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YbPd2Si2, A moderate heavy fermion system
Solid State Communications, Vo1.6l,No.8. pp.479-481,1987. Printedin Great Britain.
0038-1098/87 $3.00 + .OO PergamonJournalsLtd.
YbPd2Si2, A MODERATE HEAVY FERMION SYSTEM S.K. Dhar, E.V. Sampathkumaran and R. Vijayaraghavan Tata Institute of Fundamental Research, Bombay 400 005, India and R. Kuentzler Institut de Physique, CNRS LA 306, 3 rue de 1'Universite' 67084 Strasbourg Cedex, France (received October 22nd, 1986 by P. Wachter) Heat capacity of YbPd Sip has been measured in the temperature range 1.5 to 20 R. The coefficient of the electronic heat capacity y is found to be 203 mJ/Yb-mole, comparable to those for heavy fermion materials. It is observed that the present compound obeys the x(0)/y correlation as reported for some Yb compounds In literature.
Rare earth compounds which exhibit valence fluctuation/Kondo lattice behavior have beyn intensively studied in recent years . is interaction in these compounds gives rise to an enhanced density of quasiparticle properties of these systems below a characteristic particular, the value of the heat capacity coefficient y is large in these compounds and may, in some cases, excee g even lJ/fr mole as okserved in CeA13 , CeCu2Si2 and CeCu . Such matgrials have been termed hgavy fermions and it is of interest to look for systems with large Y. While the situation for Ce compounds is satisfactory, very few Yb compounds have been studied in this regard. In this letter we report the low temperature (1.5 to 20 K) heat capacity measurements on YbPd,Si,. The compound YbPd Si cr$gt$llizes and has in the ThCr,Si_ struc&e2tvne been studie6 efirlier by magnetic sus-edge X-ray absorption ceptibility, L 'A'photoe mission and and valence ban shows the characteristic properties of vale ce fluctuation/Kondo lattice systems 3 . The first indication for the valence instability in this compound came from room temperaturg L -edge measurements by Rao et al . I+& e susceptibility exhibits a crossover from the high temperature Curie-Weiss behavior to enhanced Pauli paramagnetism at low temperature with a ma imum at about 30 K. Below 16 K, a Ts dependence of the susceptibility has been interpreted to reflect the Fermi liquid nature of the ground state. From the analysis of susceptibility and LIII-
edge data a valence of - 2.9 has been calculated for the Yb ions. The near trivalency of Yb ions is also consistent with the absence of any lattice volume anomaly in YbPd2Si2. These properties of YbPd Si are in striking similarity with th8se20f the nearly trivalent Yb compound w which has 260 mJ/Yb-mole ’ . This ayof, provided us the primary motivation to undertake the present study. The sample used in the present measurements is the same as that was studied earlier by susceptibility and -edge X-ray absorption mentioned Heat capacity was measured by semi-adiabatic heat pulse method. The results are shown in Fig. 1 where the heat capacity is depicted in the form of C/T vs. T plot. The small peak near 2.2 K is presumably due to Yb 0 which orders magnetically at _ 2.25 %I?
T2(
K’)
Fig.l.C/T vs. ' -tteKtemp;EraFZZ ZZgti"~d~',i~ in 479
480
YbPd2Si2,A MODERATEHEAVY FERMIONSYSTEM
The entropy associated with the peak is extremely small and is less than 15 mJ/ Yb-mole, indicating that about 0.1% of the Yb ions are in the form of Yb203. The presence of such a small amount of magnetic_Yb20fi, which is often found as an impurity p ase in the preparation of Yb compounds, attests to the good quality of our sample. For a comparison, le YbCuAl studied by Pott for heat capacity, thermal expansion and resistivity had a peak in the heat capacity due to Yb 0 with an associated entropy of 0.17 3/gb-mole. The reason for making this comparison is to point out the difficulty in pre- free samples of Yb comlso that its small presence does not affect the analysis of our heat capacity d?ta. A fit of our data to C/T = y + 8T , where 8 is the lattice contribution, in the temperature range 224 to 9 K-gives y ~~40.7 mJ/gatom. K (203 mJ/Yb-mole K 1 and 6 = 0.209 mJ/g-atom. K". Above 9 K the deviation from the linear behavior might be due to lattice anharmonicity. Our heat capacity data is reasonably well rqpreseqted by the relation C/T = Y + BT + 6T in 2.42to 16 K range with y = 4046 mJ/g-atom K , 8 = 03229 mJ/gatom K and 6 = -0.378 x 10 mJ/gatom K6. It is reassuring to obtain the same value of y from the two fits. The y value of 203 mJ/Yb-mole for YbPd Si is among the highest known for Yb &mp&nds to date and, to our knowledge, is only lower than that of YbCuAL It is of interest to determine whether the correlation between the zero temperature susceptibility ~(0) and y observed in a number of intermediate valent/Kondo Yb and Ce compounds respectively extends -to YbPd2Si2 also. This correlation arises because the physical properties like x, y etc. scale with a characteristic temperture which is dependent on valence. A theoretical explanation of this scal&g has been put forward by Newns et al. using local Fermi liquid theory. Figure 2
vol. 61, No. 8
1 0
100 Y (mJ/Yb-mole.
1 200
I 300
K*)
Fig.2.Zero temperature susceptibility x(O) vs. the coefficient of electronic heat capacity y for some Yb compounds. depicts a plot of ~(0) vs. y for YbA13, YbCuAl.and YbPd2Sia. The or the first three c mpounds have been taken from Ref. 12 and ~(0) for YbPd,Si, from Ref. 7. It is observedLth&t YbPd Si obeys the x(0)/y correlation with a 2Wi.$son ratio R of 0.96. An examination of the lattice parameters of YbPd Si2 gives a Yb-Yb separation of 4.0g28. Such a large separation of rare earth atoms precludes direct 4f-4f interaction, which is considered to be a prerequisite for the occurrence of heayy fermion behavior. Meisner et al. have suggested a correlation between y and f-atom separation in such systems. From their plot the y value of YbPd Si turns out to be much larger than f8un 2 experimentally and exceeds even that of YbCuAl. This suggests that besides the 4f-4f separation, the hybridisation of the 4f shell with the neighbouring non 4f ligands hasle role in determining the y value .
REFERENCES 1. See, e.g., Proceedings of the 4th
2. 3.
4. 5.
International Conference on Valence Fluctuations, 1984, J. Magn. Magn. Mater. 47-48 (1985). K. Andres, J.E. Graebner and H.R. Ott, Phys. Rev. Lett. 35, 1979 (1975). F. Steglich, J. Aarts, C.D. Bredl, W. Lieke, D. Meschede, W. Franz and H. Schafer, Phys. Rev; Lett. fi, 1892 (1979). G.R. Stewart, Z. Fisk and M.S. Wire, Phys. Rev. E, 482 (1984). See, for a review, G.R. Stewart, Rev. Mod. Phys. 56, 755 (1984).
6. D. Rossi, R. Marazza and R. Ferro, J. Less Common Metals. 66, P17 (1979). 7. E.V. Sampathkumaran, K.H. Frank, G. Kalkowski. G. Kaindl. M. Domke and G. Wortmann, Phys. Rev. B, 5702 (1984). 8. C.N.R. Rao. D.D. Sarma. P.R. Sarode. E.V. Sampathkumaran, L:C. Gupta and' R. Vijayaraghavan, Chem. Phys. Lett. 3, 413 (1980). 9. W.C.M. Mattens, H. Holscher, G.J.M. Tuins, A.C. Moleman and F.R. de Boer, J. Magn. Magn. Mater. 15-18, 982 (1980).
vol. 61, No. 8
YbPd2Si2, A MODERATE HEAVY FERMION SYSTEM
10. R. Pott, R. Schefzyk and
D. Wohlleben, Z. Phys. 2, 17 (1981). 11. J.C.P. Klaasse, F.R. deBoer and P.F. deChate1, Physica, B 106, 178 (1981). 12. D.M. Newns, A.C. Hewson, J.W. Rasul and N. Read, J. Appl. Phys. 53, 7877 (1982).
13. G.P. Meisner, A.L. Giorgi, A.C. Lawson, G.R. Stewart, J.O. Willis, M.S. Wire and J.L. Smith, Phys. Rev. Lett. 53, 1829 (1984). 14. E.V. Sampathkumaran and R. Vijayaraghavan, Phys. Rev. Lett. 56, 2861 (1986).
481
0038-1098/87 $3.00 + .OO PergamonJournalsLtd.
YbPd2Si2, A MODERATE HEAVY FERMION SYSTEM S.K. Dhar, E.V. Sampathkumaran and R. Vijayaraghavan Tata Institute of Fundamental Research, Bombay 400 005, India and R. Kuentzler Institut de Physique, CNRS LA 306, 3 rue de 1'Universite' 67084 Strasbourg Cedex, France (received October 22nd, 1986 by P. Wachter) Heat capacity of YbPd Sip has been measured in the temperature range 1.5 to 20 R. The coefficient of the electronic heat capacity y is found to be 203 mJ/Yb-mole, comparable to those for heavy fermion materials. It is observed that the present compound obeys the x(0)/y correlation as reported for some Yb compounds In literature.
Rare earth compounds which exhibit valence fluctuation/Kondo lattice behavior have beyn intensively studied in recent years . is interaction in these compounds gives rise to an enhanced density of quasiparticle properties of these systems below a characteristic particular, the value of the heat capacity coefficient y is large in these compounds and may, in some cases, excee g even lJ/fr mole as okserved in CeA13 , CeCu2Si2 and CeCu . Such matgrials have been termed hgavy fermions and it is of interest to look for systems with large Y. While the situation for Ce compounds is satisfactory, very few Yb compounds have been studied in this regard. In this letter we report the low temperature (1.5 to 20 K) heat capacity measurements on YbPd,Si,. The compound YbPd Si cr$gt$llizes and has in the ThCr,Si_ struc&e2tvne been studie6 efirlier by magnetic sus-edge X-ray absorption ceptibility, L 'A'photoe mission and and valence ban shows the characteristic properties of vale ce fluctuation/Kondo lattice systems 3 . The first indication for the valence instability in this compound came from room temperaturg L -edge measurements by Rao et al . I+& e susceptibility exhibits a crossover from the high temperature Curie-Weiss behavior to enhanced Pauli paramagnetism at low temperature with a ma imum at about 30 K. Below 16 K, a Ts dependence of the susceptibility has been interpreted to reflect the Fermi liquid nature of the ground state. From the analysis of susceptibility and LIII-
edge data a valence of - 2.9 has been calculated for the Yb ions. The near trivalency of Yb ions is also consistent with the absence of any lattice volume anomaly in YbPd2Si2. These properties of YbPd Si are in striking similarity with th8se20f the nearly trivalent Yb compound w which has 260 mJ/Yb-mole ’ . This ayof, provided us the primary motivation to undertake the present study. The sample used in the present measurements is the same as that was studied earlier by susceptibility and -edge X-ray absorption mentioned Heat capacity was measured by semi-adiabatic heat pulse method. The results are shown in Fig. 1 where the heat capacity is depicted in the form of C/T vs. T plot. The small peak near 2.2 K is presumably due to Yb 0 which orders magnetically at _ 2.25 %I?
T2(
K’)
Fig.l.C/T vs. ' -tteKtemp;EraFZZ ZZgti"~d~',i~ in 479
480
YbPd2Si2,A MODERATEHEAVY FERMIONSYSTEM
The entropy associated with the peak is extremely small and is less than 15 mJ/ Yb-mole, indicating that about 0.1% of the Yb ions are in the form of Yb203. The presence of such a small amount of magnetic_Yb20fi, which is often found as an impurity p ase in the preparation of Yb compounds, attests to the good quality of our sample. For a comparison, le YbCuAl studied by Pott for heat capacity, thermal expansion and resistivity had a peak in the heat capacity due to Yb 0 with an associated entropy of 0.17 3/gb-mole. The reason for making this comparison is to point out the difficulty in pre- free samples of Yb comlso that its small presence does not affect the analysis of our heat capacity d?ta. A fit of our data to C/T = y + 8T , where 8 is the lattice contribution, in the temperature range 224 to 9 K-gives y ~~40.7 mJ/gatom. K (203 mJ/Yb-mole K 1 and 6 = 0.209 mJ/g-atom. K". Above 9 K the deviation from the linear behavior might be due to lattice anharmonicity. Our heat capacity data is reasonably well rqpreseqted by the relation C/T = Y + BT + 6T in 2.42to 16 K range with y = 4046 mJ/g-atom K , 8 = 03229 mJ/gatom K and 6 = -0.378 x 10 mJ/gatom K6. It is reassuring to obtain the same value of y from the two fits. The y value of 203 mJ/Yb-mole for YbPd Si is among the highest known for Yb &mp&nds to date and, to our knowledge, is only lower than that of YbCuAL It is of interest to determine whether the correlation between the zero temperature susceptibility ~(0) and y observed in a number of intermediate valent/Kondo Yb and Ce compounds respectively extends -to YbPd2Si2 also. This correlation arises because the physical properties like x, y etc. scale with a characteristic temperture which is dependent on valence. A theoretical explanation of this scal&g has been put forward by Newns et al. using local Fermi liquid theory. Figure 2
vol. 61, No. 8
1 0
100 Y (mJ/Yb-mole.
1 200
I 300
K*)
Fig.2.Zero temperature susceptibility x(O) vs. the coefficient of electronic heat capacity y for some Yb compounds. depicts a plot of ~(0) vs. y for YbA13, YbCuAl.and YbPd2Sia. The or the first three c mpounds have been taken from Ref. 12 and ~(0) for YbPd,Si, from Ref. 7. It is observedLth&t YbPd Si obeys the x(0)/y correlation with a 2Wi.$son ratio R of 0.96. An examination of the lattice parameters of YbPd Si2 gives a Yb-Yb separation of 4.0g28. Such a large separation of rare earth atoms precludes direct 4f-4f interaction, which is considered to be a prerequisite for the occurrence of heayy fermion behavior. Meisner et al. have suggested a correlation between y and f-atom separation in such systems. From their plot the y value of YbPd Si turns out to be much larger than f8un 2 experimentally and exceeds even that of YbCuAl. This suggests that besides the 4f-4f separation, the hybridisation of the 4f shell with the neighbouring non 4f ligands hasle role in determining the y value .
REFERENCES 1. See, e.g., Proceedings of the 4th
2. 3.
4. 5.
International Conference on Valence Fluctuations, 1984, J. Magn. Magn. Mater. 47-48 (1985). K. Andres, J.E. Graebner and H.R. Ott, Phys. Rev. Lett. 35, 1979 (1975). F. Steglich, J. Aarts, C.D. Bredl, W. Lieke, D. Meschede, W. Franz and H. Schafer, Phys. Rev; Lett. fi, 1892 (1979). G.R. Stewart, Z. Fisk and M.S. Wire, Phys. Rev. E, 482 (1984). See, for a review, G.R. Stewart, Rev. Mod. Phys. 56, 755 (1984).
6. D. Rossi, R. Marazza and R. Ferro, J. Less Common Metals. 66, P17 (1979). 7. E.V. Sampathkumaran, K.H. Frank, G. Kalkowski. G. Kaindl. M. Domke and G. Wortmann, Phys. Rev. B, 5702 (1984). 8. C.N.R. Rao. D.D. Sarma. P.R. Sarode. E.V. Sampathkumaran, L:C. Gupta and' R. Vijayaraghavan, Chem. Phys. Lett. 3, 413 (1980). 9. W.C.M. Mattens, H. Holscher, G.J.M. Tuins, A.C. Moleman and F.R. de Boer, J. Magn. Magn. Mater. 15-18, 982 (1980).
vol. 61, No. 8
YbPd2Si2, A MODERATE HEAVY FERMION SYSTEM
10. R. Pott, R. Schefzyk and
D. Wohlleben, Z. Phys. 2, 17 (1981). 11. J.C.P. Klaasse, F.R. deBoer and P.F. deChate1, Physica, B 106, 178 (1981). 12. D.M. Newns, A.C. Hewson, J.W. Rasul and N. Read, J. Appl. Phys. 53, 7877 (1982).
13. G.P. Meisner, A.L. Giorgi, A.C. Lawson, G.R. Stewart, J.O. Willis, M.S. Wire and J.L. Smith, Phys. Rev. Lett. 53, 1829 (1984). 14. E.V. Sampathkumaran and R. Vijayaraghavan, Phys. Rev. Lett. 56, 2861 (1986).
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