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Magnetoresistance of CeCu4Ga under pressure
MIllSlU[ El SEVIER
Physica
B 199&200
119941 181 - 1 8 2
Magnetoresistance of CeCu4Ga under pressure A. Eichler*, C. Mehls, Ch. Sutter bzstitla./'iir Technische Physik and Hoclunagnetfehtanlage. TU Braunschweig. Pos¢/ach 3329. D-38023 Braunschweig, Germany
Abstract CeCu,,Ga is known to be a heavy fermion compound with an extremely high electronic specific heat coefficient of 1.9 J/tool K 2. Up to now there are no indications of superconductivity or long-range magnetic order in this material. Specific heat, susceptibility and resistivity data show distinct maxima at about !.5 K, which may be interpreted as the sign of a coherent K o n d o state at low temperatures. We have performed resistance measurements to study the influence of pressure and magnetic field on this coherent state. Whereas in all cerium Kondo systems strong pressure effects are known, no pressure dependence is found in the present case. The result rises new questions about the ground state of this material.
Heavy fermion materials can be found in all kinds of metallic ground states: magnetically ordered, superconducting or just paramagnetic Fermi-liquid like [I]. The latter state, however, is rather scarce, CeCum, and - with some reserve CeAI3 being the only archetypical examples. These systems are ideally suited to study the pure K o n d o lattice behaviour, i.e. the ransition from the single-ion Kondo region to the correlated one at the lowest temperature, usually called coherent state. This transition is characterised by distinct features: a maximum in the electronic specific heat C/T, a crossover from the Iogarithmical temperature dependence of the electrical resistance at high temperatures to a T2-1aw (p = Po -t- A T" ) at very low temperature, a change in sign of the magnetoresistance and a maximum in the Hall coefficient. The energy scale T* of these phenomena is smaller than that of the single-ion Kondo effect ( TK l but usually taken as proportional to that, e.g. in the pressure dependence.
* Corresponding author.
Anolher material which apparently belongs to that group of paramagnetic heavy fermions is CeCu,~Ga [2]. it is distinguished by an extremely high maximum in the specific heat of about 3 J / m o l K at 1 K [3] which is suppressed by magnetic fields of 2 T. The susceptibility follows a Curie-Weiss law with an effective moment of 2.55~B at higher temperatures, while at about 1 K, a broad maximum appears. l'he resistivity shows a Kondo-like increase with falling temperature between 90 and ~ 2 K reaching a broad maximum around 1.5 K [2]. Below 0.25 K, a TZ-law can be fitted with a coefficient A = 5.8 uflcm/K which is of the same order of magnitude as in CeCum, and CeAI3. Because of the metallurgy of this material, however, it is not warranted that there is a i_c_i .~l l C_U_l~l K .i l.g. .l l.t. . . . . 3 l a r k ., ~ a, ,t . I~.u. ... t e. h y l.p e r a. t H r o. s . . T h ~ properties may be determined by disorder in the distribution of Ga on the Cu sites [2]. We have started an investigation of CeCu,~Ga under pressure by electrical transport and specific heat measurements to look if there is a correlation between the different signatures of coherence (if there is an)'!} as has been verified for CeCum, [4]. Here we present our
0921-4526 94 $07.00 c~ 1994 I-ilsevicr Science B.V. All rights reserved 5;SDI 0921-4526(93)E0232-6
A. Eichler etal. / Pltysi('a B 199&200 (1994) 181 182
182 122 120:
118 - . - - ' - - - - ' - - ~ ' ~ . . ~
o i 710TA, : I 0 . 0 T u : 14.0T -
•~ d . . , ~
116 -
~ ' ~ _
114 ca.
112
_
I
~
,t, ~ ~ .
I
0
-
1
n~
__~
~ i
0
2
4
6 T/K
8
I0
12
Fig. I. Resistance of CoCu,LGa at different magnetic fields (p = 5.5 kbar). The full lines show the corresponding data at zero pressure.
results on the resistivity under pressure and in high magnetic fields. The measurements have been performed in a 3He cryostat. Magnetic fields up to 14 T were supplied by a Bitter magnet. The polycrystalline samples had typical dimensions of 1.5 x 1.5 x 0.25 m m 3. Four potential leads were softsoldered at the corners and the resistivity was measured using the (DC-) van der Pauw method [5]. Pressure was generated in a piston--cylinder cell with a ! : 1 isopentane/n-pentane mixture as pressure transmitting medium. The resistivity at a pressure of 5.5 kbar as a function of temperature at different magnetic fields is plotted in Fig. I. The zero field data are exactly the same as those under a field of 0.5 T for T > 7 K, but show a steplike decrease near 7 K because of the contacts becoming superconducting. "lhey are, therefore, not included in the figure. The resistivity rises nearly logarithmically to low temperatures as expected for a Kondo system. A flat maximum at 1.3 K is observed. In increasing fields, p is successively reduced and the maximum is shifted to higher temperature. At 14T, the maximum is found no more below 1O K. The magnetoresistance does not become positive above 0.3 K, which is our experimental limit. Tile shape of the data at 5.5 kbar, our highest pressure reached, compares exactly to that at p = 0, with only
minor changes in the absolute value. This is d e m o n , strated by the full lines which represent numerical fits to the corresponding data under the same fields at zero pressure. There is no discernible pressure effect. This in sharp contrast to the observations on other K o n d o lattice systems, In CeCu(, and CeAI3, e.g. the temperature of the resistivity maximum is shifted by a relative amount of 0.06/kbar [6]; an effect of comparable order of magnitude should lead to a shift by 1.7 K in the present case. There is no crossover to positive magnetoresistance under pressure either, as was found in CelnCu2 [7]. One reason may be that our m a x i m u m pressure is not high enough. But even in this restricted range some quantitative changes in pIT, B) should be observed in a K o n d o system. It may be suspected, therefore, that the qualitative features observed in p are not precursors of a coherent state; the formation of the latter is presumably prohibited by atomic disorder on the copper sites. Thanks are due to Prof. K. Winzer for making the sample available to us and to the Deutsche Forschungsgemeinschaft for financial support.
References
[1] F. Steglich, J. Phys. Chem. Solids 50 I1989j 225. [2] E. Bauer, N. Pillmayr, E. Gratz, D. Gignoux, D. Schmitt, K. Winzer and J. Kohlmann, J. Magn. Magn. Mater. 71 11988) 311. [3] J. Kohlmann, E. Bauer and K. Winzcr, J. Magn. Magn. Mater. 82 (1989) 169. [4] T Penney, F.P. Milliken, F. Holtzbcrg and Z. Fisk, in: Theoretical and Experimental Aspects of Valence Fluctuations and Heavy Fermions, eds. L.C Crupta and S.K. Malik IPlenum. New York. 1987~ p. 77. [5] L.J. van der Pauw, Philtps Rcs. Rcp. 13 11958) I. [6] A. Shibata, G. Oomi, Y. Onuky and T. Komatsubara, J. Phys. Soc. Japan 55 (1986) 2086; A. Percheron, J.C. Achard, O. Gorchov, B. Corm, t, D. Jerome and B. Coqblin, Solid State Commun. 12 t1973) 1289. [7] T. Kagayama, G. Oomi, R. Yagi, Y. lye, Y. Onuki and T. Komalsubara, J. Phys. Soc. Japan 61 11992) 2632.
Physica
B 199&200
119941 181 - 1 8 2
Magnetoresistance of CeCu4Ga under pressure A. Eichler*, C. Mehls, Ch. Sutter bzstitla./'iir Technische Physik and Hoclunagnetfehtanlage. TU Braunschweig. Pos¢/ach 3329. D-38023 Braunschweig, Germany
Abstract CeCu,,Ga is known to be a heavy fermion compound with an extremely high electronic specific heat coefficient of 1.9 J/tool K 2. Up to now there are no indications of superconductivity or long-range magnetic order in this material. Specific heat, susceptibility and resistivity data show distinct maxima at about !.5 K, which may be interpreted as the sign of a coherent K o n d o state at low temperatures. We have performed resistance measurements to study the influence of pressure and magnetic field on this coherent state. Whereas in all cerium Kondo systems strong pressure effects are known, no pressure dependence is found in the present case. The result rises new questions about the ground state of this material.
Heavy fermion materials can be found in all kinds of metallic ground states: magnetically ordered, superconducting or just paramagnetic Fermi-liquid like [I]. The latter state, however, is rather scarce, CeCum, and - with some reserve CeAI3 being the only archetypical examples. These systems are ideally suited to study the pure K o n d o lattice behaviour, i.e. the ransition from the single-ion Kondo region to the correlated one at the lowest temperature, usually called coherent state. This transition is characterised by distinct features: a maximum in the electronic specific heat C/T, a crossover from the Iogarithmical temperature dependence of the electrical resistance at high temperatures to a T2-1aw (p = Po -t- A T" ) at very low temperature, a change in sign of the magnetoresistance and a maximum in the Hall coefficient. The energy scale T* of these phenomena is smaller than that of the single-ion Kondo effect ( TK l but usually taken as proportional to that, e.g. in the pressure dependence.
* Corresponding author.
Anolher material which apparently belongs to that group of paramagnetic heavy fermions is CeCu,~Ga [2]. it is distinguished by an extremely high maximum in the specific heat of about 3 J / m o l K at 1 K [3] which is suppressed by magnetic fields of 2 T. The susceptibility follows a Curie-Weiss law with an effective moment of 2.55~B at higher temperatures, while at about 1 K, a broad maximum appears. l'he resistivity shows a Kondo-like increase with falling temperature between 90 and ~ 2 K reaching a broad maximum around 1.5 K [2]. Below 0.25 K, a TZ-law can be fitted with a coefficient A = 5.8 uflcm/K which is of the same order of magnitude as in CeCum, and CeAI3. Because of the metallurgy of this material, however, it is not warranted that there is a i_c_i .~l l C_U_l~l K .i l.g. .l l.t. . . . . 3 l a r k ., ~ a, ,t . I~.u. ... t e. h y l.p e r a. t H r o. s . . T h ~ properties may be determined by disorder in the distribution of Ga on the Cu sites [2]. We have started an investigation of CeCu,~Ga under pressure by electrical transport and specific heat measurements to look if there is a correlation between the different signatures of coherence (if there is an)'!} as has been verified for CeCum, [4]. Here we present our
0921-4526 94 $07.00 c~ 1994 I-ilsevicr Science B.V. All rights reserved 5;SDI 0921-4526(93)E0232-6
A. Eichler etal. / Pltysi('a B 199&200 (1994) 181 182
182 122 120:
118 - . - - ' - - - - ' - - ~ ' ~ . . ~
o i 710TA, : I 0 . 0 T u : 14.0T -
•~ d . . , ~
116 -
~ ' ~ _
114 ca.
112
_
I
~
,t, ~ ~ .
I
0
-
1
n~
__~
~ i
0
2
4
6 T/K
8
I0
12
Fig. I. Resistance of CoCu,LGa at different magnetic fields (p = 5.5 kbar). The full lines show the corresponding data at zero pressure.
results on the resistivity under pressure and in high magnetic fields. The measurements have been performed in a 3He cryostat. Magnetic fields up to 14 T were supplied by a Bitter magnet. The polycrystalline samples had typical dimensions of 1.5 x 1.5 x 0.25 m m 3. Four potential leads were softsoldered at the corners and the resistivity was measured using the (DC-) van der Pauw method [5]. Pressure was generated in a piston--cylinder cell with a ! : 1 isopentane/n-pentane mixture as pressure transmitting medium. The resistivity at a pressure of 5.5 kbar as a function of temperature at different magnetic fields is plotted in Fig. I. The zero field data are exactly the same as those under a field of 0.5 T for T > 7 K, but show a steplike decrease near 7 K because of the contacts becoming superconducting. "lhey are, therefore, not included in the figure. The resistivity rises nearly logarithmically to low temperatures as expected for a Kondo system. A flat maximum at 1.3 K is observed. In increasing fields, p is successively reduced and the maximum is shifted to higher temperature. At 14T, the maximum is found no more below 1O K. The magnetoresistance does not become positive above 0.3 K, which is our experimental limit. Tile shape of the data at 5.5 kbar, our highest pressure reached, compares exactly to that at p = 0, with only
minor changes in the absolute value. This is d e m o n , strated by the full lines which represent numerical fits to the corresponding data under the same fields at zero pressure. There is no discernible pressure effect. This in sharp contrast to the observations on other K o n d o lattice systems, In CeCu(, and CeAI3, e.g. the temperature of the resistivity maximum is shifted by a relative amount of 0.06/kbar [6]; an effect of comparable order of magnitude should lead to a shift by 1.7 K in the present case. There is no crossover to positive magnetoresistance under pressure either, as was found in CelnCu2 [7]. One reason may be that our m a x i m u m pressure is not high enough. But even in this restricted range some quantitative changes in pIT, B) should be observed in a K o n d o system. It may be suspected, therefore, that the qualitative features observed in p are not precursors of a coherent state; the formation of the latter is presumably prohibited by atomic disorder on the copper sites. Thanks are due to Prof. K. Winzer for making the sample available to us and to the Deutsche Forschungsgemeinschaft for financial support.
References
[1] F. Steglich, J. Phys. Chem. Solids 50 I1989j 225. [2] E. Bauer, N. Pillmayr, E. Gratz, D. Gignoux, D. Schmitt, K. Winzer and J. Kohlmann, J. Magn. Magn. Mater. 71 11988) 311. [3] J. Kohlmann, E. Bauer and K. Winzcr, J. Magn. Magn. Mater. 82 (1989) 169. [4] T Penney, F.P. Milliken, F. Holtzbcrg and Z. Fisk, in: Theoretical and Experimental Aspects of Valence Fluctuations and Heavy Fermions, eds. L.C Crupta and S.K. Malik IPlenum. New York. 1987~ p. 77. [5] L.J. van der Pauw, Philtps Rcs. Rcp. 13 11958) I. [6] A. Shibata, G. Oomi, Y. Onuky and T. Komatsubara, J. Phys. Soc. Japan 55 (1986) 2086; A. Percheron, J.C. Achard, O. Gorchov, B. Corm, t, D. Jerome and B. Coqblin, Solid State Commun. 12 t1973) 1289. [7] T. Kagayama, G. Oomi, R. Yagi, Y. lye, Y. Onuki and T. Komalsubara, J. Phys. Soc. Japan 61 11992) 2632.