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Exotic magnetic and superconducting phases in CeCu2Si2 close to magnetically ordered state : Cu-NQR study
Journal of Magnetism and Magnetic Materials 226}230 (2001) 374}375
Exotic magnetic and superconducting phases in CeCu Si close to magnetically ordered state : Cu-NQR study Y. Kawasaki *, K. Obinata , K. Ishida , T. Mito , G.-q. Zheng , Y. Kitaoka , C. Geibel, F. Steglich Department of Physical Sciences, Graduate School of Engineering Sciences, Osaka University, Toyonaka, Osaka 560-8531, Japan Max-Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
Abstract We report Cu-NQR results under pressure on heavy-fermion (HF) superconductor CeCu Si close to a magnetic phase, Ce Cu Si and CeCu (Si Ge ) . It is found that an application of a minute pressure exceeding 0.3 GPa markedly suppresses low-energy magnetic #uctuations observed at ambient pressure (P"0) in Ce Cu Si . Correspondingly, anomalous superconducting (SC) characteristics evolve into conventional HF-SC ones with increasing pressure. In the latter, the Ge substitution works as a negative chemical pressure. CeCu (Si Ge ) reveals a magnetic phase transition at ¹ "0.75 K followed by the SC phase transition at ¹ &0.4 K at P"0. It is suggested , experimentally that the magnetic #uctuations seen in Ce Cu Si at P"0 possesses a quantum critical character of both SC and magnetically ordered states. 2001 Elsevier Science B.V. All rights reserved. Keywords: Heavy-fermion compounds; Superconductivity; NMR; Pressure e!ect
It has been believed that a superconducting (SC) phase in the heavy-fermion (HF) compound CeCu Si is located close to a magnetic phase and might thus be related to a new pairing state and hence mechanism [1,2]. From NQR experiments on a series of Ce Cu Si [3], we have shown that an unusual >V >W magnetic phase, so-called A phase dominated by extremely low-energy magnetic #uctuations comparable to NQR frequency &3 MHz, exists just in the crossover region between the magnetically ordered and SC states. Nearly homogeneous Ce Cu Si (denoted as Ce0.99) is dominated by the magnetic #uctuations at low temperature (¹) and exhibits small SC anomaly of speci"c heat below ¹ &0.65 K [4,5]. Any static magnetic order is not identi"ed down to 0.012 K by the NQR measurements. In order to shed further light on the magnetic and SC natures, we have carried out Cu-NQR measurements on Ce0.99 under pressure (P) and on Ge-substituted CeCu (Si Ge ) . In the latter, the * Corresponding author. Fax: #81-6-6845-4632. E-mail address: [email protected] (Y. Kawasaki).
Ge substitution corresponds to a negative chemical P. The experimental procedure was reported elsewhere [6]. We begin with the results under P on Ce0.99. An inset of Fig. 1 displays ¹ dependence of the Cu-NQR intensity (I) of Ce0.99 multiplied by ¹, r(¹)"(I;¹)/(I(4.2);4.2) normalized by I;¹ at 4.2 K. ¹ was con"rmed to stay nearly constant up to 1.44 GPa with ¹ "0.65 K as indicated by a dot line. Here ¹ was determined from AC-susceptibility measurements using the in situ NQR coil. Abrupt drop of r(¹) below ¹ at P"0.85 and 1.44 GPa is ascribed to the SC diamagnetic shielding of RF "eld for the NQR experiment. At ambient pressure (P"0), on the other hand, r(¹) starts to decrease below 1.2 K which is higher than ¹ . The decrease of r(¹) above ¹ indicates that magnetic #uctuations slow down to very low frequency comparable to the NQR one. It is remarkable that the decrease of r(¹) in the normal state becomes small by increasing P, suggesting that the lowfrequency magnetic #uctuations are suppressed by P. Correspondingly, 1/¹ exhibits a P-induced change. Fig. 1 shows ¹ variations of 1/¹ in Ce0.99 at various values of P and in the Ge-doped sample at P"0. As
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 3 4 1 - X
Y. Kawasaki et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 374}375
Fig. 1. ¹ dependences of 1/¹ in Ce Cu Si at various values of P and CeCu (Si Ge ) at P"0. Inset shows the ¹ dependence of the NQR intensity in Ce0.99 multiplied by ¹, r(¹)"(I;¹)/(I(4.2);4.2), normalized by the value at 4.2 K.
reported previously [3], the decrease of 1/¹ in Ce0.99 at P"0 becomes moderate upon cooling, deviating well below ¹ from a ¹-like dependence seen in typical HF superconductors. The enhancement of 1/¹ at low ¹ is suppressed by applying a small P such as 0.3 GPa. Eventually, its ¹ dependence evolves into the ¹-like dependence. This result is corroborated by the P-induced recovery in r(¹), ensuring that the marked suppression of extremely low-frequency magnetic #uctuations leads to the appearance of conventional HF-SC state. Next we turn to the result of the Ge-doped sample. Likewise in undoped Ce0.99 at P"0, r(¹) in the Gedoped sample starts to decrease below 1.4 K and signi"cantly reduces down to 0.75 K (not shown). This result indicates that the magnetic #uctuations are dominant at low ¹. It is, however, notable that both the Cu-NQR spectral widths increase below ¹ "0.75 K (see Fig. 2), , giving evidence for an onset of internal "elds at Cu site. Emergence of antiferromagnetic (AF) phase below ¹ is , supported by the result that 1/¹ exhibits a sharp peak at the same ¹ (see Fig. 1). In this sample, SC diamagnetic signal is observed below ¹ &0.4 K. What is the origin of the di!erence in the ground state of CeCu Si ? We note that the temperature ¹H, at which 1/¹ starts to decrease from the 1/¹ "constant behav ior in the high-¹ region, increases by applying a P and decreases by Ge doping. Since ¹H empirically corresponds to the Kondo temperature ¹ Jexp(!=/J ) ) with = being the conduction bandwidth, these results indicate that the hybridization between 4f and conduction electron, J , becomes stronger by P and weaker by Ge doping. We suggest that the strength of J is one of the most important parameters in the present system, although disordered e!ects induced by Ge doping are not negligible.
375
Fig. 2. ¹ dependence of Cu-NQR spectra in CeCu (Si Ge ) , indicating an onset of internal "elds at Cu site at ¹ "0.75 K. ,
In conclusion, nearly homogeneous Ce0.99 is the exotic HF superconductor that touches closely to the magnetic phase. In other words, this SC phase possesses quantum critical characters dominated by extremely low-frequency magnetic #uctuations. By applying a P which increases J , the exotic SC phase evolves into the conventional HF-SC one. By contrast, in the Gedoped CeCu Si where J is smaller than that in Ce0.99 at P"0, the onset of internal "elds at the observable Cu site is found at ¹ " 0.75 K. It is suggested that the , magnetic #uctuations seen in Ce0.99 at P"0 possesses the quantum critical character of both SC and magnetically ordered states. We propose that the low-energy spin-#uctuations may be regarded as the intermediate state between AF and SC state. Recently, Zhang proposed the SO(5) theory that uni"es the SC and AF order [7]. In this theory, d-wave SC and AF phase are treated on equal footing, and d-wave superconductivity is regarded as AF state viewed in the SO(5) coordinates. We propose that the SO(5) theory is one of the promising scenarios to account for the exotic SC phase at P"0 in Ce0.99. This work was supported by the COE Research (10CE2004) in Grant-in-Aid for Scienti"c Research from the Ministry of Education, Sport, Science and Culture, Japan. References [1] [2] [3] [4] [5] [6] [7]
D. Jaccard et al., Phys. Lett. 163 A (1992) 475. P. Gegenwart et al., Phys. Rev. Lett. 81 (1998) 1501. K. Ishida et al., Phys. Rev. Lett. 82 (1999) 5353. R. Modler et al., Physica B 206}207 (1995) 586. R. Feyerherm et al., Phys. Rev. B 56 (1997) 699. K. Ishida et al., Hyper"ne Interactions, to appear. S.C. Zhang et al., Science 275 (1997) 1089.
Exotic magnetic and superconducting phases in CeCu Si close to magnetically ordered state : Cu-NQR study Y. Kawasaki *, K. Obinata , K. Ishida , T. Mito , G.-q. Zheng , Y. Kitaoka , C. Geibel, F. Steglich Department of Physical Sciences, Graduate School of Engineering Sciences, Osaka University, Toyonaka, Osaka 560-8531, Japan Max-Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
Abstract We report Cu-NQR results under pressure on heavy-fermion (HF) superconductor CeCu Si close to a magnetic phase, Ce Cu Si and CeCu (Si Ge ) . It is found that an application of a minute pressure exceeding 0.3 GPa markedly suppresses low-energy magnetic #uctuations observed at ambient pressure (P"0) in Ce Cu Si . Correspondingly, anomalous superconducting (SC) characteristics evolve into conventional HF-SC ones with increasing pressure. In the latter, the Ge substitution works as a negative chemical pressure. CeCu (Si Ge ) reveals a magnetic phase transition at ¹ "0.75 K followed by the SC phase transition at ¹ &0.4 K at P"0. It is suggested , experimentally that the magnetic #uctuations seen in Ce Cu Si at P"0 possesses a quantum critical character of both SC and magnetically ordered states. 2001 Elsevier Science B.V. All rights reserved. Keywords: Heavy-fermion compounds; Superconductivity; NMR; Pressure e!ect
It has been believed that a superconducting (SC) phase in the heavy-fermion (HF) compound CeCu Si is located close to a magnetic phase and might thus be related to a new pairing state and hence mechanism [1,2]. From NQR experiments on a series of Ce Cu Si [3], we have shown that an unusual >V >W magnetic phase, so-called A phase dominated by extremely low-energy magnetic #uctuations comparable to NQR frequency &3 MHz, exists just in the crossover region between the magnetically ordered and SC states. Nearly homogeneous Ce Cu Si (denoted as Ce0.99) is dominated by the magnetic #uctuations at low temperature (¹) and exhibits small SC anomaly of speci"c heat below ¹ &0.65 K [4,5]. Any static magnetic order is not identi"ed down to 0.012 K by the NQR measurements. In order to shed further light on the magnetic and SC natures, we have carried out Cu-NQR measurements on Ce0.99 under pressure (P) and on Ge-substituted CeCu (Si Ge ) . In the latter, the * Corresponding author. Fax: #81-6-6845-4632. E-mail address: [email protected] (Y. Kawasaki).
Ge substitution corresponds to a negative chemical P. The experimental procedure was reported elsewhere [6]. We begin with the results under P on Ce0.99. An inset of Fig. 1 displays ¹ dependence of the Cu-NQR intensity (I) of Ce0.99 multiplied by ¹, r(¹)"(I;¹)/(I(4.2);4.2) normalized by I;¹ at 4.2 K. ¹ was con"rmed to stay nearly constant up to 1.44 GPa with ¹ "0.65 K as indicated by a dot line. Here ¹ was determined from AC-susceptibility measurements using the in situ NQR coil. Abrupt drop of r(¹) below ¹ at P"0.85 and 1.44 GPa is ascribed to the SC diamagnetic shielding of RF "eld for the NQR experiment. At ambient pressure (P"0), on the other hand, r(¹) starts to decrease below 1.2 K which is higher than ¹ . The decrease of r(¹) above ¹ indicates that magnetic #uctuations slow down to very low frequency comparable to the NQR one. It is remarkable that the decrease of r(¹) in the normal state becomes small by increasing P, suggesting that the lowfrequency magnetic #uctuations are suppressed by P. Correspondingly, 1/¹ exhibits a P-induced change. Fig. 1 shows ¹ variations of 1/¹ in Ce0.99 at various values of P and in the Ge-doped sample at P"0. As
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 3 4 1 - X
Y. Kawasaki et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 374}375
Fig. 1. ¹ dependences of 1/¹ in Ce Cu Si at various values of P and CeCu (Si Ge ) at P"0. Inset shows the ¹ dependence of the NQR intensity in Ce0.99 multiplied by ¹, r(¹)"(I;¹)/(I(4.2);4.2), normalized by the value at 4.2 K.
reported previously [3], the decrease of 1/¹ in Ce0.99 at P"0 becomes moderate upon cooling, deviating well below ¹ from a ¹-like dependence seen in typical HF superconductors. The enhancement of 1/¹ at low ¹ is suppressed by applying a small P such as 0.3 GPa. Eventually, its ¹ dependence evolves into the ¹-like dependence. This result is corroborated by the P-induced recovery in r(¹), ensuring that the marked suppression of extremely low-frequency magnetic #uctuations leads to the appearance of conventional HF-SC state. Next we turn to the result of the Ge-doped sample. Likewise in undoped Ce0.99 at P"0, r(¹) in the Gedoped sample starts to decrease below 1.4 K and signi"cantly reduces down to 0.75 K (not shown). This result indicates that the magnetic #uctuations are dominant at low ¹. It is, however, notable that both the Cu-NQR spectral widths increase below ¹ "0.75 K (see Fig. 2), , giving evidence for an onset of internal "elds at Cu site. Emergence of antiferromagnetic (AF) phase below ¹ is , supported by the result that 1/¹ exhibits a sharp peak at the same ¹ (see Fig. 1). In this sample, SC diamagnetic signal is observed below ¹ &0.4 K. What is the origin of the di!erence in the ground state of CeCu Si ? We note that the temperature ¹H, at which 1/¹ starts to decrease from the 1/¹ "constant behav ior in the high-¹ region, increases by applying a P and decreases by Ge doping. Since ¹H empirically corresponds to the Kondo temperature ¹ Jexp(!=/J ) ) with = being the conduction bandwidth, these results indicate that the hybridization between 4f and conduction electron, J , becomes stronger by P and weaker by Ge doping. We suggest that the strength of J is one of the most important parameters in the present system, although disordered e!ects induced by Ge doping are not negligible.
375
Fig. 2. ¹ dependence of Cu-NQR spectra in CeCu (Si Ge ) , indicating an onset of internal "elds at Cu site at ¹ "0.75 K. ,
In conclusion, nearly homogeneous Ce0.99 is the exotic HF superconductor that touches closely to the magnetic phase. In other words, this SC phase possesses quantum critical characters dominated by extremely low-frequency magnetic #uctuations. By applying a P which increases J , the exotic SC phase evolves into the conventional HF-SC one. By contrast, in the Gedoped CeCu Si where J is smaller than that in Ce0.99 at P"0, the onset of internal "elds at the observable Cu site is found at ¹ " 0.75 K. It is suggested that the , magnetic #uctuations seen in Ce0.99 at P"0 possesses the quantum critical character of both SC and magnetically ordered states. We propose that the low-energy spin-#uctuations may be regarded as the intermediate state between AF and SC state. Recently, Zhang proposed the SO(5) theory that uni"es the SC and AF order [7]. In this theory, d-wave SC and AF phase are treated on equal footing, and d-wave superconductivity is regarded as AF state viewed in the SO(5) coordinates. We propose that the SO(5) theory is one of the promising scenarios to account for the exotic SC phase at P"0 in Ce0.99. This work was supported by the COE Research (10CE2004) in Grant-in-Aid for Scienti"c Research from the Ministry of Education, Sport, Science and Culture, Japan. References [1] [2] [3] [4] [5] [6] [7]
D. Jaccard et al., Phys. Lett. 163 A (1992) 475. P. Gegenwart et al., Phys. Rev. Lett. 81 (1998) 1501. K. Ishida et al., Phys. Rev. Lett. 82 (1999) 5353. R. Modler et al., Physica B 206}207 (1995) 586. R. Feyerherm et al., Phys. Rev. B 56 (1997) 699. K. Ishida et al., Hyper"ne Interactions, to appear. S.C. Zhang et al., Science 275 (1997) 1089.