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Unusual nature of electronic and magnetic states of Sm1.85Ce0.15CuO4−δ
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Physica C 341-348 (2000) 491-492 www.elsevier.nl/Iocate/physc
Unusual N a t u r e of Electronic and Magnetic States of Sm1.s5Ceo.15CuO4-6 B. K. Cho a, Jae Hoon Kim b, Young Jin Kim b, Beom-hoan O e, J. S. Kim d, G. R. Stewart d aDepartment of Materials Science and Engineering, K-JIST, Kwangju 500-712, Korea bDepartment of Physics, Yonsei University, Seoul 120-749, Korea CDepartment of Electronic Materials and Devices Engineering, Inha University, Inchon 402-751, Korea dDepartment of Physics, University of Florida, Gainesville, Florida 32611-8440, USA Temperature-dependent magnetization (M(T)) and specific heat (Cp(T)) measurements were carried out on single-crystal Sml.ssCeo.1sCuO4-6 (Tc=16.5K). The magnetic anisotropy in the static susceptibility, X =- M/H, is apparent not only in its magnitude but also in its temperature dependence, with X± for H .L c larger than XIt for H [] c. For both field orientations, X does not follow the Curie-Weiss behavior due to the small energy gap of the J -= 7/2 multiplet above the J = 5/2 ground-state multiplet. However, with increasing temperature, XII(T) exhibits a broad minimum near 100 K and then a slow increase while X± (T) shows a monotonic decrease. A sharp peak in Cp(T) at 4.7 K manifests an antiferromagnetic ordering. The electronic contribution, 7, to Cp(T) is estimated to be "r = 103.2 (7) mJ/mole.Sm.K 2. The entropy associated with the magnetic ordering is much smaller than Rln2, where R is the gas constant, which is usually expected for the doublet ground state of Sm +3. The unusual magnetic and electronic properties evident in M(T) and Cp(T) are probably due to a strong anisotropic interaction between conduction electrons and localized electrons at Sm +3 sites.
We have studied electron-doped superconducting Sml.s5Ce0.15CuO4-6 single crystal by temperature-dependent magnetization and specific heat measurements. Figure 1 shows the temperature-dependent magnetic susceptibility, x(T), for Sml.ssCe0.15CuO4-5 with H = 5 kG. The large anisotropy in x(T) between H I] c and H _l_ c is quite clear and the temperature dependence for both field orientations clearly deviate from the typical Curie-Weiss behavior. The van Vleck contribution, due to the small energy gap of the J = 7/2 multiplet above the J = 5/2 ground state, is considered to account for the observed magnetic susceptibility according to the standard formula
[ X = NA
L3kB(--T---O) + 7k---~-EJ "
(1)
The fitting results are unsatisfactory for both field orientations but apparently quite good for the powder-average case. From the fitting results of the powder-average data, the splitting A E , the effective moment freer, and the Curie-Weiss tem-
perature O are found to be 466 K, 0.36 #B, and - 6 . 4 K, respectively. The value of A E is smaller than that of S m 2 C u O 4 , ( ~ 1150 K) [1], which is probably due to doping of electrons by Ce +4 ions. With increasing temperature, XII(T) for H II c shows a broad local minimum around T = 100 K and a slow increase whereas the XJ_(T) for H _L c shows a monotonic decrease. A natural mechanism for this effect would be a non-negligible anisotropic hybridization of conduction electrons with localized Sm +3 orbitals and its angular dependence. The temperature-dependent specific heat, Cp(T), is plotted in Figure 2(a). Clear evidence of the antiferromagnetic transition in Sm1.ssCeo.15CuO4-5 is given by the sharp peak at 4.7 K and the superconducting transition is seen near Te ~ 16.5 K as a slight jump in Cp. In order to separate the magnetic and nonmagnetic contributions to Cp, the data for 10 K < T < 18 K is fitted to the usual equation CpNM(T) = v T + /3T 3. It is found that -y = 191.0 (7) mJ/mole.K 2
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)00557-8
B.K. Cho et al./Physica C 341-348 (2000) 491-492
492
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Figure 1. Anisotropic magnetic susceptibility X vs. temperature T of single-crystal Sml.s5Ceo.15Cu04-~ for H / c, H JJ c, and powder average. Fits to Equation (1) in the text are shown by the solid curves.
~, 1 x." ~
E° ~.
~
8 4
o
and fl = 1.3 (1) mJ/mole.K 4, yielding the Debye temperature OD ~ 219 K. The observed value of "7 = 103.2 (7) mJ/mole.Sm.K 2 is significantly larger than those found in other R2CuO4 compounds (T'-phase), where R = Pr, Nd, and Eu [2]. It is natural to judge that the enhanced "7 is due to the doped electrons. Even for Sm2CuO4, the "7 value was previously found to be exceptionally large (~, 82 mJ/mole.Sm.K 2) [2]. It was speculated that the effects of magnetic correlation exist well above TN ~ 5.9 K, thereby making accurate determination of "y difficult. However, our estimated ~/for Smx.ssCe0.15Cu04-~ is still quite large even though TN is now lowered to 4.7 K. The contribution of magnetic correlation to the measured Cp(T) is calculated as Cpmag(T) = Cp(T) - CpNM(T) and is plotted in Figure 2(b). The entropy associated with the magnetic transition is calculated from the Cpmag(T) and is plotted in the inset of Figure 2(b). The magnetic entropy saturates rapidly above TN to ~, 4.1 J/mole.K, indicating that the transition is driven by localized electrons. However, the accumulated entropy is clearly smaller than 1.85Rln2, which is the usual value for the doublet ground state of Sm +3 [3]. It was reported by Stewart [4] that the magnetic entropy associated with a magnetic transition is significantly reduced if the magnetic transition is due to itinerant heavy fermionic electrons in analogy with a BCS-type transition.
2
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0
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Temperature(K)
5 10 15 Temperature (K)
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Figure 2. (a) Specific heat Cp vs. temperature T of single-crystal Sml.ssCe0.15CuO4-5 for 1.5 K _< T < 19.0 K. (b) Magnetic specific heat (Cp mag) vs. temperature T. Inset: entropy associated with the magnetic transition vs. temperature. Thus, the reduced entropy can be explained by the fact that itinerant electrons with heavy effective mass are involved in the transition. This explanation is also consistent with the anisotropic temperature-dependent behavior of magnetization and the enhanced electronic contribution to specific heat. ACKNOWLEDGEMENT This work was supported by the Korea Science & Engineering Foundation through the grant No. 1999-2-114-005-5. REFERENCES
1. C. L. Seaman et al., Physica C 159, 391 (1989). 2. M. F. Hundley et al., Physica C 158, 102 (1989) 3. V. Nekvasil, Physica C 170, 469 (1990). 4. G. R. Stewart, Rev. Mod. Phys. 56, 755 (1984).
Physica C 341-348 (2000) 491-492 www.elsevier.nl/Iocate/physc
Unusual N a t u r e of Electronic and Magnetic States of Sm1.s5Ceo.15CuO4-6 B. K. Cho a, Jae Hoon Kim b, Young Jin Kim b, Beom-hoan O e, J. S. Kim d, G. R. Stewart d aDepartment of Materials Science and Engineering, K-JIST, Kwangju 500-712, Korea bDepartment of Physics, Yonsei University, Seoul 120-749, Korea CDepartment of Electronic Materials and Devices Engineering, Inha University, Inchon 402-751, Korea dDepartment of Physics, University of Florida, Gainesville, Florida 32611-8440, USA Temperature-dependent magnetization (M(T)) and specific heat (Cp(T)) measurements were carried out on single-crystal Sml.ssCeo.1sCuO4-6 (Tc=16.5K). The magnetic anisotropy in the static susceptibility, X =- M/H, is apparent not only in its magnitude but also in its temperature dependence, with X± for H .L c larger than XIt for H [] c. For both field orientations, X does not follow the Curie-Weiss behavior due to the small energy gap of the J -= 7/2 multiplet above the J = 5/2 ground-state multiplet. However, with increasing temperature, XII(T) exhibits a broad minimum near 100 K and then a slow increase while X± (T) shows a monotonic decrease. A sharp peak in Cp(T) at 4.7 K manifests an antiferromagnetic ordering. The electronic contribution, 7, to Cp(T) is estimated to be "r = 103.2 (7) mJ/mole.Sm.K 2. The entropy associated with the magnetic ordering is much smaller than Rln2, where R is the gas constant, which is usually expected for the doublet ground state of Sm +3. The unusual magnetic and electronic properties evident in M(T) and Cp(T) are probably due to a strong anisotropic interaction between conduction electrons and localized electrons at Sm +3 sites.
We have studied electron-doped superconducting Sml.s5Ce0.15CuO4-6 single crystal by temperature-dependent magnetization and specific heat measurements. Figure 1 shows the temperature-dependent magnetic susceptibility, x(T), for Sml.ssCe0.15CuO4-5 with H = 5 kG. The large anisotropy in x(T) between H I] c and H _l_ c is quite clear and the temperature dependence for both field orientations clearly deviate from the typical Curie-Weiss behavior. The van Vleck contribution, due to the small energy gap of the J = 7/2 multiplet above the J = 5/2 ground state, is considered to account for the observed magnetic susceptibility according to the standard formula
[ X = NA
L3kB(--T---O) + 7k---~-EJ "
(1)
The fitting results are unsatisfactory for both field orientations but apparently quite good for the powder-average case. From the fitting results of the powder-average data, the splitting A E , the effective moment freer, and the Curie-Weiss tem-
perature O are found to be 466 K, 0.36 #B, and - 6 . 4 K, respectively. The value of A E is smaller than that of S m 2 C u O 4 , ( ~ 1150 K) [1], which is probably due to doping of electrons by Ce +4 ions. With increasing temperature, XII(T) for H II c shows a broad local minimum around T = 100 K and a slow increase whereas the XJ_(T) for H _L c shows a monotonic decrease. A natural mechanism for this effect would be a non-negligible anisotropic hybridization of conduction electrons with localized Sm +3 orbitals and its angular dependence. The temperature-dependent specific heat, Cp(T), is plotted in Figure 2(a). Clear evidence of the antiferromagnetic transition in Sm1.ssCeo.15CuO4-5 is given by the sharp peak at 4.7 K and the superconducting transition is seen near Te ~ 16.5 K as a slight jump in Cp. In order to separate the magnetic and nonmagnetic contributions to Cp, the data for 10 K < T < 18 K is fitted to the usual equation CpNM(T) = v T + /3T 3. It is found that -y = 191.0 (7) mJ/mole.K 2
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)00557-8
B.K. Cho et al./Physica C 341-348 (2000) 491-492
492
5
'
~
'
r
,
I
"
¢9
E
4
o
3
O 03
bT - -
_•
H±c
E F__: 2
_~__=
12
I
¢2
8
ee ee
..."
Av?.rage. . . . . . . . . . . . .
t
1
v
0
I
0
HIIc
I
I
1oo 200 300 Temperature (K)
Figure 1. Anisotropic magnetic susceptibility X vs. temperature T of single-crystal Sml.s5Ceo.15Cu04-~ for H / c, H JJ c, and powder average. Fits to Equation (1) in the text are shown by the solid curves.
~, 1 x." ~
E° ~.
~
8 4
o
and fl = 1.3 (1) mJ/mole.K 4, yielding the Debye temperature OD ~ 219 K. The observed value of "7 = 103.2 (7) mJ/mole.Sm.K 2 is significantly larger than those found in other R2CuO4 compounds (T'-phase), where R = Pr, Nd, and Eu [2]. It is natural to judge that the enhanced "7 is due to the doped electrons. Even for Sm2CuO4, the "7 value was previously found to be exceptionally large (~, 82 mJ/mole.Sm.K 2) [2]. It was speculated that the effects of magnetic correlation exist well above TN ~ 5.9 K, thereby making accurate determination of "y difficult. However, our estimated ~/for Smx.ssCe0.15Cu04-~ is still quite large even though TN is now lowered to 4.7 K. The contribution of magnetic correlation to the measured Cp(T) is calculated as Cpmag(T) = Cp(T) - CpNM(T) and is plotted in Figure 2(b). The entropy associated with the magnetic transition is calculated from the Cpmag(T) and is plotted in the inset of Figure 2(b). The magnetic entropy saturates rapidly above TN to ~, 4.1 J/mole.K, indicating that the transition is driven by localized electrons. However, the accumulated entropy is clearly smaller than 1.85Rln2, which is the usual value for the doublet ground state of Sm +3 [3]. It was reported by Stewart [4] that the magnetic entropy associated with a magnetic transition is significantly reduced if the magnetic transition is due to itinerant heavy fermionic electrons in analogy with a BCS-type transition.
2
• •
0
aid
0
2
4
6
8
Temperature(K)
5 10 15 Temperature (K)
10
20
Figure 2. (a) Specific heat Cp vs. temperature T of single-crystal Sml.ssCe0.15CuO4-5 for 1.5 K _< T < 19.0 K. (b) Magnetic specific heat (Cp mag) vs. temperature T. Inset: entropy associated with the magnetic transition vs. temperature. Thus, the reduced entropy can be explained by the fact that itinerant electrons with heavy effective mass are involved in the transition. This explanation is also consistent with the anisotropic temperature-dependent behavior of magnetization and the enhanced electronic contribution to specific heat. ACKNOWLEDGEMENT This work was supported by the Korea Science & Engineering Foundation through the grant No. 1999-2-114-005-5. REFERENCES
1. C. L. Seaman et al., Physica C 159, 391 (1989). 2. M. F. Hundley et al., Physica C 158, 102 (1989) 3. V. Nekvasil, Physica C 170, 469 (1990). 4. G. R. Stewart, Rev. Mod. Phys. 56, 755 (1984).