] tilt boundaries, we found a general T¢ depression of 610 K at the grain boundary, which is likely induced
1. Y. Zhu, M. Suenaga, and R. L. Sabatini, Appl. Phys. Lett. 65, (1994) 1832. 2. Qiang Li, Y. N. Tsay, Y. Zhu, M. Suenaga, G. D. Gu, and N. Koshizuka, IEEE transaction on Appl. Supercond. 7, (1997) 1584; Y. Zhu, Qiang Li, Y. N. Tsay, M. Suenaga, G. D. Gu, and N. Koshizuka, Phys. Rev. B 57, (1998) 8601. 3. Qiang Y. N. Tsay, G. Wirth, M. Suenaga, G. D. Gu, and N. Koshizuka, Appl. Phys. Lett. 74, (1999) 1323. 4. Qiang Li, Y. N. Tsay, M. Suenaga, R. A. Klemm, G. D. Gu, and N. Koshizuka, Phys. Rev. Lett. 83, (1999) 4160. 5. Qiang Li, Y. N. Tsay, Y. Zhu, M. Suenaga, G. D. Gu, and N. Koshizuka, Appl. Phys. Lett. 70, (1997) 1164.