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Neutron scattering studies on heavy fermion superconductors
Physica B 281&282 (2000) 987}988
Neutron scattering studies on heavy fermion superconductors N. Metoki!,*, Y. Koike!, Y. Haga, E. Yamamoto!, T. Honma!, Y. O1 nuki!,", N. Kimura#, K. Maezawa$ !Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195, Japan "Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan #Center of Low Temperature Science, Tohoku University, Sendai 980-0845, Japan $Faculty of Engineering, Toyama Prefectural University, Toyama 939-0398, Japan
Abstract We present the experimental evidence for the strong interplay between magnetism and superconductivity in heavy fermion superconductors. In UPd Al , we found the superconducting gap appearing in magnetic excitation spectra. We 2 3 also report the long-range magnetic correlation of the weak antiferromagnetism in UPt below 20 mK, which would be 3 a precursive phenomenon of the static ordering. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron scattering; Heavy fermion superconductors; UPd Al ; UPt 2 3 3
Interplay between magnetism and superconductivity in heavy fermion superconductors attract much interest (for example, [1]). In heavy fermion superconductors the superconductivity coexists with the antiferromagnetic (AFM) ordering of magnetic 5f electrons. Superconductivity appears in the heavy fermi-liquid state due to hybridization of conduction electron and f-electrons. Therefore, the interplay between superconductivity and magnetism is a key concept for the ground state properties. In this paper, we mention the superconducting gap appearing in the spin excitation of UPd Al [2,3] and 2 3 the long-range magnetic correlation in UPt at ultra-low 3 temperature [4,5]. Neutron scattering experiments were carried out using a cold neutron triple-axis spectrometer LTAS installed at the research reactor JRR-3M in Japan Atomic Energy Research Institute (JAERI). The single-crystal samples were grown by the Czochralski pulling method. The superconducting transition ¹ and the residual resistivity # ratio (o /o ) of UPd Al were determined to be 1.9 K RT 0 2 3 and 60, respectively. Three di!erent quality of UPt sin3
* Corresponding author. Fax.: #81-29-282-5939. E-mail address: [email protected] (N. Metoki)
gle crystals with o /o "300, 470, and 700 were measRT 0 ured to study the sample dependence. Fig. 1 is the low-energy spin excitation spectra of UPd Al at an AFM Bragg point (0 0 0.5). In the normal 2 3 stat (¹'¹ ) we observed a quasielastic and inelastic # response with the excitation energy *E of about 1.5 meV. The quasielastic component becomes weaker with decreasing temperature. Below ¹ , however, the intensity # increases again and we observed inelastic peak with
Fig. 1. Temperature dependence of the neutron inelastic scattering pro"le measured at Q"(0 0 0.5) in UPD Al . 2 3
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 9 0 5 - 9
988
N. Metoki et al. / Physica B 281&282 (2000) 987}988
Fig. 2. The longitudinal scattering pro"le of the (0.5 0 1) magnetic peak in sample d9 in UPt . 3
Fig. 2 is the temperature dependence of the (0.5 0 1) AFM Bragg peak. The AFM peak is observed for all samples. The estimated magnetic moment showed no systematic dependence on sample quality, of about 0.03 lB/U, hence the weak AFM ordering is concluded to be an intrinsic property. We observed a remarkable narrowing of the line width below 50 mK and resolution limited peak at 20 mK. The magnetic correlation length was typically 300}700 As above 50 mK, depending on the sample quality; the spin correlation length becomes longer with increasing the mean free path. Below 50 mK the correlation length increases and diverges at 20 mK. Therefore, we conclude that the speci"c heat anomaly is related to the AFM ordering in UPt . The narrowing of the line 3 width would be a precursive phenomenon of the static ordering.
Acknowledgements energy gap *E"0.36 meV at ¹"0.4 K. The gap opens in the superconducting state and increases continuously with decreasing temperature. In addition, the gap disappears with magnetic "eld higher than the critical "eld H . Therefore, we conclude that this magnetic gap cor#2 responds to the superconducting gap. The unusual behavior of the weak AFM ordering in UPt is one of the most mysterious problems in heavy 3 fermion superconductors. From many experimental data this weak AFM ordering is considered to be a dynamical #uctuation which has a characteristic time scale between NMR and neutron. The speci"c heat anomaly [6] would be a signature of a static ordering at ultra-low temperatures. In order to clarify this anomaly we have carried out ultra-low neutron scattering experiments on UPt [4,5]. 3
One of us (Y.O1 .) was supported by the Grant-in-Aidfor Priority Areas and COE Research (IOCE2004) of the Ministry of Education, Science, Sports and Culture.
References [1] T.E. Mason, G. Aeppli, Proc. Roy. Danish Acad. Sci. Lett. MfM 45 (1996) 231. [2] N. Metoki et al., J. Phys. Soc. Japan 66 (1997) 2560. [3] N. Metoki et al., Phys. Rev. Lett. 80 (1998) 5417. [4] Y. Koike et al., J. Phys. Soc. Japan 67 (1998) 1142. [5] Y. Koike et al., Japan J. Appl. Phys. Ser 11 (1999) 44. [6] E.A. Schuberth et al., Phys. Rev. Lett. 68 (1992) 117.
Neutron scattering studies on heavy fermion superconductors N. Metoki!,*, Y. Koike!, Y. Haga, E. Yamamoto!, T. Honma!, Y. O1 nuki!,", N. Kimura#, K. Maezawa$ !Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195, Japan "Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan #Center of Low Temperature Science, Tohoku University, Sendai 980-0845, Japan $Faculty of Engineering, Toyama Prefectural University, Toyama 939-0398, Japan
Abstract We present the experimental evidence for the strong interplay between magnetism and superconductivity in heavy fermion superconductors. In UPd Al , we found the superconducting gap appearing in magnetic excitation spectra. We 2 3 also report the long-range magnetic correlation of the weak antiferromagnetism in UPt below 20 mK, which would be 3 a precursive phenomenon of the static ordering. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron scattering; Heavy fermion superconductors; UPd Al ; UPt 2 3 3
Interplay between magnetism and superconductivity in heavy fermion superconductors attract much interest (for example, [1]). In heavy fermion superconductors the superconductivity coexists with the antiferromagnetic (AFM) ordering of magnetic 5f electrons. Superconductivity appears in the heavy fermi-liquid state due to hybridization of conduction electron and f-electrons. Therefore, the interplay between superconductivity and magnetism is a key concept for the ground state properties. In this paper, we mention the superconducting gap appearing in the spin excitation of UPd Al [2,3] and 2 3 the long-range magnetic correlation in UPt at ultra-low 3 temperature [4,5]. Neutron scattering experiments were carried out using a cold neutron triple-axis spectrometer LTAS installed at the research reactor JRR-3M in Japan Atomic Energy Research Institute (JAERI). The single-crystal samples were grown by the Czochralski pulling method. The superconducting transition ¹ and the residual resistivity # ratio (o /o ) of UPd Al were determined to be 1.9 K RT 0 2 3 and 60, respectively. Three di!erent quality of UPt sin3
* Corresponding author. Fax.: #81-29-282-5939. E-mail address: [email protected] (N. Metoki)
gle crystals with o /o "300, 470, and 700 were measRT 0 ured to study the sample dependence. Fig. 1 is the low-energy spin excitation spectra of UPd Al at an AFM Bragg point (0 0 0.5). In the normal 2 3 stat (¹'¹ ) we observed a quasielastic and inelastic # response with the excitation energy *E of about 1.5 meV. The quasielastic component becomes weaker with decreasing temperature. Below ¹ , however, the intensity # increases again and we observed inelastic peak with
Fig. 1. Temperature dependence of the neutron inelastic scattering pro"le measured at Q"(0 0 0.5) in UPD Al . 2 3
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 9 0 5 - 9
988
N. Metoki et al. / Physica B 281&282 (2000) 987}988
Fig. 2. The longitudinal scattering pro"le of the (0.5 0 1) magnetic peak in sample d9 in UPt . 3
Fig. 2 is the temperature dependence of the (0.5 0 1) AFM Bragg peak. The AFM peak is observed for all samples. The estimated magnetic moment showed no systematic dependence on sample quality, of about 0.03 lB/U, hence the weak AFM ordering is concluded to be an intrinsic property. We observed a remarkable narrowing of the line width below 50 mK and resolution limited peak at 20 mK. The magnetic correlation length was typically 300}700 As above 50 mK, depending on the sample quality; the spin correlation length becomes longer with increasing the mean free path. Below 50 mK the correlation length increases and diverges at 20 mK. Therefore, we conclude that the speci"c heat anomaly is related to the AFM ordering in UPt . The narrowing of the line 3 width would be a precursive phenomenon of the static ordering.
Acknowledgements energy gap *E"0.36 meV at ¹"0.4 K. The gap opens in the superconducting state and increases continuously with decreasing temperature. In addition, the gap disappears with magnetic "eld higher than the critical "eld H . Therefore, we conclude that this magnetic gap cor#2 responds to the superconducting gap. The unusual behavior of the weak AFM ordering in UPt is one of the most mysterious problems in heavy 3 fermion superconductors. From many experimental data this weak AFM ordering is considered to be a dynamical #uctuation which has a characteristic time scale between NMR and neutron. The speci"c heat anomaly [6] would be a signature of a static ordering at ultra-low temperatures. In order to clarify this anomaly we have carried out ultra-low neutron scattering experiments on UPt [4,5]. 3
One of us (Y.O1 .) was supported by the Grant-in-Aidfor Priority Areas and COE Research (IOCE2004) of the Ministry of Education, Science, Sports and Culture.
References [1] T.E. Mason, G. Aeppli, Proc. Roy. Danish Acad. Sci. Lett. MfM 45 (1996) 231. [2] N. Metoki et al., J. Phys. Soc. Japan 66 (1997) 2560. [3] N. Metoki et al., Phys. Rev. Lett. 80 (1998) 5417. [4] Y. Koike et al., J. Phys. Soc. Japan 67 (1998) 1142. [5] Y. Koike et al., Japan J. Appl. Phys. Ser 11 (1999) 44. [6] E.A. Schuberth et al., Phys. Rev. Lett. 68 (1992) 117.