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Crossover behavior from magnetism to superconductivity in CexCu2Si2
Physica B 259—261 (1999) 678—680
Crossover behavior from magnetism to superconductivity in Ce Cu Si V K. Ishida *, Y. Kawasaki , K. Tabuchi , K. Kashima , Y. Kitaoka , K. Asayama , C. Geibel, F. Steglich Department of Physical Science, Graduate School of Engineering Science, Osaka University, Machikaneyama-cho 1-3, Toyonaka, Osaka 560-8531, Japan Max-Planck-Institut fu( r Chemische Physik fester Stoffe, Bayreuther Str. 40, Haus 16 D-01187 Dresden, Germany
Abstract Cu—NQR studies have been performed to clarify the ground-state characteristics in a series of Ce Cu Si system. It was V found that the ground state evolves from magnetically ordered state at x"0.975 to heavy-electron superconducting state at x"1.025. At the border between the two phases (x"0.99), the novel magnetic character survives even at the lowest temperature. We propose that this magnetic state may be a dynamical SDW state oscillating with finite frequencies comparable to Cu—NQR frequency (&3 MHz). 1999 Elsevier Science B.V. All rights reserved. Keywords: CeCu Si ; Superconductivity; NMR/NQR
From the discovery of CeCu Si [1], a great deal of interest is attracted to the interplay between magnetic ordering or correlation and superconductivity, since an intriguing situation is realized in CeCu Si . In a stoi chiometric CeCu Si at ambient pressure, it is now known that superconducting phase (SC) is embedded in another phase denoted as A-phase. This A-phase, which was shown to be of magnetic origin by NMR [2,9] and lSR [3,10] measurements, was ensured by elastic and thermal expansion experiments to be of a high-quality single crystal [4]. It was anticipated from the NMR experiments that the A-phase should be distinguished from a static magnetic ordering, but rather characterized by magnetic correlations fluctuating with low frequencies comparable to Cu—NQR frequency&3 MHz (10 s\) [5]. This implication was obtained from the results that NQR intensity decreases upon cooling below ¹ &1 K
presumably due to extraordinary large spin-echo decay
* Corresponding author. Tel.:#81-6-850-6438; fax:#81-6845-4632; e-mail: [email protected].
rate 1/¹ , whereas the NQR spectral width does not exhibit any hyperfine broadening signaling an appearance of spontaneous magnetic moments [2,5,9]. This is consistent with the recent lSR experiments [6,11]. In order to highlight an interplay between the A-phase and the superconductivity and to clarify an evolution of ground state by varying the Ce nominal content x, we have made Cu—NQR studies on the same series of Ce Cu Si polycrystal samples used in the previous V >W measurements of the specific heat [7,12] and the lSR [6,11]. We used four samples with different Ce nominal content such as CeCu Si with ¹ &0.7 K (hereafter denoted as Ce1.00), Ce Cu Si with ¹ &0.6 K (Ce1.025), Ce Cu Si (Ce0.99) and non-supercon ducting Ce Cu Si (Ce0.975). We should remark that the ground state in the four samples does not depend on off-stoichiometry in Cu content y, but mainly on Ce content x, since the ¹ in CeCu Si (0)y)0.2) re >W mains in the narrow range between 0.56 K(CeCu Si ) and 0.74 K(CeCu Si ) [8]. SC transition temperature ¹ was 0.65, 0.7 and 0.6 K for Ce1.025, Ce1.00 and Ce0.99, respectively. Although the small diamagnetic signal was observed in Ce0.99, the absence of significant
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 0 3 5 - 7
K. Ishida et al. / Physica B 259—261 (1999) 678—680
679
Fig. 1. Cu—NQR spectra in Ce Cu Si . V >W
anomalies at ¹ in the specific heat and the thermal expansion data are indicative of its non-bulk nature in superconductivity. Fig. 1 shows the NQR spectra for all the samples. The NQR frequency l (&3.435 MHz) is independent of x, / whereas the NQR spectral width is about twice larger for Ce1.025 and Ce0.975 than for Ce1.00 and Ce0.99. The full-width at half-maximum (FWHM) of Cu—NQR spectrum is estimated to be 26, 13, 14 and 35 kHz for Ce1.025, Ce1.00, Ce0.99 and Ce0.975, respectively. We note that the respective narrow value of FWHM&14 kHz and 13 kHz for Ce0.99 and Ce1.00 is comparable to 11 kHz for the single crystal, assuring the high quality of Ce1.00 and Ce0.99 in a microscopic level. The anomalies relevant to the A-phase were observed in the ¹ dependence of NQR intensities. The NQR intensity produced by ¹ (I¹) starts to decrease below a temperature ¹ far above ¹ . ¹ is estimated as 0.8, 1,
1.2 and 1 K for Ce1.025, Ce1.00, Ce0.99 and Ce0.975, respectively. Since any hyperfine broadening in Cu—NQR spectrum is not observed down to 12 mK for Ce1.025, Ce1.00 and Ce0.99, a static magnetic order is ruled out in these samples. It should be noted that I¹ in Ce0.99 decreases, most remarkably, down to 5% of that at 4.2 K without an anomaly near ¹ . On the contrary, in Ce0.975 we found a distinct increase in FWHM below 0.6 K, pointing to an appearance of the static hyperfine field at the Cu sites associated with the onset of static magneticordering. Therefore, the novel magnetic-state in Ce0.99,
Fig. 2. ¹ dependence of 1/¹ in Ce Cu Si by Cu NQR. V >W
in which magnetic correlations fluctuate with low frequencies comparable to NQR frequency, seems to survive at lowest temperature. In order to demonstrate an evolution of ground state from magnetically ordered to superconducting state, we show in Fig. 2 the ¹ dependence of 1/¹ of Cu under zero field for all the samples. 1/¹ is determined with a single component except for the ¹ data below 1 K in Ce0.975 where long (¹ ) and short (¹ ) components * 1 are estimated. Below 2 K, the ¹ dependence of 1/¹ reflects the novel difference in the ground state of each compound. A clear peak in 1/¹ for Ce0.975 is observed 1 at ¹ "0.6 K due to critical magnetic fluctuations to, wards a magnetically ordered phase transition. Together with the result of the distinct increase in the FWHM of Cu—NQR spectrum, the observation of peak in 1/¹ assures the long-range nature of magnetic ordering. By contrast, 1/¹ in Ce1.025 exhibits a linear-¹ behavior below 1.2 K, signaling the formation of heavy-electron band. It should, however, be noted that 1/¹ for the 1 superconducting Ce1.025 and Ce1.00 follow a ¹ dependence in a ¹ range of 0.6—0.1 K, falling on a single curve below ¹ , although the loss of I¹ below ¹ is
much more pronounced for Ce1.00 than for Ce1.025. This result is accountable by that the A-phase is expelled
680
K. Ishida et al. / Physica B 259—261 (1999) 678—680
below ¹ by the onset of SC phase, which is consistent with the results obtained from elastic measurements on the high-quality single crystal [4]. On the contrary, the 1/¹ in Ce0.99 shows a small hump around 0.6 K where C/¹ has a broad peak. The decrease in 1/¹ below 0.6 K is more moderate than the ¹ dependence in Ce1.00 and Ce1.025, and below 0.3 K it is rather similar to that for (1/¹ ) in the magnetically ordered Ce0.975. Since the * bulk superconductivity in Ce0.99 is absent, a gradual decrease in 1/¹ even at low temperatures is seemingly relevant to the low-lying excitations in quantum critical region where the evolution from magnetic ordering to singlet d-wave superconductivity occurs. In conclusion, we have found the novel evolution from the magnetic to the superconducting phase in a series of Ce Cu Si by varying the Ce nominal content x. We V propose that the Ce0.99 of high quality, compatible with the single crystal, shows quantum critical behavior at the border between the magnetic and the SC phases, namely,
the dynamical SDW state oscillating with low frequencies (&u ). ,+0 References [1] F. Steglich et al., Phys. Rev. Lett 43 (1979) 1892. [2] H. Nakamura et al., J. Magn. Magn. Mater. 76—77 (1988) 676. [3] Y.J. Uemura et al., Phys. Rev. B 39 (1989) 4726. [4] G. Bruls et al., Phys. Rev. Lett 72 (1994) 1754. [5] Y. Kitaoka et al., J. Phys. Soc. Japan 60 (1992) 2122. [6] R. Feyerherm et al., Physica B 206—207 (1995) 596. [7] F. Steglich et al., J. Phys. Condens. Matter 8 (1996) 9909. [8] Y. O nuki et al., J. Phys. Soc. Japan 56 (1987) 1454. [9] H. Nakamura et al., J. Phys: Condens. Matter 4 (1992) 473. [10] G.M. Luke et al., Phys. Rev. Lett 59 (1994) 1853. [11] R. Feyerherm et al., Phys. Rev. B 56 (1997) 699. [12] R. Modler et al., Physica B 206—207 (1995) 586.
Crossover behavior from magnetism to superconductivity in Ce Cu Si V K. Ishida *, Y. Kawasaki , K. Tabuchi , K. Kashima , Y. Kitaoka , K. Asayama , C. Geibel, F. Steglich Department of Physical Science, Graduate School of Engineering Science, Osaka University, Machikaneyama-cho 1-3, Toyonaka, Osaka 560-8531, Japan Max-Planck-Institut fu( r Chemische Physik fester Stoffe, Bayreuther Str. 40, Haus 16 D-01187 Dresden, Germany
Abstract Cu—NQR studies have been performed to clarify the ground-state characteristics in a series of Ce Cu Si system. It was V found that the ground state evolves from magnetically ordered state at x"0.975 to heavy-electron superconducting state at x"1.025. At the border between the two phases (x"0.99), the novel magnetic character survives even at the lowest temperature. We propose that this magnetic state may be a dynamical SDW state oscillating with finite frequencies comparable to Cu—NQR frequency (&3 MHz). 1999 Elsevier Science B.V. All rights reserved. Keywords: CeCu Si ; Superconductivity; NMR/NQR
From the discovery of CeCu Si [1], a great deal of interest is attracted to the interplay between magnetic ordering or correlation and superconductivity, since an intriguing situation is realized in CeCu Si . In a stoi chiometric CeCu Si at ambient pressure, it is now known that superconducting phase (SC) is embedded in another phase denoted as A-phase. This A-phase, which was shown to be of magnetic origin by NMR [2,9] and lSR [3,10] measurements, was ensured by elastic and thermal expansion experiments to be of a high-quality single crystal [4]. It was anticipated from the NMR experiments that the A-phase should be distinguished from a static magnetic ordering, but rather characterized by magnetic correlations fluctuating with low frequencies comparable to Cu—NQR frequency&3 MHz (10 s\) [5]. This implication was obtained from the results that NQR intensity decreases upon cooling below ¹ &1 K
presumably due to extraordinary large spin-echo decay
* Corresponding author. Tel.:#81-6-850-6438; fax:#81-6845-4632; e-mail: [email protected].
rate 1/¹ , whereas the NQR spectral width does not exhibit any hyperfine broadening signaling an appearance of spontaneous magnetic moments [2,5,9]. This is consistent with the recent lSR experiments [6,11]. In order to highlight an interplay between the A-phase and the superconductivity and to clarify an evolution of ground state by varying the Ce nominal content x, we have made Cu—NQR studies on the same series of Ce Cu Si polycrystal samples used in the previous V >W measurements of the specific heat [7,12] and the lSR [6,11]. We used four samples with different Ce nominal content such as CeCu Si with ¹ &0.7 K (hereafter denoted as Ce1.00), Ce Cu Si with ¹ &0.6 K (Ce1.025), Ce Cu Si (Ce0.99) and non-supercon ducting Ce Cu Si (Ce0.975). We should remark that the ground state in the four samples does not depend on off-stoichiometry in Cu content y, but mainly on Ce content x, since the ¹ in CeCu Si (0)y)0.2) re >W mains in the narrow range between 0.56 K(CeCu Si ) and 0.74 K(CeCu Si ) [8]. SC transition temperature ¹ was 0.65, 0.7 and 0.6 K for Ce1.025, Ce1.00 and Ce0.99, respectively. Although the small diamagnetic signal was observed in Ce0.99, the absence of significant
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 0 3 5 - 7
K. Ishida et al. / Physica B 259—261 (1999) 678—680
679
Fig. 1. Cu—NQR spectra in Ce Cu Si . V >W
anomalies at ¹ in the specific heat and the thermal expansion data are indicative of its non-bulk nature in superconductivity. Fig. 1 shows the NQR spectra for all the samples. The NQR frequency l (&3.435 MHz) is independent of x, / whereas the NQR spectral width is about twice larger for Ce1.025 and Ce0.975 than for Ce1.00 and Ce0.99. The full-width at half-maximum (FWHM) of Cu—NQR spectrum is estimated to be 26, 13, 14 and 35 kHz for Ce1.025, Ce1.00, Ce0.99 and Ce0.975, respectively. We note that the respective narrow value of FWHM&14 kHz and 13 kHz for Ce0.99 and Ce1.00 is comparable to 11 kHz for the single crystal, assuring the high quality of Ce1.00 and Ce0.99 in a microscopic level. The anomalies relevant to the A-phase were observed in the ¹ dependence of NQR intensities. The NQR intensity produced by ¹ (I¹) starts to decrease below a temperature ¹ far above ¹ . ¹ is estimated as 0.8, 1,
1.2 and 1 K for Ce1.025, Ce1.00, Ce0.99 and Ce0.975, respectively. Since any hyperfine broadening in Cu—NQR spectrum is not observed down to 12 mK for Ce1.025, Ce1.00 and Ce0.99, a static magnetic order is ruled out in these samples. It should be noted that I¹ in Ce0.99 decreases, most remarkably, down to 5% of that at 4.2 K without an anomaly near ¹ . On the contrary, in Ce0.975 we found a distinct increase in FWHM below 0.6 K, pointing to an appearance of the static hyperfine field at the Cu sites associated with the onset of static magneticordering. Therefore, the novel magnetic-state in Ce0.99,
Fig. 2. ¹ dependence of 1/¹ in Ce Cu Si by Cu NQR. V >W
in which magnetic correlations fluctuate with low frequencies comparable to NQR frequency, seems to survive at lowest temperature. In order to demonstrate an evolution of ground state from magnetically ordered to superconducting state, we show in Fig. 2 the ¹ dependence of 1/¹ of Cu under zero field for all the samples. 1/¹ is determined with a single component except for the ¹ data below 1 K in Ce0.975 where long (¹ ) and short (¹ ) components * 1 are estimated. Below 2 K, the ¹ dependence of 1/¹ reflects the novel difference in the ground state of each compound. A clear peak in 1/¹ for Ce0.975 is observed 1 at ¹ "0.6 K due to critical magnetic fluctuations to, wards a magnetically ordered phase transition. Together with the result of the distinct increase in the FWHM of Cu—NQR spectrum, the observation of peak in 1/¹ assures the long-range nature of magnetic ordering. By contrast, 1/¹ in Ce1.025 exhibits a linear-¹ behavior below 1.2 K, signaling the formation of heavy-electron band. It should, however, be noted that 1/¹ for the 1 superconducting Ce1.025 and Ce1.00 follow a ¹ dependence in a ¹ range of 0.6—0.1 K, falling on a single curve below ¹ , although the loss of I¹ below ¹ is
much more pronounced for Ce1.00 than for Ce1.025. This result is accountable by that the A-phase is expelled
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K. Ishida et al. / Physica B 259—261 (1999) 678—680
below ¹ by the onset of SC phase, which is consistent with the results obtained from elastic measurements on the high-quality single crystal [4]. On the contrary, the 1/¹ in Ce0.99 shows a small hump around 0.6 K where C/¹ has a broad peak. The decrease in 1/¹ below 0.6 K is more moderate than the ¹ dependence in Ce1.00 and Ce1.025, and below 0.3 K it is rather similar to that for (1/¹ ) in the magnetically ordered Ce0.975. Since the * bulk superconductivity in Ce0.99 is absent, a gradual decrease in 1/¹ even at low temperatures is seemingly relevant to the low-lying excitations in quantum critical region where the evolution from magnetic ordering to singlet d-wave superconductivity occurs. In conclusion, we have found the novel evolution from the magnetic to the superconducting phase in a series of Ce Cu Si by varying the Ce nominal content x. We V propose that the Ce0.99 of high quality, compatible with the single crystal, shows quantum critical behavior at the border between the magnetic and the SC phases, namely,
the dynamical SDW state oscillating with low frequencies (&u ). ,+0 References [1] F. Steglich et al., Phys. Rev. Lett 43 (1979) 1892. [2] H. Nakamura et al., J. Magn. Magn. Mater. 76—77 (1988) 676. [3] Y.J. Uemura et al., Phys. Rev. B 39 (1989) 4726. [4] G. Bruls et al., Phys. Rev. Lett 72 (1994) 1754. [5] Y. Kitaoka et al., J. Phys. Soc. Japan 60 (1992) 2122. [6] R. Feyerherm et al., Physica B 206—207 (1995) 596. [7] F. Steglich et al., J. Phys. Condens. Matter 8 (1996) 9909. [8] Y. O nuki et al., J. Phys. Soc. Japan 56 (1987) 1454. [9] H. Nakamura et al., J. Phys: Condens. Matter 4 (1992) 473. [10] G.M. Luke et al., Phys. Rev. Lett 59 (1994) 1853. [11] R. Feyerherm et al., Phys. Rev. B 56 (1997) 699. [12] R. Modler et al., Physica B 206—207 (1995) 586.