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In NQR study of heavy fermion compound Ce2CoIn8
ARTICLE IN PRESS
Physica B 359–361 (2005) 181–183 www.elsevier.com/locate/physb
In NQR study of heavy fermion compound Ce2 CoIn8 Hideto Fukazawaa,b,, Hiroshi Tairaa, Yoh Kohoria,b, Genfu Chenc, Shigeo Oharac, Isao Sakamotoc a
Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan b Department of Physics, Chiba University, Chiba 263-8522, Japan c Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
Abstract We have performed nuclear quadrupole resonance (NQR) measurements on 115In nuclei of Ce2 CoIn8 : Spin–lattice relaxation rate 1=T 1 remains nearly constant at high temperatures. With decreasing temperature T; 1=T 1 decreases and is proportional to T above T SC : 1=T 1 of Ce2 CoIn8 is quite similar to that of pressurized CeIn3 at about 2.7 GPa. The sign of superconductivity was observed at around 0.4 K. r 2005 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 74.20.Mn; 76.60.Gv Keywords: NQR; Heavy fermion superconductor; Spin–lattice relaxation rate
Quantum criticality and its related superconductivity is one of the important subjects of the strongly correlated electron systems [1]. Unconventional superconductivity has been indeed discovered in the pressurized CeIn3 near the magnetic instability point [1]. In these materials, the magnetic correlation arises from competition and/or cooperation between the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and the Corresponding author. Graduate School of Science and
Technology, Chiba University, Chiba 263-8522, Japan. E-mail address: [email protected] (H. Fukazawa).
Kondo effect: both effects are based on the hybridization effect of the localized f-electrons and the itinerant conduction-electrons. Because the pressure tunes the strength of this hybridization effect, it can continuously vary the system between the magnetically ordered phase and the magnetically disordered phase without impurity. Hence, the pressure study is a powerful tool to investigate the quantum criticality and the unconventional superconductivity. On the other hand, it is also vital to examine the relation between the dimensionality of the system and the quantum criticality [2], since the magnetic
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.029
ARTICLE IN PRESS 182
H. Fukazawa et al. / Physica B 359– 361 (2005) 181–183
spin fluctuation and the superconducting-phasetransition temperature generally increases with decreasing the dimensionality of the system. Cebased heavy-fermion compounds with the formula Cen CoIn3nþ2 ðn ¼ 1; 2; 1Þ are good candidates for this investigation: the most quasi-two-dimensional CeCoIn5 is a superconductor with T SC ¼ 2:3 K [3]; Ce2 CoIn8 is reported as a superconductor with T SC ¼ 0:4 K [4]; the most three-dimensional CeIn3 is an antiferromagnet with T N ¼ 10 K and becomes superconducting under pressure of P 2:7 GPa with T SC ¼ 0:2 K [1]. Although the ground state of the pressurized CeIn3 does not contain a quantum critical point (QCP) [5], those of the other two candidates are considered to be close to the antiferromagnetic (AF) QCP [4,6]. Among these compounds, Ce2 CoIn8 whose crystal structure is Ho2 CoGa8 -type tetragonal structure is not fully studied, because it is difficult to synthesize the material. However, Chen et al. recently succeeded in synthesizing it and reported the superconductivity in this compound [4]. It exhibits zero resistivity below T SC ¼ 0:4 K at ambient pressure and exhibits a broad peak around 30 K [4]. In addition, the electronic specific-heat-coefficient g is 500 mJ=K2 mol Ce: These indicate that this compound is a heavy fermion superconductor. It is also noteworthy that the dimensionality of this compound is between that of CeCoIn5 and CeIn3 ; since its crystal structure can be viewed as the successive layers of CeCoIn5 and CeIn3 : Because the resistivity measurement reflects a bulk property, there might be influence of parasitic phases. Therefore, a sitesensitive measurement is necessary to study the relation between the dimensionality of Ce2 CoIn8 and its quantum critical behavior, and to confirm whether superconductivity in Ce2 CoIn8 is intrinsic. In order to reveal essential properties, we have performed nuclear quadrupole resonance (NQR) measurements on 115In nuclei of Ce2 CoIn8 : The 115In NQR measurements were performed using a phase-coherent pulsed NQR spectrometer in the range 7–100 MHz. Single crystals of Ce2 CoIn8 were grown by an In self-flux method. The spin–lattice relaxation rate T 1 was obtained by the recovery of the nuclear magnetization after a saturation pulse. For T 1 measurements, the
crystals were crushed into powders in order to gain the sufficient NQR signal-intensity. We confirmed that the T 1 measured with powders and that with single crystals are identical at 4.2 K. The NQR spectrum of Ce2 CoIn8 was obtained at 4.2 K. Signal intensity from a parasitic phase CeCoIn5 can be seen. By estimating the ratio of the intensity of Ce2 CoIn8 to the corresponding intensity of CeCoIn5 ; we expect that the fraction of the parasitic phase is at least less than 10%. Moreover, we also expect that the crystal distortion in Ce2 CoIn8 induced by the existence of the parasitic phases is quite small since the signal intensity width of the present Ce2 CoIn8 crystals is nearly equal to the corresponding width of the pure CeCoIn5 crystals. All the three independent In sites of Ce2 CoIn8 are assigned from NQR spectrum: a nuclear resonance frequency nQ ðMHzÞ and an assymmetric parameter Z are (9.28, 0.08) for In(1), (8.71, 0.00) for In(2) and (15.39, 0.34) for In(3). In Fig. 1 we show the temperature T dependence of spin–lattice relaxation rate 1=T 1 of Ce2 CoIn8 : 1=T 1 remains nearly constant above about 30 K. This indicates that 4f electrons of Ce ions are nearly localized above this temperature
Fig. 1. Spin–lattice relaxation rate of Ce2 CoIn8 : Solid line represents the T linear curve.
ARTICLE IN PRESS H. Fukazawa et al. / Physica B 359– 361 (2005) 181–183
and is consistent with the experimental results of resistivity [4]. With decreasing temperature below 30 K, 1=T 1 decreases. 1=T 1 is proportional to T 1=2 between 4 and 25 K, but more rapidly decreases and is proportional to T below 4 K. Deviation from T linear behavior below 4 K is ascribable to the slight heat up of the samples. This suggests that the heavy quasi particles in Ce2 CoIn8 behave as fermi liquid at low temperatures and that the system is rather far from the AF QCP, which is located near the ground state of ambient CeCoIn5 [6]. We note that T dependence of 1=T 1 of Ce2 CoIn8 is quite similar to that of pressurized CeIn3 at about 2.7 GPa [7], where the fermi liquid behavior is well established above T SC : We also note another possibility that the AF spin fluctuations cancel out because of the form factor of the In(2) site and that this leads to the T linear behavior below 4 K. The most remarkable feature of the 1=T 1 is the sudden decrease below 0.4 K with decreasing T: Since the resonance frequency for the T 1 measurements is separated from that of the parasitic phase CeCoIn5 ; this sign is an intrinsic property of Ce2 CoIn8 and may be attributable to a superconducting phase transition.
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In summary, we have performed In-NQR measurements on Ce2 CoIn8 : 1=T 1 remains nearly constant at high temperatures. With decreasing T; 1=T 1 decreases and is proportional to T above T SC : 1=T 1 of Ce2 CoIn8 is quite similar to that of pressurized CeIn3 at about 2.7 GPa. The superconducting phase transition occurs at around 0.4 K. This is consistent with the report based on the resistivity measurements. This work has been supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. References [1] N.D. Mathur, et al., Nature 394 (1998) 39. [2] T. Takimoto, T. Moriya, Phys. Rev. B 66 (2002) 134516; H. Fukazawa, K. Yamada, J. Phys. Soc. Japan 72 (2003) 2449. [3] C. Petrovic, et al., J. Phys.: Condens. Matter 13 (2001) L337. [4] G. Chen, et al., J. Phys. Soc. Japan 71 (2002) 2836. [5] S. Kawasaki, et al., cond-mat/0404376. [6] Y. Kohori, et al., Phys. Rev. B 64 (2001) 134526. [7] Y. Kohori, et al., Physica B 281 & 282 (2000) 12.
Physica B 359–361 (2005) 181–183 www.elsevier.com/locate/physb
In NQR study of heavy fermion compound Ce2 CoIn8 Hideto Fukazawaa,b,, Hiroshi Tairaa, Yoh Kohoria,b, Genfu Chenc, Shigeo Oharac, Isao Sakamotoc a
Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan b Department of Physics, Chiba University, Chiba 263-8522, Japan c Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
Abstract We have performed nuclear quadrupole resonance (NQR) measurements on 115In nuclei of Ce2 CoIn8 : Spin–lattice relaxation rate 1=T 1 remains nearly constant at high temperatures. With decreasing temperature T; 1=T 1 decreases and is proportional to T above T SC : 1=T 1 of Ce2 CoIn8 is quite similar to that of pressurized CeIn3 at about 2.7 GPa. The sign of superconductivity was observed at around 0.4 K. r 2005 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 74.20.Mn; 76.60.Gv Keywords: NQR; Heavy fermion superconductor; Spin–lattice relaxation rate
Quantum criticality and its related superconductivity is one of the important subjects of the strongly correlated electron systems [1]. Unconventional superconductivity has been indeed discovered in the pressurized CeIn3 near the magnetic instability point [1]. In these materials, the magnetic correlation arises from competition and/or cooperation between the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and the Corresponding author. Graduate School of Science and
Technology, Chiba University, Chiba 263-8522, Japan. E-mail address: [email protected] (H. Fukazawa).
Kondo effect: both effects are based on the hybridization effect of the localized f-electrons and the itinerant conduction-electrons. Because the pressure tunes the strength of this hybridization effect, it can continuously vary the system between the magnetically ordered phase and the magnetically disordered phase without impurity. Hence, the pressure study is a powerful tool to investigate the quantum criticality and the unconventional superconductivity. On the other hand, it is also vital to examine the relation between the dimensionality of the system and the quantum criticality [2], since the magnetic
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.029
ARTICLE IN PRESS 182
H. Fukazawa et al. / Physica B 359– 361 (2005) 181–183
spin fluctuation and the superconducting-phasetransition temperature generally increases with decreasing the dimensionality of the system. Cebased heavy-fermion compounds with the formula Cen CoIn3nþ2 ðn ¼ 1; 2; 1Þ are good candidates for this investigation: the most quasi-two-dimensional CeCoIn5 is a superconductor with T SC ¼ 2:3 K [3]; Ce2 CoIn8 is reported as a superconductor with T SC ¼ 0:4 K [4]; the most three-dimensional CeIn3 is an antiferromagnet with T N ¼ 10 K and becomes superconducting under pressure of P 2:7 GPa with T SC ¼ 0:2 K [1]. Although the ground state of the pressurized CeIn3 does not contain a quantum critical point (QCP) [5], those of the other two candidates are considered to be close to the antiferromagnetic (AF) QCP [4,6]. Among these compounds, Ce2 CoIn8 whose crystal structure is Ho2 CoGa8 -type tetragonal structure is not fully studied, because it is difficult to synthesize the material. However, Chen et al. recently succeeded in synthesizing it and reported the superconductivity in this compound [4]. It exhibits zero resistivity below T SC ¼ 0:4 K at ambient pressure and exhibits a broad peak around 30 K [4]. In addition, the electronic specific-heat-coefficient g is 500 mJ=K2 mol Ce: These indicate that this compound is a heavy fermion superconductor. It is also noteworthy that the dimensionality of this compound is between that of CeCoIn5 and CeIn3 ; since its crystal structure can be viewed as the successive layers of CeCoIn5 and CeIn3 : Because the resistivity measurement reflects a bulk property, there might be influence of parasitic phases. Therefore, a sitesensitive measurement is necessary to study the relation between the dimensionality of Ce2 CoIn8 and its quantum critical behavior, and to confirm whether superconductivity in Ce2 CoIn8 is intrinsic. In order to reveal essential properties, we have performed nuclear quadrupole resonance (NQR) measurements on 115In nuclei of Ce2 CoIn8 : The 115In NQR measurements were performed using a phase-coherent pulsed NQR spectrometer in the range 7–100 MHz. Single crystals of Ce2 CoIn8 were grown by an In self-flux method. The spin–lattice relaxation rate T 1 was obtained by the recovery of the nuclear magnetization after a saturation pulse. For T 1 measurements, the
crystals were crushed into powders in order to gain the sufficient NQR signal-intensity. We confirmed that the T 1 measured with powders and that with single crystals are identical at 4.2 K. The NQR spectrum of Ce2 CoIn8 was obtained at 4.2 K. Signal intensity from a parasitic phase CeCoIn5 can be seen. By estimating the ratio of the intensity of Ce2 CoIn8 to the corresponding intensity of CeCoIn5 ; we expect that the fraction of the parasitic phase is at least less than 10%. Moreover, we also expect that the crystal distortion in Ce2 CoIn8 induced by the existence of the parasitic phases is quite small since the signal intensity width of the present Ce2 CoIn8 crystals is nearly equal to the corresponding width of the pure CeCoIn5 crystals. All the three independent In sites of Ce2 CoIn8 are assigned from NQR spectrum: a nuclear resonance frequency nQ ðMHzÞ and an assymmetric parameter Z are (9.28, 0.08) for In(1), (8.71, 0.00) for In(2) and (15.39, 0.34) for In(3). In Fig. 1 we show the temperature T dependence of spin–lattice relaxation rate 1=T 1 of Ce2 CoIn8 : 1=T 1 remains nearly constant above about 30 K. This indicates that 4f electrons of Ce ions are nearly localized above this temperature
Fig. 1. Spin–lattice relaxation rate of Ce2 CoIn8 : Solid line represents the T linear curve.
ARTICLE IN PRESS H. Fukazawa et al. / Physica B 359– 361 (2005) 181–183
and is consistent with the experimental results of resistivity [4]. With decreasing temperature below 30 K, 1=T 1 decreases. 1=T 1 is proportional to T 1=2 between 4 and 25 K, but more rapidly decreases and is proportional to T below 4 K. Deviation from T linear behavior below 4 K is ascribable to the slight heat up of the samples. This suggests that the heavy quasi particles in Ce2 CoIn8 behave as fermi liquid at low temperatures and that the system is rather far from the AF QCP, which is located near the ground state of ambient CeCoIn5 [6]. We note that T dependence of 1=T 1 of Ce2 CoIn8 is quite similar to that of pressurized CeIn3 at about 2.7 GPa [7], where the fermi liquid behavior is well established above T SC : We also note another possibility that the AF spin fluctuations cancel out because of the form factor of the In(2) site and that this leads to the T linear behavior below 4 K. The most remarkable feature of the 1=T 1 is the sudden decrease below 0.4 K with decreasing T: Since the resonance frequency for the T 1 measurements is separated from that of the parasitic phase CeCoIn5 ; this sign is an intrinsic property of Ce2 CoIn8 and may be attributable to a superconducting phase transition.
183
In summary, we have performed In-NQR measurements on Ce2 CoIn8 : 1=T 1 remains nearly constant at high temperatures. With decreasing T; 1=T 1 decreases and is proportional to T above T SC : 1=T 1 of Ce2 CoIn8 is quite similar to that of pressurized CeIn3 at about 2.7 GPa. The superconducting phase transition occurs at around 0.4 K. This is consistent with the report based on the resistivity measurements. This work has been supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. References [1] N.D. Mathur, et al., Nature 394 (1998) 39. [2] T. Takimoto, T. Moriya, Phys. Rev. B 66 (2002) 134516; H. Fukazawa, K. Yamada, J. Phys. Soc. Japan 72 (2003) 2449. [3] C. Petrovic, et al., J. Phys.: Condens. Matter 13 (2001) L337. [4] G. Chen, et al., J. Phys. Soc. Japan 71 (2002) 2836. [5] S. Kawasaki, et al., cond-mat/0404376. [6] Y. Kohori, et al., Phys. Rev. B 64 (2001) 134526. [7] Y. Kohori, et al., Physica B 281 & 282 (2000) 12.