- Email: [email protected]
μSR-spectroscopy on superconducting Chevrel phase compounds
Physica B 289}290 (2000) 381}384
lSR-spectroscopy on superconducting Chevrel phase compounds F.N. Gygax , M. Pinkpank , A. Schenck *, M. Decroux, ". Fischer Institute for Particle Physics, ETH Zu( rich, CH-5232 Villigen PSI, Switzerland De& p. de Phys. de la Matie& re Cond., Univ. de Gene& ve, CH-1211 Gene& ve 4, Switzerland
Abstract We report on a systematic study of the London penetration depth j in a series of stoichiometric Chevrel phase compounds XMo S (X"Ag, La, Y, Sc, Yb, Nd, Pr, Tm) in search for a possible correlation of j with ¹ as has been found before in solid solution of SnMo S Se and PbMo S Se (Birrer et al., Phys. Rev. B 48 (1993) 16589 [1]). In \V V \V V contrast to the solid solution compounds it seems that j is more or less the same for all stoichiometric compounds with ¹ *7 K and very much larger for compounds with ¹ )7 K. In NdMo S and PrMo S fast relaxation due to #uctuations of the 4f-electron moments masked completely the e!ect of the #ux line lattice on the l>-polarization in transverse "eld experiments, but it seems that the onset of superconductivity has some minute e!ect on the 4f-spin dynamics in NdMo S . Preliminary TF-studies in TmMo S show the relaxation to be induced by the #ux line lattice and the 4f-spin dynamics. We do not "nd any evidence for magnetic ordering of the Nd, Tm, Yb or Pr sublattice at low temperature. For Yb our results clearly show that Yb is in a nonmagnetic state in contrast to some previous reports (K.N. Shrivastava, K.P. Sinha, Phys. Rep. 115 (1984) 93). 2000 Elsevier Science B.V. All rights reserved. PACS: 74.70.Ad; 74.25.Ha; 76.75.#i Keywords: Chevrel phase superconductors; London penetration depth; Rare earth spin dynamics
1. Introduction In a previous study of solid solutions of Sn Mo S Se and PbMo S Se [1] it was found \V V \V V that the zero-temperature magnetic penetration depth j scaled with the transition temperature approximately like 1 J¹. A j
(1)
* Corresponding author. Tel.: #41-56-310-3263; fax: #4156-310-4362. E-mail address: [email protected] (A. Schenck).
Since the electron mean-free path l may not be very large in the solid solution compounds the e!ective jY may be given by (m"coherence length) jY "j
m 1# l
(2)
and the change of jY with composition may just re#ect the mean-free-path behavior. In order to check on a possible correlation of j and ¹ (as in high-¹ material) in a way less a!ected by the mean-free-path problem we have studied the stoichiometric, polycrystalline compounds XMo S with X"Y (¹ "2.5 K), Sc (¹ "3.3 K),
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 4 1 6 - 6
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F.N. Gygax et al. / Physica B 289}290 (2000) 381}384
Fig. 1. Temperature dependence of the free Gaussian relaxation rate p in YMo S at 100 and 500 G. The solid lines below ¹ are "ts of Eq. (1) to the data (see text).
Fig. 3. Temperature dependence of p and frequency shift *l in ScMo S at various "elds.
masked by fast dynamically induced relaxation due to the rare earth moment #uctuations which are, of course, of interest in themselves (see Section 3 below). For TmMo S only preliminary data are avail able which show a competing in#uence of both the vortex lattice and the 4f-spin dynamics. These results will not be discussed further.
2. Results on the penetration depth in the compounds with X"Y, Sc, La, Ag and Yb Fig. 2. Temperature dependence of (a) p and (b) l in AgMo S at 100 G, 500 G and 2 kG. The solid lines below ¹ are from "ts (see text).
La (¹ "7.4 K), Ag (¹ "8.5 K), Yb (¹ "10 K), Tm (¹ "4.3 K), Pr (¹ "4 K) and Nd (¹ "4 K). It was found in TF-experiments that the vortex lattice in the compounds with X"Pr and Nd was
The TF-measurements were, as usual, performed after "eld cooling through ¹ . Various "elds were applied. The obtained spectra were "tted with a Gaussian relaxation function e\NR , following common practise. Some typical results on the temperature dependence of the "tted relaxation rates p are presented in Figs. 1}3. The temperature dependence of p was "tted for ¹)¹ by the
F.N. Gygax et al. / Physica B 289}290 (2000) 381}384
383
Fig. 4. Field dependence of p(0).
Fig. 6. Plot of ¹ versus p(0) as obtained from "ts of Eq. (1) to the data, including earlier results from Ref. [1].
Fig. 5. Amplitude of the precession signal at 2 K in the rotating frame showing essentially the time dependence of the relaxation function G(t), the Fourier transform of the actual spectral distribution of frequencies in AgMo S . While G(t) has a certain Gaussain appearance at B "100 G, it follows the theoretical expectation for a #ux line lattice with jK4000 As at 500 G.
phenomenological expression
¹ ? @ p" p(0) 1! #p , ¹
(3)
where p is a temperature independent contribu tion, not related to the vortex lattice. For a normal two #uid behavior one expects a"4, b"1. The "tted p(0) is displayed in Fig. 4 as a function of applied "eld strength. As can be seen, they depend on the "eld strength in a systematic manner, probably related to the perfectness of the established #ux line lattice. It appears to be more regular in higher applied "elds. This is demonstrated in Fig. 5 which shows for AgMo S the amplitude
Fig. 7. ZF-relaxation rate j NdMo S . Di!erent symbols refer G to measurement about one year apart. Note the irregularities around ¹ . Essentially the same results are obtained in longitu dinal "elds, proving the dynamic origin of j . G
A of the lSR signal in the rotating frame for B"100 and 500 G. While at 100 G A follows nearly a Gaussian behaviour (solid line), this is not the case at 500 G. The dashed line comes close to what is predicted for a perfect #ux line lattice with j "4000 As . Note also the very anomalous tem perature dependence of both p and the diamagnetic shift of the precession frequency *l in ScMo S (see Fig. 3). In 100 G applied "eld the e!ect of the
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F.N. Gygax et al. / Physica B 289}290 (2000) 381}384
in this compound. The relaxation rates j are inde pendent of the "eld con"guration, showing them to be of dynamic origin. The temperature dependence of the ZF-j are displayed in Fig. 7. While the slow j shows a smooth behavior across ¹ , the fast j displays an anomaly at ¹ , suggesting that the onset of superconductivity a!ects the 4f-spin dynamics of the Nd-ions. This e!ect will require further attention. Fig. 8 shows the result of TF (500 G) measurements in PrMo S . The onset of super-conduct ivity below 4 K is clearly indicated by the diamagnetic shift of l. The relaxation follows an exponential function. The temperature dependence of the "tted relaxation rate j is well described by the function (solid line in Fig. 8b). j"A
1 #C (1#¹/h)@
(4)
with b"1.40(5) and h"!2.80(34) K. The value of b is roughly in agreement with the critical exponent expected for a 3D-Heisenberg coupled system ( b";0.705). Fig. 8. (a) Temperature dependence of the precession frequency l in PrMo S (B "500 G). The onset of superconductivity at ¹ K4 K is clearly visible, (b) temperature dependence of the exponential relaxation rate j. The solid line represents a "t of Eq. (2) to the data (see text). The onset of superconductivity is not re#ected in these data.
#ux line lattice is barely visible. Fig. 6 displays the "tted high "eld p(0) versus ¹ , including also re sults from Ref. [1]. As can be seen, there seems to be no obvious relation between p(0)J1j and ¹ . It rather appears as if p(0) is nearly constant for ¹ 97 K and assumes generally rather small values for compunds with ¹ :7 K. 3. Dynamic relaxation in the compounds with X"Nd and Pr ZF, LF (500 G) and TF (500 G) measurements on NdMo S reveal the presence of two di!erently fast and exponentially relaxing components with equal amplitudes, suggesting two di!erent l> sites
4. Conclusions
(i) Contrary to previous reports (Ref. [1,2]) no obvious scaling between p(0)J1/j and ¹ is found in the family of Chevrel phase superconductors, e.g., while ¹ changes by a factor of 2 from LaMo S to PbMo S , p remains es sentially unchanged. The ScMo S -results are highly anomalous and are not understood so far. (ii) The dynamics of the 4f-spin in the Nd and Prcompounds render the e!ects of the #ux line lattice invisible. The spin dynamics in PRMo S is consistent with that of a 3D Heisenberg magnet. In contrast to the Nd and Pr compounds YbMo S does not reveal any mag netic signature, proving that the Yb-ions are in the nonmagnetic Yb> ionic state (4f S ). References [1] P. Birrer et al., Phys. Rev. B 48 (1993) 16 589. [2] Y.J. Uemura et al., Phys. Rev. Lett. 66 (1991) 2665.
lSR-spectroscopy on superconducting Chevrel phase compounds F.N. Gygax , M. Pinkpank , A. Schenck *, M. Decroux, ". Fischer Institute for Particle Physics, ETH Zu( rich, CH-5232 Villigen PSI, Switzerland De& p. de Phys. de la Matie& re Cond., Univ. de Gene& ve, CH-1211 Gene& ve 4, Switzerland
Abstract We report on a systematic study of the London penetration depth j in a series of stoichiometric Chevrel phase compounds XMo S (X"Ag, La, Y, Sc, Yb, Nd, Pr, Tm) in search for a possible correlation of j with ¹ as has been found before in solid solution of SnMo S Se and PbMo S Se (Birrer et al., Phys. Rev. B 48 (1993) 16589 [1]). In \V V \V V contrast to the solid solution compounds it seems that j is more or less the same for all stoichiometric compounds with ¹ *7 K and very much larger for compounds with ¹ )7 K. In NdMo S and PrMo S fast relaxation due to #uctuations of the 4f-electron moments masked completely the e!ect of the #ux line lattice on the l>-polarization in transverse "eld experiments, but it seems that the onset of superconductivity has some minute e!ect on the 4f-spin dynamics in NdMo S . Preliminary TF-studies in TmMo S show the relaxation to be induced by the #ux line lattice and the 4f-spin dynamics. We do not "nd any evidence for magnetic ordering of the Nd, Tm, Yb or Pr sublattice at low temperature. For Yb our results clearly show that Yb is in a nonmagnetic state in contrast to some previous reports (K.N. Shrivastava, K.P. Sinha, Phys. Rep. 115 (1984) 93). 2000 Elsevier Science B.V. All rights reserved. PACS: 74.70.Ad; 74.25.Ha; 76.75.#i Keywords: Chevrel phase superconductors; London penetration depth; Rare earth spin dynamics
1. Introduction In a previous study of solid solutions of Sn Mo S Se and PbMo S Se [1] it was found \V V \V V that the zero-temperature magnetic penetration depth j scaled with the transition temperature approximately like 1 J¹. A j
(1)
* Corresponding author. Tel.: #41-56-310-3263; fax: #4156-310-4362. E-mail address: [email protected] (A. Schenck).
Since the electron mean-free path l may not be very large in the solid solution compounds the e!ective jY may be given by (m"coherence length) jY "j
m 1# l
(2)
and the change of jY with composition may just re#ect the mean-free-path behavior. In order to check on a possible correlation of j and ¹ (as in high-¹ material) in a way less a!ected by the mean-free-path problem we have studied the stoichiometric, polycrystalline compounds XMo S with X"Y (¹ "2.5 K), Sc (¹ "3.3 K),
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 4 1 6 - 6
382
F.N. Gygax et al. / Physica B 289}290 (2000) 381}384
Fig. 1. Temperature dependence of the free Gaussian relaxation rate p in YMo S at 100 and 500 G. The solid lines below ¹ are "ts of Eq. (1) to the data (see text).
Fig. 3. Temperature dependence of p and frequency shift *l in ScMo S at various "elds.
masked by fast dynamically induced relaxation due to the rare earth moment #uctuations which are, of course, of interest in themselves (see Section 3 below). For TmMo S only preliminary data are avail able which show a competing in#uence of both the vortex lattice and the 4f-spin dynamics. These results will not be discussed further.
2. Results on the penetration depth in the compounds with X"Y, Sc, La, Ag and Yb Fig. 2. Temperature dependence of (a) p and (b) l in AgMo S at 100 G, 500 G and 2 kG. The solid lines below ¹ are from "ts (see text).
La (¹ "7.4 K), Ag (¹ "8.5 K), Yb (¹ "10 K), Tm (¹ "4.3 K), Pr (¹ "4 K) and Nd (¹ "4 K). It was found in TF-experiments that the vortex lattice in the compounds with X"Pr and Nd was
The TF-measurements were, as usual, performed after "eld cooling through ¹ . Various "elds were applied. The obtained spectra were "tted with a Gaussian relaxation function e\NR , following common practise. Some typical results on the temperature dependence of the "tted relaxation rates p are presented in Figs. 1}3. The temperature dependence of p was "tted for ¹)¹ by the
F.N. Gygax et al. / Physica B 289}290 (2000) 381}384
383
Fig. 4. Field dependence of p(0).
Fig. 6. Plot of ¹ versus p(0) as obtained from "ts of Eq. (1) to the data, including earlier results from Ref. [1].
Fig. 5. Amplitude of the precession signal at 2 K in the rotating frame showing essentially the time dependence of the relaxation function G(t), the Fourier transform of the actual spectral distribution of frequencies in AgMo S . While G(t) has a certain Gaussain appearance at B "100 G, it follows the theoretical expectation for a #ux line lattice with jK4000 As at 500 G.
phenomenological expression
¹ ? @ p" p(0) 1! #p , ¹
(3)
where p is a temperature independent contribu tion, not related to the vortex lattice. For a normal two #uid behavior one expects a"4, b"1. The "tted p(0) is displayed in Fig. 4 as a function of applied "eld strength. As can be seen, they depend on the "eld strength in a systematic manner, probably related to the perfectness of the established #ux line lattice. It appears to be more regular in higher applied "elds. This is demonstrated in Fig. 5 which shows for AgMo S the amplitude
Fig. 7. ZF-relaxation rate j NdMo S . Di!erent symbols refer G to measurement about one year apart. Note the irregularities around ¹ . Essentially the same results are obtained in longitu dinal "elds, proving the dynamic origin of j . G
A of the lSR signal in the rotating frame for B"100 and 500 G. While at 100 G A follows nearly a Gaussian behaviour (solid line), this is not the case at 500 G. The dashed line comes close to what is predicted for a perfect #ux line lattice with j "4000 As . Note also the very anomalous tem perature dependence of both p and the diamagnetic shift of the precession frequency *l in ScMo S (see Fig. 3). In 100 G applied "eld the e!ect of the
384
F.N. Gygax et al. / Physica B 289}290 (2000) 381}384
in this compound. The relaxation rates j are inde pendent of the "eld con"guration, showing them to be of dynamic origin. The temperature dependence of the ZF-j are displayed in Fig. 7. While the slow j shows a smooth behavior across ¹ , the fast j displays an anomaly at ¹ , suggesting that the onset of superconductivity a!ects the 4f-spin dynamics of the Nd-ions. This e!ect will require further attention. Fig. 8 shows the result of TF (500 G) measurements in PrMo S . The onset of super-conduct ivity below 4 K is clearly indicated by the diamagnetic shift of l. The relaxation follows an exponential function. The temperature dependence of the "tted relaxation rate j is well described by the function (solid line in Fig. 8b). j"A
1 #C (1#¹/h)@
(4)
with b"1.40(5) and h"!2.80(34) K. The value of b is roughly in agreement with the critical exponent expected for a 3D-Heisenberg coupled system ( b";0.705). Fig. 8. (a) Temperature dependence of the precession frequency l in PrMo S (B "500 G). The onset of superconductivity at ¹ K4 K is clearly visible, (b) temperature dependence of the exponential relaxation rate j. The solid line represents a "t of Eq. (2) to the data (see text). The onset of superconductivity is not re#ected in these data.
#ux line lattice is barely visible. Fig. 6 displays the "tted high "eld p(0) versus ¹ , including also re sults from Ref. [1]. As can be seen, there seems to be no obvious relation between p(0)J1j and ¹ . It rather appears as if p(0) is nearly constant for ¹ 97 K and assumes generally rather small values for compunds with ¹ :7 K. 3. Dynamic relaxation in the compounds with X"Nd and Pr ZF, LF (500 G) and TF (500 G) measurements on NdMo S reveal the presence of two di!erently fast and exponentially relaxing components with equal amplitudes, suggesting two di!erent l> sites
4. Conclusions
(i) Contrary to previous reports (Ref. [1,2]) no obvious scaling between p(0)J1/j and ¹ is found in the family of Chevrel phase superconductors, e.g., while ¹ changes by a factor of 2 from LaMo S to PbMo S , p remains es sentially unchanged. The ScMo S -results are highly anomalous and are not understood so far. (ii) The dynamics of the 4f-spin in the Nd and Prcompounds render the e!ects of the #ux line lattice invisible. The spin dynamics in PRMo S is consistent with that of a 3D Heisenberg magnet. In contrast to the Nd and Pr compounds YbMo S does not reveal any mag netic signature, proving that the Yb-ions are in the nonmagnetic Yb> ionic state (4f S ). References [1] P. Birrer et al., Phys. Rev. B 48 (1993) 16 589. [2] Y.J. Uemura et al., Phys. Rev. Lett. 66 (1991) 2665.