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Observation of the flux line lattice in (La1−xSrx)2CuO4 single crystals
Solid State Communications.
107. No. 6. pp. 291-293, 1998 0 1998 Else&v Science Lid Printed in Great Britain. All rights reserved 0038-1098/98 $19.00+.00
Pergamon
Vol.
PII: SOO38-1098(98)00204-X
OBSERVATION
OF THE FLUX LINE LATTICE
A. Vostner,a’* G. Brandstatter,’
IN (La1_,Sr,)2Cu04
SINGLE CRYSTALS
Tapan Chatterji,b’d H.W. Weber,’ T. Sasagawa,’
K. Kishio’ and H. Lauterb
‘Atominstitut
der Gsterreichischen Universidten, A-1020 Wien, Austria bILL Grenoble, F-38042 Grenoble, France ‘Department of Superconductivity, University of Tokyo, Japan ‘Max-Planck-lnstitut fiir Physik Komplexer Systeme, D-01 187 Dresden, Germany (Received 5 January 1998; accepted 23 April 1998 by P. Wachter) Small angle neutron diffraction was used to image the flux line lattice (FLL) in a large superconducting (La1_,Sr,)2Cu04 (La-2 1 4) single crystal for the first time. Diffraction patterns were obtained in fields up to 0.2 T and at several temperatures (3-26 K). We find an intensity ring instead of peaks, which proves the existence of a polycrystalline flux line lattice in La-2 1 4. We relate this observation to the structural phase transition of the compound at about 200 K, which destroys the single crystalline character of the sample in the plane perpendicular to the c-axis. 0 1998 Elsevier Science Ltd. All rights reserved Keywords:
A. high-T, superconductors,
Small angle neutron scattering is an excellent method to investigate the FLL in superconductors, since the magnetic moments of the neutrons interact with the periodic array of magnetic fields. Experiments carried out on low temperature superconductors such as Nb confirmed the existence and the bulk character of a single crystalline hexagonal FLL [l]. However, the experiments were quite tedious because of the small Bragg angles of only a few minutes or even seconds. Cold neutron sources providing neutron beams with very long wave lengths and long neutron flight paths are on high temperature supernecessary. Experiments conducting single crystals are much more complicated, since they scatter only weakly because of their large magnetic penetration depth. Thus, the intensity, I 0: X-4, is very small and diffraction patterns are visible only after background subtraction and long counting times. Diffraction patterns were already obtained from Y - 1 2 3 and Bi-2 2 1 2 superconductors and showed square and hexagonal symmetry, respectively [2-41. Investigations of other high T, compounds failed so far due to the lack of adequate samples. In this paper we will present data obtained on large high quality La-2 1 4 single crystals. We find evidence * Corresponding
author.
D. phase transitions.
for the existence of a polycrystalline FLL in this compound for the first time. The La-2 1 4 single crystals with a Sr-content x of 0.075 and 0.085 were grown by a travelling solvent floating zone method. The crystal orientations were determined by an X-ray back reflection Laue technique [5]. The dimensions of the samples were up to 6 X 8.2 X 2.2 mm3. The Sr-content and the O-deficiency 6 were determined as described in [5]. The samples are of high quality and do not contain a twin pattern at room temperature. T, was measured in a field of 0.01 mT applied parallel to the ab-plane in a highly sensitive low field SQUID magnetometer. We find T, = 35 K in agreement with data reported in the literature [6]. The superconducting fraction was calculated from magnetisation measurements and found to be more than 90%. The neutron scattering experiments were carried out at the small angle neutron scattering facility Dl 1 at ILL in Grenoble. The crystal was mounted onto an aluminium sample holder and inserted into a cryostat with a horizontal magnet. The cryostat was placed onto a table, which can be rotated in the xy-plane and tilted in the z-direction. The crystallographic c-axis of the crystal was oriented parallel to both the magnetic field and the incoming monochromatic neutron beam. The neutron
291
292
FLUX LINE LATTICE
Vol. 107, No. 6
IN (La I_xSrx)2Cu04 SINGLE CRYSTALS
wave length was set to 1 nm. The counting time for each point was - 13 h. The scattered neutrons were registered by a 2D detector located up to 35 m behind the sample. The unscattered beam was intercepted by a beam stop. The diffracted intensity in the mixed state was obtained after cooling through Tc in a certain field and after subtracting the background recorded above T,. at the same field. We measured diffraction patterns at fields ranging from 50 to 200 mT at various temperatures (3 K-26 K). A smaller companion crystal was investigated by SQUID magnetometry prior to the neutron scattering experiments. From the reversible parts of the magnetization curve we derived the magnetic penetration depth hub to be about 300 nm at 0 K. Thus, La-2 1 4 has a rather large penetration depth compared to Y- 1 2 3. A counting time of about 13 h is required to reach the same intensity as for Y- 1 2 3 after counting for 15 to 30 min. Furthermore, the applied field must be kept small, in order to avoid too much overlap of the flux lines. Figure 1 shows a typical result obtained at 0.1 T and 3 K. Contrary to Y-l 2 3 or Bi-2 2 1 2 no individual peaks are visible, but a ring. A radial integration of the intensity confirms that the maximum is situated at that position, where the Bragg peak for 0.1 T and for the corresponding detector distance (20.5 m) is expected. The vertical lines in Fig. 2 indicate the calculated positions of the diffracted intensity peaks for a square and for a hexagonal flux line lattice, respectively. The measurement was repeated at the same field, but at a
Fig. 1. Ring pattern obtained at 3 K and 0.1 T. The plot shows the counts recorded by the 2D detector on a logarithmic scale (black 370. white 0). _I
q (nm”) 0 04
0 02 50
I
0.08
0.06 14 i,
!
0.10
1
‘b
I
\ i
40
Y S =
30peak is expected
5 .
2 zo/
9 5 E
.: 10
10
15 distance
25
20 from center
30
(cm)
Fig. 2. Radial integration of the diffraction pattern obtained at 3 K and 0.1 T. The maximum occurs at the expected position, cf. the vertical lines calculated for a square and a hexagonal flux line lattice at 0.1 T, respectively. 4 is the reciprocal lattice vector, q = 2?r/d, and d is the lattice parameter (d = dlo in this case). higher temperature (26 K). We found the ring at the same position, but a decrease in intensity, as expected from the temperature dependence of the form factor. If the field is increased, but all the other parameters remain unchanged (T = 3 K, same detector position as for 0.1 T), the ring should move to a different position. Diffraction patterns recorded at 0.15 and 0.2 T confirmed this, but also showed a broadening of the ring and less intensity than for the smaller field, the latter being again in agreement with the field dependence of the form factor. Finally, the angle between the c-axis of the crystal and the direction of the incoming neutron beam was set to 10” and the above experiment repeated. We found again a ring of diffracted intensity. Thus all of our data are consistent with the occurrence of a polycrystalline flux line lattice in this single crystalline material, in marked contrast to the results reported for Y-l 2 3 and Bi-2 2 1 2 single crystals. In order to understand this result, we need to consider the specific situation of La-2 1 4 superconductors. La-2 1 4 undergoes a phase transition from a tetragonal to an orthorhombic structure at 200 K, which can be understood by the bond-length mismatch between the Cu-0 and La-O bonds [7] due to their different thermal expansions. The tetragonal structure is distorted because of the rotation of the CuOd octahedra around the [l 0 0] or [0 1 0] axis, thus reducing the symmetry and leading to the occurrence of twins. The size distribution is influenced by strain [S]. The c-axis is left invariant. In the orthorhombic phase, the crystal consists of two sets of
Vol. 107, No. 6
FLUX LINE LATTICE
IN (La,-.,Sr,)2C~04
two twin orientations sharing either the (1 1 0) or the (1 -1 0) plane. In total, four different twin orientations exist [7]. Single crystalline grains were investigated by electron diffraction [7], which showed that different sets of lamellae of platelike domains parallel to (1 1 0) or (1 - 1 0) were formed by the twins. The grain itself contains such sets of lamellae with sizes of up to several pm. These sets contain a large number of monodomains. Thus, the original single crystalline character of the sample is destroyed at the phase transition; the .sample becomes polycrystalline. The main difference between Y-l 2 3 and La-2 I 4 is the orientation of the twinning elements. In both compounds, the twinning symmetry elements are the (1 1 0) and the (1 -1 0) planes. In Y-l 2 3, they are oriented along the diagonals of the Cu-0 squares, but in La-2 1 4, they are parallel to the sides of the squares [7]. Thus, the twin boundaries lead to quite different structural situations and to different conditions for flux pinning in both compounds. In summary, neutron diffraction by the flux line lattice in a big high quality La-2 1 4 single crystal was investigated. We obtain diffraction patterns showing a ring at different magnetic fields and temperatures, which proves the existence of a polycrystalline flux line lattice in this compound. The most obvious reason for the formation of a polycrystalline FLL is the low temperature crystal structure of the sample, which becomes polycrystalline below a certain transition temperature because of uncontrolled twinning. The scattered intensity is only weak because of the polycrystallinity
SINGLE CRYSTALS
293
and because of the large magnetic penetration depth, which makes a systematic study of the intensity as a function of field and temperature very difficult for this compound. Acknowledgements-We gratefully acknowledge financial support by the Institute Laue Langevin, Grenoble. REFERENCES Weber, H.W., Schelten, J. and Lippmann, G., J. Low. Temp. Phys., 16, 1974, 367. 2. Forgan, E.M., McKPaul, D., Mook, H.A., Timmins, P.A., Keller, H., Sutton, S. and Abell, J.S., Nature, 343, 1990,735. 3. Brandstatter, G., Weber, H.W., Chattopadhyay, T., Cubitt, R., Fischer, H., Wylie, M., Emel’chenko, G.A. and Wiedenmann, A., J. Appl. Qvst., 30, 1997,571. 4. Cubit& R., Forgan, E.M., Yang, G., Lee, S.L., McKPaul, D., Mook, H.A., Yethiraj, M., Kes, P.H., Li, T.W., Menoresky, A.A., Tarnawski, Z. and Mortensen, K., Nature, 365, 1993,407. 5. Sasagawa, T., Okuya, M., Shimoyama, J., Kishio, K. and Kitazawa, K., J. Low Temp. Phys., 105, 1996, 1201. 6. Li, Q., Suenaga, M., Kimura, T. and Kishio, K., Phys. Rev., B47, 1993, 11384. 7. Braden, M., Heger, G., Schweiss, P., Fisk, Z., Gamayunov, K., Tanaka, I. and Kojima, H., Physica, C191, 1992, 455. 8. Schmidt, H., Burkhardt, E., Nan Sun, Bing and Rivera, J.P., Physica, Cl57, 1989, 555. 1.
107. No. 6. pp. 291-293, 1998 0 1998 Else&v Science Lid Printed in Great Britain. All rights reserved 0038-1098/98 $19.00+.00
Pergamon
Vol.
PII: SOO38-1098(98)00204-X
OBSERVATION
OF THE FLUX LINE LATTICE
A. Vostner,a’* G. Brandstatter,’
IN (La1_,Sr,)2Cu04
SINGLE CRYSTALS
Tapan Chatterji,b’d H.W. Weber,’ T. Sasagawa,’
K. Kishio’ and H. Lauterb
‘Atominstitut
der Gsterreichischen Universidten, A-1020 Wien, Austria bILL Grenoble, F-38042 Grenoble, France ‘Department of Superconductivity, University of Tokyo, Japan ‘Max-Planck-lnstitut fiir Physik Komplexer Systeme, D-01 187 Dresden, Germany (Received 5 January 1998; accepted 23 April 1998 by P. Wachter) Small angle neutron diffraction was used to image the flux line lattice (FLL) in a large superconducting (La1_,Sr,)2Cu04 (La-2 1 4) single crystal for the first time. Diffraction patterns were obtained in fields up to 0.2 T and at several temperatures (3-26 K). We find an intensity ring instead of peaks, which proves the existence of a polycrystalline flux line lattice in La-2 1 4. We relate this observation to the structural phase transition of the compound at about 200 K, which destroys the single crystalline character of the sample in the plane perpendicular to the c-axis. 0 1998 Elsevier Science Ltd. All rights reserved Keywords:
A. high-T, superconductors,
Small angle neutron scattering is an excellent method to investigate the FLL in superconductors, since the magnetic moments of the neutrons interact with the periodic array of magnetic fields. Experiments carried out on low temperature superconductors such as Nb confirmed the existence and the bulk character of a single crystalline hexagonal FLL [l]. However, the experiments were quite tedious because of the small Bragg angles of only a few minutes or even seconds. Cold neutron sources providing neutron beams with very long wave lengths and long neutron flight paths are on high temperature supernecessary. Experiments conducting single crystals are much more complicated, since they scatter only weakly because of their large magnetic penetration depth. Thus, the intensity, I 0: X-4, is very small and diffraction patterns are visible only after background subtraction and long counting times. Diffraction patterns were already obtained from Y - 1 2 3 and Bi-2 2 1 2 superconductors and showed square and hexagonal symmetry, respectively [2-41. Investigations of other high T, compounds failed so far due to the lack of adequate samples. In this paper we will present data obtained on large high quality La-2 1 4 single crystals. We find evidence * Corresponding
author.
D. phase transitions.
for the existence of a polycrystalline FLL in this compound for the first time. The La-2 1 4 single crystals with a Sr-content x of 0.075 and 0.085 were grown by a travelling solvent floating zone method. The crystal orientations were determined by an X-ray back reflection Laue technique [5]. The dimensions of the samples were up to 6 X 8.2 X 2.2 mm3. The Sr-content and the O-deficiency 6 were determined as described in [5]. The samples are of high quality and do not contain a twin pattern at room temperature. T, was measured in a field of 0.01 mT applied parallel to the ab-plane in a highly sensitive low field SQUID magnetometer. We find T, = 35 K in agreement with data reported in the literature [6]. The superconducting fraction was calculated from magnetisation measurements and found to be more than 90%. The neutron scattering experiments were carried out at the small angle neutron scattering facility Dl 1 at ILL in Grenoble. The crystal was mounted onto an aluminium sample holder and inserted into a cryostat with a horizontal magnet. The cryostat was placed onto a table, which can be rotated in the xy-plane and tilted in the z-direction. The crystallographic c-axis of the crystal was oriented parallel to both the magnetic field and the incoming monochromatic neutron beam. The neutron
291
292
FLUX LINE LATTICE
Vol. 107, No. 6
IN (La I_xSrx)2Cu04 SINGLE CRYSTALS
wave length was set to 1 nm. The counting time for each point was - 13 h. The scattered neutrons were registered by a 2D detector located up to 35 m behind the sample. The unscattered beam was intercepted by a beam stop. The diffracted intensity in the mixed state was obtained after cooling through Tc in a certain field and after subtracting the background recorded above T,. at the same field. We measured diffraction patterns at fields ranging from 50 to 200 mT at various temperatures (3 K-26 K). A smaller companion crystal was investigated by SQUID magnetometry prior to the neutron scattering experiments. From the reversible parts of the magnetization curve we derived the magnetic penetration depth hub to be about 300 nm at 0 K. Thus, La-2 1 4 has a rather large penetration depth compared to Y- 1 2 3. A counting time of about 13 h is required to reach the same intensity as for Y- 1 2 3 after counting for 15 to 30 min. Furthermore, the applied field must be kept small, in order to avoid too much overlap of the flux lines. Figure 1 shows a typical result obtained at 0.1 T and 3 K. Contrary to Y-l 2 3 or Bi-2 2 1 2 no individual peaks are visible, but a ring. A radial integration of the intensity confirms that the maximum is situated at that position, where the Bragg peak for 0.1 T and for the corresponding detector distance (20.5 m) is expected. The vertical lines in Fig. 2 indicate the calculated positions of the diffracted intensity peaks for a square and for a hexagonal flux line lattice, respectively. The measurement was repeated at the same field, but at a
Fig. 1. Ring pattern obtained at 3 K and 0.1 T. The plot shows the counts recorded by the 2D detector on a logarithmic scale (black 370. white 0). _I
q (nm”) 0 04
0 02 50
I
0.08
0.06 14 i,
!
0.10
1
‘b
I
\ i
40
Y S =
30peak is expected
5 .
2 zo/
9 5 E
.: 10
10
15 distance
25
20 from center
30
(cm)
Fig. 2. Radial integration of the diffraction pattern obtained at 3 K and 0.1 T. The maximum occurs at the expected position, cf. the vertical lines calculated for a square and a hexagonal flux line lattice at 0.1 T, respectively. 4 is the reciprocal lattice vector, q = 2?r/d, and d is the lattice parameter (d = dlo in this case). higher temperature (26 K). We found the ring at the same position, but a decrease in intensity, as expected from the temperature dependence of the form factor. If the field is increased, but all the other parameters remain unchanged (T = 3 K, same detector position as for 0.1 T), the ring should move to a different position. Diffraction patterns recorded at 0.15 and 0.2 T confirmed this, but also showed a broadening of the ring and less intensity than for the smaller field, the latter being again in agreement with the field dependence of the form factor. Finally, the angle between the c-axis of the crystal and the direction of the incoming neutron beam was set to 10” and the above experiment repeated. We found again a ring of diffracted intensity. Thus all of our data are consistent with the occurrence of a polycrystalline flux line lattice in this single crystalline material, in marked contrast to the results reported for Y-l 2 3 and Bi-2 2 1 2 single crystals. In order to understand this result, we need to consider the specific situation of La-2 1 4 superconductors. La-2 1 4 undergoes a phase transition from a tetragonal to an orthorhombic structure at 200 K, which can be understood by the bond-length mismatch between the Cu-0 and La-O bonds [7] due to their different thermal expansions. The tetragonal structure is distorted because of the rotation of the CuOd octahedra around the [l 0 0] or [0 1 0] axis, thus reducing the symmetry and leading to the occurrence of twins. The size distribution is influenced by strain [S]. The c-axis is left invariant. In the orthorhombic phase, the crystal consists of two sets of
Vol. 107, No. 6
FLUX LINE LATTICE
IN (La,-.,Sr,)2C~04
two twin orientations sharing either the (1 1 0) or the (1 -1 0) plane. In total, four different twin orientations exist [7]. Single crystalline grains were investigated by electron diffraction [7], which showed that different sets of lamellae of platelike domains parallel to (1 1 0) or (1 - 1 0) were formed by the twins. The grain itself contains such sets of lamellae with sizes of up to several pm. These sets contain a large number of monodomains. Thus, the original single crystalline character of the sample is destroyed at the phase transition; the .sample becomes polycrystalline. The main difference between Y-l 2 3 and La-2 I 4 is the orientation of the twinning elements. In both compounds, the twinning symmetry elements are the (1 1 0) and the (1 -1 0) planes. In Y-l 2 3, they are oriented along the diagonals of the Cu-0 squares, but in La-2 1 4, they are parallel to the sides of the squares [7]. Thus, the twin boundaries lead to quite different structural situations and to different conditions for flux pinning in both compounds. In summary, neutron diffraction by the flux line lattice in a big high quality La-2 1 4 single crystal was investigated. We obtain diffraction patterns showing a ring at different magnetic fields and temperatures, which proves the existence of a polycrystalline flux line lattice in this compound. The most obvious reason for the formation of a polycrystalline FLL is the low temperature crystal structure of the sample, which becomes polycrystalline below a certain transition temperature because of uncontrolled twinning. The scattered intensity is only weak because of the polycrystallinity
SINGLE CRYSTALS
293
and because of the large magnetic penetration depth, which makes a systematic study of the intensity as a function of field and temperature very difficult for this compound. Acknowledgements-We gratefully acknowledge financial support by the Institute Laue Langevin, Grenoble. REFERENCES Weber, H.W., Schelten, J. and Lippmann, G., J. Low. Temp. Phys., 16, 1974, 367. 2. Forgan, E.M., McKPaul, D., Mook, H.A., Timmins, P.A., Keller, H., Sutton, S. and Abell, J.S., Nature, 343, 1990,735. 3. Brandstatter, G., Weber, H.W., Chattopadhyay, T., Cubitt, R., Fischer, H., Wylie, M., Emel’chenko, G.A. and Wiedenmann, A., J. Appl. Qvst., 30, 1997,571. 4. Cubit& R., Forgan, E.M., Yang, G., Lee, S.L., McKPaul, D., Mook, H.A., Yethiraj, M., Kes, P.H., Li, T.W., Menoresky, A.A., Tarnawski, Z. and Mortensen, K., Nature, 365, 1993,407. 5. Sasagawa, T., Okuya, M., Shimoyama, J., Kishio, K. and Kitazawa, K., J. Low Temp. Phys., 105, 1996, 1201. 6. Li, Q., Suenaga, M., Kimura, T. and Kishio, K., Phys. Rev., B47, 1993, 11384. 7. Braden, M., Heger, G., Schweiss, P., Fisk, Z., Gamayunov, K., Tanaka, I. and Kojima, H., Physica, C191, 1992, 455. 8. Schmidt, H., Burkhardt, E., Nan Sun, Bing and Rivera, J.P., Physica, Cl57, 1989, 555. 1.