- Email: [email protected]
115 In NQR study of superconducting CeCoIn5 under high pressure
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 189–190
115
In NQR study of superconducting CeCoIn5 under high pressure
Y. Kohoria,*, H. Saitob, Y. Kobayashib, H. Tairab, Y. Iwamotoc, T. Koharad, T. Matsumotoe, E.D. Bauerf, M.B. Maplef, J.L. Sarraog a
Department of Physics, Faculty of Science, Chiba University, Inage-ku, Chiba 263-8522, Japan Graduate School of Science and Technology, Chiba University, Inage-ku, Chiba 263-8522, Japan c Department of Transportation System Engineering, Kobe University of Mercantile Marine, Higashi-nada 658-0022, Japan d Division of Material Science, Graduate School of Science, Himeji Institute of Technology, Ako-gun, Hyogo 678-1297, Japan e National Institute for Material Science, 1-2-1 Sengen,Tsukuba, Ibaraki 305-0047, Japan f Department of Physics and Institute of Pure and Applied Physics, University of California, San Diego, La Jolla, CA 92093, USA g Condensed Matter and Thermal Physics, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b
Abstract We have carried out 115 In nuclear quadrupole resonance measurements on CeCoIn5 under high pressure P: The temperature T dependence of the nuclear spin–lattice relaxation rate 1=T1 in the normal state, 1=T1 pT 1=4 ; indicates that CeCoIn5 is located just at an antiferromagnetic (AF) instability at ambient pressure. The value of 1=T1 decreases rapidly with increasing P; reflecting that P moves CeCoIn5 away from the AF instability. r 2004 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 76.60.Gv; 74.20.Mn Keywords: NQR; Heavy fermion superconductor; CeCoIn5
The superconductivity located near the antiferromagnetic (AF) state of heavy fermion system has become one of the fundamental issues in condensed matter physics. Analogy with the high TC cuprate superconductors suggests that the d-wave superconductivitymediated via AF spin fluctuations is realized in the system. Recently, CeTIn5 (T ¼ Co; Rh and Ir) has been discovered. In CeRhIn5 ; the ground state at ambient pressure is antiferromagnet with a Ne! el temperature TN of 3:8 K: Application of a hydrostatic pressure of about 17 kbar induces a transition to the superconducting state with the critical temperature TC of 2:1 K [1]. In CeCoIn5 ; the superconductivity with TC of 2:3 K appears at ambient pressure [2]. It is an important theme of heavy fermion *Corresponding author. Tel.: +81-43-290-2766; fax: +8143-290-2874. E-mail address: [email protected] (Y. Kohori).
system to study the interplay of magnetism and superconductivity, and CeTIn5 is suitable for the study. Nuclear quadrupole resonance (NQR) is a powerful method for the study of magnetism and superconductivity, i.e., the spectrum and the nuclear spin–lattice relaxation rate, 1=T1 ; provide valuable information on the magnetic structure, the fluctuations of the magnetic moments and also the superconducting energy gap D: In this paper, we report the study of 115 In NQR in CeCoIn5 under high pressure. The hydrostatic pressure is the ideal method for the control of spin fluctuations, without introducing extrinsic disorder, and changes the distance of the system from the quantum critical point. There are two non-equivalent In sites in CeCoIn5 : One site is located in the Ce–In plane in which In ion is surrounded by Ce, and the other site by Ce and Co. We performed 115 In NQR of In in the Ce–In plane. Using 115 In NQR, the recovery of the nuclear magnetization mðtÞ after a saturation pulse was measured. Then, the value of 1=T1 was determined by fitting the expected functional form
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.196
ARTICLE IN PRESS Y. Kohori et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 189–190
190
1000 0 kbar 1/T1 ~T1/4
2.5 kbar
TC
5.2 kbar 100 14 kbar 1/T1 (s-1)
TC 1/T1 ~T
10
CeCoIn5 1 1
10
100
T (K) Fig. 1. T-dependence of 1=T1 :
of mðtÞ for spin I ¼ 92: The pressure, P; up to 14 kbar was applied by BeCu/NiCrAl piston and cylinder cells, filled with a mixture of Fluorinert 70 and 77. The magnitude of P was determined by the superconducting transition of Sn. The T- and P-dependence of 1=T1 ’s are shown in Fig. 1. The 1=T1 divided by T; 1=T1 T reveals the spinfluctuation character from the q averaged dynamical susceptibility w00 ðq; oÞ: In the conventional Fermi liquid state, 1=T1 T becomes T independent, and the deviation from this behaviour indicates the presence of the magnetic correlation in the material. At ambient pressure, the observed T dependence is close to T 1=4 in
the normal state. This behaviour is explained in the spin fluctuation theory as the system located just at the AF instability (AF quantum critical point) [3]. In the superconducting state, 1=T1 decreases rapidly below TC ð2:3 KÞ and varies in nearly proportionally to T 3 at low temperatures. This indicates that the superconducting energy gap has line nodes. The value of 1=T1 in normal state decreases rapidly with increasing P: At the pressure range between 2.5 and 5 kbar; 1=T1 varies proportionally to T 1=4 above 6 K; and then becomes slightly increases with decreasing T below 3 K: The 1=T1 below TC decreases steeper than that observed at ambient P: The reduction of the AF spin fluctuations in the superconducting state would be responsible for this very rapid T-dependence. At P ¼ 14 kbar; 1=T1 does not show Fermi liquid behavior ð1=T1 pTÞ at low T; but its T-dependence represents that the system moves close to the Fermi liquid state. The T-dependence of 1=T1 in the superconducting state is similar to that observed at ambient pressure, which means that the ratio of the superconducting energy gap to its TC does not depend on P for Pp14 kbar: In conclusion, the T-dependence of 1=T1 in the normal state of CeCoIn5 under high P has shown that applied P moves the system from the AF critical point at ambient P to nearly Fermi liquid state (just above TC ) at 14 kbar: In the superconducting state, 1=T1 has indicated that an anisotropic superconductor with line nodes appears. The ratio of the superconducting energy gap to its TC does not depend on P for Pp14 kbar: This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] H. Hegger, et al., Phys. Rev. Lett. 84 (2000) 4986. [2] C. Petrovic, et al., J. Phys. Condens. Matter 13 (2001) L337. [3] A. Ishigaki, T. Moriya, J. Phys. Soc. Japan 65 (1996) 3402.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 189–190
115
In NQR study of superconducting CeCoIn5 under high pressure
Y. Kohoria,*, H. Saitob, Y. Kobayashib, H. Tairab, Y. Iwamotoc, T. Koharad, T. Matsumotoe, E.D. Bauerf, M.B. Maplef, J.L. Sarraog a
Department of Physics, Faculty of Science, Chiba University, Inage-ku, Chiba 263-8522, Japan Graduate School of Science and Technology, Chiba University, Inage-ku, Chiba 263-8522, Japan c Department of Transportation System Engineering, Kobe University of Mercantile Marine, Higashi-nada 658-0022, Japan d Division of Material Science, Graduate School of Science, Himeji Institute of Technology, Ako-gun, Hyogo 678-1297, Japan e National Institute for Material Science, 1-2-1 Sengen,Tsukuba, Ibaraki 305-0047, Japan f Department of Physics and Institute of Pure and Applied Physics, University of California, San Diego, La Jolla, CA 92093, USA g Condensed Matter and Thermal Physics, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b
Abstract We have carried out 115 In nuclear quadrupole resonance measurements on CeCoIn5 under high pressure P: The temperature T dependence of the nuclear spin–lattice relaxation rate 1=T1 in the normal state, 1=T1 pT 1=4 ; indicates that CeCoIn5 is located just at an antiferromagnetic (AF) instability at ambient pressure. The value of 1=T1 decreases rapidly with increasing P; reflecting that P moves CeCoIn5 away from the AF instability. r 2004 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 76.60.Gv; 74.20.Mn Keywords: NQR; Heavy fermion superconductor; CeCoIn5
The superconductivity located near the antiferromagnetic (AF) state of heavy fermion system has become one of the fundamental issues in condensed matter physics. Analogy with the high TC cuprate superconductors suggests that the d-wave superconductivitymediated via AF spin fluctuations is realized in the system. Recently, CeTIn5 (T ¼ Co; Rh and Ir) has been discovered. In CeRhIn5 ; the ground state at ambient pressure is antiferromagnet with a Ne! el temperature TN of 3:8 K: Application of a hydrostatic pressure of about 17 kbar induces a transition to the superconducting state with the critical temperature TC of 2:1 K [1]. In CeCoIn5 ; the superconductivity with TC of 2:3 K appears at ambient pressure [2]. It is an important theme of heavy fermion *Corresponding author. Tel.: +81-43-290-2766; fax: +8143-290-2874. E-mail address: [email protected] (Y. Kohori).
system to study the interplay of magnetism and superconductivity, and CeTIn5 is suitable for the study. Nuclear quadrupole resonance (NQR) is a powerful method for the study of magnetism and superconductivity, i.e., the spectrum and the nuclear spin–lattice relaxation rate, 1=T1 ; provide valuable information on the magnetic structure, the fluctuations of the magnetic moments and also the superconducting energy gap D: In this paper, we report the study of 115 In NQR in CeCoIn5 under high pressure. The hydrostatic pressure is the ideal method for the control of spin fluctuations, without introducing extrinsic disorder, and changes the distance of the system from the quantum critical point. There are two non-equivalent In sites in CeCoIn5 : One site is located in the Ce–In plane in which In ion is surrounded by Ce, and the other site by Ce and Co. We performed 115 In NQR of In in the Ce–In plane. Using 115 In NQR, the recovery of the nuclear magnetization mðtÞ after a saturation pulse was measured. Then, the value of 1=T1 was determined by fitting the expected functional form
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.196
ARTICLE IN PRESS Y. Kohori et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 189–190
190
1000 0 kbar 1/T1 ~T1/4
2.5 kbar
TC
5.2 kbar 100 14 kbar 1/T1 (s-1)
TC 1/T1 ~T
10
CeCoIn5 1 1
10
100
T (K) Fig. 1. T-dependence of 1=T1 :
of mðtÞ for spin I ¼ 92: The pressure, P; up to 14 kbar was applied by BeCu/NiCrAl piston and cylinder cells, filled with a mixture of Fluorinert 70 and 77. The magnitude of P was determined by the superconducting transition of Sn. The T- and P-dependence of 1=T1 ’s are shown in Fig. 1. The 1=T1 divided by T; 1=T1 T reveals the spinfluctuation character from the q averaged dynamical susceptibility w00 ðq; oÞ: In the conventional Fermi liquid state, 1=T1 T becomes T independent, and the deviation from this behaviour indicates the presence of the magnetic correlation in the material. At ambient pressure, the observed T dependence is close to T 1=4 in
the normal state. This behaviour is explained in the spin fluctuation theory as the system located just at the AF instability (AF quantum critical point) [3]. In the superconducting state, 1=T1 decreases rapidly below TC ð2:3 KÞ and varies in nearly proportionally to T 3 at low temperatures. This indicates that the superconducting energy gap has line nodes. The value of 1=T1 in normal state decreases rapidly with increasing P: At the pressure range between 2.5 and 5 kbar; 1=T1 varies proportionally to T 1=4 above 6 K; and then becomes slightly increases with decreasing T below 3 K: The 1=T1 below TC decreases steeper than that observed at ambient P: The reduction of the AF spin fluctuations in the superconducting state would be responsible for this very rapid T-dependence. At P ¼ 14 kbar; 1=T1 does not show Fermi liquid behavior ð1=T1 pTÞ at low T; but its T-dependence represents that the system moves close to the Fermi liquid state. The T-dependence of 1=T1 in the superconducting state is similar to that observed at ambient pressure, which means that the ratio of the superconducting energy gap to its TC does not depend on P for Pp14 kbar: In conclusion, the T-dependence of 1=T1 in the normal state of CeCoIn5 under high P has shown that applied P moves the system from the AF critical point at ambient P to nearly Fermi liquid state (just above TC ) at 14 kbar: In the superconducting state, 1=T1 has indicated that an anisotropic superconductor with line nodes appears. The ratio of the superconducting energy gap to its TC does not depend on P for Pp14 kbar: This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] H. Hegger, et al., Phys. Rev. Lett. 84 (2000) 4986. [2] C. Petrovic, et al., J. Phys. Condens. Matter 13 (2001) L337. [3] A. Ishigaki, T. Moriya, J. Phys. Soc. Japan 65 (1996) 3402.