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Temperature dependence of the lower critical field of UPt3 doped with boron
.... Journal of Magnetism and Magnetic Materials 108 (1992} 75-76 North-Holland
h l l ;,
Temperature dependence of the lower critical field of with boron
UPt 3
doped
E.A. K n e t s c h a, J.A. M y d o s h ~, T. V o r e n k a m p b and A.A. Menovsk3, ,,.b a Kamerlingh Onnes Laboratorium der Rijksunicersiteit Leiden, 2300 RA LeMen, Netherlands t, Natuurkundig Laboratorium, Unicersitdt can Amsterdan~, Valckenierstraat 6.5, 1018 XE Amsterdam, Netherhmds
Doping of a small amount of boron in UPt 3 produces samples possessing somewhat higher superconducting transition temperatures and lower residual resistivities (Po). We have performed de-magnetization measurements and determined the lower critical field (Hcl) of a boron-doped, single-crystal of UPt 3, along the b and c axes. We have observed a kink in the temperature dependence of H,,I for both b and c directions. We also observe a double transition in ZFC dc-magnctization experiments for the external field parallel to the b axis. This step in the shielding curve coincides with the kink in H¢t and the second peak in the specific heat measurements on the same sample. No such feature is observed with the field along the c axis. As an upward kink in He1 cannot be due to different metallurgical phases in the sample, it might be interpreted as a manifestation of the anisotropic superconductivity in this material. After the discovery of a double superconducting transition in the heavy-fermion superconductor UPt 3 [1], this system has become the subject of extensive research and is now believed to be an unconventional superconductor. Basal plane antiferromagnetic fluctuations have been found in some samples [2] and according to several theories [3-5], these couple to the superconducting Order-Parameter (OP) and lower its symmetry with respect to the hexagonal crystal structure. This lifting of the OP degeneracy is then argued to bc the cause of the double superconducting transition in the specific heat. Other experiments showing this double transition are ultrasonics [6] and thc lowcr critical field (Hcl) [7]. As H~l is proportional to n J m * , the sharp kink upward at the temperature of the second peak in the specific hcat can bc associated with a sudden increase in superfluid density. Earlier measurements have shown that the addition of small amounts of boron to UPt 3 leads to samples possessing higher transition temperatures and lower residual resistivities [8]. This is now interpreted as a purification effect in which at least part of the boron probably reduces the oxygen content by the formation of volatile products under plasma heating conditions [9]. The nominal concentration of added boron is 11%. The homogeneity of the boron distribution in the asgrown crystal was confirmed by electron-microprobe analysis. The absolute value of the boron content however, could not be determined accurately in this way. X-ray analysis reveals one single phase of the hexagonal UPt.~ structure with a few degrees of mosaic. The samples are of high purity as is evidenced by the low P0 value of 9.46 ixg2 cm ( l / / b ) and the high residual resistance ratio of 560. The crystal showed a clear double transition in the specific heat and high transition temperatures in both C o (To+ --- 540 mK) and resistivity (Tc = 558 mK) [10].
Wc have made measurements of the lower critical field and shielding magnetization on two parallellepipeds with their long axes in the b and c direction respectively. In these measurements we employed a fluxgate magnetometer technique in a 3He cryostat. The Hcl measurements were performed by cooling the samples in zero field and recording the virgin magnetization curve at fixed temperatures. From the first deviation from lincarity the values of Hc~ could bc inferred. For our samples a smali correction for demagnetizing effects due to the sharp edges of the samples was made (D ~- 0.05). As the sample dimension in the direction of the applied magnetic field was large compared to the pick-up coil, no further corrections wcrc necessary to obtain the correct Hc~ values. Also another method for the determination of H,I based on the Benn critical state model was used [11], giving the same results. The temperature dependence of H,: t is exhibited in fig. 1 and clearly shows that H~I is anisotropic and the c axis value is significantly smaller. This contrasts quantitatively with the measurements of Vincent et al. [12] and Zhao et al. [13], as they both find an isotropic temperature dependence of H~ and somewhat smaller slope-change ratios. The H~ values of Vincent et al., are significantly higher as a rcsult of their determination criterion. The H~ values by Zhao et al., are in good agreement with our b-axis sample. However, we note that theory gives no definite prediction on the anisotropy of the Hcl behavior [3]. Figure 1 also shows lhat the kinks occur at T~*, the temperature of the second transition in the specific heat measurements. The size of the splitting and the extrapolated value of the low temperature slopes (Tc,,t~r,.) are the same for both field directions. An overview of these parameters is collected in table 1. We have also performed measurements of the
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
E.A. Knetsch et al. / Temperan,re dependence of H,. z of UPt~
76
A
0
v
(a)
20
C~ Q
•
t-. E
4
.g
40 60
u
"~
O)
2
.m 4) ,1=
0 0.30
0.40
0.50
0.60
ca
80
"-....
2
0.50
0.60
0.70
(K)
Fig. 2. ZFC shielding magnetization for UPt 3 doped with boron for the b ([]) and c (e) direction in an applied field of 10G.
(b)
I-
0.40
T
T (K)
A
F1Pm~mlmi@ppmll~Ym 0
100 0.30
,. '...
E ,,,,,, •r
u
feature has been observed in ac-susceptibility measurements [14].
1
0 0.30
0.40
0.50
0.60
T (K)
Fig. 1. Temperature dependence of the lower critical field of UPta doped with boron for the b direction (figure l(a); []) and for the c direction (figure l(b); o). Note the kink upward in both curves at To*( = 480 inK). ZFC-magnetization in an applied field o f 10 G. Various smaller fields were used, but gave similar results. As can be seen from fig. 2, there is a very interesting difference. For the b direction, a double-stepped shielding cuwe is found, whereas the c direction shows one smooth transition. The onset temperatures are equal, and coming from the few t e m p e r a t u r e side the initial flux penetration into tiw sample starts at the same temperature. But in the b-axis sample the shielding currents are in the a - c plane, in contrast to the c-axis sample where the shielding currents are in the basal plane. The anisotropy of the order parameter, also reflected in the specific heat measurements, might be the cause of this anisotropy. W e note that in the measurements of Vincent et ai., this kink is not observed for H IIb. For some samples of UPt 3, a similar Table 1 Collected parameters for boron doped UPt 3. The parameters were derived from the //~t measurements UPt 3 (boron doped)
H IIb
H IIc
7~~ (K)
0.540 0.480 0.521 28 19 1.43
0.540 0.485 0.515 19 11 1.81
T~* (K) 7~,xtrap. (K) d ttct/dT (low T) (roT/K) 6~Hc~/dT ~high T) (roT/K) slope ratio
In conclusion we can say that the effect boron doping on the properties of UPt 3 appears to be limited to purification of the s a m p l e and there seems to be no influence on the lower critical field behavior. The shielding behavior is very peculiar and might display a d e p e n d e n c e of the shielding currents on the crystal direction. Certainly m o r e extensive research is n e e d e d on UPt 3 to fully exclude the possibility of these features being due to metallurgical problems and explain this result in the context of the existing models for heavy-fermion superconductivity. We gratefully acknowledge J.J. Petersen for his efforts during data acquisition, T.J. G o r l e n m u l d e r for technical assistance and G.J. Nieuwcnhuys for fruitful conversations. This a,ork was partially supported by the Ncderlandse Stichting F O M . References
[11 R.H. Fisher et al., Phys. Rev. Lett. 62 (1989) 1411. t2l G. Aeppli et ai., Phys. Rev. Lett. 60 (1988) 615. [31 D.W. Hess et al., J. Phys.: Condens Matter 1 (1989) 8135; T.A. Tokuyasu et al., Phys. Rev. B 41 (1990) 8891.
[41 K. Machida et al., J. Phys. Soc. Jpn. 58 (1989) 2244 & 4116.
[51 R. Joynt, Superc. Sci. Technol. 1 (1988) 210. [61 G. Bruls et al., Phys. Rev. Lett. 65 (1990) 2294; S. Adenwalla et ai., Phys. Rev. Lett. 65 (1990) 2298.
[71 B.S. Shivaram et al., Phys. Rev. Lett. 63 (1989) 63. . . . 76 & 77 [81 . . . . v,,,,,,,,v e t al., J. Magn. Magn. '. ...... I.t l T. ~,z.......... (1988) 531. [9] A.A. Menovsky, J. Magn. Magn. Mater 76 & 77 (1988) 631. [10l T. Vorenkamp et al., Physica B 165 & 166 (1990) 363. [111 M. Naito et al., Phys. Rev. B 41 (1990) 4823. [121 E. Vincent et al., J. Phys.: Condens. Matter 3 (19:q) 3517. [131 Z. Zhao et al., Physica B 165 & 166 (1990) 345. [141 P.J.C. Signore et al., to be published.
h l l ;,
Temperature dependence of the lower critical field of with boron
UPt 3
doped
E.A. K n e t s c h a, J.A. M y d o s h ~, T. V o r e n k a m p b and A.A. Menovsk3, ,,.b a Kamerlingh Onnes Laboratorium der Rijksunicersiteit Leiden, 2300 RA LeMen, Netherlands t, Natuurkundig Laboratorium, Unicersitdt can Amsterdan~, Valckenierstraat 6.5, 1018 XE Amsterdam, Netherhmds
Doping of a small amount of boron in UPt 3 produces samples possessing somewhat higher superconducting transition temperatures and lower residual resistivities (Po). We have performed de-magnetization measurements and determined the lower critical field (Hcl) of a boron-doped, single-crystal of UPt 3, along the b and c axes. We have observed a kink in the temperature dependence of H,,I for both b and c directions. We also observe a double transition in ZFC dc-magnctization experiments for the external field parallel to the b axis. This step in the shielding curve coincides with the kink in H¢t and the second peak in the specific heat measurements on the same sample. No such feature is observed with the field along the c axis. As an upward kink in He1 cannot be due to different metallurgical phases in the sample, it might be interpreted as a manifestation of the anisotropic superconductivity in this material. After the discovery of a double superconducting transition in the heavy-fermion superconductor UPt 3 [1], this system has become the subject of extensive research and is now believed to be an unconventional superconductor. Basal plane antiferromagnetic fluctuations have been found in some samples [2] and according to several theories [3-5], these couple to the superconducting Order-Parameter (OP) and lower its symmetry with respect to the hexagonal crystal structure. This lifting of the OP degeneracy is then argued to bc the cause of the double superconducting transition in the specific heat. Other experiments showing this double transition are ultrasonics [6] and thc lowcr critical field (Hcl) [7]. As H~l is proportional to n J m * , the sharp kink upward at the temperature of the second peak in the specific hcat can bc associated with a sudden increase in superfluid density. Earlier measurements have shown that the addition of small amounts of boron to UPt 3 leads to samples possessing higher transition temperatures and lower residual resistivities [8]. This is now interpreted as a purification effect in which at least part of the boron probably reduces the oxygen content by the formation of volatile products under plasma heating conditions [9]. The nominal concentration of added boron is 11%. The homogeneity of the boron distribution in the asgrown crystal was confirmed by electron-microprobe analysis. The absolute value of the boron content however, could not be determined accurately in this way. X-ray analysis reveals one single phase of the hexagonal UPt.~ structure with a few degrees of mosaic. The samples are of high purity as is evidenced by the low P0 value of 9.46 ixg2 cm ( l / / b ) and the high residual resistance ratio of 560. The crystal showed a clear double transition in the specific heat and high transition temperatures in both C o (To+ --- 540 mK) and resistivity (Tc = 558 mK) [10].
Wc have made measurements of the lower critical field and shielding magnetization on two parallellepipeds with their long axes in the b and c direction respectively. In these measurements we employed a fluxgate magnetometer technique in a 3He cryostat. The Hcl measurements were performed by cooling the samples in zero field and recording the virgin magnetization curve at fixed temperatures. From the first deviation from lincarity the values of Hc~ could bc inferred. For our samples a smali correction for demagnetizing effects due to the sharp edges of the samples was made (D ~- 0.05). As the sample dimension in the direction of the applied magnetic field was large compared to the pick-up coil, no further corrections wcrc necessary to obtain the correct Hc~ values. Also another method for the determination of H,I based on the Benn critical state model was used [11], giving the same results. The temperature dependence of H,: t is exhibited in fig. 1 and clearly shows that H~I is anisotropic and the c axis value is significantly smaller. This contrasts quantitatively with the measurements of Vincent et al. [12] and Zhao et al. [13], as they both find an isotropic temperature dependence of H~ and somewhat smaller slope-change ratios. The H~ values of Vincent et al., are significantly higher as a rcsult of their determination criterion. The H~ values by Zhao et al., are in good agreement with our b-axis sample. However, we note that theory gives no definite prediction on the anisotropy of the Hcl behavior [3]. Figure 1 also shows lhat the kinks occur at T~*, the temperature of the second transition in the specific heat measurements. The size of the splitting and the extrapolated value of the low temperature slopes (Tc,,t~r,.) are the same for both field directions. An overview of these parameters is collected in table 1. We have also performed measurements of the
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
E.A. Knetsch et al. / Temperan,re dependence of H,. z of UPt~
76
A
0
v
(a)
20
C~ Q
•
t-. E
4
.g
40 60
u
"~
O)
2
.m 4) ,1=
0 0.30
0.40
0.50
0.60
ca
80
"-....
2
0.50
0.60
0.70
(K)
Fig. 2. ZFC shielding magnetization for UPt 3 doped with boron for the b ([]) and c (e) direction in an applied field of 10G.
(b)
I-
0.40
T
T (K)
A
F1Pm~mlmi@ppmll~Ym 0
100 0.30
,. '...
E ,,,,,, •r
u
feature has been observed in ac-susceptibility measurements [14].
1
0 0.30
0.40
0.50
0.60
T (K)
Fig. 1. Temperature dependence of the lower critical field of UPta doped with boron for the b direction (figure l(a); []) and for the c direction (figure l(b); o). Note the kink upward in both curves at To*( = 480 inK). ZFC-magnetization in an applied field o f 10 G. Various smaller fields were used, but gave similar results. As can be seen from fig. 2, there is a very interesting difference. For the b direction, a double-stepped shielding cuwe is found, whereas the c direction shows one smooth transition. The onset temperatures are equal, and coming from the few t e m p e r a t u r e side the initial flux penetration into tiw sample starts at the same temperature. But in the b-axis sample the shielding currents are in the a - c plane, in contrast to the c-axis sample where the shielding currents are in the basal plane. The anisotropy of the order parameter, also reflected in the specific heat measurements, might be the cause of this anisotropy. W e note that in the measurements of Vincent et ai., this kink is not observed for H IIb. For some samples of UPt 3, a similar Table 1 Collected parameters for boron doped UPt 3. The parameters were derived from the //~t measurements UPt 3 (boron doped)
H IIb
H IIc
7~~ (K)
0.540 0.480 0.521 28 19 1.43
0.540 0.485 0.515 19 11 1.81
T~* (K) 7~,xtrap. (K) d ttct/dT (low T) (roT/K) 6~Hc~/dT ~high T) (roT/K) slope ratio
In conclusion we can say that the effect boron doping on the properties of UPt 3 appears to be limited to purification of the s a m p l e and there seems to be no influence on the lower critical field behavior. The shielding behavior is very peculiar and might display a d e p e n d e n c e of the shielding currents on the crystal direction. Certainly m o r e extensive research is n e e d e d on UPt 3 to fully exclude the possibility of these features being due to metallurgical problems and explain this result in the context of the existing models for heavy-fermion superconductivity. We gratefully acknowledge J.J. Petersen for his efforts during data acquisition, T.J. G o r l e n m u l d e r for technical assistance and G.J. Nieuwcnhuys for fruitful conversations. This a,ork was partially supported by the Ncderlandse Stichting F O M . References
[11 R.H. Fisher et al., Phys. Rev. Lett. 62 (1989) 1411. t2l G. Aeppli et ai., Phys. Rev. Lett. 60 (1988) 615. [31 D.W. Hess et al., J. Phys.: Condens Matter 1 (1989) 8135; T.A. Tokuyasu et al., Phys. Rev. B 41 (1990) 8891.
[41 K. Machida et al., J. Phys. Soc. Jpn. 58 (1989) 2244 & 4116.
[51 R. Joynt, Superc. Sci. Technol. 1 (1988) 210. [61 G. Bruls et al., Phys. Rev. Lett. 65 (1990) 2294; S. Adenwalla et ai., Phys. Rev. Lett. 65 (1990) 2298.
[71 B.S. Shivaram et al., Phys. Rev. Lett. 63 (1989) 63. . . . 76 & 77 [81 . . . . v,,,,,,,,v e t al., J. Magn. Magn. '. ...... I.t l T. ~,z.......... (1988) 531. [9] A.A. Menovsky, J. Magn. Magn. Mater 76 & 77 (1988) 631. [10l T. Vorenkamp et al., Physica B 165 & 166 (1990) 363. [111 M. Naito et al., Phys. Rev. B 41 (1990) 4823. [121 E. Vincent et al., J. Phys.: Condens. Matter 3 (19:q) 3517. [131 Z. Zhao et al., Physica B 165 & 166 (1990) 345. [141 P.J.C. Signore et al., to be published.