- Email: [email protected]
115In-NQR study of antiferromagnetism and superconductivity near magnetic criticality in CeIn3
ARTICLE IN PRESS
Physica B 359–361 (2005) 413–415 www.elsevier.com/locate/physb
115
In-NQR study of antiferromagnetism and superconductivity near magnetic criticality in CeIn3
S. Kawasakia,, T. Mitoa,1, Y. Kawasakia,2, H. Kotegawaa,3, .G.-q. Zhenga,4, ¯ nukib,c Y. Kitaokaa, H. Shishidob, S. Arakib, R. Settaib, Y. O a
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan b Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan c Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Abstract We report the discovery of novel phases in CeIn3 on the verge of antiferromagnetism (AFM) through the In-nuclear quadrupole resonance (NQR) measurements. We have found that even though a pressure (P)-induced phase separation into AFM and paramagnetism (PM) takes place, a novel superconductivity (SC), which is induced without strong spin fluctuations, has been found to coexist with AFM on a microscopic level. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order magnetic phase transition might mediate attractive interaction to form Cooper pairs. r 2005 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 74.62.Fj; 74.70.Tx; 75.30.Kz Keywords: CeIn3; Antiferromagnetism; Superconductivity; First-order phase transition
Corresponding author. Tel./fax: +81 6 6850 6438.
E-mail address: [email protected] (S. Kawasaki). 1 Department of Physics, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan. 2 Department of Physics, Faculty of Engineering, Tokushima University, Tokushima 770-8506, Japan. 3 Department of Physics, Faculty of Science, Okayama University, Okayama 700-8530, Japan. 4 Department of Physics, Faculty of Science, Okayama University, Okayama 700-8530, Japan.
Pressure (P)-induced superconductivity (SC) in Ce-based heavy-fermion (HF) antiferromagnet CeIn3 has been observed in the measurements of resistivity and nuclear quadrupole resonance (NQR) under P [1–5]. It was suggested that the SC in CeIn3 is mediated by antiferromagnetic spin fluctuations induced due to the closeness to a magnetic critical point [2–4]. However, an underlying mechanism of HF SC and its relation to antiferromagnetism (AFM) are under debate, and
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.083
ARTICLE IN PRESS S. Kawasaki et al. / Physica B 359– 361 (2005) 413–415
8 CeIn3
PM
Temperature (K)
6
2.43 GPa
4 2
TFL
TN PM +AFM
Fermi liquid
AFM
0.4 SC(PM) TC
SC(AFM) TC
0.2
AFM+SC
0.0 2.1
2.2
2.3
2.4
2.5
2.6
2.7
Pressure (GPa) Fig. 1. The P–T phase diagram of CeIn3 determined from the present results. The P and T ranges where the phase separation of AFM and PM occurs are shaded in the figure.
300 NQR intensity (a.u.)
hence more systematic experiments are highly desired. Here, we report on a novel class of SC, which is not induced by magnetic in origin, coexisting uniformly with the AFM in CeIn3 via the In-NQR study, even though the magnetic phase separation emerged into paramagnetism (PM) and AFM. Fig. 1 indicates the P–temperature (T) phase diagram in CeIn3 on the verge of AFM [6]. CeIn3 forms in the cubic AuCu3 structure and orders antiferromagnetically below TN ¼ 10.2 K at P ¼ 0 with an ordering vector Q ¼ (12; 12; 12) and Ce magnetic moment MS0.5mB, which were determined by the NQR [7,8] and the neutron-diffraction experiments on single crystals [9], respectively. The resistivity measurements of CeIn3 have clarified the P–T phase diagram of AFM and SC: TN decreases with increasing P. On the verge of AFM, SC emerges in a narrow P range, exhibiting a maximum value of Tc0.2 K at Pc ¼ 2.5 GPa where AFM is believed to disappear [1–4]. Fig. 2 inset shows the NQR spectra observed above and below TN at P ¼ 2:43 GPa: A drastic change in the NQR spectral shape is observed due to the occurrence of Hint(T) at the In nuclei below TN ¼ 2.5 K. However, the spectrum below TN
CeIn3 PM AFM
1 /T1T (sec-1K-1)
414
200 AFM
TC
2.7 K
2.43 GPa TN= 2.5 K H int~ 1.1 kOe
100 mK 8
10
12
Frequency (MHz) TN
TFL
100 PM
TC
2.43 GPa
0 0.1
1 10 Temperature (K)
100
Fig. 2. T dependence of 115(1/T1T) in CeIn3 at P ¼ 2:43 GPa: Open and solid circles indicate the respective data for PM and AFM measured at 9.8 and 8.2 MHz. The dotted, solid, and dashed arrows indicate TN, TPM for PM and TAFM for AFM, c c and TFL, respectively. The inset shows the NQR spectra above and below TN at P ¼ 2:43 GPa:
includes two kinds of spectra arising from AFM and PM, providing firm microscopic evidence for the emergence of magnetic phase separation. A volume fraction of PM and AFM at P ¼ 2:43 GPa is estimated to be comparable to one another. Thus, at P ¼ 2:43 GPa; the phase separation into AFM and PM emerges below TN. This is in marked contrast to the case for the homogeneous coexistent phase of SC and AFM observed in CeRhIn5 without any trace of magnetic phase separation [10]. The uniform coexistent phase of AFM and SC is evidenced from the T dependence of 1=T 1 T from the microscopic point of view. Fig. 2 shows the T dependences of 1/T1T for AFM and PM at P ¼ 2:43 GPa: The respective superconducting transition temperature at TAFM and TPM are c c determined as the temperature below which 1/T1 decreases markedly due to the opening of superconducting gap. These results reveal that the uniform coexistent phase of AFM and SC takes place on a microscopic level at P ¼ 2.43 GPa. In PM at P ¼ 2.43 GPa, the 1=T 1 T ¼ const: relation is valid below TFL3.2 K, indicating that the Fermi-liquid state is established [6]. The highest
ARTICLE IN PRESS S. Kawasaki et al. / Physica B 359– 361 (2005) 413–415
value of Tc ¼ 230 mK in CeIn3 is realized for the PM at P ¼ 2:43 GPa: In contrast to the 1=T 1 T ¼ const: behavior for PM, the 1=T 1 T for AFM continues to increase upon cooling below TN. It is surprising to exceed the value of 1=T 1 T at around TN in spite of antiferromagnetic spin being polarized due to the onset of AFM. This suggests that the low-lying fluctuations of staggered magnetic density are largely enhanced as if it were a precursor phenomenon for SC under the background of AFM. It may be relevant with the magnetic phase separation into AFM and PM that such low-lying magnetic excitations continue to be enhanced down to Tc as T decreases well below TN. Some spin-density fluctuations may be responsible for this feature in association with the phase separation into AFM and PM. Further evidence for SC uniformly coexisting with AFM is corroborated from the T1 results at low T and P ¼ 2:43 GPa as indicated in Fig. 2. At temperatures well below TPM ¼ 230 mK and c TAFM ¼ 190 mK, unexpectedly, the magnitude of c 1=T 1 T ¼ const: coincide with one another for both the phases that are magnetically separated into AFM and PM. This means that the quasiparticle excitations for the uniform coexistent phase of SC and AFM may be the same in origin as for the phase of SC in PM. We have provided evidence for the magnetic phase separation into AFM and PM and the
415
novel SC, which is induced by magnetic phase instability, uniformly coexisting with the AFM at P ¼ 2:43 GPa in CeIn3. The highest value of Tc ¼ 230 mK in CeIn3 has been found to take place for the PM at P ¼ 2:43 GPa where the phases of AFM and PM are separated with a nearly equal fraction. The present experiments have revealed that this novel type of SC is mediated by yet unknown pairing interaction, such as staggered magnetic density fluctuations in association with the magnetic phase separation. This work was supported by a Grant-in-Aid for Creative Scientific Research (15GS0213), MEXT and The 21st Century COE Program supported by the Japan Society for the Promotion of Science. S. K. has been supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
I.R. Walker, et al., Physica C 282–287 (1997) 303. N.D. Mathur, et al., Nature 394 (1998) 39. T. Muramatsu, Thesis, Osaka University, 2001. G. Knebel, et al., Phys. Rev. B 65 (2002) 024425. S. Kawasaki, et al., Phys. Rev. B 66 (2002) 054521. S. Kawasaki, et al., J. Phys. Soc. Jpn. 73 (2004) 1647. Y. Kohori, et al., Physica B 259–261 (1999) 103. Y. Kohori, et al., Physica B 281–282 (2000) 12. W. Knafo, et al., J. Phys.: Condens. Matter 15 (2003) 3741. S. Kawasaki, et al., Phys. Rev. Lett. 91 (2003) 137001.
Physica B 359–361 (2005) 413–415 www.elsevier.com/locate/physb
115
In-NQR study of antiferromagnetism and superconductivity near magnetic criticality in CeIn3
S. Kawasakia,, T. Mitoa,1, Y. Kawasakia,2, H. Kotegawaa,3, .G.-q. Zhenga,4, ¯ nukib,c Y. Kitaokaa, H. Shishidob, S. Arakib, R. Settaib, Y. O a
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan b Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan c Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Abstract We report the discovery of novel phases in CeIn3 on the verge of antiferromagnetism (AFM) through the In-nuclear quadrupole resonance (NQR) measurements. We have found that even though a pressure (P)-induced phase separation into AFM and paramagnetism (PM) takes place, a novel superconductivity (SC), which is induced without strong spin fluctuations, has been found to coexist with AFM on a microscopic level. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order magnetic phase transition might mediate attractive interaction to form Cooper pairs. r 2005 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 74.62.Fj; 74.70.Tx; 75.30.Kz Keywords: CeIn3; Antiferromagnetism; Superconductivity; First-order phase transition
Corresponding author. Tel./fax: +81 6 6850 6438.
E-mail address: [email protected] (S. Kawasaki). 1 Department of Physics, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan. 2 Department of Physics, Faculty of Engineering, Tokushima University, Tokushima 770-8506, Japan. 3 Department of Physics, Faculty of Science, Okayama University, Okayama 700-8530, Japan. 4 Department of Physics, Faculty of Science, Okayama University, Okayama 700-8530, Japan.
Pressure (P)-induced superconductivity (SC) in Ce-based heavy-fermion (HF) antiferromagnet CeIn3 has been observed in the measurements of resistivity and nuclear quadrupole resonance (NQR) under P [1–5]. It was suggested that the SC in CeIn3 is mediated by antiferromagnetic spin fluctuations induced due to the closeness to a magnetic critical point [2–4]. However, an underlying mechanism of HF SC and its relation to antiferromagnetism (AFM) are under debate, and
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.083
ARTICLE IN PRESS S. Kawasaki et al. / Physica B 359– 361 (2005) 413–415
8 CeIn3
PM
Temperature (K)
6
2.43 GPa
4 2
TFL
TN PM +AFM
Fermi liquid
AFM
0.4 SC(PM) TC
SC(AFM) TC
0.2
AFM+SC
0.0 2.1
2.2
2.3
2.4
2.5
2.6
2.7
Pressure (GPa) Fig. 1. The P–T phase diagram of CeIn3 determined from the present results. The P and T ranges where the phase separation of AFM and PM occurs are shaded in the figure.
300 NQR intensity (a.u.)
hence more systematic experiments are highly desired. Here, we report on a novel class of SC, which is not induced by magnetic in origin, coexisting uniformly with the AFM in CeIn3 via the In-NQR study, even though the magnetic phase separation emerged into paramagnetism (PM) and AFM. Fig. 1 indicates the P–temperature (T) phase diagram in CeIn3 on the verge of AFM [6]. CeIn3 forms in the cubic AuCu3 structure and orders antiferromagnetically below TN ¼ 10.2 K at P ¼ 0 with an ordering vector Q ¼ (12; 12; 12) and Ce magnetic moment MS0.5mB, which were determined by the NQR [7,8] and the neutron-diffraction experiments on single crystals [9], respectively. The resistivity measurements of CeIn3 have clarified the P–T phase diagram of AFM and SC: TN decreases with increasing P. On the verge of AFM, SC emerges in a narrow P range, exhibiting a maximum value of Tc0.2 K at Pc ¼ 2.5 GPa where AFM is believed to disappear [1–4]. Fig. 2 inset shows the NQR spectra observed above and below TN at P ¼ 2:43 GPa: A drastic change in the NQR spectral shape is observed due to the occurrence of Hint(T) at the In nuclei below TN ¼ 2.5 K. However, the spectrum below TN
CeIn3 PM AFM
1 /T1T (sec-1K-1)
414
200 AFM
TC
2.7 K
2.43 GPa TN= 2.5 K H int~ 1.1 kOe
100 mK 8
10
12
Frequency (MHz) TN
TFL
100 PM
TC
2.43 GPa
0 0.1
1 10 Temperature (K)
100
Fig. 2. T dependence of 115(1/T1T) in CeIn3 at P ¼ 2:43 GPa: Open and solid circles indicate the respective data for PM and AFM measured at 9.8 and 8.2 MHz. The dotted, solid, and dashed arrows indicate TN, TPM for PM and TAFM for AFM, c c and TFL, respectively. The inset shows the NQR spectra above and below TN at P ¼ 2:43 GPa:
includes two kinds of spectra arising from AFM and PM, providing firm microscopic evidence for the emergence of magnetic phase separation. A volume fraction of PM and AFM at P ¼ 2:43 GPa is estimated to be comparable to one another. Thus, at P ¼ 2:43 GPa; the phase separation into AFM and PM emerges below TN. This is in marked contrast to the case for the homogeneous coexistent phase of SC and AFM observed in CeRhIn5 without any trace of magnetic phase separation [10]. The uniform coexistent phase of AFM and SC is evidenced from the T dependence of 1=T 1 T from the microscopic point of view. Fig. 2 shows the T dependences of 1/T1T for AFM and PM at P ¼ 2:43 GPa: The respective superconducting transition temperature at TAFM and TPM are c c determined as the temperature below which 1/T1 decreases markedly due to the opening of superconducting gap. These results reveal that the uniform coexistent phase of AFM and SC takes place on a microscopic level at P ¼ 2.43 GPa. In PM at P ¼ 2.43 GPa, the 1=T 1 T ¼ const: relation is valid below TFL3.2 K, indicating that the Fermi-liquid state is established [6]. The highest
ARTICLE IN PRESS S. Kawasaki et al. / Physica B 359– 361 (2005) 413–415
value of Tc ¼ 230 mK in CeIn3 is realized for the PM at P ¼ 2:43 GPa: In contrast to the 1=T 1 T ¼ const: behavior for PM, the 1=T 1 T for AFM continues to increase upon cooling below TN. It is surprising to exceed the value of 1=T 1 T at around TN in spite of antiferromagnetic spin being polarized due to the onset of AFM. This suggests that the low-lying fluctuations of staggered magnetic density are largely enhanced as if it were a precursor phenomenon for SC under the background of AFM. It may be relevant with the magnetic phase separation into AFM and PM that such low-lying magnetic excitations continue to be enhanced down to Tc as T decreases well below TN. Some spin-density fluctuations may be responsible for this feature in association with the phase separation into AFM and PM. Further evidence for SC uniformly coexisting with AFM is corroborated from the T1 results at low T and P ¼ 2:43 GPa as indicated in Fig. 2. At temperatures well below TPM ¼ 230 mK and c TAFM ¼ 190 mK, unexpectedly, the magnitude of c 1=T 1 T ¼ const: coincide with one another for both the phases that are magnetically separated into AFM and PM. This means that the quasiparticle excitations for the uniform coexistent phase of SC and AFM may be the same in origin as for the phase of SC in PM. We have provided evidence for the magnetic phase separation into AFM and PM and the
415
novel SC, which is induced by magnetic phase instability, uniformly coexisting with the AFM at P ¼ 2:43 GPa in CeIn3. The highest value of Tc ¼ 230 mK in CeIn3 has been found to take place for the PM at P ¼ 2:43 GPa where the phases of AFM and PM are separated with a nearly equal fraction. The present experiments have revealed that this novel type of SC is mediated by yet unknown pairing interaction, such as staggered magnetic density fluctuations in association with the magnetic phase separation. This work was supported by a Grant-in-Aid for Creative Scientific Research (15GS0213), MEXT and The 21st Century COE Program supported by the Japan Society for the Promotion of Science. S. K. has been supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
I.R. Walker, et al., Physica C 282–287 (1997) 303. N.D. Mathur, et al., Nature 394 (1998) 39. T. Muramatsu, Thesis, Osaka University, 2001. G. Knebel, et al., Phys. Rev. B 65 (2002) 024425. S. Kawasaki, et al., Phys. Rev. B 66 (2002) 054521. S. Kawasaki, et al., J. Phys. Soc. Jpn. 73 (2004) 1647. Y. Kohori, et al., Physica B 259–261 (1999) 103. Y. Kohori, et al., Physica B 281–282 (2000) 12. W. Knafo, et al., J. Phys.: Condens. Matter 15 (2003) 3741. S. Kawasaki, et al., Phys. Rev. Lett. 91 (2003) 137001.