- Email: [email protected]
The crystal structure and excess conductivity of BSCCO (2212) whiskers
PHYSICA ELSEVIER
PhysicaC255 (1995) 131-135
The crystal structure and excess conductivity of BSCCO (2212) whiskers I.G. Gorlova
a,*,
V.N. Timofeev b
Institute of Radioengineering & Electronics, RAS, Mokhovaya 11, 103907, Moscow, Russian Federation b Baikov Institute of Metallurgy, RAS, Leninskii prospect 49, 117911, Moscow, Russian Federation
Received 21 June 1995
Abstract
The results of a combined study of the structural and conductive properties of BSCCO (2212) whiskers are reported. After electrical measurements the samples are examined by transmission electron microscopy, without preliminary thinning. The whiskers with transverse dimensions as small as 1 × 0.1 ixm2 are single crystals of high quality. Some of the samples are free of dislocations. It is found that the quasi-one-dimensional behavior of the paraconductivity is independent of the presence of dislocations. However, the excess conductivity is suppressed in the twinned whiskers. The existence of a superstructure along the b-direction in a BSCCO crystal could be a possible reason for the a-b-plane paraconductivity anisotropy, resulting in the one-dimensional conducting fluctuations.
1. Introduction
High-T~ compounds are known to be strongly anisotropic, and one should use perfect single crystals to study their basic physical properties. A new approach to this problem is the study of needle-like single crystals - - whiskers. Whiskers are of great interest both for their high-quality single-crystal nature and their small size in two dimensions. High-T~ superconducting whiskers have recently been grown [1], and systematic investigations of their structural and superconducting properties have started. The BiSrCaCuO (BSCCO) whiskers are shown to grow along the a direction perpendicular to the direction of the non-commensurate superstructure [2]. It was found that the temperature dependence of resistance
" Corresponding author.
R ( T ) for the BSCCO (2212) whiskers, in contrast to
bulk-like crystals, deviates downwards from a linear temperature dependence at T~ < T < 2T~ K [3]. Moreover, the excess conductivity in the a direction shows quasi-one-dimensional (1D) behavior [3,4], as do 1D Aslamasov-Larkin conductive fluctuations [5]. The nature of the effect is not yet clear. The reason for a number of unusual properties of high-T~ superconductors lies in their layered crystal structure. In particular, the contribution of two-dimensional fluctuations to conductivity was observed in BSCCO films [6] and single crystals [7,8], and also in T1BaCaCuO materials [9]. The one-dimensional behavior of the paraconductivity in BSCCO whiskers could be associated with some 1D elements of BSCCO crystal structure due to the existence of the superstructure in the b direction [3]. In the present paper we report the results of a combined study of the structural and conductive properties of
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00572-2
132
LG. Gorlova, V.N. Timofeev/Physica C 255 (1995) 131-135
BSCCO (2212) whiskers with various transverse sizes and defect types.
2. Experimental BSCCO (2212) whiskers have been grown by a Pb-free method [3]. The dimensions of the whiskers investigated were (0.5-5 mm) × (1-15 p~m) × (0.1-0.5 ixm). The a-axis resistance of the samples at 300 K was P300 ~- 0.3 m f l cm, the superconducting transition temperature Tc --- 80 K. R(T) along the whisker growth direction was measured by the standard 4-probe method. Au electrical contacts were obtained by vacuum laser evaporation. Contact resistance was about 10 -5 lq × cm 2 at room temperature. The form of the superconducting transitions was used as a criterion for the selection of single-phase whiskers for further detailed investigations. The whiskers after electrical measurements were mounted on copper grids and examined using a transmission electron microscope (TEM) JEM-1000, without preliminary thinning which could lead to additional defects. The BSCCO crystals with thickness comparable to or less then 0.5 p~m had sufficient transparency for electrons accelerated by 1 MV. We studied about 30 whiskers. Besides, five BSCCO single crystals grown by flux method [8] were also studied to compare the defect structure of
needle-like and bulk-like samples. The a-b-plane resistance of bulk crystals at room temperature was P300 ~- 0.2 mf~ cm, Tc ~ 80 K. The plan-view TEM specimens were prepared by cleaving the layered crystals to several slices of about 0.1-0.3 p~m thickness.
3. Results and discussion Fig. 1 shows a typical diffraction patterns of a BSCCO (2212) bulk single crystal (Fig. l(a)) and a whisker (Fig. l(b)) in the [001] pole. The diffraction pattems (a) and (b) are practically identical. The only difference found is that the (h000) and the (hkO0) reflections with odd h and k are clearly visible in (a) but not in (b). The notation (hklm) is used to assign the satellite spots associated with the non-commensurate modulated structure [10]. For some whisker samples the reflections are present, however weakly excited. According to Ref. [10], the reflections mentioned above are forbidden for the Bbmb space group (N. 66). A possible reason for the appearance of the forbidden reflections would be double diffraction [11]. Another explanation which, as we believe, may be associated with some ordering along the a-axis in bulk single crystals requires further investigation.
b,t,
~iiiiiiiil
a
C a3
C b3
Fig. 1. Diffraction patterns in the [001] pole of a BSCCO (2212) bulk single crystal (a) and whisker (b). The (h000) and the (hkO0) reflections with odd h and k are clearly visible in (a) but not in (b).
LG. Gorloua, V.N. Timofeev/Physica C255 (1995)131-135
Ca )
Cb)
Cc)
Cd)
133
Fig. 2 shows electron micrographs of a bulk single crystal (a) and of whiskers with different sizes (b-d). A network of disordering dislocations is usually observed in the bulk-like samples (Fig. 2(a)). The defects shown in Fig. 2(b) by arrows are typical for whiskers of width > 10 txm and are regions of magnetic vortex pinning [12]. Combined TEM and SEM studies indicated that these defects are small steps on the whisker surface. The micron width whiskers are, as a rule, stepless and contain a small number of dislocations (Fig. 2(c)). Some of the whiskers of submicron cross section are completely free of dislocations (Fig. 2(d)). The 1D excess conductivity was observed for submicron whiskers and decreased with an increase of whisker cross section [3]. The results of structure analysis shown in Fig. 2 indicate that the narrower the sample the higher is the quality of the single crystal. Therefore, the 1D excess conductivity seems to be an intrinsic property of perfect crystals. This is supported by observation of the effect for two wide ( ~ 12 Ixm) perfect whiskers. The dislocations observed in the BSCCO whiskers are mainly parallel to the a direction (Fig. 2(c), see also Ref. [3]). Some transverse dislocations were also found (Figs. 3(a) and (b)). The one-dimensional behavior of paraconductivity could be due to these linear defects. However, the combined study of the structural and conductive properties of BSCCO whiskers shows that the effect of excess conductivity does not depend either on the presence or on the orientation of dislocations. The effect is highly suppressed in three cases: (1) In mixed phase whiskers. It should be noted that the R(T) behavior of mixed-phase samples corresponds neither to the (2212) nor to the (2223) phase, but depends on the proportion of the
Fig. 2. a-b-plane electron micrographs of BSCCO crystals with different sizes. (a) Bright-field TEM image for bulk single crystal with the dislocation network. × 24000 (EMX 12000). (b) Brightfield TEM image for a whisker of 11.5 ixm width, ×8000 (EM X 5 000). Growth steps are marked by arrows. (c) Dark-field TEM image for a whisker of 1.1 I~m width. X30000 (EMX 10000). Two dislocations run along the whisker length. (d) Darkfield TEM image for a defectless whisker of 1.6 Ixm width. X 30000 (EM× 15000). Only extinction bend contours are visible.
I.G. Gorlova, V.N. Timofeeo / Physica C 255 (1995) 131-135
134
phases. The detailed analysis of the mixed-phase case is beyond the scope of this article. (2) In whiskers covered by a polycrystalline film of unknown nature. A thin film was found on the surface of two single-phase (2212) samples. (3) In the twinned whisker. Of all 30 whiskers investigated there was only one twinned. The superstructure was observed in two directins (see insert to Fig. 4(a)). Fig. 4 shows a plot of the temperature dependence of normalized resistance for the twinned whisker (Fig. 4(a)) and for a typical whisker of the same transverse sizes: ~ 1.5 × 0.1 la,m 2 (Fig. 4(b)). The corresponding electron diffraction patterns are shown in inserts to Fig. 4. The resistivity of the twinned sample decreases linearly with temperature up to ~ 100 K. The similar R(T) curves are observed for
a 0 0
\
b
H
0J 0
I
I
I
100
200
,300
'r IK)
Fig. 4. Normalized resistance vs. temperature curves for the twinned whisker (a) and for a typical whisker of the same sizes ~ 1.5×0.1 p.m 2 (b). The corresponding diffraction patterns are shown in inserts.
C a3
whiskers covered by a polycrystalline film. In contrast to Fig. 4(a), a considerable deviation, A R, from linear dependence is visible in Fig. 4(b). The excess conductivity calculated from A R measurements shows quasi-one-dimensional behavior for temperatures 105 K < T < 140 K. As T decreases, the crossover from 1D to 2D behavior is observed as was earlier found in Ref. [3]. The results shown in Fig. 4 indicate that the a-b-plane paraconductivity in BSCCO materials is strongly anisotropic. 1D superconducting fluctuations seem to be the a-direction feature. The effect may be associated with the existence of the superlattice in the b direction, as supposed in Ref. [3].
4. Conclusion c la) Fig. 3. Dark-field T E M image for whiskers with transverse dislocations in the a-b-plane (a, b). ×30000 (EM× 15000).
The defect structure of BSCCO (2212) whiskers in a wide range of transverse dimensions was studied
LG. Gorloua, V.N. Timofeev/Physica C 255 (1995) 131-135
by TEM. Dislocations, growth steps and twins were observed. The narrower the sample, the higher the quality of the single crystal. Some of the whiskers with submicron transverse dimensions were found to be defectless. The comparison of the structural and conductive properties for whiskers of various sizes and defect types indicates that the a-b-plane paraconductivity is anisotropic. 1D excess conductivity in the a-direction seems to be an intrinsic property of BSCCO materials.
Acknowledgements We would like to acknowledge the contributions of A.M. Nikitina and V.U. Antokhina, who grew the samples used in this study. We thank S.G. Zybtsev for experimental assistance and helpful discussions. The research described in this publication was made possible in part by Grant No. M8B000 from the International Science Foundation and was supported by the Russian State Program on HTSC under Project No. 93046.
135
References [1] I. Matzubara, H. Kageyama, H. Tanigawa, T. Ogura, H. Yamashita and T. Kawai, Jpn. J. Appl. Phys. 28 (1989) Ll121. [2] A. Krapf, G. Lacayo, G. Kastner, W. Kraak, N. Pruss, H. Thiele, H. Dwelk and R. Herrmann, Supercond. Sci. Technol. 4 (1991) 237. [3] Yu.I. Latyshev, I.G. Gorlova, A.M. Nikitina, V.U. Antokhina, S.G. Zybtsev, N.P. Kukhta and V.N. Timofeev, Physica C 216 (1993) 471. [4] X-F-Chen, M.J. Marone, G.X. Tessema, M.J. Skove, M.V. Nevitt, D.J. Miller and B.J. Veal, Phys. Rev. B 48 (1993) 1254. [5] L.G. Aslamazov and Larkin, Phys. Lett. 26A (1968) 238. [6] G. Balestrino, A. Nigro, R. Vaglio and M. Marinelli, Phys. Rev. B 39 (1989) 12264. [7] S. Martin, A.T. Fiory, R.M. Fleming, G.P. Espinosa and A.S. Cooper, Phys. Rev. Lett. 62 (1989) 677. [8] S.N. Artemenko, I.G. Gorlova and Yu.I. Latyshev, Phys. Lett. A 138 (1989) 428. [9] D.H. Kim, A.M. Goldman, J.H. Kang, K.E. Gray and R.T. Kampwirth, Phys. Rev. B 39 (1989) 12275. [10] M. Onoda et al., Jpn. J. Appl. Phys. 27 (1988) L833. [11] O. Eibl, Solid State Commun. 69 (1989) 509. [12] L.Ya. Vinnikov, L.A. Gurevich, Yu.I. Latyshev, A.M. Nikitina, V.U. Antokhina, M.P. Lisitskii and N.P. Kukhta, Pis'ma Zh. Exp. Teor. Fiz. 57 (1993) 247.
PhysicaC255 (1995) 131-135
The crystal structure and excess conductivity of BSCCO (2212) whiskers I.G. Gorlova
a,*,
V.N. Timofeev b
Institute of Radioengineering & Electronics, RAS, Mokhovaya 11, 103907, Moscow, Russian Federation b Baikov Institute of Metallurgy, RAS, Leninskii prospect 49, 117911, Moscow, Russian Federation
Received 21 June 1995
Abstract
The results of a combined study of the structural and conductive properties of BSCCO (2212) whiskers are reported. After electrical measurements the samples are examined by transmission electron microscopy, without preliminary thinning. The whiskers with transverse dimensions as small as 1 × 0.1 ixm2 are single crystals of high quality. Some of the samples are free of dislocations. It is found that the quasi-one-dimensional behavior of the paraconductivity is independent of the presence of dislocations. However, the excess conductivity is suppressed in the twinned whiskers. The existence of a superstructure along the b-direction in a BSCCO crystal could be a possible reason for the a-b-plane paraconductivity anisotropy, resulting in the one-dimensional conducting fluctuations.
1. Introduction
High-T~ compounds are known to be strongly anisotropic, and one should use perfect single crystals to study their basic physical properties. A new approach to this problem is the study of needle-like single crystals - - whiskers. Whiskers are of great interest both for their high-quality single-crystal nature and their small size in two dimensions. High-T~ superconducting whiskers have recently been grown [1], and systematic investigations of their structural and superconducting properties have started. The BiSrCaCuO (BSCCO) whiskers are shown to grow along the a direction perpendicular to the direction of the non-commensurate superstructure [2]. It was found that the temperature dependence of resistance
" Corresponding author.
R ( T ) for the BSCCO (2212) whiskers, in contrast to
bulk-like crystals, deviates downwards from a linear temperature dependence at T~ < T < 2T~ K [3]. Moreover, the excess conductivity in the a direction shows quasi-one-dimensional (1D) behavior [3,4], as do 1D Aslamasov-Larkin conductive fluctuations [5]. The nature of the effect is not yet clear. The reason for a number of unusual properties of high-T~ superconductors lies in their layered crystal structure. In particular, the contribution of two-dimensional fluctuations to conductivity was observed in BSCCO films [6] and single crystals [7,8], and also in T1BaCaCuO materials [9]. The one-dimensional behavior of the paraconductivity in BSCCO whiskers could be associated with some 1D elements of BSCCO crystal structure due to the existence of the superstructure in the b direction [3]. In the present paper we report the results of a combined study of the structural and conductive properties of
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00572-2
132
LG. Gorlova, V.N. Timofeev/Physica C 255 (1995) 131-135
BSCCO (2212) whiskers with various transverse sizes and defect types.
2. Experimental BSCCO (2212) whiskers have been grown by a Pb-free method [3]. The dimensions of the whiskers investigated were (0.5-5 mm) × (1-15 p~m) × (0.1-0.5 ixm). The a-axis resistance of the samples at 300 K was P300 ~- 0.3 m f l cm, the superconducting transition temperature Tc --- 80 K. R(T) along the whisker growth direction was measured by the standard 4-probe method. Au electrical contacts were obtained by vacuum laser evaporation. Contact resistance was about 10 -5 lq × cm 2 at room temperature. The form of the superconducting transitions was used as a criterion for the selection of single-phase whiskers for further detailed investigations. The whiskers after electrical measurements were mounted on copper grids and examined using a transmission electron microscope (TEM) JEM-1000, without preliminary thinning which could lead to additional defects. The BSCCO crystals with thickness comparable to or less then 0.5 p~m had sufficient transparency for electrons accelerated by 1 MV. We studied about 30 whiskers. Besides, five BSCCO single crystals grown by flux method [8] were also studied to compare the defect structure of
needle-like and bulk-like samples. The a-b-plane resistance of bulk crystals at room temperature was P300 ~- 0.2 mf~ cm, Tc ~ 80 K. The plan-view TEM specimens were prepared by cleaving the layered crystals to several slices of about 0.1-0.3 p~m thickness.
3. Results and discussion Fig. 1 shows a typical diffraction patterns of a BSCCO (2212) bulk single crystal (Fig. l(a)) and a whisker (Fig. l(b)) in the [001] pole. The diffraction pattems (a) and (b) are practically identical. The only difference found is that the (h000) and the (hkO0) reflections with odd h and k are clearly visible in (a) but not in (b). The notation (hklm) is used to assign the satellite spots associated with the non-commensurate modulated structure [10]. For some whisker samples the reflections are present, however weakly excited. According to Ref. [10], the reflections mentioned above are forbidden for the Bbmb space group (N. 66). A possible reason for the appearance of the forbidden reflections would be double diffraction [11]. Another explanation which, as we believe, may be associated with some ordering along the a-axis in bulk single crystals requires further investigation.
b,t,
~iiiiiiiil
a
C a3
C b3
Fig. 1. Diffraction patterns in the [001] pole of a BSCCO (2212) bulk single crystal (a) and whisker (b). The (h000) and the (hkO0) reflections with odd h and k are clearly visible in (a) but not in (b).
LG. Gorloua, V.N. Timofeev/Physica C255 (1995)131-135
Ca )
Cb)
Cc)
Cd)
133
Fig. 2 shows electron micrographs of a bulk single crystal (a) and of whiskers with different sizes (b-d). A network of disordering dislocations is usually observed in the bulk-like samples (Fig. 2(a)). The defects shown in Fig. 2(b) by arrows are typical for whiskers of width > 10 txm and are regions of magnetic vortex pinning [12]. Combined TEM and SEM studies indicated that these defects are small steps on the whisker surface. The micron width whiskers are, as a rule, stepless and contain a small number of dislocations (Fig. 2(c)). Some of the whiskers of submicron cross section are completely free of dislocations (Fig. 2(d)). The 1D excess conductivity was observed for submicron whiskers and decreased with an increase of whisker cross section [3]. The results of structure analysis shown in Fig. 2 indicate that the narrower the sample the higher is the quality of the single crystal. Therefore, the 1D excess conductivity seems to be an intrinsic property of perfect crystals. This is supported by observation of the effect for two wide ( ~ 12 Ixm) perfect whiskers. The dislocations observed in the BSCCO whiskers are mainly parallel to the a direction (Fig. 2(c), see also Ref. [3]). Some transverse dislocations were also found (Figs. 3(a) and (b)). The one-dimensional behavior of paraconductivity could be due to these linear defects. However, the combined study of the structural and conductive properties of BSCCO whiskers shows that the effect of excess conductivity does not depend either on the presence or on the orientation of dislocations. The effect is highly suppressed in three cases: (1) In mixed phase whiskers. It should be noted that the R(T) behavior of mixed-phase samples corresponds neither to the (2212) nor to the (2223) phase, but depends on the proportion of the
Fig. 2. a-b-plane electron micrographs of BSCCO crystals with different sizes. (a) Bright-field TEM image for bulk single crystal with the dislocation network. × 24000 (EMX 12000). (b) Brightfield TEM image for a whisker of 11.5 ixm width, ×8000 (EM X 5 000). Growth steps are marked by arrows. (c) Dark-field TEM image for a whisker of 1.1 I~m width. X30000 (EMX 10000). Two dislocations run along the whisker length. (d) Darkfield TEM image for a defectless whisker of 1.6 Ixm width. X 30000 (EM× 15000). Only extinction bend contours are visible.
I.G. Gorlova, V.N. Timofeeo / Physica C 255 (1995) 131-135
134
phases. The detailed analysis of the mixed-phase case is beyond the scope of this article. (2) In whiskers covered by a polycrystalline film of unknown nature. A thin film was found on the surface of two single-phase (2212) samples. (3) In the twinned whisker. Of all 30 whiskers investigated there was only one twinned. The superstructure was observed in two directins (see insert to Fig. 4(a)). Fig. 4 shows a plot of the temperature dependence of normalized resistance for the twinned whisker (Fig. 4(a)) and for a typical whisker of the same transverse sizes: ~ 1.5 × 0.1 la,m 2 (Fig. 4(b)). The corresponding electron diffraction patterns are shown in inserts to Fig. 4. The resistivity of the twinned sample decreases linearly with temperature up to ~ 100 K. The similar R(T) curves are observed for
a 0 0
\
b
H
0J 0
I
I
I
100
200
,300
'r IK)
Fig. 4. Normalized resistance vs. temperature curves for the twinned whisker (a) and for a typical whisker of the same sizes ~ 1.5×0.1 p.m 2 (b). The corresponding diffraction patterns are shown in inserts.
C a3
whiskers covered by a polycrystalline film. In contrast to Fig. 4(a), a considerable deviation, A R, from linear dependence is visible in Fig. 4(b). The excess conductivity calculated from A R measurements shows quasi-one-dimensional behavior for temperatures 105 K < T < 140 K. As T decreases, the crossover from 1D to 2D behavior is observed as was earlier found in Ref. [3]. The results shown in Fig. 4 indicate that the a-b-plane paraconductivity in BSCCO materials is strongly anisotropic. 1D superconducting fluctuations seem to be the a-direction feature. The effect may be associated with the existence of the superlattice in the b direction, as supposed in Ref. [3].
4. Conclusion c la) Fig. 3. Dark-field T E M image for whiskers with transverse dislocations in the a-b-plane (a, b). ×30000 (EM× 15000).
The defect structure of BSCCO (2212) whiskers in a wide range of transverse dimensions was studied
LG. Gorloua, V.N. Timofeev/Physica C 255 (1995) 131-135
by TEM. Dislocations, growth steps and twins were observed. The narrower the sample, the higher the quality of the single crystal. Some of the whiskers with submicron transverse dimensions were found to be defectless. The comparison of the structural and conductive properties for whiskers of various sizes and defect types indicates that the a-b-plane paraconductivity is anisotropic. 1D excess conductivity in the a-direction seems to be an intrinsic property of BSCCO materials.
Acknowledgements We would like to acknowledge the contributions of A.M. Nikitina and V.U. Antokhina, who grew the samples used in this study. We thank S.G. Zybtsev for experimental assistance and helpful discussions. The research described in this publication was made possible in part by Grant No. M8B000 from the International Science Foundation and was supported by the Russian State Program on HTSC under Project No. 93046.
135
References [1] I. Matzubara, H. Kageyama, H. Tanigawa, T. Ogura, H. Yamashita and T. Kawai, Jpn. J. Appl. Phys. 28 (1989) Ll121. [2] A. Krapf, G. Lacayo, G. Kastner, W. Kraak, N. Pruss, H. Thiele, H. Dwelk and R. Herrmann, Supercond. Sci. Technol. 4 (1991) 237. [3] Yu.I. Latyshev, I.G. Gorlova, A.M. Nikitina, V.U. Antokhina, S.G. Zybtsev, N.P. Kukhta and V.N. Timofeev, Physica C 216 (1993) 471. [4] X-F-Chen, M.J. Marone, G.X. Tessema, M.J. Skove, M.V. Nevitt, D.J. Miller and B.J. Veal, Phys. Rev. B 48 (1993) 1254. [5] L.G. Aslamazov and Larkin, Phys. Lett. 26A (1968) 238. [6] G. Balestrino, A. Nigro, R. Vaglio and M. Marinelli, Phys. Rev. B 39 (1989) 12264. [7] S. Martin, A.T. Fiory, R.M. Fleming, G.P. Espinosa and A.S. Cooper, Phys. Rev. Lett. 62 (1989) 677. [8] S.N. Artemenko, I.G. Gorlova and Yu.I. Latyshev, Phys. Lett. A 138 (1989) 428. [9] D.H. Kim, A.M. Goldman, J.H. Kang, K.E. Gray and R.T. Kampwirth, Phys. Rev. B 39 (1989) 12275. [10] M. Onoda et al., Jpn. J. Appl. Phys. 27 (1988) L833. [11] O. Eibl, Solid State Commun. 69 (1989) 509. [12] L.Ya. Vinnikov, L.A. Gurevich, Yu.I. Latyshev, A.M. Nikitina, V.U. Antokhina, M.P. Lisitskii and N.P. Kukhta, Pis'ma Zh. Exp. Teor. Fiz. 57 (1993) 247.