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NMR evidence of BCS superconductivity in Y(Ni1−xPtx)2B2C
Physica B 259—261 (1999) 594—595
NMR evidence of BCS superconductivity in Y(Ni Pt ) B C \V V K. Mizuno *, T. Saito , H. Fudo , K. Koyama , K. Endo, H. Deguchi Faculty of Integrated Arts and Sciences, Tokushima University, Tokushima 770-8502, Japan Department of Physics, Tokyo Medical College, Tokyo 160-8402, Japan Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu, 804-8550, Japan
Abstract C and B NMR studies have been made on the quaternary superconductor Y(Ni Pt ) B C with x"0 and 0.4. \V V Temperature dependence of Knight shift and spin-lattice relaxation rate (SLRR) in the normal state was explained consistently among all the observed nuclei by Korringa mechanism without taking account of antiferromagnetic spin fluctuation. Bellow the superconducting transition temperature we observed a Hebel—Slichter peak in case of C SLRR. These results mean that the borocarbide is a normal metal and a phonon-mediated superconductor 1999 Elsevier Science B.V. All rights reserved. Keywords: NMR; Superconductivity; Borocarbide
1. Introduction Borocarbide superconductors have a relatively high superconducting transition temperature ¹ . One of the interests is whether antiferromagnetic (AF) spin fluctuation of Ni plays an important role in the normal state [1,2] or not [3,4]. The mechanism of superconductivity in the borocarbide superconductor is still a point of controversy. The possibility of unconventional superconductivity was discussed, because no one observed the Hebel—Slichter coherence peak of SLRR just below ¹ in B NMR, while a small peak is observed in C and Pt NMR [4,5]. In this paper, we present a report of C and B NMR study on Y(Ni Pt ) B C in normal \V V and superconducting state.
2. Sample preparation Polycrystalline samples Y(Ni Pt ) B C [6] of x"0 \V V and 0.4 were prepared by conventional arc-melting as
* Corresponding author. Fax: (#81) 886 56 7298; e-mail: [email protected].
described in Ref. [4]. In order to increase the intensity of C NMR signal, C enriched amorphous carbon powder was mixed with ordinary graphite powder. The net abundance of C was chosen to be 50%. The samples were wrapped in Ta foil, evacuated to 2;10\ Torr, sealed in the quartz tube, and annealed at T"1323 K for 20 h. They were pulverized into powder and cured with epoxy (Stycast 1266) in a magnetic field, which produced aligned samples with H Nc. Resistivity and magnetiz ation measurements showed ¹ "15.1 K (12.3 K in H "1.2 T) and 13.1 K (10.3 K in 1.2 T) for x"0 and 0.4, respectively. 3.
13
C and 11B NMR in the normal state
C and B NMR signals were observed by pulse-FT technique at magnetic fields of 1.2 and 0.7 T. As seen in the inset of Fig. 1, a certain increase of (¹ ¹)\ is observed with lowering temperature in the normal state. The same increase is also observed in B SLRR. These were interpreted by the contribution of AF spin fluctuation [1,2] with some paramagnetic impurities [3]. As shown in Fig. 1, we can fit our data to K "K #(S/K(a)¹ ¹)\ ,
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 0 3 - X
(1)
K. Mizuno et al. / Physica B 259—261 (1999) 594—595
Fig. 1. Shift K versus (¹ ¹)\ in YNi B C (open circles) and in Y(Ni Pt ) B C (closed circles) in the normal state. Straight line indicates that the temperature dependence of K and ¹ is well described by the Korringa mechanism only. Inset shows the temperature dependence of (¹ ¹)\.
where S"(h/8pk ) (c/c) and K(a) is an enhancement factor due to the electron—electron interaction. This linear fit found in C and B NMR means that the observed SLRR originates from Korringa mechanism only. No contribution of AF spin fluctuation nor paramagnetic impurities is needed to be taken into account. Same slope of the two lines as seen in Fig. 1 implies that the electron—electron interaction does not change by Pt substitution. The enhancement factor a"0.64 is obtained both in C and B NMR. This value is slightly larger than that of alkali metals (e.g. a"0.60 for Na) but smaller than transient metals (e.g. a"0.68 for Cu). Therefore the normal state is well explained by Korringa mechanism with some temperature dependence of uniform susceptibility s (q"0, u"0).
4.
13
C NMR in the superconducting state
Temperature dependence of C SLRR in the superconducting state is shown in Fig. 2. Small but clear enhancement below ¹ is observed. This is an evidence for BCS s wave superconductor. Field independence of the peak height and peak position guarantees that the peak does not originate from the vortex motion. Because the enhancement does not change with the applied field, the small enhancement should be a consequence of energy gap with strong anisotropy. When the superconducting mechanism is an ordinary phonon mediated one, the enlargement of the peak might be expected if Pt acts as a non-magnetic scatterer to the carrier of electrical current. However, the fact that little change of the peak height could be observed indicates that the Pt substitution is not so effective to enlarge the
595
Fig. 2. Temperature dependence of C ¹\ in YNi B C (open circles for H "1.2 T and open squares for 0.7 T) and in Y(Ni Pt ) B C (closed circles for 1.2 T) in the superconduct ing state. Inset shows the dependence of ¹ on inverse reduced temperature.
relaxation rate of the carriers. This result is consistent with the resistivity measurement which showed almost the same value just above ¹ independent of Pt concen tration: o&5;10\ ) m. As shown in the inset of Fig. 2, the temperature dependence of C ¹ below 0.8¹ can be fitted to ¹ J exp (* /k ¹), (2) where * is the superconducting gap parameter. We obtained 2* /k ¹ "3.3$0.1, which is close to the one of the weak-coupling BCS theory: 2D /k ¹ "3.52. It is consistent with results of the other kinds of experiments and is also consistent with band theory, which predicts that this borocarbide compound is a phonon-mediated s wave BCS superconductor.
5. Summary In summary, we have studied C and B NMR on the quaternary borocarbide compounds of Y(Ni Pt ) B C in normal and superconducting states. \V V C and B NMR are well described by the Korringa mechanism in the normal state. In the superconducting state, the C SLRR showed Hebel-Slichter coherence peak just below T . These facts, suggest borocarbide is a conventional BCS superconductor with strong anisotropy of the superconducting energy gap.
References [1] [2] [3] [4] [5] [6]
K. Ikushima et al., J. Phys. Soc. Japan 63 (1994) 2878. T. Kohara et al., Phys. Rev. B 51 (1995) 3985. B.J. Suh et al., Phys. Rev B 53 (1996) R6022. T. Saito, et al., J. Magn. Magn. Mater. 177—181 (1998) 557. S. Ikeda et al., Physica C 263 (1996) 526. F. Yang et al., J. Phys.: Condens. Matter 7 (1995) 2369.
NMR evidence of BCS superconductivity in Y(Ni Pt ) B C \V V K. Mizuno *, T. Saito , H. Fudo , K. Koyama , K. Endo, H. Deguchi Faculty of Integrated Arts and Sciences, Tokushima University, Tokushima 770-8502, Japan Department of Physics, Tokyo Medical College, Tokyo 160-8402, Japan Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu, 804-8550, Japan
Abstract C and B NMR studies have been made on the quaternary superconductor Y(Ni Pt ) B C with x"0 and 0.4. \V V Temperature dependence of Knight shift and spin-lattice relaxation rate (SLRR) in the normal state was explained consistently among all the observed nuclei by Korringa mechanism without taking account of antiferromagnetic spin fluctuation. Bellow the superconducting transition temperature we observed a Hebel—Slichter peak in case of C SLRR. These results mean that the borocarbide is a normal metal and a phonon-mediated superconductor 1999 Elsevier Science B.V. All rights reserved. Keywords: NMR; Superconductivity; Borocarbide
1. Introduction Borocarbide superconductors have a relatively high superconducting transition temperature ¹ . One of the interests is whether antiferromagnetic (AF) spin fluctuation of Ni plays an important role in the normal state [1,2] or not [3,4]. The mechanism of superconductivity in the borocarbide superconductor is still a point of controversy. The possibility of unconventional superconductivity was discussed, because no one observed the Hebel—Slichter coherence peak of SLRR just below ¹ in B NMR, while a small peak is observed in C and Pt NMR [4,5]. In this paper, we present a report of C and B NMR study on Y(Ni Pt ) B C in normal \V V and superconducting state.
2. Sample preparation Polycrystalline samples Y(Ni Pt ) B C [6] of x"0 \V V and 0.4 were prepared by conventional arc-melting as
* Corresponding author. Fax: (#81) 886 56 7298; e-mail: [email protected].
described in Ref. [4]. In order to increase the intensity of C NMR signal, C enriched amorphous carbon powder was mixed with ordinary graphite powder. The net abundance of C was chosen to be 50%. The samples were wrapped in Ta foil, evacuated to 2;10\ Torr, sealed in the quartz tube, and annealed at T"1323 K for 20 h. They were pulverized into powder and cured with epoxy (Stycast 1266) in a magnetic field, which produced aligned samples with H Nc. Resistivity and magnetiz ation measurements showed ¹ "15.1 K (12.3 K in H "1.2 T) and 13.1 K (10.3 K in 1.2 T) for x"0 and 0.4, respectively. 3.
13
C and 11B NMR in the normal state
C and B NMR signals were observed by pulse-FT technique at magnetic fields of 1.2 and 0.7 T. As seen in the inset of Fig. 1, a certain increase of (¹ ¹)\ is observed with lowering temperature in the normal state. The same increase is also observed in B SLRR. These were interpreted by the contribution of AF spin fluctuation [1,2] with some paramagnetic impurities [3]. As shown in Fig. 1, we can fit our data to K "K #(S/K(a)¹ ¹)\ ,
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 0 3 - X
(1)
K. Mizuno et al. / Physica B 259—261 (1999) 594—595
Fig. 1. Shift K versus (¹ ¹)\ in YNi B C (open circles) and in Y(Ni Pt ) B C (closed circles) in the normal state. Straight line indicates that the temperature dependence of K and ¹ is well described by the Korringa mechanism only. Inset shows the temperature dependence of (¹ ¹)\.
where S"(h/8pk ) (c/c) and K(a) is an enhancement factor due to the electron—electron interaction. This linear fit found in C and B NMR means that the observed SLRR originates from Korringa mechanism only. No contribution of AF spin fluctuation nor paramagnetic impurities is needed to be taken into account. Same slope of the two lines as seen in Fig. 1 implies that the electron—electron interaction does not change by Pt substitution. The enhancement factor a"0.64 is obtained both in C and B NMR. This value is slightly larger than that of alkali metals (e.g. a"0.60 for Na) but smaller than transient metals (e.g. a"0.68 for Cu). Therefore the normal state is well explained by Korringa mechanism with some temperature dependence of uniform susceptibility s (q"0, u"0).
4.
13
C NMR in the superconducting state
Temperature dependence of C SLRR in the superconducting state is shown in Fig. 2. Small but clear enhancement below ¹ is observed. This is an evidence for BCS s wave superconductor. Field independence of the peak height and peak position guarantees that the peak does not originate from the vortex motion. Because the enhancement does not change with the applied field, the small enhancement should be a consequence of energy gap with strong anisotropy. When the superconducting mechanism is an ordinary phonon mediated one, the enlargement of the peak might be expected if Pt acts as a non-magnetic scatterer to the carrier of electrical current. However, the fact that little change of the peak height could be observed indicates that the Pt substitution is not so effective to enlarge the
595
Fig. 2. Temperature dependence of C ¹\ in YNi B C (open circles for H "1.2 T and open squares for 0.7 T) and in Y(Ni Pt ) B C (closed circles for 1.2 T) in the superconduct ing state. Inset shows the dependence of ¹ on inverse reduced temperature.
relaxation rate of the carriers. This result is consistent with the resistivity measurement which showed almost the same value just above ¹ independent of Pt concen tration: o&5;10\ ) m. As shown in the inset of Fig. 2, the temperature dependence of C ¹ below 0.8¹ can be fitted to ¹ J exp (* /k ¹), (2) where * is the superconducting gap parameter. We obtained 2* /k ¹ "3.3$0.1, which is close to the one of the weak-coupling BCS theory: 2D /k ¹ "3.52. It is consistent with results of the other kinds of experiments and is also consistent with band theory, which predicts that this borocarbide compound is a phonon-mediated s wave BCS superconductor.
5. Summary In summary, we have studied C and B NMR on the quaternary borocarbide compounds of Y(Ni Pt ) B C in normal and superconducting states. \V V C and B NMR are well described by the Korringa mechanism in the normal state. In the superconducting state, the C SLRR showed Hebel-Slichter coherence peak just below T . These facts, suggest borocarbide is a conventional BCS superconductor with strong anisotropy of the superconducting energy gap.
References [1] [2] [3] [4] [5] [6]
K. Ikushima et al., J. Phys. Soc. Japan 63 (1994) 2878. T. Kohara et al., Phys. Rev. B 51 (1995) 3985. B.J. Suh et al., Phys. Rev B 53 (1996) R6022. T. Saito, et al., J. Magn. Magn. Mater. 177—181 (1998) 557. S. Ikeda et al., Physica C 263 (1996) 526. F. Yang et al., J. Phys.: Condens. Matter 7 (1995) 2369.