- Email: [email protected]
Evidence against bulk superconductivity in the high temperature superconductor La1.85Sr0.15CuO4-y
Volume 122, number 3,4
PHYSICS LETTERS A
8 June 1987
EVIDENCE AGAINST BULK SUPERCONDUCTIVITY IN THE HIGH TEMPERATURE SUPERCONDUCTOR La1.85Sr015 CuO4~ R. WAPPLING, 0. HARTMANN Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden
J.P. SENATEUR, R. MADAR, A. ROUAULT INPG, ENSPG, Laboratoire des Materiaux et du Genie Physique. UA 1109,BP 46, 38402 Saint Martin d’Hères Cedex, France
and A. YAOUANC CENG, DRF/SPh/MDIH, 85X, 38041 Grenoble Cedex, France Received 28 April 1987; accepted for publication 1 May 1987 Communicated by J.I. Budnick
From ~tSRinvestigations of the high temperature superconductor La 1 85Sr01 5CuO4 ,. it is shown that there is a very small part of the sample that exhibits a well developed Meissner effect. This indicates that the sample, although it is showing the characteristic drop in resistivity and change in susceptibility at the superconducting transition, does not contain “bulk” superconducting regions. The penetration depth ofthe externally applied magnetic field can be estimated from the change in line width ofthe ~sSR signal to be about 2300 A and its temperature dependence can be interpreted in terms of the BCS theory.
1. Introduction During the last half year there has been a considerable interest in the electrical conductivity of the quartenary metal oxides of chemical composition RE2 _~X~Cu04 —y with RE being La or Y, X = Ba, Sr or Ca, x around 0.15 and the oxygen-deficiency y varying with the sample preparation procedure. The reason for the interest in the physical properties is to be found in the unusually strong reduction in the resistivity at temperatures above 30 K. A number of recent publications have, spurred by the first articles of the IBM Zurich group [1,2], from resistivity and susceptibility measurements presented evidence that the samples become superconductive at the unusually high temperatures quoted [3—6] and these findings have been further substantiated by structural considerations [7] as well as theoretical arguments [8,91. The yttrium compounds reveal even more exotic transition temperatures, being in the liq-
uid nitrogen range [10,111. The purpose of this letter is to show that the suggested bulk superconductivity is not supported by our j.tSR data although there is a change in the parameters, in particular the damping (line width), at the proposed phase transition that can be interpreted in terms of the development of surface superconductivity. During the last few months a large number of publications have been prepared on this subject at different laboratories as judged by the activities at the recent APS meeting. Up to now the authors agree quite well on the superconducting transition temperatures (La1.85Sro.15CuO4_~:T~—~ 38 Kby resistive measurements, T~ 35 K by conductive measurements) but there is still some discrepancy as to the mechanism of superconductivity in these materials: is the superconductivity a bulk property or a surface or interface effect? In order to check how the magnetic field falls off in these materials, we have carried ‘~
209
Volume 122, number 3,4
PHYSICS LETTERS A
out a isSR experiment on a sample with nominal stoichiometry La1 85Sr0 5Cu04 —y (y being small).
8 June 1987
isation has been carried out some days after the experiment. 3.1. X-ray dtffraction
2. Experimental The typical X-ray diffraction pattern ofthe sample after the tSR experiment is reported in fig. 1. The dominant phase has the tetragonal K2NiF4 structure, Samples with nominal metal ratio La1 85/Sr001 5/Cu with unit cell parameter in goodagreement with those have been synthesised by the powder diffusion techreported by other authors (see e.g. ref. [121): nique described in many publications. The samples —o 37760 5 1 3238 1 were obtained by mixing carefully La(OH)3 (purity a ( ) nm, c— ( ) nm, 99.9%), SrCO3 (purity 99%) and CuO (purity 99°~~), / —3 506 grinding the material and heating in air at 750°Cfor ca one day. The samples (—~40 g) were then subjected The X-ray lines of the dominant phase are very to successive heat treatments at 1000°Cduring 5 to narrow, showing a very good homogeneity and a well 10 h. Between each heat treatment, the samples were crystallised compound, with crystallites >> 500 A powdered and pressed into pellets. The last stage of [13]. Weak extra lines are also visible in the specthe synthesis was to heat the samples at 1100°Cin trum, corresponding to minute amounts of La203 and air, followed by a fast cooling in air down to room a very weak single line which cannot be indexed on temperature. The compact obtained has, finally, been the K2NiF4 type structure but which can be related powdered for the ~.tSRexperiments. either to LaSrCu2O6 or to La2CuO4. A very surprising result is that the X-ray diffraction pattern of 2.2. Characterization La203 was not visible in the pattern before the ~tSR experiments, even using a very long exposure time The purity of the samples has been checked with in the Guinier—Hagg camera. This shows that the an X-ray diffractometer using Cu Ku radiation or, sample is very sensitive to air, but it is impossible to for accurate determination of the unit cell paramesay if the La203 present comes from the decompoters, with a high precision Guinier—Hagg photosition of the K2NiF4 type compound or from the graphic camera, using Cr Ku radiation and pure decomposition of an additional phase which cannot silicon as internal standard. be seen by X-ray diffraction (small amounts or bad The samples have also been characterised with a cristallisation). scanning electron microscope equipped with an energy dispersive micro-analyser. 3.2. Scanning electron microscopy The superconducting transition temperature has been determined by ac susceptibility measurements Scanning electron microscopy shows that the pow(100 and 1000 Hz). In order obtain a rough estimate der is composed of grains with sizes ranging from 1 of the amount of the superconducting phase, the ac to 10 Ism. A tentative microanalysis has been made, susceptibility signal has been calibrated using specbut the shape ofthe grains was too irregular to allow imens of pure Nb and Pb having the same volume reliable measurements. as the sample. 3.3. ac susceptibility measurements 2.1 Preparation
—
—
.
.
—
3. Results In order to be sure that the sample has not been drastically damaged during its exposure to air before and after the ~iSRexperiment, a second character210
The ac susceptibility versus temperature curve for the sample, obtained after the ~iSR experiment is presented in fig. 2. This curve is identical to the one recorded before the j.tSR studies, with a T~onset of 35.4 K and a midpoint of 33 K. The diamagnetic sig-
Volume 122, number 3,4
<
PHYSICS LETTERS A
I~.~15
8 June 1987
Ztheta ‘1
Fig. 1. X-ray diffraction spectra of La
1 85Sr0.1 5CuO4
79.975)
after the ~tSRexperiment:
‘~
K2NiF4,• extra line. The lines related to La,03 are
marked by bars.
nal reaches its maximum value at about 17 K and remains constant down to 4.2 K. The maximum signal has been determined at different frequencies (100 Hz and 1000 Hz) on 0.2 g of powder compacted to 55% of the theoretical density. The ac susceptibility signal (without corrections) reaches 25% of the signal given by the same volume of massive niobium, showing that, even after the sample has been damaged by its exposure to air, the amount of superconducting phase has not changed drastically. Note also that the major part of XAC arbitrary units
j
/ / / /
~/ 1O1~* 18
22
26 30 34
38 42
46
Fig. 2. ac susceptibility versus temperature.
50 T K
the exposure to air has taken place after the muon experiments. 3.4. 9uSR The transverse field ~.tSR method consists of implanting positive muons into a sample and detecting the precession of the muon spins in the local magnetic field inside the material. In many cases, the local field is the externally applied magnetic field and measurable quantities are then frequency shifts, changes in the observable muon polarization (initial precession amplitude a0) and the spin relaxation (time evolution of the precession signal). The basic principles can be found in several review articles [14,15]. The sample which showed the best X-ray diffraction pattern and the highest ac susceptibility signal has been studied by the j.tSR method. The measurements were made in the UI muon beam at the 600 MeV SC at CERN, Geneva, Switzerland. The 33 g sample was inside a 13 g aluminium container and was cooled with a closed-cycle refrigerator. The minimum temperature obtained was 13 K (measured with a Si diode thermometer attached to the sample container). In the data analysis, the jsSR signal was fitted with two components, one of which (approx211
Volume 122, number 3,4
PHYSICS LETTERSA
8 June 1987
imately 1/3 of the signal) was coming from the aluminium container and the cryostat walls. This signal was explicitly determined in a separate dummy sample measurement.
volume ofthe sample should be free ofmagnetic field, while the muons in normal conducting regions ofthe sample should precess in the applied magnetic field. The results for the amplitude a0 of the precessing
The important points for this investigation of superconductors with positive muons are (i) the measurement is made in an applied magnetic field, (ii) the muons located in field-free regions will not show any precession, (iii) a sample with a mixture of normal and superconducting regions (e.g. as the result of the vortex structure in type II superconductors) has an inhomogeneous internal magnetic field distribution. A damped i.tSR signal is then obtained from the muons in the parts of the sample where the field penetrates. The main parameter of interest is then the amplitude of the precession signal in the superconducting state, which should approach zero if the Meissner effect is fully developed. For a type II superconductor both an effect on the amplitude and the damping should be expected. The amplitude parameter a0 is proportional to the non-superconducting volume of the sample since the muons are implanted at random over a large volume (several cubic cm in the present investigation). We do not know the final stopping site ofthe muon, but we assume with confidence that the muon is immobile, or at least diffusing very locally during the time of the measurements. This is supported by the non-zero damping observed also above T~. It should be noted that the muons are thermalized very quickly after entering the sample, and that no effects of local heating of the samples are to be expected in j.tSR experiments. In the metals where superconductivity has been studied with muons, e.g. Nb, V, Al, the observations have been in full agreement with known transition temperatures and other known superconducting properties.
component is seen in fig. 3a. This remains almost constant showing that only a small volume of the sample exhibits bulk Meissner effect. On the other hand, the ~.tSRdamping rate parameter a (fig. 3b) shows a strong increase in the transition region, mdicating the inhomogeneous field distribution within the sample in the superconducting state. The magnetic field dependence of the i.SR damping rate (linewidth) in the superconducting state (at 13 K) is irregular, at first increasing with field and at higher fields decreasing again. The precession frequency is shifted slightly to lower values below T~ and the shift is about 1% at 20 K. The experimental data can be well fitted by a gaussian damping function. A gaussian is the normal result from an inhomogeneous distribution of magnetic fields. It should be noted that the amplitudes given in fig. 3a may be influenced by deviations from an ideal lineshape although the gaussian fits the data very well, in particular at 100 mT.
4. Results Two series of measurements were made with external fields of 7.5 mT and 100 mT respectively, The sample was cooled in zero applied field to the lowest temperature (13—14 K) and the measurements made while going upwards in temperature. This should ensure that the superconducting bulk 212
5. Discussion As mentioned above, there is hardly any detectable Meissner effect as is evidenced by the constancy ofthe amplitude ofthe precession signal. On the other hand, the increased damping of the signal below T,~ is a clear indication that the magnetic field distribution inside the sample becomes inhomogeneous and the most straightforward interpretation of this feature is that the sample really is becoming superconducting at the transition. The irregular field dependence observed can be taken as the developement of a vortex structure. This is also in accordance with the more detailed field dependence data of the ETH Zurich group [16]. The magnetic field inhomogeneity is directly related to the vortex structure and can be utilized to estimate the magnetic field penetration depth into the superconducting parts of the sample. An early study of this effect was made by Fiory et al. [7] on Nb. A simplified model of the field penetration can be described as follows: Assume that the triangular Abrikosov flux lattice can be
Volume 122, number 3,4
PHYSICS LETTERS A 18
8 June 1987
OAO (7.5 rnT)
.
•AO (100
~,
.16
~
a
~
5
10
15
20
25
30
35
40
45
50
55
60
65
Temperature (K) 1.6
OB-7.5mT
.
•B=lOOmT
b
1
;
:
~
0
5
10
1~ 20
25 30 35 40 Temperature (K)
45
50
55
60
65
Fig. 3. (a) Temperature dependence of the amplitude of the ~tSRprecession signal. (b) The ~iSR damping parameter as function of temperature.
replaced by a square one. Since each flux quantum, 0, corresponds to 2 x 10-15 V s, the application of an external field of 7.5 mT (100 mT) leads to a vortex-to-vortex distance, d, of about 5500 A (1400 A). The mean square average ofthe inhomogeneous field <(~.B)2>05 is given by [18] /
‘AB’2
\‘.
.‘
\ /
0.5
—B —
(4 exi’.
\
x [1 + (2icA/d)2J°5
—0.5
~“/1’ ~
where A is the penetration depth. Taking the measured a to be equal to y~, Ym being the gyromagnetic ratio of the muon ~ = 2ir x 133.5 MHz/T, one can directly calculate the penetration depth from the measured muon spin relaxation rate. Since the saturation damping rate seems to be the same at the two different applied fields (fig. 3b) the formula above
indicates that d and A are of the same order of magnitude. Taking the saturation value for the damping parameter a to be 1.4 ~ss’ one finds a value for the penetration depth of 2300 A at 100 mT applied field. The same analysis gives, however, a much larger value (7000 A) at 7.5 mT. It is reasonable to assume that the penatration depth should increase with .
.
increasing applied field and the damping rate data do, therefore, favour an interpretation in terms of only a very limited fraction of the sample being superconducting. A further support for this interpretation comes from a separate experiment in which the sample was cooled in the presence of an applied field of 100 mT through the superconducting transition and the precession pattern recorded at 13 K after the field had been reduced to 5 mT. No frozen in field could be detected. 213
Volume 122, number 3,4
PHYSICS LETTERS A
The temperature variation of the damping parameter allows a determination of the temperature variation of A and the variation obtained shows the dependence expected for a BCS superconductor. 6. Conclusion The present study indicates that the superconductivity in the La—Sr—Cu oxides may be of surface or interface character although the properties of a type II superconductor is retained.
8 June 1987
[4] R.J. Cava, R.B. van Dover, B. Batlogg and E.A. Rietman, Phys. Rev. Lett. 28 (1987) 408. [5] C. Politis, J. Geerk, M. Dietrich and B. Obst, Z. Phys. B 66 (1987) 141. [6] T. Uchida, Japan. J. AppI. Phys. 26 (1987) Ll. [7]J.D. Jorgensen, H-B. SchOttler, D.G. Hinks, D.W. Capone, H.K. Zhang, M.B. Brodsky and D.J. Scalapino, Phys. Rev. Lett. 58 (1987) 1024. [8] L.F. Mattheiss, Phys. Rev. Lett. 58 (1987) 1028. [9] J. Yu, A.J. FreemanandJ.-H. Xu,Phys. Rev. Lett. 58 (1987) 1035. [10] M.K. Wu, J.R. Ashborn, C.J. Thorng, PH. Hor, R.L. Meng, L. Gao, Z.H. Huang, Y.Q. Wang and C.H. Chu, Phys. Rev. (1987) 908.R.L. Meng, Z.J. Huang, Y.Q. Wang, K. [11] Lett. P.H. 58 Hor, L. Gao, Forster, J. Vassilious, C.W. Chu, M.K. Wu, J.R. Ashburn and C.J. Thorn, Phys Rev. Lett. 58 (1987) 911.
Acknowledgement The experimental assistance from Miss S. Harris is deeply appreciated.
References [I] J.G. Bednorz and K.A. MUller, Z. Phys. B 64 (1986) 189. [21 J.G. Bednorz, M. Takashige and K.A. MUller, Europhys. Lett. 3 (1987) 379. [3] C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang and Y.Q. Wang, Phys. Rev. Lett. 58 (1987) 405.
214
[12] C. Michel and B. Raveau, Rev. Chim. Miner. 21(1984) 407. [13] A. Guinier, Theorie et technique de Ia radiocristallographie (Dunod, Paris). [14]E. Karlsson, Phys. Rep. 82(1982) 271. [15] A. Schenck, Muon spin rotation spectroscopy (Hilger, Bristol, 1985). [16] A. Schenck, private communication. [17] A.T. Fiory, D.E. Murnick, M. Leventahi and W.J. Kossler, Phys. Rev. Lett. 33 (1974) 969. [18] P. Pincus, A.C. Gossard, V. Jaccarino and J.H. Wernick, Phys. Lett. 13 (1964) 21.
PHYSICS LETTERS A
8 June 1987
EVIDENCE AGAINST BULK SUPERCONDUCTIVITY IN THE HIGH TEMPERATURE SUPERCONDUCTOR La1.85Sr015 CuO4~ R. WAPPLING, 0. HARTMANN Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden
J.P. SENATEUR, R. MADAR, A. ROUAULT INPG, ENSPG, Laboratoire des Materiaux et du Genie Physique. UA 1109,BP 46, 38402 Saint Martin d’Hères Cedex, France
and A. YAOUANC CENG, DRF/SPh/MDIH, 85X, 38041 Grenoble Cedex, France Received 28 April 1987; accepted for publication 1 May 1987 Communicated by J.I. Budnick
From ~tSRinvestigations of the high temperature superconductor La 1 85Sr01 5CuO4 ,. it is shown that there is a very small part of the sample that exhibits a well developed Meissner effect. This indicates that the sample, although it is showing the characteristic drop in resistivity and change in susceptibility at the superconducting transition, does not contain “bulk” superconducting regions. The penetration depth ofthe externally applied magnetic field can be estimated from the change in line width ofthe ~sSR signal to be about 2300 A and its temperature dependence can be interpreted in terms of the BCS theory.
1. Introduction During the last half year there has been a considerable interest in the electrical conductivity of the quartenary metal oxides of chemical composition RE2 _~X~Cu04 —y with RE being La or Y, X = Ba, Sr or Ca, x around 0.15 and the oxygen-deficiency y varying with the sample preparation procedure. The reason for the interest in the physical properties is to be found in the unusually strong reduction in the resistivity at temperatures above 30 K. A number of recent publications have, spurred by the first articles of the IBM Zurich group [1,2], from resistivity and susceptibility measurements presented evidence that the samples become superconductive at the unusually high temperatures quoted [3—6] and these findings have been further substantiated by structural considerations [7] as well as theoretical arguments [8,91. The yttrium compounds reveal even more exotic transition temperatures, being in the liq-
uid nitrogen range [10,111. The purpose of this letter is to show that the suggested bulk superconductivity is not supported by our j.tSR data although there is a change in the parameters, in particular the damping (line width), at the proposed phase transition that can be interpreted in terms of the development of surface superconductivity. During the last few months a large number of publications have been prepared on this subject at different laboratories as judged by the activities at the recent APS meeting. Up to now the authors agree quite well on the superconducting transition temperatures (La1.85Sro.15CuO4_~:T~—~ 38 Kby resistive measurements, T~ 35 K by conductive measurements) but there is still some discrepancy as to the mechanism of superconductivity in these materials: is the superconductivity a bulk property or a surface or interface effect? In order to check how the magnetic field falls off in these materials, we have carried ‘~
209
Volume 122, number 3,4
PHYSICS LETTERS A
out a isSR experiment on a sample with nominal stoichiometry La1 85Sr0 5Cu04 —y (y being small).
8 June 1987
isation has been carried out some days after the experiment. 3.1. X-ray dtffraction
2. Experimental The typical X-ray diffraction pattern ofthe sample after the tSR experiment is reported in fig. 1. The dominant phase has the tetragonal K2NiF4 structure, Samples with nominal metal ratio La1 85/Sr001 5/Cu with unit cell parameter in goodagreement with those have been synthesised by the powder diffusion techreported by other authors (see e.g. ref. [121): nique described in many publications. The samples —o 37760 5 1 3238 1 were obtained by mixing carefully La(OH)3 (purity a ( ) nm, c— ( ) nm, 99.9%), SrCO3 (purity 99%) and CuO (purity 99°~~), / —3 506 grinding the material and heating in air at 750°Cfor ca one day. The samples (—~40 g) were then subjected The X-ray lines of the dominant phase are very to successive heat treatments at 1000°Cduring 5 to narrow, showing a very good homogeneity and a well 10 h. Between each heat treatment, the samples were crystallised compound, with crystallites >> 500 A powdered and pressed into pellets. The last stage of [13]. Weak extra lines are also visible in the specthe synthesis was to heat the samples at 1100°Cin trum, corresponding to minute amounts of La203 and air, followed by a fast cooling in air down to room a very weak single line which cannot be indexed on temperature. The compact obtained has, finally, been the K2NiF4 type structure but which can be related powdered for the ~.tSRexperiments. either to LaSrCu2O6 or to La2CuO4. A very surprising result is that the X-ray diffraction pattern of 2.2. Characterization La203 was not visible in the pattern before the ~tSR experiments, even using a very long exposure time The purity of the samples has been checked with in the Guinier—Hagg camera. This shows that the an X-ray diffractometer using Cu Ku radiation or, sample is very sensitive to air, but it is impossible to for accurate determination of the unit cell paramesay if the La203 present comes from the decompoters, with a high precision Guinier—Hagg photosition of the K2NiF4 type compound or from the graphic camera, using Cr Ku radiation and pure decomposition of an additional phase which cannot silicon as internal standard. be seen by X-ray diffraction (small amounts or bad The samples have also been characterised with a cristallisation). scanning electron microscope equipped with an energy dispersive micro-analyser. 3.2. Scanning electron microscopy The superconducting transition temperature has been determined by ac susceptibility measurements Scanning electron microscopy shows that the pow(100 and 1000 Hz). In order obtain a rough estimate der is composed of grains with sizes ranging from 1 of the amount of the superconducting phase, the ac to 10 Ism. A tentative microanalysis has been made, susceptibility signal has been calibrated using specbut the shape ofthe grains was too irregular to allow imens of pure Nb and Pb having the same volume reliable measurements. as the sample. 3.3. ac susceptibility measurements 2.1 Preparation
—
—
.
.
—
3. Results In order to be sure that the sample has not been drastically damaged during its exposure to air before and after the ~iSRexperiment, a second character210
The ac susceptibility versus temperature curve for the sample, obtained after the ~iSR experiment is presented in fig. 2. This curve is identical to the one recorded before the j.tSR studies, with a T~onset of 35.4 K and a midpoint of 33 K. The diamagnetic sig-
Volume 122, number 3,4
<
PHYSICS LETTERS A
I~.~15
8 June 1987
Ztheta ‘1
Fig. 1. X-ray diffraction spectra of La
1 85Sr0.1 5CuO4
79.975)
after the ~tSRexperiment:
‘~
K2NiF4,• extra line. The lines related to La,03 are
marked by bars.
nal reaches its maximum value at about 17 K and remains constant down to 4.2 K. The maximum signal has been determined at different frequencies (100 Hz and 1000 Hz) on 0.2 g of powder compacted to 55% of the theoretical density. The ac susceptibility signal (without corrections) reaches 25% of the signal given by the same volume of massive niobium, showing that, even after the sample has been damaged by its exposure to air, the amount of superconducting phase has not changed drastically. Note also that the major part of XAC arbitrary units
j
/ / / /
~/ 1O1~* 18
22
26 30 34
38 42
46
Fig. 2. ac susceptibility versus temperature.
50 T K
the exposure to air has taken place after the muon experiments. 3.4. 9uSR The transverse field ~.tSR method consists of implanting positive muons into a sample and detecting the precession of the muon spins in the local magnetic field inside the material. In many cases, the local field is the externally applied magnetic field and measurable quantities are then frequency shifts, changes in the observable muon polarization (initial precession amplitude a0) and the spin relaxation (time evolution of the precession signal). The basic principles can be found in several review articles [14,15]. The sample which showed the best X-ray diffraction pattern and the highest ac susceptibility signal has been studied by the j.tSR method. The measurements were made in the UI muon beam at the 600 MeV SC at CERN, Geneva, Switzerland. The 33 g sample was inside a 13 g aluminium container and was cooled with a closed-cycle refrigerator. The minimum temperature obtained was 13 K (measured with a Si diode thermometer attached to the sample container). In the data analysis, the jsSR signal was fitted with two components, one of which (approx211
Volume 122, number 3,4
PHYSICS LETTERSA
8 June 1987
imately 1/3 of the signal) was coming from the aluminium container and the cryostat walls. This signal was explicitly determined in a separate dummy sample measurement.
volume ofthe sample should be free ofmagnetic field, while the muons in normal conducting regions ofthe sample should precess in the applied magnetic field. The results for the amplitude a0 of the precessing
The important points for this investigation of superconductors with positive muons are (i) the measurement is made in an applied magnetic field, (ii) the muons located in field-free regions will not show any precession, (iii) a sample with a mixture of normal and superconducting regions (e.g. as the result of the vortex structure in type II superconductors) has an inhomogeneous internal magnetic field distribution. A damped i.tSR signal is then obtained from the muons in the parts of the sample where the field penetrates. The main parameter of interest is then the amplitude of the precession signal in the superconducting state, which should approach zero if the Meissner effect is fully developed. For a type II superconductor both an effect on the amplitude and the damping should be expected. The amplitude parameter a0 is proportional to the non-superconducting volume of the sample since the muons are implanted at random over a large volume (several cubic cm in the present investigation). We do not know the final stopping site ofthe muon, but we assume with confidence that the muon is immobile, or at least diffusing very locally during the time of the measurements. This is supported by the non-zero damping observed also above T~. It should be noted that the muons are thermalized very quickly after entering the sample, and that no effects of local heating of the samples are to be expected in j.tSR experiments. In the metals where superconductivity has been studied with muons, e.g. Nb, V, Al, the observations have been in full agreement with known transition temperatures and other known superconducting properties.
component is seen in fig. 3a. This remains almost constant showing that only a small volume of the sample exhibits bulk Meissner effect. On the other hand, the ~.tSRdamping rate parameter a (fig. 3b) shows a strong increase in the transition region, mdicating the inhomogeneous field distribution within the sample in the superconducting state. The magnetic field dependence of the i.SR damping rate (linewidth) in the superconducting state (at 13 K) is irregular, at first increasing with field and at higher fields decreasing again. The precession frequency is shifted slightly to lower values below T~ and the shift is about 1% at 20 K. The experimental data can be well fitted by a gaussian damping function. A gaussian is the normal result from an inhomogeneous distribution of magnetic fields. It should be noted that the amplitudes given in fig. 3a may be influenced by deviations from an ideal lineshape although the gaussian fits the data very well, in particular at 100 mT.
4. Results Two series of measurements were made with external fields of 7.5 mT and 100 mT respectively, The sample was cooled in zero applied field to the lowest temperature (13—14 K) and the measurements made while going upwards in temperature. This should ensure that the superconducting bulk 212
5. Discussion As mentioned above, there is hardly any detectable Meissner effect as is evidenced by the constancy ofthe amplitude ofthe precession signal. On the other hand, the increased damping of the signal below T,~ is a clear indication that the magnetic field distribution inside the sample becomes inhomogeneous and the most straightforward interpretation of this feature is that the sample really is becoming superconducting at the transition. The irregular field dependence observed can be taken as the developement of a vortex structure. This is also in accordance with the more detailed field dependence data of the ETH Zurich group [16]. The magnetic field inhomogeneity is directly related to the vortex structure and can be utilized to estimate the magnetic field penetration depth into the superconducting parts of the sample. An early study of this effect was made by Fiory et al. [7] on Nb. A simplified model of the field penetration can be described as follows: Assume that the triangular Abrikosov flux lattice can be
Volume 122, number 3,4
PHYSICS LETTERS A 18
8 June 1987
OAO (7.5 rnT)
.
•AO (100
~,
.16
~
a
~
5
10
15
20
25
30
35
40
45
50
55
60
65
Temperature (K) 1.6
OB-7.5mT
.
•B=lOOmT
b
1
;
:
~
0
5
10
1~ 20
25 30 35 40 Temperature (K)
45
50
55
60
65
Fig. 3. (a) Temperature dependence of the amplitude of the ~tSRprecession signal. (b) The ~iSR damping parameter as function of temperature.
replaced by a square one. Since each flux quantum, 0, corresponds to 2 x 10-15 V s, the application of an external field of 7.5 mT (100 mT) leads to a vortex-to-vortex distance, d, of about 5500 A (1400 A). The mean square average ofthe inhomogeneous field <(~.B)2>05 is given by [18] /
‘AB’2
\‘.
.‘
\ /
0.5
—B —
(4 exi’.
\
x [1 + (2icA/d)2J°5
—0.5
~“/1’ ~
where A is the penetration depth. Taking the measured a to be equal to y~, Ym being the gyromagnetic ratio of the muon ~ = 2ir x 133.5 MHz/T, one can directly calculate the penetration depth from the measured muon spin relaxation rate. Since the saturation damping rate seems to be the same at the two different applied fields (fig. 3b) the formula above
indicates that d and A are of the same order of magnitude. Taking the saturation value for the damping parameter a to be 1.4 ~ss’ one finds a value for the penetration depth of 2300 A at 100 mT applied field. The same analysis gives, however, a much larger value (7000 A) at 7.5 mT. It is reasonable to assume that the penatration depth should increase with .
.
increasing applied field and the damping rate data do, therefore, favour an interpretation in terms of only a very limited fraction of the sample being superconducting. A further support for this interpretation comes from a separate experiment in which the sample was cooled in the presence of an applied field of 100 mT through the superconducting transition and the precession pattern recorded at 13 K after the field had been reduced to 5 mT. No frozen in field could be detected. 213
Volume 122, number 3,4
PHYSICS LETTERS A
The temperature variation of the damping parameter allows a determination of the temperature variation of A and the variation obtained shows the dependence expected for a BCS superconductor. 6. Conclusion The present study indicates that the superconductivity in the La—Sr—Cu oxides may be of surface or interface character although the properties of a type II superconductor is retained.
8 June 1987
[4] R.J. Cava, R.B. van Dover, B. Batlogg and E.A. Rietman, Phys. Rev. Lett. 28 (1987) 408. [5] C. Politis, J. Geerk, M. Dietrich and B. Obst, Z. Phys. B 66 (1987) 141. [6] T. Uchida, Japan. J. AppI. Phys. 26 (1987) Ll. [7]J.D. Jorgensen, H-B. SchOttler, D.G. Hinks, D.W. Capone, H.K. Zhang, M.B. Brodsky and D.J. Scalapino, Phys. Rev. Lett. 58 (1987) 1024. [8] L.F. Mattheiss, Phys. Rev. Lett. 58 (1987) 1028. [9] J. Yu, A.J. FreemanandJ.-H. Xu,Phys. Rev. Lett. 58 (1987) 1035. [10] M.K. Wu, J.R. Ashborn, C.J. Thorng, PH. Hor, R.L. Meng, L. Gao, Z.H. Huang, Y.Q. Wang and C.H. Chu, Phys. Rev. (1987) 908.R.L. Meng, Z.J. Huang, Y.Q. Wang, K. [11] Lett. P.H. 58 Hor, L. Gao, Forster, J. Vassilious, C.W. Chu, M.K. Wu, J.R. Ashburn and C.J. Thorn, Phys Rev. Lett. 58 (1987) 911.
Acknowledgement The experimental assistance from Miss S. Harris is deeply appreciated.
References [I] J.G. Bednorz and K.A. MUller, Z. Phys. B 64 (1986) 189. [21 J.G. Bednorz, M. Takashige and K.A. MUller, Europhys. Lett. 3 (1987) 379. [3] C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang and Y.Q. Wang, Phys. Rev. Lett. 58 (1987) 405.
214
[12] C. Michel and B. Raveau, Rev. Chim. Miner. 21(1984) 407. [13] A. Guinier, Theorie et technique de Ia radiocristallographie (Dunod, Paris). [14]E. Karlsson, Phys. Rep. 82(1982) 271. [15] A. Schenck, Muon spin rotation spectroscopy (Hilger, Bristol, 1985). [16] A. Schenck, private communication. [17] A.T. Fiory, D.E. Murnick, M. Leventahi and W.J. Kossler, Phys. Rev. Lett. 33 (1974) 969. [18] P. Pincus, A.C. Gossard, V. Jaccarino and J.H. Wernick, Phys. Lett. 13 (1964) 21.