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Persistence of superconductivity in magnetically ordered SmRh4B4
Volume 75A, number 3
PHYSICS LETTERS
7 January 1980
PERSISTENCE OF SUPERCONDUCTiVITY IN MAGNETICALLY ORDERED SmRh4B4 H.R. OTT and W. ODONI Laboratoriuni für Festkörperphysik, ETH-Honggerberg, 8093 Zurich, Switzerland
and H.C. HAMAKER and M.B. MAPLE Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA 92093, USA Received 20 September 1979
Measurements of the thermal conductivity and the electrical resistivity between 50 mK and 4 K in zero magnetic field and in fields exceeding the superconducting critical field Hc2 indicate the persistence of bulk superconductivity in magnetically ordered SmRh4B4.
In recent work [1,2] it was determined experimentally how the thermal conductivity is affected when a material loses its superconductivity upon magnetic ordering as the temperature is decreased. The material investigated was ErRh4B4 which is superconducting below 8.7 K but returns to a normal ferromagnetically ordered stateconvincingly at 0.9 K [3]. that The the thermal conductivity data showed superconducting energy gap is quenched at the magnetic phase transition and that the conduction electrons at the Fermi level resume their participation in the heat transport below the magnetic ordering temperature. Previous work on material with a similarly close interaction of superconductivity and magnetism, namely the rare-earth Chevrel phase compounds, has shown that superconductivity persists below the ordering temperature if the magnetic ordering is antiferromagnetic in nature [4—6].Very recently it was found that, below their superconducting transition temperatures, NdRh4B4 [7] and SmRh4B4 [8] also exhibit pronounced anomalies in specific heat and upper critical field, indicating the occurrence of magnetic ordering without destruction of superconductivity, as revealed by measurements of the electrical resistivity, Additional and reliable information on the persistence of bulk superconductivity in the presence of mag-
netic ordering may be obtained from measurements of the thermal conductivity X. We have therefore measured X of SmRh4B4 between 0.05 K and 4 K in zero magnetic field and in external fields up to a value exceeding the maximum critical field. The sample was a parallelopiped of polycrystalline material with dimen3 and the thermal conductivity sionsdetermined 5.5 X 1 X 1mm was from the temperature gradient across the sample caused by a steady heat flow Q. In addition, we measured the electrical resistivity on this particular specimen in the same temperature and magnetic field region as cited above. In fig. 1 we show the temperature dependence of A of SmRh 4 B4 in zero magnetic field and in a field of 2 kOe. The extrapolated critical field at zero temperature is 1.85 kOe [81. Inferring that the linear temperature dependence of A for H = 2 kOe above 1 K represents the normal state thermal conductivity due to electronic heat transport it may readily be seen that A decreases gradually below its normal state behaviour as is expected for a superconductor. For our sample we determined a zero field superconducting transition temperature Tc = 2.68 K by resistivity measurements (see fig. 2). The most interesting feature is that the magnetic phase transition at 0.87 K does not influence the thermal conductivity in the same drastic way as in ErRh4B4. Below the magnetic ordering temperature of SmRh4B4 243
Volume 75A, number 3
PHYSICS LETTERS
to’ 4B4
thermal Conductivity
4/,
0°
I
/
/~
T~
/
/~> o°
J 102
usually not observed in metals because of the dominating electronic contribution. However, as far as thermal conductivity is concerned, a superconductor well below its transition temperature T~can be regarded as an insulator in the sense that only phonons contribute to A, and the spin wave contribution to the thermal conductivity may indeed become observable. The temperature dependence of A is then given by the dispersion relation of the spin waves. It should also be mentioned that contributions to the thermal conductivity due to spurious unknown and nonsuperconducting parts of the sample might lead to the observed behaviour. In fig. 2 we show the temperature dependence of the resistance of our SmRh4B4 sample in zero magnetic field and in a supercritical magnetic field. Our data
~0
I
[K)
Fig. 1. Thermal conductivity of SmRh4B4 between 0.05 K and 4 K for magnetic field H = 0 and H 2 kOe.
(probably antiferromagnetic, according to the work of Hamaker et al. [8]) A continues to decrease with decreasing temperature, contrary to the steep rise back to the normal state thermal conductivity observed in ErRh4B4 at its ferromagnetic phase transition around 0.9 K. The temperature dependence of A below TN is not that of an ordinary superconductor, however. From the logarithmic plot of A versus temperature T (see fig. 1) it may be seen that at the lowest temperatures the thermal conductivity again approaches an almost linear T dependence. This particular feature may have different causes. Considering the ordered magnetic moments of the rare-earth ions it seems possible that the persisting superconducting state is a gapless state implying a finite electronic thermal conductivity at very low temperatures. According to Ambegaokar and Griffin [9] no sharp features in the temperature dependence of A would be expected in this case. It also cannot be ruled out that the energy gap remains finite below TN and that we observe a contribution to the thermal conductivity other than those from electrons and phonons. In the antiferromagnetically ordered state, 244
may be carried by spin waves [101. This effect is
heat SmRh
7 January 1980
confirm the results obtained in ref. [81which suggested that the drop of the resistance in the normal state below I K may be associated with a magnetic phase transition. Our measurements of the normal state thermal conductivity corroborate this suggestion. Below 1 K we observe a gradual shift of A with decreasing temperature, turning into a linear T dependence again below 0.4K. The relative changes of the values of R and A from above 1 K to below 0.4 K are the same, although opposite in sign. The drop of R and the concomitant increase of A on the same sample indicate that one is not dealing with spurious superconducting phases but with a magnetic phase transition instead.
I I
50 Ho 2 kOe 40
Sm Rh4B4 30
H
20
0
—
0
I I
0
Fig. 2. Electrical resistance H = 0 and H = 2 kOe.
I
I
2.0
3.0
of SmRh4B4
I
1 1K)
for magnetic field
Volume 75A, number 3
PHYSICS LETTERS
This research was supported by the Schweizerische Nationalfonds zur Forderung der wissenschaftlichen Forschung and by the U.S. Department of Energy under contract number Ey-76-S~O3-O034-PA227-3. References [1] W. Odoni and H.R. Ott, Phys. Lett. 70A (1979) 480. [2] H.R. Ott and W. Odoni, Proc. Third Conf. on Superconductivity in d- and f-band metals (La Jolla, CA, 1979) (Academic Press). [3] W.A. Fertig, D.C. Johnston, L.E. DeLong, R.W. Mc Callum, M.B. Maple and B.T. Matthias, Phys. Rev. Lett. 38 (1977)
7 January 1980
[4] R.W. Mc Callum, D.C. Johnston, R.N. Shelton, W.A. Fertig and M.B. Maple, Solid State Commun. 24 (1977) 501; U. Azevedo et al., J. Phys. Suppl. C-6, 1(1978) 365. [5] M. Ishikawa (1977) 747. and 0. Fischer, Solid State Commun. 24 [6] D.E. Moncton, G. Shirane, W. Thomlinson, M. Ishikawa and 0. Fischer, Phys. Rev. Lett. 41(1978)1133. [7] H.C. Hamaker, L.D. Woolf, H.B. MacKay, Z. Fisk and M.B. Maple, Solid State Commun. 31(1979)139. [8] H.C. Hamaker, L.D. Woolf, H.B. MacKay, Z. Fisk and M.B. Maple, Solid State Commun., to be published. [91 V. Ambegaokar and A. Griffin, Phys. Rev. 137 (1965) 1151. [10] B. Lüthi, J. Phys. Chem. Solids 23(1962) 35.
987.
245
PHYSICS LETTERS
7 January 1980
PERSISTENCE OF SUPERCONDUCTiVITY IN MAGNETICALLY ORDERED SmRh4B4 H.R. OTT and W. ODONI Laboratoriuni für Festkörperphysik, ETH-Honggerberg, 8093 Zurich, Switzerland
and H.C. HAMAKER and M.B. MAPLE Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA 92093, USA Received 20 September 1979
Measurements of the thermal conductivity and the electrical resistivity between 50 mK and 4 K in zero magnetic field and in fields exceeding the superconducting critical field Hc2 indicate the persistence of bulk superconductivity in magnetically ordered SmRh4B4.
In recent work [1,2] it was determined experimentally how the thermal conductivity is affected when a material loses its superconductivity upon magnetic ordering as the temperature is decreased. The material investigated was ErRh4B4 which is superconducting below 8.7 K but returns to a normal ferromagnetically ordered stateconvincingly at 0.9 K [3]. that The the thermal conductivity data showed superconducting energy gap is quenched at the magnetic phase transition and that the conduction electrons at the Fermi level resume their participation in the heat transport below the magnetic ordering temperature. Previous work on material with a similarly close interaction of superconductivity and magnetism, namely the rare-earth Chevrel phase compounds, has shown that superconductivity persists below the ordering temperature if the magnetic ordering is antiferromagnetic in nature [4—6].Very recently it was found that, below their superconducting transition temperatures, NdRh4B4 [7] and SmRh4B4 [8] also exhibit pronounced anomalies in specific heat and upper critical field, indicating the occurrence of magnetic ordering without destruction of superconductivity, as revealed by measurements of the electrical resistivity, Additional and reliable information on the persistence of bulk superconductivity in the presence of mag-
netic ordering may be obtained from measurements of the thermal conductivity X. We have therefore measured X of SmRh4B4 between 0.05 K and 4 K in zero magnetic field and in external fields up to a value exceeding the maximum critical field. The sample was a parallelopiped of polycrystalline material with dimen3 and the thermal conductivity sionsdetermined 5.5 X 1 X 1mm was from the temperature gradient across the sample caused by a steady heat flow Q. In addition, we measured the electrical resistivity on this particular specimen in the same temperature and magnetic field region as cited above. In fig. 1 we show the temperature dependence of A of SmRh 4 B4 in zero magnetic field and in a field of 2 kOe. The extrapolated critical field at zero temperature is 1.85 kOe [81. Inferring that the linear temperature dependence of A for H = 2 kOe above 1 K represents the normal state thermal conductivity due to electronic heat transport it may readily be seen that A decreases gradually below its normal state behaviour as is expected for a superconductor. For our sample we determined a zero field superconducting transition temperature Tc = 2.68 K by resistivity measurements (see fig. 2). The most interesting feature is that the magnetic phase transition at 0.87 K does not influence the thermal conductivity in the same drastic way as in ErRh4B4. Below the magnetic ordering temperature of SmRh4B4 243
Volume 75A, number 3
PHYSICS LETTERS
to’ 4B4
thermal Conductivity
4/,
0°
I
/
/~
T~
/
/~> o°
J 102
usually not observed in metals because of the dominating electronic contribution. However, as far as thermal conductivity is concerned, a superconductor well below its transition temperature T~can be regarded as an insulator in the sense that only phonons contribute to A, and the spin wave contribution to the thermal conductivity may indeed become observable. The temperature dependence of A is then given by the dispersion relation of the spin waves. It should also be mentioned that contributions to the thermal conductivity due to spurious unknown and nonsuperconducting parts of the sample might lead to the observed behaviour. In fig. 2 we show the temperature dependence of the resistance of our SmRh4B4 sample in zero magnetic field and in a supercritical magnetic field. Our data
~0
I
[K)
Fig. 1. Thermal conductivity of SmRh4B4 between 0.05 K and 4 K for magnetic field H = 0 and H 2 kOe.
(probably antiferromagnetic, according to the work of Hamaker et al. [8]) A continues to decrease with decreasing temperature, contrary to the steep rise back to the normal state thermal conductivity observed in ErRh4B4 at its ferromagnetic phase transition around 0.9 K. The temperature dependence of A below TN is not that of an ordinary superconductor, however. From the logarithmic plot of A versus temperature T (see fig. 1) it may be seen that at the lowest temperatures the thermal conductivity again approaches an almost linear T dependence. This particular feature may have different causes. Considering the ordered magnetic moments of the rare-earth ions it seems possible that the persisting superconducting state is a gapless state implying a finite electronic thermal conductivity at very low temperatures. According to Ambegaokar and Griffin [9] no sharp features in the temperature dependence of A would be expected in this case. It also cannot be ruled out that the energy gap remains finite below TN and that we observe a contribution to the thermal conductivity other than those from electrons and phonons. In the antiferromagnetically ordered state, 244
may be carried by spin waves [101. This effect is
heat SmRh
7 January 1980
confirm the results obtained in ref. [81which suggested that the drop of the resistance in the normal state below I K may be associated with a magnetic phase transition. Our measurements of the normal state thermal conductivity corroborate this suggestion. Below 1 K we observe a gradual shift of A with decreasing temperature, turning into a linear T dependence again below 0.4K. The relative changes of the values of R and A from above 1 K to below 0.4 K are the same, although opposite in sign. The drop of R and the concomitant increase of A on the same sample indicate that one is not dealing with spurious superconducting phases but with a magnetic phase transition instead.
I I
50 Ho 2 kOe 40
Sm Rh4B4 30
H
20
0
—
0
I I
0
Fig. 2. Electrical resistance H = 0 and H = 2 kOe.
I
I
2.0
3.0
of SmRh4B4
I
1 1K)
for magnetic field
Volume 75A, number 3
PHYSICS LETTERS
This research was supported by the Schweizerische Nationalfonds zur Forderung der wissenschaftlichen Forschung and by the U.S. Department of Energy under contract number Ey-76-S~O3-O034-PA227-3. References [1] W. Odoni and H.R. Ott, Phys. Lett. 70A (1979) 480. [2] H.R. Ott and W. Odoni, Proc. Third Conf. on Superconductivity in d- and f-band metals (La Jolla, CA, 1979) (Academic Press). [3] W.A. Fertig, D.C. Johnston, L.E. DeLong, R.W. Mc Callum, M.B. Maple and B.T. Matthias, Phys. Rev. Lett. 38 (1977)
7 January 1980
[4] R.W. Mc Callum, D.C. Johnston, R.N. Shelton, W.A. Fertig and M.B. Maple, Solid State Commun. 24 (1977) 501; U. Azevedo et al., J. Phys. Suppl. C-6, 1(1978) 365. [5] M. Ishikawa (1977) 747. and 0. Fischer, Solid State Commun. 24 [6] D.E. Moncton, G. Shirane, W. Thomlinson, M. Ishikawa and 0. Fischer, Phys. Rev. Lett. 41(1978)1133. [7] H.C. Hamaker, L.D. Woolf, H.B. MacKay, Z. Fisk and M.B. Maple, Solid State Commun. 31(1979)139. [8] H.C. Hamaker, L.D. Woolf, H.B. MacKay, Z. Fisk and M.B. Maple, Solid State Commun., to be published. [91 V. Ambegaokar and A. Griffin, Phys. Rev. 137 (1965) 1151. [10] B. Lüthi, J. Phys. Chem. Solids 23(1962) 35.
987.
245