Encina, S., and P. Pedrazzini. "Low Temperature Thermoelectric Power of Ce(Pd{1-x}Cux)2Si2." Journal of Low Temperature Physics 179, no. 1-2 (2015): 21–27.
Abstract: We present the thermoelectric power S(T) of the Ce(Pd 1−x Cu x ) 2 Si 2 alloy for temperatures \(1.5\,\mathrm{K} K. We observe three characteristic features across the \(0 substitution range: two positive maxima and a negative minimum, that are typical for Ce compounds that display, or lie close to, magnetism. Our analysis of the data shows that the high- T maximum is related to the Kondo effect on excited crystal-field levels, but that the low- T one cannot be simply associated with the Kondo scale, TK . We speculate that disorder induced by alloying can be at the origin of this discrepancy and can also be responsible for the low S(T) measured at low temperatures in the \(0.2 concentration range. We have extended electrical resistivity measurements on Ce(PdCu)Si 2 ( x=0.5 ) down to T∼40 mK in applied fields as high as 16 T.
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Sereni, J. G. "Entropy Bottlenecks at T->0 in Ce-Lattice and Related Compounds." Journal of Low Temperature Physics 179, no. 1-2 (2015): 126–137.
Abstract: A number of specific heat Cm anomalies are reported in Ce- and Yb-lattice compounds around 1 K which cannot be associated to usual phase transitions despite of their robust magnetic moments. Instead of a Cm(T) jump, these anomalies show coincident morphology: (i) a significant tail in Cm/T , with similar power law decay above their maxima ( T>Tm ), (ii) whereas a Cm(T2) dependence is observed below Tm . (iii) The comparison of their respective entropy gain Sm(T) indicates that ≈0.7R ln2 is condensed within the T>Tm tail, in coincidence with an exemplary spin-ice compound. Such amount of entropy arises from a significant increase of the density of low energy excitations, reflected in a divergent Cm(T>Tm)/T dependence. (iv) Many of their lattice structures present conditions for magnetic frustration. The origin of these anomalies can be attributed to an interplay between the divergent density of magnetic excitations at T→0 and the limited amount of degrees of freedom: Sm = R ln2 for a doublet-ground state. Due to this “entropy bottleneck,†the paramagnetic minimum of energy blurs out and the system slides into an alternative minimum through a continuous transition. A relevant observation in these very heavy fermion systems is the possible existence of an upper limit for Cm/TLimT→0 ≈7 J/mol K 2 observed in four Yb- and Pr-based compounds.
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Sereni, J. G. "Analysis of Entropy trajectories in very Heavy Fermions for Adiabatic Demagnetization Refrigeration at the T<1K range." Materials Today: Proceedings 14 (2019): 67–71.
Abstract: With the aim to improve classical paramagnetic salts performances for adiabatic magnetization refrigeration processes, relevant thermodynamic parameters of recently discovered very heavy fermion Yb-based intermetallic compounds are investigated within the sub-Kelvin range of temperature. Focusing on fixed thermal points, required for e.g. photonic devices operating within the 100 mK range in orbital satellites, the nature of respective magnetic ground states are discussed together with the effect of an 'entropy bottleneck' imposed by the Nernst postulate the entropy S(T) trajectories. To gain insight into the main criteria for a proper choice of suitable materials for alternative applications, thermomagnetic S(T,B) trajectories and respective derivatives: δS/δT and δS/δB, are analyzed as key parameters for this magnetocaloric process and compared with those from the classical salt Cerium-Magnesium-Nitride (CMN).
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Sereni, J. G. "Thermomagnetic properties of very heavy fermions suitable for adiabatic demagnetisation refrigeration at low temperature." Philosophical Magazine 100, no. 10 (2020): 1211.
Abstract: With the aim to improve the performance of classical paramagnetic salts for adiabatic refrigeration processes at the sub-Kelvin range, relevant thermodynamic parameters of some new Yb-based intermetallic compounds are analysed and compared. Two main applications are considered: (i) those requiring temperature fixed-points to be reached applying magnetic fields of moderate intensity for satellite applications, and (ii) those which can be used for controlled thermal drifts in laboratory applications. Different thermomagnetic trajectories of the entropy are identified depending on respective specific heat behaviours and the constraints imposed by the Nernst postulate at the sub-Kelvin range. Some simple relationships are proposed to compare the diverse magnetocaloric characteristics of different systems. This procedure allows to include the classical Cerium-Magnesium-Nitride (CMN) salt in that comparison.
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