- Email: [email protected]
Two energy scales in YbInCu4 from specific heat in high magnetic fields
Physica B 312–313 (2002) 344–345
Two energy scales in YbInCu4 from specific heat in high magnetic fields Marcelo Jaime*, Roman Movshovich, Neil Harrison, John L. Sarrao Los Alamos National Laboratory, MS E536, Los Alamos, NM 87545, USA
Abstract Metallic YbInCu4 undergoes a first-order valence transition at 42 K; where the specific volume increases by 0.5% upon cooling. It is believed that the valence transition is driven by band structure effects that help quench Yb localized moments via a Kondo mechanism. The known phase diagram indicates that the transition can be suppressed with external magnetic fields. We used an adiabatic calorimeter to measure the specific heat in the high-field phase of the material. Data obtained in magnetic fields up to 50 T show an enhanced Sommerfeld coefficient and anomalies in the specific heat that are compatible with a much depressed Kondo temperature. Our data indicate quite different energy scales in the low-field and high-field phases of YbInCu4 : r 2002 Elsevier Science B.V. All rights reserved. Keywords: Specific heat; High magnetic fields; Mixed valence
YbInCu4 is a mixed valence semimetal that undergoes a first-order phase transition at TV ¼ 42 K; where the specific volume in the FCC lattice increases by 0.5% upon cooling [1]. Above TV ; the magnetic susceptibility follows Curie–Weiss behavior with y ¼ 10 K; and meff ¼ 4:37mB corresponding to the Yb3þ ðJ ¼ 7=2Þ ground-state. Below TV ; the compound is non-magnetic, with a Sommerfeld coefficient g ¼ C=TjT-0 ¼ 50 mJ=mol Yb K2 : At TV ; the specific heat shows a sharp anomaly, where 80 % of the entropy corresponding to Yb3þ (Rln8) is recovered [2]. Applied magnetic fields and pressure effectively suppress TV : Complete suppression is accomplished at HV E34 T; and PV E20 kbar: A similar effect is observed with chemical substitutions, and data from samples of different composition collapse in the same line of the phase diagram ðH=HV ðT ¼ 0Þ; T=TV ðH ¼ 0ÞÞ [3]. It has been proposed that the first-order phase transition is driven by band structure effects. The Fermi energy, located in a region of low DOS at T > TV ; moves to increase NðEF Þ and reaches nearly complete Kondo compensation of Yb localized moments, reducing the cost in entropy paid for the free magnetic moments. This *Corresponding author. E-mail address: [email protected] (M. Jaime).
interpretation is supported by inelastic neutron scattering [4] and thermal expansion [5] results that show two different energy scales, i.e. TK ¼ 25 K above TV ; and TK ¼ 425 K below. Large high-quality single crystals have been grown using the flux method [2]. Magnetic susceptibility and specific heat measurements performed in DC magnetic fields (H ¼ 0; 18 T) show a sharp transition at TV ¼ 42 K: Because of the elliptic shape of the phase diagram, TV in H ¼ 18 T only moves slightly to 35 K [6]. Complete suppression of the first-order phase transition allows the study of YbInCu4 ground state in greater detail. This has been accomplished with specific heat measurement in pulsed magnetic fields at the NHMFLLANL pulsed field facility. An adiabatic calorimeter working up to 60 T was used for this purpose [7]. Reduction of TV to 12 K is observed in a magnetic field of 32 T; and complete suppression in H ¼ 36 T: The Sommerfeld coefficient C=T; displayed in Fig. 1, shows an 8-fold increase in the high-field phase to 400 mJ=mol Yb K2 : In addition to specific heat, the changes in the sample temperature as the magnetic field is increased at constant entropy (magnetocaloric effect) are recorded with the adiabatic calorimeter. In the high-field phase, the magnetic phase shows a linear increase in temperature
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 3 0 4 - 7
M. Jaime et al. / Physica B 312–313 (2002) 344–345 1.0
2.5 2.0
31.9 T
0.8
35.9 T
0.6
34 T 40 T 50 T
2
C/T (J/mol K )
345
M/M sat
1.5 1.0 0.5
34 T
0.4
50 T
H=0 0.2
0.0
0
50
100
200
150 2
250
300
dM/dT
350
2
T (K )
Fig. 1. Specific heat divided by temperature vs. temperature for YbInCu4 at constant magnetic fields H ¼ 0; 31:9 and 35:9 T showing a large enhancement of the Sommerfeld coefficient in the high-field phase.
with magnetic field [6]. The reversibility of the thermodynamic state in the high-field phase allows the use of Maxwell’s equations to calculate the high-field specific heat as C=T ¼ ðqM=qTjH Þ=ðqT=qHjS Þp qM=qTjH ; where M is the sample magnetization, since qT=qHjS is constant in the high-field phase. The magnetization vs. field up to 50 T was obtained in a capacitor-driven pulsed magnet, at temperatures between 1:4 and 100 K: From these curves, M vs. T at constant field has been extracted, see Fig. 2 and used to estimate the magnetic component in the specific heat of YbInCu4 : The socalculated specific heat shows a broad symmetric anomaly, resembling a Kondo anomaly, that moves to higher temperatures with magnetic field. The temperatures at which the anomaly is centered, C41 K in 34 T and C44 K in 50 T; are consistent with the Sommerfeld coefficient measured at constant field at 36 T and indicate an energy scale much smaller in the high-field phase magnetic phase than in the low field spinquenched phase. In conclusion, we have measured the specific heat of YbInCu4 in magnetic fields up to 36 T using an adiabatic calorimeter in a 60 T long pulse magnet. This experiment shows an enhanced Sommerfeld coefficient
0.0 0
20
40
60
80
100
T (K)
Fig. 2. Magnetization vs temperature at constant magnetic fields H ¼ 34; 40 and 50 T from which the specific heat is calculated using the equation C=Tp qM=qTjH : The Kondolike anomaly indicates TK C45 K when H ¼ 50 T:
in the high-field phase of YbInCu4 : We have also measured the high-field magnetization in the same single crystal and used it together with magnetocaloric data to calculate the specific heat at 34 and 50 T: Both experiments give consistent results, and indicate different energy scales in YbInCu4 low-field and high-field phases. Experiments at the NHMFL are supported by the US national Science Foundation through Cooperative Grant No. DMR 9016241, the State of Florida, and the US Department of Energy.
References [1] I. Felner, I. Nowik, Phys. Rev. B 37 (1988) 5633, and references therein. [2] J.L. Sarrao, et al., Phys. Rev. B 58 (1998) 409. [3] C.D. Immer, et al., Phys. Rev. B 56 (1997) 71. [4] J.M. Lawrence, et al., Phys. Rev. B 55 (1997) 14 467. [5] A.L. Cornelius, et al., Phys. Rev. B 56 (1997) 7993. [6] M. Jaime, et al., unpublished. [7] M. Jaime, et al., Nature 405 (2000) 160.
Two energy scales in YbInCu4 from specific heat in high magnetic fields Marcelo Jaime*, Roman Movshovich, Neil Harrison, John L. Sarrao Los Alamos National Laboratory, MS E536, Los Alamos, NM 87545, USA
Abstract Metallic YbInCu4 undergoes a first-order valence transition at 42 K; where the specific volume increases by 0.5% upon cooling. It is believed that the valence transition is driven by band structure effects that help quench Yb localized moments via a Kondo mechanism. The known phase diagram indicates that the transition can be suppressed with external magnetic fields. We used an adiabatic calorimeter to measure the specific heat in the high-field phase of the material. Data obtained in magnetic fields up to 50 T show an enhanced Sommerfeld coefficient and anomalies in the specific heat that are compatible with a much depressed Kondo temperature. Our data indicate quite different energy scales in the low-field and high-field phases of YbInCu4 : r 2002 Elsevier Science B.V. All rights reserved. Keywords: Specific heat; High magnetic fields; Mixed valence
YbInCu4 is a mixed valence semimetal that undergoes a first-order phase transition at TV ¼ 42 K; where the specific volume in the FCC lattice increases by 0.5% upon cooling [1]. Above TV ; the magnetic susceptibility follows Curie–Weiss behavior with y ¼ 10 K; and meff ¼ 4:37mB corresponding to the Yb3þ ðJ ¼ 7=2Þ ground-state. Below TV ; the compound is non-magnetic, with a Sommerfeld coefficient g ¼ C=TjT-0 ¼ 50 mJ=mol Yb K2 : At TV ; the specific heat shows a sharp anomaly, where 80 % of the entropy corresponding to Yb3þ (Rln8) is recovered [2]. Applied magnetic fields and pressure effectively suppress TV : Complete suppression is accomplished at HV E34 T; and PV E20 kbar: A similar effect is observed with chemical substitutions, and data from samples of different composition collapse in the same line of the phase diagram ðH=HV ðT ¼ 0Þ; T=TV ðH ¼ 0ÞÞ [3]. It has been proposed that the first-order phase transition is driven by band structure effects. The Fermi energy, located in a region of low DOS at T > TV ; moves to increase NðEF Þ and reaches nearly complete Kondo compensation of Yb localized moments, reducing the cost in entropy paid for the free magnetic moments. This *Corresponding author. E-mail address: [email protected] (M. Jaime).
interpretation is supported by inelastic neutron scattering [4] and thermal expansion [5] results that show two different energy scales, i.e. TK ¼ 25 K above TV ; and TK ¼ 425 K below. Large high-quality single crystals have been grown using the flux method [2]. Magnetic susceptibility and specific heat measurements performed in DC magnetic fields (H ¼ 0; 18 T) show a sharp transition at TV ¼ 42 K: Because of the elliptic shape of the phase diagram, TV in H ¼ 18 T only moves slightly to 35 K [6]. Complete suppression of the first-order phase transition allows the study of YbInCu4 ground state in greater detail. This has been accomplished with specific heat measurement in pulsed magnetic fields at the NHMFLLANL pulsed field facility. An adiabatic calorimeter working up to 60 T was used for this purpose [7]. Reduction of TV to 12 K is observed in a magnetic field of 32 T; and complete suppression in H ¼ 36 T: The Sommerfeld coefficient C=T; displayed in Fig. 1, shows an 8-fold increase in the high-field phase to 400 mJ=mol Yb K2 : In addition to specific heat, the changes in the sample temperature as the magnetic field is increased at constant entropy (magnetocaloric effect) are recorded with the adiabatic calorimeter. In the high-field phase, the magnetic phase shows a linear increase in temperature
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 3 0 4 - 7
M. Jaime et al. / Physica B 312–313 (2002) 344–345 1.0
2.5 2.0
31.9 T
0.8
35.9 T
0.6
34 T 40 T 50 T
2
C/T (J/mol K )
345
M/M sat
1.5 1.0 0.5
34 T
0.4
50 T
H=0 0.2
0.0
0
50
100
200
150 2
250
300
dM/dT
350
2
T (K )
Fig. 1. Specific heat divided by temperature vs. temperature for YbInCu4 at constant magnetic fields H ¼ 0; 31:9 and 35:9 T showing a large enhancement of the Sommerfeld coefficient in the high-field phase.
with magnetic field [6]. The reversibility of the thermodynamic state in the high-field phase allows the use of Maxwell’s equations to calculate the high-field specific heat as C=T ¼ ðqM=qTjH Þ=ðqT=qHjS Þp qM=qTjH ; where M is the sample magnetization, since qT=qHjS is constant in the high-field phase. The magnetization vs. field up to 50 T was obtained in a capacitor-driven pulsed magnet, at temperatures between 1:4 and 100 K: From these curves, M vs. T at constant field has been extracted, see Fig. 2 and used to estimate the magnetic component in the specific heat of YbInCu4 : The socalculated specific heat shows a broad symmetric anomaly, resembling a Kondo anomaly, that moves to higher temperatures with magnetic field. The temperatures at which the anomaly is centered, C41 K in 34 T and C44 K in 50 T; are consistent with the Sommerfeld coefficient measured at constant field at 36 T and indicate an energy scale much smaller in the high-field phase magnetic phase than in the low field spinquenched phase. In conclusion, we have measured the specific heat of YbInCu4 in magnetic fields up to 36 T using an adiabatic calorimeter in a 60 T long pulse magnet. This experiment shows an enhanced Sommerfeld coefficient
0.0 0
20
40
60
80
100
T (K)
Fig. 2. Magnetization vs temperature at constant magnetic fields H ¼ 34; 40 and 50 T from which the specific heat is calculated using the equation C=Tp qM=qTjH : The Kondolike anomaly indicates TK C45 K when H ¼ 50 T:
in the high-field phase of YbInCu4 : We have also measured the high-field magnetization in the same single crystal and used it together with magnetocaloric data to calculate the specific heat at 34 and 50 T: Both experiments give consistent results, and indicate different energy scales in YbInCu4 low-field and high-field phases. Experiments at the NHMFL are supported by the US national Science Foundation through Cooperative Grant No. DMR 9016241, the State of Florida, and the US Department of Energy.
References [1] I. Felner, I. Nowik, Phys. Rev. B 37 (1988) 5633, and references therein. [2] J.L. Sarrao, et al., Phys. Rev. B 58 (1998) 409. [3] C.D. Immer, et al., Phys. Rev. B 56 (1997) 71. [4] J.M. Lawrence, et al., Phys. Rev. B 55 (1997) 14 467. [5] A.L. Cornelius, et al., Phys. Rev. B 56 (1997) 7993. [6] M. Jaime, et al., unpublished. [7] M. Jaime, et al., Nature 405 (2000) 160.