
Correa, V. F., P. Pedrazzini, D. G. Franco, A. J. Rosa, B. Rubrecht, N. Haberkorn. "Lowtemperature thermal expansion of the topological material candidates βPtBi2 and βPdBi2." Physica B – Condensed Matter 641 (2022): 414102.
Abstract: We report on the lowtemperature ($T <$ 120 K) thermal expansion of the bismuthbased topological semimetal $\beta$PtBi$2$ and topological superconductor $\beta$PdBi$2$ candidates.
The linear thermalexpansion coefficient of tetragonal $\beta$PdBi$2$ shows a pronounced anisotropy between the $a$ and $c$axis while the volume thermalexpansion coefficient $\alphaV$($\beta$PdBi$2$) is considerable larger than $\alphaV$($\beta$PtBi$_2$).
The coefficient $\alphaV$($\beta$PtBi$2$) nearly matches the experimental specific heat, from which a Debye temperature $\thetaD =$ 199 K is obtained. On the other hand, $\alphaV$($\beta$PdBi$2$) reasonably fits the Debye model with $\thetaD =$ 138 K, extracted from the lowtemperature specific heat.
An almost constant Gr\"uneisen parameter $\Gamma \approx$ 2 is obtained for both compounds.
No magnetostriction is observed in any of both compounds up to $\mu_0 H =$ 16 T.
We compare our results with other Bibased topological materials.



Correa, V. F., A. G. Villagrán Asiares, D. Betancourth, S. Encina, P. Pedrazzini, P. S. Cornaglia, García D.J., J. G. Sereni, B. Maiorov, N. CarocaCanales et al. "Strong magnetoelastic effect in CeCo1âˆ’xFexSi as Néel order is suppressed." Physical Review B 100 (2019): 184409.
Abstract: A very strong magnetoelastic effect in the CeCo1âˆ’xFexSi alloys is reported. The strength of the magnetostrictive
effect can be tuned upon changing x. The moderate lowtemperature linear magnetostriction observed at low
Fe concentrations becomes very large (L
L (16T, 2K) = 3 Ã— 10âˆ’3) around the critical concentration xc â‰ˆ 0.23 at
which the longrange antiferromagnetic order vanishes. Upon increasing doping through the nonmagnetic region
(x > xc), the magnetostriction strength gradually weakens again. The interplay between magnetic order and the
Kondo screening appears to cause an enhanced valence susceptibility slightly changing the Ce ions valence,
ultimately triggering the large magnetostriction observed around the critical concentration. Previous studies
of the evolution of the lattice parameters with x as well as magnetization and xray absorption spectroscopy
measurements support this hypothesis.



Betancourth, D., V. F. Correa, J. I. Facio, Fernández J., V. Vildosola, R. LoraSerrano, J. M. Cadogan, A. A. Aligia, P. S. Cornaglia, and García D.J. "Magnetostriction reveals orthorhombic distortion in tetragonal Gd compounds." Physical Review B 99 (2019): 134406.
Abstract: We report detailed thermal expansion and magnetostriction experiments on GdCoIn5 and GdRh(In1âˆ’xCdx )5
(x = 0 and 0.025) singlecrystal samples that show a sudden change in the dilation at a field B for temperatures
below the NÃ©el transition temperature TN . We present a firstprinciples model including crystalfield effects,
dipolar and exchange interactions, and the dependence of the latter with lattice distortions in order to fully
account for the magnetostriction and magnetic susceptibility data. The meanfield solution of the model shows
that a transition between metastable states occurs at the field B. It also indicates that two degenerate phases
should coexist at temperatures below TN , which may explain the lack of observation, in highresolution
xray experiments, of an orthorhombic distortion at the NÃ©el transition, even though the experimentally
determined magnetic structure breaks the tetragonal symmetry and the magnetoelastic coupling from our model
is significant. These conclusions could be extended to other tetragonal Gdbased compounds that present the
same phenomenology.



Facio, J. I., D. Betancourth, P. Pedrazzini, V. F. Correa, V. Vildosola, D. J. García, and P. S. Cornaglia. "Why the Cobased 115 compounds are different: The case study of GdMIn5 (M = Co,Rh,Ir)." Physical Review B 91 (2015): 014409.
Abstract: The discovery in 2001 of superconductivity in some heavy fermion compounds of the RMIn5 (R = 4f or
5f elements; M = Co, Rh, Ir) family, has triggered an enormous amount of research into understanding the
physical origin of superconductivity and its relation with magnetism. Although many properties have been
clarified, there are still crucial questions that remain unanswered. One of these questions is the particular role
of the transition metal in determining the value of critical superconducting temperature (TC). In this work, we
analyze an interesting regularity that is experimentally observed in this family of compounds, where the lowest
NÂ´eel temperatures are obtained in the Cobased materials. We focus our analysis on the GdMIn5 compounds
and perform densityfunctionaltheorybased totalenergy calculations to obtain the parameters for the exchange
coupling interactions between the magnetic moments located at the Gd3+ ions. Our calculations indicate that
the ground state of the three compounds is a Ctype antiferromagnet determined by the competition between
the first and secondneighbor exchange couplings inside GdIn3 planes and stabilized by the couplings across
MIn2 planes. We then solve a model with these magnetic interactions using a meanfield approximation and
quantum Monte Carlo simulations. The results obtained for the calculated NÂ´eel and CurieWeiss temperatures,
the specific heat, and the magnetic susceptibility are in very good agreement with the existent experimental data.
Remarkably, we show that the firstneighbor interplane exchange coupling in the Cobased material is much
smaller than in the Rh and Ir analogs which leads to a more twodimensional magnetic behavior in the former.
This result explains the observed lower NÂ´eel temperature in Co115 systems and may shed light on the fact that
the Cobased 115 superconductors present the highest TC.



LoraSerrano, R., V. F. Correa, C. Adriano, C. Giles, J. G. S. Duque, E. Granado, P. G. Pagliuso, T. P. Murphy, E. C. Palm, S. W. Tozer et al. "First order magnetic transition and magnetoelastic effects in Sm2IrIn8." Physica B 403, no. 59 (2008): 1365–1367.
Abstract: We report measurements of temperature dependent heat capacity, thermal expansion and high resolution Xray diffraction (XRD) taken on single crystals of Sm2IrIn8 intermetallic compound. This compound belongs to the RmMnIn3m+2n family (R=rare earth, m=1, 2, n=0, 1 and M=Rh, Ir and Co) which includes a number of heavy fermion superconductors for R=Ce. Particularly, Sm2IrIn8 is the only member of this family to present a first order magnetic phase transition (FOMT). Both thermal expansion and heat capacity data show very pronounced sharps peaks at View the MathML source consistent with an FOMT. The linear thermalexpansion coefficient is anisotropic and both caxis and basal ab plane coefficients change discontinuously at 14.2 K. This change is negative for both direction in contrast to what was found for other members of family such as Ce2RhIn8 and CeRhIn5. The zerofield high resolution XRD data at 14.2 K shows no evidence for a tetragonaltoorthorhombic structural phase transition. We discuss our results considering tetragonal crystalline field effects (CEF), quadupolar interactions, antiferromagnetic domains and magnetoelastic effects.



Betancourth, D., J. I. Facio, P. Pedrazzini, C. B. R. Jesus, P. G. Pagliuso, V. Vildosola, P. S. Cornaglia, D. J. García, and V. F. Correa. "Low Temperature magnetic properties of GdCoIn5." Journal of Magnetism and Magnetic Materials 374 (2015): 744–747.
Abstract: A comprehensive experimental and theoretical study of the low temperature properties of GdCoIn5 was performed. Specific heat, thermal expansion, magnetization and electrical resistivity were measured in good quality single crystals down to 4He temperatures. All the experiments show a secondorderlike phase transition at 30 K probably associated with the onset of antiferromagnetic order. The magnetic susceptibility shows a pronounced anisotropy below T N with an easy magnetic axis perpendicular to the crystallographic Ä‰axis. Total energy GGA+U calculations indicate a ground state with magnetic moments localized at the Gd ions and allowed a determination of the GdGd magnetic interactions. Band structure calculations of the electron and phonon contributions to the specific heat together with Quantum Monte Carlo calculations of the magnetic contributions show a very good agreement with the experimental data. Comparison between experiment and calculations suggests a significant anharmonic contribution to the specific heat at high temperature (View the MathML sourceTâ‰³100K).



Correa, V. F., and F. J. Castro. "FirstOrder Phase Transformation at Constant Volume: A Continuous Transition?" Entropy 24 (2022): 31.
Abstract: We describe a firstorder phase transition of a simple system in a process where the
volume is kept constant. We show that, unlike what happens when the pressure is constant, (i) the
transformation extends over a finite temperature (and pressure) range, (ii) each and every extensive
potential (internal energy U, enthalpy H, Helmholtz energy F, and Gibbs energy G), and the entropy
S is continuous across the transition, and (iii) the constantvolume heat capacity does not diverge
during the transition and only exhibits discrete jumps. These nonintuitive results highlight the
importance of controlling the correct variables in order to distinguish between continuous and
discontinuous transitions. We apply our results to describe the transition between ice VI and liquid
water using thermodynamic information available in the literature and also to show that a firstorder
phase transition driven in isochoric condition can be used as the operating principle of a
mechanical actuator.

