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Nuclear quadrupole resonance studies in Bi2Sr2CaCu2Ox
0038-1098/89$3.00+.00 Pergamon Press plc
Solid State Communications,Vol. 71, No. 10, pp. 835-837, 1989. Printed in Great Britain.
NuclearQuadrupoleResonanceStudiesin Bi2Sr2CaCu20x A.K.
Rajarajan, V.R. Palkar, N.C. Mishra*, M.S. Multani, R. Vijayaraghavan and L.C. Gupta
* Tata Instituteof FundamentalResearch,Bombay 400 005, INDIA Departmentof Physics,Utkal University,Bhubaneswar751 004, INDIA (ReceivedJune 23, 1989 by C.N.R.Rae)
at 4.2K in Bi Sr CaCu20x Nuclear quadrupole resonancemeasurements (T = 70K) are reported. A broad resonanceextendingfr& 13 MHz to 3O'MHzis observed.Relaxationtimes T, and T2 measuredat 22.5MHzare 30 msec. and 120 usec. respectively
Cu-0 chains, but some of the crystallographic phases of these systemshave T much higher than that of l-2-3sytems. It Es, therefore, of considerable interest to study NMR properties of Bi- and Tl-based systems and comparethem with those of l-2-3system. As a part of our program on NMR and NQR investigations of high-Tccuprates,we recently reported[lOI resultsof our Tl-NMRstudies in T12Ba~Ca2Cua0~0. In this note, we report resul s of u NQR-studiesin a polycrystalline sampleof Bi2Sr2CaCu20x (Tc= 70K) at 4.2K.
1. INTRODUCTION Nuclear quadrupole resonance (NQR) technique [l] offersa uniqueopportunity of studying the staticas well as the dynamical aspects of the superconductingstate. The principal advantage of this technique over nuclear magnetic resonance(NMR)is that the NQR experiment is performedin zero applied magneticfield unlikeNMR which is performedin the presenceof a magneticfield. This enables one to avoid any complications arisingdue to the application of a magnetic field. Nonequivalenceof the crystallographic sites of a given nuclear species results into a corresponding numberof resonance lines. The width of these lines could be taken as an index of the qualityof the material;the sharperthe lines, the better is the quality of the material. The dynamicsof the systemcould be studied by measuringthe relaxationtimes T1 and T2 of the nuclearspeciesof interestas a function of temperature. Througha detailed analysis of the relaxationdata it may be possible to infer whether or not the superconductor under investigation is a 'BCS'superconductor [2]. Considerable amountof NQR 'i-2-3' work has been reported on superconductors(YBa Cu 0 ) [3] and distinct resonanceshave been bs r ed from 'plane' and 'chain' copper ato!s '[:I. Energy gap as obtained from the relaxationstudieshas been found to be differentfor the two structurally non-equivalent coppersites [51.
2. EXPERIMENTAL Individually. dried Bi.&, CaCOJ, ,SrCOZ, CuO were welghed in the req i ed prop rtions o obtain the stoichiometryof Bi Sr CaCu 0 . Purityof all the compoundswas be&er2than2$. Intimately mixed polJderof the ingredients
Other cuprate-systems with T higher than 80K include those based on bism%h [61 and thallium[7]. The basic structuraldifference between 'I-2-3' system and those based on bismuth and thallium is that unlike in YBa Cu 0 -systems,there are no Cu-0 chainsin This difference is thege&xerials [8, 91. of fundamentalsignificance with respectto the chemical aspects of superconductivity in cuprates. Not only do the Bi- and Tl-based compoundsexhibitsuperconductivity without the
TEMPERATURE
(K)
Fig.1.AC-susceptibilityof the sampleused in this work. The diamagnetic signal, which sets in helm 70K, is in arbitrary units and appearsin our instruments as a positivevoltage. 835
NUCLEAR QUADRUPOLE RESONANCE STUDIES IN Bi2Sr2CaCu20x
836
Vol. 71, No. 10
was calcinedat 800 OC for 10 hours and the resulting powderwas pressedinto a disc under a pressureof 10 tons/inchcand sinteredat 860 % for 10 hours. The pelletwas then crushed into a fine pmder, re-pelletized and sintered again at 800 % for a further10 hours. X-ray diffraction of our sample was measured using Cu-Kol radiationwith a nickel filter and was found to be in agreement with that publishedin literature. No secondphase was discerniblefrom the observed diffraction pattern. AC-susceptibility, Fig.1, shcws an onset of diamagnetism at 70 K whereasT , as determined from the four-probe resisgivity measurements is about 68 K (zeroresistance). FREQ
NQR measurementswere carriedout on a well crushedsampleenclosedin a glass capsule of about 81mndiameter. The samplecoil was dippeddirectlyin liquidHe bath. A coherent pulsednuclearmagneticresonance spectrometer was used to observethe nuclearquadrupolespin echo signalsas a functionof frequency. T, and T were measuredusing standard pulseseque&es. 3. RESULTS AND DISCUSSIONS
Nuclear quadrupole resonance spin echo (SE) signalswere observedat 4.2 K. Figure 2 shows an SE signal, which is an averageof 500 scans, at 22.5 MHz. Amplitudeof the-echowas recorded as a function of frequency the interval18 MHz - 30 I%-lz.Figure 3 in shows the profileof the resonancespectrum as a functionof frequency. Relaxation times T, (snin lattice relaxationtime) and Tz (transverse . relaxation time) were measureda 22.5 MHz using standard pulse sequences. Spin latticerelaxationtime was determined using a three pulse Tl - 'TI, and monitoring the sequence,71- Tl- m/2--r ? magnetization as a recovery of nuclea function of the delayTl. These measurements led to a value of 30 msec. for T at 4.2K. T2 was measuredby monitoringthe stJe&h of the
TIME
(/AS)
Fig.2 Spin echo signalobservedat 22.5 MHz. The signalhas to be averagedover many scans beforeit can be observedwith a good S/N. The signalin this figureis an averageof 500 scans.
(MHz)
Fig.3.Resonance spectrum as a function of frequggcy. ThE5arrcwsmark the position of cu and Cu -NQR signals. The large width of the spectr& suggests -a distribution of the electric field gradients at the copper sites. The solid line is a guide to the eye. spin echo signal as a functionof the delay between the two pulses ( n/2- T -n) used to produce the SE signal.A value of about 120 !&ec.was obtainedfor T2. The largewidth of the spectrum, Fig.3, could be due to the poorly resolved lines originating from the coppersites having a distributionof the electric field gradients (EFG'S). Consideringthat the spectrumarises from cooper nuclei (see below also), one abundantj. If thesbectra are broadeneddue-to the spatialfluctuationof the EFG's, as seems to be the case in the presentwork, one may not see the distinctpeaks for the two isotopes. The frequenciesof the NQR signalsfor the two isotopes quadrupoleofm~~~~~~ T;;61;3e;b3 rii;82Tf . . ""2; distribution of the EFG'smay partlybe due to the fact that the sample used in these investigationsmay not have an ideal chemical composition corresponding to the 2-2-l-2phase of Bi- Sr- Ca- Cu- 0 system. Further, there seems to be a partialdisorderamong Ca and Sr atoms [81 as is evidentfrom the distribution of atoms in the unit cell of the 2-2-l-2 phase as shcwn in Fig.4. Oxygenvacanciesat certain sites also may contribute, at leastpartially, to the resonance width, Various kinds of crystallographic structureshave been suggested of the 2-2-l-2 system [ll] which depend sensitivelyon the precisechemicalcomposition of the system. Therefore, fluctuationin the chemicalcomposition may be responsiblefor the largewidth of the observedresonance. We have broadening of observed similar line Ga-substituted material Cu-NQR [12], in This is essentiallydue to the ~~2cu~ii~~~~~~g10~isorder caused by introduction of Ga atoms into the lattice. The possibility of more than one crystallographic sites of copper in 2-2-I-2phasealso should not be ruled out.
Vol. 71, No. 10
NUCLEAR
QLJADRUPOLE RESONANCE
hrlrz
Sr CU
-
Cal%) CU
-
Sr
-
Bi
-
l -
Bi
.-
Sr,Ca
l
Bi
o-
- cu 0
-Sr
-
837
IN Bi2Sr2CaCu20x
We would like to point out that bismuth nucleus (I q 9/2) also is knckJnto give rise to nuclear ouadrupole resonances. For example in and Bi (GeO j _ Bi4(Si04)d, ,one observes- four NQ# s&!&s [I?] corresn ndinn to four nuclear transitions among the quadrupole split levels of Bi-nucleus. Tne possibility of observing Bi-resonance in Bi-based cuprates is also being examined.
Bi
-
STUDIES
cu
-Ca(Sr) -clJ
In conclusion, we have observed an NQR spectrum at 4.2K in a sample of Bi2Sr CaCu 0 in frequency range 18 MHz - 30 M$Iz" the The large width could be due to poorly resolved resonances originating from copper-sites with closely lying electric field gradients. Tne temperature dependence of the spin lattice T being relaxation time across is investigated. Similar work 8n samples of chemical composition close to that of
of Bi&ksonance in these system is also being examined.
-Sr -Bi
Fig.4. Unit cell of the structure of the Bi Sr CaCu 0 -system. Partial distribu&og of* ??aand Sr is suggested at certain sites of the unit cell.
REFERENCES
1. R.
Vijayaraghavan, A.K. Rajarajan and L.C. Temperature Gupta in Studies of High Superconductors, Vo1.2, Ed. A.V. Narlikar , Nova Science Publishers, Inc. New York (1989).
2. D.E. MacLaughlin, Solid State Phys. 3,
1,
(19%). 3. See for example, many papers on nuclear quadrupole resonance on YBa Cu 0 - system presented at the Internationa1 C&?erence on High Temperature Superconductors and Materials and Mechanisms of Superconductivity, Interlaken, .&Switzerland 1988 Feb. 28, Eds. J. Muller and J.L. Olsen, North Holland, Amsterdam (1988). 4. Y. Kitaoka, S. Hiramatsu, T. Kondo and K. Asayama, J. Phys. Sot. Japan. 57, 30 (1988). Warren Jr., R.E. Walstedt, G.F. 5. W.W. Brennert, G.P. Espinosa, and J.P. Remeika, Phys. Rev. Lett. 59 1860 (1987). 6. H. Maeda, Y. Tanaka, M. Fukutoni and T. Asano, Japan J. Appl. Phys. 27_,L209 (1987). 7. Z.Z. Sheng and A.M. Hermann, Nature 55, 332, (1988).
M.A. Subramanian, C.C. Torardi, J.C. Calabrese, Gopalakrishnan,. K.J. J. Morrissev, T.R. Askew. R.B. FliDDen. U. Chmdhry- 'and A.W. Sieight, Science (USA), 239, 1015 (1988). S.S.P. Parkin, V.Y. Lee, E.M. Engler, A.I. Nazzal, T.C. Huang, G. German, R. Savoy and A. Beyers, Phys. Rev. Lett. @, 2539 (1988) 10.A.K. Rajarajan, K.V. Gopalakrishnan, R. Vijayaraghavan and L.C. Gupta, Solid State Communications,69, 213, (1989). ll.See,
for example O.N. Srivastava in the Proceedings of the International Symposium High Temperature Superconductivity, gford & IBM Publishing Co., New Delhi, India (1988).
12.A.K.Rajarajan,L.C. Gupta, R.Vijayaraghavan and N.C. Mishra, Submitted to the International Conference on Materials and Mechanisms of Superconductivity, High Temperature Superconductors, Stanford University, Stanford, July 23-28, (1989). 13.K.V. Gopalakrishnan, L.C. Gupta and Vijayaraghavan,Pramana 5, 243 (1976).
R.
Solid State Communications,Vol. 71, No. 10, pp. 835-837, 1989. Printed in Great Britain.
NuclearQuadrupoleResonanceStudiesin Bi2Sr2CaCu20x A.K.
Rajarajan, V.R. Palkar, N.C. Mishra*, M.S. Multani, R. Vijayaraghavan and L.C. Gupta
* Tata Instituteof FundamentalResearch,Bombay 400 005, INDIA Departmentof Physics,Utkal University,Bhubaneswar751 004, INDIA (ReceivedJune 23, 1989 by C.N.R.Rae)
at 4.2K in Bi Sr CaCu20x Nuclear quadrupole resonancemeasurements (T = 70K) are reported. A broad resonanceextendingfr& 13 MHz to 3O'MHzis observed.Relaxationtimes T, and T2 measuredat 22.5MHzare 30 msec. and 120 usec. respectively
Cu-0 chains, but some of the crystallographic phases of these systemshave T much higher than that of l-2-3sytems. It Es, therefore, of considerable interest to study NMR properties of Bi- and Tl-based systems and comparethem with those of l-2-3system. As a part of our program on NMR and NQR investigations of high-Tccuprates,we recently reported[lOI resultsof our Tl-NMRstudies in T12Ba~Ca2Cua0~0. In this note, we report resul s of u NQR-studiesin a polycrystalline sampleof Bi2Sr2CaCu20x (Tc= 70K) at 4.2K.
1. INTRODUCTION Nuclear quadrupole resonance (NQR) technique [l] offersa uniqueopportunity of studying the staticas well as the dynamical aspects of the superconductingstate. The principal advantage of this technique over nuclear magnetic resonance(NMR)is that the NQR experiment is performedin zero applied magneticfield unlikeNMR which is performedin the presenceof a magneticfield. This enables one to avoid any complications arisingdue to the application of a magnetic field. Nonequivalenceof the crystallographic sites of a given nuclear species results into a corresponding numberof resonance lines. The width of these lines could be taken as an index of the qualityof the material;the sharperthe lines, the better is the quality of the material. The dynamicsof the systemcould be studied by measuringthe relaxationtimes T1 and T2 of the nuclearspeciesof interestas a function of temperature. Througha detailed analysis of the relaxationdata it may be possible to infer whether or not the superconductor under investigation is a 'BCS'superconductor [2]. Considerable amountof NQR 'i-2-3' work has been reported on superconductors(YBa Cu 0 ) [3] and distinct resonanceshave been bs r ed from 'plane' and 'chain' copper ato!s '[:I. Energy gap as obtained from the relaxationstudieshas been found to be differentfor the two structurally non-equivalent coppersites [51.
2. EXPERIMENTAL Individually. dried Bi.&, CaCOJ, ,SrCOZ, CuO were welghed in the req i ed prop rtions o obtain the stoichiometryof Bi Sr CaCu 0 . Purityof all the compoundswas be&er2than2$. Intimately mixed polJderof the ingredients
Other cuprate-systems with T higher than 80K include those based on bism%h [61 and thallium[7]. The basic structuraldifference between 'I-2-3' system and those based on bismuth and thallium is that unlike in YBa Cu 0 -systems,there are no Cu-0 chainsin This difference is thege&xerials [8, 91. of fundamentalsignificance with respectto the chemical aspects of superconductivity in cuprates. Not only do the Bi- and Tl-based compoundsexhibitsuperconductivity without the
TEMPERATURE
(K)
Fig.1.AC-susceptibilityof the sampleused in this work. The diamagnetic signal, which sets in helm 70K, is in arbitrary units and appearsin our instruments as a positivevoltage. 835
NUCLEAR QUADRUPOLE RESONANCE STUDIES IN Bi2Sr2CaCu20x
836
Vol. 71, No. 10
was calcinedat 800 OC for 10 hours and the resulting powderwas pressedinto a disc under a pressureof 10 tons/inchcand sinteredat 860 % for 10 hours. The pelletwas then crushed into a fine pmder, re-pelletized and sintered again at 800 % for a further10 hours. X-ray diffraction of our sample was measured using Cu-Kol radiationwith a nickel filter and was found to be in agreement with that publishedin literature. No secondphase was discerniblefrom the observed diffraction pattern. AC-susceptibility, Fig.1, shcws an onset of diamagnetism at 70 K whereasT , as determined from the four-probe resisgivity measurements is about 68 K (zeroresistance). FREQ
NQR measurementswere carriedout on a well crushedsampleenclosedin a glass capsule of about 81mndiameter. The samplecoil was dippeddirectlyin liquidHe bath. A coherent pulsednuclearmagneticresonance spectrometer was used to observethe nuclearquadrupolespin echo signalsas a functionof frequency. T, and T were measuredusing standard pulseseque&es. 3. RESULTS AND DISCUSSIONS
Nuclear quadrupole resonance spin echo (SE) signalswere observedat 4.2 K. Figure 2 shows an SE signal, which is an averageof 500 scans, at 22.5 MHz. Amplitudeof the-echowas recorded as a function of frequency the interval18 MHz - 30 I%-lz.Figure 3 in shows the profileof the resonancespectrum as a functionof frequency. Relaxation times T, (snin lattice relaxationtime) and Tz (transverse . relaxation time) were measureda 22.5 MHz using standard pulse sequences. Spin latticerelaxationtime was determined using a three pulse Tl - 'TI, and monitoring the sequence,71- Tl- m/2--r ? magnetization as a recovery of nuclea function of the delayTl. These measurements led to a value of 30 msec. for T at 4.2K. T2 was measuredby monitoringthe stJe&h of the
TIME
(/AS)
Fig.2 Spin echo signalobservedat 22.5 MHz. The signalhas to be averagedover many scans beforeit can be observedwith a good S/N. The signalin this figureis an averageof 500 scans.
(MHz)
Fig.3.Resonance spectrum as a function of frequggcy. ThE5arrcwsmark the position of cu and Cu -NQR signals. The large width of the spectr& suggests -a distribution of the electric field gradients at the copper sites. The solid line is a guide to the eye. spin echo signal as a functionof the delay between the two pulses ( n/2- T -n) used to produce the SE signal.A value of about 120 !&ec.was obtainedfor T2. The largewidth of the spectrum, Fig.3, could be due to the poorly resolved lines originating from the coppersites having a distributionof the electric field gradients (EFG'S). Consideringthat the spectrumarises from cooper nuclei (see below also), one abundantj. If thesbectra are broadeneddue-to the spatialfluctuationof the EFG's, as seems to be the case in the presentwork, one may not see the distinctpeaks for the two isotopes. The frequenciesof the NQR signalsfor the two isotopes quadrupoleofm~~~~~~ T;;61;3e;b3 rii;82Tf . . ""2; distribution of the EFG'smay partlybe due to the fact that the sample used in these investigationsmay not have an ideal chemical composition corresponding to the 2-2-l-2phase of Bi- Sr- Ca- Cu- 0 system. Further, there seems to be a partialdisorderamong Ca and Sr atoms [81 as is evidentfrom the distribution of atoms in the unit cell of the 2-2-l-2 phase as shcwn in Fig.4. Oxygenvacanciesat certain sites also may contribute, at leastpartially, to the resonance width, Various kinds of crystallographic structureshave been suggested of the 2-2-l-2 system [ll] which depend sensitivelyon the precisechemicalcomposition of the system. Therefore, fluctuationin the chemicalcomposition may be responsiblefor the largewidth of the observedresonance. We have broadening of observed similar line Ga-substituted material Cu-NQR [12], in This is essentiallydue to the ~~2cu~ii~~~~~~g10~isorder caused by introduction of Ga atoms into the lattice. The possibility of more than one crystallographic sites of copper in 2-2-I-2phasealso should not be ruled out.
Vol. 71, No. 10
NUCLEAR
QLJADRUPOLE RESONANCE
hrlrz
Sr CU
-
Cal%) CU
-
Sr
-
Bi
-
l -
Bi
.-
Sr,Ca
l
Bi
o-
- cu 0
-Sr
-
837
IN Bi2Sr2CaCu20x
We would like to point out that bismuth nucleus (I q 9/2) also is knckJnto give rise to nuclear ouadrupole resonances. For example in and Bi (GeO j _ Bi4(Si04)d, ,one observes- four NQ# s&!&s [I?] corresn ndinn to four nuclear transitions among the quadrupole split levels of Bi-nucleus. Tne possibility of observing Bi-resonance in Bi-based cuprates is also being examined.
Bi
-
STUDIES
cu
-Ca(Sr) -clJ
In conclusion, we have observed an NQR spectrum at 4.2K in a sample of Bi2Sr CaCu 0 in frequency range 18 MHz - 30 M$Iz" the The large width could be due to poorly resolved resonances originating from copper-sites with closely lying electric field gradients. Tne temperature dependence of the spin lattice T being relaxation time across is investigated. Similar work 8n samples of chemical composition close to that of
of Bi&ksonance in these system is also being examined.
-Sr -Bi
Fig.4. Unit cell of the structure of the Bi Sr CaCu 0 -system. Partial distribu&og of* ??aand Sr is suggested at certain sites of the unit cell.
REFERENCES
1. R.
Vijayaraghavan, A.K. Rajarajan and L.C. Temperature Gupta in Studies of High Superconductors, Vo1.2, Ed. A.V. Narlikar , Nova Science Publishers, Inc. New York (1989).
2. D.E. MacLaughlin, Solid State Phys. 3,
1,
(19%). 3. See for example, many papers on nuclear quadrupole resonance on YBa Cu 0 - system presented at the Internationa1 C&?erence on High Temperature Superconductors and Materials and Mechanisms of Superconductivity, Interlaken, .&Switzerland 1988 Feb. 28, Eds. J. Muller and J.L. Olsen, North Holland, Amsterdam (1988). 4. Y. Kitaoka, S. Hiramatsu, T. Kondo and K. Asayama, J. Phys. Sot. Japan. 57, 30 (1988). Warren Jr., R.E. Walstedt, G.F. 5. W.W. Brennert, G.P. Espinosa, and J.P. Remeika, Phys. Rev. Lett. 59 1860 (1987). 6. H. Maeda, Y. Tanaka, M. Fukutoni and T. Asano, Japan J. Appl. Phys. 27_,L209 (1987). 7. Z.Z. Sheng and A.M. Hermann, Nature 55, 332, (1988).
M.A. Subramanian, C.C. Torardi, J.C. Calabrese, Gopalakrishnan,. K.J. J. Morrissev, T.R. Askew. R.B. FliDDen. U. Chmdhry- 'and A.W. Sieight, Science (USA), 239, 1015 (1988). S.S.P. Parkin, V.Y. Lee, E.M. Engler, A.I. Nazzal, T.C. Huang, G. German, R. Savoy and A. Beyers, Phys. Rev. Lett. @, 2539 (1988) 10.A.K. Rajarajan, K.V. Gopalakrishnan, R. Vijayaraghavan and L.C. Gupta, Solid State Communications,69, 213, (1989). ll.See,
for example O.N. Srivastava in the Proceedings of the International Symposium High Temperature Superconductivity, gford & IBM Publishing Co., New Delhi, India (1988).
12.A.K.Rajarajan,L.C. Gupta, R.Vijayaraghavan and N.C. Mishra, Submitted to the International Conference on Materials and Mechanisms of Superconductivity, High Temperature Superconductors, Stanford University, Stanford, July 23-28, (1989). 13.K.V. Gopalakrishnan, L.C. Gupta and Vijayaraghavan,Pramana 5, 243 (1976).
R.