Specific heat of Ce3Bi4Pt3 at 60 T

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Abstract

Kondo insulator materials such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3, and SmB6, are 3d, 4f and 5f intermetallic compounds. At high temperatures they behave like metals but a gap Δ in the conduction band opens at the Fermi energy as the temperature is reduced. It has been proposed that the formation of the low-temperature gap is a consequence of the hybridization between the conduction band and the f-electron levels. If this is true, Kondo metal physics should be recovered when the gap is closed at high magnetic fields. We report here specific heat results of Ce3Bi4Pt3 in DC and pulsed magnetic fields up to 60 T. We see evidence for the reduction of the gap in 18 T and a rapid increase of the Sommerfeld coefficient CH/TT→0 in 30T>H>40T. Numerical results and the analysis of the data with the Coqblin–Schrieffer model prove a field-induced Kondo insulator-to-Kondo metal crossover.

Introduction

Kondo insulator materials (KI) [1] have attracted considerable attention during recent years in part because the synthesis of quality single-crystal samples has allowed the measurement of the temperature dependence of the spin gap with high-resolution photoemission techniques [2], [3], [4]. Another way to study the nature of the spin gap in KI is to measure the magnetic field dependence of their transport properties. The electrical resistivity [5], [6], Hall effect [6], and magnetization [7] were measured in cubic Ce3Bi4Pt3 (Fig. 1) as a function of the magnetic field in DC and capacitor-bank driven pulsed magnets. A rather large negative magnetoresistance was observed at low temperatures in single-crystal samples together with a recovery of the charge carrier concentration in fields up to 60 T, evidence that strongly suggest that the spin gap can be closed in such magnetic fields [6]. More recent low-temperature/high-field magnetization measurements in powder samples showed no indication of insulator-to-metal crossover [7], in apparent contradiction with transport results. Measurements of the specific heat in magnetic field directly probe the evolution of the excitation spectrum, and Kondo gap in particular, and can therefore provide the key to understanding the physical origins of the very distinctive ground-state properties of KI.

The 60 T long pulse (60 TLP) magnet, at the National High Magnetic Field Laboratory-Los Alamos Pulsed Field Laboratory, is driven by a 1.4 GW synchronous power generator and produces a flat top field for a period of 100+ ms at 60 T and for longer time at lower fields. We have built a calorimeter out of plastic materials that enables us to perform heat capacity measurements at temperatures between 1.4 and 20 K in this magnet. We have also used the relaxation technique to measure the heat capacity of our sample at higher temperatures in a 20 T superconducting magnet, in a standard calorimeter.

Section snippets

Low magnetic fields

The presence of the spin gap in Ce3Bi4Pt3 should be evident in the temperature dependence of the specific heat at temperatures comparable to the gap as the conduction band is depopulated, if the phonon contribution is not too large and the sensitivity of the experiment is high. For magnetic fields available in DC superconducting (SC) magnets, we expect the value of the gap to remain high, and therefore, the experimental temperature range must be extended to values comparable with the zero-field

High magnetic fields

For high enough magnetic fields the spin gap should be totally suppressed and as the sample changes from insulator to metal, the low-temperature specific heat should change accordingly. During the magnetic field pulse produced by the 60 TLP magnet, which lasts for about 2 s, our plastic calorimeter can be regarded as thermally isolated, i.e. in an adiabatic condition. We used a heat pulse method, where a known amount of heat is delivered to the calorimeter using a chip resistor, to measure the

Discussion

In order to tell, whether our results indicate the recovery of the metallic Kondo state in high fields, the increase in γH observed in Ce3Bi4Pt3 needs to be put in perspective. Magnetic susceptibility and the high temperature neutron quasielastic line width measurements can be used to estimate a zero-field Kondo temperature of TK0=240–320 K. In turn, the Sommerfeld coefficient for a metal with such TK can be estimated using the expression for a single-impurity Kondo system [9] γ0=3×1.29πR/6TK0=

Conclusions

These results of thermodynamic measurements add new and essential information to what we know about Kondo insulators from transport measurements in high fields, and will aid in the construction of a theoretical model for this important class of strongly correlated compounds. Additionally, in the course of these studies we have demonstrated the feasibility of direct specific heat measurements in the extreme conditions of the pulsed magnetic fields produced by the 60 TLP magnet at the NHMFL-LANL

Acknowledgments

We thank Z. Fisk, J.D. Thompson and P. Schlottmann for discussions; J. Kim for his assistance with the thermometry calibration in the 30 T d.c. magnet at the NHMFL/Tallahassee; D. Rickel, C. Mielke, J. Betts, J. Schillig, M. Gordon, J. Sims and M. Pacheco for technical assistance and operation of the 60 TLP magnet.

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