- Email: [email protected]
Magnetic and superconducting phase diagram in oxybromite cuprate Ca2-xNaxCuO2Br2
ARTICLE IN PRESS
Physica B 374–375 (2006) 75–78 www.elsevier.com/locate/physb
Magnetic and superconducting phase diagram in oxybromite cuprate Ca2x NaxCuO2 Br2 S. Kuroiwaa,, Y. Zenitania, M. Yamazawaa, Y. Tomitaa, J. Akimitsua, K. Ohishib, A. Kodab, S.R. Sahab, R. Kadonob,1, I. Watanabec, S. Ohirac a Department of Physics and Mathematics, Aoyama-Gakuin University, Fuchinobe 5-10-1, Sagamihara, Kanagawa 229-8558, Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan c Advanced Meson Science Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b
Abstract A comprehensive magnetic and superconducting phase diagram determined by muon spin rotation/relaxation ðmSRÞ is presented for Ca2x Nax CuO2 Br2 which has apical bromine atoms. Evidence for antiferromagnetic (AF) order in lightly doped samples and that for quasi-static spin glass (SG)-like state in moderately doped ones are obtained by ZF-mSR at low temperatures. While the phase diagram is qualitatively similar to that in typical 2-1-4 cuprates including Ca2x Nax CuO2 Cl2 and La2x Srx CuO4 , it exhibits a slight shift (expansion) over the x axis. r 2005 Elsevier B.V. All rights reserved. Keywords: Ca2x Nax CuO2 Br2 ; Halogen atom; Magnetic phase diagram; mSR
1. Introduction Since the discovery of copper oxide superconductors, numerous experiments and theoretical studies have been performed to elucidate the role of apical atoms over the CuO2 planes. Al-Mamouri et al. [1] reported the superconductivity in Sr2 CuO2 F2þd at T c ¼ 46 K by substituting apical O2 with F . Subsequently, Hiroi et al. [2] demonstrated superconductivity in Ca2x Nax CuO2 Cl2 (Na–CCOC) with T c ¼ 29 K over an optimally hole-doped region ð0:15oxo0:20Þ. Recently, Zenitani et al. [3] succeeded in mapping out a comprehensive phase diagram of superconductivity in Ca2x Nax CuO2 Cl2y Bry , focusing on the relationship between the superconductivity and Cu and Cl/Br distance. The crystal structure of this compound is similar to that of La2 CuO4 , but all the apical oxygen atoms are replaced by Cl/Br atoms.
Corresponding author. Tel.: +81 42 759 6545; fax: +81 42 759 6287.
E-mail address: [email protected] (S. Kuroiwa). Also at School of Mathematical and Physical Science, The Graduate University for Advanced Studies. 1
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.11.019
In the process of this study, a peculiar feature has been observed in Ca2x Nax CuO2 Br2 (Na–CCOB) that definitive superconductivity with T c ¼ 19 K develops over an optimally hole-doped region x0:25 which is substantially shifted from that of typical copper-oxide systems. Unfortunately, it is difficult to determine the magnetic critical temperature ðT N Þ for the lightly hole doped Na–CCOB (including x ¼ 0, CCOB) by the conventional magnetic susceptibility measurements because of extremely weak magnetic response due to the non-distorted CuO2 plane. In order to elucidate the magnetic properties in the lowdoping region, we have conducted mSR experiment to map out the comprehensive magnetic phase diagram of Na–CCOB. In this paper, we report on the magnetic and superconducting phase diagrams of Na–CCOB determined, respectively, by mSR and magnetic susceptibility measurements. The spontaneous precession signal in the ZF-mSR time spectra expected for the occurrence of AF long range order has been observed in the undoped and slightly Na-doped samples ð0oxo0:03Þ at low temperatures. While no precession signal was identified in those with 0:04oxo0:15, evidence was found for a quasi-static spin
ARTICLE IN PRESS S. Kuroiwa et al. / Physica B 374–375 (2006) 75–78
76
glass (SG)-like state at lower temperatures. Moreover, development of superconducting state was observed in the sample with xX0:15. These characteristic doping levels are higher than those found in typical 2-1-4 cuprates, suggesting a shift (or expansion) of the entire phase diagram along the x axis.
notable that the precession signal is relatively weak probably due to smaller volume fraction of AF state. These spectra are well reproduced by the following form, Pz ðtÞ ¼
n X
Ai exp½ðsi tÞ2 cosð2pf i t þ fÞ
i¼1
þ An exp½ðsn tÞb ,
2. Experimental results A polycrystalline sample of CCOB was prepared by the solid state reaction under Ar atmosphere with stoichiometric mixture of CaBr2 ð99:9%Þ and CuO(99.9%) as a starting material. Those of Na–CCOB were synthesized under an ultra-high pressure ð5:5 GPaÞ. The samples were characterized by means of magnetization and powder X-ray diffraction measurements. We have confirmed that the no impurity phase was detected for any composition in the Na doping range and that the lattice parameter systematically decreased with increasing x. Unfortunately, the actual carrier concentration is yet to be determined quantitatively at this stage, and therefore the sodium concentration x in this paper refers only to a nominal composition. mSR measurements were performed at the Tri-University Meson Facility (TRIUMF, Canada), Meson Science Laboratory (KEK-MSL, Japan) and RIKEN-RAL Muon Facility (RIKEN-RAL, UK). ZF- and LF-mSR spectra were obtained at temperatures between 2 K and ambient temperature. TF-mSR measurements have been employed at ambient temperature for the correction of instrumental asymmetry. The superconducting phase was determined by a superconducting quantum interference device (SQUID) magnetometer. Fig. 1 shows the ZF-mSR time spectra in the parent material CCOB at several temperatures. The time spectra at low temperatures exhibit spontaneous oscillation expected for the occurrence of AF long range order. However, compared with the cases of other halogen cuprates (CCOC [4] and Sr2 CuO2 Cl2 (SCOC) [5]), it is
ð1Þ
where Ai and An refer to the asymmetry for the oscillating and non-oscillating components. si;n is the relaxation rate, f i is the muon spin precession frequency, f is the initial phase and b is the power of the exponent. The temperature dependence of high ðf 1 Þ and low ðf 2 Þ frequency components is shown in Fig. 2. The f 1 exhibits a steep enhancement with decreasing temperature below 200 K, while its oscillating amplitude disappears above 200 K. Moreover, it turned out that a precession signal obtained by fitting analysis exhibits further splitting into two components below 180 K. It is interesting to note that one of those components ðf 2 Þ takes a value common to that in CCOC and LSCO [6], and f 1 is close to that in CCOC and SCOC. Solid curve for f 1 in Fig. 1 is the fitting result with a form, AðT N TÞg , which yields T N ¼ 186:5ð5:0Þ K. The obtained T N is considerably smaller than that in both CCOC and SCOC which exhibit T N 260 K as determined by mSR measurements [4,5]. The small T N in CCOB implies that the interplanar exchange interaction in CCOB is suppressed in comparison with that in CCOC and SCOC ˚ by substituting an due to the increase of c axis ð2 AÞ apical Cl with Br . Moreover, ZF-mSR data for the lightly Na doped compounds ðx ¼ 0:02; 0:03Þ show clear oscillating signal at low temperatures. A fitting procedure similar to that for CCOB was employed to the data analysis for these samples. As a result, we found a steep decrease of T N with increasing sodium concentration; T N ¼ 168:5ð7:0Þ K for x ¼ 0:02 and 117:1ð8:0Þ K for x ¼ 0:03. 20 Ca2CuO2Br2 ZF-µSR
CCOB
Asymmetry
Frequency [MHz]
220K
0.2
100K
TN = 186.48 (5.0) K
15
ZF-SR
f1
10
f2
5
0.1 2K
0 0 0
0.2
0.4
0.6
1
2
3
4
50
100 150 Temperature [K]
200
250
Time [s] Fig. 1. ZF-mSR time spectra in parent material CCOB at several temperatures. Solid curves are results of fitting by Eq. (1).
Fig. 2. Temperature dependence of muon spin precession frequency f 1 (solid symbol) and f 2 (open symbol) in Eq. (1). Solid curve is the result of fitting by AðT N TÞg .
ARTICLE IN PRESS S. Kuroiwa et al. / Physica B 374–375 (2006) 75–78
Asymmetry
100K 10K
8K
0.14 0.12
ZF-SR T = 2K 0
0.16
6K
2K
Ca2-xNaxCuO2Br2 (x = 0.04)
0.4
0.16
0.4
0.2 0.8 1.2 Time [s]
1.6
0 Ca2-xNaxCuO2Br2 (x = 0.05)
5K
0.12
Longitudinal relaxation rate [s-1]
Asymmetry
0.20
ZF-SR x = 0.05
(a) 0.08 0.20 Asymmetry
Ca2-xNaxCuO2Br2 x = 0.04
0.18
0.24
77
100mT 10mT
0.16 0mT
0.12
LF-SR
0.4 0.2
0 0.6
Ca2-xNaxCuO2Br2 (x = 0.08)
0.4 0.2
x = 0.05
0
2
4
6
8
10
0
Time [s]
(b)
Ca2-xNaxCuO2Br2 (x = 0.15)
0.4 Fig. 3. (a) ZF-mSR time spectra in Na–CCOB with x ¼ 0:05. Inset shows a ZF-mSR time spectrum with x ¼ 0:04 at 2 K. (b) LF-mSR time spectra at 2 K for various applied magnetic fields.
Fig. 3 shows ZF-mSR time spectra at several temperatures (inset shows the data with x ¼ 0:04 at 2 K) and LFmSR time spectra under several applied magnetic fields in Na–CCOB with x ¼ 0:05. Since the ZF-mSR time spectra over a region of intermediate sodium concentration ð0:03oxo0:15Þ situated between AF and superconducting phases (see below) exhibit no clear precession signal, they suggest a strongly disordered magnetic ground state dominating over the AF long range order. These spectra were analyzed by the following form: Pz ðtÞ ¼
2 X
Ai G KT ðDdip ; tÞ expðli tÞ þ AB ,
(2)
i¼1
where Ai is the decay asymmetry for signal from the samples, AB is that from sample holder, li is the relaxation rate and Ddip is the nuclear dipolar width 0:188 ms1 determined from the data above 150 K. The second component ði ¼ 2Þ is additionally needed to fit the data only below 10 K with a strongly damped oscillation seen in the spectra at 2 K (which is not included explicitly in the above equation). This suggests that a disordered magnetic ground state like spin-glass (or spin density wave) state is present, which is either static or dynamically fluctuating. Therefore, we have performed LFð100 mTÞ-mSR measurements in order to distinguish the two possibilities. As shown in Fig. 4, the longitudinal relaxation rate increases at low temperatures for every Na doped samples due to the slowing down of the thermal fluctuation of Cu moments. Since the enhancement of longitudinal relaxation rate occurs over a temperature region which is wider than
0.2
0 0
5
10
15 20 Temperature [K]
25
30
Fig. 4. The temperature dependence of longitudinal relaxation rate under LF ¼ 100 mT for several Na concentration.
that associated with the AF transition, such behavior in those samples ð0:03oxo0:15Þ is attributed to the SG-like magnetic state. We tentatively define the peak top as an onset of spin glass-like behavior at T D 6:9, 6.3, 4.2 and 1.7 K in x ¼ 0:04, 0.05, 0.08 and 0.15 specimens, respectively. The superconducting critical temperature ðT c Þ were determined by means of magnetic susceptibility measurements. While the superconducting volume fraction is very small ð51%Þ in those with xo0:15 (see the inset of Fig. 5), the Meissner effect can be unambiguously observed above x0:15. The maximum superconducting volume fraction is observed when x0:275. Interestingly, it decreases at higher values of x. We have also found that the nominally optimal hole concentration for superconductivity is x ¼ 0:2520:30 with corresponding T c 19 K. However, the value of x is known as that upon preparation at this stage, and it may need verification by some other method. As a summary, the magnetic and superconducting phase diagrams of Na–CCOB as a function of x is shown in Fig. 5. In the obtained phase diagram, the region of magnetic phase is shifted (expanded) to the direction of higher doping. Actually, Mattheiss reported that the energy band of Br at the apical sites in CCOB calculated by using
ARTICLE IN PRESS S. Kuroiwa et al. / Physica B 374–375 (2006) 75–78
78
be trapped on the energy band of apical Br rather than the O-2p orbital in CuO2 plane.
Temperature [K]
100
AF
[10-3emu/g]
0 x = 0.10 x = 0.15 x = 0.175 x = 0.20 x = 0.25 x = 0.275 x = 0.35
-2 -4 -6 8
12
16
Acknowledgements
20
T [K]
10
SC
TN TD TC
SG-like 1
0
0.1
0.2 Na concentration x
0.3
Fig. 5. Magnetic and superconducting phase diagrams of Na–CCOB. The inset shows the temperature dependence of magnetic susceptibility at 1 mT.
tight-binding approximation is located slightly close to the Fermi level than that of oxygen in the CuO2 plane, while that of Cl in CCOC is located at a level lower than Fermi level [7]. Therefore, we speculate that the hole carrier may
We thank all the staff of KEK-MSL, TRIUMF and RIKEN-RAL for their technical support. The AoyamaGakuin University group was partly supported by the 21st COE program, ‘‘High-Tech Research Center’’ Project for Private Universities: matching fund subsidy for 2002–2004, and the Grant-in-Aid for Scientific Research on Priority Areas, both provided by the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] [2] [3] [4] [5] [6] [7]
M. Al-Mamouri, et al., Nature 369 (1994) 382. Z. Hiroi, et al., Nature 371 (1994) 139. Y. Zenitani, et al., Physica C 419 (2005) 32. K. Ohishi, et al., J. Phy. Soc. Jpn. 74 (2005) 2408. L.P. Le, et al., Phys. Rev. B 42 (1990) 2182. Y.J. Uemura, et al., Phys. Rev. Lett. 59 (1987) 1045. L.F. Mattheiss, Phys. Rev. B 42 (1990) 354.
Physica B 374–375 (2006) 75–78 www.elsevier.com/locate/physb
Magnetic and superconducting phase diagram in oxybromite cuprate Ca2x NaxCuO2 Br2 S. Kuroiwaa,, Y. Zenitania, M. Yamazawaa, Y. Tomitaa, J. Akimitsua, K. Ohishib, A. Kodab, S.R. Sahab, R. Kadonob,1, I. Watanabec, S. Ohirac a Department of Physics and Mathematics, Aoyama-Gakuin University, Fuchinobe 5-10-1, Sagamihara, Kanagawa 229-8558, Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan c Advanced Meson Science Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b
Abstract A comprehensive magnetic and superconducting phase diagram determined by muon spin rotation/relaxation ðmSRÞ is presented for Ca2x Nax CuO2 Br2 which has apical bromine atoms. Evidence for antiferromagnetic (AF) order in lightly doped samples and that for quasi-static spin glass (SG)-like state in moderately doped ones are obtained by ZF-mSR at low temperatures. While the phase diagram is qualitatively similar to that in typical 2-1-4 cuprates including Ca2x Nax CuO2 Cl2 and La2x Srx CuO4 , it exhibits a slight shift (expansion) over the x axis. r 2005 Elsevier B.V. All rights reserved. Keywords: Ca2x Nax CuO2 Br2 ; Halogen atom; Magnetic phase diagram; mSR
1. Introduction Since the discovery of copper oxide superconductors, numerous experiments and theoretical studies have been performed to elucidate the role of apical atoms over the CuO2 planes. Al-Mamouri et al. [1] reported the superconductivity in Sr2 CuO2 F2þd at T c ¼ 46 K by substituting apical O2 with F . Subsequently, Hiroi et al. [2] demonstrated superconductivity in Ca2x Nax CuO2 Cl2 (Na–CCOC) with T c ¼ 29 K over an optimally hole-doped region ð0:15oxo0:20Þ. Recently, Zenitani et al. [3] succeeded in mapping out a comprehensive phase diagram of superconductivity in Ca2x Nax CuO2 Cl2y Bry , focusing on the relationship between the superconductivity and Cu and Cl/Br distance. The crystal structure of this compound is similar to that of La2 CuO4 , but all the apical oxygen atoms are replaced by Cl/Br atoms.
Corresponding author. Tel.: +81 42 759 6545; fax: +81 42 759 6287.
E-mail address: [email protected] (S. Kuroiwa). Also at School of Mathematical and Physical Science, The Graduate University for Advanced Studies. 1
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.11.019
In the process of this study, a peculiar feature has been observed in Ca2x Nax CuO2 Br2 (Na–CCOB) that definitive superconductivity with T c ¼ 19 K develops over an optimally hole-doped region x0:25 which is substantially shifted from that of typical copper-oxide systems. Unfortunately, it is difficult to determine the magnetic critical temperature ðT N Þ for the lightly hole doped Na–CCOB (including x ¼ 0, CCOB) by the conventional magnetic susceptibility measurements because of extremely weak magnetic response due to the non-distorted CuO2 plane. In order to elucidate the magnetic properties in the lowdoping region, we have conducted mSR experiment to map out the comprehensive magnetic phase diagram of Na–CCOB. In this paper, we report on the magnetic and superconducting phase diagrams of Na–CCOB determined, respectively, by mSR and magnetic susceptibility measurements. The spontaneous precession signal in the ZF-mSR time spectra expected for the occurrence of AF long range order has been observed in the undoped and slightly Na-doped samples ð0oxo0:03Þ at low temperatures. While no precession signal was identified in those with 0:04oxo0:15, evidence was found for a quasi-static spin
ARTICLE IN PRESS S. Kuroiwa et al. / Physica B 374–375 (2006) 75–78
76
glass (SG)-like state at lower temperatures. Moreover, development of superconducting state was observed in the sample with xX0:15. These characteristic doping levels are higher than those found in typical 2-1-4 cuprates, suggesting a shift (or expansion) of the entire phase diagram along the x axis.
notable that the precession signal is relatively weak probably due to smaller volume fraction of AF state. These spectra are well reproduced by the following form, Pz ðtÞ ¼
n X
Ai exp½ðsi tÞ2 cosð2pf i t þ fÞ
i¼1
þ An exp½ðsn tÞb ,
2. Experimental results A polycrystalline sample of CCOB was prepared by the solid state reaction under Ar atmosphere with stoichiometric mixture of CaBr2 ð99:9%Þ and CuO(99.9%) as a starting material. Those of Na–CCOB were synthesized under an ultra-high pressure ð5:5 GPaÞ. The samples were characterized by means of magnetization and powder X-ray diffraction measurements. We have confirmed that the no impurity phase was detected for any composition in the Na doping range and that the lattice parameter systematically decreased with increasing x. Unfortunately, the actual carrier concentration is yet to be determined quantitatively at this stage, and therefore the sodium concentration x in this paper refers only to a nominal composition. mSR measurements were performed at the Tri-University Meson Facility (TRIUMF, Canada), Meson Science Laboratory (KEK-MSL, Japan) and RIKEN-RAL Muon Facility (RIKEN-RAL, UK). ZF- and LF-mSR spectra were obtained at temperatures between 2 K and ambient temperature. TF-mSR measurements have been employed at ambient temperature for the correction of instrumental asymmetry. The superconducting phase was determined by a superconducting quantum interference device (SQUID) magnetometer. Fig. 1 shows the ZF-mSR time spectra in the parent material CCOB at several temperatures. The time spectra at low temperatures exhibit spontaneous oscillation expected for the occurrence of AF long range order. However, compared with the cases of other halogen cuprates (CCOC [4] and Sr2 CuO2 Cl2 (SCOC) [5]), it is
ð1Þ
where Ai and An refer to the asymmetry for the oscillating and non-oscillating components. si;n is the relaxation rate, f i is the muon spin precession frequency, f is the initial phase and b is the power of the exponent. The temperature dependence of high ðf 1 Þ and low ðf 2 Þ frequency components is shown in Fig. 2. The f 1 exhibits a steep enhancement with decreasing temperature below 200 K, while its oscillating amplitude disappears above 200 K. Moreover, it turned out that a precession signal obtained by fitting analysis exhibits further splitting into two components below 180 K. It is interesting to note that one of those components ðf 2 Þ takes a value common to that in CCOC and LSCO [6], and f 1 is close to that in CCOC and SCOC. Solid curve for f 1 in Fig. 1 is the fitting result with a form, AðT N TÞg , which yields T N ¼ 186:5ð5:0Þ K. The obtained T N is considerably smaller than that in both CCOC and SCOC which exhibit T N 260 K as determined by mSR measurements [4,5]. The small T N in CCOB implies that the interplanar exchange interaction in CCOB is suppressed in comparison with that in CCOC and SCOC ˚ by substituting an due to the increase of c axis ð2 AÞ apical Cl with Br . Moreover, ZF-mSR data for the lightly Na doped compounds ðx ¼ 0:02; 0:03Þ show clear oscillating signal at low temperatures. A fitting procedure similar to that for CCOB was employed to the data analysis for these samples. As a result, we found a steep decrease of T N with increasing sodium concentration; T N ¼ 168:5ð7:0Þ K for x ¼ 0:02 and 117:1ð8:0Þ K for x ¼ 0:03. 20 Ca2CuO2Br2 ZF-µSR
CCOB
Asymmetry
Frequency [MHz]
220K
0.2
100K
TN = 186.48 (5.0) K
15
ZF-SR
f1
10
f2
5
0.1 2K
0 0 0
0.2
0.4
0.6
1
2
3
4
50
100 150 Temperature [K]
200
250
Time [s] Fig. 1. ZF-mSR time spectra in parent material CCOB at several temperatures. Solid curves are results of fitting by Eq. (1).
Fig. 2. Temperature dependence of muon spin precession frequency f 1 (solid symbol) and f 2 (open symbol) in Eq. (1). Solid curve is the result of fitting by AðT N TÞg .
ARTICLE IN PRESS S. Kuroiwa et al. / Physica B 374–375 (2006) 75–78
Asymmetry
100K 10K
8K
0.14 0.12
ZF-SR T = 2K 0
0.16
6K
2K
Ca2-xNaxCuO2Br2 (x = 0.04)
0.4
0.16
0.4
0.2 0.8 1.2 Time [s]
1.6
0 Ca2-xNaxCuO2Br2 (x = 0.05)
5K
0.12
Longitudinal relaxation rate [s-1]
Asymmetry
0.20
ZF-SR x = 0.05
(a) 0.08 0.20 Asymmetry
Ca2-xNaxCuO2Br2 x = 0.04
0.18
0.24
77
100mT 10mT
0.16 0mT
0.12
LF-SR
0.4 0.2
0 0.6
Ca2-xNaxCuO2Br2 (x = 0.08)
0.4 0.2
x = 0.05
0
2
4
6
8
10
0
Time [s]
(b)
Ca2-xNaxCuO2Br2 (x = 0.15)
0.4 Fig. 3. (a) ZF-mSR time spectra in Na–CCOB with x ¼ 0:05. Inset shows a ZF-mSR time spectrum with x ¼ 0:04 at 2 K. (b) LF-mSR time spectra at 2 K for various applied magnetic fields.
Fig. 3 shows ZF-mSR time spectra at several temperatures (inset shows the data with x ¼ 0:04 at 2 K) and LFmSR time spectra under several applied magnetic fields in Na–CCOB with x ¼ 0:05. Since the ZF-mSR time spectra over a region of intermediate sodium concentration ð0:03oxo0:15Þ situated between AF and superconducting phases (see below) exhibit no clear precession signal, they suggest a strongly disordered magnetic ground state dominating over the AF long range order. These spectra were analyzed by the following form: Pz ðtÞ ¼
2 X
Ai G KT ðDdip ; tÞ expðli tÞ þ AB ,
(2)
i¼1
where Ai is the decay asymmetry for signal from the samples, AB is that from sample holder, li is the relaxation rate and Ddip is the nuclear dipolar width 0:188 ms1 determined from the data above 150 K. The second component ði ¼ 2Þ is additionally needed to fit the data only below 10 K with a strongly damped oscillation seen in the spectra at 2 K (which is not included explicitly in the above equation). This suggests that a disordered magnetic ground state like spin-glass (or spin density wave) state is present, which is either static or dynamically fluctuating. Therefore, we have performed LFð100 mTÞ-mSR measurements in order to distinguish the two possibilities. As shown in Fig. 4, the longitudinal relaxation rate increases at low temperatures for every Na doped samples due to the slowing down of the thermal fluctuation of Cu moments. Since the enhancement of longitudinal relaxation rate occurs over a temperature region which is wider than
0.2
0 0
5
10
15 20 Temperature [K]
25
30
Fig. 4. The temperature dependence of longitudinal relaxation rate under LF ¼ 100 mT for several Na concentration.
that associated with the AF transition, such behavior in those samples ð0:03oxo0:15Þ is attributed to the SG-like magnetic state. We tentatively define the peak top as an onset of spin glass-like behavior at T D 6:9, 6.3, 4.2 and 1.7 K in x ¼ 0:04, 0.05, 0.08 and 0.15 specimens, respectively. The superconducting critical temperature ðT c Þ were determined by means of magnetic susceptibility measurements. While the superconducting volume fraction is very small ð51%Þ in those with xo0:15 (see the inset of Fig. 5), the Meissner effect can be unambiguously observed above x0:15. The maximum superconducting volume fraction is observed when x0:275. Interestingly, it decreases at higher values of x. We have also found that the nominally optimal hole concentration for superconductivity is x ¼ 0:2520:30 with corresponding T c 19 K. However, the value of x is known as that upon preparation at this stage, and it may need verification by some other method. As a summary, the magnetic and superconducting phase diagrams of Na–CCOB as a function of x is shown in Fig. 5. In the obtained phase diagram, the region of magnetic phase is shifted (expanded) to the direction of higher doping. Actually, Mattheiss reported that the energy band of Br at the apical sites in CCOB calculated by using
ARTICLE IN PRESS S. Kuroiwa et al. / Physica B 374–375 (2006) 75–78
78
be trapped on the energy band of apical Br rather than the O-2p orbital in CuO2 plane.
Temperature [K]
100
AF
[10-3emu/g]
0 x = 0.10 x = 0.15 x = 0.175 x = 0.20 x = 0.25 x = 0.275 x = 0.35
-2 -4 -6 8
12
16
Acknowledgements
20
T [K]
10
SC
TN TD TC
SG-like 1
0
0.1
0.2 Na concentration x
0.3
Fig. 5. Magnetic and superconducting phase diagrams of Na–CCOB. The inset shows the temperature dependence of magnetic susceptibility at 1 mT.
tight-binding approximation is located slightly close to the Fermi level than that of oxygen in the CuO2 plane, while that of Cl in CCOC is located at a level lower than Fermi level [7]. Therefore, we speculate that the hole carrier may
We thank all the staff of KEK-MSL, TRIUMF and RIKEN-RAL for their technical support. The AoyamaGakuin University group was partly supported by the 21st COE program, ‘‘High-Tech Research Center’’ Project for Private Universities: matching fund subsidy for 2002–2004, and the Grant-in-Aid for Scientific Research on Priority Areas, both provided by the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] [2] [3] [4] [5] [6] [7]
M. Al-Mamouri, et al., Nature 369 (1994) 382. Z. Hiroi, et al., Nature 371 (1994) 139. Y. Zenitani, et al., Physica C 419 (2005) 32. K. Ohishi, et al., J. Phy. Soc. Jpn. 74 (2005) 2408. L.P. Le, et al., Phys. Rev. B 42 (1990) 2182. Y.J. Uemura, et al., Phys. Rev. Lett. 59 (1987) 1045. L.F. Mattheiss, Phys. Rev. B 42 (1990) 354.