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139La NQR and NMR studies of the structural phase transitions in La1.8−xEu0.2SrxCuO4
PHYSICA® ELSEVIER
Physica C 341-348 (2000) 2127-2128 www.elsevier.nl/Iocate/physc
139La NQR and NMR studies of the structural phase transitions in Lal.s_xEuo.2SrxCuO4 B. J. Suh a* and P. C. Hammel b $ aDepartment of Physics, The Catholic University of Korea, Puchon, Korea bCondensed Matter and Thermal Physics, Los Alamos National Laboratory, Los Alamos, NM 87545 139La NQR and NMR relaxation measurements in the title compound have been used to investigate the dynamical properties of the structural phase transitions (SPT): HTT -+ LTO and LTO -+ LTT of lathanum cuprate. We present the data for thermodynamic fluctuations of critical modes in the vicinity of the SPT's which clearly reveal the character of each SPT and its dependence on doped hole concentration.
Understanding of the rich structural anomalies in high-Tc cuprates is crucial to understaning superconductivity itself. The observation of the intrinsic structural inhomogeneity and the sensitivity of superconductivity to structure in La2-z(Ba,Sr)xCuO4 [1] and in rare-earthdoped La2_~_uMuSr~CuO4 (M = Nd, Eu)[2] compounds emphasizes the critical role of structure. In La2_z_uMySr~Cu04, a sequence of structural phase transitions (SPT) occurs on lowering temperature: high-temperature tetragonal (HTT) --+ low-temperature orthorhombic (LTO) -+ low-temperature tetragonal (LTT) phase. The H T T ~ LTO is known as a second order SPT while the LTO ~ LTT as a first order due to the fact that the space groups D 18 for LTO and D41~ for LTT are not in the group-subgroup relation [3,4]. In addition, for small x, the LTO --+ LTT transition occurs from LTO to a less orthorhombic phase with P c c n symmetry followed by a continuous change to LTT phase [3,5]. We have performed 139La nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) relaxation measurements in Lal.s-~Eu0.2SrzCuOa to better understand the *This work was su pported by the Korea Science & Engineering Foundation (KOSEF) through the Grant No. 1999-2-114-005-5. t T h e work at Los Alamos was performed under the auspices of US D e p a r t m e n t of Energy.
character of the SPT's. The H T T -+ LTO transition is characterized as a second order SPT while the LTO -+ LTT in heavily hole-doped one as a first order and in lightly hole-doped one as a quasi secend order transition, respectively. Two polycrystalline samples of lightly St-doped Lal.s-zEu0.2Sr~Cu04 with x = 0.01 and 0.015 and two heavily doped single crystals with x = 0.13 and 0.2 were investigated. 139La nuclear spin-lattice relaxation rate T1-1 was measured by monitoring the recovery of the magnetization after saturation with single ~r/2 pulse. In lightly doped one, 139La (I = 27-)T1-1 was measured at the 2VQ(q -5 ~ -t-3) NQR transition, and in heavily doped one, T1-1 was obtained at the NMR central line (m = +1 ~ _½), respectively. Figures 1 show the results of the relaxation rate T~-1 vs temperature. Main features are: (i) strong enhancement of T1-1 with a peak at low T, and (ii) distinct behaviors of T1-1 in the vicinity of each SPT. The anomalous behavior of T1--1 at low T demonstrating an inhomogeneous slowing of spin dynamics is well discussed elsewhere [6]. In the following discussion, we will focus only on characterizing the structural phase trantions (SPT). The H T T --+ LTO transition was observable only for x = 0.2 sample among our selections in the temperature range (T < 300 K) investigated. As seen in Fig. l(b), the transition is character-
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)01023-6
2128
B.J Suh, PC. Hammel/Physica C 341-348 (2000) 2127-2128
.
.
.
.
T
i
1000
• Sr = 0.015
"7,
100
10
~
' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' '
(b) • Sr = 0 . 1 3 a Sr = 0.20
1000
'v--
THT
100
[]
10
0
50
100
150
200
250
300
Temperature (K) Figure 1. A semilog plot of 139La T11 vs T: (a) in lightly Sr-doped Lal.s-=Euo.2SrxCuO4 by NQR, and (b) in heavily Sr-doped one by NMR.
800
•
'1
. . . .
/LT 600
v
. . . .
I ' ' '
0 Sr : 0.01 • Sr = 0.13
0
%
400
0
o
.p.
0o
200
o
o 0
I
0
50
100
150
ized by a sharp peak at THT = 225 -I- 5 K. NMR and NQR T1-1 probes thermodynamic fluctuations of the critical modes at a phase transition. The enhancement of T1-1 is a typical result for phonon softening at a SPT reflecting the slowing down of the critical-mode fluctuations as the SPT is approached [7]. This implies that the HTT LTO transition is continuous, i.e., a second order. The sudden decrease in T~ ] of heavily doped samples at the LTO -+ LTT transition temperature TLT with essentially no enhancement is in contrast to the relative sharp peak o f T ~ 1 at THT. The contrasting behaviors of T~ 1 are in good agreement with the observation by other experiments and theory [2-5] that the LTO ~ LTT in heavily doped sample is discotinuous (first-order) transition in which there is no well-defined Tdependent, critical behavior of order parameter, while the HTT -+ LTO is a seconder order as discussed above. In lightly Sr-doped one, the LTO --+ LTT transition is accompanied by a strong enhancement of T1-1 with a sharp peak [Fig. l(a)]. This is obviously distinct from the behavior of heavily doped one as seen in Fig. 2. This strong and gradual increase of T]-1 is not expected from a discontinuous change of order parameter although the transition is observed to be very abrupt by xray diffraction [5]. Therefore, we characterize the LTO ~ LTT actually, the LTO -+ P c c n in lightly Sr-doped one as a "quasi" second order transition. We thank B. Bfichner and M. Hficker for sample preparation and helpful discussions.
TN
o
REFERENCES
l
%000 OOoo o 200
250
300
Temperature (K) Figure 2. A linear plot of 139La T1-1 for demonstrating the distinct features of TLT between lightly and heavily St-doped one. The small peak marked as TN is N6el temperature associated with magnetic ordering.
1. J.D. Axe et al., Phys. Rev. Lett. 62, 2751 (1989). 2. B. Bfichner et al., Phys. Rev. Lett. 73, 1841 (1994); Europhys. Lett. 21,953 (1993). 3. J.D. Axe and M.K. Crawford, J. Low Temp. Phys. 95,217 (1994). 4. A.Y. Cherny, Physica C 244, 129 (1995). 5. M.K. Crawford et aL, Phys. Rev. B 47, 11623 (1993). 6. B.J. Suh et al., Phys. Rev. B 59, R3952 (1999); unpublished. 7. A. Rigamonti, Adv. Phys. 33, 115 (1984).
Physica C 341-348 (2000) 2127-2128 www.elsevier.nl/Iocate/physc
139La NQR and NMR studies of the structural phase transitions in Lal.s_xEuo.2SrxCuO4 B. J. Suh a* and P. C. Hammel b $ aDepartment of Physics, The Catholic University of Korea, Puchon, Korea bCondensed Matter and Thermal Physics, Los Alamos National Laboratory, Los Alamos, NM 87545 139La NQR and NMR relaxation measurements in the title compound have been used to investigate the dynamical properties of the structural phase transitions (SPT): HTT -+ LTO and LTO -+ LTT of lathanum cuprate. We present the data for thermodynamic fluctuations of critical modes in the vicinity of the SPT's which clearly reveal the character of each SPT and its dependence on doped hole concentration.
Understanding of the rich structural anomalies in high-Tc cuprates is crucial to understaning superconductivity itself. The observation of the intrinsic structural inhomogeneity and the sensitivity of superconductivity to structure in La2-z(Ba,Sr)xCuO4 [1] and in rare-earthdoped La2_~_uMuSr~CuO4 (M = Nd, Eu)[2] compounds emphasizes the critical role of structure. In La2_z_uMySr~Cu04, a sequence of structural phase transitions (SPT) occurs on lowering temperature: high-temperature tetragonal (HTT) --+ low-temperature orthorhombic (LTO) -+ low-temperature tetragonal (LTT) phase. The H T T ~ LTO is known as a second order SPT while the LTO ~ LTT as a first order due to the fact that the space groups D 18 for LTO and D41~ for LTT are not in the group-subgroup relation [3,4]. In addition, for small x, the LTO --+ LTT transition occurs from LTO to a less orthorhombic phase with P c c n symmetry followed by a continuous change to LTT phase [3,5]. We have performed 139La nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) relaxation measurements in Lal.s-~Eu0.2SrzCuOa to better understand the *This work was su pported by the Korea Science & Engineering Foundation (KOSEF) through the Grant No. 1999-2-114-005-5. t T h e work at Los Alamos was performed under the auspices of US D e p a r t m e n t of Energy.
character of the SPT's. The H T T -+ LTO transition is characterized as a second order SPT while the LTO -+ LTT in heavily hole-doped one as a first order and in lightly hole-doped one as a quasi secend order transition, respectively. Two polycrystalline samples of lightly St-doped Lal.s-zEu0.2Sr~Cu04 with x = 0.01 and 0.015 and two heavily doped single crystals with x = 0.13 and 0.2 were investigated. 139La nuclear spin-lattice relaxation rate T1-1 was measured by monitoring the recovery of the magnetization after saturation with single ~r/2 pulse. In lightly doped one, 139La (I = 27-)T1-1 was measured at the 2VQ(q -5 ~ -t-3) NQR transition, and in heavily doped one, T1-1 was obtained at the NMR central line (m = +1 ~ _½), respectively. Figures 1 show the results of the relaxation rate T~-1 vs temperature. Main features are: (i) strong enhancement of T1-1 with a peak at low T, and (ii) distinct behaviors of T1-1 in the vicinity of each SPT. The anomalous behavior of T1--1 at low T demonstrating an inhomogeneous slowing of spin dynamics is well discussed elsewhere [6]. In the following discussion, we will focus only on characterizing the structural phase trantions (SPT). The H T T --+ LTO transition was observable only for x = 0.2 sample among our selections in the temperature range (T < 300 K) investigated. As seen in Fig. l(b), the transition is character-
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)01023-6
2128
B.J Suh, PC. Hammel/Physica C 341-348 (2000) 2127-2128
.
.
.
.
T
i
1000
• Sr = 0.015
"7,
100
10
~
' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' '
(b) • Sr = 0 . 1 3 a Sr = 0.20
1000
'v--
THT
100
[]
10
0
50
100
150
200
250
300
Temperature (K) Figure 1. A semilog plot of 139La T11 vs T: (a) in lightly Sr-doped Lal.s-=Euo.2SrxCuO4 by NQR, and (b) in heavily Sr-doped one by NMR.
800
•
'1
. . . .
/LT 600
v
. . . .
I ' ' '
0 Sr : 0.01 • Sr = 0.13
0
%
400
0
o
.p.
0o
200
o
o 0
I
0
50
100
150
ized by a sharp peak at THT = 225 -I- 5 K. NMR and NQR T1-1 probes thermodynamic fluctuations of the critical modes at a phase transition. The enhancement of T1-1 is a typical result for phonon softening at a SPT reflecting the slowing down of the critical-mode fluctuations as the SPT is approached [7]. This implies that the HTT LTO transition is continuous, i.e., a second order. The sudden decrease in T~ ] of heavily doped samples at the LTO -+ LTT transition temperature TLT with essentially no enhancement is in contrast to the relative sharp peak o f T ~ 1 at THT. The contrasting behaviors of T~ 1 are in good agreement with the observation by other experiments and theory [2-5] that the LTO ~ LTT in heavily doped sample is discotinuous (first-order) transition in which there is no well-defined Tdependent, critical behavior of order parameter, while the HTT -+ LTO is a seconder order as discussed above. In lightly Sr-doped one, the LTO --+ LTT transition is accompanied by a strong enhancement of T1-1 with a sharp peak [Fig. l(a)]. This is obviously distinct from the behavior of heavily doped one as seen in Fig. 2. This strong and gradual increase of T]-1 is not expected from a discontinuous change of order parameter although the transition is observed to be very abrupt by xray diffraction [5]. Therefore, we characterize the LTO ~ LTT actually, the LTO -+ P c c n in lightly Sr-doped one as a "quasi" second order transition. We thank B. Bfichner and M. Hficker for sample preparation and helpful discussions.
TN
o
REFERENCES
l
%000 OOoo o 200
250
300
Temperature (K) Figure 2. A linear plot of 139La T1-1 for demonstrating the distinct features of TLT between lightly and heavily St-doped one. The small peak marked as TN is N6el temperature associated with magnetic ordering.
1. J.D. Axe et al., Phys. Rev. Lett. 62, 2751 (1989). 2. B. Bfichner et al., Phys. Rev. Lett. 73, 1841 (1994); Europhys. Lett. 21,953 (1993). 3. J.D. Axe and M.K. Crawford, J. Low Temp. Phys. 95,217 (1994). 4. A.Y. Cherny, Physica C 244, 129 (1995). 5. M.K. Crawford et aL, Phys. Rev. B 47, 11623 (1993). 6. B.J. Suh et al., Phys. Rev. B 59, R3952 (1999); unpublished. 7. A. Rigamonti, Adv. Phys. 33, 115 (1984).